Chapter3 The Cellular Level of Organization

A primary responsibility of each cell is to contribute to homeostasis, a term used in biology that refers to a dynamic state of balance within parameters that are compatible with life. For example, living cells require a water-based environment to survive in, and there are various physical (anatomical) and physiological mechanisms that keep all the trillions of living cells in the human body moist. This is one aspect of homeostasis. When a particular parameter, such as blood pressure or blood oxygen content, moves far enough out of homeostasis (generally becoming too high or too low), illness or disease—and sometimes death—inevitably results.

In this chapter, you will learn about the major components and functions of a prototypical, generalized cell and discover some of the different types of cells in the human body.

3.1 The Cell Membrane

Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.

Structure and Composition of the Cell Membrane

The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”) composed of a head (phosphate group) and tails (fatty acids). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions.

A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 3.1). The phosphate group is negatively charged, making the head polar and hydrophilic. A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic. A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids, and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region.

Figure 3.1 is a diagram of the structure of a phospholipid. The hydrophilic head is shown as a pink sphere with a structure diagram of its component molecules, phosphate and glycerol. Two hydrophobic tails are shown as rectangles. One is saturated fatty acid, and the other is unsaturated fatty acid, which is an angled rectangle on the right. — Figure 3.1 **Phospholipid Structure.** A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic, and a nonpolar lipid “tail,” which is hydrophobic. The tails can be any mixture of saturated and unsaturated fatty acids. Unsaturated fatty acids result in kinks in the hydrophobic tails.

The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 3.2). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.

Figure 3.2 is a diagram of a phospholipid bilayer. Two layers of phospholipids are arranged such that the hydrophobic tails are facing each other, and the hydrophilic heads are facing apart. The hydrophilic heads of the top layer face the extracellular environment, and the hydrophilic heads of the bottom layer face the intracellular environment. — Figure 3.2 **Phospholipid Bilayer.** The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads of the phospholipids contact the fluid inside and outside of the cell.

Membrane Proteins

The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins.

Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral proteins (Figure 3.3). As its name suggests, an integral protein is a protein that is embedded in the membrane. A channel protein is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell.

Another important group of integral proteins is cell recognition proteins, which serve to mark a cell’s identity so that it can be recognized by other cells. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell (e.g., hormones, neurotransmitters), and this binding induces a chemical reaction within the cell. A ligand is the specific molecule that binds to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel.

Example: A receptor-ligand interaction is seen in the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell.

Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate molecules on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles.

Figure 3.3 is an illustration of a lipid bilayer with different types of proteins, lipids, and cholesterol embedded in it. These structures are distributed throughout the lipid bilayer and include tubular channel protein, columnar integral membrane protein, conical peripheral membrane protein, cholesterol molecules, glycoprotein or a protein with a carbohydrate attached, and glycolipid or a lipid with a carbohydrate attached. — Figure 3.3 **Cell Membrane.** The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.

Example: It may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients.

The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defence cells “know” not to attack the person’s own body cells, but it is also the reason organs donated by another person might be rejected.

Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.

Transport Across the Cell Membrane

One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca⁺⁺, Na⁺, K⁺, and Cl⁻; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO₂), which must leave the cell.

The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using cellular energy from adenosine triphosphate (ATP).

Passive Transport

In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where ions are more concentrated to where ions are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient.) Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration of themselves.

Example: Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually, the sugar will diffuse throughout the tea until no concentration gradient remains.

In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules bump into each other and spread out faster than at cooler temperatures. Having an internal body temperature around 37°C thus also aids in the diffusion of particles within the body.

Whenever a substance exists in greater concentration on one side of a selectively permeable membrane, such as a cell membrane, any substance that can move down its own concentration gradient across the membrane will do so.

Example: Substances that can easily diffuse through the lipid bilayer of the cell membrane include the gases oxygen (O₂) and CO₂. O₂ generally diffuses into cells because it is more concentrated outside of them, and CO₂ typically diffuses out of cells because it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane.

Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O₂ inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO₂ as a by-product of metabolism, CO₂ concentrations rise within the cytoplasm; therefore, CO₂ will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower. This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion (Figure 3.4).

Figure 3.4 is a three-part diagram of the diffusion of small uncharged molecules across the lipid bilayer or plasma membrane over time. First, all the uncharged molecules are above the lipid bilayer in the extracellular fluid. Then, a few uncharged molecules have traversed the lipid bilayer into the cytoplasm below. Finally, the uncharged molecules are evenly distributed between the extracellular fluid and the cytoplasm on either side of the lipid bilayer. — Figure 3.4 **Simple Diffusion Across the Cell (Plasma) Membrane.** The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.

Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Very small polar molecules, such as water, can cross via simple diffusion due to their size. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion, as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size, charge, and/or polarity (Figure 3.5).

Example: Facilitated diffusion moves glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.

Figure 3.5 is a two-part illustration of the different means of facilitated diffusion across the plasma membrane. The first illustration shows a channel protein allowing the transport of solutes across a membrane through its tubular structure. The second illustration shows carrier proteins that selectively transmit solutes across a membrane. — Figure 3.5 **Facilitated Diffusion.** (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins and usually mildly discriminate what they allow through based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.

To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate (assist) its inward diffusion.

Example: Even though sodium ions (Na⁺) are highly concentrated outside of cells, these electrolytes are charged and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”) so that Na⁺ ions can move down their concentration gradient from outside the cells to inside the cells.

There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.

Water can also move freely across the cell membranes of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. Osmosis is the diffusion of water from a biologic compartment with a lower concentration of solutes to one with a higher concentration of solutes (down its concentration gradient) through a selectively permeable membrane (Figure 3.6).

Tonicity is a measure of a solution’s ability to change the volume of cells by altering their water content. The movement of water molecules is not itself regulated by some cells, so it is important that these cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be isotonic (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).

Figure 3.6 is a two-part illustration of the diffusion of water by osmosis through a semipermeable membrane. First, the water level is equal on either side of the membrane, and the solutes are more concentrated in the right half of the beaker. Then water is moved through osmosis from the left side to the right side of the beaker to balance the concentration of solutes, raising the water level on the right side of the membrane. — Figure 3.6 **Osmosis.** Osmosis is the diffusion of water through a selectively permeable (semipermeable) membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic. Water “follows” salt.

Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 3.7). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all the body’s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.

Figure 3.7 is an illustration of how red blood cells respond to the tonicity of a solution. In the hypertonic case, the red blood cells are shrivelled as water molecules leave the cell. In the isotonic case, red blood cells look normal as water molecules are shown entering and leaving the cell at the same rate. In the hypotonic case, red blood cells are overinflated to the point of bursting as water molecules enter the cell. — Figure 3.7 **Concentration of Solutions.** A hypertonic solution has a higher solute concentration than another solution. An isotonic solution has a solute concentration equal to that of another solution. A hypotonic solution has a lower solute concentration than another solution.

Active Transport

There are two types of active transport, primary and secondary.

For all the transport methods described in the previous section, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During primary active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers and usually against its concentration gradient.

One of the most common types of active transport involves proteins that serve as pumps. The word pump probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration of itself to an area of high concentration of itself).

The sodium-potassium pump, which is also called Na⁺/K⁺ ATPase, transports sodium ions out of a cell while moving potassium ions into the cell. The Na⁺/K⁺ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively charged (at around −70 mV) relative to the outside.

The negative electrical gradient is maintained because each Na⁺/K⁺ pump moves three Na⁺ ions out of the cell and two K⁺ ions into the cell for each ATP molecule that is used (Figure 3.8). This process is so important for nerve cells that it accounts for the majority of their ATP usage.

Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane.

Example: The sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell.

Figure 3.8 is a diagram of the action of sodium potassium pumps embedded in a plasma membrane. Refer to the extended description for more details. — Figure 3.8 **Sodium-Potassium Pump.** The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.
Extended description

Secondary active transport harnesses an electrochemical gradient that was established by primary active transport to drive the transport of a second molecule. Symporters are secondary active transporters that move two substances in the same direction.

Example: The sodium-glucose symporter uses sodium ions to “pull” glucose molecules into the cell. Because cells store glucose for energy, glucose is typically at a higher concentration inside the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.

Conversely, antiporters are secondary active transport systems that transport substances in opposite directions.

Example: The sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H⁺) out of the cell. The sodium-hydrogen antiporter is used to maintain the pH of the cell’s interior.

Other Types of Active Transport

Other forms of active transport do not involve membrane carriers. Endocytosis is the process of a cell ingesting material by enveloping it in a portion of its cell membrane and then pinching off that portion of membrane (Figure 3.9). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane embedded with proteins. Endocytosis often brings materials into the cell that must be broken down or digested.

Phagocytosis is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like a little Pac-Man, an immune cell’s job is to patrol body tissues for unwanted matter such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, pinocytosis brings fluid containing dissolved substances into a cell through membrane vesicles.

Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis, or endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will bring in the entire part of the cell membrane containing the receptor-ligand complexes.

Figure 3.9 is a diagram of three types of endocytosis, which are phagocytosis, pinocytosis, and receptor-mediated endocytosis. Refer to the extended description for more details. — Figure 3.9 **Three Forms of Endocytosis.** Endocytosis is a form of active transport in which a cell envelops extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in solution. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.
Extended description

Example: Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes.

In contrast with endocytosis, exocytosis is the process of a cell exporting material using vesicles (Figure 3.10). Many cells manufacture substances that must be secreted; these substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases its contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis. Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.

Figure 3.10 is an illustration of exocytosis. A vesicle is shown fusing with the membrane from within the cell and then releasing its contents into the extracellular fluid. — Figure 3.10 **Exocytosis.** Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.

3.2 The Cytoplasm and Cellular Organelles

All living cells in multicellular organisms contain an internal cytoplasmic compartment and a nucleus within the cytoplasm. Cytosol, the jellylike substance within the cell, provides the fluid medium necessary for biochemical reactions. Eukaryotic cells, including all animal cells, also contain various cellular organelles. An organelle is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function. Just as the various bodily organs work together in harmony to perform all of a human’s functions, the many different cellular organelles work together to keep the cell healthy and performing all its important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm. The nucleus is a cell’s central organelle, which contains the cell’s DNA (Figure 3.11).

Figure 3.11 is an illustration of a human cell with the intracellular organelles and structures labelled. These include mitochondria, plasma membrane, microtubule, centrosome, microfilament, lysosome, smooth endoplasmic reticulum, secretory vesicle, peroxisome, vacuole, cytoplasm, Golgi vesicle, Golgi apparatus, chromatin, nucleolus, nucleus, rough endoplasmic reticulum, ribosomes, and intermediate filament. — Figure 3.11 **Prototypical Human Cell.** While this image is not indicative of any one particular human cell, it is a prototypical example of a cell containing the primary organelles and internal structures.

Organelles of the Endomembrane System

A set of three major organelles together forms a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions (Figure 3.12).

Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. Rough ER (RER) is so called because its membrane is dotted with embedded granules—organelles called ribosomes—giving the RER a bumpy appearance. A ribosome is a tiny, non-membrane-bound, free-floating organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA subunits consisting of ribosomal RNA (rRNA) and proteins that wrap around messenger RNA (mRNA) to start the process of translation, followed by protein synthesis for use in the cytosol of the cell.

Figure 3.12 is a three-part diagram of the structure of the endoplasmic reticulum. The first part is an illustration of the rough and smooth endoplasmic reticulum relative to the nucleus. The upper right micrograph shows the rough endoplasmic reticulum. Dark granules along the edges of the endoplasmic reticulum are labelled as ribosomes. The lower-left micrograph shows the smooth endoplasmic reticulum amid other unlabelled organelles. — Figure 3.12 **Endoplasmic Reticulum (ER).** (a) The ER is a winding network of thin membranous sacs found in close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance and function (source: mouse tissue). (b) Rough ER is studded with numerous ribosomes, which are sites of protein synthesis (source: mouse tissue). EM × 110,000. (c) Smooth ER synthesizes phospholipids, steroid hormones, regulates the concentration of cellular Ca⁺⁺, metabolizes some carbohydrates, and breaks down certain toxins (source: mouse tissue). EM × 110,510. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

Smooth ER (SER) lacks these ribosomes. One of the main functions of the SER is in the synthesis of lipids. The SER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities of steroid hormones, such as those of the female ovaries and male testes, contain large amounts of SER. In addition to lipid synthesis, the SER also sequesters (i.e., stores) and regulates the concentration of cellular Ca⁺⁺, a function extremely important in skeletal muscle cells, where Ca⁺⁺ is the trigger for muscle contraction. The SER additionally metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.

In contrast with the SER, the primary job of the RER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of RER). Typically, a protein is synthesized within the ribosome and released inside the channel of the RER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.

The Golgi Apparatus

The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post office.

The Golgi apparatus looks like stacked, flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released from the opposite side after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted (Figure 3.13).

Lysosomes

Some of the protein products packaged by the Golgi include digestive enzymes that are meant to remain inside the cell for use in breaking down certain materials. The enzyme-containing vesicles released by the Golgi may form new lysosomes or fuse with existing lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as damaged organelles. Autophagy is the process of a cell digesting its own structures.

Figure 3.13 is a two-part diagram of the Golgi apparatus. An illustration shows the location and structure of the Golgi apparatus between the plasma membrane and the rough endoplasmic reticulum and nucleus. A micrograph shows the folds of the Golgi in detail. Refer to the extended description for more details. — Figure 3.13 **Golgi Apparatus.** (a) The Golgi apparatus manipulates products from the rough ER and also produces new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell for inclusion in cell membranes, including the plasma membrane, and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes (or packaged and sent for fusion with existing lysosomes). (b) An electron micrograph of the Golgi apparatus.
Extended description

Example: When certain immune defence cells (white blood cells) phagocytize bacteria, the bacterial cell is transported into a lysosome and digested by the enzymes inside. As one might imagine, such phagocytic defence cells contain large numbers of lysosomes.

Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. This “self-destruct” mechanism is called autolysis and makes the process of cell death controlled (a mechanism called apoptosis).

Organelles for Energy Production and Detoxification

In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert to chemical energy that can be used to power biochemical reactions. Another important function of the cell is detoxification. Humans take in all sorts of toxins from the environment and also produce harmful chemicals as by-products of cellular processes. Cells called hepatocytes in the liver detoxify many of these toxins.

Mitochondria

A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of two lipid bilayer membranes (Figure 3.14). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. The middle of the mitochondrion contains a fluid-filled substance called the matrix. The inner membrane and the matrix are locations for many of the reactions that convert energy stored in nutrient molecules (such as glucose) into adenosine triphosphate (ATP), which provides usable energy to the cell.

Cells use ATP constantly, so the mitochondria are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly breathe them in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria. Nerve cells also need large quantities of ATP to run their sodium-potassium pumps. Therefore, an individual neuron will be loaded with over a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically active, might only have a couple hundred mitochondria.

Figure 3.14 is a two-part diagram of the structure of a mitochondrion. An illustration shows how the outer membrane envelopes the mitochondrion. Its inner membrane fills most of the interior space of the organelle and is lined with deep groove-like cristae. Between the outer and inner membranes is the intermembrane space. A micrograph shows the structure of a mitochondrion in detail. — Figure 3.14 **Mitochondrion.** The mitochondria are the energy-conversion factories of the cell. (a) A mitochondrion is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. (b) An electron micrograph of mitochondria. EM × 236,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Peroxisomes

Like lysosomes, a peroxisome is a single-layer membrane-bound cellular organelle that contains mostly enzymes. Peroxisomes perform a couple of different functions, including lipid metabolism and chemical detoxification. In contrast to the digestive enzymes found in lysosomes, the enzymes within peroxisomes serve to transfer hydrogen atoms from various molecules to oxygen, producing hydrogen peroxide (H₂O₂). In this way, peroxisomes neutralize poisons such as alcohol.

The Cytoskeleton

Much like the bony skeleton structurally supports the human body, the cytoskeleton helps the cells maintain their structural integrity. The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and the transportation of substances within the cell.

The cytoskeleton forms a complex threadlike network throughout the cell, consisting of three different kinds of protein-based filaments: microfilaments, intermediate filaments, and microtubules (Figure 3.15).

A microtubule is the thickest of the three; it is a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Microtubules also make up two types of cellular appendages important for motion: cilia and flagella. Cilia are found on many cells of the body, including the epithelial cells that line the airways of the respiratory system. Cilia move rhythmically; they beat constantly, moving waste materials such as dust, mucus, and bacteria upward through the airways, away from the lungs, and toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary toward the uterus. A flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated cell in humans is the sperm cell that must propel itself toward female egg cells.

A very important function of microtubules is to set the paths (somewhat like railroad tracks) along which the genetic material can be pulled (a process requiring ATP) during cell division so that each new daughter cell receives the appropriate set of chromosomes. Two short, identical microtubule structures called centrioles are found near the nuclei of cells. A centriole can serve as the cellular origin point for microtubules, extending outward as cilia or flagella, or can assist with the separation of DNA during cell division. Microtubules grow out from the centrioles by adding more tubulin subunits, like adding additional links to a chain.

Figure 3.15 is a three-part diagram of the different cytoskeletal components in a cell. These are the tubulin dimer, actin subunit, and fibrous subunit. Refer to the extended description for more details. — Figure 3.15 **The Three Components of the Cytoskeleton.** The cytoskeleton consists of (a) microtubules, (b) microfilaments, and (c) intermediate filaments. The cytoskeleton plays an important role in maintaining cell shape and structure, promoting cellular movement, and aiding cell division.
Extended description

In contrast with microtubules, microfilaments, or actin filaments, are a thinner type of cytoskeletal filament (see Figure 3.15b). Like microtubules, actin filaments are long chains of single subunits (called actin subunits). Actin fibres as well as the protein called myosin are responsible for muscle contraction. Actin also has an important role during cell division. When a cell is about to split in half during cell division, actin filaments work with myosin to create a cleavage furrow that eventually splits the cell down the middle, forming two new cells from the original cell.

An intermediate filament, as its name would suggest, is a filament intermediate in thickness between the microtubules and microfilaments (see Figure 3.15c). Intermediate filaments are made up of long fibrous subunits of proteins such as keratin, which are wound together like the threads that compose a rope. Intermediate filaments, in concert with the microtubules, are important for maintaining cell shape and structure. Unlike the microtubules, which resist compression, intermediate filaments resist tension—the forces that pull apart cells. There are many cases in which cells are prone to tension, such as when epithelial cells of the skin are compressed, tugging them in different directions. Intermediate filaments help anchor organelles together within a cell and also link cells to other cells by forming special cell-to-cell junctions.

3.3 The Nucleus and DNA Replication

The nucleus is the largest and most prominent of a cell’s organelles (Figure 3.16). The nucleus is generally considered the control centre of the cell because it stores as DNA most of the genetic instructions for manufacturing protein. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 3.17); these are known as multinucleated cells. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 3.18). Without nuclei, the lifespan of RBCs is short, so the body must produce new ones constantly.

Figure 3.16 is an illustration of the structure of the nucleus. The nucleolus sits as the centre of the nucleus and looks like a dense ball of yarn. It is surrounded by strands of condensed chromatin. The nucleus is surrounded by a nuclear envelope that is covered in numerous nuclear pores. External to the nuclear envelope are the cisternae of the rough endoplasmic reticulum. — Figure 3.16 **The Nucleus.** The nucleus is the control centre of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell.

Figure 3.17 is a micrograph of multiple muscle cells. They are long, thin structures sandwiched alongside each other. One muscle cell is highlighted amid the others to demonstrate that there are multiple nuclei present in each cell. — Figure 3.17 **Multinucleate Muscle Cell.** Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei and is referred to as “multinucleated.” These muscle cells are long and fibrous (often referred to as muscle fibres). During development, many smaller cells fuse to form a mature muscle fibre. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM × 104.3. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Inside the nucleus lies the blueprint that dictates everything a cell will do and all the products it will make. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that each new cell receives a full complement of DNA through DNA replication. The nucleus is also the site of RNA production. Messenger RNA, or mRNA, carries the “message” of the DNA out of the nucleus to the cytosol. This is known as transcription. The following section will explore the structure of the nucleus and its contents as well as the process of DNA replication.

Figure 3.18 is a pair of micrographs of a young red blood cell or erythroblast extruding its nucleus. In the left micrograph, the nucleus is partially extruded from the red blood cell. The nucleus is enveloped by a thin layer of cellular fluid and contiguous with the rest of the cell. In the right micrograph, the nucleus is surrounded by an envelope of cellular fluid but is completely extruded from the cell. — Figure 3.18 **Red Blood Cell Extruding Its Nucleus.** Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School © 2012)

Organization of the Nucleus and Its DNA

Like most other cellular organelles, the nucleus is surrounded by a membrane, called the nuclear envelope. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There can also be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for the construction of ribosomes, called ribosomal or rRNA. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. DNA in the nucleus is always found with proteins called histones. This DNA and protein structure is called chromatin (Figure 3.19).

Along the chromatin threads, the DNA is wrapped around a set of histone proteins. When a cell is in the process of division, the chromatin condenses into chromosomes so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.

Figure 3.19 is a diagram of the macrostructure of DNA. An X-shaped chromosome is shown with its component chromatin, itself a tightly wound helical structure. The chromatin is expanded to show its nucleosomes with their component histones. These structures unravel further to reveal the DNA double helix. Finally, the DNA helix is flattened out to show its nucleotide base pairs. — Figure 3.19 **DNA Macrostructure.** Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide.

DNA Replication

In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal muscle fibres, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off the surface by friction.

A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. This double helix can be constructed easily because the two strands are antiparallel, meaning the two strands run in opposite directions. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 3.20). The two sides of the ladder are not identical but complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member. The four nitrogen-containing bases of a nucleotide in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.

Figure 3.20 is a three-part diagram of the molecular structure of the nitrogenous bases of DNA and the bonds they form to create base pairs between the helical sugar phosphate backbones. Refer to the extended description for more details. — Figure 3.20 **Molecular Structure of DNA.** The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds.
Extended description

DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 3.21 and described here:

Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA.
Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct nucleotides to complement the template strand, synthesizing a new strand nucleotide by nucleotide. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.
Stage 3: Termination. Once the two original strands are bound to their own finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.

Figure 3.21 is a diagram of the process of DNA replication. A chromosome is shown expanding into the original template DNA, which splits into two halves at the replication fork. A wedge-shaped helicase is present at the replication fork. DNA polymerases are shown adding free nucleotides to the leading and lagging strands to form new DNA produced with the pattern established by the original template DNA. — Figure 3.21 **DNA Replication.** DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one preexisting strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.”

Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is semiconservative, because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is replicated. As you might imagine, it is very important that DNA replication takes place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and correct them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.

3.4 Protein Synthesis

It was mentioned earlier that DNA provides a “blueprint” for the cell structure and physiology. This refers to the fact that DNA contains the information necessary for the cell to build one very important type of molecule: the protein. Most structural components of the cell are made up, at least in part, by proteins, and virtually all the functions that a cell carries out are completed with the help of proteins. One of the most important classes of proteins is enzymes, which help speed up necessary biochemical reactions that take place inside the cell. Some of these critical biochemical reactions include building larger molecules from smaller components (such as occurs during DNA replication or synthesis of microtubules) and breaking down larger molecules into smaller components (such as when harvesting chemical energy from nutrient molecules). Whatever the cellular process may be, it is almost sure to involve proteins. A gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein. Gene expression, which transforms the information coded in a gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are made.

From DNA to RNA: Transcription

DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis. This intermediate messenger is messenger RNA (mRNA), a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus and into the cytoplasm, where it is used to produce proteins.

There are several different types of RNA, each having different functions in the cell.

Gene expression begins with the process called transcription, which is the synthesis of a strand of mRNA that is complementary to the gene of interest. This process is called transcription because the mRNA is like a transcript, or copy, of the gene’s DNA code. Transcription begins in a fashion somewhat like DNA replication, in that a region of DNA unwinds and the two strands separate; however, only that small portion of the DNA will be split apart. The triplets within the gene on this section of the DNA molecule are used as the template to transcribe the complementary strand of RNA (Figure 3.22). A codon is a three-base sequence of mRNA, so called because it directly encodes amino acids. Like DNA replication, there are three stages to transcription: initiation, elongation, and termination.

Stage 1: Initiation. A region at the beginning of the gene called a promoter—a particular sequence of nucleotides—triggers the start of transcription.
Stage 2: Elongation. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A, C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA. This process builds a strand of mRNA.

Figure 3.22 is a diagram of RNA polymerase transcribing a DNA template strand into its corresponding RNA transcript. A chromosome is shown expanding into the double helix DNA, which splits into two halves in the lower part of the diagram. As one half passes through an RNA polymerase, an RNA transcript is produced. The two halves of DNA are rejoined and connect back into the chromosome to form a closed loop. — Figure 3.22 **Transcription: From DNA to mRNA.** In the first of the two stages of making protein from DNA, a gene on the DNA molecule is transcribed into a complementary mRNA molecule.

Stage 3: Termination. At the end of the gene, a sequence of nucleotides called the terminator sequence causes the new RNA to fold up on itself. This fold causes the RNA to separate from the gene and from RNA polymerase, ending transcription.

From RNA to Protein: Translation

Like translating a book from one language into another, the codons on a strand of mRNA must be translated into the amino acid alphabet of proteins. Translation is the process of synthesizing a chain of amino acids called a polypeptide. Ribosomal RNA (rRNA) is a type of RNA that, together with proteins, composes the structure of the ribosome. Ribosomes exist in the cytoplasm as two distinct components, a small and a large subunit. When an mRNA molecule is ready to be translated, the two subunits come together and attach to the mRNA. The ribosome provides a substrate for translation, bringing together and aligning the mRNA molecule with the molecular “translators” that must decipher its code.

The other major requirement for protein synthesis is the translator molecules that physically “read” the mRNA codons. Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino acids to the ribosome and attaches each new amino acid to the last, building the polypeptide chain one by one. Thus, tRNA transfers specific amino acids from the cytoplasm to a growing polypeptide. The tRNA molecules must be able to recognize the codons on mRNA and match them with the correct amino acid. The tRNA is modified for this function. On one end of its structure is a binding site for a specific amino acid. On the other end is a base sequence that matches the codon specifying its particular amino acid. This sequence of three bases on the tRNA molecule is called an anticodon.

Much like the processes of DNA replication and transcription, translation consists of three main stages: initiation, elongation, and termination. The initiation stage takes place with the binding of a ribosome to an mRNA transcript. The elongation stage involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and codon sequences are bound (remember, they are complementary base pairs), the tRNA presents its amino acid cargo, and the growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various enzymes and requires energy. The tRNA molecule then releases the mRNA strand, the mRNA strand shifts one codon over in the ribosome, and the next appropriate tRNA arrives with its matching anticodon. This process continues until the final codon on the mRNA is reached, which provides a “stop” message that signals termination of translation and triggers the release of the complete, newly synthesized protein. Thus, a gene within the DNA molecule is transcribed into mRNA, which is then translated into a protein product (Figure 3.23).

Commonly, an mRNA transcription will be translated simultaneously by several adjacent ribosomes. This increases the efficiency of protein synthesis. A single ribosome might translate an mRNA molecule in approximately one minute; so multiple ribosomes aboard a single transcript could produce multiple times the number of the same protein in the same minute. A polyribosome is a string of ribosomes translating a single mRNA strand.

Figure 3.23 is a diagram of the processes of transcription and translation in a cell. DNA to mRNA transcription takes place inside the nucleus of the cell, and translation from mRNA to protein takes place in the cytoplasm of the cell. — Figure 3.23 **From DNA to Protein: Transcription Through Translation.** Transcription within the cell nucleus produces an mRNA molecule, which is modified and then sent into the cytoplasm for translation. The transcript is decoded into a protein (red thick line in cytoplasm) with the help of a ribosome (yellow structure connected to the protein in cytoplasm) and tRNA molecules.

3.5 Cell Growth and Division

So far in this chapter, you have read numerous times about the importance and prevalence of cell division. While there are a few cells in the body that do not undergo cell division (such as gametes, red blood cells, most neurons, and some muscle cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells except for the cells that produce eggs and sperm (which are referred to as germ cells) are somatic cells. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent). A homologous pair of chromosomes is the two copies of a single chromosome found in each somatic cell. The human is a diploid organism, having 23 homologous pairs of chromosomes in each of the somatic cells. The condition of having pairs of chromosomes is known as diploidy. Ova and sperm contain only half the DNA of a diploid cell and are called haploid.

Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two new cells.

The Cell Cycle

One “turn” or cycle of the cell cycle consists of two general phases: interphase, followed by mitosis and cytokinesis. Interphase is the period of the cell cycle during which the cell is not dividing. The majority of cells are in interphase most of the time. Mitosis is the division of genetic material, during which the cell nucleus breaks down and two new, fully functional nuclei are formed. Cytokinesis divides the cytoplasm into two distinctive cells.

Interphase

A cell grows and carries out all normal metabolic functions and processes in a period called G₁ (Figure 3.24). G₁ phase (gap 1 phase) is the first gap, or growth phase, in the cell cycle. For cells that will divide again, G₁ is followed by replication of the DNA during the S phase. The S phase (synthesis phase) is the period during which a cell replicates its DNA.

Figure 3.24 is a diagram of the different stages of the cell cycle. It is a circular graph divided into four wedges. The first two are equally sized, labelled G1 phase and S phase. The third is slightly smaller and labelled G2. The smallest wedge is labelled M. The G0 phase, where the cells are not actively dividing, is exterior to the circular graph. G0 feeds into the starting G1 phase. — Figure 3.24 **Cell Cycle.** The two major phases of the cell cycle include mitosis (designated M), when the cell divides, and interphase, when the cell grows and performs all its normal functions. Interphase is further subdivided into G₁, S, and G₂ phases.

After the synthesis phase, the cell proceeds through the G₂ phase. The G₂ phase is a second gap phase, during which the cell continues to grow and makes the necessary preparations for mitosis. Between the G₁, S, and G₂ phases, cells will vary the most in their duration of the G₁ phase. It is here that a cell might spend a couple of hours or many days. The S phase typically lasts between 8 and 10 hours, and the G₂ phase lasts approximately 5 hours. In contrast to these phases, the G₀ phase is a resting phase of the cell cycle. Cells that have temporarily stopped dividing and are resting (a common condition), and cells that have permanently ceased dividing (like nerve cells) are said to be in G₀.

The Structure of Chromosomes

DNA is in the form of chromatin during interphase but condenses to form chromosomes during cell division.

Billions of cells in the human body divide every day. During the S phase of interphase, the amount of DNA within the cell precisely doubles. Therefore, after DNA replication but before cell division, each cell actually contains two copies of each chromosome. Each copy of the chromosome is referred to as a sister chromatid and is physically bound to the other copy. (Note that the term sister chromatid is used regardless of the sex of the person.) The centromere is the structure that attaches one sister chromatid to another. Because a human cell has 46 chromosomes, during this phase, there are 92 chromatids (46 × 2) in the cell. Make sure not to confuse the concept of a pair of chromatids (one chromosome and its exact copy attached during mitosis) and a homologous pair of chromosomes (two paired chromosomes that were inherited separately, one from each parent; Figure 3.25).

Mitosis and Cytokinesis

The mitotic phase of the cell typically takes between 1 and 2 hours. During this phase, a cell undergoes two major processes. First, it completes mitosis, during which the contents of the nucleus are equitably pulled apart and distributed between its two halves. Cytokinesis then occurs, dividing the cytoplasm and cell body into two new cells. Mitosis is divided into four major stages that take place after interphase (Figure 3.26) and in the following order: prophase (with prometaphase), metaphase, anaphase, and telophase. The process is then followed by cytokinesis.

Prophase is the first phase of mitosis, during which the loosely packed chromatin coils and condenses into visible chromosomes. During prophase, each chromosome becomes visible with its identical partner attached, forming the familiar X shape of sister chromatids. The nucleolus disappears early during this phase, and the nuclear envelope also disintegrates.

Figure 3.25 is an illustration of a pair of chromosomes. The chromosomes are roughly X-shaped with elongated lower legs. The upper legs are homologous chromosomes. The narrow part of the X where the legs come together is the centromere. Two kinetochores are embedded in the centromere of each chromosome. The longer lower legs are sister chromatids. — Figure 3.25 **A Homologous Pair of Chromosomes with Their Attached Sister Chromatids.** The red and blue colours correspond to a homologous pair of chromosomes. Each member of the pair was separately inherited from one parent. Each chromosome in the homologous pair is also bound to an identical sister chromatid, which is produced by DNA replication, and results in the familiar “X” shape.

Figure 3.26 is a table showing the different stages of mitosis and cytokinesis using both drawings and text. The stages of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase. These are followed by cytokinesis, which differs in animal and plant cells. Refer to the extended description for more details. — Figure 3.26 **Cell Division: Mitosis Followed by Cytokinesis.** The stages of cell division oversee the separation of identical genetic material into two new nuclei, followed by the division of the cytoplasm.
Extended description

A major occurrence during prophase concerns a very important structure that contains the origin site for microtubule growth. Recall the cellular structures called centrioles that serve as origin points from which microtubules extend. These tiny structures also play a very important role during mitosis. A centrosome is a pair of centrioles together. The cell contains two centrosomes side by side, which begin to move apart during prophase. As the centrosomes migrate to two different sides of the cell, microtubules begin to extend from each like long fingers from two hands extending toward each other. The mitotic spindle is a structure composed of the centrosomes and their emerging microtubules.

Near the end of prophase, there is an invasion of the nuclear area by microtubules from the mitotic spindle. The nuclear membrane has disintegrated, and the microtubules attach themselves to the centromeres that adjoin pairs of sister chromatids. The kinetochore is a protein structure on the centromere that is the point of attachment between the mitotic spindle and the sister chromatids. This stage is referred to as late prophase or “prometaphase” to indicate the transition between prophase and metaphase.

Metaphase is the second stage of mitosis. During this stage, the sister chromatids, with their attached microtubules, line up along a linear plane in the middle of the cell. A metaphase plate forms between the centrosomes that are now located at either end of the cell. The metaphase plate is the name for the plane through the centre of the spindle on which the sister chromatids are positioned. The microtubules are now poised to pull apart the sister chromatids and bring one from each pair to each side of the cell.

Anaphase is the third stage of mitosis. Anaphase takes place over a few minutes, when the pairs of sister chromatids are separated from one another, forming individual chromosomes once again. These chromosomes are pulled to opposite ends of the cell by their kinetochores as the microtubules shorten. Each end of the cell receives one partner from each pair of sister chromatids, ensuring that the two new daughter cells will contain identical genetic material.

Telophase is the final stage of mitosis. Telophase is characterized by the formation of two new daughter nuclei at either end of the dividing cell. These newly formed nuclei surround the genetic material, which uncoils such that the chromosomes return to loosely packed chromatin. Nucleoli also reappear within the new nuclei, and the mitotic spindle breaks apart, each new cell receiving its own complement of DNA, organelles, membranes, and centrioles. At this point, the cell is already beginning to split in half as cytokinesis begins.

The cleavage furrow is a contractile band made up of microfilaments that forms around the midline of the cell during cytokinesis. (Recall that microfilaments consist of actin.) This contractile band squeezes the two cells apart until they finally separate. Two new cells are now formed. One of these cells (the “stem cell”) enters its own cell cycle, able to grow and divide again at some future time. The other cell transforms into the functional cell of the tissue, typically replacing an “old” cell there.

Imagine a cell that completed mitosis but never underwent cytokinesis. In some cases, a cell may divide its genetic material and grow in size but fail to undergo cytokinesis. This results in larger cells with more than one nucleus.

Usually, this is an unwanted aberration and can be a sign of cancerous cells.

3.6 Cellular Differentiation

How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.

Stem Cells

A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to their potential to differentiate.

The first embryonic cells that arise from the division of the zygote are the ultimate stem cells; these stem cells have the potential to differentiate into any of the cells needed to enable an organism to grow and develop.

The embryonic cells that develop from totipotent stem cells and are precursors to the fundamental tissue layers of the embryo are classified as pluripotent. A pluripotent stem cell is one that has the potential to differentiate into any type of human tissue but cannot support the full development of an organism. These cells then become slightly more specialized and are referred to as multipotent cells.

A multipotent stem cell has the potential to differentiate into different types of cells within a given cell lineage or a small number of lineages, such as a red blood cell or white blood cell.

Stem cells are unique in that they can also continually divide and regenerate new stem cells instead of further specializing. There are different stem cells present at different stages of a human’s life. They include the embryonic stem cells of the embryo, fetal stem cells of the fetus, and adult stem cells in the adult. One type of adult stem cell is the epithelial stem cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin. Adult bone marrow has three distinct types of stem cells: hematopoietic stem cells, which give rise to red blood cells, white blood cells, and platelets; endothelial stem cells, which give rise to the endothelial cell types that line blood and lymph vessels; and mesenchymal stem cells, which give rise to the different types of muscle cells.

Differentiation

When a cell goes through a process of differentiation (becomes more specialized), it may undertake major changes in its size, shape, metabolic activity, and overall function. Because all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the different cell types come to be so different? The answer is analogous to a movie script. The different actors in a movie all read from the same script; however, they are each only reading their own part of the script.

Similarly, all cells contain the same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function. In biology, this is referred to as the unique genetic expression of each cell.

Key Terms

active transport:: Form of transport across the cell membrane that requires input of cellular energy.
amphipathic:: Describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in water solubility.
anaphase:: Third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of a dividing cell.
anticodon:: Consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on an mRNA molecule.
antiporters:: Secondary active transporters that transport substances in opposite directions.
apoptosis:: Programmed cell death.
autolysis:: Breakdown of cells by their own enzymatic action.
autophagy:: Lysosomal breakdown of a cell’s own components.
cell cycle:: Life cycle of a single cell, from its birth until its division into two new daughter cells.
cell membrane:: Membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various molecules; also known as plasma membrane.
centriole:: Small, self-replicating organelle that provides the origin for microtubule growth and moves DNA during cell division.
centromere:: Region of attachment for two sister chromatids.
centrosome:: Cellular structure that organizes microtubules during cell division.
channel protein:: Membrane-spanning protein that has an inner pore that allows the passage of one or more substances.
chromatin:: Substance consisting of DNA and associated proteins.
chromosome:: Condensed version of chromatin.
cilia:: Small appendage on certain cells formed by microtubules and modified for movement of materials across the cellular surface.
cleavage furrow:: Contractile ring that forms around a cell during cytokinesis and pinches the cell into two halves.
codon:: Consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid.
concentration gradient:: Difference in the concentration of a substance between two regions.
cytokinesis:: Final stage in cell division, where the cytoplasm divides to form two separate daughter cells.
cytoplasm:: Internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles, cellular solutes, and suspended materials.
cytoskeleton:: “skeleton” of a cell; formed by rodlike proteins that support the cell’s shape and provide, among other functions, locomotive abilities.
cytosol:: Clear, semifluid medium of the cytoplasm, made up mostly of water.
differentiation:: Process in which a cell may undertake major changes in its size, shape, metabolic activity, and overall function.
diffusion:: Movement of a substance from an area of higher concentration to one of lower concentration.
diploid:: Condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one set inherited from each of two parents).
DNA polymerase:: Enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA replication.
DNA replication:: Process of duplicating a molecule of DNA.
electrical gradient:: Difference in the electrical charge (potential) between two regions.
endocytosis:: Import of material into the cell by formation of a membrane-bound vesicle.
endoplasmic reticulum (ER):: Cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively).
exocytosis:: Export of a substance out of a cell by formation of a membrane-bound vesicle.
extracellular fluid (ECF):: Fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other reservoirs in the body.
facilitated diffusion:: Diffusion of a substance with the aid of a membrane protein.
flagellum:: Appendage on certain cells formed by microtubules and modified for movement.
G₀phase:: Phase of the cell cycle usually entered from the G₁ phase; characterized by long or permanent periods where the cell does not move forward into the DNA synthesis phase.
G₁phase:: First phase of the cell cycle, after a new cell is born.
G₂phase:: Third phase of the cell cycle, after the DNA synthesis phase.
gene:: Functional length of DNA that provides the genetic information necessary to build a protein.
gene expression:: Active interpretation of the information coded in a gene to produce a functional gene product.
genome:: Entire complement of an organism’s DNA; found within virtually every cell.
glycocalyx:: Coating of sugar molecules that surrounds the cell membrane.
glycoprotein:: Protein that has one or more carbohydrates attached.
Golgi apparatus:: Cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport.
haploid:: Contain only half the DNA of a diploid cell; examples are ova and sperm cells.
histone:: Family of proteins that associate with DNA in the nucleus to form chromatin.
homologous:: Describes two copies of the same chromosome (not identical), one inherited from each parent.
hydrophilic:: Describes a substance or structure attracted to water.
hydrophobic:: Describes a substance or structure repelled by water.
hypertonic:: Describes a solution concentration that is higher than a reference concentration.
hypotonic:: Describes a solution concentration that is lower than a reference concentration.
integral protein:: Keep membrane-associated protein that spans the entire width of the lipid bilayer.
intermediate filament:: Type of cytoskeletal filament made of keratin, characterized by an intermediate thickness, and playing a role in resisting cellular tension.
interphase:: Entire life cycle of a cell, excluding mitosis.
interstitial fluid (IF):: Fluid in the small spaces between cells not contained within blood vessels.
intracellular fluid (ICF):: Fluid in the cytosol of cells.
isotonic:: Describes a solution concentration that is the same as a reference concentration.
kinetochore:: Region of a centromere where microtubules attach to a pair of sister chromatids.
ligand:: Molecule that binds with specificity to a specific receptor molecule.
lysosome:: Membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes.
messenger RNA (mRNA):: Nucleotide molecule that serves as an intermediate in the genetic code between DNA and protein.
metaphase:: Second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the centre of the cell.
metaphase plate:: Linear alignment of sister chromatids in the centre of the cell, which takes place during metaphase.
microfilament:: The thinnest of the cytoskeletal filaments; composed of actin subunits that function in muscle contraction and cellular structural support.
microtubule:: The thickest of the cytoskeletal filaments, composed of tubulin subunits that function in cellular movement and structural support.
mitochondrion:: One of the cellular organelles bound by a double lipid bilayer that functions primarily in the production of cellular energy (ATP).
mitosis:: Division of genetic material, during which the cell nucleus breaks down and two new, fully functional nuclei are formed.
mitotic phase:: Phase of the cell cycle in which a cell undergoes mitosis.
mitotic spindle:: Network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes during mitosis.
multipotent:: Describes the condition of being able to differentiate into different types of cells within a given cell lineage or a small number of lineages, such as a red blood cell or white blood cell.
nuclear envelope:: Membrane that surrounds the nucleus, consisting of a double lipid bilayer.
nuclear pore:: One of the small, protein-lined openings found scattered throughout the nuclear envelope.
nucleolus:: Small region of the nucleus that functions in ribosome synthesis.
nucleus:: Cell’s central organelle; contains the cell’s DNA.
organelle:: Any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell.
osmosis:: Diffusion of water molecules down their concentration gradient across a selectively permeable membrane.
passive transport:: Form of transport across the cell membrane that does not require input of cellular energy.
peripheral protein:: Membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane.
peroxisome:: Membrane-bound organelle that contains enzymes primarily responsible for detoxifying harmful substances.
phagocytosis:: Endocytosis of large particles.
pinocytosis:: Endocytosis of fluid.
pluripotent:: Describes the condition of being able to differentiate into a large variety of cell types.
polypeptide:: Chain of amino acids linked by peptide bonds.
polyribosome:: Simultaneous translation of a single mRNA transcript by multiple ribosomes.
promoter:: Region of DNA that signals transcription to begin at that site within the gene.
prophase:: First stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of the chromatin to form chromosomes.
receptor:: Protein molecule that contains a binding site for another specific molecule (called a ligand).
receptor-mediated endocytosis:: Endocytosis of ligands attached to membrane-bound receptors.
ribosomal RNA (rRNA):: RNA that makes up the subunits of a ribosome.
ribosome:: Cellular organelle that functions in protein synthesis.
RNA polymerase:: Enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the transcription phase of protein synthesis.
rough ER (RER):: Organelle that function as the site for protein synthesis; many ribosomes attach to the ER giving it the studded appearance of rough ER.
secondary active transport:: Process harnessing an electrochemical gradient established by primary active transport to drive the transport of a second molecule.
selective permeability:: Feature of any barrier that allows certain substances to cross but excludes others.
sister chromatid:: One of a pair of identical chromosomes, formed during DNA replication.
smooth ER (SER):: Organelle lacking attached ribosomes that are specialized for lipid synthesis, detoxification, carbohydrate metabolism, and calcium ion storage.
sodium-potassium pump (also Na⁺/K⁺ATPase):: Membrane-embedded protein pump that uses ATP to move Na⁺ out of a cell and K⁺ into the cell.
somatic cell:: All cells of the body excluding gamete cells.
S phase:: Stage of the cell cycle during which DNA replication occurs.
stem cell:: Cell that is oligo-, multi-, or pluripotent and has the ability to produce additional stem cells rather than becoming further specialized.
symporters:: Secondary active transporters that move two substances in the same direction.
telophase:: Final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new daughter nuclei.
tonicity:: A measure of a solution’s ability to change the volume of cells by altering their water content.
transcription:: Process of producing an mRNA molecule that is complementary to a particular gene of DNA.
transfer RNA (tRNA):: Molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly place them into the sequence.
translation:: Process of producing a protein from the nucleotide sequence code of an mRNA transcript.
vesicle:: Membrane-bound structure that contains materials within or outside of the cell.

Chapter Review

3.1 The Cell Membrane

The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment. It is composed of a phospholipid bilayer, with hydrophobic internal lipid “tails” and hydrophilic external phosphate “heads.” Various membrane proteins are scattered throughout the bilayer, both inserted within it and attached to it peripherally. The cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer. All materials that cross the membrane do so using passive (non-energy-requiring) or active (energy-requiring) transport processes. During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient. Water passes through the membrane in a diffusion process called osmosis. During active transport, energy is expended to assist materials’ movement across the membrane in a direction against their concentration gradient. Active transport may take place with the help of protein pumps or through the use of vesicles.

3.2 The Cytoplasm and Cellular Organelles

The internal environment of a living cell is made up of a fluid, jellylike substance called cytosol, which consists mainly of water but also contains various dissolved nutrients and other molecules. The cell contains an array of cellular organelles, each one performing a unique function and helping maintain the health and activity of the cell. The cytosol and organelles together compose the cell’s cytoplasm.

Most organelles are surrounded by a lipid membrane similar to the cell membrane of the cell. The endoplasmic reticulum (ER), Golgi apparatus, and lysosomes share a functional connectivity and are collectively referred to as the endomembrane system. There are two types of ER: smooth and rough. While the smooth ER performs many functions, including lipid synthesis and ion storage, the rough ER is mainly responsible for protein synthesis using its associated ribosomes. The rough ER sends newly made proteins to the Golgi apparatus, where they are modified and packaged for delivery to various locations within or outside of the cell. Some of these protein products are enzymes destined to break down unwanted material and are packaged as lysosomes for use inside the cell.

Cells also contain mitochondria and peroxisomes, which are the organelles responsible for producing the cell’s energy supply and detoxifying certain chemicals, respectively. Biochemical reactions within mitochondria transform energy-carrying molecules into the usable form of cellular energy known as ATP. Peroxisomes contain enzymes that transform harmful substances such as free radicals into oxygen and water. Cells also contain a miniaturized “skeleton” of protein filaments that extends throughout their interior. Three different kinds of filaments compose this cytoskeleton (in order of increasing thickness): microfilaments, intermediate filaments, and microtubules. Each cytoskeletal component performs unique functions as well as provides a supportive framework for the cell.

3.3 The Nucleus and DNA Replication

The nucleus is the command centre of the cell, containing the genetic instructions for all the materials a cell will make (and thus all the functions it can perform). The nucleus is encased within a membrane of two interconnected lipid bilayers, side by side. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of the nucleus. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis. The nucleus houses the genetic material of the cell: DNA. DNA is normally found as a loosely contained structure called chromatin within the nucleus, where it is wound up and associated with a variety of histone proteins. When a cell is about to divide, the chromatin coils tightly and condenses to form chromosomes.

There is a pool of cells constantly dividing within your body. The result is billions of new cells being created each day. Before any cell is ready to divide, it must replicate its DNA so that each new daughter cell will receive an exact copy of the organism’s genome. A variety of enzymes are enlisted during DNA replication. These enzymes unwind the DNA molecule, separate the two strands, and assist with the building of complementary strands along each parent strand. The original DNA strands serve as templates from which the nucleotide sequences of the new strands are determined and synthesized. When replication is completed, two identical DNA molecules exist. Each one contains one original strand and one newly synthesized complementary strand.

3.4 Protein Synthesis

DNA stores the information necessary for instructing the cell to perform all its functions. Cells use the genetic code stored within DNA to build proteins, which ultimately determine the structure and function of the cell. This genetic code lies in the particular sequence of nucleotides that make up each gene along the DNA molecule. To “read” this code, the cell must perform two sequential steps. In the first step, transcription, the DNA code is converted into an RNA code. A molecule of messenger RNA that is complementary to a specific gene is synthesized in a process similar to DNA replication. The molecule of mRNA provides the code to synthesize a protein. In the process of translation, the mRNA attaches to a ribosome. Next, tRNA molecules shuttle the appropriate amino acids to the ribosome one by one, coded by sequential triplet codons on the mRNA, until the protein is fully synthesized. When completed, the mRNA detaches from the ribosome, and the protein is released. Typically, multiple ribosomes attach to a single mRNA molecule at once, such that multiple proteins can be manufactured from the mRNA concurrently.

3.5 Cell Growth and Division

The life of a cell consists of stages that make up the cell cycle. After a cell is born, it passes through an interphase before it is ready to replicate itself and produce daughter cells. This interphase includes two gap phases (G₁ and G₂) as well as an S phase, during which its DNA is replicated in preparation for cell division. The cell cycle is under precise regulation by chemical messengers both inside and outside the cell that provide “stop” and “go” signals for movement from one phase to the next. Failures of these signals can result in cells that continue to divide uncontrollably, which can lead to cancer.

Once a cell has completed interphase and is ready for cell division, it proceeds through four separate stages of mitosis (prophase, metaphase, anaphase, and telophase). Telophase is followed by the division of the cytoplasm (cytokinesis), which generates two daughter cells. This process takes place in all normally dividing cells of the body except for the germ cells that produce eggs and sperm.

3.6 Cellular Differentiation

One of the major areas of research in biology is that of how cells specialize to assume their unique structures and functions, since all cells essentially originate from a single fertilized egg. Cell differentiation is the process of cells becoming specialized as the body develops. A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are divided into several categories according to their potential to differentiate. While all somatic cells contain the exact same genome, different cell types only express some of those genes at any given time.

These differences in gene expression ultimately dictate a cell’s unique morphological and physiological characteristics. The use of different transcription factor proteins, which bind to DNA and promote or hinder the transcription of different genes, is the primary mechanism that determines which genes will be expressed and which ones will not. Through the action of these transcription factors, cells specialize into one of hundreds of different cell types in the human body.

4. The Tissue Level of Organization