Chapter2 The Chemical Level of Organization
The smallest, most fundamental material components of the human body are basic chemical elements. In fact, chemicals called nucleotide bases are the foundation of the genetic code, with the instructions on how to build and maintain the human body from conception through old age. There are about three billion of these base pairs in human DNA. Life cannot exist without many of the elements that are part of the earth. All the elements that contribute to chemical reactions, to the transformation of energy, and to electrical activity and muscle contraction—elements that include phosphorus, carbon, sodium, and calcium, to name a few—originated in stars. These elements, in turn, can form both the inorganic and organic chemical compounds important to life, including, for example, water, glucose, and proteins. This chapter begins by examining elements and how the structures of atoms, the basic units of matter, determine the characteristics of elements by the number of protons, neutrons, and electrons in the atoms. The chapter then builds the framework of life from there.
2.1 Elements and Atoms: The Building Blocks of Matter
Elements and Compounds
The substance of the universe—from a grain of sand to a star—is called matter. All matter in the natural world is composed of one or more of the 92 fundamental substances called elements. An element is a pure substance that is distinguished from all other matter by the fact that it cannot be created or broken down by ordinary chemical means. While your body can assemble many of the chemical compounds needed for life from their constituent elements, it cannot make elements. They must come from the environment.
Example: An element that you must take in is calcium (Ca). Calcium is essential to the human body; it is absorbed and used for a number of processes, including strengthening bones. When you consume dairy products, your digestive system breaks down the food into components small enough to cross into the bloodstream. Among these is calcium, which, because it is an element, cannot be broken down further. The elemental calcium in cheese, therefore, is the same as the calcium that forms your bones. Some other elements you might be familiar with are oxygen, sodium, and iron. The elements in the human body are shown in Figure 2.1, beginning with the most abundant: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Each element’s name can be replaced by a one- or two-letter symbol; you will become familiar with some of these during this course. All the elements in your body are derived from the foods you eat and the air you breathe.
Figure 2.1 Elements of the Human Body. The main elements that compose the human body are shown from most abundant to least abundant.
In nature, elements rarely occur alone. Instead, they combine to form compounds. A compound is a substance composed of two or more elements joined by chemical bonds.
Example: The compound glucose is an important body fuel. It is always composed of the same three elements: carbon, hydrogen, and oxygen. Moreover, the elements that make up any given compound always occur in the same relative amounts. In glucose, there are always six carbon and six oxygen units for every twelve hydrogen units. But what, exactly, are these “units” of elements?
Atoms and Subatomic Particles
An atom is the smallest quantity of an element that retains the unique properties of that element. In other words, an atom of hydrogen is a unit of hydrogen—the smallest amount of hydrogen that can exist. As you might guess, atoms are almost unfathomably small. The period at the end of this sentence is millions of atoms wide.
Atomic Structure and Energy
Atoms are made up of even smaller subatomic particles, three types of which are important: the proton, neutron, and electron. The sum of positively charged protons and noncharged (“neutral”) neutrons found in an atom’s nucleus is its mass number, and the number of protons determines the elements’ atomic number. The number of negatively charged electrons that “spin” around the nucleus at close to the speed of light equals the number of protons. An electron has about 1/2000th the mass of a proton or neutron.
Example: Figure 2.2 shows two models that can help you imagine the structure of an atom—in this case, helium (He). In one model, helium’s two electrons are shown circling the nucleus in a fixed orbit depicted as a ring. Although this model is helpful in visualizing atomic structure, in reality, electrons do not travel in fixed orbits but whiz around the nucleus erratically in a so-called electron cloud.
Figure 2.2 Two Models of Atomic Structure. (a) In the planetary model, the electrons of helium are shown in fixed orbits, depicted as rings, at a precise distance from the nucleus, somewhat like planets orbiting the sun. (b) In the electron cloud model, the electrons of helium are shown in the variety of locations they would have at different distances from the nucleus over time.
An atom’s protons and electrons carry electrical charges. Electrons, which have a negative charge, are designated e−. An atom’s neutrons have no charge: They are electrically neutral because they have an equal number of electrons and protons.
Atomic Number and Mass Number
The atomic number, which is the number of protons in the nucleus of the atom, identifies the element. Because an atom usually has the same number of electrons as protons, the atomic number identifies the usual number of electrons as well.
In their most common form, many elements also contain the same number of neutrons as protons. An element’s mass number is the sum of the number of protons and neutrons in its nucleus.
The periodic table of the elements is a chart identifying the 92 elements found in nature as well as several larger, unstable elements discovered experimentally.
Figure 2.3 The Periodic Table of the Elements. (credit: R. A. Dragoset, A. Musgrove, C. W. Clark, W. C. Martin)
The Behaviour of Electrons
In the human body, atoms do not exist as independent entities. Rather, they are constantly reacting with other atoms to form and break down more complex substances. To fully understand anatomy and physiology, you must grasp how atoms participate in such reactions. The key is understanding the behaviour of electrons.
Although electrons do not follow rigid orbits a set distance away from the atom’s nucleus, they do tend to stay within certain regions of space called electron shells. An electron shell is a layer of electrons that encircle the nucleus at a distinct energy level.
The atoms of the elements found in the human body have from one to five electron shells, and all electron shells hold eight electrons except the first shell, which can only hold two. This configuration of electron shells is the same for all atoms. The precise number of shells depends on the number of electrons in the atom.
Example: Hydrogen and helium have just one and two electrons, respectively. If you take a look at the periodic table of the elements, you will notice that hydrogen and helium are placed alone on either side of the top row; they are the only elements that have just one electron shell (Figure 2.4). A second shell is necessary to hold the electrons in all elements larger than hydrogen and helium.
The factor that most strongly governs the tendency of an atom to participate in chemical reactions is the number of electrons in its valence shell. A valence shell is an atom’s outermost electron shell. If the valence shell is full, the atom is stable, meaning its electrons are unlikely to be pulled away from the nucleus by the electrical charges of other atoms. If the valence shell is not full, the atom is reactive, meaning it will tend to react with other atoms in ways that make the valence shell full.
Figure 2.4 Electron Shells. Electrons orbit the atomic nucleus at distinct levels of energy called electron shells. (a) With one electron, hydrogen only half fills its electron shell. Helium also has a single shell, but its two electrons completely fill it. Disregard b. and c. parts of this figure.
Example: Consider hydrogen, with its one electron only half filling its valence shell. This single electron is likely to be drawn into relationships with the atoms of other elements so that hydrogen’s single valence shell can be stabilized.
All atoms (except hydrogen and helium with their single electron shells) are most stable when there are exactly eight electrons in their valence shell. This principle is referred to as the octet rule, and it states that an atom will give up, gain, or share electrons with another atom so that it ends up with eight electrons in its own valence shell.
2.2 Chemical Bonds
Atoms link by forming a chemical bond. A bond is a weak or strong electrical attraction that holds atoms in the same vicinity. The new grouping is typically more stable—less likely to react again—than its component atoms were when they were separate. A more or less stable grouping of two or more atoms held together by chemical bonds is called a molecule. The bonded atoms may be of the same element, as in the case of H2, which is called molecular hydrogen or hydrogen gas. When a molecule is made up of two or more atoms of different elements, it is called a chemical compound. Thus, a unit of water, or H2O, is a compound, as is a single molecule of the gas methane, or CH4.
Three types of chemical bonds are important in human physiology because they hold together substances that are used by the body for critical aspects of homeostasis, signalling, and energy production, to name just a few important processes. These are ionic bonds, covalent bonds, and hydrogen bonds.
Ions and Ionic Bonds
When an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons, the atom will then become positively or negatively charged. This happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either by gaining electrons to fill a shell that is more than half full or by giving away electrons to empty a shell that is less than half full, thereby leaving the next smaller electron shell as the new, full valence shell. An atom that has an electrical charge—whether positive or negative—is an ion. For example, a potassium ion is written K+, indicating that it has lost a single electron. A positively charged ion is known as a cation. If an element accepts one electron, its electrons will outnumber its protons by one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride and is written as F−. A negatively charged ion is known as an anion
Atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. A cation that has donated two electrons has a net charge of +2. Using magnesium (Mg) as an example, this can be written Mg++ or Mg2+. An anion that has accepted two electrons has a net charge of −2. The ionic form of selenium (Se), for example, is typically written Se2−.
The opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity, forming an ionic bond. An ionic bond is an ongoing close association between ions of opposite charge. As shown in Figure 2.5, sodium commonly donates an electron to chlorine, becoming the cation Na+. When chlorine accepts the electron, it becomes the chloride anion, Cl−. With their opposing charges, these two ions strongly attract each other.
Water is an essential component of life because it is able to break the ionic bonds in salts to free the ions. In fact, in biological fluids, most individual atoms exist as ions. These dissolved ions produce electrical charges within the body. The behaviour of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram (EKG or ECG) or an electroencephalogram (EEG). The electrical activity that derives from the interactions of the charged ions is why they are also called electrolytes.
Covalent Bonds
A covalent bond shares electrons in a mutually stabilizing relationship. The atoms do not lose or gain electrons permanently; instead, the electrons move back and forth between the elements. Because of the close sharing of pairs of electrons (one electron from each of two atoms), covalent bonds are stronger than ionic bonds.
Nonpolar Covalent Bonds
The sharing of the negative electrons is relatively equal, as is the electrical pull of the positive protons in the nuclei of the atoms involved. This is why covalently bonded molecules that are electrically balanced in this way are described as nonpolar; that is, no region of the molecule is either more positive or more negative than any other (Figure 2.6). In covalent bonds, electrons in the two atoms’ overlapping atomic orbitals are shared to fill the valence shells of both atoms, ultimately stabilizing both of the atoms involved. In a single covalent bond, a single electron pair is shared between two atoms, while in a double covalent bond, two pairs of electrons are shared between two atoms. There are even triple covalent bonds, where three electron pairs are shared between two atoms.
Figure 2.5 Ionic Bonding. (a) Sodium readily donates the solitary electron in its valence shell to chlorine, which needs only one electron to have a full valence shell. (b) The opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of a bond of attraction called an ionic bond. (c) The attraction of many sodium and chloride ions results in the formation of large groupings called crystals.
Polar Covalent Bonds
A polar molecule contains regions that have partial opposite electrical charges. Some elements have a strong “pull” on the electrons of other elements. In a covalent bond, elements do not take electrons from other elements but pull them closer to their own nucleus. Take, for example, a water molecule (Figure 2.7). In this molecule, the nucleus of the oxygen attracts the electrons more strongly than the nuclei of the hydrogen atoms. Because electrons are negative, the oxygen end of their bond becomes slightly more negative than the hydrogen end of their bond. This gives the water molecule partially charged ends.
Hydrogen Bonds
A hydrogen bond is a bond between molecules or part of a large molecule. It is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. In other words, hydrogen bonds always include hydrogen that is already part of a polar molecule.
Figure 2.6 Covalent Bonding.
Figure 2.7 Polar Covalent Bonds in a Water Molecule.
Example: The most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens before your eyes whenever two raindrops merge into a larger bead or a creek spills into a river. Hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules (Figure 2.8).
Water molecules also strongly attract other types of charged molecules as well as ions.
Example: This explains why “table salt”—a molecule called a “salt” in chemistry, which consists of equal numbers of positively charged sodium (Na+) and negatively charged chloride (Cl−) ions—dissolves so readily in water, in this case forming dipole-ion bonds between the water and the electrically charged ions (electrolytes). Water molecules also repel molecules with nonpolar covalent bonds, like fats, lipids, and oils.
Figure 2.8 Hydrogen Bonds Between Water Molecules. Notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms. Hydrogen bonds are relatively weak and therefore are indicated with a dotted (rather than a solid) line.
2.3 Chemical Reactions
One characteristic of a living organism is metabolism, which is the sum of all the chemical reactions that go on to maintain that organism’s health and life.
The Role of Energy in Chemical Reactions
Chemical reactions require a sufficient amount of energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ones formed. In general, kinetic energy is the form of energy powering any type of matter in motion. Imagine you are building a brick wall. The energy it takes to lift and place one brick atop another is kinetic energy—the energy matter possesses because of its motion. Once the wall is in place, it stores potential energy. Potential energy is the energy of position, or the energy matter possesses because of the positioning or structure of its components. If the brick wall collapses, the stored potential energy is released as kinetic energy as the bricks fall. In the human body, potential energy is stored in the bonds between atoms and molecules. Chemical energy is the form of potential energy in which energy is stored in chemical bonds. When those bonds are formed, chemical energy is invested, and when they break, chemical energy is released. Notice that chemical energy, like all energy, is neither created nor destroyed; rather, it is converted from one form to another. When you eat an energy bar before heading out the door for a hike, the honey, nuts, and other foods the bar contains are broken down and rearranged by your body into molecules that your muscle cells convert to kinetic energy.
Chemical reactions that release more energy than they absorb are characterized as exergonic. The catabolism of the foods in your energy bar is an example. Some of the chemical energy stored in the bar is absorbed into molecules your body uses for fuel, but some of it is released—for example, as heat. In contrast, chemical reactions that absorb more energy than they release are endergonic. These reactions require energy input, and the resulting molecule stores not only the chemical energy in the original components but also the energy that fuelled the reaction.
Because energy is neither created nor destroyed, where does the energy needed for endergonic reactions come from? In many cases, it comes from exergonic reactions.
Forms of Energy Important in Human Functioning
You have already learned that chemical energy is absorbed, stored, and released by chemical bonds. In addition to chemical energy, mechanical, radiant, and electrical energy are important in human functioning.
- • Mechanical energy, which is stored in physical systems such as machines, engines, or the human body, directly powers the movement of matter. When you lift a brick into place on a wall, your muscles provide the mechanical energy that moves the brick.
- • Radiant energy is energy emitted and transmitted as waves rather than matter. These waves vary in length from long radio waves and microwaves to short gamma waves emitted from decaying atomic nuclei. The full spectrum of radiant energy is referred to as the electromagnetic spectrum.
- • Electrical energy, supplied by electrolytes in cells and body fluids, contributes to the voltage changes that help transmit impulses in nerve and muscle cells.
Characteristics of Chemical Reactions
All chemical reactions begin with a reactant, the general term for one or more substances that enter into a reaction. Sodium and chloride ions, for example, are the reactants in the production of table salt. The one or more substances produced by a chemical reaction are called the product.
Like energy, mass cannot be created or destroyed in a chemical reaction. This is called the law of conservation of mass. In chemical reactions, the components of the reactants—the elements involved and the number of atoms of each—are all present in the product(s). Similarly, there is nothing present in the products that is not present in the reactants.
A synthesis reaction is a chemical reaction that results in the synthesis (joining) of components that were formerly separate. A decomposition reaction is a chemical reaction that breaks down or “decomposes” something larger into its constituent parts.
An exchange reaction is a chemical reaction in which both synthesis and decomposition occur, chemical bonds are both formed and broken, and chemical energy is absorbed, stored, and released.
Enzymes and Catalysts
In chemistry, a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change. You can think of a catalyst as a chemical change agent. They help increase the rate and force at which atoms, ions, and molecules collide, thereby increasing the probability that their valence shell electrons will interact. The most important catalysts in the human body are enzymes. An enzyme is a biological catalyst primarily composed of either protein (amino acid chains) or ribonucleic acid (RNA) and is specific for a single substrate. More details later in this chapter.
All reactions, whether they are endergonic or exergonic, require a little bit of energy to “get going.” This is activation energy. Enzymes work by lowering the level of energy that needs to be invested in a chemical reaction. A chemical reaction’s activation energy is the “threshold” level of energy needed to break the bonds in the reactants. Once those bonds are broken, new arrangements can form. Without an enzyme to act as a catalyst, a much larger investment of energy is needed to ignite a chemical reaction (Figure 2.9). Enzymes are critical to the body’s healthy functioning. They assist, for example, with the breakdown of food and its conversion to energy. In fact, most of the chemical reactions in the body are facilitated by enzymes.
Figure 2.9 Energy Required in Chemical Reactions. Enzymes decrease the activation energy required for a given chemical reaction to occur. (a) Without an enzyme, the energy input needed for a reaction to begin is high. (b) With the help of an enzyme, less energy is needed for a reaction to begin.
2.4 Inorganic Compounds Essential to Human Functioning
The concepts you have learned so far in this chapter govern all forms of matter and would work as a foundation for geology as well as biology. This section of the chapter narrows the focus to the chemistry of human life—that is, the compounds important for the body’s structure and function. In general, these compounds are either inorganic or organic.
- • An inorganic compound is a substance that does not contain both carbon and hydrogen. A great many inorganic compounds do contain hydrogen atoms, such as water (H2O) and the hydrochloric acid (HCl) produced by your stomach. In contrast, only a handful of inorganic compounds contain carbon atoms. Carbon dioxide (CO2) is one of the few examples.
- • An organic compound, then, is a substance that contains both carbon and hydrogen. Organic compounds are synthesized via covalent bonds within living organisms, including the human body. Recall that carbon and hydrogen are the second- and third-most-abundant elements in your body.
The following section examines the three groups of inorganic compounds essential to life: water, salts, and acids and bases. Organic compounds are covered later in the chapter.
Water
As much as 70 percent of an adult’s body weight is water. This water is contained both within the cells and between the cells that make up tissues and organs. Its several roles make water indispensable to human functioning.
Water as a Lubricant and Cushion
Water is a major component of many of the body’s lubricating fluids.
Example: Water in synovial fluid lubricates the actions of body joints, and water in pleural fluid helps the lungs expand and recoil with breathing. Watery fluids help keep food flowing through the digestive tract and ensure that the movement of adjacent abdominal organs is friction-free.
Water also protects cells and organs from physical trauma.
Example: Water’s protective functions include cushioning the brain within the skull, protecting the delicate nerve tissue of the eyes, and cushioning a developing fetus in the mother’s womb as well.
Water as a Heat Sink
A heat sink is a substance or object that absorbs and dissipates heat but does not experience a corresponding increase in temperature. In the body, water absorbs the heat generated by chemical reactions without greatly increasing in temperature. Moreover, when the environmental temperature soars, the water stored in the body helps keep the body cool. This cooling effect happens as warm blood from the body’s core flows to the blood vessels just under the skin and is transferred to the environment. At the same time, sweat glands release warm water in sweat. As the water evaporates into the air, it carries away heat, and then the cooler blood from the periphery circulates back to the body core.
Water as a Component of Liquid Mixtures
A mixture is a combination of two or more substances, each of which maintains its own chemical identity. In other words, the constituent substances are not chemically bonded into a new, larger chemical compound.
Example: The concept is easy to imagine if you think of powdery substances such as flour and sugar; when you stir them together in a bowl, they obviously do not bond to form a new compound. The air you breathe is a gaseous mixture containing three discrete elements—nitrogen, oxygen, and argon—and one compound, carbon dioxide.
There are three types of liquid mixtures, all of which contain water as a key component. These are solutions, colloids, and suspensions.
For cells in the body to survive, they must be kept moist in a water-based liquid called a solution. In chemistry, a liquid solution consists of a solvent (water) that dissolves a substance called a solute. An important characteristic of solutions is that they are homogeneous; that is, the solute molecules are distributed evenly throughout the solution.
Example: If you were to stir a teaspoon of sugar into a glass of water, the sugar would dissolve into sugar molecules separated by water molecules. The ratio of sugar to water in the left side of the glass would be the same as the ratio of sugar to water in the right side of the glass. If you were to add more sugar, the ratio of sugar to water would change, but the distribution—provided you had stirred well—would still be even.
Water is certainly the most abundant solvent in the body; essentially all the body’s chemical reactions occur among compounds dissolved in water. Water is considered the “universal” solvent, and it is thought that life cannot exist without water because of this. Because water molecules are polar, with regions of positive and negative electrical charge, water readily dissolves ionic compounds and, to a lesser degree, polar covalent compounds. Such compounds are referred to as hydrophilic, or “water-loving.” As mentioned in the sugar example, sugar dissolves well in water. This is because sugar molecules contain regions of hydrogen-oxygen polar bonds, making them hydrophilic. Nonpolar molecules, which do not readily dissolve in water, are called hydrophobic, or “water-fearing.”
Concentrations of Solutes
The concentration of a given solute is the number of particles of that solute in a given space; for example, oxygen makes up about 21 percent of atmospheric air.
A colloid is a mixture that is somewhat like a heavy solution. The solute particles consist of tiny clumps of molecules large enough to make the liquid mixture opaque (because the particles are large enough to scatter light). Familiar examples of colloids are milk and cream. In the thyroid glands, the thyroid hormone is stored as a thick protein mixture also called a colloid.
A suspension is a liquid mixture in which a heavier substance is suspended temporarily in a liquid but, over time, settles out. This separation of particles from a suspension is called sedimentation.
Example: Sedimentation occurs in a blood test that establishes a sedimentation rate, or sed rate. The test measures how quickly red blood cells in a test tube settle out of the watery portion of blood (known as plasma) over a set period of time. Rapid sedimentation of blood cells does not normally happen in the healthy body, but aspects of certain diseases can cause blood cells to clump together, and these heavy clumps of blood cells settle to the bottom of the test tube more quickly than do normal blood cells.
The Role of Water in Chemical Reactions
Two types of chemical reactions involve the creation or the consumption of water: dehydration synthesis and hydrolysis.
- • In dehydration synthesis, one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group (OH) in the synthesis of a new product. In the formation of their covalent bond, a molecule of water is released as a by-product. This is also sometimes referred to as a condensation reaction.
- • In hydrolysis, a molecule of water disrupts a compound, breaking its bonds. The water is itself split into H and OH. One portion of the severed compound then bonds with the hydrogen atom, and the other portion bonds with the hydroxyl group.
These reactions are reversible and play an important role in the chemistry of organic compounds (which will be discussed shortly).
Salts
Salts (broader meaning) are formed when ions form ionic bonds. In these reactions, one atom gives up one or more electrons and thus becomes positively charged, whereas the other accepts one or more electrons and becomes negatively charged—for example, potassium chloride or sodium chloride. You can define a salt as a substance that, when dissolved in water, dissociates into ions other than H+ or OH−. This fact is important in distinguishing salts from acids and bases, discussed next.
Example: A typical salt, NaCl, dissociates completely in water. The positive and negative regions on the water molecule (the hydrogen and oxygen ends, respectively) attract the negative chloride and positive sodium ions, pulling them away from each other. Again, whereas nonpolar and polar covalently bonded compounds break apart into molecules in solution, salts dissociate into ions. These ions are electrolytes; they are capable of conducting an electrical current in solution. This property is critical to the function of ions in transmitting nerve impulses and prompting muscle contraction.
Many other salts are important in the body. Bile salts produced by the liver help break apart dietary fats, and calcium phosphate salts form the mineral portions of teeth and bones.
Acids and Bases
Acids and bases, like salts, dissociate in water into electrolytes. Acids and bases can very much change the properties of the solutions in which they are dissolved.
Acids
An acid is a substance that releases hydrogen ions (H+) in solution (Figure 2.10a). Because an atom of hydrogen has just one proton and one electron, a positively charged hydrogen ion is simply a proton. This solitary proton is highly likely to participate in chemical reactions. Strong acids are compounds that release all their H+ in solution; that is, they ionize completely.
Example: Hydrochloric acid (HCl), which is released from cells in the lining of the stomach, is a strong acid because it releases all its H+ in the stomach’s watery environment. This strong acid aids the stomach in killing ingested microbes.
Weak acids do not ionize completely; that is, some of their hydrogen ions remain bonded within a compound in solution. An example of a weak acid is vinegar, or acetic acid; it is called acetate after it gives up a proton.
Bases
A base is a substance that releases hydroxyl ions (OH−) in solution or one that accepts H+ already present in solution (see Figure 2.10b). Strong bases release most or all of their hydroxyl ions; weak bases release only some hydroxyl ions or absorb only a few H+.
Example: Food mixed with hydrochloric acid from the stomach would burn the small intestine, the next portion of the digestive tract after the stomach, if it were not for the release of bicarbonate (HCO−) released by the pancreas, a weak base that attracts H+. Bicarbonate accepts some of the H+ protons, thereby reducing the acidity of the solution.
Figure 2.10 Acids and Bases. (a) In aqueous solution, an acid dissociates into hydrogen ions (H+) and anions. Nearly every molecule of a strong acid dissociates, producing a high concentration of H+. (b) In aqueous solution, a base dissociates into hydroxyl ions (OH−) and cations. Nearly every molecule of a strong base dissociates, producing a high concentration of OH−.
The Concept of pH
The relative acidity or alkalinity of a solution can be indicated by its pH. A solution’s pH is the negative, base-10 logarithm of the hydrogen ion (H+) concentration of the solution.
Example: A pH 4 solution has a concentration of H+ that is ten times greater than that of a pH 5 solution. That is, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5.
The concept of pH will begin to make more sense when you study the pH scale, as shown in Figure 2.11. The scale consists of a series of increments ranging from 0 to 14. A solution with a pH of 7 is considered neutral—neither acidic nor basic. Pure water has a pH of 7. The lower the number below 7, the more acidic the solution, or the greater the concentration of H+. The concentration of hydrogen ions at each pH value is 10 times different than the next pH. The higher the number above 7, the more basic (alkaline) the solution, or the lower the concentration of H+.
Figure 2.11 The pH Scale.
Example: Human urine is ten times more acidic than pure water, and HCl is 10,000,000 times more acidic than water.
Buffers
The pH of human blood normally ranges from 7.35 to 7.45, although it is typically identified as pH 7.4. At this slightly basic pH, blood can reduce the acidity resulting from the carbon dioxide (CO2) constantly being released into the bloodstream by the trillions of cells in the body. Homeostatic mechanisms (along with exhaling CO2 while breathing) normally keep the pH of blood within this narrow range. This is critical because fluctuations—either too acidic or too alkaline—can lead to life-threatening disorders.
All cells of the body depend on homeostatic regulation of acid-base balance at a pH of approximately 7.4. The body therefore has several mechanisms for this regulation, involving breathing, the excretion of chemicals in urine, and the internal release of chemicals collectively called buffers into body fluids. A buffer is a solution of a weak acid and its conjugate base. A buffer can neutralize small amounts of acids or bases in body fluids (details in Chapter 25).
Example: If there is even a slight decrease below 7.35 in the pH of a body fluid, the buffer in the fluid—in this case, acting as a weak base—will bind the excess hydrogen ions. In contrast, if pH rises above 7.45, the buffer will act as a weak acid and contribute hydrogen ions.
Excessive acidity of the blood and other body fluids is known as acidosis. Acidosis can also be caused by metabolic problems that reduce the level or function of buffers that act as bases or that promote the production of acids.
Example: With severe diarrhea, too much bicarbonate can be lost from the body, allowing acids to build up in body fluids. In people with poorly managed diabetes (ineffective regulation of blood sugar), acids called ketones are produced as a form of body fuel. These can build up in the blood, causing a serious condition called diabetic ketoacidosis. Kidney failure, liver failure, heart failure, cancer, and other disorders can also prompt metabolic acidosis.
In contrast, alkalosis is a condition in which the blood and other body fluids are too alkaline (basic). As with acidosis, respiratory disorders are a major cause; however, in respiratory alkalosis, carbon dioxide levels fall too low. Lung disease, aspirin overdose, shock, and ordinary anxiety can cause respiratory alkalosis, which reduces the normal concentration of H+. Metabolic alkalosis often results from prolonged, severe vomiting, which causes a loss of hydrogen and chloride ions (as components of HCl). Medications can also prompt alkalosis. These include diuretics that cause the body to lose potassium ions as well as antacids when taken in excessive amounts—for instance, by someone with persistent heartburn or an ulcer.
2.5 Organic Compounds Essential to Human Functioning
Organic compounds typically consist of groups of carbon atoms covalently bonded to hydrogen, usually oxygen, and often other elements as well. They are found throughout the world in soils and seas, commercial products, and every cell of the human body. The four types most important to human structure and function are carbohydrates, lipids, proteins, and nucleotides.
Carbohydrates
A carbohydrate is a molecule composed of carbon, hydrogen, and oxygen. The chemical formula for a “generic” molecule of carbohydrate is (CH2O)n.
Carbohydrates are referred to as saccharides, a word meaning “sugars.” Three forms are important in the body. Monosaccharides are the monomers of carbohydrates. Disaccharides (di- = “two”) are made up of two monomers. Polysaccharides are the polymers of carbohydrates and can consist of hundreds to thousands of monomers.
Monosaccharides
A monosaccharide is a monomer of carbohydrates. Five monosaccharides are important in the body. Three of these are the hexose sugars, so called because they each contain six atoms of carbon. These are glucose, fructose, and galactose. The remaining monosaccharides are the two pentose sugars, each of which contains five atoms of carbon. They are ribose and deoxyribose.
A disaccharide is a pair of monosaccharides. Disaccharides are formed via dehydration synthesis, and the bond linking them is referred to as a glycosidic bond (glyco- = “sugar”). Three disaccharides are important to humans. These are sucrose, commonly referred to as table sugar; lactose, or milk sugar; and maltose, or malt sugar. As you can tell from their common names, you consume these in your diet; however, your body cannot use them directly. Instead, in the digestive tract, they are split into their component monosaccharides via hydrolysis.
Polysaccharides
Polysaccharides can contain a few to a thousand or more monosaccharides. Three are important to the body:
- • Starches are polymers of glucose. They occur in long chains called amylose or branched chains called amylopectin, both of which are stored in plant-based foods and are relatively easy to digest.
- • Glycogen is also a polymer of glucose, but it is stored in the tissues of animals, especially in the muscles and liver. It is not considered a dietary carbohydrate because very little glycogen remains in animal tissues after slaughter; however, the human body stores excess glucose as glycogen, again, in the muscles and liver.
- • Cellulose, a polysaccharide that is the primary component of the cell wall of green plants, is the component of plant food referred to as “fibre.” In humans, cellulose/fibre is not digestible; however, dietary fibre has many health benefits. It helps you feel full so you eat less, it promotes a healthy digestive tract, and a diet high in fibre is thought to reduce the risk of heart disease and possibly some forms of cancer.
Functions of Carbohydrates
The body obtains carbohydrates from plant-based foods. Grains, fruits, and legumes and other vegetables provide most of the carbohydrates in the human diet, although lactose is found in dairy products.
While most body cells can break down other organic compounds for fuel, all body cells can use glucose. Moreover, nerve cells (neurons) in the brain, spinal cord, and peripheral nervous system, as well as red blood cells, can use only glucose for fuel. In the breakdown of glucose for energy, molecules of adenosine triphosphate, better known as ATP, are produced. Adenosine triphosphate (ATP) is composed of a ribose sugar, an adenine base, and three phosphate groups. ATP releases free energy when its phosphate bonds are broken and thus supplies ready energy to the cell. More ATP is produced in the presence of oxygen (O2) than in pathways that do not use oxygen. The overall reaction for the conversion of the energy in glucose to energy stored in ATP can be written as follows:
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
In addition to being a critical fuel source, carbohydrates are present in very small amounts in cells’ structure. For instance, some carbohydrate molecules bind with proteins to produce glycoproteins, and others combine with lipids to produce glycolipids, both of which are found in the membrane that encloses the contents of body cells.
Lipids
A lipid is one of a highly diverse group of compounds made up mostly of hydrocarbons. The few oxygen atoms they contain are often at the periphery of the molecule. Their nonpolar hydrocarbons make all lipids hydrophobic. In water, lipids do not form a true solution, but they may form an emulsion, which is the term for a mixture of solutions that do not mix well.
Triglycerides
A triglyceride is one of the most common dietary lipid groups, and the type found most abundantly in body tissues. This compound, which is commonly referred to as a fat, is formed from the synthesis of two types of molecules:
- • A glycerol backbone at the core of triglycerides consists of three carbon atoms.
- • Three fatty acids, long chains of hydrocarbons with a carboxyl group and a methyl group at opposite ends, extend from each of the carbons of the glycerol.
Triglycerides form via dehydration synthesis. As a group, triglycerides are a major fuel source for the body. Glycerol gives up hydrogen atoms from its hydroxyl groups at each bond, and the carboxyl group on each fatty acid chain gives up a hydroxyl group. A total of three water molecules are thereby released.
Fatty acid chains that have no double carbon bonds anywhere along their length and therefore contain the maximum number of hydrogen atoms are called saturated fatty acids. These straight, rigid chains pack tightly together and are solid or semisolid at room temperature. Butter and lard are examples, as is the fat found on a steak or in your own body. In contrast, unsaturated fatty acids are fatty acids with one double carbon bond that are kinked at that bond. These monounsaturated fatty acids are therefore unable to pack together tightly and are liquid at room temperature. Polyunsaturated fatty acids contain two or more double carbon bonds and are also liquid at room temperature. Plant oils such as olive oil typically contain both mono- and polyunsaturated fatty acids.
Finally, trans fatty acids—found in certain processed foods, including some stick and tub margarines—are thought to be even more harmful to the heart and blood vessels than saturated fatty acids. Trans fats are created from unsaturated fatty acids (such as corn oil) when chemically treated to produce partially hydrogenated fats.
Fatty acids are also components of glycolipids, which are sugar-fat compounds found in the cell membrane. Lipoproteins are compounds in which the hydrophobic triglycerides are packaged in protein envelopes for transport in body fluids.
Phospholipids
As its name suggests, a phospholipid is a bond between the glycerol component of a lipid and a phosphorus molecule. In fact, phospholipids are similar in structure to triglycerides. However, instead of having three fatty acids, a phospholipid is generated from a diglyceride, a glycerol with just two fatty acid chains. The third binding site on the glycerol is taken up by the phosphate group, which in turn is attached to a polar “head” region of the molecule. Recall that triglycerides are nonpolar and hydrophobic. This still holds for the fatty acid portion of a phospholipid compound. However, the head of a phospholipid contains charges on the phosphate groups as well as on the nitrogen atom. These charges make the phospholipid head hydrophilic. Therefore, phospholipids are said to have hydrophobic tails, containing the neutral fatty acids, and hydrophilic heads, containing the charged phosphate groups and nitrogen atom.
Steroids
A steroid compound (referred to as a sterol) has as its foundation a set of four hydrocarbon rings bonded to a variety of other atoms and molecules. Although both plants and animals synthesize sterols, the type that makes the most important contribution to human structure and function is cholesterol, which is synthesized by the liver in humans and animals and is also present in most animal-based foods. Like other lipids, cholesterol’s hydrocarbons make it hydrophobic; however, it has a polar hydroxyl head that is hydrophilic. Cholesterol is an important component of bile acids, compounds that help emulsify dietary fats. In fact, the word root chole- refers to bile. Cholesterol is also a building block of many hormones, signalling molecules that the body releases to regulate processes at distant sites. Finally, like phospholipids, cholesterol molecules are found in the cell membrane, where their hydrophobic and hydrophilic regions help regulate the flow of substances into and out of the cell.
Prostaglandins
Like a hormone, a prostaglandin is one of a group of signalling molecules, but prostaglandins are derived from unsaturated fatty acids. One reason that the omega-3 fatty acids found in fish are beneficial is that they stimulate the production of certain prostaglandins that help regulate aspects of blood pressure and inflammation, thereby reducing the risk for heart disease. Prostaglandins also sensitize nerves to pain.
Example: One class of pain-relieving medications called nonsteroidal anti-inflammatory drugs (NSAIDs) works by reducing the effects of prostaglandins.
Proteins
You might associate proteins with muscle tissue, but in fact, proteins are critical components of all tissues and organs. A protein is an organic molecule composed of amino acids linked by peptide bonds. Proteins include the keratin in the epidermis of skin that protects underlying tissues as well as the collagen found in the dermis of skin, in bones, and in the meninges that cover the brain and spinal cord. Proteins are also components of many of the body’s functional chemicals, including digestive enzymes in the digestive tract, antibodies, the neurotransmitters that neurons use to communicate with other cells, and the peptide-based hormones that regulate certain body functions (for instance, growth hormone). While carbohydrates and lipids are composed of hydrocarbons and oxygen, all proteins also contain nitrogen (N), and many contain sulfur (S) in addition to carbon, hydrogen, and oxygen.
Microstructure of Proteins
Proteins are polymers made up of nitrogen-containing monomers called amino acids. An amino acid is a molecule composed of an amino group and a carboxyl group together with a variable side chain. Just 20 different amino acids contribute to nearly all the thousands of different proteins important in human structure and function. Body proteins contain a unique combination of a few dozen to a few hundred of these 20 amino acid monomers. All 20 of these amino acids share a similar structure. Amino acids join via dehydration synthesis to form protein polymers. The unique bond holding amino acids together is called a peptide bond. A peptide bond is a covalent bond between two amino acids that forms by dehydration synthesis. A peptide, in fact, is a very short chain of amino acids. Strands containing fewer than about 100 amino acids are generally referred to as polypeptides rather than proteins.
The body is able to synthesize most of the amino acids from components of other molecules; however, nine cannot be synthesized and have to be consumed in the diet. These are known as the essential amino acids.
Free amino acids available for protein construction are said to reside in the amino acid pool within cells. Structures within cells use these amino acids when assembling proteins. If a particular essential amino acid is not available in sufficient quantities in the amino acid pool, however, synthesis of proteins containing it can slow or even cease.
Shape of Proteins
A protein’s shape is essential to its function, and it is determined, most fundamentally, by the sequence of amino acids of which it is made. The sequence is called the primary structure of the protein. Although some polypeptides exist as linear chains, most are twisted or folded into more complex secondary structures that form when bonding occurs between amino acids with different properties at different regions of the polypeptide.
When they are exposed to extreme heat, acids, bases, and certain other substances, proteins will denature. Denaturation is a change in the structure of a molecule through physical or chemical means. Denatured proteins lose their functional shape and are no longer able to carry out their jobs. An everyday example of protein denaturation is the curdling of milk when acidic lemon juice is added.
The contribution of the shape of a protein to its function can hardly be exaggerated.
Example: The long, slender shape of protein strands that make up muscle tissue is essential to their ability to contract (shorten) and relax (lengthen); bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are deposited. These elongated proteins, called fibrous proteins, are strong, durable, and typically hydrophobic.
In contrast, globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic.
Example: Hemoglobin proteins are packed into red blood cells; however, globular proteins are abundant throughout the body, playing critical roles in most body functions; enzymes are examples of this.
Proteins Function as Enzymes
Without enzymes to catalyze chemical reactions, the human body would be nonfunctional. It functions only because enzymes function.
Enzymatic reactions—chemical reactions catalyzed by enzymes—begin when substrates bind to the enzyme. A substrate is a reactant in an enzymatic reaction. This occurs on regions of the enzyme known as active sites (Figure 2.12). Any given enzyme catalyzes just one type of chemical reaction. This characteristic, called specificity, is due to the fact that a substrate with a particular shape and electrical charge can bind only to an active site corresponding to that substrate.
Due to this jigsaw puzzle–like match between an enzyme and its substrates, enzymes are known for their specificity. In fact, as an enzyme binds to its substrate(s), the enzyme structure changes slightly to find the best fit between the transition state (a structural intermediate between the substrate and product) and the active site. This active-site modification in the presence of a substrate, along with the simultaneous formation of the transition state, is called induced fit. Overall, there is a specifically matched enzyme for each substrate and, thus, for each chemical reaction; however, there is some flexibility as well. Some enzymes have the ability to act on several different structurally related substrates.
The binding of a substrate produces an enzyme-substrate complex. It is likely that enzymes speed up chemical reactions in part because the enzyme-substrate complex undergoes a set of temporary and reversible changes that cause the substrates to be oriented toward each other in an optimal position to facilitate their interaction. This promotes increased reaction speed. The enzyme then releases the product(s) and resumes its original shape. The enzyme is then free to engage in the process again and will do so as long as substrate remains.
Figure 2.12 Steps in an Enzymatic Reaction. According to the induced-fit model, the active site of the enzyme undergoes conformational changes upon binding with the substrate. (a) Substrates approach active sites on the enzyme. (b) Substrates bind to active sites, producing an enzyme-substrate complex. (c) Changes internal to the enzyme-substrate complex facilitate interaction of the substrates. (d) Products are released, and the enzyme returns to its original form, ready to facilitate another enzymatic reaction.
Other Functions of Proteins
Protein is important in building, repairing, and maintaining muscle tissue, but the truth is that proteins contribute to all body tissues, from the skin to the brain cells. Also, certain proteins act as hormones, chemical messengers that help regulate body functions; for example, growth hormone is important for skeletal growth, among other roles.
Their basic and acidic components enable proteins to function as buffers in maintaining acid-base balance, but they also help regulate fluid-electrolyte balance. Proteins attract fluid, and a healthy concentration of proteins in the blood, the cells, and the spaces between cells helps ensure a balance of fluids in these various “compartments.” Moreover, proteins in the cell membrane help transport electrolytes in and out of the cell, keeping these ions in a healthy balance. Like lipids, proteins can bind with carbohydrates. They can thereby produce glycoproteins or proteoglycans, both of which have many functions in the body.
The body can use proteins for energy when carbohydrate and fat intake is inadequate and stores of glycogen and adipose tissue become depleted. However, since there is no storage site for protein except in functional tissues, using protein for energy causes tissue breakdown and results in body wasting.
Nucleotides
The fourth type of organic compound important to human structure and function is the nucleotide. A nucleotide is one of a class of organic compounds composed of three subunits:
- • one or more phosphate groups
- • a pentose sugar: either deoxyribose or ribose
- • a nitrogen-containing base: adenine (A), cytosine (C), guanine (G), thymine (T), or uracil (U)
Nucleotides can be assembled into nucleic acids (DNA or RNA) or the energy compound adenosine triphosphate.
Nucleic Acids
The nucleic acids differ in their type of pentose sugar. Deoxyribonucleic acid (DNA) is a polynucleotide that stores genetic information; it serves as long-term storage of information for protein synthesis. DNA contains deoxyribose (so called because it has one less atom of oxygen than ribose) plus one phosphate group and one nitrogen-containing base. The “choices” of base for DNA are adenine, cytosine, guanine, and thymine. Ribonucleic acid (RNA) is a ribose-containing nucleotide that helps manifest the genetic code as protein. RNA contains ribose, one phosphate group, and one nitrogen-containing base, but the “choices” of base for RNA are adenine, cytosine, guanine, and uracil (not thymine).
Bonds formed by dehydration synthesis between the sugar of one nucleic acid monomer and the phosphate group of another form a “backbone” from which the components’ nitrogen-containing bases protrude. In DNA, two such backbones attach at their protruding bases via hydrogen bonds. These twist to form a shape known as a double helix (Figure 2.13). The sequence of nitrogen-containing bases within a strand of DNA forms the genes that act as a molecular code, instructing cells in the assembly of amino acids into proteins. Humans have almost 22,000 genes in their DNA, locked up in the 46 chromosomes inside the nucleus of each cell (except red blood cells, which lose their nuclei during development). These genes carry the genetic code to build one’s body and are unique for each individual, except identical twins.
Figure 2.13 DNA. In the DNA double helix, two strands attach via hydrogen bonds between the bases of the component nucleotides.
In contrast, RNA consists of a single strand of RNA nucleotides of sugar-phosphate backbone studded with bases. Messenger RNA (mRNA) is created during protein synthesis to carry the genetic instructions from the DNA to the cell’s protein manufacturing plants in the cytoplasm, the ribosomes.
Adenosine Triphosphate
The nucleotide adenosine triphosphate (ATP) is composed of a ribose sugar, an adenine base, and three phosphate groups. ATP is classified as a high-energy compound because the two covalent bonds linking its three phosphates store a significant amount of potential energy. In the body, the energy released from these high-energy bonds helps fuel the body’s activities, from muscle contraction to the transport of substances in and out of cells to anabolic chemical reactions.
When a phosphate group is cleaved from ATP, the products are adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis reaction can be written as follows:
ATP + H2O → ADP + Pi + energy
Removal of a second phosphate leaves adenosine monophosphate (AMP) and two phosphate groups. Again, these reactions also liberate the energy that had been stored in the phosphate-phosphate bonds. They are reversible, too, as when ADP is formed back into ATP with the addition of energy. Phosphorylation is the addition of a phosphate group to an organic compound, in this case resulting in ATP. In such cases, the same level of energy that had been released during hydrolysis must be reinvested to power dehydration synthesis.
Cells can also transfer a phosphate group from ATP to another organic compound. For example, when glucose first enters a cell, a phosphate group is transferred from ATP, forming glucose phosphate (C6H12O6—P) and ADP.
Key Terms
- acid:
- Compound that releases hydrogen ions (H+) in solution.
- activation energy:
- Amount of energy greater than the energy contained in the reactants, which must be overcome for a reaction to proceed.
- adenosine triphosphate (ATP):
- Nucleotide containing ribose, an adenine base, and three phosphates; essential in energy transfer.
- amino acid:
- Building block of proteins; characterized by an amino functional group, carboxyl functional groups, and a variable side chain.
- anion:
- Atom with a negative charge.
- atom:
- Smallest unit of an element that retains the unique properties of that element.
- atomic number:
- Number of protons in the nucleus of an atom.
- base:
- Compound that accepts hydrogen ions (H+) in solution.
- bond:
- Electrical force linking atoms.
- buffer:
- Solution containing a weak acid or a weak base that opposes wide fluctuations in the pH of body fluids.
- carbohydrate:
- Class of organic compounds built from sugar molecules containing carbon, hydrogen, and oxygen in a 1-2-1 ratio.
- catalyst:
- Substance that increases the rate of a chemical reaction without itself being changed in the process.
- cation:
- Atom with a positive charge.
- chemical energy:
- Form of energy that is absorbed as chemical bonds form, stored as they are maintained, and released as they are broken.
- colloid:
- Liquid mixture in which the solute particles consist of clumps of molecules large enough to scatter light.
- compound:
- Substance composed of two or more different elements joined by chemical bonds.
- covalent bond:
- Chemical bond in which two atoms share electrons, thereby completing their valence shells.
- decomposition reaction:
- Type of catabolic reaction in which one or more bonds within a larger molecule are broken, resulting in the release of smaller molecules or atoms.
- denaturation:
- Change in the structure of a molecule through physical or chemical means.
- deoxyribonucleic acid (DNA):
- Deoxyribose-containing nucleotide that stores genetic information.
- disaccharide:
- Pair of carbohydrate monomers bonded by dehydration synthesis via a glycosidic bond.
- double helix:
- Characteristic of DNA where two strands attach via hydrogen bonds between the bases of the component nucleotides.
- electron:
- Subatomic particle having a negative charge and nearly no mass; found orbiting the atom’s nucleus.
- electron shell:
- Area of space a given distance from an atom’s nucleus in which electrons are grouped.
- element:
- Substance that cannot be created or broken down by ordinary chemical means.
- enzyme:
- Protein or RNA that catalyzes chemical reactions.
- exchange reaction:
- Type of chemical reaction in which bonds are both formed and broken, resulting in the transfer of components.
- hydrogen bond:
- Very weak bond between molecules or between parts of large polar molecules like water.
- inorganic compound:
- Substance that does not contain both carbon and hydrogen.
- ion:
- Atom with an overall positive or negative charge.
- ionic bond:
- Attraction between an anion and a cation.
- kinetic energy:
- Energy that matter possesses because of its motion.
- lipid:
- Class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water.
- mass number:
- Sum of the number of protons and neutrons in the nucleus of an atom.
- matter:
- Physical substance; that which occupies space and has mass.
- molecule:
- Two or more atoms covalently bonded together.
- monosaccharide:
- Monomer of carbohydrate; also known as a simple sugar.
- neutron:
- Heavy subatomic particle having no electrical charge and found in the atom’s nucleus.
- nucleotide:
- Class of organic compounds composed of one or more phosphate groups, a pentose sugar, and a nitrogen-containing base.
- organic compound:
- Substance that contains both carbon and hydrogen.
- peptide bond:
- Covalent bond formed by dehydration synthesis between two amino acids.
- periodic table of the elements:
- Arrangement of the elements in a table according to their atomic number; elements having similar properties because of their electron arrangements compose columns in the table, while elements having the same number of valence shells compose rows in the table.
- pH:
- Negative logarithm of the hydrogen-ion (H+) concentration of a solution.
- phospholipid:
- A lipid compound in which a phosphate group is combined with a diglyceride.
- phosphorylation:
- Addition of one or more phosphate groups to an organic compound.
- polar molecule:
- Molecule covalently bonded where one atom is more electronegative and draws electrons toward its nucleus, resulting in slight negative and positive charges.
- potential energy:
- Stored energy matter possesses because of the positioning or structure of its components.
- product:
- One or more substances produced by a chemical reaction.
- prostaglandin:
- Lipid compound derived from fatty acid chains; important in regulating several body processes.
- protein:
- Class of organic compounds that are composed of many amino acids linked together by peptide bonds.
- proton:
- Heavy subatomic particle having a positive charge and found in the atom’s nucleus.
- reactant:
- One or more substances that enter into the reaction.
- ribonucleic acid (RNA):
- Ribose-containing nucleotide that helps manifest the genetic code as protein.
- solution:
- Homogeneous liquid mixture in which a solute is dissolved into molecules within a solvent.
- steroid (also sterol):
- Lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and molecules.
- substrate:
- Reactant in an enzymatic reaction.
- suspension:
- Liquid mixture in which particles distributed in the liquid settle out over time.
- synthesis reaction:
- Type of anabolic reaction in which two or more atoms or molecules bond, resulting in the formation of a larger molecule.
- triglyceride:
- Lipid compound composed of a glycerol molecule bonded with three fatty acid chains.
- valence shell:
- Outermost electron shell of an atom.
Chapter Review
2.1 Elements and Atoms: The Building Blocks of Matter
The human body is composed of elements, the most abundant of which are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). You obtain these elements from the foods you eat and the air you breathe. The smallest unit of an element that retains all the properties of that element is an atom. But atoms themselves contain many subatomic particles, the three most important of which are protons, neutrons, and electrons. These particles do not vary in quality from one element to another; rather, what gives an element its distinctive identification is the quantity of its protons, called its atomic number. Protons and neutrons contribute nearly all of an atom’s mass; the number of protons and neutrons is an element’s mass number. Heavier and lighter versions of the same element can occur in nature because these versions have different numbers of neutrons. Different versions of an element are called isotopes.
The tendency of an atom to be stable or to react readily with other atoms is largely due to the behaviour of the electrons within the atom’s outermost electron shell, called its valence shell. Helium and larger atoms with eight electrons in their valence shell are unlikely to participate in chemical reactions because they are stable. All other atoms tend to accept, donate, or share electrons in a process that brings the electrons in their valence shell to eight (or in the case of hydrogen, to two).
2.2 Chemical Bonds
Each moment of life, atoms of oxygen, carbon, hydrogen, and the other elements of the human body are making and breaking chemical bonds. Ions are charged atoms that form when an atom donates or accepts one or more negatively charged electrons.
Cations (ions with a positive charge) are attracted to anions (ions with a negative charge). This attraction is called an ionic bond. In covalent bonds, the participating atoms do not lose or gain electrons but rather share them. Molecules with nonpolar covalent bonds are electrically balanced and have a linear three-dimensional shape. Molecules with polar covalent bonds have “poles”—regions of weakly positive and negative charge—and have a triangular three-dimensional shape. An atom of oxygen and two atoms of hydrogen form a water molecule by means of polar covalent bonds. Hydrogen bonds link hydrogen atoms already participating in polar covalent bonds to anions or electronegative regions of other polar molecules. Hydrogen bonds link water molecules, resulting in the properties of water that are important to living things.
2.3 Chemical Reactions
Chemical reactions, in which chemical bonds are broken and formed, require an initial investment of energy. Kinetic energy, the energy of matter in motion, fuels the collisions of atoms, ions, and molecules that are necessary if their old bonds are to break and new ones to form. All molecules store potential energy, which is released when their bonds are broken.
Four forms of energy essential to human functioning are chemical energy, which is stored and released as chemical bonds are formed and broken; mechanical energy, which directly powers physical activity; radiant energy, emitted as waves such as in sunlight; and electrical energy, the power of moving electrons.
Chemical reactions begin with reactants and end with products. Synthesis reactions bond reactants together, a process that requires energy, whereas decomposition reactions break the bonds within a reactant and thereby release energy. In exchange reactions, bonds are both broken and formed, and energy is exchanged.
The rate at which chemical reactions occur is influenced by several properties of the reactants: temperature, concentration, pressure, and the presence or absence of a catalyst. An enzyme is a catalytic protein that speeds up chemical reactions in the human body.
2.4 Inorganic Compounds Essential to Human Functioning
Inorganic compounds essential to human functioning include water, salts, acids, and bases. These compounds are inorganic; that is, they do not contain both hydrogen and carbon. Water is a lubricant and cushion, a heat sink, a component of liquid mixtures, a by-product of dehydration synthesis reactions, and a reactant in hydrolysis reactions. Salts are compounds that, when dissolved in water, dissociate into ions other than H+ or OH−. In contrast, acids release H+ in a solution, making it more acidic. Bases accept H+, thereby making the solution more alkaline (caustic). The pH of any solution is its relative concentration of H+. A solution with pH 7 is neutral. Solutions with a pH below 7 are acids, and solutions with a pH above 7 are bases. A change in a single digit on the pH scale (e.g., from 7 to 8) represents a tenfold increase or decrease in the concentration of H+. In a healthy adult, the pH of blood ranges from 7.35 to 7.45. Homeostatic control mechanisms important for keeping blood in a healthy pH range include chemicals called buffers, weak acids and weak bases released when the pH of blood or other body fluids fluctuates in either direction outside of this normal range.
2.5 Organic Compounds Essential to Human Functioning
Organic compounds essential to human functioning include carbohydrates, lipids, proteins, and nucleotides. These compounds are said to be organic because they contain both carbon and hydrogen. Carbon atoms in organic compounds readily share electrons with hydrogen and other atoms, usually oxygen and sometimes nitrogen. Carbon atoms also may bond with one or more macromolecules such as carboxyls, hydroxyls, aminos, or phosphates.
Monomers are single units of organic compounds. They bond by dehydration synthesis to form polymers, which can in turn be broken by hydrolysis.
Carbohydrate compounds provide essential body fuel. Their structural forms include monosaccharides such as glucose, disaccharides such as lactose, and polysaccharides, including starches (polymers of glucose), glycogen (the storage form of glucose), and fibre. All body cells can use glucose for fuel. It is converted via an oxidation-reduction reaction to ATP.
Lipids are hydrophobic compounds that provide body fuel and are important components of many biological compounds. Triglycerides are the most abundant lipid in the body and are composed of a glycerol backbone attached to three fatty acid chains. Phospholipids are compounds composed of a diglyceride with a phosphate group attached at the molecule’s head. The result is a molecule with polar and nonpolar regions. Steroids are lipids formed of four hydrocarbon rings. The most important is cholesterol. Prostaglandins are signalling molecules derived from unsaturated fatty acids.
Proteins are critical components of all body tissues. They are made up of monomers called amino acids, which contain nitrogen joined by peptide bonds. Protein shape is critical to its function. Most body proteins are globular. An example is enzymes, which catalyze chemical reactions.
Nucleotides are compounds with three building blocks: one or more phosphate groups, a pentose sugar, and a nitrogen-containing base. DNA and RNA are nucleic acids that function in protein synthesis. ATP is the body’s fundamental molecule of energy transfer. Removal or addition of phosphates releases or invests energy.