Organic Substances

Organic Substances

Organic substances include carbohydrates, lipids, proteins, and nucleic acids. Many organic molecules are made up of long chains of carbon atoms linked by covalent bonds. The carbon atoms usually form addi-tional covalent bonds with hydrogen or oxygen atoms and less commonly with nitrogen, phosphorus, sulfur, or other elements.

Carbohydrates

Carbohydrates provide much of the energy requiredby the body’s cells and help to build cell structures. Carbohydrate molecules consist of carbon, hydro-gen, and oxygen molecules. The carbon atoms they contain join in chains that vary with the type of car-bohydrate. The hydrogen and oxygen atoms usually occur in a 2:1 ratio, which is the same as in water. In most cases, the overall carbon to hydrogen to oxygen ratio is 1:2:1. Carbohydrates with shorter chains are called sugars. Carbohydrates also include starches. Collectively, carbohydrates represent 1% to 2% of cell mass in the body. The term carbohydrateactu-ally means “hydrated carbon.” Usually, the larger the carbohydrate molecule, the less soluble it is in water. Carbohydrate molecules are the body’s most readily available source of energy.

Monosaccharides

Simple sugars have 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms (C₆H₁₂O₆). They are also known as monosaccharides. Simple sugars include glucose, fructose, galactose, ribose, and deoxyribose. Ribose and deoxyribose differ from the others in that they each contain five atoms of carbon. The most important metabolic fuel molecule in the body is glu-cose. Monosaccharides are single chain or single ring structures. They may contain between three and seven carbon atoms. Monosaccharides are generally named based on the number of carbon atoms they contain. In the human body, the most important ones are the pentose (five-carbon) and hexose (six-carbon) sugars (FIGURE 2-7).

Disaccharides

Complex carbohydrates include sucrose (table sugar) and lactose (milk sugar). Some of these carbohydrates are double sugars or disaccharides and are formed when two monosaccharides are joined by dehydra-tion synthesis. A water molecule is lost as the bondis made. Another important disaccharide is malt-ose(malt sugar). Disaccharides cannot pass throughcell membranes because of their size, so instead are digested to simple sugar units for absorption from the digestive tract. They decompose via hydrolysis, which is basically the reverse process of dehydration synthe-sis. A water molecule is added, which breaks the bond and releases the simple sugar units.

Polysaccharides

Other types of complex carbohydrates contain many simple joined sugar units such as plant starch, and are known as polysaccharides. They are polymers of simple sugars, linked together via dehydration synthesis, and function as storage products because they are large and fairly insoluble. They are less sweet than the simple and double sugars. Humans and other animals synthesize a polysaccharide called glycogen.

In all animal tissues, glycogen is the storage carbo-hydrate. It is mostly stored in the skeletal muscle and liver, and is highly branched (like starch) and made up of large molecules. When the blood sugar level drops quickly, the liver cells break down glycogen releas-ing its glucose units into the blood. Because of many branch ends that can release glucose at the same time, body cells can have almost immediate stores of glu-cose to use as fuel.

Only glycogen and starch are of major impor-tance in the human body. They are glucose polymers with different forms of branching. Starch is the storage carbohydrate that is formed by plants, with high and variable amounts of glucose units. Starches include potatoes and grain products. Starches must be digested for absorption. Humans cannot digest cellulose, which is another polysaccharide found in plants, but it functions as bulk, a form of fiber, which aids in peristalsis of feces.

Carbohydrates are primarily used by the body for ever-ready, easy- to-use cellular fuel. Glucose is the primary form of fuel used by most cells, which in general can only use a few types of other simple sugars. Remember that glucose is broken down and oxidized inside cells, during which time electrons are transferred. This releases the bond energy that is stored in the glucose, and ATP can be synthesized. When ATP is sufficiently present, carbohydrates from the diet can be converted to glycogen (or fat) and stored in the body. For structural needs, only tiny amounts of carbohydrates are used. There are some sugars in human genes, whereas others are attached to external cell surfaces and used to guide interactions between cells.

Lipids

Lipids are insoluble in water, but may dissolve inother lipids, oils, ether, chloroform, or alcohol. Lipids include a variety of compounds such as triglycerides, phospholipids, and steroids with vital cell functions. Fats are the most common type of lipids. They provide­ roughly twice the energy of carbohydrates. Lipids help to maintain body temperature. Like carbohy-drates, fat molecules also contain carbon, hydrogen, and oxygen but have far fewer oxygen atoms than do carbohydrates. Some complex lipids also contain phosphorus. Lipoproteins are complexes or com-pounds that contain lipids and proteins. Nearly all lipids in the plasma are present as lipoproteins. There are five types of lipoproteins:

■■High-density lipoproteins (HDL): Good cholesterol

■■Low-density lipoproteins (LDL): Bad cholesterol

■■Very-high-density lipoproteins (VHDL)

■■Very-low-density lipoproteins (VLDL)

■■Intermediate-density lipoproteins (IDL)

Triglycerides

Fatty acids and glycerol are the building blocks of fat molecules. A single fat molecule consists of one glycerol­ molecule bonded to three fatty acid molecules­. These fat molecules are known as ­triglycerides, also called neutral fats, a subcate-gory of lipids that includes fats (when solid) and oils (when liquid). These molecules are formed by the condensation of one molecule of glycerol, which is a three -carbon sugar alcohol (a modified simple sugar) . A triglyceride contains three fatty acid mol-ecules and glycerol. Triglycerides contain different saturated and unsaturated fatty acid combinations. Those with the most saturated fatty acids are called saturated fats and those with the most unsaturatedfatty acids are called unsaturated fats. In general, the ratio of fatty acids to glycerol in a triglyceride is 3:1. Via dehydration synthesis, fat synthesis involves the attachment of three fatty acid chains to just one glycerol molecule. An E-shaped molecule is devel-oped. The fatty acid chains vary, but the glycerol is always the same in all triglycerides.

Fatty acids are linear chains of carbon and hydro-gen atoms known as hydrocarbon chains, with an organic acid group located at one end. They consist of a long hydrocarbon tail and a smaller area consisting of a carboxyl group known as the head (FIGURE 2-8). Triglycerides may be made up of hundreds of atoms. Fats and oils, after being consumed, must be broken down to their simpler building blocks before they can be absorbed. Nonpolar molecules are made from their hydrocarbon chains. Oils (fats) and water cannot mix because polar and nonpolar molecular molecules cannot interact. Triglycerides provide the body’s best type of stored energy. Upon oxidizing, they release large amounts of energy. Deeper body tissues are protected from heat loss and mechanical trauma by triglycerides, which are mostly found beneath the skin. Women have a thicker subcutaneous fatty layer than men, which helps to insulate them from colder temperatures.

Saturated fat is defined as containing carbonatoms that are bound to as many hydrogen atoms as possible becoming saturated with them. The degree of saturation determines how solid the molecule is at various temperatures. Saturated fats have fatty acid changes with single covalent bonds between carbon atoms (FIGURE 2-9). These straight fatty acid chains have saturated fat molecules packed closely together at room temperature making them solid. Longer fattyacid chains and fatty acids with more saturation are commonly found in animal fats and butterfat, which are solid at room temperature.

Fatty acid molecules with one double bond between carbon atoms are called unsaturated. Double bonds cause fatty acid chains to form “kinks,” mean-ing they cannot be packed closely enough to solid-ify. Therefore, triglycerides with either short fatty acid chains or unsaturated fatty acids are oils. They are ­liquid at room temperature, a typical factor of plant lipids. Examples include oils from corn, olives,peanuts, safflowers, and soybeans. Unsaturated fats (especially olive oil) are healthier. Fatty acid molecules with many double-bonded carbon atoms are called polyunsaturated. FIGURE 2-10compares the differencesbetween saturated and unsaturated fats.

Many types of margarines and baked products contain trans fats, which are oils solidified by adding hydrogen atoms at the sites of carbon double bonds. Trans fats are now known to increase risks for heart disease even more significantly than solid animal fats. Oppositely, the omega-3 fatty acids from coldwater fish are known to decrease the risk of heart disease and certain inflammatory diseases.

Phospholipids

Similar to a fat molecule, a phospholipid consists of a glycerol portion with fatty acid chains. They are structurally related to glycolipids and are actu-ally modified triglycerides. Human cells can synthe-size both types of lipids, primarily from fatty acids. A phospholipid includes a phosphate group that is soluble in water and two molecules of fatty acids. They are an important part of cell structures. The distinc-tive chemical properties of phospholipids come from the phosphorus-containing group. The tails of these molecules (the hydrocarbon portion) are nonpolar; they react only with nonpolar molecules. The heads of these molecules (the phosphorus-containing part) are polar, attracting other polar or charged particles (including ions or water). The unique phospholipids can be used as the primary material for the building of cell membranes.

Steroids

Steroid molecules are large, basically flat lipid mole-cules that share a distinctive carbon framework in com-parison with fats or oils. Steroids have four connected rings of carbon atoms. All steroid molecules have the same basic structure: three six-carbon rings joined to one five-carbon ring. They include cholesterol, estro-gen, progesterone, testosterone, cortisol, and estradiol (FIGURE 2-11). Steroids are also fat soluble and have little to no oxygen. Steroid hormones are vital for homeosta-sis. The sex hormones include the sex steroids, which are essential for reproduction. If no corticosteroids were produced by the adrenal glands, it would be fatal.

Cholesterol is the most important steroid and isingested in animal foods such as cheese, eggs, and meat. The liver also produces certain amounts of cho-lesterol. Although essential for human life, excessive cholesterol participates in atherosclerosis and related disease. In the cell membranes, cholesterol is the raw material that helps to synthesize vitamin D, bile salts, and steroid hormones.

Eicosanoids

Eicosanoids are lipids that are mostly derived from arachidonic acid, a 20-carbon fatty acid existing inall cell membranes, the most important of which are the prostaglandins and related acids. Prostaglandins are important for blood clotting, inflammation, labor contractions, regulation of blood pressure, and many other body processes. Prostaglandin synthesis and inflammatory effects are blocked by medications such as the cyclooxygenase inhibitors and nonsteroidal anti-inflammatory drugs.

1. Explain the most common type of lipids and list the components their molecules contain.

2. Distinguish between saturated and unsaturated fats.

3. Define the terms phospholipid and steroid.

4. What are the roles of prostaglandins?

Proteins

Proteins are the most abundant organic componentsof the human body and in many ways the most import-ant. They make up between 10% and 30% of cell mass and are the basic structural materials of the body. Pro-teins are vital for many body functions. On cell sur-faces, some proteins combine with carbohydrates to become glycoproteins. They allow cells to respond to certain molecules that bind to them. Proteins include biologic catalysts (enzymes), contractile proteins of muscles, and the hemoglobin of the blood.

There are more than 200,000 types of proteins in the human body, the full set known as the proteome­. Antibodies are proteins that detect and destroy ­foreign substances. All proteins contain carbon, hydrogen, oxygen, and nitrogen atoms, with small quantities of sulfur also present. Proteins always con-tain nitrogen atoms. Twenty common amino acids, both essential and nonessential, make up the proteins that exist in humans and most other living organ-isms (TABLE 2-2).

Amino acids are the building blocks of proteins, with two ­primary groups: amines and organic acids. Amino acids act as either bases (protonacceptors) or acids (proton donors). All amino acids are exactly the same except for one group of atoms, known as the amino acid’sR group. Differences in the R group determine the chemical uniqueness of each amino acid (FIGURE 2-12).

Nucleic Acids

Nucleic acids are large organic molecules (macro-molecules) that carry genetic information or form ­structures within cells. They are composed of car-bon, hydrogen, oxygen, nitrogen, and phosphorus. Nucleic acids are actually the largest molecules in the body. Nucleic acids store and process information at the molecular level inside the cells. The two classes of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are found in allliving things, cells, and viruses. Individual strands of DNA and RNA have a similar structure (FIGURE 2-14).

Nucleotides are the structural units of nucleicacids. These complex units consist of a nitrogen-­ containing base, a pentose sugar, and a phosphate group. The nucleotide structure is based on five major types of nitrogen-containing bases:

■■ Adenine (A): A large, two-ring base (purine)

■■ Guanine (G): Also a purine

■■ Cytosine(C): A smaller, single-ring base (pyrimidine)

■■ Thymine (T): Also a pyrimidine

■■ Uracil (U): Also a pyrimidine

Enzymes

Enzymes are globular proteins that promote chem-ical reactions by lowering the activation energy requirements. Activation energy is the energy that must be overcome for a chemical reaction to occur. Therefore, they make chemical reactions possible and catalyze the reactions that sustain life. This means that enzymes are catalysts. Enzyme molecules are manufactured by cells to promote specific reac-tions. Enzymes are among the most important of all the body’s proteins. Nearly everything that occurs in the human body relies on a specific enzyme. In the body, enzymes assist in the digestion of food, drug metabolism, protein formation, and many other types of reactions. Enzymes make metabolic reac-tions possible inside cells by controlling tempera-ture conditions that otherwise would be too mild for them to occur.