The Carbon Atom and the Macromolecules
I. All living things contain the same types of molecules.
A. There is also diversity among the exact molecules each kind of organism contains.
B. About 95% of the body’s weight is made up of carbon, hydrogen, nitrogen, and oxygen.
C. Both the sameness and diversity of life are dependent on the chemical characteristics of carbon.
1. Carbon’s chemistry is essential to living things.
2. Organic molecules must contain carbon and hydrogen.
a) Organic molecules also often contain oxygen, nitrogen, and other elements.
b) Carbon dioxide is an inorganic molecule, since it contains no hydrogen.
c) Organic molecules characterize the structure and function of living things.
D. Inorganic molecules constitute nonliving matter but also play important roles in living things.
II. The chemistry of the cell and the organism are dependent on carbon.
A. Carbon contains 4 electrons in its outer shell.
1. This allows it to bond with as many as 4 other atoms.
2. The covalent bonds that carbon forms are quite strong.
3. Carbon usually bonds to hydrogen, oxygen, nitrogen, or another carbon atom.
4. Long chains of carbon atoms are not unusual in living systems.
5. Carbon can also form double covalent bonds by sharing 2 pairs of electrons.
6. Ring compounds are sometimes formed by carbon-to-carbon bonding.
B. Small organic molecules include sugars, fatty acids, amino acids, and nucleotides.
1. In addition to carbon backbones, these molecules have characteristic functional groups.
a) A functional group is a cluster of atoms with a certain pattern that always behaves in a particular way.
b) Functional groups help determine the characteristics of organic molecules.
2. An organic molecule’s characteristics include whether it is hydrophilic or hydrophobic.
a) Polar molecules with positive or negative charges are hydrophilic, that is, attracted to water because of water’s polar nature.
b) A hydrocarbon (with only carbon and hydrogen) is always hydrophobic but may also contain a functional group that can ionize to form a hydrophilic region.
c) Ionizing functional groups include -COOH, -OH, -CO, and -NH2.
3. Isomers are molecules that have identical molecular formulas (same number and kind of atoms) but different arrangement of the atoms structurally.
C. Large organic molecules or macromolecules are made up of subunits of the smaller organic molecules.
1. A subunit is called a monomer, and the macromolecule is a polymer.
2. Simple sugars are the monomers within polysaccharides.
3. Fatty acids and glycerol are the subunits within a fat.
4. Amino acids join to form proteins.
5. Nucleotides are the subunits of nucleic acids.
D. Condensation is a dehydration synthesis reaction that removes the components of water to create a bond to form a polymer.
1. The proper enzyme is required for the dehydration synthesis to occur.
2. When a bond is formed, energy is used.
E. Hydrolysis is the process by which a polymer is broken down.
1. It is essentially the reverse of the condensation process.
2. A water molecule is broken apart, and the -OH group is attached to one monomer and the -H attaches to the other monomer, which had been joined by the bond that is broken.
3. It is called hydrolysis because water is used to break a bond apart.
4. Breaking a bond releases energy, making it available for further use.
III. Simple sugars to polysaccharides are carbohydrates.
A. Carbohydrates usually contain carbon, hydrogen, and oxygen in a ratio of 1:2:1.
1. The general formula for any carbohydrate is (CH2O)n, where n is the number of groups in the molecule.
2. Simple sugars are monosaccharides, with a carbon backbone of 3 to 7 carbons.
a. The best-known monosaccharides are hexoses, with 6 carbons, such as glucose, fructose, and galactose, all of which are isomers of each other.
b. Important 5-carbon sugars (pentoses) are ribose and deoxyribose, found in nucleic acids.
3. A disaccharide contains 2 monosaccharides joined by condensation.
a. Sucrose contains glucose and fructose and is used as table sugar.
b. Lactose contains glucose and galactose and is found in milk.
c. Maltose contains 2 glucose monosaccharides and is produced by starch digestion.
B. Starch and glycogen differ only slightly in structure and are used as storage compounds.
1. Glycogen has many side branches of glucose.
2. Starch has few side chains and a very long main chain.
3. Plant cells store extra carbohydrates as complex sugars and starches.
4. Animal cells store extra carbohydrates as glycogen, or “animal starch,” in the liver; between meals the glycogen releases glucose to keep the blood sugar level stable.
C. Cellulose and chitin are structural carbohydrate molecules.
1. Cellulose contains glucose molecules with a slightly different type of linkage than that found in starch and glycogen.
a) In starch and glycogen, bond orientation allows polymers to form compact spirals.
b) In cellulose, bonds make the polymer straight and fibrous.
c) Long, unbranched polymers of cellulose are held together by hydrogen bonds to form microfibrils.
d) Several microfibrils make up a fibril, and crossed parallel layers of fibrils make up plant cell walls.
2. Humans use cellulose in cotton clothing and in wood for buildings and furniture, but humans and most animals cannot digest the cellulose in plant foods.
a) Humans lack the enzyme needed to break the linkage between glucose molecules in cellulose but have the enzyme to break the bonds in starch.
b) Cellulose is important in the diet because it provides bulk (fiber or roughage) that maintains regular elimination.
3. Chitin is found in the exoskeleton of insects and related animals.
a) The glucose that forms the monomers of chitin has an amino group attached to each monomer.
b) Chitin has the same kind of bond linkage as cellulose, so it is not digestible.
IV. Fatty acids to lipids are mainly insoluble in water because they lack polar groups.
A. Lipids are often used as long-term energy storage compounds, while others are components of plasma membranes.
B. Fats and oils contain 2 types of subunits, fatty acids and glycerol.
1. Each fatty acid is a long hydrocarbon chain with a carboxyl (acid) group at one end.
a) The carboxyl is a polar group, so the fatty acid is soluble in water.
b) Most fatty acids in cells contain 16 to 18 carbon atoms per molecule.
c) Fatty acids are saturated (with no double bonds) or unsaturated (with at least one double bond between the carbon atoms, so it is not saturated with hydrogens).
2. Glycerol has 3 polar hydroxyl groups, each of which can react with a fatty acid to form a neutral fat molecule or triglyceride by condensation.
a) Triglycerides with unsaturated fatty acids melt at a lower temperature than those containing saturated chains.
b) A double bond in the unsaturated chain makes a kink that prevents close packing among the chains.
3. Fats contain mostly C—H bonds, which contain more energy than the C—OH bonds in carbohydrates, so fats contain more energy per molecule than do carbohydrates.
a) Animal fat contains twice as much energy per molecule as glycogen.
b) Fat stores 6 times as much energy per gram as glycogen, because the fats do not contain water associated with the molecule and carbohydrates do.
C. Waxes have a long-chain fatty acid bonded to a long-chain alcohol.
1. Waxes have a high melting point so are solid at normal temperatures.
2. Hydrophobic waxes are waterproof and resistant to degradation.
3. They form protective coverings for plants and animals, as in the ear canal of humans.
D. Phospholipids contain a phosphate group that is polar and can ionize.
1. Their structure is similar to that of neutral fat but with a phosphate group in place of the third fatty acid.
2. The phosphate group forms the polar head group that contains phosphate and nitrogen.
3. Phospholipids in water form a double sheet with the polar heads facing outward toward the water and the nonpolar tails facing each other, making an interface between 2 watery layers, as the plasma membrane of cells separate the interior and exterior watery solutions.
E. Steroids are lipids with an entirely different structure from fats and phospholipids.
1. The steroid backbone consists of 4 fused carbon rings.
2. Cholesterol is the precursor of several other steroids, including several hormones.
3. Saturated fats and cholesterol in the diet can lead to deposits of fatty materials on the linings of blood vessels.
V. Amino acids are the subunits of proteins.
A. Proteins have both structural and metabolic functions.
1. Myosin and actin are the main components of muscle.
2. Insulin is a hormone that regulates blood sugar levels.
3. Hemoglobin transports oxygen in the blood.
4. Collagen fibers support many organs.
5. The most important cell proteins are enzymes that act as organic catalysts to speed up chemical reactions within cells.
6. The role of a protein depends on its structure and biological function.
B. Amino acids and peptides are involved in the structure of proteins.
1. Twenty different amino acids are commonly found in cells.
a) All amino acids contain 2 functional groups, a carboxyl or acid group—COOH and an amino group—NH2.
b) The amino and carboxyl groups both ionize at normal body pH.
c) Each amino acid has a different R group (remainder of the molecule) which gives it its unique chemical properties.
2. A peptide is 2 or more amino acids joined together, and a polypeptide is a chain of many amino acids joined by peptide bonds.
a) A protein can contain more than one polypeptide chain, so it can have many amino acids.
b) A peptide bond forms by a condensation reaction between the amino group of one amino acid and the carboxyl group of the next amino acid in the chain.
c) The polarity of the peptide bond allows hydrogen bonds to form between different parts of a polypeptide chain.
C. There are up to 4 levels of structure in a protein molecule.
1. The primary structure is the sequence of amino acids joined by peptide bonds.
2. The secondary structure is due to the particular orientation of the polypeptide sequence in space.
a) These orientations are called the alpha helix and the beta sheet.
b) Secondary structures are due to hydrogen bonding between oxygen and nitrogen atoms of different amino acids.
c) The alpha helix forms a fibrous protein such as keratin of hair and wool, while the beta pleated sheet is found in a fibrous protein such as silk.
3. The tertiary structure is produced by folds and twists to make a globular protein that contains regions of both alpha helix and beta sheet.
a) The folding and twisting is determined by interactions of the R groups of different amino acids.
b) Hydrogen bonds, ionic bonds, and covalent bonds are involved in giving stability to the tertiary structure of a protein, along with disulfide bonds and hydrophobic interactions.
4. Proteins with more than one polypeptide have a quaternary structure related to the interaction of the different polypeptide chains.
D. Denaturation of proteins can be caused by changes in temperature or pH.
1. These produce a change in the protein shape, as in milk curdling or cooking egg white.
2. A denatured protein loses its normal configuration.
3. Normal bonding patterns between different parts of the molecule are disturbed.
4. If denaturation conditions are not severe and can be removed, some proteins may be able to regain their normal structure.
5. This shows that the primary structure of the protein determines its final configuration.
VI. A nucleotide contains 3 types of unit molecules, phosphoric acid, a pentose sugar, and a nitrogen-containing base.
A. Nucleotides can have metabolic functions in cells.
1. Some are components of coenzymes, which help facilitate enzymatic reactions.
2. ATP (adenosine triphosphate) supplies energy for energy-requiring processes in cells.
B. Nucleic acids are made up of nucleotide subunits.
1. DNA (deoxyribonucleic acid) is the genetic material of the cell.
2. RNA (ribonucleic acid) works with DNA to control protein synthesis.
3. DNA and RNA have differences in their structures.
a) DNA contains the sugar deoxyribose, while RNA contains ribose.
b) In DNA, the bases are the purines adenine and guanine, and the pyrimidines cytosine and thymine; in RNA all the bases are the same except for uridine replacing thymine.
4. Nucleotides join in a particular sequence by condensation reactions.
a) This forms a linear strand with a backbone of phosphate-sugar-phosphate-sugar.
b) The bases project to one side of the backbone.
5. DNA is a double-stranded molecule, while RNA is single-stranded.
6. DNA forms a double helix, with the bases oriented toward the center and joined by complementary base pairing.
a) Adenine always binds to thymine (A to T) and guanine binds to cytosine (G to C).
b) The number of purine bases is always equal to the number of pyrimidine bases.