Chapter 3 Carbon Compounds in Cells

I. Ancient Carbon Treasures
A. Millions of years ago organic matter was submerged by geologic forces.
1. Coal, peat, and other fossil fuels are the result of ancient photosynthesis that trapped the sunlight’s energy.
2. Now, humans are extracting these nonrenewable resources from the earth.
B. Present-day organic substances —biological molecules—are the foundations of the structure and function of living cells.

II. Properties of Carbon Compounds
A. Oxygen, hydrogen, and carbon are the most abundant elements in living matter.
1. Much of the H and O are linked as water.
2. Carbon can form four covalent bonds with other atoms to form organic molecules of several configurations.
3. Carbon compounds in cells are called organic molecules to distinguish them from simple inorganic molecules, which have no carbon chains or rings.
B. Families of Small Organic Compounds
1. These include compounds with at most twenty carbon atoms.
2. They include simple sugars, fatty acids, amino acids, and nucleotides.
3. They are used as an energy source or as building blocks for the synthesis of macromolecules: polysaccharides, lipids, proteins, and nucleic acids.
C. Functional Groups
1. Functional groups are atoms or groups of atoms covalently bonded to a carbon backbone.
2. Functional groups convey distinct properties, such as solubility and chemical reactivity, to the complete molecule.
D. Condensation and Hydrolysis
1. Small molecules can combine to form large ones because of special proteins called enzymes that can speed up a chemical reaction.
2. In condensation, one molecule is stripped of its H+, another is stripped of its OH–. The two molecule fragments join to form a new compound and the H+ and OH– form water.
3. Hydrolysis is the reverse: one molecule is split by the addition of H+ and OH– (from water) to the components.

III. Carbohydrates—Energy and Structural Roles
A. Monosaccharides
1. Monosaccharides—one sugar unit—are the simplest carbohydrates.
2. They are characterized by solubility in water, sweet taste, and several —OH groups.
3. Ribose and deoxyribose (five-carbon backbones) are building blocks for nucleic acids.
4. Glucose and fructose (six-carbon backbones) are used in assembling larger carbohydrates.
B. Oligosaccharides
1. An oligosaccharide is a short chain of two or more sugar monomers.
2. Disaccharides—two sugar units—are examples.
a. Sucrose (glucose + fructose) is a transport form of sugar used by plants and harvested by humans for use in food.
b. Lactose (glucose + galactose) is present in milk.
c. Maltose (two glucose units) is present in germinating seeds.
3. Oligosaccharides with three or more sugar monomers are attached as short side chains to proteins where they participate in membrane function.
C. Polysaccharides
1. A polysaccharide is a straight or branched chain of hundreds or thousands of sugar monomers.
2. Starch is a plant storage form of energy, arranged as unbranched coiled chains, easily hydrolyzed to glucose units.
3. Cellulose is a fiberlike structural material—tough, insoluble—used in plant cell walls.
4. Glycogen is a highly-branched chain used by animals to store energy in muscles and liver.
5. Chitin is a specialized polysaccharide with nitrogen attached to the glucose units; it is used as a structural material in arthropod exoskeletons and fungal cell walls.

IV. Lipids
A. Lipids are greasy or oily compounds with little tendency to dissolve in water.
1. They can be broken down by hydrolysis reactions.
2. They function in energy storage, membrane structure, and coatings.
B. Lipids With Fatty Acids
1. A fatty acid is a long chain of mostly carbon and hydrogen atoms with a —COOH group at one end.
2. When they are part of complex lipids, the fatty acids are like long, flexible tails.
3. Glycerides are commonly called fats and oils.
a. They are formed by the attachment of one (mono-), two (di-), or three (tri-) fatty acids to a glycerol.
b. They are a rich source of energy, yielding more than twice the energy per weight basis as carbohydrates.
c. Saturated fats (triglycerides) have only single C—C bonds in their fatty acid tails and are solids at room temperature.
d. Unsaturated fats are liquids (oils) at room temperature because one or more double bonds between the carbons in the fatty acids permits “kinks” in the tails.
4. Phospholipids are the main structural material of membranes.
a. They are formed by attachment of two fatty acids to a glycerol.
b. Attachment of a small polar group produces a molecule with a hydrophilic head and two hydrophobic tails.
5. Waxes form water-repellant coatings.
a. They are formed by attachment of long-chain fatty acids to long-chain alcohols.
b. They serve as coatings for plant parts and as animal coverings.
C. Lipids Without Fatty Acids
1. Steroids have a backbone of four carbon rings.
2. Cholesterol is a component of cell membranes in animals and can be modified to form sex hormones.

V. Proteins
A. Proteins are polymers of amino acids.
1. Amino acids are small organic molecules with an amino group, a carboxyl group, and one of twenty varying R groups.
2. Proteins function as enzymes, in cell movements, as storage and transport agents, as hormones, as antibodies, and as structural material.
B. Protein Structure
1. Primary structure is defined as ordered sequences of amino acids each linked together by peptide bonds to form polypeptide chains.
2. Three-dimensional structure is determined by how amino acid sequences present their atoms for hydrogen bonding.
a. Secondary structure refers to the helical coil (as in hemoglobin) or sheetlike array (as in silk) that results from hydrogen bonding of side groups on the amino acid chains.
b. Tertiary structure is the result of folding due to interactions among R groups along the polypeptide chain.
c. Quaternary structure describes the complexing of two of more polypeptide chains to form globular (example: hemoglobin) or fibrous proteins.
C. Lipoproteins and Glycoproteins
1. Lipoproteins have both lipid and protein components; they transport fats and cholesterol in the blood.
2. Glycoproteins consist of oligosaccharides covalently bonded to proteins; they are abundant on the exterior of animal cells, as cell products, and in the blood.
D. Protein Denaturation
1. High temperatures (>60ûC) or changes in pH can cause a loss of a protein’s normal three-dimensional shape (denaturation).
2. Normal functioning is lost upon denaturation, which is often irreversible.

VI. Nucleotides and Nucleic Acids
A. Each nucleotide has a five-carbon sugar (ribose or deoxyribose), a nitrogen-containing base, and a phosphate group.
B. Three basic kinds of nucleotide-based molecules exist:
1. Adenosine phosphates are chemical messengers (cAMP) or energy carriers (ATP).
2. Nucleotide coenzymes transport hydrogen atoms and electrons (examples: NAD+ and FAD).
3. Nucleic acids are polymers of nucleotides.
a. DNA is a double-stranded helix carrying encoded hereditary instructions.
b. RNA is single stranded and functions in translating the code to build proteins.