CHAPTER 3: Carbon Compounds in Cells -- Macromolecules   A. Plants use carbon dioxide to make sugars and starches.  B. Is there a relationship between global warming and the concentrations of carbon dioxide in the atmosphere?  I. Properties of Organic Compounds   A. The Molecules of Life 1. These include carbohydrates, lipids, proteins, and nucleic acids.  2. They are used as energy sources, structural materials, metabolic workers, and carriers of hereditary information.  3. These molecules are organic compounds, with hydrogen and other elements covalently bonded to carbon atoms. B. Carbon's Bonding Behavior 1. Oxygen, hydrogen, and carbon are the most abundant elements in living things.   a. Much of the hydrogen and oxygen are linked as water.   b. Carbon can form four covalent bonds with other atoms to form organic molecules of several configurations.  2. The orientations of the atoms attached to a carbon backbone give rise to the three-dimensional shapes and functions of biological molecules.  C. Functional Groups 1. A hydrocarbon, which has only hydrogen atoms attached to a carbon backbone, does not break apart easily; they form very stable portions of most biological molecules.  2. Functional groups (such as the &endash;OH of alcohols) are atoms or groups of atoms covalently bonded to a carbon backbone; they convey distinct properties, such as solubility, to the complete molecule. D. How Do Cells Build Organic Compounds? 1. Enzymes speed up specific metabolic reactions by these mechanisms:   a. Functional-group transfer: one molecule gives up a functional group, which another molecule accepts. b. Electron transfer: one or more electrons stripped from one molecule are donated to another molecule.  c. Rearrangement: a juggling of internal bonds converts one type of organic compound into another.  d. Condensation: through covalent bonding, two molecules combine to form a larger molecule.  e. Cleavage: a molecule splits into two smaller ones.   2. In condensation, small molecules can combine to form larger ones; for example, sugar monomers combine to form starch polymers.  3. In hydrolysis, one larger molecule is split by the addition of H+ and OH&emdash; (from water) to the components.   II. Carbohydrates A. A carbohydrate is a simple sugar or a larger molecule composed of sugar units. 1. Carbohydrates are the most abundant biological molecules. 2. Carbohydrates have structural roles and serve as forms of transportable and stored energy.   B. The Simple Sugars 1. A monosaccharide, one sugar unit, is the simplest carbohydrate. 2. Simple sugars are soluble in water and may be sweet-tasting. 3. Ribose and deoxyribose (five-carbon backbones) are building blocks 4. Glucose (six-carbon backbone) is a primary energy source and precursor of many organic molecules. C. Short-Chain Carbohydrates 1. A disaccharide is a short chain resulting from the covalent bonding of two monosaccharides.a. a. Sucrose (table sugar) is glucose plus fructose.   b. Lactose (milk sugar) is glucose plus galactose. c. Maltose (grain sugar) is composed of two glucose units.  2. Oligosaccharides may be attached to proteins where they have roles in membrane functions and immunity. D. Complex Carbohydrates 1. A polysaccharide consists of many sugar units (same or different) covalently linked.  2. The most common polysaccharides are chains of glucose:   a. Starch (energy storage in plants) and cellulose (structure of plant cell walls) are made of glucose units but in different bonding arrangements.  b. Glycogen is a storage form of glucose found in animal tissues.  c. Chitin, which has nitrogen atoms attached to its backbone, is the main structural material in the external skeletons of arthropods. III. Lipids A. Lipids are characterized by their inability to dissolve in water. 1. Lipids are composed mostly of hydrocarbon.  2. They form the basic structures of membranes and have roles in energy metabolism. B. Fats and Fatty Acids 1. A fatty acid is a long, unbranched hydrocarbon with a &endash;COOH group at one end.   a. Unsaturated fatty acids are liquids (oils) at room temperature because one or more double bonds between the carbons in the tails permit "kinks."  b. Saturated fatty acids have only single C&endash;C bonds in their tails and are solids at room temperatures.  2. Triglycerides, such as butter, lard, and oils, are rich sources of energy.  a. These lipids have fatty acid tails attached to a molecule of glycerol.  b. On a per weight basis, triglycerides yield more than twice as much energy as carbohydrates. C. Phospholipids 1. Phospholipids have a glycerol backbone, two fatty acids, a phosphate group, and a small hydrophilic group.  2. They are important components of cell membranes, where the hydrophilic heads face toward the inner and outer surfaces and the hydrophobic tails face inward. D. Sterols and Their Derivatives 1. Sterols have a backbone of four carbon rings, but no fatty acids.  2. Cholesterol is a component of cell membranes in animals and can be modified to form sex hormones. E. Waxes 1. Waxes are special molecules with fatty acid chains attached to alcohols.  2. They confer extraordinary waterproofing qualities. IV. Amino Acids and the Primary Structure of Proteins A. Proteins function as enzymes, in cell movements, as storage and transport agents, as hormones, as antidisease agents, and as structural material throughout the body. B. Structure of Amino Acids 1. Amino acids are small organic molecules with an amino group, an acid group, a hydrogen atom, and an "R" group.  2. The 20 different R groups determine the twenty naturally-occurring amino acids in humans. C. Primary Structure of Proteins 1. Primary structure is defined as the chain (polypeptide) of amino acids each linked together in a definite sequence by peptide bonds between an amino group of one unit and an acid group of another. 2. Three or more amino acids linked together in this way forms a polypeptide chain. V. How Does a Protein's Three-Dimensional Structure Emerge? A. Second Level of Protein Structure 1. Hydrogen bonds join the side groups of the amino acids in the primary chains.  2. The result is a helical coil (alpha helix) or sheetlike array (beta pleated sheet). B. Third Level of Protein Structure 1. Interactions among R groups results in a complex three-dimensional shape.  2. Globular proteins have extensive tertiary structure. C. Fourth Level of Protein Structure 1. Hemoglobin consists of four folded chains called globins, each with a heme group.  2. Quaternary structure describes the complexing of two or more polypeptide chains.  3. Keratin and collagen are examples of complex structural proteins.   D. Glycoproteins and Lipoproteins 1. Some proteins have other organic molecules attached to their polypeptide chains.  2. Lipoproteins and glycoproteins transport lipids and oligosaccharides, respectively. E. Structural Changes by Denaturation 1. High temperatures or chemicals can cause the three-dimensional shape to be disrupted.  2. Normal functioning is lost upon denaturation, which is often irreversible. VI. Focus on the Environment: Food Production and a Chemical Arms Race  VII. Nucleotides and Nucleic Acids A. Nucleotides are small organic molecules. 1. Each nucleotide has a five-carbon sugar (ribose or deoxyribose), a nitrogen-containing base (single- or double-ringed), and a phosphate group.  2. Some nucleotides are involved in metabolism:  a. Adenosine phosphates are energy carriers (ATP).  b. Nucleotide coenzymes transport hydrogen atoms and electrons (examples: NAD+ and FAD). B. Nucleic Acids&endash;DNA and RNA 1. In nucleic acids, four different kinds of nucleotides are bonded together in large macromolecules.  2. RNA is single-stranded; it functions in the assembly of proteins.  3. DNA is double-stranded; genetic messages are encoded in its base sequences.