Carbon Chemistry A carbon atom has 4 electrons in an outer shell and completes its outer shell by sharing electrons with other atoms in four covalent bonds. The simplest organic compounds are hydrocarbons, which containonly carbon and hydrogen atoms the simplest hydrocarbon is methane.. Larger hydrocarbons (such as octane, with eight carbons) are the main molecules in the gasoline we burn in cars and other machines. Hydrocarbons are also important fuels in your body.Giant Molecules from Smaller Building BlocksThreecategories of macromolecules are- carbohydrates (eg. starchy foods), proteins (eg. enzymes and the molecules of your hair), nucleic acids (eg. DNA)onomers combine togetherthrough adehydration reaction removing a molecule of water. !iant molecules are hydrolysed to ma"e their monomers available to cells.#arbohydratesThese are classified as$ onosaccharides%imple sugars, or monosaccharides cannot be bro"endown by hydrolysis into smaller sugars. They are the main fuel molecules for cellular wor". &isaccharidesTo form a disaccharide, two simple sugars are 'oined by a dehydration reaction. (olysaccharides #omple) carbohydrates, or polysaccharides, are long chains of sugar units, polymers of monosaccharides. %tarch and glycogen are storage polysaccharides in plants and animals, respectively. The cellulose of plant walls is an e)ample of a structural polysaccharide.Lipids*n contrast to carbohydrates and most other biologicalmolecules, lipids are organic compounds that are hydrophobic.Lipids are neither macromolecules nor polymers.+ats A typical fat consists of a glycerol molecule 'oined withthree fatty acid molecules via dehydration reactionsThe resulting fat is called a triglyceride, ,hen there is a double bond in the carbon s"eleton of the fatty acid it is said to be unsaturated. The fatty acids in the fat molecule that lac" double bonds in theirhydrocarbon portion are saturated. -nsaturated fats are converted to saturated fats by adding hydrogen, a process called hydrogenation which creates trans fat(roteins (rotein is a polymer constructed from amino acid monomers.Their most important role is as en.ymes and chemicals that change the rate of a chemical reaction. All proteins are macromolecules constructed from a common set of /0 "inds of amino acids. 1ach amino acid consists of a central carbon atom bonded to four covalent partners (roteins as (olymers#ells lin" amino acid monomers together by dehydration reactions. The bond between ad'acent amino acids is called a peptide bond. The resulting long chain of amino acids is called polypeptide. (rotein %hapeA functional protein is one or more polypeptide chains precisely twisted, folded, and coiled into a molecule of uni2ue shape.The levels of protein structure are3 primary,secondary, and tertiary. (roteins with more than one polypeptide chain have a fourth level3 2uaternary structure. 4$ ucleic Acids5 cleic acids are macromolecules that provide thedirections for building proteins.There are actually two types of nucleic acids3 &6Awhich stands for deo)yribonucleic acid) and 76A (forribonucleic ilcid).6ucleic acids are polymers made from monomerscalled nucleotides (+igure 8./8). 1ach nucleotide containsthree parts. At the center of each nucleotide is afive$carbon sugar, deo)yribose in &6A and ribose in76AA molecule of cellular &6A is double$stranded, withtwo polynucleotide strands wrapped around each otherto form a double heli)The en.yme lactase, li"e all proteins, is encoded by a& A gene. A reasonable hypothesis is that lactoseintolerantpeople have a defect in their lactase gene.Evolution and LactoseIntolerance in Humans9 you:ll recall from the ;iology and %ociety section, mostof the world:s population are lactose intolerant as adultsand thus do not easily digest the mil sugar lactose.