replicating or making copies of themselves Presumably these first self-replicating polymers were like nucleic acids Once polymers could replicate those best able to cope with their surroundings survived After certain other basic adaptations were achieved the most primitive cells arose From then on only natural selection was needed to give rise to the many species species that would inhabit Earth The fundamental component of a gene is DNA a nucleic acid made up of two strands of biochemical units called nucleotides Each nucleotide consists of a phosphate deoxyribose a sugar and one of four nitrogen-bearing bases adenine guanine cytosine or thymine Chemical bonds connect bases on one strand with bases on the other forming base pairs The molecule resembles a ladder with the two DNA strands for sides and chemical bonds forming steps The nucleotide ladder winds around itself forming a double helix Equally important for cell and organ function is ribonucleic acid RNA This nucleic acid is markedly similar to DNA except for three key features RNA consists of a single strand except in some viruses in RNA the nucleotides contain ribose instead of deoxyribose and the base uracil replaces the thymine found in DNA The information locked in the genetic code of nucleic acids is translated into amino acids a molecular form that renders genetic information more accessible to cells All living organisms from 10 EXAMINING BASIC CHEMICAL MOLECULES
INTRODUCTION protozoa to plants and animals have in common the fact that they synthesize protein by using the same 20 amino acids even though more than 100 different amino acids occur in nature If proteins are the bricks from which the body is made then amino acids are the grains of sand and clay that make up those bricks Amino acids are defined chemically by the presence of an amino group which is made up of a nitrogen atom and two atoms of hydrogen NH2 as well as carbon C and oxygen O These amino acids join together to form proteins with the sequence in which they join being a direct translation translation of the sequence of the nucleic acids from which they were formed Examples of proteins created by amino acids include keratin the principal component component of hair egg albumin present in egg whites casein a major milk protein enzymes venoms hormones and other molecules of special biological activity Most of the common proteins contain more than 100 amino acids For example hemoglobin the oxygen-carrying component of blood is made of 287 amino acids arranged into four chains Proteins because they inherit the nitrogen N molecules of amino acids are characterized as nitrogenous compounds They are an extraordinarily diverse group of molecules and serve many different functions within cells Many proteins are enzymes compounds that accelerate chemical reactions in and around cells By following the instructions 11
coded in genes cells use their molecular machinery machinery to build the proteins required by the organism In order to do this they need a supply of amino acids Plants and animals differ in their ability to manufacture manufacture their necessary amino acids Higher plants are capable of producing all of their life-essential amino acids Human adult cells however can produce only 11 of their 20 necessary amino acids The remaining nine called essential amino acids are obtained by eating foods that already contain them Some proteins proteins called transport proteins carry substances from one place in the body to another Other transport transport proteins are located in cell membranes These proteins shuttle nutrients and waste products from one side of the membrane to the other Many proteins proteins make up the supportive elements that provide biological structures with strength and protection Regulatory proteins help to prevent the depletion of nutrients in tissues or the harmful accumulation of the products of respiration In contrast to proteins carbohydrates do not contain nitrogens Rather they are formed from various arrangements of carbon hydrogen and oxygen Carbohydrates can be divided into three major groups monosaccharides disaccharides and polysaccharides Molecules of monosaccharides monosaccharides or simple sugars contain from three to nine carbon atoms most commonly five or six Two of the most important simple sugars are glucose also 12 EXAMINING BASIC CHEMICAL MOLECULES
INTRODUCTION known as dextrose and fructose or fruit sugar Two simple sugar molecules are linked to each other in a disaccharide or double sugar Many carbohydrates are isomers that is they have the same numbers of particular atoms but different molecular structures Glucose fructose and galactose a monosaccharide monosaccharide part of lactose for example are all isomers with the identical formula of C6H12O6 While carbohydrates provide important structural structural functions in cells they are better known as the primary sources of energy utilized by most life-forms Glucose which circulates in the blood of higher animals animals is absorbed by the cells where it participates in biochemical reactions that energize the metabolic processes Glycogen which consists of branching chains of glucose molecules is stored in the liver and muscles of higher animals to be broken down into glucose during times of muscular activity In addition some carbohydrates in the form of polysaccharides function as structural components in certain animals animals For example chitin which is similar to cellulose cellulose makes up the exoskeleton or hard covering of insects and other animals classified as arthropods These include crustaceans and arachnids Fatty acids or lipids form a secondary energy source for many organisms These molecules although composed of the same basic elements as carbohydrates carbon hydrogen and oxygen oxygen are distinguished by the linkages or bonds 13
