GACE Biology (6–12) (710)

Living organisms contain four major classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These large molecules are polymers built from smaller monomer subunits through dehydration synthesis (condensation) reactions. Hydrolysis reactions break polymers into monomers by adding water.

Carbohydrates contain carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources, energy storage molecules, and structural components.

Monosaccharides are simple sugars with three to seven carbon atoms. Glucose (C₆H₁₂O₆) is the primary energy source for cellular respiration. Fructose is found in fruits. Galactose is a component of lactose. Ribose and deoxyribose are pentose sugars found in nucleic acids.

Disaccharides form when two monosaccharides join through glycosidic bonds. Sucrose (glucose + fructose) is common table sugar. Lactose (glucose + galactose) is milk sugar. Maltose (glucose + glucose) results from starch digestion.

Polysaccharides are long chains of monosaccharides. Starch is the storage polysaccharide in plants, consisting of amylose (unbranched) and amylopectin (branched). Glycogen is the storage polysaccharide in animals, stored primarily in liver and muscle tissue. Cellulose is a structural polysaccharide in plant cell walls, containing beta-glucose linkages that most animals cannot digest. Chitin is a structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Lipids are hydrophobic molecules containing carbon, hydrogen, and oxygen, but with much less oxygen than carbohydrates. They serve as energy storage, membrane components, hormones, and insulation.

Triglycerides (fats and oils) consist of glycerol bonded to three fatty acid chains. Saturated fatty acids contain no carbon-carbon double bonds and are typically solid at room temperature. Unsaturated fatty acids contain one or more carbon-carbon double bonds and are typically liquid at room temperature. Trans fats contain artificially produced double bonds that create health risks.

Phospholipids contain glycerol, two fatty acid chains, and a phosphate group. The phosphate head is hydrophilic. The fatty acid tails are hydrophobic. This amphipathic nature enables phospholipids to form cell membranes.

Steroids have a four-ring carbon structure. Cholesterol is a membrane component and precursor for steroid hormones. Testosterone, estrogen, and cortisol are steroid hormones derived from cholesterol.

Waxes are highly hydrophobic lipids providing protective coatings on plant leaves and animal integuments.

Proteins are polymers of amino acids linked by peptide bonds. They serve as enzymes, structural components, transport molecules, antibodies, hormones, and signaling molecules.

Amino acids contain a central carbon bonded to an amino group, carboxyl group, hydrogen atom, and variable R group (side chain). Twenty standard amino acids exist, classified by R group properties: nonpolar, polar, acidic, or basic.

Protein structure exists at four levels. Primary structure is the linear sequence of amino acids. Secondary structure involves local folding patterns: alpha helices and beta pleated sheets, stabilized by hydrogen bonds between backbone atoms. Tertiary structure is the overall three-dimensional shape of a single polypeptide, stabilized by interactions between R groups including hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions. Quaternary structure involves the arrangement of multiple polypeptide subunits.

Protein function depends on proper folding. Denaturation disrupts protein structure through heat, pH changes, or chemical agents, causing loss of function. Chaperone proteins assist proper folding.

Enzymes are protein catalysts that accelerate biochemical reactions by lowering activation energy. The active site is the region where substrates bind. Enzyme-substrate specificity follows the induced fit model. Enzyme activity is influenced by temperature, pH, substrate concentration, and the presence of inhibitors or activators. Competitive inhibitors bind the active site. Noncompetitive inhibitors bind elsewhere and alter enzyme shape.

Nucleic acids are polymers of nucleotides that store and transmit genetic information.

Each nucleotide contains a pentose sugar, phosphate group, and nitrogenous base. DNA nucleotides contain deoxyribose sugar. RNA nucleotides contain ribose sugar.

DNA contains the bases adenine, guanine, cytosine, and thymine. RNA contains adenine, guanine, cytosine, and uracil. Purines (adenine and guanine) have two-ring structures. Pyrimidines (cytosine, thymine, and uracil) have one-ring structures.

DNA forms a double helix with complementary base pairing: adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds). The two strands run antiparallel (5' to 3' in opposite directions).

RNA is typically single-stranded. Messenger RNA (mRNA) carries genetic information from DNA to ribosomes. Transfer RNA (tRNA) brings amino acids to ribosomes during translation. Ribosomal RNA (rRNA) forms structural and catalytic components of ribosomes.