Cell Biology and Biochemistry
Domain 2 - Cell Biology and Biochemistry
Items here test the chemistry that makes cells work and the structures that carry out cellular functions: macromolecule structure and function, enzyme behavior, membrane transport, organelle roles, and energy transformation.
Biochemistry fundamentals
- Water's polarity gives it cohesion, adhesion, a high specific heat and heat of vaporization, and solvent power; these properties underlie transpiration, thermoregulation, and most metabolic chemistry.
- Polymers are built by dehydration synthesis and broken by hydrolysis. Carbohydrates: monosaccharides joined by glycosidic bonds - energy and structure (cellulose, chitin). Lipids: glycerol plus fatty acids via ester bonds; saturated chains pack tightly, unsaturated chains kink; phospholipids are amphipathic and form bilayers; steroids act as signaling molecules.
- Proteins: amino acids joined by peptide bonds; primary through quaternary structure is determined by R groups and the surrounding environment. Denaturation by heat or pH disrupts noncovalent interactions but not peptide bonds.
- Nucleic acids: nucleotides joined 5′→3′ by phosphodiester bonds; ATP is the universal energy currency.
- Enzymes lower activation energy, are unchanged by the reaction, and are specific to their substrate at the active site. Competitive inhibitors compete for the active site (raise apparent Km, Vmax unchanged); noncompetitive/allosteric inhibitors reduce Vmax. Rate rises with temperature and substrate until denaturation or saturation.
Membranes and organelles
The fluid mosaic model: a phospholipid bilayer with embedded proteins and cholesterol modulating fluidity. Small nonpolar molecules diffuse freely; water and ions pass through channels; large or charged molecules need transporters. Passive transport (diffusion, facilitated diffusion, osmosis) runs down a gradient with no ATP; active transport (Na⁺/K⁺ pump) runs against it using ATP; bulk transport uses vesicles. Prokaryotes lack membrane-bound organelles; eukaryotes compartmentalize, with mitochondria and chloroplasts carrying their own DNA as endosymbiotic remnants.
Worked example
A celery stalk is placed in distilled water and another in 5% salt solution. In distilled water the external solute concentration is lower than that of the vacuolar sap, so water enters by osmosis, the vacuole swells against the wall (turgor), and the stalk stiffens. In salt solution the exterior is hypertonic, water leaves, the membrane pulls away from the wall (plasmolysis), and the stalk wilts. Reasoning rule: water always moves toward the higher solute concentration, and the cell wall, not the membrane, limits expansion.
Traps
Reversing the direction of osmosis; saying active transport uses ATP but omitting the required transport protein; calling a cell in a hypotonic solution "shrunken"; confusing competitive with noncompetitive inhibition; assuming enzymes are consumed; attributing protein shape to peptide bonds alone.