Cell Biology and Biochemistry
Cell Biology and Biochemistry
This domain tests the molecular basis of life and the structures that carry out cellular functions. Expect questions on water properties, macromolecules, enzyme kinetics, membrane transport, organelles, the cell cycle, and the major energy pathways. You must connect structure to function and predict how cells respond to changes in their environment.
Core rules and concepts
- Water is polar and forms hydrogen bonds, giving it cohesion, adhesion, high specific heat, and solvent properties.
- Macromolecules: carbohydrates (monosaccharides to polysaccharides), lipids (glycerol and fatty acids; hydrophobic), proteins (amino acids; peptide bonds), nucleic acids (nucleotides; phosphodiester bonds).
- Enzymes lower activation energy, are not consumed, and have optimal temperature and pH; denaturation changes active-site shape.
- Membranes are fluid mosaics. Diffusion and osmosis are passive; facilitated diffusion uses channels; active transport uses ATP. Osmosis is water movement across a selectively permeable membrane toward higher solute concentration.
- Prokaryotes lack membrane-bound organelles; eukaryotes have a nucleus, mitochondria, chloroplasts, ER, Golgi, and lysosomes. Mitochondria and chloroplasts have their own DNA.
- Cellular respiration includes glycolysis, the Krebs cycle, and electron transport; photosynthesis includes the light reactions and the Calvin cycle.
Worked example
An animal cell is placed in a 5% NaCl solution. The cytosol is about 0.9% solute. Because the outside is hypertonic, water leaves the cell by osmosis and the cell shrinks. In distilled water, the outside is hypotonic, water enters, and the cell may lyse. In an isotonic solution, net water movement is near zero. The reasoning: water moves from lower solute concentration, higher water potential, to higher solute concentration, and animal cells lack a cell wall to resist pressure.
Traps that cost marks
- Saying osmosis moves solutes rather than water.
- Confusing facilitated diffusion with active transport; only active transport directly requires ATP.
- Claiming enzymes are used up or change the equilibrium.
- Treating photosynthesis and respiration as exact reverses; they differ in pathways and location.