Molecules and Cells
Molecules and Cells tests whether you can connect molecular structure to cellular function: water's properties, the four macromolecule classes, membrane architecture and transport, enzyme kinetics, energy pathways, and the cell cycle. Questions usually ask you to predict an outcome from a structural change rather than recall a definition.
Water and macromolecules
- Water's polarity and hydrogen bonding explain cohesion, adhesion, high specific heat, and its role as a solvent.
- Carbohydrates store energy and build structural polymers; lipids are hydrophobic and form bilayers; proteins depend on primary sequence for folding; nucleic acids store and transmit information.
- Polymer synthesis is a condensation reaction that releases water; hydrolysis reverses it.
Membranes and transport
- The fluid-mosaic model describes a phospholipid bilayer with embedded proteins; cholesterol modulates fluidity.
- Simple diffusion follows a concentration gradient; facilitated diffusion uses channels or carriers without ATP; active transport uses ATP or an electrochemical gradient.
- Osmosis moves water toward the higher solute concentration, which is the lower water potential.
Enzymes and energy
- Enzymes lower activation energy, are not consumed, and have an active site whose shape determines specificity.
- Competitive inhibitors raise apparent Km without changing Vmax; noncompetitive inhibitors lower Vmax.
- Glycolysis occurs in the cytosol; pyruvate oxidation and the Krebs cycle occur in the mitochondrial matrix; oxidative phosphorylation occurs on the inner mitochondrial membrane. Photosynthesis runs light reactions in thylakoid membranes and the Calvin cycle in the stroma.
Worked example
A reaction has Vmax = 120 µmol min⁻¹ and Km = 4 mM. At [S] = 12 mM, v = Vmax[S]/(Km + [S]) = (120 × 12)/(4 + 12) = 90 µmol min⁻¹. Because [S] is three times Km, the enzyme is 75% saturated. Doubling enzyme concentration doubles Vmax but leaves Km unchanged.
Traps that cost marks
- Reversing the direction of osmosis; water moves toward higher solute concentration.
- Claiming all active transport hydrolyzes ATP directly; cotransport uses an existing gradient.
- Mixing up competitive and noncompetitive inhibition effects on Km and Vmax.
- Placing glycolysis in the mitochondrion.
- Treating the Calvin cycle as light-dependent; it uses ATP and NADPH but does not require light directly.