Cell Biology and Biochemistry
Cell Biology and Biochemistry
This domain tests how cells are built and how their molecules behave: membrane organization, transport, enzyme kinetics, and the chemistry that powers metabolism. Questions often ask you to predict the direction of a process or interpret a graph rather than recall a definition.
Core rules
- The plasma membrane is a fluid mosaic: a phospholipid bilayer with integral and peripheral proteins, cholesterol modulating fluidity, and glycolipids and glycoproteins facing the extracellular space.
- Simple diffusion follows concentration gradients and needs no energy; facilitated diffusion uses channels or carriers and is saturable; active transport uses ATP or an electrochemical gradient.
- The Na+/K+ ATPase moves 3 Na+ out and 2 K+ in per ATP, creating a net negative interior and driving secondary transport such as glucose uptake in the intestine.
- Enzyme rate rises with substrate concentration until saturation; Km is the substrate concentration at half Vmax. Competitive inhibitors raise apparent Km with unchanged Vmax; non-competitive inhibitors lower Vmax.
Worked example
An enzyme has Vmax = 120 µmol min⁻¹ and Km = 5 mM. At [S] = 15 mM, use v = Vmax[S]/(Km + [S]) = 120 × 15/(5 + 15) = 90 µmol min⁻¹. At [S] = Km, v is exactly Vmax/2 = 60. Reasoning: saturation makes the denominator approach [S], so v approaches Vmax but never exceeds it.
Traps
- Km is not a direct measure of affinity: lower Km means higher affinity.
- Do not say active transport moves molecules against a gradient without noting the energy source.
- Osmosis is water movement down a water-potential gradient, not solute movement.
- In graphs, check axis units and whether the curve is initial rate, not product accumulated.