Zellbiologie / Cell biology: membranes, transport, compartments and enzymes
Cell-biology items in the BW Abitur test how structure enables function: membrane architecture, transport across membranes, the division of labour between organelles, and the control of reactions by enzymes. Expect data-based tasks - uptake curves, osmosis experiments, inhibition graphs - where you must name a mechanism and justify it from the evidence rather than describe the picture.
Membranes and compartmentalisation
- Fluid mosaic model: phospholipid bilayer with hydrophobic tails facing inward, integral and peripheral proteins, cholesterol, and glycolipids/glycoproteins on the outer face.
- Cholesterol reduces fluidity at high temperature and prevents rigid packing at low temperature, broadening the working range of the membrane.
- Compartmentalisation keeps incompatible reactions apart (hydrolytic enzymes inside lysosomes) and concentrates substrates; the endomembrane system (nuclear envelope, ER, Golgi, vesicles, plasma membrane) is connected through vesicle traffic.
- Mitochondria and chloroplasts are double-membraned with their own DNA and ribosomes (endosymbiotic origin); their inner membranes are folded (cristae, thylakoids) to increase surface area.
Transport across membranes
- Diffusion and facilitated diffusion follow the electrochemical gradient and need no ATP; carrier-mediated uptake saturates, so the rate plateaus as external concentration rises.
- Osmosis: water moves towards the higher solute concentration (lower water potential). In plant cells the wall permits turgor; plasmolysis follows water loss.
- Primary active transport: the Na+/K+-ATPase moves 3 Na+ out and 2 K+ in per ATP, building gradients that secondary active transport (cotransport) then spends.
- Bulk transport: endocytosis and exocytosis via vesicles.
Enzymes
- Induced fit and active-site specificity; Km is the substrate concentration at half Vmax and is inversely related to affinity; Vmax reflects enzyme amount and turnover.
- Competitive inhibitors raise Km with Vmax unchanged; non-competitive inhibitors lower Vmax; heat or extreme pH denatures tertiary structure.
Worked example
A graph shows 14C-glucose uptake into intestinal cells rising steeply at low concentration and flattening above about 8 mmol/L, with unchanged cellular ATP. Reasoning: the initial steep rise indicates a concentration gradient driving entry; the plateau indicates carrier saturation, not membrane damage; the unchanged ATP argues against primary active transport. Conclusion: facilitated diffusion through a specific carrier. A competition trial with a structurally similar sugar would test that specificity claim.
Traps that cost marks
Reversing the osmotic direction (water goes to the higher solute concentration); claiming facilitated diffusion hydrolyses ATP; writing that the Na+/K+ pump 'creates the membrane potential' without mentioning the K+ leak; confusing surface-area-to-volume reasoning with magnification.