Cell Biology: Structure, Organelles, Membranes and Transport
Structure, organelles, membranes and transport
The 5-unit exam tests structure–function reasoning: given a cell type, deduce which organelles dominate; given an organelle, predict the defect. Transport is tested rigorously - direction, energy source and the protein machinery involved.
Core rules
- Prokaryote vs eukaryote: bacteria lack membrane-bound organelles, have 70S ribosomes, circular DNA and peptidoglycan walls; eukaryotes have a nucleus, 80S ribosomes and internal membranes.
- Compartmentalisation: transcription in the nucleus; secretory proteins on rough ER; lipid/steroid synthesis and detox on smooth ER; modification and sorting in the Golgi; acid hydrolases in lysosomes; H₂O₂ breakdown in peroxisomes; oxidative phosphorylation in mitochondria; photosynthesis in chloroplasts.
- Endosymbiotic evidence: double membrane, circular DNA, 70S ribosomes, independent division.
- Membrane: fluid mosaic; amphipathic phospholipids; cholesterol buffers fluidity; integral vs peripheral proteins; glycoproteins for recognition.
- Transport: simple diffusion (small, nonpolar, down-gradient); facilitated diffusion (carriers/channels, down-gradient, saturable, no ATP); osmosis (water toward higher solute concentration, via aquaporins); active transport (Na⁺/K⁺ ATPase: 3 Na⁺ out, 2 K⁺ in, ATP); cotransport; endocytosis and exocytosis.
Worked example
A cell equilibrated in 0.3 M sucrose is transferred to 0.3 M glucose. Glucose enters through a carrier, so internal solute concentration rises and water follows osmotically; turgor increases. Sucrose at the same molarity is osmotically equivalent but impermeant, so no sustained water entry occurs. Reasoning: equal osmolarity does not mean equal effect - permeability decides.
Marks lost
Naming the Golgi as the site of protein synthesis; claiming facilitated diffusion uses ATP; describing osmosis without stating the direction of water movement; forgetting that plant cells become turgid rather than lysing.