Cell Biology
Core concepts
The cell is the unit of structure and function. A candidate must be able to reason about membrane structure (fluid mosaic model, integral versus peripheral proteins, cholesterol as a fluidity buffer), organelle function (rough versus smooth ER, Golgi processing and sorting, lysosomal acid hydrolases, peroxisomal catalase and β-oxidation), and the cytoskeleton (microfilaments, intermediate filaments, microtubules, plus motor proteins kinesin and dynein). Mitochondrial structure links to oxidative phosphorylation: outer membrane, intermembrane space, cristae-bearing inner membrane, matrix. Know the cell cycle phases and checkpoints, the stages of mitosis, and how meiosis differs in pairing and crossing over. Transport (simple diffusion, facilitated diffusion, primary and secondary active transport, the Na⁺/K⁺-ATPase moving 3 Na⁺ out and 2 K⁺ in), cell junctions, and signal transduction (receptor types, G proteins, second messengers such as cAMP) recur constantly.
Common traps
Questions often swap smooth and rough ER roles, or confuse lysosome with peroxisome. Do not assume all active transport is ATP-driven directly - secondary active transport uses an electrochemical gradient. Remember that prokaryotes lack membrane-bound organelles but possess ribosomes and a nucleoid. In the cell cycle, DNA replication occurs in S phase, not G2. Distinguish apoptosis (programmed, caspase-mediated, no inflammation) from necrosis (passive, inflammatory).
Worked example
Rule of thumb: trace the secretory pathway in order - ribosome on rough ER → ER lumen → Golgi cis and trans faces → vesicle → plasma membrane or lysosome. If a protein functions in the cytosol, it is made on free ribosomes; if secreted or membrane-bound, on rough ER. A question asking where a newly synthesized digestive enzyme is glycosylated is answered: rough ER and Golgi, not the cytosol.