Cell Biology and Molecular Biology
Cell Biology and Molecular Biology on the LU Faculty of Medical Sciences second-year entrance paper tests structure–function links: membrane organisation, organelle roles, transport, and information flow from DNA to protein. Items are usually short "which statement is correct" questions or one-step calculations.
Core rules
- Membrane: fluid mosaic of phospholipids, cholesterol (buffers fluidity), integral/transmembrane and peripheral proteins, glycocalyx facing outward.
- Transport: passive movement down an electrochemical gradient (simple diffusion for O₂, CO₂, steroids; facilitated via GLUT and channels). Primary active transport burns ATP - the Na⁺/K⁺-ATPase exports 3 Na⁺ and imports 2 K⁺ per ATP. Secondary active transport borrows that Na⁺ gradient (SGLT).
- Replication: semiconservative; the new strand is synthesised 5′→3′; leading strand continuous, lagging strand as Okazaki fragments sealed by ligase.
- Expression: transcription in the nucleus reads the template strand; mRNA matches the coding strand except that U replaces T. Translation at the ribosome (A, P, E sites); AUG = Met; UAA, UAG, UGA are stop codons.
Worked example
A double-stranded DNA sample is 30% adenine. What fraction is guanine? Chargaff pairs A with T, so T = 30% and A + T = 60%. The remaining 40% is G + C, split equally because G pairs with C. Guanine = 20%.
Traps
- Reading the coding strand when the question asks what the template encodes.
- Assuming a fixed ATP yield per NADH without checking the shuttle (≈2.5 via malate–aspartate, ≈1.5 via glycerol phosphate).
- Confusing facilitated diffusion (saturable, no ATP) with active transport.
- Forgetting the nucleolus builds rRNA, not ribosomal proteins.