B2: Movement Across Membranes and Surface Area to Volume
Official Cambridge & Imperial ESAT specification requirements for Movement Across Membranes and Surface Area to Volume on the Biology paper. Understand the syllabus boundaries, spot distractor traps engineered by examiners, and master the 2-step Sub-90 shortcuts that compress solve time well below the 88.8-second cutoff.
Official Syllabus Subtopics
Requirements specified in the UAT-UK framework
- 1Fluid mosaic membrane model: phospholipid bilayer with embedded intrinsic and extrinsic proteins
- 2Simple diffusion down a concentration gradient (non-polar small molecules like O₂ and CO₂)
- 3Facilitated diffusion via channel and carrier proteins (ions and polar glucose, no ATP)
- 4Osmosis: passive net movement of water molecules down a water potential (Ψ) gradient across a partially permeable membrane
- 5Turgor pressure, flaccidity, and plasmolysis in plant cells
- 6Active transport requiring ATP hydrolysis against concentration gradient (e.g. sodium-potassium pump)
- 7Surface area to volume ratio (SA:V) scaling: SA ∝ r², Volume ∝ r³, SA:V ∝ 1/r
Sub-90 Exam Speed Tips
How top candidates solve this spec under the 88.8s clock
As cell diameter or organism size doubles, surface area quadruples while volume increases eightfold, cutting SA:V ratio in half.
Water always moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential.
Examiners engineer distractor options using common algebraic expansions and sign slips. Always test structural ratios or invariants before calculating arithmetic values.
Tested Topics in Specification B2
Master the individual question patterns and Sub-90 shortcuts for this specification.
Osmosis Water Potential Gradients, Active Transport & Cube SA:V Diffusion Ratios
For a cube of side L, SA:V = 6/L (halving side length doubles SA:V). On an osmosis % mass change graph, the x-intercept (0% change) is the exact isotonic intracellular solute concentration.
Ready to Master Specification B2 Under Exam Conditions?
Practice timed questions under the authentic Pearson VUE interface with live split stopwatch pacing, dry-wipe scratchpad, and distractor forensics.