The key distinction is that phospholipids move within the membrane, while substances entering or leaving must cross it. Phospholipids mainly move sideways, keeping their hydrophobic tails facing inward. This maintains a continuous, water-repelling interior as the molecules move—normal fluidity doesn’t create open passageways through the membrane. Control comes from the bilayer’s chemistry and its transport proteins: - Small nonpolar molecules, such as oxygen and carbon dioxide, dissolve in the lipid interior and diffuse across, following their concentration gradients. - Ions and larger polar molecules, such as glucose, cannot readily cross that interior. They generally require specific channels or carriers. Cells regulate transport by controlling these proteins’ activity and abundance. Temperature can affect this balance. Warming generally increases fluidity and can increase leakage; cooling makes the membrane more rigid and can impair transport protein function. Large temperature changes can disrupt the barrier, particularly when lipids change between fluid and more ordered states. In animal cells, cholesterol helps maintain a membrane that is both fluid and resistant to leakage by influencing how the phospholipids pack together. Fluidity and permeability are related, but they are not the same property—a membrane can allow sideways movement while strongly restricting movement across it.

@Minh Nguyen Duong Bao 6/6 for the work submitted! (1 mark per MCQ or FRQ part.) - MCQs: 4/4 - FRQ (a): 1/1 — Correct calculations and units. - FRQ (b): 1/1 — Correct relationship supported by numerical data. The repeated “Cube 1” labels are a minor typo; no marks deducted. Next time you can just post a picture of the workbook 😅 must of taken ages to type all of that

@Anh Luu No worries, Bao Anh! It’s technically still on time since you submitted before this afternoon’s test, so there’s no late penalty. 😊 3/3 for parts (b)–(d)! ✅ - (b) 1/1: Reasonable estimate and correct explanation of equal water potentials. Remember to include M. - (c) 1/1: Correctly explains why water leaves the tissue. - (d) 1/1: Correctly predicts slower water loss, with outward movement continuing. One correction for (d): water can still pass through the remaining aquaporins; it does not all have to cross the phospholipid bilayer. A clearer prediction is: “The initial rate decreases, but the direction remains unchanged because the water-potential gradient is unchanged.” Good luck with your test this afternoon! ✍️

4/5 — good work! Your answers to (b), (c), and (d) are correct. You clearly connect reduced membrane folding to lower ATP production and distinguish functional reasoning from evolutionary evidence. Part (a) is missing. Add: “Organelle Q likely evolved from a bacterium engulfed by an ancestral eukaryotic cell through endosymbiosis.”