@Anh Luu Cumulative Practice, MCQ 3.C-06–10 — page 149 Score: 4/5 ✅ Well done! Questions 6, 7, 8, and 10 are correct. You show a good understanding of ATP coupling, photosystem location, the source of photosynthetic oxygen, and energy storage in the proton gradient. Question 9: The correct answer is B. Inhibiting PSII reduces water splitting and electron flow, so O₂ and NADPH production decrease. The mitochondrial matrix is not the compartment involved in photosynthetic electron transport. Review how PSII supplies electrons to the light reactions—good work overall!

@Viet Bach Cumulative Practice, MCQ 3.C-16–20 Score: 5/5 ✅ Excellent work—all five answers are correct! You clearly understand proton flow down an electrochemical gradient, how uncoupling reduces ATP yield and increases heat release, and how shared energy mechanisms support common ancestry. You also correctly interpret the error bars: overlap alone cannot establish whether a difference is statistically significant. Your final answer accurately traces energy from light through carbohydrates and respiration to ATP and cellular work. Strong understanding of the connections across Unit 3!

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.