Worked examplesThree real compartmentalization questions
From the practice pool, not the mock papers — each with the reasoning that produces the answer.
57% of them come with a figure. Reading the graph or diagram correctly is most of the work here before any content knowledge applies.
88% test a single AP skill: Concept Explanation. That makes this topic unusually predictable to prepare for.
Concept Explanation
Glucose moves into a cell through a membrane protein from an area of higher glucose concentration to an area of lower glucose concentration. ATP is not used. Which transport process is occurring?
- AFacilitated diffusioncorrect
- BEndocytosis
- CActive transport
- DExocytosis
Why A is correct
Facilitated diffusion uses a transport protein to move substances down their concentration gradients without ATP input.
The stem asks students to explain a specific mechanism, not just recall a unit term: "Glucose moves into a cell through a membrane protein from an area of higher glucose concentration to an area of...". The correct choice is supported by Facilitated diffusion uses a transport protein to move substances down their concentration gradients without ATP input. The tempting alternatives such as Endocytosis or Active transport fail because they do not produce the same outcome described in the stem.
Concept Explanation
The sodium-potassium pump hydrolyzes ATP while moving Na+ out of a cell and K+ into the cell against their concentration gradients. Which process is described?
- AFacilitated diffusion through an ion channel
- BSimple diffusion through the lipid bilayer
- CActive transport by a membrane proteincorrect
- DOsmosis through aquaporins
Why C is correct
Moving solutes against their concentration gradients requires energy. Because ATP hydrolysis powers the pump, the process is active transport.
The stem asks students to explain a specific mechanism, not just recall a unit term: "The sodium-potassium pump hydrolyzes ATP while moving Na+ out of a cell and K+ into the cell against their...". The correct choice is supported by Because ATP hydrolysis powers the pump, the process is active transport. The tempting alternatives such as Facilitated diffusion through an ion channel or Simple diffusion through the lipid bilayer fail because they do not produce the same outcome described in the stem.
Concept Explanation
Lysosomal enzymes function best at low pH. Which process most directly helps maintain the acidic interior of lysosomes?
- AH+ leakage maintains low lysosome pH
- BATP-driven transport of H+ into the lysosomecorrect
- CReplication of lysosomal DNA inside the organelle
- DDiffusion of ribosomes through lysosomal membranes
Why B is correct
Proton pumps use ATP to move H+ into lysosomes, helping maintain the low internal pH required by lysosomal enzymes.
The stem asks students to explain a specific mechanism, not just recall a unit term: "Lysosomal enzymes function best at low pH. Which process most directly helps maintain the acidic interior of lysosomes". The correct choice is supported by Proton pumps use ATP to move H+ into lysosomes, helping maintain the low internal pH required by lysosomal enzymes. The tempting alternatives such as Passive leakage of H+ out of the lysosome maintains the low internal pH or Replication of lysosomal DNA inside the organelle fail because they do not produce the same outcome described in the stem.
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Last reviewed 2026-08-28. Topic and unit names follow the
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