Question 1 · 2.1 Cell Structure and Function
In a gland cell that exports a digestive enzyme, researchers apply a compound that leaves protein synthesis fully active but blocks vesicles from budding off the rough endoplasmic reticulum. Where would the newly made enzyme be expected to pile up?
- A In the lumen of the rough ER
- B In the Golgi, where sugar groups are added
- C In vesicles docked at the plasma membrane
- D In the cytosol, on free ribosomes
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A — A protein headed for export is threaded into the ER interior while it is still being assembled by a ribosome bound to that membrane, so it arrives in the ER lumen automatically. Every step after that is vesicle traffic. Shut down the first vesicle step and the enzyme is finished, folded, and stranded in the compartment it was delivered into.
Question 2 · 2.2 Cell Size
In the human small intestine, which folds of the lining greatly increase the surface area for absorption?
- A Alveoli
- B Cilia
- C Flagella
- D Villi
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D — Villi are folds of the small intestine lining that multiply the surface available for absorbing digested nutrients. Alveoli also increase surface area, but they are the tiny air sacs of the lungs used for gas exchange.
Question 3 · 2.3 Plasma Membrane
A membrane protein of a green alga carries a short tag at each end of its chain. Fluorescent antibodies that bind one tag or the other are too large to cross a membrane. Binding was scored on intact cells and on cells whose membranes had been punctured with a mild solvent. Which conclusion about the chain do these results support?
- A It crosses an even number of times, so the two ends lie on the same face
- B It crosses an even number of times, so the two ends lie on opposite faces
- C It crosses an odd number of times, so the two ends lie on opposite faces
- D It flips across the bilayer, so each end spends part of its time outside
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C — Antibodies reach only the outer face of intact cells. The N-terminal tag labels intact cells, so the N-terminus is outside; the C-terminal tag labels only after puncturing, so the C-terminus is inside. Each membrane-spanning segment carries the chain to the other face, so ends on opposite faces require an odd number of crossings (one, three, five and so on). Membrane proteins also keep a fixed orientation: if the chain flipped, the C-terminal tag would label intact cells at least some of the time.