A researcher treats isolated chloroplasts with a drug that destroys all thylakoid membranes while leaving the outer and inner membranes intact. Which photosynthetic activity would be most directly disrupted?
AThe Calvin cycle, because it requires thylakoid membranes for CO₂ fixation by RuBisCO
BThe light reactions, because photosystems I and II and ATP synthase are embedded in the thylakoid membranes
CBoth stages equally, since ATP and NADPH produced in the stroma feed back into the thylakoids
DCarbon fixation only, since RuBisCO is attached to the outer thylakoid surface
The light reactions are exclusively located in the thylakoid membranes: the photosystems that absorb light, the electron transport chain, and the ATP synthase that harnesses the proton gradient are all embedded there. Destroying the thylakoids eliminates the entire light-capture and energy-conversion machinery. The Calvin cycle enzymes (including RuBisCO) are in the stroma and would remain physically intact, but they would quickly halt due to the absence of ATP and NADPH supplied by the light reactions.
Question 2 Multiple Choice
A student claims that the stroma is analogous to the mitochondrial intermembrane space — both are enclosed regions between two membranes. What is wrong with this analogy?
AThe stroma is outside the outer membrane, not enclosed within any membrane system
BThe stroma is inside the inner membrane, making it analogous to the mitochondrial matrix, not the intermembrane space
CChloroplasts have only one bounding membrane, so there is no valid mitochondrial analogy
DThe stroma is equivalent to the thylakoid lumen, which corresponds to the mitochondrial intermembrane space
The stroma is the aqueous matrix enclosed by the inner membrane — structurally and functionally analogous to the mitochondrial matrix (not the intermembrane space). Both are enzyme-rich soluble compartments where major metabolic cycles occur: the Calvin cycle in the stroma, and the citric acid cycle in the mitochondrial matrix. The mitochondrial intermembrane space has its chloroplast counterpart in the thylakoid lumen, where protons accumulate during the light reactions, just as they accumulate in the mitochondrial intermembrane space during oxidative phosphorylation.
Question 3 True / False
Chloroplasts contain their own circular DNA and 70S ribosomes, which is consistent with their evolutionary origin as endosymbiotic cyanobacteria.
TTrue
FFalse
Answer: True
The endosymbiotic theory holds that chloroplasts descended from cyanobacteria engulfed by an ancestral eukaryote. The evidence includes: circular DNA resembling bacterial genomes, 70S ribosomes matching bacterial (not eukaryotic) size, double membranes (the inner from the bacterium, the outer from the host's engulfing vesicle), and the ability to divide by binary fission. These features are inexplicable if chloroplasts arose de novo but are predicted by the endosymbiotic hypothesis.
Question 4 True / False
The Calvin cycle takes place in the thylakoid membrane, where enzymes can directly access the light energy captured by the photosystems.
TTrue
FFalse
Answer: False
The Calvin cycle occurs in the stroma, not the thylakoid membrane. The thylakoid membrane is the site of the light reactions (photosystems, electron transport, proton pumping, and ATP synthesis). The Calvin cycle enzymes — including RuBisCO, which fixes CO₂ — are dissolved in the stroma. The two stages are spatially separated: light reactions in the thylakoid membrane produce ATP and NADPH, which diffuse into the stroma where they power the Calvin cycle. This separation is a deliberate compartmentalization, not a flaw.
Question 5 Short Answer
Explain why the orientation of ATP synthase in the thylakoid membrane is critical for coupling the light reactions to the Calvin cycle.
Think about your answer, then reveal below.
Model answer: The light reactions pump H⁺ into the thylakoid lumen, building a proton gradient. ATP synthase spans the thylakoid membrane with its catalytic F₁ head protruding into the stroma. As protons flow down their gradient through ATP synthase, ATP is synthesized and released directly into the stroma — exactly where Calvin cycle enzymes are located. If ATP synthase faced the other way (F₁ into the lumen), ATP would be produced inside the thylakoid, inaccessible to the stroma without additional transport. The orientation ensures that energy conversion and carbon fixation are spatially coupled.
This question highlights why structure determines function in organelles. The analogy with mitochondria is useful here: in mitochondria, the ATP synthase F₁ head faces the mitochondrial matrix (where ATP is needed for biosynthesis), not the intermembrane space. In chloroplasts, the equivalent arrangement places ATP production in the stroma. Both organelles use the same design principle: orient ATP synthase so that ATP is synthesized where it is immediately consumed.