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Photosynthesis: Question 1

Syllabus 13.1, 13.2

Multiple choice A2 1 mark

A biology student is comparing the light-dependent stage and the light-independent stage (the Calvin cycle) of photosynthesis inside a chloroplast.

Which row correctly identifies the membrane system or region in which each stage takes place, together with one substance produced there?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall where each stage of photosynthesis occurs

A chloroplast has two main compartments relevant to photosynthesis:

  • The thylakoid membranes (stacked into grana) contain chlorophyll, accessory pigments and the proteins of photosystem I and photosystem II. This is where the light-dependent stage takes place: light energy is absorbed, water is split by photolysis (releasing oxygen), and ATP and reduced NADP are generated.
  • The stroma is the fluid-filled region surrounding the thylakoids. This is where the light-independent stage (Calvin cycle) takes place: carbon dioxide is fixed onto RuBP by rubisco, and the ATP and reduced NADP made in the light-dependent stage are used to convert the product (GP) into triose phosphate (TP), which is used to make glucose and to regenerate RuBP.

So the light-dependent stage happens on the thylakoid membranes and yields (among other things) reduced NADP, while the light-independent stage happens in the stroma and yields (among other things) triose phosphate.

Step 2: Evaluate each option

  • A: correct, thylakoid membranes for the light-dependent stage (producing reduced NADP) and stroma for the light-independent stage (producing triose phosphate) matches the true locations and products.
  • B: incorrect, this swaps the two locations around; the light-dependent stage cannot occur in the stroma because it needs the photosystems embedded in the thylakoid membranes, and the light-independent stage cannot occur on the thylakoid membranes because rubisco and the other Calvin cycle enzymes are dissolved in the stroma.
  • C: incorrect. The light-dependent stage does not produce glucose directly (glucose is only built up later, in the Calvin cycle, from triose phosphate), and the light-independent stage does not produce oxygen (oxygen comes only from the photolysis of water in the light-dependent stage).
  • D: incorrect. The thylakoid membranes are not merely structural; they house the photosystems and electron carriers that actually carry out the light-dependent stage, which cannot happen in the stroma.

Final answer

  • The light-dependent stage occurs on the thylakoid membranes (producing reduced NADP, among other products) and the light-independent stage occurs in the stroma (producing triose phosphate), option A.