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A2 · Practice questions · Chloroplasts and pigments

Membranes, pigments, spots.

Six original Cambridge-style questions on the chloroplast and its pigments, from labelling a micrograph to calculating Rf values.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Sites, spectra, ratios.

01
Recall
[2 marks]

State the site within the chloroplast of the light-dependent stage and of the light-independent stage of photosynthesis.

  • Light-dependent stage: the thylakoids (thylakoid membranes and thylakoid spaces), which are stacked to form grana. ✓
  • Light-independent stage (Calvin cycle): the stroma. ✓
02
Explain
[4 marks]

Describe how the structure of a chloroplast, as seen in an electron micrograph, is related to its function.

  • Many thylakoid membranes stacked into grana give a large surface area for photosystems / pigments, electron carriers and ATP synthase. ✓
  • The thylakoid space is small, so a proton gradient builds up quickly for ATP synthesis. ✓
  • The stroma contains the enzymes of the Calvin cycle (such as rubisco), close to the thylakoids that supply ATP and reduced NADP. ✓
  • Circular DNA and 70S ribosomes make some chloroplast proteins; starch grains and lipid droplets store products. ✓
03
Explain
[3 marks]

Explain why photosynthesis is described as an energy transfer process.

  • Light energy is absorbed by chloroplast pigments in the thylakoids. ✓
  • In the light-dependent stage it is transferred to chemical energy in ATP and reduced NADP. ✓
  • In the Calvin cycle the energy in ATP and reduced NADP is used to make complex organic molecules (such as triose phosphate, then sugars) from carbon dioxide. ✓
04
Data
[3 marks]

An absorption spectrum for chlorophyll a has peaks at 430 nm and 662 nm. The action spectrum for the same leaf shows a high rate from 430 nm to 490 nm and a low rate from 520 nm to 600 nm. Interpret these data.

  • The action spectrum is high where chlorophyll a absorbs strongly (blue), so absorbed light is used for photosynthesis. ✓
  • The high rate up to 490 nm, beyond the chlorophyll a peak, is due to accessory pigments (carotene, xanthophyll, chlorophyll b) absorbing these wavelengths and passing the energy to chlorophyll a. ✓
  • The rate is low from 520 to 600 nm because little green / yellow light is absorbed by any pigment; it is reflected or transmitted. ✓
05
Calculation
[3 marks]

On a chromatogram the solvent front is 9.5 cm from the origin. Spot X is 6.2 cm and spot Y is 4.3 cm from the origin. Known Rf values in this solvent are: carotene 0.95, xanthophyll 0.71, chlorophyll a 0.65, chlorophyll b 0.45. Identify X and Y.

  • Rf of X = 6.2 ÷ 9.5 = 0.65, so X is chlorophyll a. ✓
  • Rf of Y = 4.3 ÷ 9.5 = 0.45, so Y is chlorophyll b. ✓
  • Both distances are measured from the origin line to the centre of the spot / to the solvent front. ✓
06
Analysis
[3 marks]

A student's chromatogram shows the pigments as one long smear with no separate spots. Suggest three improvements to the method.

  • Make the origin spot small and concentrated: apply many small drops, drying between each. ✓
  • Make sure the solvent level is below the origin, so the pigments do not dissolve into the solvent in the jar. ✓
  • Cover the jar so the atmosphere is saturated with solvent vapour; or use a thin-layer plate / a more suitable non-polar solvent for better separation; or remove the paper before the solvent reaches the top. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.