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Biology 20

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Photosynthesis

Ten questions of mixed difficulty, covering Photosynthesis. Print it, or work through it on screen — the answer key starts on its own page.

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Photosynthesis

Biology 20 · maddyhelps.com

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  1. The overall equation for photosynthesis is

    1. a) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
    2. b) 6CO₂ + 6O₂ → C₆H₁₂O₆ + 6H₂O
    3. c) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
    4. d) C₆H₁₂O₆ → 6CO₂ + 6H₂O

  2. ATP is made in the thylakoid by chemiosmosis, which means

    1. a) ATP is made directly by an enzyme in the stroma
    2. b) electrons are transferred straight to ADP
    3. c) light energy is converted directly into ATP
    4. d) protons pumped into the thylakoid space flow back out through ATP synthase

  3. Photosynthesis takes place in the

    1. a) ribosome
    2. b) chloroplast
    3. c) mitochondrion
    4. d) nucleus

  4. The oxygen released by photosynthesis comes from

    1. a) glucose
    2. b) the atmosphere
    3. c) carbon dioxide
    4. d) water

  5. When light intensity is increased, the rate of photosynthesis eventually stops rising because

    1. a) the plant runs out of water permanently
    2. b) chlorophyll is destroyed by bright light
    3. c) the Calvin cycle stops needing energy
    4. d) some other factor, such as CO₂ concentration, has become limiting

  6. The light-dependent reactions supply the Calvin cycle with

    1. a) CO₂ and water
    2. b) oxygen and NADP⁺
    3. c) glucose and oxygen
    4. d) ATP and NADPH

  7. The Calvin cycle consumes

    1. a) CO₂ and light
    2. b) O₂, ATP and NADPH
    3. c) glucose and O₂
    4. d) CO₂, ATP and NADPH

  8. The Calvin cycle takes place in the

    1. a) stroma
    2. b) cell wall
    3. c) thylakoid space
    4. d) thylakoid membrane

  9. The enzyme rubisco is responsible for

    1. a) making ATP in the stroma
    2. b) breaking glucose down for energy
    3. c) splitting water in the thylakoid
    4. d) attaching CO₂ to RuBP at the start of the Calvin cycle

  10. Chlorophyll appears green because it

    1. a) reflects green light and absorbs red and blue
    2. b) contains a green metal ion
    3. c) absorbs green light
    4. d) emits green light

Answer key · Photosynthesis

Biology 20 · maddyhelps.com

  1. c) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — Carbon dioxide and water go in, glucose and oxygen come out, with light supplying the energy. The second option is cellular respiration, which is the same equation run backwards.
  2. d) protons pumped into the thylakoid space flow back out through ATP synthase — The electron transport chain uses energy from excited electrons to pump protons, building a concentration gradient. The gradient is the stored energy, and letting it collapse through ATP synthase is what makes the ATP.
  3. b) chloroplast — Chloroplasts contain the pigments and the membranes the process needs. Mitochondria do cellular respiration — plant cells have both, and use both.
  4. d) water — Photolysis splits water to replace the electrons chlorophyll loses, and oxygen is the leftover. Tracing photosynthesis with a heavy isotope of oxygen was how this was settled — labelled water produced labelled oxygen gas.
  5. d) some other factor, such as CO₂ concentration, has become limiting — A rate is set by whatever is in shortest supply. Once light is plentiful, adding more cannot help until the next bottleneck is relieved — which is why commercial greenhouses raise CO₂ as well as light.
  6. d) ATP and NADPH — The first stage converts light into two chemical carriers — energy in ATP and reducing power in NADPH. The Calvin cycle spends both to build sugar, and returns ADP and NADP⁺ to be recharged.
  7. d) CO₂, ATP and NADPH — Carbon dioxide provides the carbon; ATP and NADPH provide the energy and electrons needed to reduce it. The cycle needs no light directly, which is why calling it the dark reactions is misleading — it stops when the light stops because the supplies do.
  8. a) stroma — The stroma is the fluid surrounding the thylakoids, and it holds the enzymes of the cycle. The two stages are separated in space, which lets the products of one be handed to the other.
  9. d) attaching CO₂ to RuBP at the start of the Calvin cycle — Carbon fixation — joining inorganic CO₂ to a five-carbon sugar — is the cycle's first and defining step. Rubisco is thought to be the most abundant protein on Earth, and it is also famously slow.
  10. a) reflects green light and absorbs red and blue — The colour you see is the light a pigment does not use. Green is reflected, which is why it is the least useful colour for growing plants under.