Photosynthesis and Limiting Factors · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

What holds the leaf back

Science · Organisms & Life Processes · ages 15-16
Name ______________________   Date ____________

  1. A grow lamp sits 1 metre from a seedling. You move it to 2 metres. What happens to the light hitting the leaf?

    • It halves
    • It doubles
    • It drops to a quarter

  2. At any moment, one factor sets the ceiling on the photosynthesis rate.

    Circle one:   True   False

  3. A rate graph flattens as light keeps rising. What does the flat part mean?

    • Another factor
    • Light still limits
    • The plant died

  4. A grow lamp sits 1 metre from a tomato seedling. You move the lamp to 2 metres away. What happens to the light intensity hitting the leaf?

    • It stays the same
    • It drops to half
    • It drops to a quarter
    • It doubles

  5. A greenhouse already has strong lamps. Why add carbon dioxide instead of more lamps?

    • Lamps are illegal
    • Carbon dioxide is the current ceiling, so raising it lifts the rate
    • Plants prefer darkness

  6. On the climbing part of a light graph, what limits the rate?

    • Light
    • Temperature
    • Water always

  7. A lamp gives a light intensity of 800 lux when it sits 1 metre from a seedling. What is the intensity, in lux, when the lamp is moved to 4 metres away?

    Answer: ______________

  8. A lamp gives 800 lux at 1 metre. What is the intensity in lux at 4 metres?

    Answer: ______________

  9. It gets far too hot and the rate falls. What happened?

    • Light ran out
    • Enzymes fail in the heat
    • Carbon dioxide turned solid

  10. Moving a lamp dims its light to one ninth. How many times farther away is the lamp now?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

What holds the leaf back W1-mt_PDn831oIih-s1

  1. It drops to a quarter · Doubling the distance divides the light by 2 squared, which is 4.
  2. True · Light, carbon dioxide or temperature: one of them always limits.
  3. Another factor · Extra light changes nothing once a different ceiling takes over.
  4. It drops to a quarter · Light intensity follows an inverse square law, so doubling the distance divides the intensity by 2 squared, which is 4.
  5. Carbon dioxide is the current ceiling, so raising it lifts the rate · Raise the ceiling that actually limits, not the one already plenty.
  6. Light · While the line climbs with light, light is the ceiling.
  7. 50 · Moving the lamp 4 times farther away divides the intensity by 4 squared, which is 16.
  8. 50 · Four times farther divides the light by 4 squared, which is 16.
  9. Enzymes fail in the heat · Past some heat the reaction machinery breaks instead of quickening.
  10. 3 · One ninth means distance squared is 9, so the distance tripled.
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