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

What holds the leaf back

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

  1. A point-like grow lamp sits 1 metre from a seedling. You move it to 2 metres. Using the inverse-square model, what happens to the light hitting the leaf?

    • It falls to half as much light
    • It becomes twice as intense
    • It drops to one quarter of the original intensity

  2. In the simple limiting-factor model, one main factor sets the photosynthesis ceiling at a given moment.

    Circle one:   True   False

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

    • Another factor is now limiting the photosynthesis rate
    • Light still limits the rate in exactly the same way
    • The measured rate must be ignored because the line is flat

  4. A lamp is moved from 2 metres to 6 metres from a seedling. Under the inverse-square model, what fraction of the original light intensity reaches the leaf?

    • One third
    • One ninth
    • One sixth
    • Three times as much

  5. A greenhouse already has strong lamps and a suitable temperature. Why might the grower add carbon dioxide instead of more lamps?

    • More light will always fix every limit, even when carbon dioxide or temperature is still low
    • Carbon dioxide is the current ceiling, so raising it can lift the rate
    • Plants stop using carbon dioxide whenever strong lamps are switched on

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

    • Light, the factor on the horizontal axis
    • Temperature is the only factor that can limit photosynthesis
    • Water always limits every leaf on the climbing part of the graph

  7. A lamp gives 200 lux at 2 metres. What intensity does the leaf receive at 8 metres under the inverse-square model?

    Answer: ______________

  8. A point-like lamp gives 800 lux at 1 metre. Under the inverse-square model, what is the intensity in lux at 4 metres?

    Answer: ______________

  9. The temperature becomes far too high and the photosynthesis rate falls. What is the best explanation?

    • Light ran out before the leaf could finish the reaction
    • Heat damaged enzymes involved in photosynthesis
    • Carbon dioxide became a solid inside the leaf at high temperature

  10. Under the inverse-square model, moving a point-like 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 one quarter of the original intensity · Doubling the distance divides the light by 2 squared, which is 4.
  2. True · The model identifies the factor that would raise the rate if it increased. Other factors can become limiting when conditions change.
  3. Another factor is now limiting the photosynthesis rate · If more light no longer raises the rate, another factor has become the current limit.
  4. One ninth · The distance becomes three times larger. The inverse-square rule divides intensity by 3 squared, or 9.
  5. Carbon dioxide is the current ceiling, so raising it can lift the rate · With plenty of light and a suitable temperature, carbon dioxide is the factor left holding the rate down. Raising that ceiling can increase the rate.
  6. Light, the factor on the horizontal axis · While the line climbs with light, light is the ceiling.
  7. 12.5 · The lamp is four times farther away, so divide 200 lux by 4 squared, or 16. The result is 12.5 lux.
  8. 50 · Four times farther divides the light by 4 squared, which is 16.
  9. Heat damaged enzymes involved in photosynthesis · Excess heat can change the enzymes' active sites, so the reactions slow instead of speeding up.
  10. 3 · One ninth means the square of the distance ratio is 9, so the distance tripled.
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