A student investigated how the direction from which light comes affects the way a shoot bends. Bean seedlings were grown in identical pots and then lit in s...
Question 1 Report
A student investigated how the direction from which light comes affects the way a shoot bends. Bean seedlings were grown in identical pots and then lit in several ways: from the left side, from the right side, from directly above, with even light from all directions at once, and in complete darkness. After 48 hours the mean angle through which each shoot had bent was measured, and a shoot that grew straight up was recorded as a curvature of 0 degrees. The apparatus is shown in the diagram and the mean curvature for each lighting condition is given in the table.
| Direction of light | Mean curvature of shoot / degrees |
|---|---|
| from the left side | 42 |
| from the right side | 40 |
| from directly above | 3 |
| all-round even light | 2 |
| complete darkness | 1 |
(a) State the direction of light that produced the largest curvature. [1]
(b) Describe the effect of the direction of light on the curvature of the shoots. [2]
(c) Calculate the difference in mean curvature between light from the left side and complete darkness. [2]
(d) Explain why the shoots showed almost no curvature in all-round even light. [3]
(e) Suggest why the complete darkness result was included. [1]
Answer Details
This question tests phototropism - the growth of a shoot towards light - using results from different lighting directions.
(a) Direction giving the largest curvature [1] From the left side (42 degrees, the highest value in the table).
(b) Effect of light direction on curvature [2] Light from the side (left or right) gives a large curvature (1); while light from above, all-round even light and darkness give little or no curvature (1). Shoots only bend when light comes from one side.
(c) Difference between left light and darkness [2] Subtract the darkness value from the left-side value: \( 42 - 1 = 41 \) degrees (1 mark working, 1 mark answer).
(d) Why almost no curvature in all-round even light [3] The light reaches all sides of the shoot equally (1); so auxin is distributed evenly around the shoot (1); therefore all sides elongate equally and the shoot grows straight up (1). Bending only happens when auxin collects on the shaded side.
(e) Why the darkness result was included [1] As a control, to show the curvature is caused by the light and not by some other factor.
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