The bending of a shoot towards the light is caused by the uneven spreading of a plant hormone across the shoot. To show this, a shoot was set up so that lig...
Question 1 Report
The bending of a shoot towards the light is caused by the uneven spreading of a plant hormone across the shoot. To show this, a shoot was set up so that light shone strongly on one side only, leaving the opposite side in shade. The way the hormone was spread across the shoot is represented in the diagram by a scatter of small dots, where more dots mean a greater amount of hormone. The two sides of the shoot are labelled P and Q, one facing the light and the other in shadow. Comparing how many dots lie on each side reveals the link between the hormone and the bending of the shoot.
(a) State which side, P or Q, is the shaded side. [1]
(b) Describe how the amount of hormone on side P compares with side Q. [2]
(c) Explain, in terms of the hormone, why the shoot will bend towards the light. [3]
(d) Suggest why the hormone gives no bending when the shoot is lit equally from all sides. [2]
Answer Details
This question tests phototropism, the growth of a shoot towards light caused by the uneven spread of the hormone auxin. The side with more auxin elongates more.
(a) Which side is shaded [1] - side P (it has more hormone, shown by more dots, because auxin accumulates away from the light).
(b) How the hormone on side P compares with side Q [2]
Side P has more hormone (auxin) than side Q (1); the hormone has moved to and accumulated on the shaded side (1).
(c) Why the shoot bends towards the light [3]
- Auxin is made in the tip of the shoot (1).
- The higher concentration of auxin on the shaded side (P) increases cell elongation there (1).
- Side P therefore grows longer than side Q, so the shoot bends towards the light (1).
(d) Why equal lighting gives no bending [2]
Light reaches all sides equally (1), so the auxin is spread evenly and all sides elongate by the same amount, so the shoot grows straight up (1).
Exam tip: remember the chain: auxin made in the tip, accumulates on the shaded side, causes more elongation there, so the shoot bends towards the light. The bending is a growth response, not muscle movement.
Download The App On Google Playstore
Everything you need to excel in your exams
