The leaves of Mimosa pudica plants are made of a number of structures known as pinnae. The pinnae fold when the leaf is touched. This closes the leaf.
Fig. 4.1 shows an open leaf of M. pudica before it is touched. Fig. 4.2 shows the same leaf that has closed after being touched.
(a) A touch stimulus to an M. pudica leaf causes an action potential to be generated. The action potential results in changes in cells, which cause the leaf to close.
Fig. 4.3 shows the mechanism in M. pudica cells that causes the leaf to close.
The leaves of M. pudica and the leaves of Venus fly traps move in response to touch stimuli, but the mechanisms that cause the responses are different.
Describe the differences between the mechanism shown in Fig. 4.3 and the mechanism that causes the closure of the modified leaves in Venus fly traps.
(b) The rate of photosynthesis decreases by 40% when the leaves of M. pudica close.
Explain why the rate of photosynthesis decreases when the leaves of M. pudica close.
(c) Plants can carry out cyclic photophosphorylation and non-cyclic photophosphorylation during the light-dependent stage of photosynthesis. These processes occur at the grana of chloroplasts.
Outline the similarities and differences between cyclic photophosphorylation and non-cyclic photophosphorylation.
▶️ Answer/Explanation
(a)
In Venus fly traps:
- Two sensory hairs need to be touched simultaneously to trigger the response, unlike M. pudica which responds to a single touch.
- Protons leave the hinge cells (rather than entering as in M. pudica), moving into the cell walls.
- Calcium ions (Ca²⁺) move into the hinge cells, which isn’t mentioned in the M. pudica mechanism.
- Water moves into the hinge cells (the opposite of M. pudica where water moves out).
- The hinge cells become turgid (swell) rather than flaccid, causing the rapid closure of the trap.
(b)
The rate of photosynthesis decreases because:
- When the leaves close, there is less surface area exposed to light, reducing the amount of light energy that can be absorbed by chloroplasts.
- Fewer stomata are exposed to the air when the leaves are closed, limiting the intake of carbon dioxide which is essential for the Calvin cycle.
- The folded leaf structure may shade some chloroplasts, further reducing light absorption.
(c)
Similarities:
- Both processes involve the photoactivation of chlorophyll in Photosystem I.
- Both involve the movement of energetic electrons along an electron transport chain.
- Both utilize chemiosmosis to produce ATP.
- Both occur in the thylakoid membranes of chloroplasts.
Differences:
- Non-cyclic photophosphorylation uses both Photosystem I and II, while cyclic only uses Photosystem I.
- Non-cyclic produces oxygen through photolysis of water; cyclic does not.
- Non-cyclic produces both ATP and reduced NADP; cyclic only produces ATP.
- In non-cyclic, electrons end up in NADP; in cyclic, electrons return to the same photosystem.
- Non-cyclic involves the splitting of water molecules to replace electrons; cyclic does not require this.
- Non-cyclic results in a linear electron flow; cyclic results in a circular electron flow.