Organisms respond to changes in their environments

1. In a shoot lit from one side, how does IAA cause bending towards light?

1. It inhibits elongation on the shaded side
2. It causes cell elongation on the shaded side
3. It is destroyed by light on the shaded side
4. It causes cell division on the lit side

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B: It causes cell elongation on the shaded side. IAA accumulates on the shaded side, cells there elongate more, and the shoot bends towards light.

2. Woodlice move faster and turn less in dry conditions. What kind of response is this?

1. Taxis
2. Tropism
3. Kinesis
4. Reflex

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C: Kinesis. The change in movement is non-directional, so it is a kinesis.

3. Why do rods give high sensitivity to light?

1. They contain iodopsin
2. Several rods share one bipolar neurone, allowing spatial summation
3. They are only in the fovea
4. Each has its own neurone

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B: Several rods share one bipolar neurone, allowing spatial summation. Many small generator potentials combine to reach threshold in dim light.

4. How does a Pacinian corpuscle respond to pressure?

1. Calcium channels open
2. Stretch-mediated sodium ion channels open
3. Potassium ions enter
4. Acetylcholine is released

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B: Stretch-mediated sodium ion channels open. Deformation opens stretch-mediated Na⁺ channels, producing a generator potential.

5. In a horizontal root, what does IAA do on the lower side?

1. Stimulates elongation
2. Inhibits elongation, so the root bends down
3. Stops photosynthesis
4. Causes cell death

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B: Inhibits elongation, so the root bends down. In roots IAA inhibits cell elongation, so the upper side grows more and the root curves downwards.

6. Which receptors give high visual acuity?

1. Rods
2. Cones
3. Pacinian corpuscles
4. Baroreceptors

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B: Cones. Each cone connects to a single bipolar neurone, so the brain can separate close points of light.

7. What is the correct order of a simple reflex arc?

1. Receptor, motor, relay, sensory, effector
2. Receptor, sensory, relay, motor, effector
3. Effector, motor, relay, sensory, receptor
4. Sensory, receptor, relay, effector, motor

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B: Receptor, sensory, relay, motor, effector. The impulse goes from receptor to sensory neurone, relay neurone, motor neurone and then effector.

8. Why does a shoot grown in unilateral light bend towards it?

1. IAA accumulates on the light side, inhibiting growth there
2. IAA accumulates on the shaded side, promoting cell elongation there
3. IAA is destroyed entirely
4. No hormone is involved

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B: IAA accumulates on the shaded side, promoting cell elongation there. This is an example of positive phototropism.

9. What is spatial summation?

1. One neurone firing repeatedly in quick succession
2. Several presynaptic neurones combining their input to reach threshold on one postsynaptic neurone
3. A reflex arc
4. A type of receptor

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B: Several presynaptic neurones combining their input to reach threshold on one postsynaptic neurone. This can help trigger an action potential from otherwise sub-threshold signals.

10. A shoot tip is covered with an opaque foil cap and lit from one side only. What is the most likely result?

1. Bends towards the light
2. Bends away from the light
3. Grows straight upwards with no bending
4. Stops growing completely

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C: Grows straight upwards with no bending. The tip detects the light. When it is covered, IAA stays evenly distributed, so both sides elongate equally and the shoot grows straight up. Growth does not stop.