1. How can changes in the environment be detected by an organism ?
Answer
By receptors.
2. What name is given to the growth factor that allows plants to respond to stimuli ? How does it allow a plant shoot to show positive phototropism ?
Answer
IAA (indole acetic acid). More IAA is found on the shaded side of the shoot. This stimulates faster cell elongation, causing the shoot to bend and grow towards the light.
3. The figure below shows the results of some investigations into the response of shoots to light. All the investigations used the same kind of shoot. In each case, light was shone on the shoots from the left-hand side only. Explain what the scientists could conclude from comparing investigations A-H.
Answer
Intact shoots grow towards light. B and C show that the tip is required to detect or respond to stimulus. D and E show that the tip must produce a chemical in response to stimulus. This chemical can pass through agar but not an impermeable substance. F bends towards light showing chemical passes down shaded side. No bending in G because chemical can't pass down shaded side. H shows that the tip perceives the stimulus, as only the tip is above the dark sand. Curvature still occurs because the tip releases a chemical that passes down the shaded side. The light does not destroy the chemical, since both sides of the shoot are not exposed to light.
4. A trained male Olympic athlete had a resting heart rate of 43 beats per minute. An untrained man, of the same age and height, had a resting heart rate of 77 beats per minute. Explain this difference.
Answer
Training increases stroke volume. At rest, both men require a similar cardiac output, so the trained athlete has a lower heart rate.
5. The figure below shows a simple reflex arc. Identify structures 1–4.
Answer
1 = receptor,
6. Explain the advantage of a spinal reflex arc.
Answer
A spinal reflex arc provides a quick, immediate response to a harmful stimulus because the CNS is not fully involved. The pathway only requires two synapses, allowing the response to occur rapidly.
7. The figure below shows the response of different kinds of receptors to a continuous stimulus. Compare the response of a Pacinian corpuscle with the response of a stretch receptor in muscle.
Answer
Pacinian corpuscle responds rapidly to a stimulus but stops responding very rapidly (within a second). Stretch receptor produces a slower rate of impulses as time goes on (until 2 seconds) but continues to generate impulses at a slower and continuous rate.
8. Explain the advantage of a stretch receptor in muscle showing this pattern of response.
Answer
Muscles may need to remain contracted for a long time, so impulses must continue to pass to the CNS throughout the contraction.
9. The figure below shows the distribution of receptor cells in the retina of a human. Identify regions X, Y and receptor cells A, B.
Answer
X = fovea,
10. Explain why it is difficult to read a newspaper in dim light.
Answer
It is difficult to read a newspaper in dim light because cones are not stimulated in low light, so rods are used instead. Cones provide better visual acuity, while rods have poor acuity, making fine detail harder to see.