1. The Pacinian corpuscle is an example of a receptor. What is the structure of this receptor ?
Answer
Consists of many layers of membrane which surround the end of the axon of sensory neurone. Has stretch mediated sodium channels which open when pressure is applied.
2. What is the role of the chemoreceptors and pressure receptors (baroreceptors) in controlling heart rate and where are they located ?
Answer
Chemoreceptors detect changes in pH of blood and either increase or decrease impulses to the medulla.
Pressure receptors (baroreceptors) detect changes in blood pressure and either increase or decrease impulses to the medulla.
Baroreceptors found in carotid artery and aorta.
Chemoreceptors found in medulla (oblongata), carotid artery and aorta.
3. A student investigated the response of a seedling root to gravity. The student germinated the seeds in a moist container until the roots were 15 mm long. All the seeds were supported by a damp piece of filter paper in a lidded container to keep the seeds moist. The roots were aligned horizontally as shown in the figure below.
The seeds were divided into two groups. Group 1 was left intact, and the position of the root tip was marked on the paper. Group 2 was treated exactly the same as Group 1, but the final 1 mm of the root tip was removed using a scalpel. The position of the cut tip was marked on the paper. After 4 hours the student measured the position of each root tip and its angle from the original horizontal position. The results of the student's investigation can be seen in the table below. A negative figure means the root curved upwards and a positive figure means the root curved downwards. Calculate the mean root angle for each investigation.
Answer
tip removed: −3; tip intact: +37
4. Explain what these results show about the response of a seedling root to gravity.
Answer
When the root tip is removed, the root either does not bend at all or bends upwards. This shows that it is unable to respond to gravity without the tip.
5. Explain how the removal of the tip in Group 1 caused the roots to grow differently from the roots in Group 2.
Answer
Removal of the tip eliminates the source of auxin. Without auxin, it cannot accumulate on the side of the root nearest to the force of gravity. Because a high concentration of auxin inhibits cell elongation in roots, the side facing away from gravity elongates, causing the root to bend towards gravity.
6. A student set up an investigation as shown in the figure below. Name the type of response.
Answer
Positive geotropism.
7. Explain the appearance of the seedlings after 2 days.
Answer
Roots growing towards gravity. Auxin settles on lower side of root. Inhibits cell elongation (or growth) on lower side of root.
8. IAA enters cells via a specific membrane protein and leaves by facilitated diffusion through a different protein. It causes cell elongation. 2.4 D is a synthetic auxin. It can enter cells through the same receptor as IAA but it cannot leave. 2.4 D is used as a selective weedkiller on lawns. It kills off broadleaved plants but not narrow-leaved plants, such as grass. Use this information to explain why 2.4 D is more effective as a weedkiller than IAA would be.
Answer
2.4-D is more effective as a weedkiller than IAA because it enters the cell but cannot leave, so it accumulates and lasts longer.
9. 2.4 D is sprayed onto lawns to kill weeds and is absorbed through the leaves. Explain how 2.4 D kills broad-leaved plants but not narrow-leaved plants.
Answer
Broad-leaved plants have a larger surface area and therefore absorb more 2.4 D than narrow-leaved plants. In narrow-leaved plants, cell elongation is less marked, so they survive, whereas in broad-leaved plants excess elongation occurs, causing them to die.
10. Explain why it may be impossible to see a dim star in the night sky when you look at it directly, although it may be seen if you do not look directly at it.
Answer
It may be impossible to see a dim star when looking directly at it because the image falls on the fovea, which contains mainly cones and few or no rods. Cones are not sensitive in dim light. If the star is viewed indirectly, the image falls on the peripheral retina, where there are more rods, so the star can be detected.