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Plant responses: tropisms, auxins and the classic experiments questions
Tropisms and how they are named, the classic coleoptile experiments and the single conclusion each supports, auxin and cell elongation through wall acidification, why shoots and roots respond to gravity in opposite directions, gibberellins and abscisic acid, and the commercial uses of plant growth substances.
6 original questions · 20 marks · the plant responses: tropisms, auxins and the classic experiments notes · Hormonal communication, plant responses and homeostasis
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Describe the experiment carried out by Boysen-Jensen on coleoptiles, and describe what its result establishes about the signal that passes from the tip.
Mark scheme
- B1 the tip was cut off and replaced, with a thin block of gelatine inserted between the tip and the stump, and the coleoptile was then lit from one side
- B1 bending towards the light still happened when gelatine was used
- B1 when the gelatine was replaced by mica, which nothing can diffuse through, bending stopped
- A1 so whatever passes from the tip to the growing region is a chemical that diffuses, rather than an electrical signal or a mechanical pull; the experiment does not identify the chemical
Explain how auxin arriving at a cell in the growing region of a shoot causes that cell to become longer.
Mark scheme
- B1 auxin binds to receptors in the cell and stimulates the proton pumps in the cell surface membrane
- B1 the pumps actively transport hydrogen ions out into the cell wall, so the pH of the wall falls to about 5
- B1 at that pH enzymes called expansins become active and loosen the bonds between the cellulose microfibrils, so the wall becomes more plastic
- A1 the turgor pressure already present can then stretch the loosened wall, water enters by osmosis, and the cell lengthens along its axis
A germinating seedling is laid on its side in complete darkness. Explain why its shoot grows upwards and its root grows downwards, even though auxin collects on the lower side of both.
Mark scheme
- B1 gravity is detected by statoliths, dense starch-filled organelles that sediment to the lower side of particular cells, such as the columella cells of the root cap
- B1 auxin is then redistributed to the lower side of both the shoot and the root
- B1 in the shoot that concentration stimulates elongation, so the lower cells elongate more than the upper ones and the shoot curves upwards, which is negative gravitropism
- A1 root cells are inhibited at concentrations that stimulate a shoot, so the lower cells of the root elongate less than the upper ones and the root curves downwards, which is positive gravitropism
Explain how a synthetic auxin sprayed over a field can kill broad-leaved weeds without seriously harming the cereal crop growing among them.
Maltsters soak barley grains in water and add gibberellin to the steeping tank. Suggest why this makes the starch in the grains turn into sugars sooner and more evenly across the batch.
Give the name of the tropism shown by a shoot growing towards light from one side, and give the name of the plant growth substance responsible for the bending.
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