Biology: Plant Hormones and Tropisms
How chemical signals guide plant growth and responses
Biology: Plant Hormones and Tropisms
How chemical signals guide plant growth and responses
Biology - Grade 9-12
- 1
Define a plant hormone and explain why plant hormones can affect cells far from where they are produced.
Think about how a signal made in one part of an organism can travel to another part.
A plant hormone is a chemical messenger made by a plant that helps regulate growth, development, or responses to the environment. Plant hormones can affect distant cells because they may be transported through plant tissues, such as vascular tissue, or move from cell to cell until they reach target cells with the correct receptors. - 2
A bean seedling is placed near a window, and after several days its shoot bends toward the light. Identify the tropism and explain the role of auxin in this response.
This response is positive phototropism because the shoot grows toward light. Auxin collects more on the shaded side of the shoot, causing cells on that side to elongate more, so the shoot bends toward the light source. - 3
Roots usually grow downward even when a seed germinates on its side. Identify the tropism involved and explain how this response helps the plant survive.
Positive means growing toward the stimulus.
This response is positive gravitropism because roots grow in the direction of gravity. Growing downward helps roots anchor the plant and reach water and minerals in the soil. - 4
Compare positive phototropism and negative gravitropism in shoots.
Positive phototropism in shoots means the shoot grows toward light. Negative gravitropism in shoots means the shoot grows away from gravity, upward. Both responses help the shoot reach light for photosynthesis. - 5
A gardener cuts the top bud off a young plant, and later the side branches grow more actively. Which hormone is most responsible for the original inhibition of side branch growth, and what is this effect called?
The tip of the shoot can control the growth of buds below it.
Auxin is most responsible for inhibiting side branch growth when it is produced by the top bud. This effect is called apical dominance. - 6
Explain how cytokinins and auxins can have different effects on plant growth even though both are plant hormones.
Cytokinins generally promote cell division and can encourage the growth of lateral buds, while auxins often promote cell elongation and apical dominance. Their effects depend on the target tissue, hormone concentration, and the balance between different hormones. - 7
A farmer applies gibberellins to a crop and notices that the stems become taller. Explain why gibberellins can cause this effect.
Gibberellins are often associated with growth in stems and seed germination.
Gibberellins can promote stem elongation by stimulating cell division and cell elongation in certain plant tissues. This can make stems grow taller than they would without the added hormone. - 8
Some fruits release ethylene as they ripen. Explain how one ripe banana can cause nearby unripe bananas to ripen faster.
A ripe banana releases ethylene gas, which can spread through the air to nearby bananas. Ethylene acts as a plant hormone that speeds up ripening processes, so the nearby unripe bananas may ripen faster. - 9
During a drought, a plant closes many of its stomata. Which hormone is most directly involved, and how does this response help the plant?
This hormone is strongly linked to stress responses and water conservation.
Abscisic acid, or ABA, is most directly involved in closing stomata during drought. Closing stomata reduces water loss through transpiration, helping the plant conserve water. - 10
A vine wraps around a fence after touching it. Identify the tropism and explain what stimulus causes the response.
This response is thigmotropism because the vine grows in response to touch. Contact with the fence acts as the stimulus, causing growth patterns that allow the vine to coil around the support. - 11
Distinguish between a tropism and a nastic movement using one example of each.
Focus on whether the direction of the stimulus controls the direction of the response.
A tropism is directional growth toward or away from a stimulus, such as a shoot bending toward light in phototropism. A nastic movement is a non-directional response that does not depend on the direction of the stimulus, such as a Venus flytrap closing when touched. - 12
A student places two identical seedlings in boxes. One box has a hole on the left side that lets in light, and the other box is fully open under even light. What is the independent variable in this experiment?
The independent variable is the direction or pattern of light exposure. The student changes whether the seedling receives light from one side or evenly from above and all around. - 13
In the same seedling experiment, list two variables that should be kept constant to make the test fair.
Constants are conditions that stay the same in both groups.
Two variables that should be kept constant are the type and age of the seedlings and the amount of water they receive. Other good constants include soil type, temperature, and length of the experiment. - 14
A plant root grows toward a moist area of soil. Identify the tropism and explain why this response is adaptive.
This response is positive hydrotropism because the root grows toward water. It is adaptive because water is needed for photosynthesis, transport, cell structure, and many chemical reactions in the plant. - 15
A researcher removes the tip from a grass coleoptile and then shines light from one side. The coleoptile does not bend toward the light. Explain what this result suggests about the shoot tip.
Consider what part of the shoot controls the growth response.
This result suggests that the shoot tip detects light or produces a signal needed for bending. In classic phototropism experiments, the tip produces auxin, and uneven auxin distribution causes cells to elongate differently so the shoot bends toward light.