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- Define gravitropism as a response in which parts of a plant grow towards or away from gravity
Gravitropism is a response in which parts of a plant grow towards or away from gravity. Note: gravitropism used to be called geotropism, which is why some past papers may mention geotropism instead.
- Define phototropism as a response in which parts of a plant grow towards or away from the direction from which light is coming
Phototropism is a response in which parts of a plant grow towards or away from the direction from which light is coming.
- Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth
Certain chemicals in plants regulate plant growth.
Phototropism and geotropism are examples of how chemicals like plant hormones regulate plant growth.
In phototropism, these chemicals move to the part of the plant receiving less light. This causes that part of the plant to grow more than the side receiving light, making the plant bend towards the light.
In gravitropism, the plant hormone gathers on the lower side of the plant. This has different effects on different parts of the plant:
- In the shoot, the chemical stimulates growth, so the stem curves upwards
- In the root, the chemical slows growth, so the root curves down.
- Investigate gravitropism and phototropism in shoots and roots
Note: auxin is the main plant hormone that regulates gravitropism and phototropism.
I think the images summarises all that you need to know in regard to the investigation pretty well, but if you want a more detailed experiment plan then use this link:
Take two cardboard boxes. In one, cut a small hole (about 2’’ in diameter) on the top and in the other, cut a hole of the same size on the side of the box, about 3cm up from the bottom. Cover both holes with clear plastic.
Take a photo of two small plant shoots growing in their own separate plant pots from the front. Place one in each box, lining them up with the holes. You can grow your own bean plant shoots for this experiment. Take a photo of both plants from above.
Position the boxes in different parts of the same room, and place lamps next to the boxes. Each one should be lined up with the holes on the boxes, so that maximum light can get inside from the direction of the hole.
Every morning, turn on both lamps and every night, turn them off. Make sure they are properly watered and fertilized (both should be provided water and fertilizer in the exact same amounts). Do this for one week. Open the boxes and take pictures of the plants from above and in front. You will notice that both plants grew in the direction of the holes.
Explanation: In phototropism, the auxins move away from the light, towards the shaded side, so this side of the plant will grow more, causing the plant to bend towards the light.
- Explain the role of auxin in controlling shot growth, limited to:
- Auxin made in shoot tip (only)
- Auxin spreads through the plant from the shoot tip
- Auxin is unequally distributed in response to light and gravity
- Auxin stimulates cell elongation
The hormones that regulate plant growth are called auxins.
Note that auxins in the shoot stimulate cell elongation (by stimulating the absorption of water, forcing cells to expand) and that auxins in the root slow down cell growth.
Auxins are produced in the shoot tips of growing plants. These regions are called ‘meristems’. More specifically, a meristem is a region of cell division in a plant, but I don’t think you’ll have to know this term at the IGCSE level. All you need to know for your exam is that auxins are produced in shoot tips.
Auxin spreads through the plant from the shoot tip, however, its distribution throughout the plant is unequal. This is because of its response to light and gravity.
In phototropism, auxin moves to the part of the plant receiving less light. This causes the cells on the shaded part of the plant to elongate, making the plant bend towards the light.
In gravitropism, auxin gathers on the lower side of the plant. This has different effects on different parts of the plant:
- In the shoot, auxin stimulates cell elongation, so the stem curves upwards
- In the root, auxin slows growth, so the root curves down.
Notes submitted by Sarah
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