IGCSE Coordinated Science: Tropic Responses Tropic Responses 1. Define and investigate geotropism (as a response in which a plant grows towards or away from gravity) and phototropism (as a response in which a plant grows towards or away from the direction from which light is coming). Plants can’t move, we all know that, unless you watch some weird TV show where they can. Then, they need to have some other abilities to receive the benefits of sunlight, hence maximum photosynthesis. Plants have the ability to detect stimulus and then respond to the stimulus by either bending or growing in its direction or away from it. We call these responses Tropism. In GCSE, there are essentially two types of tropism, Geotropism and Phototropism. ● Geotropism: This is the response where a plant grows either towards or away from gravity. ● Phototropism: The response where a plant grows towards or away from the direction from which light is coming at. Tropisms can either be Positive or Negative. A tropism that is positive is in the direction of the stimulus, whilst a negative tropism is one where the direction is away from the stimulus. 2 Explain the chemical control of plant growth by auxins including geotropism and phototropism in terms of auxins regulating differential growth. Auxin Auxin is a type of chemical which control the tropism. To be more specific, Auxin is a plant hormone. Auxin is produced by cells at the tip of the roots and shoots of plants. At the top 0f the cell, there is a section where cells are being produced by dividing so the shoot grows.
The growth of the cells are controlled by “auxin”. Auxin is what allows a plant to go, so if you cut off the auxin, the plant will not grow. Auxin in Phototropism If sun shines on the right side of plants shoot, auxins will accumulate on the dark opposite left. Auxin accumulating on the dark side will obviously grow at a faster rate than the left side of the plant. When the right side of the plant is growing faster than the left side, the plant starts to bend towards the right side towards sunlight. We call this phototropism. ( Might need editing) Auxin in Geotropism Auxin like to settle at the bottom of the root. This does not make the tip of the root grow taller. Interestingly, Auxin prevents the cells at the bottom tip of the root from growing, allowing the cells in the middle to grow faster. When the cells in the middle of the root start to grow faster, what this does it that it pushes the root deeper into the soil and the root gets longer. Additionally, the root grows in the direction of gravitational pull. We call this geotropism. Whats good about Phototropism? ● Flowers can be seen by insects for pollination ● Plants grow taller, which is better for seed dispersal ● Leaves are exposed to more sunlight and more photosynthesis is available What is good about Geotropism? ● More water can reach the roots ● Roots have larger surface area for more diffusion and osmosis ● By growing very deep into the soil, the roots are able to be held very firmly.