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Biology - Plant Sensitivity and Control (AQA Syllabus A)

In GCSE Science one topic studied is how organisms use nerves and hormones. This is the last of five quizzes on that subject and it looks in particular at how plants use hormones for both sensitivity and control.

We all recognise the main parts of a plant but have you ever wondered how the stem of a plant knows to grow upwards and how the roots know that they should grow downwards?

One of the characteristics of life on earth is sensitivity to the surroundings. In the case of animals how they react to external stimuli is easy to see, however, it is not always so obvious for plants. Plants don't have a nervous system or muscles but they do have hormones which they use to give them sensitivity to their surroundings. The sensitivity granted by the hormones enables plants to grow towards light and makes their roots push down into the soil.

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Certain plants living in areas where conditions mean that there are few nutrients have developed carnivorous habits. Not like the giant 'man-eating' plants of science fiction stories - they only eat insects. You probably know about the venus fly trap which has small hairs on its 'traps' that detect insects and close the trap, but there are others such as the sundew and pitcher plants. These plants have evolved to trap flies and other insects. They secrete digestive juices that slowly break down the insects into the nutrients that the plant needs. These are then absorbed by the plant cells and transported in the sap to where they are needed.

As well as for sensitivity, plants also use hormones to control their biological processes. Growth of plants is controlled by hormones but plants also use them to control their formation of flowers, stems and leaves, and for their shedding of leaves.

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  1. Which of the following options applies to the plant hormones, auxins?
    Auxins are a group of plant hormones that control phototropism and geotropism
  2. If a plant pot with a seedling is placed on its side with a bright light at 45 degrees to the pot and higher, what will happen?
    The stems are positively phototropic and negatively geotropic so they will grow towards the bright light source as well as upwards. The roots are positively geotropic which is why they grow downwards
  3. The ripening of fruit is a response to hormones. Certain hormones can be sprayed on plants to make the fruit all ripen at the same time. Which of the following is not a disadvantage with this?
    In this type of question you are looking for an advantage. Picking all of the fruit in one short period of time makes the process cheaper for big companies
  4. Phototropism is caused by a hormone. What is phototropism?
    Plants growing next to a window need to be turned round regularly otherwise their leaves will all lean towards the light
  5. What happens in a plant shoot that makes it grow towards the light?
    In the cells in plant shoots, the auxins are hormones that stimulate growth
  6. Some students did an experiment on plant shoots. They cut off the tips of the shoots on 10 seedlings, covered the tips of 10 more with kitchen foil and left 10 seedlings as they were as a control. The seedlings were then placed in a large box with a hole cut in one side. Which of the following would describe their results?
    Auxins are made in the growing tips of plants - no tips, no growth. In the dark, auxin levels are higher so those seedlings grew taller. The control seedlings behaved normally and grew towards the light
  7. Which of these combinations describes the tropisms of shoots correctly?
    This ensures that they grow in the right direction and towards the light
  8. What are the three main external stimuli to which plants respond?
    Some carnivorous plants respond to touch
  9. Which of these combinations describes the tropisms of roots correctly?
    This ensures that they grow downwards, anchoring the plant in the soil and finding water and nutrients
  10. Gravitropism (geotropism) is also caused by a hormone. What is gravitropism?
    If you plant a seed upside-down it doesn't matter. Gravitropism will ensure that the shoot and root grow the right way

Author: Kev Woodward

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