Photosynthesis and Plant Respones Section 3

Biology iGCSE topic 6 plants as organisms, photosynthesis and plant responses section 3

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  • Created by: emma
  • Created on: 24-11-12 16:48

Targets for Section 3

  • Recall the meaning of phototropism, hydrotropism and gravitropism.
  • Recall that plants produce hormones that coordinate and control growth
  • Recall that the hormone controlling tropisms is called auxin. 
  • Explain how shoots grow towards the light but roots do not respond to the direction of light. 
  • Explain how roots grow towards moisture, but shoots do not respond to the direction of water.
  • Explain how auxin causes different rates of growth in the cells in root and tem tips that causes them to grow in different directions. 
  • Interpret experimental data on the effects of hormones 
  • Recall agricultural use of plant hormones as weedkillers and to stimulate rooting. 
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Plant Responses

When you plant a seed, do you think about which way up it should go?

What would you say if the roots started coming up out of the ground and the stem grew down?

Why do plants have to grow towards the things they need?

Plants are unable to move long distances so they need to grow towards the things they need.

Plants need to grow towards light and water so they can photosynthesise and grow as fast as possible. They also need to grow the right way up with the shoots growing upwards and the roots growing downwards.

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A tropism is a growth movement of a plant towards or away from an external stimulus.

The three main stimuli that plants responds to are:

- Light     =     phototropism

- Gravity  =      gravitropism

- Water   =      hyrdrotropism

 Tropic responses are caused by plant hormones; the most importatnt one is auxin.                  

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Phototropism - Response to Light

Shoots grow towards light - they are positively phototropic.

Roots don't respond to light - they are not phototropic.

The hormone auxin, always moves away from the light
Where it is most concentrated, it causes cells to elongate and divide faster, so the plant grows.

Since the light naturally comes from above the auxin becomes concentrated below the tip of the shoot. It causes cells elongate and divide and so the plant grows upwards.

If the light comes from the side, the auxin moves to the shaded side of the shoot.


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Gravitropism - response to gravity

Shoots grow away from the direction of gravity. They are negatively gravitropic.

Roots grow towards the direction of gravity - they are positvely gravitropic.

The hormone auxin aways moves down, in the direction of gravity,

In shoots, auxin causes cells to elongate, so the shoot grows upwards.

In roots the reverse happens - a low concentration of auxin causes elongation of the cells, but a high concentration of auxin inhabits it.

This can be seen when a plant is put on it's side.

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Hydrotropism - response to water

Roots grow towards moisture - they are positively hydrotropic.

Shoots don't respond to moisture - they are not hyrdrotropic.

In root tips the hormone auxin moves away from water.

The auxin cayses cells to elongate and divide, so the root bends towards the water.

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Commercial Uses of Plant Hormones

Plant hormones increase the rate of growth by promoting cell elongation and cell division. This effect has been exploited in the horticultural industry.

Rooting hormones

Stem cuttings will naturally produce roots, given time. Adding rooting hormone speeds up the production of roots, which means the plant can be quickly established.

What is the commercial advantage to using rooting hormone?

If the plants can grow quicker you can make more money because you've got more plants to sell.


An ideal weed killer will kill the weeds but not the other plants. Hormone weed killer work by stimulating growth of the weed at a rate so fast that it cannot sustain it and dies.

Why is it commercially important to kill weeds?
Weeds compete with crop plants for water , nutrients, sunlight and space.

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