AQA GCSE Control and coordination in plants using hormones(Biology)
Control and coordination in plants using hormones
Plants need to control their growth and respond to changes in their environment. They do this using chemical messengers called plant hormones. These hormones move through the plant and affect how different parts grow and develop.
Hormones in Plants
Plants produce hormones to:
Control growth
Respond to light (phototropism)
Respond to gravity (gravitropism or geotropism)
Unlike animals, plants do not have a nervous system. Hormones are therefore very important for coordinating plant responses.
Auxin and Plant Growth
Auxin is a key plant hormone that controls growth in roots and shoots.
Auxin works by causing cells to grow longer (elongate). However, auxin is not always evenly distributed throughout the plant. When auxin is unevenly distributed, it causes different parts of the plant to grow at different rates.
This unequal growth allows plants to bend towards or away from stimuli such as light or gravity.
Auxin in Shoots
When the shoot is evenly illuminated
Stage 1: Auxin is produced in the tip of the shoot (seen by red dots in image below.
Stage 2: Auxin will diffuse evenly down the shoot because the shoot is evenly illuminated from above.
Stage 3: Auxin causes cell elongation evenly across the whole width of the the shoot. As a result the whole shoot grows directly upwards.
In the image below compared the length of the cells im image 2 and 3.
Auxin in unevenly illuminated shoots
1.Auxin is produced in the tip of the shoot.
2.Auxin builds up on the shaded side of the shoot
3.Cells on this side grow more
3.The shoot bends towards the light
This is called phototropism.
Auxin in Roots
Auxin in the root, experiences the effect of gravity, it moves to the lower side of the root. Look at the right hand side of the diagram below.Auxin slows down growth in root cells
The auxin will inhibit growth on the lower side of the root, but growth of the upper root will continue.
As a result the root will bend and grow downwards into the soil.
The root bends in response to gravity
This is called gravitropism (or geotropism).
If you look at the left side of the diagram you will see the red dots of auxin on the lower side of the shoot, where they cause growth, so the shoot will bend and grow upwards.
Gibberellins
Gibberellins are plant hormones involved in starting seed germination.
They help:
Break seed dormancy (allows seeds to germinate)
Trigger enzymes that help the seed start growing
Ethene
Ethene is a plant hormone that controls:
Cell division
Fruit ripening
For example, ethene causes fruits to soften and become sweeter as they ripen.
Practice Questions
1.Describe how auxin causes a shoot to bend towards light.
2.A scientist removes the tip of a plant shoot.
Predict what will happen to the plant’s growth and explain why.
3.State two roles of ethene in plants.
Factors affecting the rate of photosynthesis
Measuring & calculating rates of photosynthesis
Inverse square law and photosynthesis
Economics of enhancing the conditions in greenhouses
Investigating the effect of light intensity on the rate of photosynthesis