In I/GCSE Biology, Plant responses is commonly asked in the exam! 👍

• Plants need light for photosynthesis, can’t move to obtain the most light

1. Light coming in all directions ➡️ normally grown (A)
2. No light ➡️ stems much longer, stems grown straight up (B)
3. Light coming from direction ➡️ stems grown towards the light – unidirectional light  (phototropism) (C)

• Plant stems – positively phototropic (grow towards the light stimulus)

Do you remember the topic regarding plant in I/GCSE Biology?

Which part of the stem detects the stimulus?

• Tip of stem is the receptor for the light stimulus

Unidirectional light ➡️ unequal growth

• Cells on shaded side grow longer than side nearest to light (stem bends)
• Growth of cells = hormones called auxins
• Auxins ➡️ produced at tip of stem and are transported downwards

Unidirectional light ➡️ light in one direction only

• More auxin transported down shaded side ➡️ faster cell growth ➡️ stem bends towards light

It is important to memorize what gravitropism and hydrotropism are in I/GCSE Biology!

• Plant stems – negatively gravitropic = grow away from the direction of the force of gravity
• Plant roots – positively gravitropic = grow in the direction of the force of gravity
• Stem beneath soil will eventually find light   

Roots ➡️ help keep plant anchored in soil

• Causes unequal distribution of auxin (like phototropism)
• Auxin accumulates on underside of horizontal stem
• Cells on underside grow faster than on upper side, stem grows upwards

If root in horizontal position – grow downwards

• Cells on upper side grow faster than on underside but auxin accumulates on underside as well
• Roots grow down because the concentration of auxin that stimulates stem growth, inhibits root growth

• Root is stimulated by force of gravity AND directional water stimulus
• Root is positively hydrotropic - grows towards water.

1. Stems

• Positively phototropic
• Negatively gravitropic

1. Roots

• Positively gravitropic
• Positively hydrotropic

References:

1. "Auxins", https://i2.wp.com/orbitbiotech.com/wp-content/uploads/2018/04/phototropism.png?fit=584%2C300&ssl=1
2. "Auxins", https://betterbonsai.co.za/wp-content/uploads/2019/07/Auxin1.jpg
3. "Copy Of Plants Responses To Stimuli", https://image.slidesharecdn.com/ks4plantresponses-150907042315-lva1-app6892/95/ks4-plant-responses-17-638.jpg?cb=1441599869
4. "B1: Influences on Life GCSE Revision", http://1.bp.blogspot.com/--ap8Rvg-1ls/UJLYNfiZCVI/AAAAAAAAAb0/Glla-GpGaN0/s1600/tropism+experiment.jpg

And we're done with this topic! Well Done!

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Drafted by Alyssa (Biology)