Explain why plants need to respond to their environment 🌿
Do you still remember the content in A-level biology ?
Like animals,plants must also need to respond to external stimuli. This is important to:
- Avoid predation.
- Avoid abiotic (non-living) stress.
- Maximize photosynthesis
- Obtain more light, water and minerals.
- Seed set/seed dispersal.
- Ensure germination in suitable conditions/pollination.
Tropism
A directional growth response in which the direction of the response is determined by the direction of the external stimulus. Tropisms maybe positive (a growth response towards the stimulus) or negative (a growth response away from the stimulus).
Different Types of Tropism
- Phototropism (light) – shoots grow towards light– they are positively phototrophic.
- Geotropism (gravity) – roots grow towards the pull of gravity.
- Chemo ropism (chemicals) – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilization can take place.
- Thigmotropism (touch) – shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
Revision of hormone
Do you still remember what is meaning of hormone in A-level biology ?
Hormones,also referred to as plant growth regulators, coordinate plant responses to environmental stimuli. Like animal hormones, plant hormones are chemical messengers that can be transported away from their site of manufacture, by active transport, diffusion and mass flowing the phloem sap or in xylem vessels, to act at target cells or tissues of the plant. They bind to receptors on the plasma membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular cells. This specific binding makes sure that the complementary shapes on the membranes of particular cells.
Different types of plant hormones
Auxin 🌱
Do you still remember what is Auxin in A-Level biology?
Auxins (such as IAA) are produced at the apex.Auxin stimulates shoot growth by causing cell elongation – process is called acid growth hypothesis.
- Auxin stimulates an ATPase enzyme in plasma membrane.
- Hydrogen ions are actively pumped into the cell wall, decreasing the pH.
- Wall-lossening enzyme (expansions) become active.
- Bonds break within the cellulose (including hydrogen bonds).
- Cell wall becomes less rigid allowing cell elongation.
Phototropism 🔦
In a phototropic response, a shoot bends towards a light source. This happens between the shaded side elongates faster than the illuminated side, which pushes the end of the shoot towards the light.
- A shoot is eliminated from one side.
- Auxin produced from the tip of the shoot accumulates on the shaded side.
- Cells on the shaded side elongated.
- Shoot bends towards the light(positive phototropism).
- Auxin broken down by enzyme.
References:
1.https://getrevising.co.uk/resources/ocr-f215-plant-responses
That's the end of the topic! Well Done!
Drafted by Jacob (Biology)