Flashcards in 5.5 - Plant and Animal Responses Deck (72)
Why do plants respond to stimuli?
To increase their chance of survival by responding to changes in their environment.
Give an example of plants responding to a biotic stress.
Herbivory – being eaten by plants.
Give an example of plants responding to an abiotic stress.
Anything that is non-living – drought, extreme cold.
Describe 3 chemical defences of plants to herbivory.
Tannins – both taste bitter & make the plant hard to digest.
Alkaloids - taste bitter, noxious smell & poisonous characteristics.
Pheromones - signalling chemicals that produce a response in other organisms.
Describe a physical defence of plants to herbivory & name a plant that does this.
Some plants fold up in response to being touched e.g. Mimosa pudica.
Why do plants fold up in response to herbivory?
May knock off the insect eating them or scare the organism eating them.
What is the name given to a plant responding to touch or contact?
What are tropisms?
A directional growth response to a stimulus.
What is geotropism, phototropism & hydrotropism?
A directional growth response to gravity, light or water.
Describe a positive and negative tropism.
Positive – grows towards the stimulus.
Negative – grows away from the stimulus.
Where are plant hormones produced? Where do they act?
Produced in growing regions & move to regions where they are needed. Specific hormones have specific shapes that only bind to complementary receptors on the plasma membrane of target cells.
Where does cell division take place in a plant?
Only in meristematic regions of the plant – tip of shoots & roots, cambium & pericycle.
How does growth occur elsewhere in the plant? What is this called?
Growth occurs by increasing the size of the cell by making cell walls loose & stretchy & absorbing water into the vacuole. This is called cell elongation.
What effect does Indoleacetic acid (IAA) have on the growth of shoots & roots?
Stimulates elongation in shoots. Inhibits elongation in roots.
Where is IAA produced?
Meristematic tissue – tips of shoots and roots.
How is IAA moved around the plant to control growth over short & long distances?
Over short distances - moves by diffusion & active transport.
Over long distances - moves via the phloem.
Where does IAA move to in shoots in response to light?
To the shaded areas to stimulate elongation in these cells.
Where does IAA move to in response to gravity?
It accumulates on the lower side & stimulates cell elongation in shoots or inhibits cell elongation in roots.
What are the roles of plant hormones?
Role in: Leaf loss. Stimulated by: Ethene. Inhibited by: Auxins.
Role in: Seed germination. Stimulated by: Gibberellins. Inhibited by: Abscisic acid.
Role in: Stomatal closure. Stimulated by: Abscisic acid. Inhibited by: Not inhibited, only stimulated.
Role in: Apical dominance. Stimulated by: Auxins stimulate the growth of the apical bud. Inhibited by: Auxins inhibit the growth of side shoots from the lateral buds.
Role in: Stem elongation. Stimulated by: Gibberellins. Inhibited by: Not inhibited, only stimulated.
What are the commercial uses of plant hormones?
Hormone: Ethene. Commercial use: Fruit ripening. Effect: Stimulates enzymes that - break down cell walls; break down chlorophyll; convert starch into sugars.
Hormone: IAA (auxin). Commercial use: Rooting powder. Effect: Stimulates cuttings to grow roots; lots of the same plant (cloning) can be grown quickly & cheaply.
Hormone: High levels of IAA. Commercial use: Selective weed killer. Effect: Stimulates the plant shoots of broad-leaved plants (dandelion) to grow so rapidly that it exhausts the plant & it dies. IAA has little effect on narrow leaved plants.
Explain the control of leaf loss by plant hormones.
Ethene stimulates abscission cells (layer of cells at the bottom of the leaf stalk) to expand, breaking cell walls & cause the leaf to fall.
Auxins are produced by young leaves & inhibit leaf loss.
Explain the control of seed germination by plant hormones.
Gibberellins are produced by the embryo. Gibberellic acid (GA) stimulates production of amylase (and other enzymes) by the aleurone layer. Amylase breaks down starch to maltose in the endosperm. Maltose is then broken down into glucose (by maltase) & used to provide energy for growing embryo.
Explain the control stomatal closure by plant hormones.
Abscisic acid binds to receptor on guard cell membrane. The receptors activate a cascade of events that result in specific ion channels opening. Ions move out of the guard cell, raising the water potential. Water leaves guard cells by osmosis. Guard cells become flaccid & stomata close.
What would trigger the stomata to close?
When there is too much water loss through transpiration or in response to detecting an invading pathogen.
Darkness / night time when no photosynthesis takes place so no need for gas exchange.
What is apical dominance?
The growth of the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems. Apical dominance prevents side shoots from growing.
In what way does apical dominance help a plant to survive?
Allows a plant to grow very fast, past the smaller plants where there is competition for light. Saves energy as prevents the side shoots from the same plant competing with the shoot for light.
Why do side shoots grow when you remove the apical bud?
The apical bud produces auxins. If removed side shoot growth will no longer be inhibited and start growing by cell division & elongation.
Why do tall trees have large side shoots towards the base?
Auxins become less concentrated as they move away from the apical bud to the rest of the plant. At the base of the tree there is a very low auxin concentration and so side shoot growth is not inhibited.
Why are dwarf varieties of plants short?
A mutation will result in a non-functional gene for gibberellin. Without this hormone, stem elongation cannot be stimulated.