Homeostasis 3 Flashcards
Plant hormones
Why do plants produce hormones?
To coordinate and control growth, responses to light (phototropism), and responses to gravity (geotropism).
What does unequal distributions of auxin cause?
Unequal growth rates in plant roots and shoots.
What is auxin?
A plant hormone that controls growth near the tips of roots and shoots.
Where is auxin produced?
The tips.
It moves backwards to stimulate the cell elongation process which occurs in the cells just behind the tips.
What happens if the tip of a shoot is removed?
No auxin will be available and so the shoot may stop growing.
What does auxin do?
-Promote growth in the shoot
-Inhibit growth in the root
Phototropism
The growth of plant shoots towards the light.
Why do shoots bend towards the light?
-When a shoot tip is exposed to light, more auxin accumulates on the side that’s in the shade than that’s in the light.
-This makes the cells grow (elongate) faster on the shaded side, so the shoot bends towards the light
Geotropism
The growth of plant roots towards gravity.
Negative geotropism
The growth of plant shoots away from gravity.
Negative phototropism
The growth of plant roots away from the light.
Why do roots grow downwards, towards gravity?
-When a root is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
-In roots, extra auxin inhibits growth.
-So, the cells on the top side of the root elongate faster and the root bends downwards
Why do shoots grow upwards, away from gravity?
-When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
-In shoots, extra auxin promotes growth
-This causes the lower side to grow faster, bending the shoot upwards.
What are gibberellins important in?
Initiating seed germination.
What does ethene control?
Cell division and ripening of fruits.
What are auxins useful for?
Controlling plant growth:
-Killing weeds
-Growing from cuttings with rooting powder
-Growing cells in tissue culture
Auxins in weed killers
-A too high concentration of auxin can kill a plant
-They disrupt the growth patterns of of broad-leaved plants (weeds) and kill them
-But spare narrow-leaved grasses and cereals (crops)
What happens if we add rooting powder containing auxins to cuttings?
The cuttings will grow roots rapidly and start growing as new plants.
This enables growers to produce lots of clones of useful plants very quickly.
How can auxins promote growth in tissue culture?
-Tissue culture can be used to grow clones of a plant from a few of its cells
-Hormones such as auxins need to be added to the growth medium (along with nutrients) to stimulate the cells to divide and form both roots and shoots
Plant growth hormones are used in…
…agriculture and horticulture.
What is ethene used for commercially?
It is used in the food industry to control the ripening of fruit during storage and transport.
What is ethene?
A gas produced by ageing parts of a plant.
It influences the growth of the plant by controlling cell division.
It also stimulates enzymes that cause fruit to ripen.
(Some fruit will produce more ethene as it ripens)
How is ethene used in food ripening?
-Fruit is picked while it’s still unripe
-So, it’s firmer and less easily damaged
-Ethene gas is added to the fruit during transport
-So, it will be perfectly ripe as it reaches the shelves of a supermarket
How can the effects of ethene be stopped?
-Using chemicals that react with ethene to remove it from the air
-Adding chemicals to fruits that limit the amount of ethene that can be produced or block its effect
(this delays ripening on supermarket shelves)
How can gibberellins be used?
-To end seed dormancy
-To promote flowering
-To increase fruit size
What is gibberellin?
A type of plant hormone that stimulates seed germination, stem growth, and flowering.
What is seed germination?
When a seed starts to grow into a plant.
What is seed dormancy?
Seeds not germinating until they’ve been through certain conditions (e.g. a period of cold or dryness).
How can gibberellins be used to control seed dormancy?
They are used to treat seeds, making them germinate at times of the year they wouldn’t normally.
It also helps to ensure all the seeds in a batch germinate at the same time.
How can gibberellin be used to induce flowering?
-Some plants require certain conditions to flower, such as longer days or low temperatures.
-If these plants are treated with gibberellin, they will flower without any change in their environment
-It can also be used to grow bigger flowers
How can gibberellin be used to grow larger fruit?
Gibberellin can be added to fruits that do not grow large (e.g. to seedless grapes which do not grow as large as seeded grapes) to make them match the normal varieties.
Where will auxins always diffuse to?
The shaded part of a plant.
