Plant Responses

Question 1

How do plants respond to external stimuli?

Answer 1

• tannins
• alkaloids
• pheromones

Question 2

What are tannins?

Answer 2

Toxic to microorganisms and larger herbivores. They are found in the upper epidermis of leaves which makes them have a bitter taste.
In roots they prevent infiltration by pathogens.

Question 3

What are alkaloids?

Answer 3

Derived from amino acids.
It is a feeding detergent to animals as it tastes bitter.
They are located in growing tips and flowers, and peripheral layers of stems and roots.

Question 4

Pheromones

Answer 4

Chemicals that are realised by one individual and which can affect the behaviour or physiology of another.

Question 5

Phototropism

Answer 5

Shoots grow towards light, which enable photosynthesis.

Question 6

Geotropism

Answer 6

Roots grow towards the pull of gravity. This anchors them in the soil and helps them to take up water which is needed for support. [turgidity]

Question 7

Chemotropism

Answer 7

On flower pollen tubes grow down the style attracted by the chemical towards the ovary, so fertilisation can take place.

Question 8

Thigmotropism

Answer 8

Shoots of climbing plants such as ivy.
They wind around other plants or solid structure to gain support.

Question 9

explain three structural defences a plant may have to discourage consumption by herbivores.

Answer 9

Horns, spikes, barbs = cause pain or introduce poison into the predators.
Lignin - wood my, impermeable molecules that are difficult to digest.

Question 10

What is a thigmonastic response?

Answer 10

Non-directional responses to an external stimuli [nastic responses]
E.g. Mimosa Pudica responds to touch with a sudden folding of the leaves.

Question 11

Cytokinins.

Answer 11

Promotes cell division
Delays leaf senescence (leaf aging)
Overcomes apical dominance, by being applied artificially to apical buds.

Question 12

What is apical dominance?

Answer 12

It ensures that tres and plants grows tall and abstain as much light as possible.

When the auxins at the top of the plant stem cause the stem to grow upwards and stop any lateral buds from growing sideways.

Question 13

Absicisic acid

Answer 13

In the roots = to inhibits cell division and promotes stomata closure to reduce water loss.

In the seeds it inhibits germination

Question 14

Auxins

Answer 14

In shoot tips = promotes cell elongation
Roots= inhibits cell elongation
Shoots= inhibits leaf abscission [when plants drops its leaves]
Apical buds = remotes apical dominance

Question 15

Gibberellins

Answer 15

Promotes seed germination and growth of stems.
Can promote stem elongation by working with auxxi8ns
Promotes lateral shoot growth

Question 16

Ethene

Answer 16

Stimulates a fission/ fruit drop
Involved in fruit ripening

Question 17

Why are plant growth regulators called hormones?

Answer 17

Plant hormones still need to bind to receptors.
They are transported away from their site of synthesis to where they act.

Question 18

Explain the mechanism of cell elongation with auxins.

Answer 18

Auxin is released from meristematic cells at shoot tip.
The auxin diffuses down the shoot tip down a concentration gradient.
The auxin binds to specific receptors on plasma membranes of cells in the shoots. This activates the proton pump to actively transport H+ into the cellulose cell wall.
This allows the cell wall to be reduce the pH and activate expansins.
Cellulose cell wall e comes flexible and elastic and therefore water can enter cells and elongate them as it enters the vacuole.

Question 19

Explain the mechanisms of Germination involving gibberellins

Answer 19

[It requires O2, warm temps and water to activates the gibberellins]
1. Water is absorbed y seed and softens seed coat, to allow young roots [radicles] and young shoots [plumules] to break out of seed.
2. Gibberellins are activated in embryo, they diffuse from embryo to the aleurone layer. They then bind to the specific receptors on plasma membranes of cells and initiate transcription/translation of genes encoding digestive enzymes. E.g. amylase, protease. Maltese.
3. Starch in the seed is digested by the amylase into maltose, then the maltose is digested by multisession into alpha-glucose. And the proteins are digested by protease to produce amino acids.

Question 20

What is the commercial use of auxins ?

Answer 20

Taking cuttings= Taken between 2 nodes at an angle. Lower leaves removed, and cut stem is dipped in rooting powder containing auxin. Replanted in moist soil and covered with a transparent polythene bag.

To produce seedless fruit [parthenocarpy] = treat unpollinated flowers with auxin, this promotes ovule growth and auxin release in the developing fruit, no seeds remain in the fruit.

As a herbicide= promotes excessive shoot growth so stem cannot support itself, buckles and dies.

Question 21

What is the commercial uses for gibberellins?

Answer 21

Citrus fruits= applying gibberellins can delay senescence in citrus fruits, which improve shelf life.

Synergy with cytokinins = work synergistically with cytokinins to elongate and improve the shape of apples.

Brewing = speeds up germination of barley seeds, barley then turns into malt.

Sugar cane growth= gibberellins cause stem elongation between the nodes, sugar cane stores sugar in cells in the internodes, so elongation makes more available from each plant.

Question 22

What is the commercial uses of cytokinins?

Answer 22

Delay leaf senescence = prevents yellowing of lettuce leaves after they have been picked.

Plant tissue culture = used as a growth hormone in micropropagation, promotes bud and shoot growth by stimulating cell division in explants.

Question 23

What is the commercial use of ethene

Answer 23

• speeds up fruit ripening in tomatoes =, citrus fruits and apples.
• promotes fruit drop in cherry, cotton and walnut.
  -promotes lateral growth in some plants
  -restricts ethene and can prevent fruit ripening which is useful for storage and transport of bananas.