Plant Responses

Question 1

How do plants increase their survival chances against herbivores?

Answer 1

They protect themselves using toxic chemicals, communication with other plants and physical responses

Question 2

How do some plants deter pests?

Answer 2

Plants produce alkaloids and tannins in their leaves that deter pests

Alkaloids are found in nicotine and taste bitter and are poisonous

Tannins taste bitter and interfere with digestion

Question 3

What are alarm pheromones?

Answer 3

plants release alarm pheromones to warn other plants of herbivores

This stimulates them to synthesise toxic chemicals.

Other pheromones attract predators to eat pests

Question 4

What does the mimosa pudica do in response to touch?

Answer 4

It folds up its leaves in response to touch

This helps to knock off or scare any insects

Question 5

How do plants respond to cold temperatures?

Answer 5

Plants synthesise antifreeze proteins that prevent ice crystals from forming

Question 6

What are tropisms?

Answer 6

They are a response by an organism in a particular direction caused by external stimulus

Plants show phototropism and geotropism

Question 7

What is phototropism?

Answer 7

a directional response to sunlight

Plant shoots show positive phototropism and grow towards the sun

Roots show negative phototropism and grow away from the sun

Question 8

What is geotropism

Answer 8

A directional response to gravity

Plant shoots show negative geotropism and grow against the force of gravity

Plant roots show positive geotropism and grow with the force of gravity

Question 9

what do auxins do?

Answer 9

Auxins promote cell elongation in shoots but have opposite effect in roots

One of the most important growth factors is auxins such as indoleacetic acid

Question 10

Other than auxins, what growth factors are also important?

Answer 10

Gibberellins - stimulate seed germination and flowering

Abscisic acid (ABA) - helps plants respond to environmental stress and is involved in stomatal closure

Cytokinins - stimulate cell division and cell differentiation

Ethene - stimulates flowering and fruit ripening

Question 11

What is indoleacetic acid (IAA)?

Answer 11

a type of auxin which allows plants to respond to light and gravity

Enters the nucleus and binds to promoter region of DNA

Acts as transcription factor which activates or inhibits the transcription genes for proteins involved in cell elongation of growth

Transported in phloem

Question 12

What does IAA do in shoots?

Answer 12

IAA accumulates on the shaded side of the shoot and activates genes involved in cell elongation

Activated genes are transcribed into proteins

Cell walls become looser and stretchy, causing the cells to become longer than the cells on the sunny side

Shoot bends towards the sun

Question 13

What does IAA do in roots?

Answer 13

IAA accumulates on the more shaded side but it inhibits the growth of cells

More cells on non shaded side so root grows away from sun

Question 14

How does IAA regulate geotropism?

Answer 14

IAA accumulates on the underside of shoots and roots

In shoots, IAA causes cell elongation which causes it to bend upwards against gravity

In roots, IAA inhibits cell growth which causes roots to grow downwards with gravity

Question 15

What is apical dominance?

Answer 15

The growth of the apical bud at the expense of the side shoots growing

Promoted by auxins

Question 16

How do auxins promote apical dominance?

Answer 16

Auxins prevent the growth of side shoots to save energy and prevent competition for light between the main shoot and side shoots

Less energy used for side shoots so more used so the plants grows taller and maximise sunlight absorbtion

Question 17

What happens when the apical bud is removed?

Answer 17

Auxin levels drop which causes side shoots to start growing.

If the tip of shoot is replaced by an agar block containing auxin, the growth of side shoots is inhibited

Question 18

What are gibberellins?

Answer 18

a group of plant hormones that stimulate seed germination, growth of side shoots, stem elongation and flowering

They stimulate the breakdown of starch into glucose in the seed which can be used in respiration to produce energy to grow

Question 19

What is leaf abscission?

Answer 19

Controlled by hormones of deciduous trees in winter

Trees loose leaves in colder temperatures to conserve water during when there is less photosynthesis

Question 20

How does leaf abscission occur?

Answer 20

Auxin inhibits leaf abscission and ethene promotes abscission

Auxin levels drop and ethene rises

when ethene reaches a certain levels, the abscission layer develops at the base of the leaf stalk

The cells in the layer expand in response to ethene and the cell walls break and the leaf breaks off

Question 21

How do guard cells control the closing and opening of stomata?

Answer 21

When guard cells of full of water they become turgid which pushes the stomatal pore open

When guard cells lose water they turn flaccid and close the stomatal pore

Question 22

How is the guard cell controlled?

Answer 22

Controlled by the plant hormone abscisic acid (ABA)

ABA binds to receptors on guard cell membrane causing calcium ion channels to open

Influx of calcium ions causes potassium ion channels to open and potassium ions leave the guard cell

Water potential in guard cell increases and water moves out of cell by osmosis. Guard cell becomes flaccid and pores close

Question 23

What are commercial uses of plant hormones?

Answer 23

Auxins are used in rooting powders to stimulate root growth on plant cuttings

Auxins used in herbicides by stimulating stem elongation so that the weed grows fast it cant survive

Ethene is used to control fruit ripening by breaking down cell walls and converting starch into sugars