Plant Responses

Question 1

Define tropism

Answer 1

A directional growth response in which the direction of the response is determined by the direction of the external stimulus.

Question 2

Give an example of phototropism

Answer 2

Shoots grow towards light (they are positively phototropic), which enables them to photosynthesise.

Question 3

Give an example of geotropism

Answer 3

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 (to keep cells turgid)

Question 4

Give an example of chemotropism

Answer 4

Pollen tubes grow towards chemicals given off by the ovary in a flower

Question 5

Give an example of thigmotrophisms

Answer 5

Shoots respond to touch by growing around other plants.

Question 6

Define a nastic response

Answer 6

An instant response

Question 7

What is meristematic tissue

Answer 7

Growing points in a plant where immature, undifferentiated cells are still capable of dividing

Question 8

What are plant hormones/growth factors

Answer 8

Co-ordinate plant growth, produced in one part of the plant and transported to the target cells by active transport and diffusion in the xylem and phloem vessels

Question 9

What are cytokinins

Answer 9

Growth factors which promotes cell division(mitosis) in the meristem

Question 10

What are auxins

Answer 10

Growth factors stimulating shoot growth

Question 11

How do auxins work

Answer 11

H+ is actively pumped into the cell by an ATPase enzyme.
The resulting low pH allows wall loosening enzymes to break bonds within the cellulose so that the wall becomes less rigid and can expand as water moves into the cell by osmosis
Inhibits cell growth of lateral buds
Inhibits leaf fall

Question 12

What are gibberellins

Answer 12

Growth factors which Promotes elongation of stems - mitosis cell elongation

Question 13

What is abscisic acid

Answer 13

A growth factor which Causes stomotal closure when plants are stressed by low water availability

Question 14

What is ethene

Answer 14

Growth factor which Stimulates leaf fall by increasing cellulase production in the abscission zone
Promotes fruit ripening

Question 15

What happens to a shoot when light is shone from above

Answer 15

The auxins will be spread equally along the side of the plant, spreading from the tip, causing cell elongation across the zones of elongation.

Question 16

What happens to a shoot when light is shone only on one side

Answer 16

Auxins will move down from the shoot tip, towards the shaded side of the shoot. Only those cells on the shaded side elongate and the shoot will bend towards the light.

Question 17

Why does a shoot bend towards light

Answer 17

Auxins produced in the tips of the plant and are transported down the shoot to bring about cell elongation
Photosensitive enzymes in the tip absorb blue wavelengths of light and become more active causing the auxins to redistribute towards the shaded area
Because there are more auxins on the shaded side the cells elongate more and the shoot bends towards the light.

Question 18

What are 3 roles of auxin in apical dominance

Answer 18

high concentrations of auxins inhibit lateral bud growth and low levels of auxins encourage lateral bud growth

Auxins are produced by the apical bud and move down the stem

At the top of the stem the auxin concentration is high and constantly produced, this inhibits the growth of lateral buds which remain dormant.

Further down the stem the concentration of auxins is lower, the lateral buds are stimulated to grow, this means that plants grow into a cone shape, so higher leaves don’t shade lower ones.

If the apical bud is removed, the dormant lateral buds grow due to the lack of auxin

If the apical bud is removed and auxins are placed on the cut tip, the lateral buds will remain dormant

Question 19

What is the role of Gibberellins

Answer 19

Gibberlellin brings about stem elongation by loosening cell walls which causes cell elongation.

Question 20

Why do Gibberellins cause stem elongation

Answer 20

Gibberlellin works by affecting gene expression. The hormone moves through the plasma membrane and into the cell nucleus where it binds to a series of receptor proteins eventually resulting in the breakdown of DELLA proteins allowing transcription of the gene.

(Della proteins bind to transcription factors inhibit cell division and expansion)

Question 21

Define etiolated

Answer 21

When plants are grown without a light source so continuously grow upwards in the search for light.

Question 22

Define deciduous plants

Answer 22

Plants which lose their leaves in autumn and replace them again in the spring/summer

Question 23

What is the role of ethene in senescence

Answer 23

As leafs age the production of auxin falls and the production of ethene increases.

Layer of cells called the abscission layer develops at the bottom of the leaf petiole.

Ethene stimulates the production of the enzyme cellulase which hydrolyses the bonds in cell walls in the abscission layer and the cells expand

A protective layer of cells with wax in their cell walls forms and seals off the xylem and phloem from the leaf, preventing against fungal infections.

Leaf is broken off by the wind and under goes decomposition where the nitrates from the leaf are eventually returned to the leaf via the nitrogen cycle

Question 24

Define senescence

Answer 24

Leaf loss

Question 25

Give 3 commercial uses of Auxins

Answer 25

Taking cuttings - dipping the end of the cutting in auxins promotes the growth of roots

Producing seedless fruit - treating unpollinated flowers with auxins stimulates fruit growth without fertilisation; consequently the fruit has no seeds.

Herbicides - Auxins promote shoot growth to an extent that the stem falls over and dies

Question 26

Give 2 commercial uses of Gibberellin

Answer 26

Extending shelf life of fruit - Gibberlellin delay the ripening of citrus fruit and stimulate seed germination (barley malting)

Sugar production - Gibberlellin elongate the stems of sugar canes meaning more sugar

Question 27

Give 2 commercial uses of Cytokinins

Answer 27

Keeping lettuce green - cytokinins delay leaf senesence so leafy vegetables and cut flowers stay green for longer

Micro propagation - cytokinins promote bud and shoot growth in small tissue samples taken from the parent plant

Question 28

Give 2 commercial uses of ethene

Answer 28

Ripening of the plant - Fruit is picked and shipped before it is ripe, then it is hung in rooms with ethene gas in the atmosphere to promote ripening

Encourage fruit drop - ethene is released in orchards to make all fruits ready for picking at the same time

Question 29

Define abscission

Answer 29

Shedding of a leaf

Question 30

Define apical dominance

Answer 30

The inhibition of the growth of lateral buds by the presence of an active apical meristem

Question 31

What are the stages of apical dominance

Answer 31

1- ATPase enzyme on the plasma membrane
2- H+ ions are atcively transported into the cell wall
3- The pH is then lowered
4- This stimulates expansins (wall loosening enzyme) now has optimum conditions
5- The bands between cellulose are broken
6- Walls become less rigid and can expand so cells can divide

Question 32

Define petiole

Answer 32

The leaf ‘stem’ that breaks away from main stem

Question 33

What are the stages of leaf abscission

Answer 33

1- Auxins inhibit abscission
2- Cytokinins stop leaves senescing
3- Cytokinin level drops so senescing begins
4- Leaf senescence causes auxin production to drop
5- Cells in abscission zone become sensitive to ethene
6- Ethene production increases due to decrease in auxin concentration
7- Enzyme celluclase is increased which digests cell walls in abscission zone
8- The petiole separates from the stem
9- Leaf lost

Question 34

Where does the most cell division occur

Answer 34

Near the tip

Question 35

Where does the maximum cell elongation occur

Answer 35

Just below the tip

Question 36

Define coleoptile

Answer 36

A sheath and enclosed leaf

Question 37

What is an intact coleoptile’s response to light

Answer 37

Positive phototropism

Question 38

What materials prevent the shoot growing towards the light

Answer 38

Materials not permeable to water

Question 39

Why do plants respond to stimuli

Answer 39

Helps the plants to avoid stress, to avoid being eaten and to survive long enough to reproduce

Question 40

How do plant hormones move around the plant

Answer 40

Active transport
Diffusion
Mass flow in phloem or in xylem vessels