PLANT RESPONSES

Question 1

Directional growth

Answer 1

=tropism

Controlled by plant hormones

Question 2

Non-directional growth

Answer 2

=nastic response

Fast + controlled by temporary changes (e.g. plant turgidity- osmosis)

Question 3

Example of non-directional growth response

Answer 3

Daisies:

Open during day for pollination, close during the night for protection

Question 4

Physical responses to herbivores

Answer 4

Thorns
Spiny leaves
Tough fibrous tissue

Question 5

Chemical responses to herbivores

Answer 5

Alkaloids- bitter + toxic (e.g. nicotine), increase production in response to damage
Tannins- bitter + bind to digestive enzymes of animals + inactivate them
Terpenoids- toxic + smell deters insects

Question 6

Pheromones (response to herbivores)

Answer 6

Chemical produced by an individual to affect behaviour of another

Question 7

Example of pheromone

Answer 7

When plant is damaged, release chemicals causing nearby plants to release chemicals
Chemicals attract parasites to kill attacker (e.g. caterpillar)

Question 8

Thigmonasty (response to herbivores)

Answer 8

Response to touch

Question 9

Responses to abiotic stress

Answer 9

Leaves fall off due to low temps (no photosynthesis, so wasting energy)
Antifreeze production- stops ice crystals forming in cytoplasm (e.g. carrots)
Stomatal closure to reduce transpiration (drought)

Question 10

Types of tropisms

Answer 10

Geotropism (gravity)
Phototropism (light)
Chemotropism
Thigmotropism (touch)

Question 11

State some roles of plant hormones

Answer 11

Produced in variety of tissue
Move via AT, diffusion + mass flow in phloem + xylem
Slow response
Permanent changes
Work synergistically + antagonistically with other hormones

Question 12

Auxin effect

Answer 12

E.g. IAA
Promote cell elongation
Inhibits growth of side shoots
Inhibits leaf abscission

Question 13

Cytokinins effect

Answer 13

Promote cell division

Works synergistically with auxin

Question 14

Gibberellins effect

Answer 14

Stem elongation

Seed germination

Question 15

Absicic acid effect

Answer 15

E.g. ABA
Seed dormancy
Stimulates stomatal closure (guard cells= flat + flaccid)
Stimulates anti-freeze production

Question 16

Ethene effect

Answer 16

Promote fruit ripening

Leaf abscission

Question 17

Conclusions to plant hormone experiments

Answer 17

Shows shoots grow towards light
Tip is needed to grow + bend
Light sensors must be in tip of shoot
Auxin diffuses down shaded part of plant

Question 18

How does auxin work

Answer 18

Made in meristem cells near tips of roots + shoots
Causes cell elongation
Binds to receptors on cell surface membrane + promote AT of H+ into cell wall (=acidic)
Reduces pH= weakens cellulose bonds + activates enzymes to break cellulose bonds
Osmosis, elongation is permanent as cellulose cell wall is weakened
Plant ages + destroys auxin, cellulose cell wall; becomes rigid= permanent change

Question 19

Auxin + phototropism

Answer 19

Cells in tip contain phototropin in cell membrane
When hit by light they become phosphorylated
This causes auxin produced in shoot + tip to move to shaded side (via transporter proteins)
Cell elongates- shaded side bends

Question 20

Explain how roots are +ve geotropic + -ve phototrophic

Answer 20

Gravity causes auxin to move to lower side of roots + shoots

• in root auxin inhibits cell elongation. Cells on lower side elongates less + root grows downwards
• in stem auxin promotes elongation. Cells on lower side elongate + stem grows upwards

Question 21

Apical dominance

Answer 21

Region near top of terminal bud contains apical meristem = dominance over lateral buds= plants grow straight without wasting energy on side branches, allows them to compete for light (photosynthesis)
If tip of plant is cut off, lateral buds start growing + plant bushes out

Question 22

Control of apical dominance

Answer 22

Auxin made in aphical bud where it causes cell elongation

Auxin diffuses down lateral buds, where it inhibits them

Question 23

Why will lateral buds grow if plant is upside down?

Answer 23

Upside down= diffusion of auxin upwards against gravity, therefore auxin to lateral buds + growth

Question 24

Gibberellins + stem elongation

Answer 24

Produced in young leaves + seeds

Causes growth in cell internodes

Question 25

Auxin + Gibberellin can be...

Answer 25

Synergistic- grow tall | Antagonistic- gibberellins stimulate lateral shoots, auxin inhibits them

Question 26

Gibberellin + germination (Barley seed)

Answer 26

When seed absorbs water embryo releases gibberellins Gibbs travel to aleurone layer (membrane) of endosperm (food store) This switches on genes which code for amylase + protease (hydrolytic enzymes) Stored starch hydrolysed to glucose (using enzymes) This provides substrate for respiration so embryo can grow

Question 27

Why does leaf abscission occur

Answer 27

Less light in winter (shorter days, lower intensity) + lower temps reduces photosynthesis Water lost through leaves, fungal infections + frost= damaged plant= leaf loss Wastes energy

Question 28

Leaf abscission in deciduous plants

Answer 28

Auxin produced in young leaves, inhibits abscission Reduce light= reduced auxin concentration Ethene produced as leaves age To balance shift between two hormones, leaf abscission starts (aux + eth= antagonistic) Abscission layer of leaf have thin walls (weakened by enzymes), breaking cellulose bonds At same time- Suberin develops underneath for protection Vascular bundle sealed off Leaf blown away with wind

Question 29

When is absicic acid produced

Answer 29

Under abiotic stress (e.g. temp) | Causes stomata to close, reducing transpiration

Question 30

Absicic acid + stomatal closure

Answer 30

ABA binds to receptors on cell surface membrane of guard cells Ca2+ enters cell causing other ions to exit (e.g. K+ + Cl-) Water potential of cell increases Water leaves cell via osmosis Guard cells= flaccid (straight)= closed stomata

Question 31

Commercial use of auxin

Answer 31

Selective weed killer Rooting powder Pathonocarpy fruit development without pollination

Question 32

Commercial use of gibberellins

Answer 32

Improve fruit shape Delay fruit dropping Bring forward seed production Stopping= shorter plants= saves resources + reduces wind damage

Question 33

Commercial use of cytokinins

Answer 33

Promote cell division Stops ageing (reduced waste) Promote shoot growth in tissue culture

Question 34

Commercial use of ethene

Answer 34

Fruit ripening Fruit dropping at same time (easier harvesting) Speed up ripening Promote growth of female flowers (for pollination) Stopping increase CO2 which reduces ethene= delay fruit ripening