3.4.5 Control Systems in Plants

Question 1

Why do flowering plants respond to changes in their environment?

Answer 1

Increases their chances of survival

Question 2

Give examples of how flowering plants respond to changes in environment

Answer 2

• They sense direction of light and grow towards it to maximise light absorption for photosynthesis
• Sense gravity so roots and shoots grow in right direction
• Climbing plants have a sense of touch = find things to climb up and reach sunlight

Question 3

What is tropism?

Answer 3

Response of plant to directional stimulus

Question 4

How do plants respond to stimuli?

Answer 4

By regulating their growth

Question 5

What is positive tropism?

Answer 5

Growth towards stimulus

Question 6

What is negative tropism?

Answer 6

Growth away from stimulus

Question 7

Name 4 types of tropisms

Answer 7

• Phototropism
• Gravitropism
• Hydrotropism
  
  • Responds to water
• Chemotropism
  
  • Responds to chemicals

Question 8

What is phototropism?

Answer 8

Growth of plant in response to light

Question 9

Describe how the shoots and roots respond to light

Answer 9

• Shoots are positively phototropic and grow towards light
• Roots are negatively phototropic and grow away from light

Question 10

What is gravitropism (aka geotropism)?

Answer 10

Growth of plant in response to gravity

Question 11

Describe how the shoots and roots respond to gravity

Answer 11

• Shoots are negatively gravitropic and grow upwards
• Roots are positively gravitropic and grow downwards

Question 12

What do plants use to respond to directional stimuli?

Answer 12

growth factors

Question 13

What are growth factors?

Answer 13

Hormone-like chemicals that speed up or slow down plant growth

Question 14

Where are growth factors produced?

Answer 14

In growing regions of plant (e.g. shoot tips, leaves)

& move to where they’re needed in other parts of plant

Question 15

What do auxins (growth factors) do?

Answer 15

• Stimulate growth of shoots by cell elongation
  
  • Where cell walls become loose and stretchy so cells get longer

Question 16

What does high concentrations of auxin result in?

Answer 16

Inhibits growth in roots

Question 17

What is indoleacetic acid (IAA)?

Answer 17

Important auxin that’s produced in tips of shoots in flowering plants

Question 18

Describe how IAA is moved around a plant

Answer 18

Moves by diffusion and active transport over short distances & via phloem over long distances

Question 19

What does an uneven distribution of IAA mean?

Answer 19

There’s uneven growth of plant

Question 20

Describe where IAA moves to control phototropism

Answer 20

IAA moves to more shaded parts of shoots and roots so there’s uneven growth

Question 21

Phototropism

State what happens to IAA and its effect in a shoot

Answer 21

• IAA concentration increases on the shaded side
  
  • cells elongate and the shoot bends towards the light

Question 22

Phototropism

State what happens to IAA and its effect in a root

Answer 22

• IAA concentration increases on the shaded side
  
  • Growth is inhibited so root bends away from light

Question 23

Describe where IAA moves to control gravitropism

Answer 23

IAA moves to the underside of shoots and roots, so there’s uneven growth

Question 24

Gravitropism

State what happens to IAA and its effect in a shoot

Answer 24

• IAA concentration increases on lower side
  
  • cells elongate so shoot grows upwards

Question 25

Gravitropism

State what happens to IAA and its effect in a root

Answer 25

• IAA concentration increases on the lower side
  
  • growth is inhibited so the root grows downwards

Question 26

Describe how bananas are artificially ripened before they are sold

Answer 26

• Placed in ripening rooms where temperature and humidity are raised optimum levels
• Ethene concentration is raised using a catalytic generator
• Rooms are airtight to prevent gases escaping

Question 27

Describe the role of ethene in fruit ripening

Answer 27

Ethene switch on genes for enzymes responsible for changing colour (hydrolyses), softening fruit (pectinases), increasing sugar content (amylases)

Synthesis of these enzymes require ATP and hence linked to increased respiration rate

Question 28

Describe the role of abscisic acid (ABA) on a plant

Answer 28

• Key role in plants response to environmental stress
• Closing the stomata in response to low soil water or high salinity

Question 29

When is abscisic acid (ABA) needed? Describe how ABA is transported

Answer 29

When soil water is low, ABA is transported from roots where it is synthesised up to shoots via xylem

Question 30

Describe how stomata open

Answer 30

1. H⁺ are actively transported out of guard cells causing inside to become more negative
2. Negative charge causes inward K⁺ gates to open and allows K⁺ to enter guards cells
3. K⁺ lowers water potential and water moves in by osmosis
4. Guard cells become turgid

Question 31

Describe how abscisic acid (ABA) causes the stomata to close

Answer 31

1. ABA binds to receptors = production of H₂O₂ and other reactive oxygen species (ROS)
2. ROS then triggers an increase in Ca²⁺ in cytoplasm = Ca²⁺ enters from both outside the cell and from the vacuole
   
   • ROS inhibits H⁺ pumps
3. Rise in Ca²⁺ opens anion channels with Cl^- and other negatively charged ions to leave
4. This caused inward K⁺ channels to close and outward K⁺ channels to open = causing K⁺ to leave the guard cell
5. Water then leaves guard cells via osmosis, so they become flaccid and close the stomata

Question 32

Describe how you would investigate the effect of light intensity on the direction of shoot growth

Answer 32

1. Place same amount of compost into each small transparent plastic containers (about 2 spatulas)
2. Add same volume of water to each container
3. Then add 10 cress seeds to each container, distribute them evenly
4. Wrap black card around one container and leave a square hole in the card so light can pass through
5. Leave both containers in sunlight for a day
6. Then observe the results/measure the curvature (degrees) of the plant

Question 33

Describe how you would investigate the effect of gravity on the direction of root growth

Answer 33

1. Cut blotting paper to fit into a CD jewel case & place into case
   
   • 6 cases in total
2. Moisten bolting paper
   
   • No surface water
3. Place 4 seeds in middle of moist paper, evenly spaced from each other
   
   1. None of the seeds should be close to sides of paper
   2. Close CD case and repeat for each CD case
4. Label each CD case and mark the 4 edges on the front side of each CD case: Up, Down, Left and Right
5. Using modelling clay to hold them in place, set 2 CD cases upright each in their own shallow dish with Up on top edge
6. Place 2 CD cases horizontally into 2 shallow dishes
   
   1. (Roots can’t grow in direction of gravity)
   2. Use modelling clay under the CD cases to lift them up
      
      • (Any water added can drip into dish to dry)
7. 2 CD cases set vertically in modelling clay on 2 more shallow dishes, label Up at top
   
   1. Rotate these CD cases 90° clockwise every 2 days
8. Put all of your CD cases inside a dark cupboard
9. Check seeds daily
   
   1. Add water using dropper
10. Once roots start to grow measure angle of growth daily with protractor
    
    1. ‘Down’ = 0°, left = 90°, up = 180°, right = 270°

Question 34

What does ABA do in plants cells besides closing stomatas? (linked to fruit development)

Answer 34

• ABA (is a hormone) that regulates the expression of genes that matures fruit and causes abscission (fruit to drop off)
  
  • ABA increases while the fruit is maturing, causing some fruit to drop off