Topic 6—A: Stimuli and Responses- 3. Responses in plants

Question 1

How do plants increase their chances of survival?

Answer 1

• by responding to changes in their environment

Question 2

Examples of plants responding to changes in their environment

Answer 2

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

Question 3

Definition of tropism

Answer 3

• It is the response of a plant to a directional stimulus (a stimulus coming from a particular direction)

Question 4

How do plants respond to stimuli?

Answer 4

• by regulating their growth

Question 5

What is a positive tropism?

Answer 5

• Growth towards the stimulus

Question 6

What is a negative tropism?

Answer 6

• Growth away from the stimulus

Question 7

Phototropism

Answer 7

• The growth of a plant in response to light
• Shoots are positively phototropic and grow towards light
• Roots are negatively phototropic and grow away from light

Question 8

Gravitropism

Answer 8

• The growth of a plant in response to gravity
• Shoots are negatively gravi tropic and grow upwards
• Roots are positively gravitropic And grow downwards

Question 9

Auxins

Answer 9

• plants respond to directional stimuli using specific growth factor- these are hormone-like chemicals that speed up or slow down plant growth
• Plant growth factors are produced in the growing regions of the plant (e.g. shoot and root tips.) and they move to where they’re needed in the other parts of the plant.
• auxins are produced in the tips of shoots and diffuse backwards to stimulate the cell just behind the tips to elongate- this is where cell walls become loose and stretchy so the cell gets longer
• if the tip of a shoot is removed, no auxin will be available and the shoot stop growing
• Auxins stimulate growth in shoots but higher concentrations inhibit growth in roots

Question 10

Indoleacetic acid

Answer 10

• An important auxin that produced in the tips of shoots and roots in flowering plants
• It’s moved around the plant to control tropisms
• It moves by diffusion and active transport over short distances and via the phloem over long distances from the shoots to the roots
• This results in different parts of the plant having different concentrations of IAA
• The uneven distribution of IAA means there’s uneven growth of the plant

Question 11

IAA (roots)

Answer 11

• Inhibits growth (elongation)
• inhibit elongation in the roots
• i AA redistributes to the gravity/lower side of the root so only the top side elongates

Question 12

IAA (shoots)

Answer 12

• Growth
• IAA causes elongation in the shoots
• IAA redistributes to the shaded side of the shoots so only the shaded side elongates

Question 13

IAA- phototropism

Answer 13

• IAA moves to the more shaded parts of the shoots and roots so there’s uneven growth
  Shoots
• IAA concentration increases on the shaded side- cells elongate and the shoot bends towards the light
  Roots
• IAA concentration increases on the shaded side- growth is inhibited so the root bends away from the light

Question 14

IAA- gravitropism

Answer 14

• IAA moves to the underside of shoots and roots so there’s uneven growth
  Shoots
• IAA concentration increases on the lower side cells elongate so the shoot grows upwards
  Roots
• IAA concentration increases on the lower side- growth is inhibited so the root grows downwards