P1 Plant Responses

Question 1

What are the two types of plant responses to stimuli?

Answer 1

1. Abiotic stress - caused by abiotic factors such as temperature, light or water. Eg. when less water is available plants close their stomata to reduce the rate of transpiration.
2. Herbivory - will respond with either physical defences, chemical defences or they respond to touch (mimosa plant - folds it’s leaves when touched).

Question 2

What are a plants physical defences to herbivory?

Answer 2

Producing thorns or spikes

Question 3

What are a plants chemical defences to herbivory?

Answer 3

1. Alkaloids:
   - eg. tobacco plants produce an alkaloid called nicotine.
   - to most herbivores, alkaloids are toxic, resulting in the herbivores dyeing, enabling the plant to survive and reproduce.
2. Pheromones:
   - trigger nearby plant leaves to produce more callose
   - therefore these leaves become harder for herbivores to eat enabling to plant to survive and reproduce.
3. Tannins:
   - acidic chemicals
   - most tannins are toxic to herbivores

Question 4

What is a tropism?

Answer 4

When part of a plant has a directional growth response to a stimulus:
- the plant will either grow towards the stimulus, or away from it
- therefore tropisms are enabled by cell elongation, which is controlled by auxins

Question 5

Describe phototropism

Answer 5

• In shoot tips, auxins concentrates in cells that are not facing the light (because a higher concentration of auxins stimulates cell elongation in shoot tips).
• This causes the shoot tips to bend towards the light.

Question 6

Describe geotropism

Answer 6

• In root tips, auxins concentrates in cells that are facing gravity (a higher auxin concentration inhibits cell elongation in root tips).
• This causes the root tip to bend towards gravity.

Question 7

What are plant hormones?

Answer 7

• Signalling molecules that enable plants to respond to changes in the environment.
• They enable plants to: avoid abiotic stress (eg. cold or heat), avoid being eaten by predators and access resources such as light, minerals and water.

Question 8

What are the roles of auxins?

Answer 8

1. Stimulates cell elongation.
2. Inhibits leaf loss in deciduous plants (plants that loose their leaves) in spring and summer. This ensures plants are able to photosynthesise and build up a glucose store for autumn and winter.
3. Maintains apical dominance (when the tip of the plant’s stem is dominant over it’s lateral branches). Higher concentrations of auxins in the tip stimulates the plant to grow upwards, while inhibiting lateral growth. Removing the plants tip causes the lateral branches to grow.

Question 9

What are the roles of gibberellins?

Answer 9

1. Stimulates seed germination:
   - When a seed absorbs water the embryo inside the seed produces gibberellins.
   - These gibberellins activate genes that code for amylases and proteases.
   - These enzymes break down the food sources inside the seed into glucose.
   - Glucose is transported back to the embryo to provide energy for it’s growth.
2. Stimulates cell stem elongation:
   - It does so by stimulating cell elongation and cell division in the stem.
3. Stimulate stomatal closure when there is a lack of water, ensuring plants don’t loose even more water by transpiration.

Question 10

What are the roles of ethene?

Answer 10

1. Stimulates leaf loss in deciduous plants in autumn (has the opposite effect of auxins).
2. Stimulates fruit ripening in late summer.

Question 11

Differences between plant and animal hormones

Answer 11

1. Plant hormones can be produced in many plant tissues, while animal hormones are only produced in endocrine glands.
2. Plant hormones are transported cell to cell, while animal hormones are only transported in the blood stream.
3. Plant hormones act on most cells, while animal hormones only act on specific cells.
4. The response triggered by plant hormones is slower than the response triggered by animal hormones.

Question 12

What is the commercial use of ethene?

Answer 12

Control fruit ripening, as ethene stimulates fruit ripening:
1. Farmers harvest fruit before it is ripe, because hard, unripe fruit is easier to transport than soft, ripe fruit meaning they won’t get as damaged.
2. Once they have arrived and are ready to be sold, the unripe fruit is exposed to ethene gas, which stimulates ripening.

Question 13

What are the commercial uses of auxins?

Answer 13

1. To grow cuttings, as auxins stimulate shoot growth:
   - Cut a small piece from a plant stem (the cutting).
   - Dip the cutting into rooting powder that contains auxins.
   - Plant the cutting into the soil, allowing a new plant to grow.
2. As a weedkiller, as auxins stimulate shoot growth, however when applied at high concentrations, auxins causes uncontrolled shoot growth, causing the plant to die:
   - apply a high concentration of auxins to a weed plant
   - as a result, the plant grows too fast and dies
   - the crop plant is then freed from its competition and grows normally