5.4 Plant Hormones

The flashcards cover the specification point 4.5.4 Plant Hormones (Biology Only). This includes: 4.5.4.1 Control & Coordination, Required Practical 8 -- Plant Responses, and 4.5.4.2 Use of Plant Hormones.

What is a tropism?

The response of a plant to a specific stimulus.

What is phototropism?

The response of a plant to light.

What is gravitropism/geotropism?

The response of a plant to gravity.

Name 3 plant hormones.

• Auxin

• Gibberellins

• Ethene

• Plants produce hormones to coordinate and control growth and responses to light (___________) and gravity (__________ or __________).

• Unequal distributions of _______ cause unequal growth rates in plant roots and shoots.

• phototropism

• gravitropism

• geotropism

• auxin

Most plants show __________ ______________ because they grow towards the light source.

positive phototropism

Positive Phototropism in Plant Shoots:

• The plant is exposed to ______ on one side.

• ________, a hormone, moves to the ________ side of the shoot.

• _______ stimulates cells to grow _____ here.

• This means the shoot bends ______ the light.

• The plant receives ______ light, meaning ____________ can occur at a ______ rate.

• light

• Auxin

• shaded

• Auxin

• more

• towards

• more

• photosynthesis

• faster

Most shoots show ________ ___________ as they grow away from gravity.

negative gravitropism

Negative Gravitropism in Plant Shoots:

If a shoot is horizontal:

• Auxin moves to the ______ side.

• The cells of the shoot grow ______ on the side with most auxin, so it stimulates cells to grow _____ here.

• This makes the shoot _______ and grow away from the _______.

• This is beneficial as light levels are likely to be higher ________ ______ from the ground.

• lower

• more

• more

• bend

• ground

• further away

Most roots show ________ ______________ as they grow towards gravity.

positive gravitropism

Positive Gravitropism in Plant Roots:

If a root is horizontal:

• Auxin moves to the _______ side.

• The cells of the root grow more on the side with ______ auxin, so it stimulates cells to grow on the ______ side.

• This makes the root bend and grow _________.

• This is beneficial as there are more likely to be increased levels of _______ and ________ lower down, and it provides _______ for the plant.

• lower

• less

• upper

• downwards

• water

• nutrients

• stability

When the auxin distribution becomes equal on both sides it grows ________ in that direction.

straight

What do gibberellins do?

Initiates seed germination

What does ethene do?

• Controls cell division

• Controls ripening of fruits

What is the independent variable in RP 8?

Light levels

What is the dependent variable in RP 8?

Height of seedling

What are the control variables in RP 8?

• Amount of water that seeds are watered with

• Type of seed

• Number of germinated seeds

What are the sources of error in RP 8? (3)

• Temperature could also affect growth

• Sample size for each environmental condition could be too small

• Seedlings in the same petri dish might not receive the same exposure to sunlight

In RP 8, which group of seedlings would you expect to grow the most and why?

• Full sunlight

• Light is needed for photosynthesis

  • These plants don’t have light intensity as a limiting factor

In RP 8, why is it necessary to find the mean height of the seedlings?

• Some seedlings might be shorter/longer than others in the same dish.

• Calculating a mean makes the values more representative and allows comparison between groups.

In RP 8, why must you hold the seedlings straight when you are measuring them?

• Seedlings bend towards the light when they grow (phototropism)

• Holding them straight allows you to measure their full length

RP 8 — Plant Responses:

1. Incubate 10 seeds in 3 petri dishes and allow them to __________.

• Number of ___________ seeds and amount of water should be the same.

• Water daily with _______ __________ of water to each dish.

2. Measure the _____ ________ of each seedling in mm using a _____.

4. Put one petri dish in each place:

• _____ ___________

• ______ __________

• __________

5. Measure the height of every seedling for _ days.

• Remove ________ and calculate the ______ ________ for each day.

• germinate

• germinated

• equal amounts

• initial height

• ruler

• full sunlight

• partial sunlight

• darkness

• 5

• anomalies

• mean height

What are auxins used for? (3)

• Weed killers

• Rooting powder

• Tissue cultures

What is ethene used for?

• Food ripening

What are gibberellins used for? (3)

• Ends seed dormancy

• Promotes flowering

• Increases fruit size

Why can auxins be used for rooting powders?

It causes a new plant to grow very quickly.

Why can auxins be used for weed killers?

• It causes cells to grow at a rapid rate

  • Causes plant death

Why is ethene used for food transport?

• Fruit can be harvested while it is unripe

• Then it can be ripened just before it is sold

• A student investigated the effect of gravity on the growth of bean seedlings.

• The student set up apparatus A and apparatus B.

• Figure 2 shows both sets of apparatus

Give the reason why the student placed both sets of apparatus in the dark. [1 mark]

To prevent the direction of light affecting the results.

• Students investigated the response of plant shoots to one-sided light box.

• Figure 13 shows how the students set up three experiments.

Suggest two control variables the students should have used in their investigation. [2 marks']

• Same volume of water

• Same temperature

• Students investigated the response of plant shoots to one-sided light.

• Figure 13 shows how the students set up three experiments.

Describe how experiment B and experiment C acted as controls for the investigation. [2 marks]

• B shows that the response in A is due to light as B has no light.

• C shows that the response in A is due to one-sided light as C has light from all directions.

• Auxin is a plant hormone.

• Auxin is made in the shoot tip.

• Scientists investigated the role of auxin in the response of shoot tips to light.

• This is the method used:

  • Grow four seedlings in the dark for a few days.

  • Cut the tip off the shoot of each seedling.

  • Place each shoot tip on a small block of agar jelly.

  • Place the shoot tips and agar in different conditions as shown in Figure 14.

  • After 24 hours, measure the mass of auxin in the agar blocks.

• The numbers under each block show the mass of auxin that diffused into the blocks from the shoot tips.

• The mass of auxin is given in arbitrary units.

A scientist made a hypothesis:

‘Light causes auxin to move from the side of the shoot nearest to the light to the side furthest from the light.’

Describe the evidence from Figure 14 which supports the hypothesis. [3 marks]

• In F there is more auxin in the side furthest from the light

• In G there is the same mass of auxin in each side

• In F auxin can move through the shoot but in G the glass prevents this

A farmer sprayed an apple tree with gibberellin.

Suggest two reasons why the farmer sprayed the apple tree with gibberellin. [2 marks]

1. To initiate flowering

2. To grow bigger fruits