Plant Responses

Question 1

Where does growth in a plant occur?

Answer 1

In the meristems where there are groups of immature cells still capable of dividing

Question 2

What are the 4 types of meristems and where are they found in the plant and what type of growth do they allow?

Answer 2

1. Apical meristems = found at tips/apex of roots and shoots = roots/shoots getting longer
2. Lateral bud meristems = give rise to side shoots/ branches
3. Lateral meristem = cylinders of meristem tissue near outside of roots/shoots = them getting wider
4. Intercalary meristems = between the nodes where leaves and buds branch off the stem = responsible for shoots getting longer between the nodes

Question 3

Define trophic responses

Answer 3

-A DIRECTIONAL growth response to an external environmental stimulus, either positive (towards) or negative (away) from that stimulus, brought about by plant hormones

Question 4

Why does much of the work on tropisms involve very young seedlings?

Answer 4

-easy to work with and manipulate
-they grow and respond quickly to environmental changes
-responses are easy to see and measure
-(monocotyledonous plants only have one shoot so is easier to observe)

Question 5

Explain what the 5 tropic responses are

Answer 5

1) Phototropism = a response to light, ensures plant gets access to enough light to maximise photosynthesis, glucose production, growth

2) Geotropism = a response to gravity, shoots show negative geotropism and roots show positive geotropism. This ensures that when a seed germinates the shoot and root grow in the right directions

3) Hydrotropism = a response to moisture, root tips tend to grow towards damper areas of soil, increasing their access to water

4) Thigmotropism = a response to touch, this is important in climbing plants as it allows them to detect a support and curl around it

5) Chemotropism = a response to chemicals, e.g pollen tubes grow down the flowers stigma towards the ovules due to chemotropism

Question 6

What is meant by a nastic response in plants?

Answer 6

-the response is NON directional (doesn’t respond towards or away from the stimulus) as the whole plant responds

Question 7

Give an example of a plant with a nastic response and what the adaptive significance of this type of response are

Answer 7

-E.g mimosa pudica or Venus fly trap

-may scare off leaf eating insects/cause them to be knocked off
-may be involved leaves closing as the sun goes down to reduce transpiration

Question 8

Explain some of the chemical defences that plants use to reduce the effects of herbivory and other stresses

Answer 8

1) Alkaloids = chemicals with bitter taste/ poisonous that deter or kill herbivores e.g nicotine
2) Tannins = bitter taste, can bind to proteins in the gut making it hard to digest
3) Pheromones (signalling chemicals that produce a response in another organism) = when corn plants are being eaten by caterpillars can produce pheromones which attract parasitic wasps to lay their eggs in the caterpillars, killing them
4) chemicals that act as antifreeze to stop plant cells being destroyed

Question 9

Explain what plant hormones are and how they work in plants/ what they have an effect on

Answer 9

-plant hormones are chemical messengers that are transported from the cells where they are made to other parts of the plant where they have an effect. They can move from cell to cell by diffusion. When they reach their target cells, they bind to protein receptors on the plasma membrane
-some plant hormones amplify each other’s effects, whilst some cancel them out
-they can influence cell division, cell elongation and cell differentiation

Question 10

What are the key roles of the plant hormone auxin?

Answer 10

-promote cell elongation (more made on dark side)
-maintains apical dominance
-inhibits side shoots
-inhibits leaf fall
-stimulates ethene production

Question 11

What are the key roles of the plant hormone gibberellins?

Answer 11

-promotes seed germination
-causes stem elongation

Question 12

What are the key roles of the plant hormone cytokinins

Answer 12

-promotes cell division
-inhibits leaf aging

Question 13

What are the key roles of the plant hormone abscisic acid?

Answer 13

-inhibits seed germination
-close stomatal closure
-stimulates cold protection e.g antifreeze chemicals made

Question 14

What are the key roles of the plant hormone ethene?

Answer 14

-promotes fruit ripening
-promotes leaf fall

Question 15

Explain the step by step process of how a seed germinated so it can start growing

Answer 15

-seed absorbs water
-this activates the embryo plant inside
-this produces gibberellins which activate genes coding for amylase and proteases
-the proteases are used to break down proteins in the seed, and the amino acids are then used to make new enzymes
-the amylase is used to break down starch stored in the seed to produce glucose
-the glucose released is then respired to produce ATP
-the ATP is used for protein synthesis/ cell division/ growth
-the embryo plant breaks through the seed coat and a shoot tip is formed at one end and appears through the soil, whilst a root tip forms at the other end and grows into the soil

Question 16

What experimental evidence is there to support the role of gibberellins in the germination of seeds?

Answer 16

1) mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins, which did not germinate. If gibberellins applied externally they germinate normally

2) if gibberellin biosynthesis inhibitors are applied to seeds they don’t germinate as can’t make the gibberellins needed to break dormancy. If the inhibition is removed or gibberellins applied the seeds germinate

Question 17

Where is the hormone auxin made in the plant and what effect does it have on the plants structure?

Answer 17

-it’s made in the cells of the shoot tip and root tip (meristems), moving by diffusion/ active transport to where they act
-auxin affects the plasticity (flexibility) of the cell walls, so if present the cell wall can stretch more easily, so the cell can grow

Question 18

Explain the step by step process of how auxin works

Answer 18

1. It binds to specific receptors on plant cell walls
2. This causes the release of hydrogen ions into the plant cell wall
3. This lowers the pH of the plant cell wall to around pH 5.0
4. This makes cellulose more flexible, increasing plasticity
5. This activates “expansin” enzymes
6. These allow the cells to elongate

-once the cell matures auxin is destroyed, the pH rises again, expansin is inactivated, and the cell wall becomes a fixed shape/ size and growth is complete

Question 19

In which area of a plant is the most auxin produced and explain why/ it’s effect on the plant

Answer 19

-more auxin produced on the shaded/dark side of the plant
-if shoot tips are covered/ completely in dark, auxin from the shoot tip is made in higher concentrations and moves downwards in an equally distributed way
-this causes the whole stem to grow upwards

Question 20

Explain the changes in direction of shoot growth related to auxin distribution, and what auxin distribution is affected by

Answer 20

-Auxin distribution affected by light intensity
-equal auxin distribution = shoot grows straight up
-unequal auxin distribution = side with the higher concentration grows a lot more then the other side, causing the shoot to bend to one way

Question 21

State what type of phototropism and geotropism plant shoots are

Answer 21

-positively phototrophic = grow towards light
-negatively geotrophic = grow away from gravity

Question 22

Explain why in an experiment a control plant which was illuminated from all sides grew straight upwards whereas the experimental plant which was only illuminated from one side grew to one side

Answer 22

-control = uniform light shone on to the plant, so there is uniform distribution of auxin down the whole shoot tip. All cells underneath shoot tip elongate at an equal rate causing the shoot to grow straight upwards

-experimental = unilateral light shone from one side, causes more auxin to pass from the tip to the shaded side. Higher auxin concentration in these cells causes them to elongate more than the cells on the illuminated side, causing the shoot to bend towards the light

Question 23

What effect does auxin have on the roots of a plant compared to the shoots?

Answer 23

-with roots, very low concentrations of auxin stimulate growth, but then as concentration increases growth is inhibited

Question 24

Explain what type of phototropism and geotrophism plant roots are

Answer 24

-negatively phototrophic = grow away from the light
-positively geotrophic = grow towards gravity

Question 25

Explain what apical dominance is and what effect auxin has on this

Answer 25

-high concentrations of auxin suppress the growth of side shoots, resulting in apical dominance
-growth of the main shoot is stimulated by auxin produced as the tip, so it grows quickly
-growth of the lateral shoots is inhibited by the auxin as it moves down the stem

Question 26

What experimental evidence is there for the effect of auxin on apical dominance?

Answer 26

-if the apical shoot is removed, the auxin producing cells are removed, so no auxin is present
-therefore the side shoots grow freely and quickly
-if auxin is applied artificially to the cut stem apical dominance is restored and the side shoots stop growing again

Question 27

What are the advantages of apical dominance in a plant?

Answer 27

-allows plants to grow tall quickly
-helps them to absorb more sunlight for photosynthesis, so they can out compete their neighbours
-plants that can establish faster than their neighbours have a survival advantage

Question 28

Explain the effect of the plant hormone gibberellins on stem elongation

Answer 28

-they affect the lengths of internodes- the regions between the leaves on a stem
-dwarf plants have the alleles for low gibberellin production, and tall plants have the allele for high gibberellin production

Question 29

Why is it useful if farmers are able to breed many crop varieties with shorter stems, by inhibiting the gibberellin synthesis pathway?

Answer 29

-shorter plants are less likely to be damaged or fall over in high winds/ heavy rainfall
-financially better for the farmer if the plant puts more energy resources into developing the grain rather than the stem

Question 30

Define synergism

Answer 30

When plant hormones work together, complementing each other, giving a greater response then each one would on their own

Question 31

Define antagonism

Answer 31

When plant hormones have opposite effects to each other e.g one promotes growth and one inhibits growth. It is the balance between them that then determines the plant responses

Question 32

What effect does a plant being grown in the dark have on its growth

Answer 32

-if a plant is in the dark, it needs to grow rapidly upwards to try reach a light source in order to photosynthesise
-following germination, seedlings that break through the soil first will have less competition from other seedlings so will be able to establish themselves more easily
-once the plant reaches the light a slowing down of the upwards growth is valuable, allowing for resources to be used by growing leaves and strengthening the stem

Question 33

What plant hormone is responsible for the extreme elongation of the internodes when a plant is grown in the dark?

Answer 33

Gibberellins

Question 34

Why are plants that are forced to grow in the dark pale in colour?

Answer 34

-because the plant is deprived of light, so little chlorophyll is made

Question 35

Which crop plant is often forced to grow in the dark and why?

Answer 35

-rhubarb
-because we eat the stems so we want the stem to grow quickly