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OCR A level Biology HJ > Plant responses > Flashcards

Plant responses Flashcards

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1
Q

why do all multicellular organisms need to respond to changes in the environment?

A

in order to survive — these changes are called stimuli.

Stimuli can be external or internal and are detected by receptors.

Responding to stimuli allows organisms to avoid harmful external changes whilst also maintaining an optimal internal environment for metabolic reactions.

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2
Q

define stimulus

A

A detectable change in the external or internal environment of an organism that leads to a response from the organism.

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3
Q

how do plants respond to environmental changes?

A

Plants respond to environmental changes by producing chemicals or altering their growth

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4
Q

what are some chemical defences to external stimuli?

A
  • tannins
  • alkaloids
  • pheromones
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5
Q

how do tannins act chemical responses to external stimuli?

A

Tannins — toxic to microorganisms and herbivores

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6
Q

how do alkaloids act chemical responses to external stimuli?

A

Alkaloids — found in the growing shoot tips and the peripheral layers of stems and roots. They are derived from amino acids and taste bitter, therefore deterring animals from eating the plant

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7
Q

how do pheromones act chemical responses to external stimuli?

A

Pheromones — chemically released by plants which can affect the behaviour or physiology of other organisms

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8
Q

what is meant by tropism?

A

A directional growth response in which the direction of the response is determined by the direction of the external stimulus.

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9
Q

what are the 4 different types of tropism?

A
  • phototropism
  • geotropism
  • chemotropism
  • thigmotropism
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10
Q

what is meant by phototropism?

A

part of a plant grows in a particular direction in response to a light stimulus

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11
Q

what is meant by geotropism?

A

part of a plant grows in a particular direction in response to the force of gravity

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12
Q

what is meant by chemotropism?

A

part of a plant grows in response to the location of chemicals

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13
Q

what is meant by thigmotropism?

A

part of a plant (normally shoots) that grow via winding around other plants/structures to gain support

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14
Q

what are the 2 types of tropism?

A

plant can respond towards a stimulus (positive tropic response) or away from a stimulus (negative tropic response).

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15
Q

what is meant by nastic responses and give example?

A

Nastic responses are defined as a non-directional response to external stimuli.

For example, the plant Mimosa pudica responds to touch in such a way that its leaves fold up. This response is often referred to as thigmonasty

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16
Q

why does growth only occur in meristems?

A
  • The cell wall in plant cells limits the cell’s ability to divide and grow.
  • Growth therefore only occurs in groups of immature cells in the plant, called meristems.
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17
Q

what is meant by apical meristems?

A

present at the tip of roots and shoots and responsible for its expansion

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18
Q

what is meant by lateral bud meristem?

A

present in buds and give rise to side shoots

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19
Q

what is meant by lateral meristems?

A

form a cylinder outside the roots and shoots and are responsible for the widening of the roots and shoots

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20
Q

what is meant by intercalary meristem?

A

located between the nodes and responsible for the shoot getting longer

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21
Q

what are plant responses controlled by and why?

A

Unlike animals, plants do not have a nervous system. Instead, their responses are controlled by plant hormones

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22
Q

what are plant hormones produced by?

A

Plant hormones are produced by many plant tissues and can be transported to different parts of the plant.

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23
Q

what are some processes that plant hormones can influence?

A

-cell division,
-cell elongation
-cell differentiation
=====all of which affect the growth of the plant.

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24
Q

what is the role of auxins?

A

Auxins are responsible for controlling plant phototropic and geotropic responses by regulating cell elongation

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25
Q

what are 4 things that Auxins cause in plants?

A
  • Shoots to grow towards light (positively phototropic)
  • Roots to grow in the opposite direction to light (negatively phototropic)
  • Shoots to grow in the opposite direction to gravity (negatively geotropic)
  • Roots to grow in the direction of gravity (positively geotropic)
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26
Q

does auxin cause cell elongation in shoots or cell division?

A

auxin causes cell elongation in shoots rather than cell division.

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27
Q

what impact does auxins have on cell walls?

A
  • Auxin causes an increase in the plasticity of plant cell walls, allowing them to stretch more easily when the cell elongates.
  • Auxin causes protons to be actively transported by a plasma membrane ATPase (enzyme) into spaces within the cell wall.
  • This activates proteins called expansins (due to the fall in pH),which loosen the cellulose in the cell wall, making the cell wall less rigid (able to expand).
  • These enzymes break bonds in cellulose, allowing cell walls to expand as the cell takes water in.
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28
Q

describe the 5 stem mechanism underlying phototropism

A
  1. -Cells in the tip of the shoot produce auxin
    • Initially, auxin is transported from the tip of the shoot and distributed evenly throughout all regions down the shoot
    • Light causes auxin to move from the illuminated side of the shoot to the shaded side (so there’s a greater concentration of auxin on the shaded side of the shoot)
    • Auxin causes elongation of shoot cells — therefore cell elongation occurs more quickly on the shaded side of the shoot
    • This causes the shoot to bend towards the light;this is a positive phototropism
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29
Q

what impact does auxin have in roots?

A
  • high concentrations of auxin inhibit cell elongation.
  • Therefore, auxin inhibits cell elongation on the shaded side but occurs more quickly on the illuminated side.
  • The root bends away from the light source;this is a negative phototropism.
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30
Q

what are the 2 enzymes that have been identified which cause the redistribution of auxin in response to light?

A
  • Phototropin 1

- Phototropin 2

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31
Q

what are PIN proteins?

A
  • PIN proteins are transmembrane proteins found on all sides of the plant cells’ plasma membrane.
  • PIN proteins control the efflux of auxin from cells. Depending on which side of the plant cell they are located they send auxin in different directions in the shoot, thus essentially controlling its distribution.
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32
Q

what impact does blue light have on both Phototropin enzymes?

A
  • The activity of both enzymes is enhanced by blue light, therefore it’s thought higher phototropin activity occurs on the light side than the shaded side.
  • This gradient leads to the redistribution of auxin through its effects on PIN proteins.
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33
Q

what are PIN proteins controlled by?

A

PIN proteins are controlled by a different molecule, PINOID.

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34
Q

why is it not confirmed whether phototropins effect the activity of this molecule?

A

Phototropins are said to have effect the activity of this molecule. However, there is still no general consensus about the mechanism as this process is said to only work when there are short bursts of light.

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35
Q

describe the 5 step mechanism underlying geotropism

A
  1. -Auxin is produced at the root tip and distributed evenly throughout the root
  2. -Gravity causes the auxin to move from the upper side of the root to the lower side, resulting in a higher concentration of auxin in the lower side
  3. -Auxin inhibits root cell elongation — therefore cell elongation occurs more quickly on the upper side
  4. -This causes the root to bend down in the direction of gravity; this is a positive geotropism
    • In shoots, auxin has the opposite effect — high concentrations of auxin promote cell elongation. Therefore, auxin promotes cell elongation on the lower side, and cell elongation occurs more quickly on this side. The shoot grows upwards in the opposite direction to gravity; this is a negative geotropism.
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36
Q

what 2 phenomena did a series of experiments carried out by Darwin and Boysen- Jensen confirm?

A
  • The shoot tip was responsible for phototropic responses

- Water and solutes need to move backwards from the shoot tip for phototropism to occur

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37
Q

what happens to the shoot tip if:

  • a gelatine block was placed on it
  • a mica block was paced on it
A
  • When a gelatine block (permeable to solutes) was placed on a shoot tip, the shoot showed positive phototropism,
  • but when a mica block (impermeable to solutes) was placed, the shoot showed no phototropic response.
  • The messenger for the response was therefore shown to be a diffusible compound.
38
Q

describe and explain what happened when a scientist discovered the presence of a chemical messenger (auxin)?

A

=it stimulated the phototropic response seen:

  • When the shoot tip was placed on an agar block and the agar block then placed onto the top of the cut plant, the shoot still grew. This is because the chemical messenger diffused into the agar and could then diffuse into the plant
  • When a plain agar block was placed on top of the cut plant however, no growth was seen
39
Q

how are the chemical messengers in plants produced?

A

These chemical messengers are produced by cells in a variety of tissues and are transported around the plant via active transport, diffusion or mass flow in the phloem/xylem.

40
Q

how are hormones specific? why is this useful?

A
  • Hormones are specific; they have specific shapes which only bind to complementary receptors found on the plasma membrane of their target cells.
  • This ensures hormones only act upon the correct tissues.
41
Q

what are the 5 main types of plant hormones?

A
  • cytokinins
  • auxins
  • abscisic acids
  • gibberellins
  • ethene
42
Q

describe and explain how cytokinins work

A
  • In cold climates, deciduous plants shed their leaves to reduce transpiration and the loss of water.
  • Cytokinins normally delay leaf senescence (ageing) as they help maintain the nutrient supply in the leaves, and hence have a direct effect on cell division and cell expansion.
  • However, when cytokinin levels fall, leaf senescence is increased.
43
Q

describe and explain how auxins work

A
  • Auxins promote cell elongation and are involved in the inhibition of side-shoot growth.
  • They also inhibit leaf abscission (the falling off of leaves).
  • They are present in leaves and, similarly to cytokinins, when auxin levels decrease, leaf abscission increases.
44
Q

describe and explain how abscisic work

A
  • When there is low water availability, abscisic acid causes the stomata in the leaves to close, further reducing the loss of water through transpiration.
  • In addition, abscisic acid inhibits seed germination and growth.
45
Q

describe auxins and how it leads to apical dominance

A
  • Auxins are a type of plant growth factor which controls phototropism and geotropism in plant shoots and roots.
  • Auxin also plays a role in inhibiting the growth of side branches from lateral buds present further down the shoot.
  • This inhibition of lateral buds is called apical dominance.
46
Q

what happens to the lateral buds if the shoot tip is removed?

A

If the shoot tip is removed, side branches from lateral buds start to grow.

47
Q

where is auxin produced?

A

Auxin is produced in the tip.

48
Q

what is the suggested theory on the effects of auxin?

A

It was suggested that auxin prevents the lateral buds from growing.

49
Q

how do we determine the exact effect of auxins on lateral bud growth?

A
  • The ends of a shoot were cut,and a layer of auxins applied to the cut end
  • No lateral bud growth was observed
50
Q

what does cutting the plant cause?

A

cutting the plant exposes the cells to oxygen, which may have caused them to synthesise another hormone which stimulates lateral bud growth.

51
Q

how do we confirm the absence of auxin was responsible for lateral bud growth?

A
  • A ring of auxin transport inhibitors was placed below the tip of the shoot instead of cutting it
  • Lateral bud growth was observed
52
Q

upon observations, what does normal auxin levels and low auxin levels lead to?

A

it was suggested that normal auxin levels in lateral buds inhibits growth, whereas low auxin levels promote growth.

53
Q

what was a later discovery how the auxin levels in lateral buds increase?

A

It was later discovered that auxin levels in lateral buds increased upon tip cutting.

54
Q

what are the 2 other hormones involved auxin levels in lateral buds increasing upon tip cutting and how?

A
  • ABSCISIC ACID inhibits bud growth. High levels of auxin in shoots keeps the level of abscisic acid high, inhibiting bud growth, therefore, when the tip is cut, abscisic acid levels drop so the buds can grow
  • CYTOKININS— promotes bud growth. Application of cytokinin directly to buds can override the apical dominance effect. High levels of auxin makes the shoot tip a cytokinin sink. Upon auxin’s removal, cytokinin spreads evenly around the plant
55
Q

what are gibberellins?

A

Gibberellins are plant hormones which control stem elongation and seed germination in plants.

56
Q

what is a theory suggesting gibberellin is responsible for plant stem growth?

A

-This is because the fungus secretes a type of gibberellin called gibberellic acid (GA3) which can be used to make dwarf varieties of plants grow taller, suggesting gibberellin is responsible for plant stem growth.

57
Q

what is GA1 responsible for?

A

GA1, another type of gibberellin, was found in plants. This was directly responsible for causing stem elongation.

58
Q

what are the differences in GA1 levels in tall and dwarf pea plants?

A

GA1 levels in tall pea plants and dwarf pea plants were compared. Taller plants had higher GA1 levels

59
Q

how is GA1 formed? the enzyme and what it encoded by?

A
  • It had been discovered that GA1 is formed from the conversion of another type of gibberellin called GA20.
  • This process was discovered to be catalysed by an enzyme encoded by the Le gene.
60
Q

explain the involvement of gibberellins in seed germination

A
  • Gibberellin is also released by the plant embryo during seed germination.
  • When the seed absorbs water, it causes the embryo to release gibberellin.
  • The gibberellin enables the production of the enzyme amylase which can break down starch into glucose.
  • This allows glucose to be used for respiration and also be used by the seed for protein synthesis, also allowing it to grow.
61
Q

why are plant hormones used commercially?

A

Plant hormones can be exploited to manipulate the growth of plants for commercial use in order to maximise profits

62
Q

why is auxin used commercially?

A

Auxin plays a role in preventing leaf and fruit abscission, therefore, artificial auxins are often used to prevent leaf and fruit drop.

63
Q

what are some commercial uses of auxin?

A
  • To treat unpollinated flowers allowing seedless fruits to be produced — artificial auxin stimulates ovule growth, triggering automatic production of natural auxin. The plant can therefore complete its developmental process
  • Used as herbicides to kill weeds — artificial auxin stimulates shoot growth so the plant cannot support itself and dies. It is also a man-made hormone,so, the plant finds it difficult to break-down and the effects last for longer
  • Auxins are also put in rooting powder to encourage the downwards root growth of plant cuttings
64
Q

why are cytokinins used commercially?

A

Cytokinins play a role in preventing leaf senescence — preventing the decay and yellowing of leaves. Commercially, cytokinins are used to prevent the yellowing of plant leaves, such as lettuce leaves, after they have been picked.

65
Q

why are cytokinins used in tissue culture?

A

Cytokinins also play a role in promoting bud and shoot growth, therefore they are used in tissue culture to help mass-produce plants.

66
Q

how are cytokinins used in tissue culture?

A
  • Cytokinins are applied to small pieces of tissue taken from the parent plant, which goes on to produce a short shoot with many side branches
  • These side branches can then be split into many small plants which can be grown separately
67
Q

how and why are gibberellins used commercially? (5)

A
  • Gibberellins play a role in stem elongation and activating seed germination.
  • Commercially, gibberellins are used in fruit production
  • Gibberellins are also used to speed up beer production
  • Gibberellins are used to increase sugar production in sugar canes by causing stem elongation
  • Gibberellins are also used to speed up plant breeding
68
Q

give an example of why gibberellins are used in fruit production

A

they can be used to elongate the stems of fruits so that the individual fruits have more space to grow bigger.

69
Q

how and why are gibberellins used to speed up beer production?

A

=Gibberellins are also used to speed up beer production because they stimulate the production of the enzyme amylase.

  • When barley seeds germinate, their starch can be broken down to maltose by amylase
  • Malt, which is obtained from maltose, is needed for beer production as artificial gibberellins are added to barley seeds to produce amylase. This causes more starch to be broken down to maltose
70
Q

how are gibberellins used to increase sugar production in canes?

A
  • Gibberellins are used to increase sugar production in sugar canes by causing stem elongation.
  • Sugar canes store sugar in subsections of the stems called internodes, hence the use of gibberellins means more sugar can be produced and stored.
71
Q

why are gibberellins used to speed up plant breeding?

A
  • Gibberellins are also used to speed up plant breeding because they induce seed formation in young plants.
  • Spraying plants with gibberellin synthesis inhibitors can keep flowers short, preventing plant stems bending;harvesting the crop is therefore easier.
72
Q

what is ethene and what does it do?

A

Ethene is a gaseous plant hormone which controls ripening and rotting in plants.

73
Q

how is ethene used commercially?

-give an example

A

Ethene is used commercially to speed up the ripening of fruits and promote fruit drop from trees.

-It is often used in cucumbers to reduce the chance of self-pollination which causes cucumbers to taste bitter

74
Q

what is a benefit of ethene for commercial benefit?

A
  • Commercially, fruit ripening can also be prevented by inhibiting ethene synthesis in fruits by storing fruits at a low temperature, with low oxygen and high carbon dioxide levels.
  • When ripening is prevented, fruit can be stored for longer, increasing shelf-life.
75
Q

what is meant by abiotic stress?

A
  • its anything that’s potentially harmful to a plant that’s natural, but nonliving, like a drought (water stress).
  • plants can respond respond to abiotic stress, e.g.- some respond to extreme cold by producing their own form of antifreeze
76
Q

what is meant by herbivory?

A

herbivory is when plants are eaten by animals (incl ding insects).

  • plants have chemical defences that they can use against herbivory.
  • e.g.- they can produce toxic chemicals in response to being eaten
77
Q

what are pheromones?

A

pheromones are signalling chemicals that produce a response in other organims

78
Q

describe the 5 step method into phototropism (how plant shoots respond to light)

A

1- take 9 wheat shoots that are roughly equal in height and plant them in individual pots in the same type of soil.

2-next, prepare the shoots as follows:

  • cover the tips of 3 shoots with a foil cap (shoot A)
  • leave 3 shoots without foil (shoot )
  • wrap the bases of the final 3 shoots with foil, leaving only the tip exposed (shoot C)

3- set up the shoots in front of a light source. Make sure that the shoots are all the same distance from the light source and experience the same intensity of light. You should also make sure that all other variables, including temperature and exposure to moisture, and controlled ( e.g. by growing all the shoots in the same propagator and watering them all equally).

4- leave the shoots for 2 days

5- after they’ve been left for 2 days, record the amount of growth (in mm) and the direction of growth of your shoots. This gives you both quantitative and qualitative data.

79
Q

what results do you expect from an investigation into phototropism?

A
  • the shoots with exposed tips (B and C) should have grown towards the light source (positive phototropism).
  • covering the tip with a foil cap (shoot A) prevents growth towards the light- it’s the tip (where IAA is produced) that’s most sensitive to light and because it’s covered the shoo should have continued to grow straight up.
  • covering the base of the shoot with foil (shoot C) should still allow the tip to grow towards the light.
80
Q

describe the 3 step method for a practical investigation into geotropism

A

1- line 3 petri dishes with moist (but not soaking wet) cotton wool. You should use the same volume f water and the same amount of cotton wool in each dish.

2- space out 10 cress seeds on the surface of the cotton wool in each dish and push each seed down into the wool slightly.

3- tape a lid onto each dish and wrap each one in foil, making sure there are no gaps. The dishes are wrapped in foil to prevent any light reaches the seeds- this would affect you results because shoots from the germinating seeds would grow towards any light entering the dish (due to phototropism)

81
Q

what is an apical bud?

A

the shoot tip at the top of a flowering plant is called apical bud

82
Q

what is meant by apical dominance?

A
  • auxins stimulate the growth of the apical bud and inhibit the growth of side shoots from lateral buds.
  • this is called apical dominance- the apical bud is dominant over the lateral buds
83
Q

what is the benefit of apical dominance?

A
  • apical dominance prevents side shoots from growing- this saves energy and prevents side shoots from the same plant competing with the shoot tip for light.
  • Because energy isn’t being used to grow side shoots apical dominance allows a plant in an area where there are loads of other plants to grow tall very fast, past the smaller plants, to reach the sunlight.
84
Q

what happens to a plants auxin production if you remove the apical bud?

A
  • if you remove the apical bud then the plant won’t produce auxins, so the side shoots will start growing by cell division and cell elongation.
  • however, if you replace the tip with a source of auxin, side shoot development is inhibited.
  • this demonstrates that apical dominance is controlled by auxins
85
Q

describe a 7 step method for investigating the role of auxins in apical dominance

A

1- plant 30 plants pf the same type (e.g. pea plants) that are a similar age, height and weight in pots containing the same type of soil.

2-cont and record the number of side shoots growing form the main stem of each plant. These might e tricky to spot- they may be located where the stalks of leaves join to the main stem.

3- for 10 plants, remove the tip of the shoot and apply a paste containing auxins to the top of the stem.

4- for another 10 plants, remove the tip of the shoot and apply a paste without auxins to the top of the stem.

5- leave the final 10 plants as they are- these are your untreated controls. remember, you always need to have controls ( e.g. without the hormone, untouched) for comparison- so you know the effect you see is likely to be due to the hormone and not any other factor.

6- leave your plants to grow for 6 days. You need to keep all he plants in the same conditions- the same light intensity, water, e.t.c. This makes sure any variables that may affect your results are controlled, which makes your experiment valid.

7- after 6 days, count the number of side shoots growing form the main stem of each of your plants.

86
Q

describe a 5 step method to investigating the role of gibberellins in stem elongation

A

1- plant 40 plants (e.g. dwarf pea plants) that are a similar age, height and mass in pots containing the same type of soil.

2- leave 20 plants to grow, watering them all in the same way and keeping all other conditions the same. These plats are your negative controls.

3- leave the other 20 plants to grow in the same conditions, except water them with a dilute solution of gibberellin (e.g. 100 mg dm-3 gibberellin).

4- let all the plants grow for 28 days

5- every 7 days measure the length of the stem of each plant. calculate the mean stem length for the plants watered normally and the plants watered with gibberellin.

87
Q

explain what deciduous plants are

A
  • they are plants that lose their leaves in winter.
  • losing their leaves helps plants to conserve water (lost from leaves) during the cold part of the year, when it might be difficult to absorb water form the soil (the soil water may be frozen) and when there’s less light for photosynthesis
88
Q

define abscisson

A

leaf loss

89
Q

what hormones is leaf loss controlled by and how?

A
  • auxins inhibit leaf loss

- ethene stimulates leaf loss

90
Q

how is the plant hormone abscisic acid (ABA) able to trigger stomatal closure?

A

1- ABA binds to receptors on the guard cell membranes. This causes specific channels to open, which allows calcium ions to enter the cytosol from the vacuole.

2- the increased conc of calcium ions in the cytosol causes other ion channels to open. These ion channels allow ions (such as potassium ions) to leave the guard cells, raising the water potential of the cells.

3- water then leaves the guard cells by osmosis.

4- the guard cells become flaccid and the stomata close

91
Q

explain stomatal closure

A
  • plants need to be able to close their stomata in order to reduce water loss through transpiration.
  • they do this using guard cells.
  • guard cells are found either side of a stomatal pore
  • when the guard cells are full of water, they are plump and turgid and the pore is open
  • when the guard cells lose water, they become flaccid, making the pre close
92
Q

give a 4 step method on how you would carry out 5 serial dilutions with a dilution factor of 2, starting with an initial glucose concentration of 40mM

A

1- line up 5 test tubes in a rack

2- add 10cm3 of the initial 40nM sucrose solution to the first test tube and 5cm3 of distilled water to the other 4 test tubes.

3- then, using a pipette, draw 6cm3 of the solution from the first test tube, add it to the distilled water in the 2nd test tube and mix the solution thoroughly. You now have 10cm3 of solution that’s half as concentrated as the solution in the 1st test tube (it’s 20mM) .

4- repeat this process 3 more times to create solutions of 10mM, 5mM and 2.5mM

OCR A level Biology HJ (23 decks)

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