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OCR Biology > 5.1.5a - Plant Responses > Flashcards

5.1.5a - Plant Responses Flashcards

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1
Q

What are some examples of plant responses?

A
  • tropisms
  • responses to touch
  • responses to herbivory
  • responses to abiotic stress
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2
Q

What is a tropism?

A

A growth movement of a part of a plant in response to a directional stimulus

Positive tropism - toward the stimulus
Negative tropism - away from stimulus

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3
Q

What is phototropism?

A
  • In response to light (abiotic)

* advantage - ensures plant gets access to as much light as possible, maximising photosynthesis

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4
Q

What is geotropism?

A
  • in response to gravity (abiotic)
  • shoots show negative geotropism (grow away from the pull of gravity)
  • roots show positive geotropism (grow towards the pull of gravity
  • ensures roots/shoots from germinating seeds grow in the right directions, regardless of the orientation of the seed
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5
Q

What is hydrotropism?

A
  • in response to moisture (abiotic)

* root tips normally grow towards damper areas of soil, increasing their access to water

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6
Q

What is thigmotropism?

A
  • in response to touch (biotic or abiotic)
  • important to climbing plants, which are adapted to climb as it provides them with certain advantages
  • these plants can detect living/non-living support and curl around it
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7
Q

How can plants respond to touch?

A

Eg Mimosa pudica
• when touched, leaflets of the touch-sensitive leaves fold in rapidly

  • caused by water uptake from base of leaves (therefore increase volume) and rapid loss of water from adjacent cells
  • the movement occurs rapidly and most likely due to bioelectrical signals
  • potentially an adaptation to protect leaflets from herbivorous insects

Eg Venus fly traps
• traps fold together rapidly when stimulated by touch of an insect, trapping it inside
• enzymes are secreted and the insect is digested

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8
Q

What is herbivory?

A

The consumption of plants by herbivores

Many plants have adaptations to avoid herbivory, both physical and chemical

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9
Q

What are some physical defences against herbivory?

A
  • thorns
  • barbs
  • spikes
  • spiny leaves
  • fibrous/inedible tissue (eg bark)
  • stings
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10
Q

What are some chemical defences against herbivory?

A
  • tannins
  • alkaloids
  • terpenoids
  • pheromones
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11
Q

What are tannins?

A
  • group of compounds called phenols
  • stored in vacuoles of plant cells
  • give a bitter taste to put off herbivores
  • toxic to insects (binds digestive enzyme in saliva and inactivates them)

Eg tea

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12
Q

What are alkaloids?

A
  • bitter tasting, nitrogenous compounds
  • work by affecting metabolism of plants (sometimes poisoning them)
  • eg caffeine - toxic to fungi and insects. Can also prevent germination of seeds of other plants
  • eg nicotine - produced in roots of tobacco plants + stored in vacuoles. Released when eaten (toxic)
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13
Q

What are terpenoids?

A

Group of compounds produced by plants, often forming essential oils.

Act as toxins to insects and fungi

Eg pyrethrin is an insect neurotoxin
Eg citronella acts as an insect repellent

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14
Q

What are pheromones?

A

Affect the social behaviour of other plants in the same species

Eg maple leaves:
• release pheromones when attacked by insects
• pheromones absorbed by leaves on other branches
• leaves then produce protective chemicals (eg callose)

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15
Q

What are volatile organic compounds (VOCs)?

A

Act like pheromones, but between themselves and other species

Eg cabbages - when attacked by certain insects, release VOCs that attract parasites to destroy the insect attacking them

Some VOCs will cause neighbouring plants to produce VOCs before they’ve been attacked

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16
Q

What is a coleoptile?

A

A sheath that surrounds the young growing shoots of grass plants

Often used in experiments investigating how phototropism works

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17
Q

What experiments where conducted to investigate the process by which phototropism occurs?

A

Darwin’s experiment (1880)

Boysen-Jensen’s experiments (1913)

Paàl;s experiment (1919)

Wne’ts experiment (1926)

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18
Q

What was Darwin’s experiment (1880)?

A
  • Darwin discovered that removing the tip of a coleoptile stopped phototropic response to unidirectional light source (light coming from one side)
  • to ensure this wasn’t due to the wounding of the plant, he covered the tip of a coleoptile with an opaque cap to block out the light. This all stepped the phototropic response occurring
  • this shows that the tip of the coleoptile is responsible for detecting light
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19
Q

What was Boysen-Jensen’s experiment (1913)?

A

He found that of he replaced the cut tip back on top of the coleoptile and inserted a gelatine block as a barrier in between, the phototropic response was restored
•this showed that the stimulus for growth was a chemical (hormone) which was able to travel through the block

He then inserted a mica barrier (impermeable to chemicals) halfway through the coleoptile, first on the lit side and then on the shaded side
• when it was on the lit side, the phototropic response occurred
• when it was on the shaded side, the phototropic response didn’t occur
• this confirmed that the stimulus for growth was a hormone
• showed that it was produced at the tip before travelling down the coleoptile on the side opposite to the stimulus (the shaded side)
•showed that the stimulus caused growth on the shaded side, rather than inhibiting growth on the lit side

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20
Q

What was Paál’s experiment (1919)?

A

Paál cut the tip of the coleoptile and replaced it off-centre in the dark
• the side of the coleoptile that the tip was placed on grew more than the other side, causing it to curve (similar to phototropic response)
• this showed that, in the light, phototropic response was caused by a hormone diffusing through the plant tissue and stimulating growth of the tissue

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21
Q

What was Went’s experiment (1926)?

A

Went placed the cut tip of a coleoptile on a gelatine block, allowing hormones from the tip to diffuse into the block. The block was then placed on the coleoptile, off-centre and in the dark
• as in Paál’s experiment, the side with the gelatine grew more than the other side, causing it to curve
• the greater the concentration of the hormone present, the more the coleoptile curved

22
Q

What are auxins?

A

A class of plant hormones that control cell elongation, produce tropisms, prevent abscission, maintain apical dominance and stimulate the production of ethene

23
Q

What is IAA?

A
  • a type of auxin
  • coordinates phototropism and geotropism by controlling growth by elongation
  • synthesised in the growing tips of roots and shoots (ie meristem) and passes down the stem
  • IAA molecules activate proteins in the cell wall called expansins, which loosen the bonds between cellulose microfibrils
  • the cell walls are now more flexible and the cel can elongate
24
Q

What is the mechanism by which phototropism occurs?

A
  • phototropism affects the shoots and the top of the stems
  • the conc of IAA determines the rate of cell elongation
  • if the conc of IAA isn’t uniform, then uneven growth can occur
  • shoots grow towards the light - this is positive phototropism
  • in shoots, higher conc IAA = greater rate of cell elongation
  • IAA moves from the illuminated side of a shoot to the shaded side
  • the higher conc on shaded side causes a faster rate of cell elongation
  • this causes the shoot to bend toward the light
25
Q

What is the mechanism by which geotropism occurs?

A

Shoots grow away from gravity - negative geotropism
• gravity means there is a high concentration of IAA on the lower side of the shoot
• IAA increases the rate of cell elongation in shoots
• the bottom of the shoot elongates faster and the shoot bends upwards

Roots grow towards gravity - positive geotropism
• gravity means there is a high concentration of IAA on the lower side of the root
• IAA decreases the rate of cell elongation in roots
•the lower side grows at a slower rate than the upper side of the root
•the root bends downwards

26
Q

What abiotic stresses do plants need to respond to?

A
  • low temp
  • low daylight
  • low water availability
  • high win
  • changes in salinity
27
Q

What is photoperiodism?

A

A plants growth response to seasonal changes in the length of daylight

Evidence suggests that it is the lack of light that triggers a plant response, not the length of daylight as previously thought

Importance of photoperiodism:
•breaking dormancy
•timing of flowers
• formation of tubers

28
Q

What makes plants sensitive to the lack of light?

A

Phytochromes (light sensitive pigments)

2 forms - Pr and Pfr
•Each absorb different wavelengths of light
• ratio of Pr to Pfr changes depending on the levels of light

29
Q

Why do plants lose leaves due to abiotic stress?

A
  • plants in temperate climates can see great changes in temperature and daylight hours depending on the season
  • if temperature and daylight hours are too low, more glucose is required for respiration and protecting the leaves against freezing than is produced by photosynthesis
  • trees in leaf are also more likely to be damaged/blown over by winter gales

As a result, deciduous trees lose all of their leaves in winter and remain dormant until days lengthen and temperatures rise

30
Q

What are the 2 parts of the abscission zone?

A

SEPARATION ZONE
• small cells with thin cell walls
• cellulose breaks down the cell wall of the cells
• the wall disintegrates and the leaf separates (falls off)

PROTECTIVE LAYER
• larger cells, which have fatty substances (suberin) in their cell walls
• this part remains on the plant
• provides a protective scar when the leaf falls off

31
Q

How does abscission (leaf fall) occur?

A

• falling light results in a reduction of auxin
• in response, the leaves release the hormone ethene
• at the base of the leaf stalk is the abscission zone
• ethene causes the cells to weaken the cells in the separation zone of the abscission zone
• vascular bundles are sealed off
• fatty material deposited in the cells on the stem side of the separation layer, forming a protective scar when the leaf falls off
•cells in the separation zone start to retain water and swell
(Putting strain on weakened outer layer)
•low temps and strong winds put strain on the zone and the leaf falls off

32
Q

How do hormones control stomatal closure?

A
  • during times of water stress, ABA is produced
  • guard cells have ABA receptors on cell surface membrane
  • ABA binds with receptors, inhibiting proton pumps and stopping active transport of H+ out of guard cells
  • ABA also causes Ca2+ to move into cytoplasm through membrane
  • Ca2+ causes channel proteins to open, allowing -ve ions to leave guard cell. This stimulates opening opening of channels to let K+ leave and closing channels that let K+ into guard cells
  • loss of ions increases water potential of guard cells
  • water leaves guard cells via osmosis• guard cells come flaccid, causing stomata to close
33
Q

How do plants protect themselves from freezing?

A

If plant cells freeze, their membranes will become disrupted and they’ll die

Cytoplasm and vacuoles contain solutes which lower the freezing point

Some plants produce sugars, polysaccharides, amino acids and proteins that act as antifreeze

Different genes can be activated/repressed at different times - plant responds to temperature/daylight reduction

34
Q

What are giberellins?

A

Plant hormones that control stem elongation, trigger mobilisation of food stored in a seed at germination, stimulate pollen tube growth in fertilisation

35
Q

What is ethene?

A

Plant hormone that causes fruit ripening, promotes abscission in deciduous trees

36
Q

What is abscisic acid (ABA)?

A

Plant hormone that maintains dormancy of sees and buds, stimulates cold protective responses (eg antifreeze production), stimulates stomatal closing

37
Q

Why is it difficult to study the effects of plant hormones?

A
  • work at very low concentration, so difficult to isolate and measure changes in concentration
  • multiple interactions between the hormones makes it difficult to isolate the role of a single chemical in a specific response
38
Q

What is the difference between synergism and antagonism?

A

Synergism - different hormones working together, complementing each other and giving a greater response compared to on their own

Antagonism - different hormones have opposite effects (eg one promoting growth, one inhibiting it)

39
Q

How are hormones involved in seed germination?

A
  • seed absorbs water, activating embryo to release gibberellin
  • gibberellin stimulates enzyme production (amylase, protease)
  • enzymes break down starchy food store in cotyledon (dicot)/endosperm (monocot)
  • embryo uses substrates for respiration to make ATP
  • ATP used to build materials for growth
  • evidence ABA acts as an antagonist - the relative levels of both hormones determine when the seed will germinate
40
Q

What is apical dominance?

A

The growth and dominance of the main shoot (so the plant grows upwards) as a result of the suppression of lateral shoots by auxin

41
Q

Why is apical dominance important?

A
  • best for plants to grow upwards towards the light so plant has more access to energy for photosynthesis
  • sideways growth less useful and takes energy and resources
42
Q

How are auxins involved in apical dominance?

A
  • growth in main shoot stimulated by the auxin produced a the tip so it grows quickly
  • lateral shoots are inhibited by the hormone that moves back down the stem so don’t grow much
  • further down shoot, there is a lower concentration of auxin so lateral shoots grow more strongly
43
Q

How can the effect of auxins in apical dominance be shown experimentally?

A
  • apical bud (tip) of first test plant is removed, resulting in lateral buds growing
  • a second genetically identical plant is decapitated and replaced by an agar block containing auxin, restoring inhibition of lateral bud growth.
44
Q

What is some experimental evidence for the role of gibberellins in seed germination?

A

Mutant varieties of seeds lacking for the gene for producing gibberellins do not germinate
If gibberellins are applied externally to these seeds they will germinate normally

If gibberellin biosynthesis inhibitors are applied to normal seeds, they will not germinate
If you remove the inhibitor or gibberellins are applied, the seeds will germinate

45
Q

What is some experimental evidence for the role of gibberellin in stem elongation?

A

Gibberellins help plants grow by stimulating cell division and elongation in the stem

  • Dwarf plant varieties have been shown to have very low levels of gibberellin, often due to mutations
  • Treating dwarf varieties with gibberellins results in them growing to the same height as normal varieties
46
Q

Describe a practical to investigate the effect of plant hormones on growth

A 
EQUIPMENT
• IBA - a type of auxin
• alcohol
• measuring cylinder
• beakers
• plant eg Busy Lizzie (Impatiens walleriana)
• soil/plant compost
• scalpel
• digital balance

METHOD
1) make a 5% solution of IBA by dissolving it in alcohol (IBA is insoluble in water). Label it solution A

2) add 1cm^3 of A to 9cm^3 alcohol to make solution B (0.5%) - this is a serial dilution. Add 1cm^3 of B to 9cm^3 alcohol to make solution C (0.05%). Repeat to make solution D (0.005%) an E (0.0005%)
3) 60 stem cutting should be taken from the same plant
4) dip the ends of 10 cuttings into solution and place in compost, dipped end down. Repeat with solutions B, C, D and E. Dip the remaining 10 shoots in the alcohol as a control (ensures effect on root growth is due to hormone,not alcohol)
5) after set time period (eg 1 week) carefully remove all cuttings and weigh them. Find a mean for each set of 10
6) plot the results on a graph worth concentration of auxin on -axis and mass of roots grown on y-axis

LIMITATIONS
• certain factors should be controlled - eg volume of water provided to each set of cuttings during growth, temperature, humidity, light intensity - this is to ensure conc of IBA is the only variable affecting root growth

• while it may not be possible to obtain cuttings from the same plant, they should all be same species (if possible, genetically identical). This is to ensure all cuttings respond to hormone in the same way, and that generics aren’t having an effect

47
Q

What are climatic vs non-climatic fruits?

A

CLIMATIC:
• continue to ripen after harvesting
• ripening linked to ethene
• eg banana, avocado, mango, tomato

NON-CLIMATIC:
• don’t ripen much after picking
• don’t produce much ethene
• eg oranges, strawberries, watermelon

48
Q

How can plant hormones be used commercially?

A
  • control ripening
  • rooting powders
  • weed killers
49
Q

How are plant hormones used to control ripening?

A

Hormone used:
• ethene

How:
• fruit harvested when formed but not ripe
• unripe fruit are hard so less easily damaged during transport
•fruit is then exposed to ethene gas
• ensures each batch of fruit ripen at the same rate and are all at the same stage when put in supermarket

Benefit:
• reduces wastage of fruit
•increases time available for fruit to be sold

50
Q

How are plant hormones used in rooting powders and micropropagation?

A

Hormone:
• auxin

How:
• applying auxin to cut shoots stimulates root production
• placing a cut stem in hormone rooting powder increases the chance of root forming

Benefit:
• can develop cuttings to sell/use
• large scale micropropagation can result in hundreds of new plants formed from one original plant

51
Q

How are hormones used in weed killers?

A

Hormone:
• auxin

How:
•synthetic auxin is applied as weed killer
• weeds (mostly broad-leaved dicots) will absorb the weed killer
•weed killer affects metabolism by increasing growth rate to an unsustainable level, so the plant dies
• crops (narrow-leaved monocots) are not affected and continue to grow as normal

Benefit:
•synthetic auxins are simple and cheap to produce
•selective in their work - don’t affect crops

52
Q

What are some other commercial uses of plant hormones?

A
  • seedless fruit (auxin)
  • fruit dropping (ethene)
  • preventing ageing of ripening fruit (cytokinins)
  • control tissue development in micropropagation (cytokinins)
  • delay ripening and ageing in fruit (gibberellins)

OCR Biology (25 decks)

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