plant responses

Question 1

what is a tropism?

Answer 1

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

Question 2

what is phototropism in response to, the type of stimulus and advantage of tropism

Answer 2

light, abiotic. Ensures plants get access to as much light as possible, maximising photosynthesis

Question 3

what is geotropism in response to and the type of stimulus and advantage of tropism

Answer 3

gravity, abiotic. Shoots show negative geotropism and roots show positive geotropism and this ensures shoots/ roots from the germinating seeds grow in the right directions regardless of the orientation of the seed

Question 4

what is hydrotropism in response to and the type of stimulus and advantage of tropism

Answer 4

moisture, abiotic. Root tips normally grow towards damper areas of soil which increases access to water

Question 5

what is thigmotropism in response to and the type of stimulus and advantage of tropism

Answer 5

touch, abiotic or biotic. Important in climbing plants & allows these plants to detect a living or nonliving support and curl around it

Question 6

what is chemotropism in response to and the type of stimulus

Answer 6

chemicals, abiotic or biotic

Question 7

give an example of thigmotropism

Answer 7

Mimosa pudica which has leaflets that rapidly fold inwards when touched. This is caused by rapid water uptake (and increase in volume) in cells at the base of each leaflet & rapid loss of water and therefore collapse of adjacent cells

Question 8

what is a nastic movement?

Answer 8

when there is a non directional response

Question 9

what are tannins and it’s mode of action. give 2 examples

Answer 9

• water soluble carbon compounds in the chemical group known as flavonoids
• stored in the vacuoles. can be fatal to insects bc chemicals are produced during the breakdown of tannin in the insect gut
• bitter taste puts off herbivores
• gallic acid (rhubarb), flavan 3 ol (tea and cocao)

Question 10

give 3 examples of herbivore-repellent chemicals

Answer 10

Tannins, Alkaloids, Pheromones

Question 11

what are alkaloids and it’s mode of action and 2 examples

Answer 11

• nitrogenous compounds derived from amino acids
• bitter tasting and can be toxic
• caffeine and nicotine

Question 12

what are pheromones and it’s mode of action and give an example

Answer 12

• chemicals released by one member of a species that affect the physiology or behaviour of another member of the same species
• could be directly toxic or trigger other chemical defences
• in some plants, oxides of ethene appear to switch on the genes for producing chemicals that deter insects

Question 13

what could be some reasons for abiotic stress?

Answer 13

freezing, drought, increased soil water salinity, presence of heavy metals (e.g. lead, copper,zinc)

Question 14

how can plants react to drought?

Answer 14

closing the stomata or by dropping leaves

Question 15

how do some plants react to a drop in temperature below freezing point?

Answer 15

producing an antifreeze chemical in their cells, that decreases the formation of ice crystals that can destroy plant cells if allowed to form within them

Question 16

what was darwins’ experiment? (1880)

Answer 16

discovered that removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source from occurring. to check this was not due to the wounding to the plant, he covered the tip of a coleoptile with an opaque cover to block out light. phototropic response stopped –> shows the tip coleoptile was responsible for detecting light

Question 17

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

Answer 17

found that if he replaced the cut tip back on top of the coleoptile & inserted a gelatin block as a barrier, the phototropic response was restored. shows the stimulus for growth was a hormone, which was able to travel through the gelatin block. then a mica barrier was inserted (mica is impermeable to chemicals) halfway through the coleoptile just below the tip, first on the lit side & then on the shaded side. when mica barrier was inserted into the lit side, the phototropic response occurred. when mica barrier was inserted into the shaded side, the phototropic response didn’t occur. confirmed that the stimulus for growth was a hormone and showed it was made at the tip, before going down the coleoptile on opposite side to the stimulus (i.e. the shaded side). also showed that the stimulus acted by causing growth on the shaded side (rather than inhibiting growth on the lit side)

Question 18

what was Paál’s Experiment (1919)?

Answer 18

cut off the tip of a coleoptile and then 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 the coleoptile to curve (similar to a phototropic response). showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue

Question 19

what was Went’s Experiment (1926)?

Answer 19

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

Question 20

what is Indole-3-acetic acid?

Answer 20

• IAA is an auxin & a specific growth factor found in plants
  It’s synthesised in the growing tips of roots and shoots (i.e. in the meristems)
• IAA coordinates phototropisms in plants by controlling growth by elongation

Question 21

How is IAA synthesised?

Answer 21

IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth. The IAA molecules activate proteins in the cell wall known as expansins, which loosen the bonds between cellulose microfibrils, making cell walls more flexible. The cell can then elongate