3.5.1 - PHOTOSYNTHESIS

Question 1

describe what happens during photoionisation in the light-dependent reaction (2)

Answer 1

1. chlorophyll absorbs light OR light excited electrons in chlorophyll
2. electrons are lost OR chlorophyll becomes positively charged

Question 2

suggest and explain the advantage of having different coloured pigments in leaves (1)

Answer 2

1. absorb different/more wavelengths of light for photosynthesis

Question 3

heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time.

heat stress is a major factor in limiting the rate of photosynthesis.

heat stress decreases the light-dependent reaction of photosynthesis.

explain why this leads to a decrease in the light independent reaction (2)

Answer 3

1. less ATP
2. less reduced NADP

Question 4

heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time.

heat stress is a major factor in limiting the rate of photosynthesis.

heat stress leads to a decrease in the activity of rubisco. a decrease in the activity of an enzyme means that the rate of the reaction it catalyses becomes slower

explain why a decrease in the activity of rubisco would limit the rate of photosynthesis (2)

Answer 4

1. less CO2 reacts with RuBP
2. so less GP

Question 5

where precisely is rubisco found in a cell? (1)

Answer 5

stroma of chloroplast

Question 6

the solution that the student used to produce the chloroplast suspension had the same water potential as the chloroplasts.

explain why it was important that these water potentials were the same (2)

Answer 6

1. so osmosis does not occur
2. so chloroplast does not burst

Question 7

the light absorbed by chlorophyll is used in the light-dependent reaction.

name the two products of the light-dependent reaction that are required for the light-independent reaction (2)

Answer 7

1. ATP
2. reduced NADP

Question 8

atrazine binds to proteins in the electron transfer chain in chloroplasts of weeds, reducing the transfer of electrons down the chain.

explain how this reduced the rate of photosynthesis in weeds (4)

Answer 8

1. reduced transfer of protons across thylakoid membrane
2. so less ATP produced
3. so less NADP produced
4. so light-independent reaction slows OR less reduction of GP to triose phosphate

Question 9

when treated with atrazine, weeds have been shown to give off small amounts of heat.

suggest an explanation for this observation (1)

Answer 9

1. energy is released from excited electrons that were lost from chlorophyll

Question 10

the stomata of this plant close when the light is turned off.

explain the advantage of this to the plant (2)

Answer 10

1. because water is lost through the stomata
2. closure prevents/reduces this water loss
3. maintains water content of the cells

Question 11

crops use light energy to produce photosynthetic products.

describe how crop plants use light energy during the light-independent reaction (5)

Answer 11

1. excites electrons/electrons removed from chlorophyll
2. electrons move along electron transfer chain releasing energy
3. energy is used to join ADP + Pi to form ATP
4. photolysis of water produces protons, electrons and oxygen
5. NADP is reduced by electrons

Question 12

explain the relationship between stomatal opening and photosynthesis (2)

Answer 12

1. stomata allow uptake of CO2
2. CO2 used in photosynthesis

Question 13

where precisely in a cell does the Calvin cycle take place? (1)

Answer 13

1. stroma of chloroplasts

Question 14

during the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. describe how (6)

Answer 14

1. rubisco combines CO2 with RuBP
2. which produces 2 molecules of GP
3. GP is reduced triose phosphate - using reduced NADP and using energy from ATP
4. triose phosphate is converted to glucose (useful organic substance)

Question 15

iron deficiency reduces electron transport.

use this information and your knowledge of photosynthesis to explain the decrease in production of triose phosphate in the iron-deficient plants (4)

Answer 15

1. less ATP produced
2. less reduced NADP produced
3. ATP and reduced NADP are produced during the light-dependent reaction
4. less reduction of GP to triose phosphate

Question 16

iron deficiency results in a decrease in the uptake of CO2. explain why (2)

Answer 16

1. less triose phosphate converted to RuBP
2. CO2 combines with RuBP

Question 17

suggest why few species of plant can live below large trees in a forest (3)

Answer 17

1. less light energy reaches ground
2. smaller range of wavelengths passes through leaves
3. little light for chlorophyll to absorb
4. so insufficient photosynthesis for growth
5. photosynthesis unlikely to excide respiration

Question 18

suggest and explain one reason why in the leaves at the top of trees in a forest, CO2 is often the limiting factor for photosynthesis (2)

Answer 18

ANY PAIR:

1. lots of light, so light is not limiting
2. light dependent reaction is not limiting

OR

3. high use of CO2
4. light independent reaction is limiting

Question 19

name the substance which combines with carbon dioxide in a chloroplast (1)

Answer 19

RuBP (ribulose bisphosphate)

Question 20

scientists measured the oxygen produced by the light-independent reaction of photosynthesis.

name two other substances produced by the light-dependent reaction (2)

Answer 20

1. reduced NADP
2. ATP

Question 21

the investigation was carried out in bright light.

explain why reducing the light intensity would affect the amount of oxygen released by the seaweeds (3)

Answer 21

1. light intensity is a limiting factor
2. fewer electrons released from chlorophyll
3. less photolysis therefore less O2 from water

Question 22

plants produce ATP in their chloroplasts during photosynthesis. they also produce ATP during respiration.

explain why it is important for plants to produce ATP during respiration in addition to during photosynthesis (5)

Answer 22

1. in the dark, no light energy, so no ATP production in photosynthesis
2. some tissues unable to photosynthesise
3. ATP cannot be stored
4. plants use more ATP than what is produced by photosynthesis
5. ATP used for active transport

Question 22

during photosynthesis, oil-pal trees convert CO2 into organic substances.

describe how (6)

Answer 22

1. rubisco combines CO2 with ribulose bisphosphate (RuBP)
2. which produces 2 molecules of GP
3. which are reduced to triose phosphate
4. using reduced NADP and using energy from ATP
5. triose phosphate converted to other organic substances, e.g. glucose
6. in Calvin cycle (light independent reaction)

Question 23

the concentration of CO2 in the air at different heights above ground changes over a period of 24 hours.

use your knowledge of photosynthesis to describe these changes and explain why they occur (5)

Answer 23

1. high concentration of CO2 linked with night/darkness
2. no photosynthesis in dark, light is required for the light dependent reaction
3. in dark, plants respire
4. in light, net uptake of CO2 is greater than rate of respiration
5. decrease in CO2 concentration as height increases
6. at ground level, less photosynthesising tissues

Question 24

in the light independent reaction of photosynthesis, the carbon in CO2 becomes carbon in triose phosphate.

describe how (5)

Answer 24

1. rubisco combines CO2 and RuBP
2. to produce 2 molecules of GP
3. which are reduced to triose phosphate
4. using reduced NADP
5. and energy from ATP

Question 25

describe how light energy absorbed by chlorophyll molecules is used to synthesise ATP (5)

Answer 25

1. excitation of chlorophyll electrons
2. electrons emitted from chlorophyll molecule
3. to electron transport chain
4. loss of energy by electrons along the chain
5. this energy is used to synthesise ATP from ADP + Pi

Question 26

in the light dependent reaction, describe how light energy generates ATP (5)

Answer 26

1. light energy excites electrons in chlorophyll
2. electrons pass down electron transfer chain
3. electrons reduce carriers
4. energy released from electron as it passes through chain
5. ATP is generated from ADP + Pi (phosphorylation of ATP)

Question 27

describe the light independent reactions of photosynthesis and explain how they allow the continued synthesis of hexose sugars (6)

Answer 27

1. RuBP combines with CO2
2. to form GP
3. using ATP
4. and reduced NADP
5. 2 molecules of TP form hexose
6. all RuBP is regenerated

Question 28

describe the role of electron transport chains in the light dependent reactions (6)

Answer 28

1. ETC accepts excited electrons
2. from chlorophyll
3. electrons lose energy along chain
4. ATP produced
5. from ADP + Pi
6. reduced NADP formed
7. when electrons and H+ combine with NADP
8. H+ from photolysis

Question 29

the photolysis of water is an important part of the process of photosynthesis.

describe what happens in the photolysis of water (2)

Answer 29

ANY TWO:

1. water forms H+ and e-
2. O2 formed
3. light excited electrons

Question 30

ATP and reduced NADP are two products of the light dependent reactions.

describe one function of each of these substances in the light independent reaction (2)

Answer 30

ATP: provides energy for GP to be converted to triose phosphate

reduced NADP: reduces GP to triose phosphate

Question 31

in which part of a chloroplast do light dependent reactions occur? (1)

Answer 31

grana / thylakoids / lamellae

Question 32

describe the part played by chlorophyll in photosynthesis (3)

Answer 32

1. absorbs light energy
2. excites electrons / raises energy level of electrons
3. ATP formed

Question 33

describe how NADP is reduced in the light dependent reaction of photosynthesis (2)

Answer 33

1. electrons
2. from chlorophyll

Question 34

explain why there is twice the amount of GP as RuBP when CO2 concentration is high (1)

Answer 34

RuBP combines with CO2 to produce 2x GP

Question 35

in the experiment the cells were supplied with radioactively labelled 14CO2.

explain why (1)

Answer 35

used in photosynthesis, allows detection of products

Question 36

explain how lack of light caused the amount of GP to rise (2)

Answer 36

1. ATP and reduced NADP not formed ( because no light dependent reaction)
2. GP is not being used to form RuBP

Question 37

explain why glucose concentration decreased after the light was switched off (1)

Answer 37

used in respiration

Question 38

which process is the source of the ATP used in the conversion of GP to TP (1)

Answer 38

light dependent reaction

Question 39

lowering temperature has little effect on the light dependent reaction, but it slows down the light independent reaction.

explain why the light independent reaction slows down at low temperatures (2)

Answer 39

1. enzymes are involved
2. slow rate of chemical reaction at low temperature
3. fewer collisions