Photosynthesis

Question 1

where does the light-dependent reaction occur

Answer 1

thylakoids of chloroplasts

Question 2

where does light-independent reaction occur

Answer 2

stroma of chloroplasts

Question 3

explain the role of light in photoionisation

Answer 3

chlorophyll molecules absorb energy from photons of light

this ‘excites’ 2 electrons, raising them to a higher energy level, causing them to be released from chlorophyll

Question 4

name the 2 main stages involved in ATP production in the light-dependent reaction

Answer 4

1. electron transfer chain
2. chemiosmosis

Question 5

what happens in the electron transfer chain

Answer 5

electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which release energy

Question 6

how is proton concentration gradient established during chemiosmosis

Answer 6

some energy released from the ETC is coupled to the active transport of H+ ions from stroma into the thylakoid space

Question 7

how does chemiosmosis produce ATP in the light-dependent stage

Answer 7

H+ ions move down concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase

ATP synthase catalyses ADP + Pi –> ATP

Question 8

explain the role of light in photolysis

Answer 8

light energy splits molecules of water
2H2O –> 4H+ + 4e- + O2

Question 9

what happens to the produces of the photolysis of water

Answer 9

H+ ions move out of thylakoid space via ATP synthase and are used to reduce the coenzyme NADP

e- replace electrons lost from chlorophyll

O2 used for respiration or diffuses out of leaf as waste gas

Question 10

how and where is reduced NADP produced in the light-dependent reaction

Answer 10

NADP + 2H+ + 2e- –> NADPH

catalysed by dehydrogenase enzymes

stroma of chloroplasts

Question 11

where do the H+ ions and electrons used to reduce NADP come from

Answer 11

H+ ions : photolysis of water

electrons: NADP acts as the final electron acceptor of the electron transfer chain

Question 12

name the 3 stages in calvin cycle

Answer 12

1. carbon fixation
2. reduction
3. regeneration

Question 13

what happens during carbon fixation

Answer 13

reaction between CO2 and ribulose biphosphate (RuBP) catalysed by rubisco
forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)

Question 14

what happens during reduction in calvin cycle

Answer 14

2x GP are reduced to 2x triose phosphate (TP)

requires 2x reduced NADP and 2x ATP

forms 2x NADP and 2x ADP

Question 15

how does the light-independent reaction result in the production of useful organic substances

Answer 15

1C leaves the cycle (i.e. some of the TP is converted into useful organic molecules)

Question 16

what happens during regeneration (in the Calvin cycle)

Answer 16

• after 1C leaves the cycle, the 5C compound RuP forms
• RuBP is regenerated from RuP using 1x ATP
• forms 1x ADP

Question 17

state the roles of ATP and NADPH in the light-independent reaction

Answer 17

• ATP: reduction of GP to TP and provides phosphate group to convert RuP into RuBP
• NADPH : coenzyme transports electrons needed for reduction of GP to TP

Question 18

state the number of carbon atoms in RuBP, GP and TP

Answer 18

RuBP = 5
GP = 3
TP = 3

Question 19

structure of a chloroplast

Answer 19

• usually disc-shaped
• double membrane
• thylakoids = flattened discs stack to form grana
• intergranal lamellae = tubular extensions attach thylakoids in adjacent grana
• stroma = fluid filled matrix

Question 20

how does the structure of chloroplast maximise rate of light-dependent reaction

Answer 20

• ATP synthase channels within granal membrane
• large surface area of thylakoid membrane for ETC
• photosystems position chlorophyll to enable maximum absorption of light

Question 21

how does the structure of the chloroplast maximise the rate of the light-independent reaction

Answer 21

• own dna and ribosome for synthesis of enzymes e.g. rubisco
• concentration of enzymes and substrates in stroma is high

Question 22

define ‘limiting factor’

Answer 22

factor that determines maximum rate of a reaction, even if other factors change to become more favourable

Question 23

name 4 environmental factors that can limit the rate of photosynthesis

Answer 23

• light intensity (light-dependent stage)
• CO2 levels (light independent stage)
• temperature (enzyme-controlled steps)
• mineral/magnesium levels (maintain normal functioning of chlorophyll)

Question 24

outline some common agricultural practices used to overcome the effect of limiting factors in photosynthesis

Answer 24

• artificial light, especially at night
• artificial heating
• addition of CO2 to greenhouse atmosphere

Question 25

why do farmers try to overcome the effect of limiting factors

Answer 25

• to increase yield
• additional cost must be balanced with yield to ensure maximum profit

Question 26

suggest how a student could investigate the effect of a named variable on the rate of photosynthesis

Answer 26

dependent variable: rate of O2 production / CO2 consumption

1. use a potometer
2. place balls of calcium alginate containing green algae in hydrogen carbonate indicator (colour change orange –> magenta as CO2 is consumed and pH increases)

Question 27

state the purpose and principle of paper chromatography

Answer 27

molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper)

Question 28

outline a method for extracting photosynthetic pigments

Answer 28

use a pestle and mortar to grind a leaf with an extraction solvent e.g. propanone

Question 29

outline how paper chromatography can be used to separate photosynthetic pigments

Answer 29

1. use a capillary tube to spot pigment extract onto pencil ‘start line’ 1 cm above bottom of paper
2. place chromatography paper in solvent (origin should be above solvent level)
3. allow solvent to run until it almost touches the other end of the paper. pigments move different distances