5.1 Photosynthesis

Question 1

Where do the light-dependent & light-independent reactions occur in plants?

Answer 1

light-dependent - thylakoids of chloroplasts
light-independent - stroma of chloroplasts

Question 2

Explain the role of light in photoionization

Answer 2

Chlorophyll molecules absorb energy from photons of light.
This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll

Question 3

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

Answer 3

1. Electron-transfer chain
2. Chemiosmosis

Question 4

What happens in the electron transfer chain (ETC)?

Answer 4

Electrons released from the chlorophyll move down a series of carrier proteins embedded in the thylakoid membranes & undergo a series of redox reactions, which releases energy.

Question 5

How is a proton concentration gradient established during chemiosmosis?

Answer 5

thylakoidhas a higher concentration of protons than the stroma which froms a proton concentration gradient.

Question 6

How does chemiosmosis produce ATP in the light-dependent stage?

Answer 6

H+ ions move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase.
ATP synthase catalyses ADP +Pi –> ATP

Question 7

Explain the role of light in photolysis

Answer 7

Light energy splits molecules of water
2H20 –> 4H+ + 4E- +O2

Question 8

What happens to the products of the photolysis of water?

Answer 8

. H+ ions: move out of the thylakoid space via ATP synthase & are used to reduce the coenzyme NADP
. e-: reduce electrons lost from the chlorophyll
. 02: used for respiration or diffuses out of leaf as waste gas.

Question 9

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

Answer 9

.NADP +2H + 2e- –> reduced NADP
. Catalysed by dehydrogenase enzymes
. stroma of chloroplasts

Question 10

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

Answer 10

. H+ ions: photolysis of water
Electrons: NADP acts as the final electron acceptor of the electron transfer chain

Question 11

Name the 3 main stages of the Calvin cycle

Answer 11

1. Carbon Fixation
2. Reduction
3. Regeneration

Question 12

What happens during carbon fixation?

Answer 12

. Reaction between CO2 & ribulose biphosphate (RuBP) catalysed by rubisco
. Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)

Question 13

What happens during reduction (in the Calvin cycle)?

Answer 13

. 2x GP are reduced to 2x triose phosphate (TP)
. Requires 2x reduced NADP & 2x ATP
. Forms 2x NADP & 2X ATP

Question 14

How does the light independent reaction result in the production of useful organic substances?

Answer 14

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

Question 15

What happens during regeneration (in the Calvin cycle)?

Answer 15

. After 1C leaves the cycle, the 5C compound RuP forms
. RuBP is regenerated from RuP using 1X ATP
. Forms 1x ADP

Question 16

State the role of ATP & (reduced) NADP in the light-independent reaction

Answer 16

. ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP
. (reduced) NADP : coenzyme transports electrons needed for reduction of GP to TP

Question 17

State the number of carbon atoms in RuBP, GP & TP

Answer 17

RuBP = 5
GP = 3
TP = 3

Question 18

Describe the structure of a chloroplast

Answer 18

. Usually disc-shaped
. Double membrane (envelope)
. Thylakoids: flattened disc stack to form grana
. Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
Stroma: fluid-filled matrix

Question 19

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

Answer 19

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

Question 20

How does the structure of the chloroplast maximise the rate of the light independent reaction?

Answer 20

. Owns DNA & ribosomes for synthase of enzymes e.g. rubisco
. concentration of enzymes & substrates in stroma is high

Question 21

Define “limiting factor”

Answer 21

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

Question 22

Name 4 environmental factors that can limit the rate of photosynthesis

Answer 22

. light intensity
. co2 levels
. temperature
. magnesium levels

Question 23

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

Answer 23

. Artificial light, especially at night
. Artificial heating
. Addition of CO2 to greenhouse atmosphere

Question 24

Why do farmers try to overcome the effect of limiting factors?

Answer 24

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

Question 25

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

Answer 25

Dependent variable: rate of 02 production/ CO2 consumption
1. use of potometer
2. Place balls of calcium alginate containing green algae in hydrogen carbonate indicator ( colour change from orange to magenta)

Question 26

State the purpose and principle of paper chromatography

Answer 26

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

Question 27

Outline a method for extracting photosynthetic pigments

Answer 27

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

Question 28

Outline how paper chromatography can be used to separate photosynthetic pigments.

Answer 28

1. Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin) 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.

Question 29

What are RF values? How are they calculated?

Answer 29

. Ratios that allow comparisons of how far molecules have moved in chromatograms.
. Rf values = distance between origin and centre of pigment spot/ distance between origin and solvent front.