5.1 Photosynthesis ❗️

Question 1

Where do light-dependent reactions occur in plants?

Answer 1

In the thylakoids of chloroplasts

Question 2

Where do light-independent reactions occur in plants?

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 releases energy.

Question 6

How is a proton concentration gradient established during chemiosmosis?

Answer 6

Some energy released from the electron transport chain is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space

Question 7

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

Answer 7

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

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 products of the photolysis of water?

Answer 9

H+ ions move out of the thylakoid space via ATP synthase and are used to reduce the coenzyme NADP.
The electrons replace electrons lost from chlorophyll
O2 is used for respiration or diffuses out of the leaf as waste gas

Question 10

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

Answer 10

• NADP + 2H+ + 2e- ——> reduced NADP
• 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 main stages in the 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 bisphosphate (RuBP) catalysed by rubisco.
Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)

Question 14

What happens during reduction in the Calvin cycle

Answer 14

2 x GP are reduced to 2 X triose phosphate (TP)
This requires 2 X reduced NADP and 2 X ATP
Forms 2 X NADP and 2 X 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

Outline the sequence of events in the light-independent reaction(Calvin cycle)

Answer 17

Slide 34 on pmt 5.1 photosynthesis

Question 18

State the roles of ATP and reduced NADP in the light-independent reaction

Answer 18

ATP: reduction of GP to TP and provides phosphate group to convert RuP into RuBP

Reduced NADP: coenzyme transports electrons needed for reduction of GP to TP

Question 19

State number of carbon atoms in RuBP, GP and TP

Answer 19

RuBP: 5

GP:3

TP:3

Question 20

Describe the structure of a chloroplast

Answer 20

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

Question 21

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

Answer 21

-ATP synthase channels within granal membrane
- large surface area of thylakoid membrane for electron transport chain
- photosystems position chlorophyll to enable maximise absorption of light

Question 22

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

Answer 22

Own DNA and ribosomes for synthesis of enzymes e.g. rubisco

Concentration of enzymes and substrates in stroma is high

Question 23

Define ‘limiting factor’

Answer 23

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

Question 24

Name 4 environmental factors that can limit the rate of photosynthesis

Answer 24

Light intensity (light-dependent stage)
CO2 levels (light-independent stage)
Temperature(enzyme-controlled steps)
Mineral/magnesium levels(maintain normal functioning of chlorophyll)

Question 25

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

Answer 25

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

Question 26

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

Answer 26

To increase yield

Additional cost must be balanced with yield to ensure maximum profit

Question 27

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

Answer 27

Dependent variable: rate of O2 production/ CO2 consumption

1. Use potometer
2. Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange —> magenta as CO2 is consumed and pH increases)

Question 28

State the purpose and principle of paper chromatography

Answer 28

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

Question 29

Outline a method for extracting photosynthetic pigments

Answer 29

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

Question 30

Outline how paper chromatography can be used to separate photosynthetic pigments

Answer 30

1. Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin) 1cm 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 31

What are Rf values? How can they be calculated?

Answer 31

Ratios that allow comparison of how far molecules have moved on chromatograms.

Rf values = distance between origin and centre of pigments spot/ distance between origin and solvent front