Photosynthesis

Question 1

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

Answer 1

Light-dependent: in the thylakoids of chloroplasts
Light-independent: stroma of chloroplasts

Question 2

Explain the role of light in photoionisation

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 chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which releases energy.

Question 5

How is a proton concentration gradient established during chemiosmosis?

Answer 5

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

Question 6

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

Answer 6

H+ ions (protons) 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 and are used to reduce the coenzyme NADP
e-: replaces electrons lost from chlorophyll
O2: 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
the 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

What happens during carbon fixation?

Answer 11

The reaction between CO2 and ribulose bisphosphate (RuBP) catalysed by Rubisco.
Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)

Question 11

Name the 3 main stages in the Calvin cycle.

Answer 11

1. Carbon fixation
2. Reduction
3. Regeneration

Question 11

What happens during reduction in the Calvin cycle?

Answer 11

2x GP is reduced to 2X triose phosphate (TP).
Requires 2x reduced NADP and 2X ATP
Forms 2x NADP and 2x ADP

Question 12

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

Answer 12

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

Question 13

What happens during regeneration in the Calvin cycle?

Answer 13

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

Question 14

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

Answer 14

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 15

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

Answer 15

RuBP:5
GP: 3
TP:3

Question 16

Describe the structure of a Chloroplast.

Answer 16

-Usually disc-shaped
-Double membrane
-Thylakoids: flattened discs stacks to form grana.
- Intergranal Lamellae: tubular extension attaches thylakoids in adjacent grana
- Stroma: fluid-filled matrix

Question 17

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

Answer 17

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

Question 18

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

Answer 18

-Own DNA and ribosomes for the synthesis of enzymes e.g. rubisco
-Concentration of enzymes and substrates in stroma is high.

Question 19

Define Limiting factor.

Answer 19

Factors that determine the maximum rate of a reaction, even if other factors change to become more favourable.

Question 20

Name 4 environmental factors that can limit the rate of photosynthesis.

Answer 20

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

Question 21

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

Answer 21

• Artificial light, especially at night
  -Artificial heating
• Addition of CO2 to green house atmosphere.

Question 22

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

Answer 22

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

Question 23

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

Answer 23

Dependent variable: rate of O2 production / CO2 consumption
1. use a potometer
2. place balls of calcium alginate containing green algae in hydrocarbonate indicator ( colour change orange→ magenta as CO2 is consumed and pH increases).

Question 24

State the purpose and principle of paper chromatography.

Answer 24

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

Question 25

Outline a method for extracting photosynthetic pigments

Answer 25

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

Question 26

Outline how paper chromatography can be used to separate photosynthetic pigments

Answer 26

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

Question 27

What are Rf values? How can they be calculated?

Answer 27

• Ratios that allow comparison of how far molecules have moved in chromatograms.
  -Rf value= distance between origin and center of pigment spot/ distance between origin and solvent front.