Photosynthesis

Question 1

What is photosynthesis?

Answer 1

A process in which plants absorb light energy and convert it to chemical potential energy. Simple inorganic molecules are converted into organic molecules.

Question 2

What is the features of chloroplasts?

Answer 2

• Outer and inner membrane
• Stroma
• Thylakoids in a stack called a granum

Question 3

What is the equation for photosynthesis?

Answer 3

6CO₂ + 6H₂O –(light)–> C₆H₁₅O₆ + 6O₂
Carbon dioxide + water –(light)–> Glucose + oxygen

Question 4

What is OILRIG?

Answer 4

O - Oxidation
I - is
L - loss of electrons / hydrogen

R - Reduction
I - is
G - gain of electrons / hydrogen

Question 5

What is a redox reaction?

Answer 5

The gain or loss of hydrogen or electrons.

Question 6

What is NADP?

Answer 6

It is a molecule which acts as a hydrogen acceptor that becomes reduced to NADPH.
NADPH provides reducing power within reactions as it loses the hydrogen, becoming oxidised to NADP.

Question 7

Where does the light dependent reaction occur?

Answer 7

In the thylakoids of chloroplasts.

Question 8

What are the stages of the LDR?

Answer 8

• Photoionisation
• Chemiosmosis
• Photolysis
• Reduction of NADP

Question 9

What happens during photoionisation?

Answer 9

Chlorophyll absorb a photon of light. This causing the electrons to gain energy, reaching a higher energy level (get excited) and leave the chlorophyll.

Question 10

What happens during chemiosmosis?

Answer 10

Electrons move along electron transport chain releasing energy, this energy is used to pump H+ across the thylakoid membrane from the stroma into the intermembrane space.

H+ flow back through via ATP synthase and energy is used to join ADP and Pi to form ATP (Phosphorylation

Question 11

What happens during photolysis?

Answer 11

Photolysis of water produces H+, electrons and oxygen, the electron replace those lost by the chlorophyll.

2H₂O -> O₂ + 4H+ + 4e-

Question 12

What is the role of NADP in LDR?

Answer 12

The electrons from the electron Transport chain combine with NADP and the H+ ions from photolysis and the H+ that have passed back into the stroma to form reduced NADP (NADPH

Question 13

Where does the light independent reaction/calvin cycle occur?

Answer 13

Stroma

Question 14

What molecules does the LIR use from LDR?

Answer 14

ATP and NADPH

Question 15

What happens during the Calvin cycle?

Answer 15

1. Carbon dioxide combines with ribulose bisphosphate / RuBP;
2. Produces two glycerate (3-)phosphate (GP)
3. GP reduced to triose phosphate
4. Using reduced NADP;
5. Using energy from ATP;
6. Triose phosphate converted to glucose / hexose / RuBP / ribulose bisphosphate / named organic substance;

Question 16

What factors affect photosynthesis?

Answer 16

· Temperature
· Carbon Dioxide concentration
· Light intensity

Question 17

How do commercial glasshouses work?

Answer 17

Growers can apply knowledge of limiting factors to enhance temperature, carbon dioxide concentration and light intensity. Manipulation of these factors may increase productivity and therefore produce higher yields. Glasshouses can be used to grow crops in areas which otherwise the crop would not grow and also allow crops to be grown all year round.

1. Artificial light may be used to increase light intensity to the optimum levels. Blinds may be used to shade plants if light intensity is very high as very high light intensity may damage the chlorophyll
2. Carbon dioxide levels can be increased by burning fossil fuels ( e.g. paraffin heaters ) or pumping CO2 into the glasshouse
3. Heaters can be used to raise the temperature to optimum levels ( also used at night to prevent temperatures falling to levels where plants may be damaged)

Question 18

Explain the role of chlorophyll in photolysis. (3)

Answer 18

Absorbs light / energy;
Loses electrons / becomes positively charged / is oxidised; Accepts electrons from water / from OH– which causes more water to dissociate / pulls equilibrium to the right;

Question 19

Explain how the energy of light is converted into chemical energy in the light-dependent reactions. (3)

Answer 19

Electrons raised to higher energy level / electrons excited; Use of electron carriers / cytochromes / acceptors; For production of ATP

Question 20

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

Answer 20

1. 5C / RuBP combines with CO2;
2. to form 3C compound / TP / GP;
3. using ATP;
4. and reduced NADP / eq;
5. 2 molecules of 3C compound / TP / GP form hexose;
6. all RuBP is regenerated;
7. 10 molecules of 3C / TP / GP form 6 molecules of 5C / RuBP

Question 21

Describe the role of electron transport chains in. the light-dependent reactions of photosynthesis

Answer 21

1. electron transport chain accepts excited electrons;
2. from chlorophyll / photosystem;
3. electrons lose energy along chain;
4. ATP produced;
5. from ADP and Pi;
   6 reduced NADP formed;
6. when electrons (from transport chain) and H+ combine with NADP;
7. H+ from photolysis

Question 22

Crops use light energy to produce photosynthetic products. Describe how crop plants use light energy during the light-dependent reaction. (5)

Answer 22

**1.     Excites electrons / electrons removed (from chlorophyll); **
Accept: higher energy level as ‘excites’.
**2.      Electrons move along carriers/electron transfer chain releasing energy; **
Accept: movement of H+/protons across membrane releases energy.
Reject: ‘produces energy’ for either mark but not for both.
**3.      Energy used to join ADP and Pi to form ATP; **
Reject: ‘produces energy’ for either mark but not for both.
Accept: energy used for phosphorylation of ADP to ATP
Do not accept P as Pi but accept phosphate.
**4.      Photolysis of water produces protons, electrons and oxygen; **
**5.      NADP reduced by electrons / electrons and protons / hydrogen; **
Accept: NADP to NADPH (or equivalent) by addition of electrons/hydrogen.
Do not accept NADP reduced by protons on its own.

Question 23

During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how. (5 marks)

Answer 23

1.      Carbon dioxide combines with ribulose bisphosphate / RuBP;
2.      Produces two glycerate (3-)phosphate / GP;
Accept: any answer which indicates that 2 x as much GP produced from one RuBP.
3.      GP reduced to triose phosphate / TP;
Must have idea of reduction. This may be conveyed by stating m.p. 4.
4.      Using reduced NADP;
Reject: Any reference to reduced NAD for m.p.4 but allow reference to reduction for m.p. 3.
5.      Using energy from ATP;
Must be in context of GP to TP.
6.      Triose phosphate converted to glucose / hexose / RuBP / ribulose bisphosphate / named organic substance;

Question 24

ATP is useful in many biological processes. Explain why.(4 marks

Answer 24

1.      Releases energy in small / manageable amounts;
Accept less than glucose
2.      (Broken down) in a one step / single bond broken immediate energy compound / makes energy available rapidly;
Accept easily broken down
3.      Phosphorylates / adds phosphate makes (phosphorylated substances) more reactive / lowers activation energy;
Do not accept phosphorus or P on its own
4. Reformed / made again;
Must relate to regeneration

Question 25

The concentrations of carbon dioxide in the air at different heights above ground in a forest changes over a period of 24 hours. Use your knowledge of photosynthesis to describe these changes and explain why they occur. (5 marks)

Answer 25

1.      High concentration of carbon dioxide linked with night / darkness;
Accept: converse of low in day
2.      No photosynthesis in dark / night / light required for photosynthesis / light-dependent reaction;
Ignore references to rate of photosynthesis in day / night
Accept day = light
3.      (In dark) plants (and other organisms) respire;
Must be a reference to plants or all organisms
4.      In light net uptake of carbon dioxide by plants / plants use more carbon dioxide than they produce / rate of photosynthesis greater
than rate of respiration;
Do not allow converse for this point
Accept description of compensation point
5.      Decrease in carbon dioxide concentration with height;
Accept: converse of increase closer to ground
6.      At ground level fewer leaves / less photosynthesising
tissue / more animals / less light;

Question 26

Describe the role of electron transport chains in the light-dependent reactions of photosynthesis. (5 marks)

Answer 26

1       electron transport chain accepts excited electrons;
2       from chlorophyll / photosystem;
3       electrons lose energy along chain;
4       ATP produced;
5       from ADP and Pi;
6       reduced NADP formed;
7       when electrons (from transport chain) and H+ combine with NADP;
8       H+ from photolysis;