Topic 5.1 - Photosynthesis

Question 1

What are the 2 stages of photosynthesis?

Answer 1

1) Light dependent reaction
2) Light independent reaction

Question 2

Where does the light dependent reaction occur?

Answer 2

Thylakoid membrane of chloroplast

Question 3

Where does the light independent reaction occur?

Answer 3

Stroma of chloroplast

Question 4

Describe photoionisation in the light dependent reaction (2)

Answer 4

• Chlorophyll absorbs light energy which excites its electrons (higher energy level)
• So electrons are released from chlorophyll (chlorophyll becomes positively charged)

Question 5

Describe what happens after photoionisation in the light dependent reaction (5)

Answer 5

1) Electrons move along electron transfer chain (electron carriers), releasing energy
2) This energy is used to actively pump protons from stroma into thylakoid
3) Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase
4) Energy used to join ADP + Pi to form ATP (photophosphorylation)
5) NADP accepts a proton and an electron to become reduced NADP

Question 6

Describe photolysis of water in the light dependent reaction (1+1)

Answer 6

• Water splits to produce protons, electrons and oxygen
  (Electrons replace those lost from chlorophyll)

Question 7

Describe the light independent reaction of photosynthesis (Calvin cycle) (5)

Answer 7

1) CO2 reacts with ribulose biphosphate (RuBP)
- Catalysed by the enzyme rubisco

2) Forming 2 glycerate 3-phosphate (GP) molecules

3) GP reduced to triose phosphate (TP)
- Using products from light dependent reaction - reduced NADP and energy from ATP

4) Some TP converted to useful organic substances (E.g. glucose)

5) Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)

Question 8

What are the 3 main factors affecting rate of photosynthesis?

Answer 8

1) Temperature

2) Light intensity

3) CO2 concentration

Question 9

Describe and explain how temperature affects rate of photosynthesis (1+2, 1+2)

Answer 9

1) As temperature increases, the rate increases
- Enzymes E.g. rubisco gain kinetic energy
- So more enzyme-substrate complexes form

2) Above an optimum temperature, the rate decreases]
- Enzymes denature as H bonds in tertiary structure break
- So fewer enzyme-substrate complexes form

Question 10

Describe and explain how light intensity affects rate of photosynthesis (1+2, 1+1)

Answer 10

1) As light intensity increases, rate increases
- Light dependent reaction increases so more ATP and reduced NADP produced
- So light independent reaction increases as more GP reduced to TP and more TP regenerates RuBP

2) Above a certain light intensity, rate stops increasing
- Another factor is limiting e.g. temperature / CO2 concentration

Question 11

Describe and explain how CO2 concentration affects rate of photosynthesis (1+4, 1+1)

Answer 11

1) As CO2 concentration increases, rate increases
- Light independent reaction increases
- As more CO2 combines with RuBP to form GP
- So more GP reduced to TP
- So more TP converted to organic substances and more RuBP regenerated

2) Above a certain CO2 concentration, rate stops increasing
- Another factor is limiting E.g. temperature / light intensity

Question 12

Explain the key consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors (2)

Answer 12

1) Agricultural practice should increase rate of photosynthesis, leading to increased yield
- As more glucose produced for faster respiration
- So more ATP to release energy for growth e.g cell division / protein synthesis

2) But profit from extra yield should be greater than costs