5.1 Photosynthesis

Question 1

What are the stages of photosynthesis?

Answer 1

1. Light dependent reaction
   ○ Thylakoid membrane of chloroplast
2. Light independent reaction
   ○ Stroma of chloroplast

Question 2

Describe the structure of a chloroplast

Answer 2

Question 3

Describe photoionisation in the light-dependent reaction (LDR)

Answer 3

Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP (chemiosmotic theory):

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 and Pi to form ATP (photophosphorylation)
5. NADP accepts a proton and an electron to become reduced NADP

Question 4

Describe photolysis of water in the LDR

Answer 4

● Water splits to produce protons, electrons and oxygen (H2O → 1⁄2 O2 + 2e- + 2H+)
○ Electrons replace those lost from chlorophyll

Question 5

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

Answer 5

1. CO2 reacts with ribulose bisphosphate (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 (eg. glucose)
5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)

Question 6

Describe image for the Calvin cycle

Answer 6

Question 7

Describe and explain how temperature affects rate of photosynthesis

Answer 7

1. As temperature increases, rate increases
   ○ Enzymes eg. rubisco gain kinetic energy
   ○ So more enzyme-substrate complexes form
2. Above an optimum temperature, rate decreases
   ○ Enzymes denature as H bonds in tertiary structure break
   ○ So fewer enzyme-substrate complexes form

Question 8

Describe and explain how light intensity affects rate of photosynthesis

Answer 8

1. As light intensity increases, rate increases
   ○ Light-dependent reaction increases (eg. more photoionisation of chlorophyll) 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 eg. temperature / CO2 concentration

Question 9

Describe and explain how CO2 concentration affects rate of photosynthesis

Answer 9

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 eg. temperature / light intensity

Question 10

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

Answer 10

● 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 eg. cell division, protein synthesis

● But profit from extra yield should be greater than costs (money & environmental costs)