Photosynthesis

Question 1

what are the two stages of photosynthesis?

Answer 1

• light dependent reaction
• light independent reaction

Question 2

where does the light-dependant reaction take place?

Answer 2

in the thylakoid membrane in the chloroplast

Question 3

where does the light-independant reaction take place?

Answer 3

in stroma of the chloroplast

Question 4

equation for the light-dependent reaction?

Answer 4

light energy + water -> ATP + reduced NADP (+oxygen)

Question 5

what are the four key stages for the light-dependent reaction?

Answer 5

1. Photolysis
2. Photoionisation of chlorophyll
3. Chemiosmosis
4. Photophosphorylation

Question 6

what happens during the photolysis of water stage?

Answer 6

• light energy is absorbed by chlorophyll and splits into o2, H+ and e-
• 2h2- -> o2 + 4e- + 4h+
• the e- are passed along a chain of electron carrier proteins
• the oxygen is either used for respiration or diffuses out of the leaf through the stomata

Question 7

what happens during the photoionisation of chlorophyll stage?

Answer 7

• light energy is absorbed by chlorophyll and the energy results in the electrons becoming excited and raising up an energy level to leave chlorophyll
• some energy from the reduced electrons is used to make ATP and reduced NADP in chemiosmosis

Question 8

what happens during chemiosmosis?

Answer 8

• electrons move down the electron transfer chain from PSII to PSI via redox reactions, losing energy at each step
• some energy is used to actively transport protons from the stroma to thylakoid membrane
• this creates an electrochemical gradient across the thylakoid membrane (higher in thylakoid than in stroma)
• protons move via facilitated diffusion down the gradient into the stroma via the enzyme ATP synthase embedded in the thylakoid membrane (energy from this allows ADP + Pi -> ATP - photophosphorylation)

Question 9

how is reduced NADP produced?

Answer 9

in PSI electrons are excited and transferred to NADP (with a proton from photolysis) to reduce NADP to form reduced NADP

Question 10

what else is the light-independent reaction known as?

Answer 10

the calvin cycle

Question 11

what does the fluid in the stroma contain?

Answer 11

the enzyme rubisco
- because of this, the reaction is temperature sensitive as it involves an enzyme (no light energy needed)

Question 12

equation for the calvin cycle?

Answer 12

co2 + reduced NADP + ATP -> hexose sugar

Question 13

what is the role of atp and reduced nadp in the calvin cycle?

Answer 13

atp provides energy for GP -> TP
reduced NADP provides H/electrons for GP -> TP

Question 14

what are the stages of the calvin cycle?

Answer 14

1. co2 (1C) reacts with rubp (ribulose biphosphate) [5C] catalysed by the enzyme rubisco
2. this produces 2 molecules of glycerate 3-phosphate (GP) [3C]
3. GP is reduced to triose phosphate (TP) [3C] using energy from the hydrolysis of ATP and H+ from reduced NADP
   - some TP is converted into useful organic substance such as glucose
   - TP is used to regenerate rubp [5C] (using the rest of ATP)

Question 15

what is a limiting factor?

Answer 15

a factor is limiting if when it’s made a more favourable value, the rest of photosynthesis increases - until photosynthesis is limited by a different factor

Question 16

how does temperature affect the rate of photosynthesis?

Answer 16

• rate of photosynthesis increases as temperature increases up to an optimum, decreases after
• limits light independent reaction as it’s enzyme controlled (rubisco)
• increasing temperature up to optimum means more kinetic energy for enzymes and more e-s complexes (rubisco)
• when above optimum, h bonds in tertiary structure break -> active site changes shape / enzyme denatured (rubisco), fewer e-s complexes

Question 16

how does carbon dioxide concentration affect the rate of photosynthesis?

Answer 16

• rate of photosynthesis increases as carbon dioxide concentration increases (then plateaus)
• if carbon dioxide concentration dramatically decreased:
  1. limits light independent reaction
  2. less co2 to combine with RuBP to form GP
• less GP reduced to TP
• less TP (and GP) converted to organic substances e.g, hexose to regenerate RuBP

Question 17

what are some common agricultural practices used to overcome the effect of these limiting factors?

Answer 17

• growing plants under artificial lighting to maximise light intensity or heating a greenhouse to increase the temperature and burning fuel, such as paraffin burners to release more carbon dioxide
• if limiting factors are minimal, rate of photosynthesis will increase so:
  1. faster production of glucose allowing faster respiration
  2. more ATP to provide energy for growth, e.g, cell division, protein synthesis
  3. therefore a higher yield so more profit