5.1 - Photosynthesis

Question 1

Describe what happens during photoionisation in the light-dependent reaction. (2)

Answer 1

• Chlorophyll absorbs light
  /Light excites/moves electrons in chlorophyll
• Electron/s are lost
  /(Chlorophyll) becomes positively charged

Question 2

Heat stress decreases the light dependent reaction of photosynthesis. Explain why this leads to a decrease in the light-independent reaction. (2)

Answer 2

• (Less/no) ATP
• (Less/no) reduced NADP

Question 3

A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis. Explain why. (2)

Answer 3

• (Less/no) carbon dioxide (reacts) with RuBP
• (Less/no) GP

Question 4

Where precisely is rubisco found in a cell? (1)

Answer 4

• Stroma (of/in chloroplast)

Question 5

Atrazine binds to proteins in the electron transfer chain in chloroplasts of weeds, reducing the transfer of electrons down the chain. Explain how this reduces the rate of photosynthesis in weeds. (4)

Answer 5

• Reduced transfer of protons across thylakoid membrane / Reduced chemiosmotic gradient / proton gradient across thylakoid membrane
• (So) less ATP produced
• (So) less reduced NADP produced
• (So) light-independent reaction slows / stops
  /Less reduction of GP to triose phosphate

Question 6

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

Answer 6

• Excites electrons / electrons removed (from chlorophyll)
  ‘accept higher energy level for excites’
• Electrons move along carriers/electron transfer chain releasing energy
  ‘accept movement of H+/protons across membrane releases energy’
• Energy used to join ADP and Pi to form ATP
  ‘accept energy used for phosphorylation of ADP to ATP’
• Photolysis of water produces protons, electrons and oxygen
• NADP reduced by electrons / electrons and protons / hydrogen

Question 7

Explain the relationship between stomatal opening and photosynthesis. (2)

Answer 7

• Stomata allow uptake of carbon dioxide
• Carbon dioxide used in / required for photosynthesis

Question 8

Describe what happens in photosynthesis during the light-dependent reaction. (5)

Answer 8

• The 5-carbon molecule / RuBP combines/binds with CO2
• To form a 3-carbon compound/triose phosphate/glycerate 3 phosphate
• Using 2 ATP and reduced 2 NADP
• 2 molecules of the 3-carbon compound/triose phosphate/glycerate 3-phosphate form a 6-carbon compound/hexose sugar
• All the ribulose bisphosphate/RuBP is regenerated
  -10 molecules of the 3-carbon compound/triose phosphate/glycerate 3-phosphate are formed from 6 molecules of 5-carbon compound/RuBP