LIR

Question 1

Where does Light indepdent reaction take place?

Answer 1

• In stroma.

Question 2

How many carbons do RUBP/ CO2 contain?

Answer 2

• RUBP: 5 carbons
• CO2: 1 carbon.

Question 3

What does the light- independent reaction do?

Answer 3

• Uses reduced NADP, from light-dependent reaction, to form simple sugar - RUBP.

Question 4

What provides the additional energy required for the light-indepdent reaction? Why is energy required?

Answer 4

• Hydrolysis of ATP, from light-dependent reaction, provides the additional energy required for LIR.
• Energy is required in LIR process as it’s enzyme-controlled process.
• Energy required to convert glycerate-3-phosphate into triose phosphate.

Question 5

What enzyme catalyses the reaction between RUBP and CO2 in LIR?

Answer 5

• Rubisco.

Question 6

Different steps of LIR.

Answer 6

1.) CO2 reacts with RUBP to form 2 molecules of GP, catalysed by enzyme-rubisco.
2.) Energy from ATP hydrolysis/ reduced NADP (from LDR) are used to reduce GP to triose phosphate.
3.) Some triose phosphate used to regenerate RUBP in Calvin’s cycle.
4.) Some triose phosphate converted to useful organic substances.

MS response

Question 7

Abbreviated = GP. Give full name.

Answer 7

• Glycerate-3-phosphate.
• U CANNOT ABBREVIATE THIS IN EXAM!

Question 8

Abbreviated = RUBP. Give full name.

Answer 8

• Ribulose biphosphate

Question 9

Explain why two glycerate-3-phosphate molecules are formed when RUBP and CO2 react together.

Answer 9

• RUBP = 5 carbons/ CO2 = 1 carbons.
• Glycerate-3-phosphate = 3 carbons, so, 2 must be formed to balance out the 6 in RUBP/ CO2!

Question 10

How is glycerate-3-phosphate reduced to triose phosphate?

Answer 10

• Reduced to triose phosphate by gaining protons/ electrons from reduced NADP, formed in light-dependent reaction.
• Reduced NADP becomes NADP.
• NADP becomes involved in light-dependent reaction again.

Question 11

What is the cycle where triose phosphate is used to regenerate RUBP in the light-independent reaction called?

Answer 11

• Calvin’s cycle.

Question 12

How many triose phosphate molecules are formed when the GP molecules are reduced? and why?

Answer 12

• 2 GP molecules reduced to form 2 triose phosphate molecules.
• Triose phosphate = 3 carbons, so, need 2 to balance out the 6 carbons in 2 GP molecules.

Question 13

Give examples of the useful organic substances triose phosphate could be converted to.

Answer 13

• Glucose
• Joined glucose monomers can form starch/ cellulose.

Question 14

PMT 2016-2021 Photosynthesis.

Why does faster LIR lead to faster growth in plants?

Answer 14

• More/ faster synthesis of organic compounds (ie. glucose –> used to make amino acids.)
• Produce more sugars that can be used in respiration –> so have more energy for growth.

Question 15

PMT 2022-2024 photosynthesis

If very high light intensities can inhibit the release of electrons from chlorophyll.
Why would increased light intensity lead to a decrease mean CO2 uptake?

(4 marks.)

Answer 15

1.) Less ATP/ NADPH produced.
2.) Less glycerate-3-phosphate converted into triose phosphate.
3.) Less RuBP is regenerated.
4.) Less RuBP to react with CO2 (decreasing CO2 uptake.)

() - extra info for clarification.

Question 16

Consequence of less CO2 uptake.

Answer 16

• Less CO₂ uptake –> less RuBP regenerated.
• So, less RuBP reacts with CO₂