Light-Independent Reaction

Question 1

Describe and explain The Calvin Cycle

Answer 1

• the light-independent reaction
• it takes place in the stroma of the chloroplasts
• it makes a molecule called triose phosphate from CO2 and ribulose bisphosphate (a 5- carbon compound)
• triose phosphate can be used to make glucose and other useful organic substances
• as its a cycle, ribulose bisphosphate is regenerated

Question 2

Describe and explain the first stage of the Calvin Cycle

Answer 2

1) Formation of glycerate 3-phosphate
Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast
Here its combined with ribulose bisphosphate (RuBP), a 5-carbon compound
This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP)
Ribulose bisphosphate carboxylase (RuBisCO) catalyses the reaction between CO2 and RuBP

Question 3

Describe and explain the second stage of the Calvin Cycle

Answer 3

2) Formation of triose phosphate

The 3-carbon compound GP is reduced to a different carbon compound called triose phosphate (TP).
ATP (from the light-dependent reaction) provides energy to do this
This reaction also requires H+ ions, which comes from NADPH
Reduced NADP is recycled to NADP (for use in the light-dependent reaction again)
TP is then converted into many useful organic compounds

Question 4

Describe and explain the third stage of the Calvin Cycle

Answer 4

3) Regeneration of ribulose bisphosphate

Five out of every six molecules of TP produced in the cycle arent used to make useful compounds, but to regenerate RuBP.
Regeneration RuBP uses the rest of the ATP produced by the light-dependent reaction

Question 5

Describe and explain hexose sugars

Answer 5

• a hexose sugar is a monosaccharide that has 6 carbon atoms e.g. glucose
• one hexose sugar is made by joining two molecules of TP together.
• The Calvin Cycle needs to turn six times to make one hexose sugar. This is because 3 turns of the cycle produces 6 molecules of TP ( as two molecules of TP are made for every one CO2). Five out of six of these TP molecules are used to regenerate RuBP.
  This means that for three turns of the cycle only one TP is produced that used to make a hexose sugar (a hexose sugar is made up of two TP molecules as one TP molecule has 3 carbon atoms)

Six turns of the cycle needs 18 ATP and 12 reduced NADP from the light-dependent reaction

Question 6

What other organic compounds are made from TP and GP?

Answer 6

• Carbohydrates - hexose sugars are made from two TP molecules and larger carbohydrates are made by joining hexose sugars together
• Lipids - these are made using glycerol, which is synthesised from TP, and fatty acids which are synthesised from glycerate 3-phosphate (GP)
• Amino acids- some amino acids are made from GP