Topic 5-A: Photosynthesis and Respiration-3. Photosynthesis and the light independent reaction

Question 1

Where does the light-independent reaction occur?

Answer 1

• In the stroma of the chloroplasts

Question 2

What does the light-independent reaction make?

Answer 2

• It makes a molecule called triose phosphate from carbon dioxide and ribulose phosphate

Question 3

What are the stages of the light-independent reaction?

Answer 3

1. C02 enters the leaf through the stomata and diffuses into the stroma of the chloroplast
2. Carbon dioxide (1 carbon compound) combines with ribulose biphosphate (RuBP) (5 carbon compound) which is catalysed by the enzyme rubisco. 6 carbon compound breaks down into 2 molecules of a 3 carbon compound called glycerate phosphate.
3. ATP is hydrolysed (from light-dependant reaction) providing the energy needed to reduce the 3 carbon compound (GP) to a different 3 carbon compound called triose phosphate (2x)
4. GP is converted to triose phosphate with energy released from ATP->ADP + Pi and with the reducing power of NADPH. NADP and ADP and pi are reused in the light dependant reaction
5. ATP from the light dependant reaction is used to regenerate ribulose biphosphte from triose phosphate. Some Tp molecules are converted into glucose and modified into fats/proteins.

Question 4

What is a hexose sugar?

Answer 4

• Simple 6-carbon sugars e.g. glucose
• One hexose sugar is made by joining 2 molecules of triose phosphate together
• They can be used to make larger carbohydrates
• The light independent reaction needs to turn 6 times to make one hexose sugar
• Reason for this is that 3 turns of the cycle produces 6 molecules of triose phosphate as 2 molecules of TP are made for every C02 molecule used
• 5 out of the 6 TP molecules are used to regenerate ribulose biphosphate
• This means for the 3 turns of thr cycle only one TP is produced that’s used to make a hexose sugar
• Due to a hexose sugar having 6 carbons, two TP molecules would be needed to form one hexose sugar so it must turn 6 times to produce 2 molecules of TP

Question 5

How are hexose sugars made?

Answer 5

• They are made from two triose phosphate molecules

Question 6

How are larger carbohydrates made?

Answer 6

• They are made by joining hexose sugars together in different ways

Question 7

How are lipids made?

Answer 7

• These are made by using glycerol, which is synthesised from triose phosphate, and fatty acids which are synthesised from glycerate 3-phosphate