L25 - Photosynthesis 2

Question 1

What is another name for the Light Independent reactions in photosynthesis?

Answer 1

The Calvin Cycle

Question 2

Who was the Calvin cycle named after?

Answer 2

One of 2 scientists Benson and Calvin who established the reactions pathways

Question 3

Where does the Calvin take place?

Answer 3

In the Stroma of chloroplasts

Question 4

what does the first reaction of the Calvin cycle involve? What is it commonly called?

Answer 4

It involves the fixation of CO2

It is called the C3 reaction - as the first compound identified following CO2 fixation has 3C atoms

Question 5

describe the reaction of how CO2 is fixed

Answer 5

CO2 + ribulose-1,5-bisphosphate (RBP) —>

2(3C compound 3-phosphoglycerate)

Question 6

What is the C3 reaction catalysed by?

Answer 6

Rubisco

Question 7

After the C3 reaction what has to happen?

Answer 7

• RBP has to be regenerated
• And produce something from the cycle - Net gain of carbon

This is done by using the ATP and NADPH formed in the light reactions

Question 8

Describe how RBP is regenerated

Answer 8

It is done in 2 reactions:

1) The hydrogen group on both 3C molecules is replaced by a phosphate using an ATP molecule for each:

2 3-phospho-glycerate + 2Pi —–> 2 1,3-bisphospho-glycerate
(Basically COOH goes to COOP on each)

2) Then H+ is donated from NADPH to each:

2 1,3-bisphospho-glycerate + 2H+ —-> 2 Glyceraldehyde-
3-phosphate (G3P) + 2Pi
(COOP goes to CHO)

Question 9

What does Triose-phosphate isomerase do?

Answer 9

Glyceraldehyde-3-P (G3P) —–> Dihydroxyacetone-P (DHAP)

Question 10

The enzymes Aldolase and Phosphatase work together, describe what they do

Answer 10

Aldolase:
Aldose-P + DHAP ——-> Ketose-bisP

Phosphatase:
Ketose-bisP + H2O ——-> Ketose-P + Pi

(basically aldose phosphate goes to Ketose phosphate and a spare Pi)

Question 11

What does Transketolase do?

Answer 11

Transketolase:

Ketose-P + Aldose-P ——-> Aldose-P + Ketose-P

(Basically it turns a K into an A and an A into a K by adding/removing 2 carbons)

Question 12

What does Isomerase do?

Answer 12

Converts an aldose to its corresponding Ketose:

Ribose-5-P ——–> Ribulose-5-P

Question 13

What does Epimerase do?

Answer 13

Epimerase: moves a hydroxyl group fro one side of the carbon backbone to the other side

Xylulose-5-P ———-> Ribulose-5-P

Question 14

What does Ribulose-5-P kinase do?

Answer 14

Ribulose-5-P kinase:

Ribulose-5-P + ATP ——–> Ribulose-1,5-bisP + ADP

Basically reforms RBP

Question 15

Describe the Calvin Cycle in 4 steps

Answer 15

STEP 1

• start with 6RubP reacts with 6CO2
• forms 6 unstable 6C compounds which immediately turn into 12 3PG

STEP 2

• 12 3PG uses 12 ATP and 12 NADPH to form 12 G3P
• 2 of these G3P are then taken out of the cycle and are used to create a 6C sugar

STEP 3
- 10 G3P is converted into 6 Ru5P

STEP 4
- 6 Ru5P then uses 6 ATP to form the original 6RubP

Question 16

Give a modification of the Calvin Cycle in CAM plants

Answer 16

CO2 is stored as malate in vacuoles, so that in the day malate can be broken down to CO2 and Pyruvate to be used for photosynthesis when their stomata are closed in the day

Question 17

What is the Hatch-Slack or C4 reaction?

Answer 17

If CO2 in atmosphere is low, rubisco carries out an oxidation reaction instead of fixating CO2.

To combat this :
- CO2 is converted to malate in the mesophyll cells

• this is then taken to the bundle sheath cell where it breaks down into CO2 and Pyruvate
• this increases the concentration of CO2 to a level that rubisco will work so it can be then used for photosynthesis