Chapter 10 Photosynthesis 3 Calvin cycle

Lecture 28

Photosynthesis 2 phases

1.nThe light reaction (“photo”)

The calvin cycle (“synthesis”)

The calvin cycle (dark reaction)

Uses ATP and NADPH from the light reaction to convert CO2 to sugar

Where does the calvin cycle occur

Stroma

3 phases of the calvin cycle

1. Carbon fixation

2. Reduction

3. Regeneration of RuBP (the CO2 acceptor)

RuBP

CO2 acceptor

G3P, glyceraldehyde-3-phosphate

A 3 carbon molecule

What sugar leaves while carbon enters the cycle as CO2

G3P

How many times must the calvin cycle take place for the synthesis of one G3P, fixing 3 carbon molecules of CO2

3 times

1. Carbon fixation

Rubisco enzyme catalyzes the CO2 fixation reaction

CO2 binds ribulose bisphosphate (RuBP)

The resulting 6-carbon molecule is unstable and immediately falls apart into 2 3 carbon molecules

2. Reduction

6 ATP are used to phosphorylate the six 3-carbon molecules

Electrons from 6 NADPH reduce the 6 molecules to G3P

6 G3P are produced, one G3P leaves the cycle as a product

3. Regeneration of RuBP

The other 5 G3P remain in the cycle and are rearranged to become 3 RuBP

Calvin cycle inputs

6 NADPH

9 ATP

3 CO2

Calvin cycle outputs 

G3P

Cyclic Electron Flow

for making extra ATP

Only PSI is used and the first ETC

Only ATP is produced

Dehydration

Stomata when open:

CO2 enters the leaf

H2O and O2 exit

Photorespiration

An energy-wasteful process in C3 plants

In C3 plants, initial fixation of CO2 is by ——- which has a weak affinity for CO2

Rubisco

1.      Which of the inputs to the Calvin cycle come from the light reaction? How many are made per one light reaction?

1 ATP and 1 NADPH

C4 plants

Separate CO2 collection from the calvin cycle using space 

CAM plants

Seperate CO2 from the calvin cycle using time

C4 plants bind CO2 in ——-

Mesophyll cells

PEP carboxylase

Enzyme that fixes CO2 onto PEP

Where is C4 exported to

Bundle sheath cells

CAM plants (Crassulacean acid metabolism) 

Open thier stomata at night when cooler to minimize water loss; incorporate CO2 into organic acids

In what conditions would your plant need to close its stomata? 

They close on hot dry days to prevent water loss through transpiration

How do you conserve water

By closing the stomata

Why does closing stomata lead to photorespiration?

Closing stomata lowers CO2 and raises O2 inside the leaf

1. What enzyme contributes to

photorespiration and why? 

This causes rubisco to bind O2 instead of CO2 which begins photorespiration because it can act as both a carboxylase and an oxygenase.