photosynthesis hour 3

7/30/2019 Photosynthesis Hour 3

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Powerpoint@lecture Slides Are Prepared By Biology Lecturer, KMPk
mailto:point@lecturemailto:point@lecture


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16.4 Light Independent

Reaction/Calvin cycle


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At the end of this topic, students should be able to :

- Describe Calvin cycle involving carbon fixation,

reduction of PGAL and regeneration of RuBP.


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Calvin Cycle

The Calvin Cycle uses the

chemical energy (ATP and

NADPH) to reduce CO2 to

sugar.

NADPH and ATP are

products from light

dependent reaction

RuBP regenerate afterCO2 enter the cycle and

leave the cycle as sugar.

Occur at stroma.


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Calvin Cycle

Anabolic pathway

- the cycle builds sugar from smaller molecules (CO2)

- by using ATP as an energy source

- and NADPH as reducing powerof electron carrier.


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Calvin Cycle

Carbohydrate produced

directly from the Calvin

cycle is actually not

glucose, but three

carbon sugar.

This sugar isGlyceraldehyde 3-

phosphate (G3P) /

PGAL


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Calvin Cycle

The Calvin cycle has

THREE phases

i. Carbon fixation(catalyzed by rubisco)

ii. Reduction of PGAL

iii. Regeneration of the

CO2 acceptor(RuBP)


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Calvin Cycle

For net synthesis of

1 molecules of G3P

- the cycle must

take place THREE

times

- fixing THREEmolecules of CO2.


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Phase 1: Carbon Fixation

Each CO2 (one at time) to attach with 5C sugar named

ribulose bisphosphate (RuBP).

Catalyzed by RuBP carboxylase (rubisco)


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Phase 1: Carbon Fixation

Produce six-carbon intermediate which is unstable.

So, it immediately split in half.

Form TWO molecules of3-phosphoglycerate/PGA.

(3C molecules)


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Phase 2: Reduction of PGAL

Each molecule of 3-phosphoglycerate receives an

additional phosphate group from ATP.

Forming 1,3-bisphosphoglycerate.

Spends 6 ATP.


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Phase 2: Reduction of PGAL

A pair of electron from each NADPH reduces 1,3-

bisphophoglycerate.

Which also loss a phosphate group.

Forming Glyceraldehyde 3-phosphate / PGAL.


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Phase 2: Reduction

of PGAL

For every THREE

molecules of CO2, SIX

molecules of G3P

formed. Only ONE molecules of

G3P counted as a net

gain of carbohydrate and

use by plant cell. Other FIVE molecules

must be recycled to

regenerate the 5C RuBP.


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Phase 3: Regeneration of RuBP

FIVE molecules of G3P (3C)

are rearranged into THREE

molecules of RUBP (5C). Cycle spends THREE more

molecules of ATP.

RuBP is ready to receives

another CO2. Cycle continues.


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Calvin Cycle

For the net synthesis of

ONE G3P molecule, theCalvin cycle consumed:

- NINE molecules of

ATP

- SIX molecules ofNADPH


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Calvin Cycle

To form ONE molecules of sugar (glucose), the cycle

needs SIX molecules of CO2.

The complete cycle must occurTWICE.


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Calvin Cycle: Phase 1


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1) With the aid of labeled diagram, explain the main

stages of Calvin Cycle.

2) What is the function of NADP+?


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16.5 Alternative mechanisms of

carbon fixation : Hatch-Slack

(C4) and Crassulacean Acid

Metabolism (CAM) pathways

photosynthesis hour 3

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