PHOTOSYNTHESISPlant Physiology

Photosynthesis is a chemical process in which light energy is converted into chemical energy.

Photosynthesis equation:

CO2 + H2O+ light energy →Sugar (glucose) + O2

Leaves of plants have chloroplasts (found in mesophyll cell) that are filled with chlorophyll, which capture the energy from the sunlight).

Chloroplast is consists of:1. Outer membrane.2. Innermembrane.3. Thylakoid membrane (in which light reaction take place).4. Granum ( stack of thylakoid).5. Stroma ( in which dark reaction take place).

Photosynthesis has to main phases:1.Light Reaction2.Dark Reaction ( Light Independent Reaction)

Light Reaction Required light and water (H2O).

ATP & NADPH molecules are form which utilize in Calvin Cycle.

O2 is released as a by product through stomata.

Calvin Cycle (Dark Reaction)

Discovered by Melvin Calvin & other scientists.

Takes place in stroma.

Doesn’t required light.

Other names C3 cycle or light independent reaction.

Use CO2 to form Glucose.

ATP use as energy.

NADPH use as reducing power.

6 turn of cycle make one molecule of Glucose.

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Involves 3 steps:

1. Carboxylation ( Carbon Fixation).

2. Reduction.

3. Regeneration.

1. Carboxylation ( Carbon Fixation ) Begins with 5 carbon sugar call ribulose

bisphosphate (RuBP).

CO2 combines with RuBP to form 2 molecules of 3-Phosphoglycerate (3-PGA or PGA).

First CO2 converts to a very unstable 6 carbon intermediate that is immediately converted to 2 molecules of 3-phosphoglycerate or 3-phosphoglyceric acid (3-PGA).

Carboxylation catalyzed by an enzyme called ribulose 1,5-bisphosphate carboxylase (RuBisCO= most abundant enzyme found on earth).

3RuBP + 3CO2 → 6 glycerate-3-phosphate

2.Reduction ATP and NADPH are used to reduce 3-PGA

(synthesized in phase 1) into glyceraldehyde 3-phosphate (G3P or PGAL), then ATP and NADPH are converted to ADP and NADP+, respectively.

An enzyme  3-phosphoglycerate kinase adds a phosphate from ATP to 3-PGA to make 1,3-bisphosphoglycerate.

Then another enzyme glyceraldehyde-3-phosphate dehydrogenase removes hydrogen from NADPH and adds it to the 1,3-bisphosphoglycerate to make glyceraldehyde-3-phosphate (G3P or PGAL)

The resulting NADP+, ADP, and Pi can be recycled to the light reactions.

2 3-PGA + 2ATP + 2NADPH

2G3P + 2ADP + NADP++ 2Pi

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3.Regeneration Regeneration of RuBP: G3P is converted back to RuBP to keep the

Calvin cycle going.

A complex series of reactions rearranges the carbon skeletons of 5 glyceraldehyde-3-phosphate molecules (G3P) into 3 RuBP molecules (using ATP).

G3P serves as the starting material for the synthesis of glucose and fructose.

One molecule of G3P leaves the Calvin Cycle and is used to make other organic compounds including amino acids, lipids, and carbohydrates.

RuBP is thus regenerated to begin the cycle again. 

5 G3P + 3 ATP 3 RuBP + 3 ADP + 2 Pi

Presented By:

Mehwish Akhter

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