Photosyn UG

7/31/2019 Photosyn UG

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Agricultural and Food EngineeringDepartment

Indian Institute of Technology Kharagpur

PHOTOSYNTHESIS

Prof. S. Dutta Gupta


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Photosynthesis is the process by which

autotrophic organisms use light energy to

make sugar and oxygen gas from carbon

dioxide and water

Definition of Photosynthesis

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS


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Light Energy Harvested by Plants & Convertedto Chemical Energy

6 CO2 + 6 H2O + light energy C6H12O6 + 6

O2


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internal leaf structure

chloroplastsouter membrane

inner membrane

thylakoid

Plant Leaves & Chloroplast(Sites of Photosynthesis)


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Chloroplast Structure

Thickened disclike structures arecalled thylakoids.A stack ofthylakoids iscalled a granum.(Plural grana)

Stroma is a liquid

surrounding thethylakoids.


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Chloroplasts

absorb light

energy andconvert it to

chemical energy

Light Reflectedlight

Absorbedlight

Transmittedlight Chloroplast


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Photosynthetic pigments are arranged asphotosystems


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Different pigments absorb light

differently


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The Calvin cycle makessugar from carbon

dioxide ATP generated by the light

reactions provides the energyfor sugar synthesis

The NADPH produced by the

light reactions provides the

electrons for the reduction of

carbon dioxide to glucose

Light

Chloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

The light reactions

convert solarenergy to chemicalenergy Produce ATP & NADPH

Overall Reactions of Photosynthesis


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Chlorophyll a & bChl a has a methyl

group

Chl b has a carbonyl

group

Porphyrin ring

delocalized e-

Phytol tail


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The Light Reactions

(light dependent)

Photosystem Icyclic

photophosphorylation

Photosystem IInoncyclic

photophosphorylation

Photolysis


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Cyclic Photophosphorylation Process for ATP generation associated with

some Photosynthetic Bacteria

Reaction Center => 700 nm


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Primaryelectron acceptor

Primaryelectron acceptor

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis

Non-Cyclic Photophosphorylation Photosystem II regains electrons by splitting

water, leaving O2 gas as a by-product


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The Calvin Cycle in

Photosynthesis

The Calvin cycle is the mechanismfor turning CO2 into carbohydrate


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The Calvin-Benson/C3 cycle

Has 3 phases:

1. carbon fixation phase

2. reduction of CO2 phase3. regeneration of RuBP phase

Light-independent reactions

(Dark Reactions)


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The free energy of cleavage of ~P bonds ofATP, andreducing power ofNADPH, are used to fix and

reduce CO2 to form carbohydrate.

Enzymes & intermediates of the Calvin Cycle arelocated in the chloroplast stroma, a compartment

somewhat analogous to the mitochondrial matrix.

grana disks

(thylakoids)

stromacompartment

2 outer

membranes

Chloroplast

Calvin Cycle,earlier designated

the photosynthetic"dark reactions," isnow called thecarbon reactionspathway:


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Ribulose Bisphosphate Carboxylase (RuBP Carboxylase),catalyzes CO2 fixation:

ribulose-1,5-bisphosphate + CO2 2 3-phosphoglycerate

Because it can alternatively catalyze an oxygenase reaction,the enzyme is also called RuBP Carboxylase/Oxygenase(RuBisCO). It is the most abundant enzyme on earth.

Ribulose-1,5-bisphosphate(RuBP)

OH

H2C

CH

C

C

OHH

H2C OPO 32-

OPO 32-

O

3-Phosphoglycerate

(3PG)

OH

H2C

CH

C

OO

OPO 32-

-


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carbon fixation phase


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Reduction of CO2

phase


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Regeneration of RuBPphase


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3-PGA

1,3 bisPO4 glycerate

Glyceraldehyde-3-PO4

Fructose 1,6bisPO4

Glucose-6-PO4

DHAP

Calvin

Calvin

Calvin

ATP

ADP

NADPH + H+

NADP+

Fructose-6-PO4

12c

18c

6c

24c 6c

12c

36c

36c

Carbon Balance in

Calvin Cycle


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Synthesis of Sucrose and Starch

F6P G6P G1P

ADP-glucose

ATP

PPi

(glucose)n

ADP + (glucose)n+1

Starch(amylose)

UDP-glucosefructose-6-PO4

UDP + Sucrose-6-PO4

H2O

Pi

UTP

PPi

Stroma

Cytosol

Sucrose


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Thank goodness for

photosynthesis


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CO2 + ATP + NADPH

Glyceraldehyde-3-P + ADP + Pi + NADP

+

Glyceraldehyde-3-P may be converted to other CHO:

metabolites (e.g., fructose-6-P, glucose-1-P)

energy stores (e.g., sucrose, starch)

cell wall constituents (e.g., cellulose).Glyceraldehyde-3-P can also be utilized by plant cells ascarbon source forsynthesis ofother compounds such asfatty acids & amino acids.

glyceraldehyde-

3-phosphate

OH

H2C

CH

CHO

OPO32

OCO

carbon

dioxide

Summary of

Calvin Cycle


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Summary of

Calvin Cycle


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CO


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Separationin space (C4 cycle)

Vascular tissue

Malate

Bundle sheath cell

Mesophyll cell

surrounding

bundle sheath

CO2

PEP (3C)

C3CO2

OAA (4C)

Pyruvate (C3)

ATPADP

sugar

CO


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Separationin time (CAM cycle)

Mesophyll

Malat

In the light

In the dark

CO2

PEP (3C)

C3CO2

OAA (4C)

Pyruvate (C3)

ATPADP

sugar


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CO2

directly

RuBP recipient

3 PGA is the first

product

O2 can interfere Photorespiration

CO2

indirectly

PEP recipient

4 C organic acid is the

first product (Malate,

Aspartate)

O2 cannot interfere

No photorespriation

C3 vs C4 Plants

A Lesson in PhotoefficiencyC3 C4

C d C Pl t


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C3 and C4 Plants

soybean

wheat

rice

sugar beet

alfalfa

spinach tobacco

sunflower

corn

sorghum

sugar cane millet

crab grass

Bermuda grass pigweed

C3 C4

OO2 O2


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PHOTORESPIRATION

Definition 1:

An interference with carboxylation caused by

the deviant interaction of RUBISCO with oxygen

The aberrant use of oxygen by chloroplasts

A process that leads to only one 3PGA being

produced in the dark reaction in

chloroplasts

Definition 2:

O2O2

O2 O2

O2

O2


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When O2 reacts with ribulose-1,5-bisphosphate, theproducts are 3-phosphoglycerate plus the 2-Ccompound 2-phosphoglycolate.

This reaction is the basis for the name RuBPCarboxylase/Oxygenase (RuBisCO).

OH

H2C

CH

C

OO

OP O32

H2C

C

OP O32

O

O

3-phospho- phosphoglycolateglycerate

Photorespiration:

O2 can compete with

CO2for binding to RuBisCO,especially when [CO2] is low& [O2] is high.

Photorespiration is a wasteful process, substantiallyreducing efficiency of CO2 fixation, even at normal ambient

CO2


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Overview of Photosynthesis
http://www.flickr.com/photos/ghonet/27421952/


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It's not thateasy bein'

green but itis essential forlife on earth!
http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/http://www.flickr.com/photos/ghonet/27421952/


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Why are leaves green?

What is chlorophyll?

Which cell organelle contains chlorophyll?

What are the units of light?

How are photons captured?

QUIZ


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Which photosystem is involved insplliting of water in photosynthesis

Where in the chloroplast the Calvin cyclereactions occur?

What is the most prevalent enzyme onthe planet? Why is it so important?

Quiz


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What is the first product of photosynthesisin C4 plants?

Name the key enzyme of C4 metabolism

What is Kranz anatomy?

What are the advantages of C4 plants?

Quiz


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What is chlorophyll fluorescence?

What is non-photochemical quenching

Quiz

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