Vital metabolism for survival of

life in the earth

Prof Adinpunya MitraAgricultural & Food Engineering Department

THE SUN: MAIN SOURCE OF

ENERGY FOR LIFE ON EARTH

• Almost all plants are photosynthetic autotrophs, as

are some bacteria and protists

– Autotrophs generate their own organic matter through

photosynthesis

– Sunlight energy is transformed to energy stored in the

form of chemical bonds

(a) Mosses, ferns, andflowering plants

(b) Kelp

(c) Euglena (d) Cyanobacteria

THE BASICS OF PHOTOSYNTHESIS

Light Energy Harvested by Plants &

Other Photosynthetic Autotrophs

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Different wavelengths of visible light are seen by the human eye as different colors

WHY ARE PLANTS GREEN?

Gamma

raysX-rays UV Infrared

Micro-

waves

Radio

waves

Visible light

Wavelength (nm)

Transmitted light

Sunlight minus absorbed

wavelengths or colors

equals the apparent color of

an object.

WHY ARE PLANTS GREEN?

Structure of Plant Cell and Chloroplast

Electron Microscopic View

Image of plant cells

Light Microscopic View

WHY ARE PLANTS GREEN?

Plant Cells

have Green

Chloroplasts

The thylakoid

membrane of the

chloroplast is

impregnated with

photosynthetic

pigments (i.e.,

chlorophylls,

carotenoids).

Chloroplast Structure• Inner membrane

called the thylakoidmembrane.

• Thickened regions calledthylakoids. A stack of thylakoids is called a granum. (Plural – grana)

• Stroma is a liquid surrounding the thylakoids.

• Chloroplasts

absorb light

energy and

convert it to

chemical energy

LightReflected

light

Absorbedlight

Transmittedlight

Chloroplast

THE COLOR OF LIGHT SEEN IS THE

COLOR NOT ABSORBED

• Photosynthesis is the process by which

autotrophic organisms use light energy to

make sugar and oxygen gas from carbon

dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

• The Calvin cycle makes

sugar from carbon

dioxide– ATP generated by the light

reactions provides the energy

for 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 solar energy to chemical energy– Produce ATP & NADPH

AN OVERVIEW OF PHOTOSYNTHESIS

Chloroplasts: Sites of Photosynthesis

• Photosynthesis

– Occurs in chloroplasts, All green plant parts

have chloroplasts and carry out

photosynthesis

• The leaves have the most chloroplasts

• The green color comes from chlorophyll in the

chloroplasts

• The pigments absorb light energy

• In most plants, photosynthesis occurs

primarily in the leaves, in the chloroplasts

• A chloroplast contains:

– stroma, a fluid

– grana, stacks of thylakoids

• The thylakoids contain chlorophyll

– Chlorophyll is the green pigment that captures

light for photosynthesis

Photosynthesis occurs in chloroplasts

The location and structure of chloroplasts

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana

• Chloroplasts contain several pigments

Chloroplast Pigments

– Chlorophyll a

– Chlorophyll b

– Carotenoids

Figure 7.7

Chlorophyll a & b•Chl a has a methyl

group

•Chl b has a carbonyl

group

Porphyrin ring

delocalized e-

Phytol tail

Different pigments absorb light

differently

Pigment Absorption

How Light is Captured?

The Light Reactions

(light dependent)

• Photosystem I…cyclic

photophosphorylation

• Photosystem II…noncyclic

photophosphorylation

• Photolysis

The Z scheme (Light Reactions)

Cyclic Photophosphorylation• Process for ATP generation associated with

some Photosynthetic Bacteria

• Reaction Center => 700 nm

Primaryelectron acceptor

Primaryelectron acceptor

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis

Noncyclic Photophosphorylation• Photosystem II regains electrons by splitting

water, leaving O2 gas as a by-product

Water-splittingphotosystem

NADPH-producingphotosystem

ATPmill

• Two types of

photosystems

cooperate in the

light reactions

Concept of Light Reaction

• The O2 liberated by photosynthesis is made

from the oxygen in water (H+ and e-)

Plants produce O2 gas by splitting H2O

2 H + 1/2

Water-splittingphotosystem

Reaction-center

chlorophyll

Light

Primaryelectronacceptor

Energyto make

Primaryelectronacceptor

Primaryelectronacceptor

NADPH-producingphotosystem

Light

NADP

1

2

3

How the Light Reactions Generate ATP and NADPH?

Z-scheme of Photosynthesis

• Two connected photosystems collect

photons of light and transfer the energy to

chlorophyll electrons

• The excited electrons are passed from the

primary electron acceptor to electron

transport chains

– Their energy ends up in ATP and NADPH

In the light reactions, electron transport

chains generate ATP, NADPH, & O2

Chemiosmosis powers ATP synthesis in

the light reactions

• The electron transport chains are arranged

with the photosystems in the thylakoid

membranes and pump H+ through that

membrane

– The flow of H+ back through the membrane is

harnessed by ATP synthase to make ATP

– In the stroma, the H+ ions combine with NADP+

to form NADPH

Chemiosmosis powers ATP

synthesis in the light reactions

• The production of ATP by chemiosmosis in

photosynthesis

Thylakoidcompartment(high H+)

Thylakoidmembrane

Stroma(low H+)

Light

Antennamolecules

Light

ELECTRON TRANSPORT

CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

• A Photosynthesis Road Map

Chloroplast

Light

Stack ofthylakoids ADP

+ P

NADP

Stroma

Lightreactions

Calvincycle

Sugar used for

Cellular respiration

Cellulose

Starch

Other organic compounds

The Calvin Cycle in

Photosynthesis

The Calvin cycle is the mechanism

for turning CO2 into glucose

Calvin Cycle Strategy

6CO2 Glucose

(Ideal)

(Real)

6CO2 + 6RuBP G6P + 6RuBP6C 30C 6C 30C

(Mechanism)

6CO2 + 6RuBP 12, 3PGA

The Calvin/Benson/C3 cycle

• Has 3 phases:

1. carbon fixation phase

2. reduction of CO2 phase

3. regeneration of RuBP phase

Light-independent reactions

(Dark Reactions)

The free energy of cleavage of ~P bonds of ATP, and reducing power of NADPH, are used to fix and reduce CO2 to form carbohydrate.

Enzymes & intermediates of the Calvin Cycle are located in the chloroplast stroma, a compartment somewhat analogous to the mitochondrial matrix.

grana disks

(thylakoids)

stroma

compartment

2 outer

membranes

Chloroplast

Calvin Cycle, earlier designated the photosynthetic "dark reactions," is now called the carbon reactionspathway:

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 OPO32-

OPO32-

O

3-Phosphoglycerate

(3PG)

OH

H2C

CH

COO

OPO32-

-

carbon fixation phase

Reduction of CO2

phase

Regeneration of RuBP

phase

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

Synthesis of Sucrose and Starch

F6P G6P G1P

ADP-glucose

ATP

PPi

(glucose)n

ADP + (glucose)n+1

Starch

(amylose)

UDP-glucose

fructose-6-PO4

UDP + Sucrose-6-PO4

H2O

Pi

UTP

PPi

Stroma

Cytosol

Sucrose

Thank goodness for

photosynthesis

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 as carbon source for synthesis of other compounds such as fatty acids & amino acids.

glyceraldehyde-

3-phosphate

OH

H2C

CH

CHO

OPO32

OCO

carbon

dioxide

Summary of

Calvin Cycle

Summary of

Calvin Cycle

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

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

OH

H2C

CH

COO

OPO 32

H2C

C

OPO 32

O

O

3-phospho- phosphoglycolate glycerate

Photorespiration:

O2 can compete with CO2 for binding to RuBisCO,

especially when [CO2] is low & [O2] is high.

Photorespiration is a wasteful process, substantially

reducing efficiency of CO2 fixation, even at normal ambient

CO2

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

Strategy for Preventing

Photorespiration

Fix CO2 in an environment shielded from O2

Use an enzyme that does not react with O2

Avoid RUBISCO

PLAN

Strategy (cont.)

SOLUTION

CO2 fixation occurs in Mesophyll cells

CO2 fixing enzyme is not RUBISCO

PEP carboxylase will not react with O2

RUBISCO never changedTake

HomeInstead plant anatomy changed

CO2 fixing enzyme is PEP carboxylase

Separation

in space (C4 cycle)

Vascular tissue

Malate

Bundle sheath cell

Mesophyll cell

surrounding

bundle sheath

CO2

PEP (3C)

C3CO2

OAA (4C)

Pyruvate (C3)

ATPADP

sugar

• CO2 directly

• RuBP recipient

• RUBISCO open

• O2 can interfere

• Photorespiration

likely

• CO2 indirectly

• PEP recipient

• RUBISCO shielded

• O2 cannot interfere

• No photorespriation

C3 vs C4 Plants

A Lesson in Photoefficiency

C3 C4

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

What did you Learn?

• Photorespiration is to be avoided

• RUBISCO is an oxidase/carboxylase

• Oxygen cuts photoefficiency in half

• C3 and C4 plants differ in anatomy

• Mesophyll cells fix CO2 to PEP

• Bundle sheath cells have Calvin cycle

• C4 plants grow more ferociously

Separation

in time (CAM cycle)

Mesophyll

Malat

In the light

In the dark

CO2

PEP (3C)

C3CO2

OAA (4C)

Pyruvate (C3)

ATPADP

sugar

• Carbon dioxide is broken and “fixed” into

glucose or fructose molecules in the CALVIN

CYCLE!!!!

• Glucose subunits can make cellulose or other

polysaccharides, such as fruit sugars.

• The carbon skeleton in glucose also helps to

synthesize other important biochemical

compounds such as, lipids, amino acids, and

nucleic acids.

Overview of the Dark Reactions

Overview of Photosynthesis

Summary Diagram of Photosynthesis

It's not that

easy bein'

green… but it

is essential for

life on earth!

•Why are leaves green?•What is chlorophyll?•Which cell organelle contains chlorophyll?• What are the ‘ units’ of light?•How are photons ‘captured’?

QUIZ

•What’s produced in PS I?

•What are the products of the Calvin cycle?

•Where in the chloroplast the Calvin cycle reactions occur?

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

Quiz