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