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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