Download photosynthesis in higher plants

PHOTOSYNTHESIS IN HIGHER PLANTS
Topic
1. Introduction
2. Where does photosynthesis takes
place?
3. What is Light reaction?
4. Cyclic Photophosphorylation
5. Where are the ATP and NADPH
used?
6. Factors affecting Photosynthesis
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03
05
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Syllabus
Photosynthesis in higher Plants
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PHOTOSYNTHESIS IN HIGHER PLANTS
INTRODUCTION
“Photosynthesis is a Physico - chemical or photo-biochemical process in which the organic
compound (carbohydrates) are synthesised from the inorganic raw material (H2O & CO2) in the
presence of light & pigments. O2 is evolved as by product or one of the net products”.
6CO2 + 12H2O





Sunlight
Chlorophyll
C 6H 12O 6 + 6H2O + 6CO 2
Light energy is converted into chemical energy by photosynthesis.
First of all photosynthesis started in eubacteria.
First true & oxygenic (O2 evolving) photosynthesis started in cyanobacteria. (BGA)
Photosynthesis is a redox reaction during which oxidation of H2O occurs (as it provides H
+
and e–) during light reaction and reduction of CO2 occurs (as it accepts H+ & e–) during
dark reaction (biosynthetic phase)
Photosynthesis is a Anabolic (synthesising) & Endergonic (Energy absorbing) process.
WHAT DO WE KNOW



Some simple experiments show that chlorophyll (green pigment of the leaf), light and CO2 are
required for photosynthesis to occur.
Look for starch formation in two leaves - a variegated leaf or a leaf that was partially
covered with black paper and one that was exposed to light. On testing these leaves for starch
it was clear that photosynthesis occured only in the green parts of the leaves in the
presence of light.
Another experiment is the half-leaf experiment, where a part of a leaf is enclosed in a test
tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed
to air. The setup is then place in light for some time. On testing for starch later in the two halves
of the leaf, the exposed part of the leaf test positive for starch while the portion that was in the
tube, tested negative. This shows that CO2 is required for photosynthesis.
EARLY EXPERIMENTS

Stephen Hales (1727) :- Recognised the important of air (CO2) and light for photosynthesis
(nourishment) in plants. He is considered as discoverer of photosynthesis and “ Father of
plant physiology”.
(Father of Indian plant physiology - J.C. Bose)
J. Priestley (1770) :- He carried out very interesting exp. with Bell jar, Rat, Pudina &
Candle. He came conclusion that plants purify the air (gaseous exchange), during
photosynthesis. Discoverer of oxygen Phlogiston ⇒ Bad air from candles, Dephlogiston
⇒ Good air
Experiment by Joseph Priestley :
Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential
role of air in the growth of green plants.
Priestley observed that a candle curning in a closed space - a bell jar, soon gets extinguished.
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Photsynthesis in Higher Plants # 1
Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or
an animal that breathe the air, both some how ,damage the air. But when he placed a mint plant in
the same bell jar, he found that the mouse stayed alive and the candle continued to burn.
Prisetley hypothesised as follows : plants restore to the air whatever breathing animals
and burning candles remove.
Jan Ingenhousz (1779) :- Importance of light and green colour. O2 is released in the presence
of light by green parts.
Experiment by Jan Ingenhouzs :
Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the
sunlight, Jan Ingenhouzs (1730 - 1799) showed that sunlight is essential to the plant process
that somehow purifies the air fouled by burning candles or breathing animals.
Ingenhouzs in an elegant experiment with an aquatic plant showed that in bright sunlight, small
bubbles were formed around the green parts while in the dark they did not . Later he identified these
bubbles to be of oxygen. Hence he showed that it is only the green part of the plants
that could release oxygen.
J.V. Sachs (1854) :- Recognised the relation among photosynthesis, chloroplast and starch. First
visible product of photosynthesis is starch.
Engelmann (1888) :- Described Action spectrum of photosynthesis by using Cladophora, Spirogyra algae and aerobic bacteria experiment.
Experiment by Engelmann :
T.W. Engelmann (1843 - 1909) Used a prism to split light into its spectral components and
then illuminated a green algae. Cladophora placed in a suspension of aerobic bacteria. The
bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated
mainly in the region of red and blue light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption spectrum of chlorophyll a and b.
Van Neil (1931) :- O2 release from water not from CO2 . Experiment on purple and green
sulphur bacteria.
H2A = suitable oxidisable compound or H-Donor
Light
2H2O + CO2 
 2A + CH2O + H2O
Bacteria
6CO2 + 12H2S 
 C6H12O6 + 12S + 6H2O
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Photosynthesis in Higher Plants # 2
Ruben, Hassid & Kamen (1941) :- Used O18 (radioisotopic technique) to show
experimentally that O2 in photosynthesis released from water.
Light
6CO162  12H 2 O18 Chlorophyll
C 6 H12 O166  6H 2 O16  6O 218
Robert Hill & Bendal (1937) :- Detailed study of light reaction in isolated chloroplast of Stellaria
plant.
Photolysis of H2O is the chief role of chloroplast and evolution of O2 occurs only in the presence of
suitable e– acceptor from water in photosynthesis. (Hill-reaction). Electron acceptors = Potassium
Ferricyanide. NADP+ and DCPIP (Dichlorophenol Indophenol -a dye)
WHERE DOES PHOTOSYNTHESIS TAKE PLACE
Photosynthesis does take place in the green leaves of plants but it does so also in other
green parts of the plants.
Within the chloroplast there is the fluid stroma and the membranous system consisting of grana
and the stroma lamelle. There is a clear division of lobour within the chloroplast.
The membrane system is responsible for trapping the light energy and also for the synthesis
of ATP and NADPH + H+ [Light reaction]. In stroma, enzymatic reactions incorporate CO2 into
the plant leading to the synthesis ofsugar, which in turn forms starch. (Biosynthetic phase/
Dark reaction)
400 - 700 nm light is used in photosynthesis, so it is known as PAR (Photosynthetic active radiation).
HOW MANY PIGMENTS ARE INVOLVED IN PHOTOSYNTHESIS
1.
2.
3.
1.
Photosynthetic pigments are special molecules those absorb, transmitt and reflect different
colours of light from the visible spectrum of light. pigment appears in the colour which it reflect
and uses the colour which it absorbs.
Photosynthetic Pigments are of following types :Chlorophylls
Carotenoids
Phycobillins - > Present in lower plant
Chlorophylls Chlorophyll are of following types
Chl. a, Chl - b, Chl - c, Chl - d, Chl - e, Bacterio Chl. - a, Bacterio Chl. - b and Bacterioviridin
(Chlorobium chlorophyll)
Bacterio Chl. - a, Bacterio Chl. - b, are of purple colour. Remaining chlorophylls are of green
colour of various shades (Chl. a - bright or blue green in chromatogram and Chl. b - yellow
green).
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Photsynthesis in Higher Plants # 3
Structure of Chlorophyll Chlorophylls are magnesium porphyrin compounds. Porphyrin ring consist 4-pyrrole rings
(Tetrapyrrole).
Chlorophyll molecule has a Mg - porphyrin head & phytol tail. Head is hydrophillic & phytol
tail is lipophillic (Hydrophobic).
Size of chlorophyll molecule - head (15 Ao × 15 Ao) and tail (20 Ao)
Cholorophyll (a)
Chlorophyll - a C55H72O5N4Mg

CH3 group in IInd pyrrol ring
Chlorophyll - b C55H70O6N4Mg

CHO group in IInd pyrrole ring
Chl -a is universal pigment, which is found in all O2 librating photosynthetic organisms.
Chlorophyll synthesis Succinyl CoA + Glycine  Protochlorophyll (Protochlorophyllide)
light
2H
2.
Chlorophyll
Carotenoids (Yellow to yellow orange) The first carotenoid was discovered in carrot by Wackenroder and was named carotene.
Carotenoids are the first type of Carotenoids. These are yellow orange in colour. It contains
carbon and hydrogen.
Another carotenoid is xanthophyll that contains carbon, hydrogen and oxygen. These are usually yellow in colour.
Among carotenoids, - carotene (type of carotene) and luttein (type of xanthophyll) are
common in plants
Graph (a) showing the ability of pigments to absorb lights of different wavelengths (absorption
spectrum) Chl. - a shows maximum absorption at blue light. It show another absorption at
blue light. It show another absorption peak at red light.
Graph (b) showing the wavelengths at which photosynthesis occurs in a plant (Action spectrum).
The wavelengths at which there is maximum absorption by chlorophyll a, i.e. in the blue
and the red regions, also show higher rate of photosynthesis. Hence, we can conclude that
chlorophyll - a is the chief pigment associated with photosynthesis.
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Photosynthesis in Higher Plants # 4
Absorption spectrum of photosynthesis - Blue red.
Action spectrum of photosynthesis - Red blue
These graphs, together show that most of the photosynthesis takes place in the red and blue
region of the spectrum; some photosynthesis does take place at the other wavelengths of
the visible spectrum. It happens because accessory pigment also absorb light of these
regions and transfer the energy chlorophyll.
Accessory pigments (mainly carotenes), also protect chlorophyll -a (reaction centre) from
photo-oxidation
WHAT IS LIGHT REACTION
Emerson & Arnold worked on chlorella and gave concept of two photosystem or two pigment system.
The groups of photosynthetic pigments in thylakoid membranes are known as photosystem.
The pigments are organised as two discrete photochemical light harvesting complexes (LHC)
within the Photosystem I (PS I) and Photosystem II (PS II).
Photosystem = Reaction centre + LHC
In every photosystem there is a reaction centre (molecule of chlorophyll -a) surrounded by
accessory pigments. The accessory pigments absorb light energy and transfer it to the
reaction centre. These pigments help to make photosynthesis more efficient by absorbing
different wavelengths of light. These molecules are known as antenna molecules or LHC. The
LHC are made up of hundreds (250 - 400 molecules) of pigment molecules bound to proteins.
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Photsynthesis in Higher Plants # 5
The reaction centre is different in both the photosystem. In PS I the reaction centre chlorophylla has an absorption peak at 700 nm. hence is called P700, while in PS II it has absorption peak
at 680 nm, and is called P680.
These are named in the sequence of their discovery, and not in the sequence in which
they function during the light reaction.
The PS II is located in the appressed region of granal thylakoids and PS I is non appressed
region of grana and in stroma thylakoids. (In the other way we can say that granal thylakoids
have both PS I and PS II whereas stroma thylakoids have only PS I).
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Photosynthesis in Higher Plants # 6
THE ELECTRON TRANSPORT, SPLITTING OF WATER AND PHOTOPHOSPHORYLATION
(A)
Light Rxn./Hill reaction/Photochemical reaction/ Generation of assimilatory power
(NADPH2 + ATPS) photophase.
(I)
Z-Scheme/Non-cyclic PhotophosphorylationBoth PS - I and PS - II are involved in Non cyclic photophosphorylation. So It occurs at
grana thylakoids on not in stroma thylaloids because stroma thylakoids lack PS -II as
well as NADP reductase enzymes
Primary e– acceptor from PS -II is pheophytin, which passes electrons to an electrons
transported system (ETS) consisting of cytochromes. This movement of electrons is
downhill, in terms of oxidation-reduction or redox potential.
The electrons emitted from PS -II pass through the electron transport chain to the pigments
of PS I
Simultaneously, electrons in the reaction centre of PS I are also excited when they
receive red light wavelength 700 nm and are transferred to another acceptor molecule (FRS)
and then to Fd. The electrons then move downhill again, this time to a molecule of energy
rich NADP+. The addition of these electrons reduces NADP+ to NADPH + H+ (Protons
from stroma). At every oxidation/reduction of e– carrier take place and energy released which
is utilized in creation of proton gradient to form ATP
This whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down
the electron transport chain to PS I, excitation of electrons, transfer to another accepter, and
finally down hill to NADPH called the Z-scheme, due to its characteristic shape.
This Z-shape is formed when all the carriers are placed in a sequence on a redox
potential scale
The e– ejected from PS- II by absorption of light energy never comeback to Chl-a-680
(reaction centre), gap of e– in PS-II is filled by photolysis of water, as a result, oxygen
evolution occurs in Z-scheme.
Photolysis or splitting of water occurs on the inner side of thylakoid membrane thus
protons and O2 are released in the lumen of thylakoid.
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Photsynthesis in Higher Plants # 7
Quantum Requirement
The no. of light Quanta or photons required for the evolution of 1 molecule of O2 in photosynthesis photons.
Quantum Yield
The no. of oxygen molecule evolved by one quantum of light in photosynthesis is called as
Quantum yield. 8 quanta evolve 1 O2. Hence the qunatum yield is 1/8 or 0.125 or 12.5 %)
(II)
Cyclic Photophosphorylation
In cyclic photophosphorylation, only PS-I (LHC-I) works.
A possible location where this could be happening is in the stroma lamellae.
During Cyclic ETS, the electron ejected from reaction centre of PS- I, returns back to
its reaction centre. At every step oxidation and reduction of each e– carrier take place which
release the energy and this energy is utilized in creation of proton gradient to form ATP.
In cyclic ETS, no oxygen evolution occurs beacuse photolysis of water is absent.
NADPH + H+ (Reducing power) is not formed in cyclic process. There is only formation of
ATP.
S. No
Cyclic photophosphorylation
1 Only PS- I is involved in cyclic process
2 First e– acceptor from PS-I is FRS
Non Cyclic photophosphorylation
Both PS-II & PS-I are involved in non-cyclic process
–
First e aceptor from PS-II is pheophytin
–
3
The e– expelled from P700 is cycled back.
The e expelled from reaction center is not cycled
–
back. Its loss is compensated by e from H2O
4
Photolysis of water and evolution of O2 does Photolysis
not take place
of water and evolution of O2 takes place.
5
NADP+ is not reduce
+
NADP is reduced to NADPH + H
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Photosynthesis in Higher Plants # 8
Chemiosmotic Hypothesis
Proposed by Peter Mitchell (1961) to explain the mechanism of ATP formation (Phosphorylation) in chloroplast (During respiration).
According to this hypothesis ATP synthesis is linked to development of a proton
gradient (Difference) of proton (H+) concentration) across a membrane. In chloroplast
these are membranes of the thylakoid.
The steps that cause a proton gradient to develop
(a)
Since splitting of water molecule takes place on the inner side of the membrane, the hydrogen
ions that are produces by the splitting of water accumulate within the lumen of the
thylakoids.
(b)
As electron move through the photosystems, protons are transported across the membrane.
This happen because the primary accepter of electron (Pheophytin) which is located
towards the outer side the membrane transfers its electron not to an electron carrier but to an
H carrier (Plastoquinon). Hence this molecule removes a proton from the stroma while
transporting an electron. When Plastoquinon passes on its electron to the electron carrier (Fe
protein ‘or’ Cyto-chrome) on the inner side of membrane, the proton is released into the inner
side of the membrane or the lumen side of the membrane
(c)
The NADP reductase enzyme is located on the stroma side of the membrane.Along
with electron that come from the accepter of electrons (Fd) of PS I, protons are necessary
for the reduction NADP+ to NADPH+ + H+. These protons are also removed from the
stroma.
Hence within the chloroplast, protons in the stroma decrease in number, while in the lumen the accumulation of protons. This creates a proton gradient across the thylakoid
mem-brane as well measurable decrease in pH in the lumen.
This gradient is important because it is breakdown of this gradient that leads to release
of energy. The gradient is broken down due to the movement of protons across the
membrane to the stroma through the transmembrane channel of the F0 of the ATPase
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The ATPase enzyme consists of two parts: one called the F0 is embedded in the membrane
and and forms a transmembrane channel that carries out facilitated diffusion of protons
across the membrane. The other portion is called F1 and produced on the outer surface of
the thylakoid membrane on the side that faces the stroma.
The break down of the gradient provides enough energy to cause a conformational change in the
F1 particle of the ATPase, which makes the enzyme, synthesise several molecules of
energy -packed ATP.
Chemiosmosis require a membrane, a proton pump/H-carrier, a proton gradient and
ATPase.
Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen.
WHERE ARE THE ATP AND NADPH USED ? (Dark Reaction/ Biosynthetic phase)
In this process CO2 is reduced to sugar. It is known as dark reaction but it doesn`t mean that
it occurs in dark. This process does not directly depends on the presence of light but is
dependent on the products of the light reaction like ATP and NADPH2 so immediately after light
becomes unavailable, the biosynthetic process continues for some time and then stops.
Reduction of CO2 is a multistep process which waas described by Melvin Calvin and Benson.
Calvin presented these reactions in cyclic manner & thus called as Calvin cycle.
Biosynthetic phase completes in different plants by 3 different pathways 1.
C3 pathway
2.
C4 pathway
3.
CAM pathway
In C3 pathway, Biosynthetic phase has only Calvin cycle. In C4 and CAM pathway, some
additional reactions also occur before Calvin cycle, during biosynthetic phase.
1.
The CO2 reduction or assimilation into glucose in all photosynthetic plants take place by Calvin
cycle as it is the ultimate pathway of glucose synthesis operated in C3, C4 & CAM Plants.
C3 pathway or The Calvin cycle :- /Dark reaction/ Blackman reaction/Biochemical Phase
C3 pathway occurs in stroma of chloroplast of mesophyll cells.
Ist stable compound of Calvin cycle is 3 carbon compound -PGA (Phosphoglyceric acid or
phospho-glycerate) thus Calvin cycle is called C3-cycle. (First compound is unstable, 6C keto
acid-carboxy ketoribitol biosphosphate).
Calvin studies the dark reaction in green algae Chlorella & Scenedesmus. During his experiment
he used chromatography & radioisotopy (C14 radioisotope) techniques for detecting reactions of
C3-cycle
RuBisCO (Ribulose-1, 5-bis-Phosphate carboxylase-oxygenase) is main enzyme in C3-cycle
which is present in stroma. RuBisCO is the most abudant enzyme and protein on earth.
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Photosynthesis in Higher Plants # 10
6 turns of Calvin cycle are required for the formation of one glucose or Fructose as 6 CO2 are
required for the synthesis of one hexose.
12 NADPH + H+ 18 ATP are required as assimilatroy power to produce one Glucose or Fructose
in dark reaction in C3 cycle.
2.
C4 pathway/CO2 concentration mechanism/Co-oprative photosynthesis/Dicarboxylic acid
cycle(DCA cycle)/Hatch & Slack Pathway
Kortschak and Hartt first observed that 4C OAA (Oxaloacetic Acid) is formed during dark
reaction in Sugarcane leaves.
Hatch & Slack (1967), Studies it in detail and proposed cyclic pathway for dark reactions in
sugar-cane & maize leaves.
First stable product of this pathway is OAA, which is a 4C, DCA (Dicarboxylic Acid), thus Hatch
& Slack pathway is called C4 cycle or DCA cycle.
C4-cycle occurs in about 1500 sps. of 19 families of Angiosperm, but most of the plants are
monocots (Tropical grasses), which belong to Gramineae & Cyperaceae families. C4 plants are
adapted to hot and dry environment.
E.g. of C4 plants- sugarcane, Maize, Sorghum.
Wheat, Rice and Barley (monocot) are C3 species.
Kranz (Wreath) Anatomy - Presentin leaves of C4 plants. In these plants special green large
cells are found around the vascular bundles in leaves , these are called bundle sheath cells, and the
leaves, which have such anatomy are said to have ‘Kranz anatomy’. ‘Kranz’ means ‘wreath’ and
is reflec-tion of the arrangement of cells.
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Photsynthesis in Higher Plants # 11
(i)
(ii)
(iii)
The bundle sheath cells may form several layers around the vascular bundles, they are characterised
byhaving a large number of chloroplasts
thick walls impervious to gaseous exchange
no intercellular spaces
Dimorphric chloroplasts are present in leaf cells. Chloroplast of B.S. cells or Kranz cells are larger
& without grana (Agranal) but the thylakoids are present only as stroma lamellae. Mesophyll chloroplast are small & with grana.
Two types of photosynthetic cells involved in dark reaction of C4 plants, Bundle sheath cells and
mesophyll cells. Thus operation of Hatch and slack pathway requires cooperation of both photosynthetic cells i.e. Mesophyll cells and BS cells. Initial CO2 fixation occurs in mesophyll cells By PEPcase
and final CO2 fixation in BS cells by RuBisCO.
Detail form of C4 Pathway/Hatch and Slack Pathway
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First CO2 acceptor in C4 plants is PEP (Phosphoenol Pyruvate) (3C-compound) in mesophyll
cells, while second CO2 acceptor in RuBP (5C-compound), in bundle sheath cells.
Initial fixation of CO2 in mesophyll cells is catalysed by PEPcase (PEP carboxylase), which results
in the formation of OAA (4C), So called C4 pathway.
Then reduction of OAA take place by NADPH2 results in formation of malic Acid (4C) or transamination of OAA resulting in formation of Aspartic acid (4C).
The malic acid or Aspartic acid (4C) formed in mesophyll cells is transfered to bundle sheath
cells. In B.S. cells the oxidative decarboxylation of malate takes place and CO2 is released
along with pyruvic Acid (3C).
Released CO2 in B.S. Cells isaccepeted by RUBP, catalysed by RuBisCO. The C3 cycle/calvin
cycle operates in B.S. cells with utilization of assimilatory power (18 ATP & 12 NADPH2) resulting
in formation of glucose or fructose.
Pyruvic acid from B.S. cells return to mesophyll cells. It regenerate the PEP (pri. CO2 acceptor) by
utilization of 12 ATP, catalysed by enzymes PPDK (Pyruvate phosphate dikinase). So in C4 plants
total 30 ATP and 12 NADPH2 aree utilized for synthesis of one glucose or Fructose.
C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). C4 plant loses
only half as much water as a C3 plant for the same amount of CO2 fixed.
(i)
(ii)
(iii)
(iv)
C4 plants are special :
They have a special type of leaf anatomy (Kranz anatomy)
They tolerate higher temperature
They show a response to high light intensities
They lack a process called photorespiration so have greater productivity of biomass.
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Photsynthesis in Higher Plants # 13
Photosynthetic carbon oxidation (PCO) cycle/C2 Cycle/Photorespiration/Glycolate-Metabolism
The light dependent uptake of O2 & released of CO2 in C3 photosynthetic cells is called
Photorespiration. RuBisCO is characterised by the fact that its active site can binding to both
CO2 andO2 -hence the name. This binding is competitive. It is the relative concentration of O2
and CO2 that determines which of the two will bind to the enzyme. (Usually RuBisCO has a much
greater affinity for CO2 than for O2). Internal CO2 conc. is constant around RubisCO, but
internal O2 conc. is variable due to variable rate of light reaction. When O2 conc. is higher than
CO2 conc. then RubisCO perform oxygenation of RuBP, which leads to phenomenon of
photorespiration.
The reason of photorespiration is same site of O2 evolution/photolysis of H2O and catalytic
activity of RubisCO, this site is Chloroplast of Mesophyll cells.
Conditions for photorespiration - High O2, Low CO2, High light intensity and high temperature.
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Photosynthesis in Higher Plants # 14
Characteristics
Cell type in which the Calvin
cycle take`s place
Cell type in which the initial
carboxylation reaction occurs
How many cell types does
the leaf have that fix CO2
Which is the primary CO2 acceptor
Number of carbons in the
primary CO2 acceptor
Which is the primary CO2
fixation product
No. of carbons in the primary CO2
fixation product
Does the plant have RuBisCO ?
Does the plant have PEP Case ?
Which cells in the plant have Rubisco ?
CO2 fixation rate under high
light conditions
Whether phorepiration is present
at low light intensities
Whether phorepiration is present
at high light intensities
Whether photorespiration would
be present at low CO2 concentrations
Whether photorespiration would
be present at high CO2 concentrations
C3 Plants
C4 Plants
Choose from
Mesophyll
Bundle sheath
Mesophyll/Bundle sheath/both
Mesophyll
Mesophyll
Mesophyll/Bundle sheath/both
Two: Bundle sheath and
mesophyll
One: Mesophyll
Three: Bundle sheath, palisade,
spongy mesophyll
One
Two
RuBP
PEP
RuBP/PEP/PGA
5
3
5/4/3
PGA
OAA
PGA/OAA/RuBP/PEP
3
4
3/4/5
Yes
Yes
Mesophyll
Yes
Yes
Bundle sheath
Yes/No/Not always
Yes/No/Not always
Mesophyll/Bundle sheath/none
Medium
High
Low/high/medium
Negligible
Negligible
High/neglibible/sometimes
High
Negligible
High/neglibible/sometimes
High
Negligible
High/neglibible/sometimes
Negligible
Negligible
High/neglibible/sometimes
o
o
Temperature optimum
20-25 C
30-40 C
Examples
Wheat Rice
Maize Sugarcane Sorghum
30-40 C/20-25C/ above 40 C
In C3 plants some O2 does bind to RuBisCO and hence CO2 fixation is decreased. Here the RuBP
instead of being converted to 2 molecules of PGA, binds with O2 to form one molecule of
phospho-glycerate (3C) and one mole of phosphoglycolate (2C).
In the photo-respiratory pathway there is neither synthesis of sugars, nor of ATP and
NADPH2. Rather it results in the release of CO2 with the utilization of ATP. Therefore, photo
respiration is a wasteful process.
It occurs in chloroplast, peroxisome & mitochondria.
In C4 plants photo-respiration does not occur. This is because they have a mechanism that
increases the concentration of CO2 at the enzyme site. This takes place when the C4 acid (malic
or aspartic acid) from the mesophyll cell is broken down in the bundle sheath cells to release CO2
(CO2 pump-ing), this results in increasing the intracellular concentration of CO2. In turn, this
ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity.
Also, in C4 plants, site of O2 evolution (mesophyll cell) and site of RuBisCO activity (Bundle sheath
cell) are different.
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3.
CAM-Pathway/ Crassulacean Acid Metabolisn / Dark CO2 fixation
Oleary and Ruhani discovered CAM-process in membranes of of Crassulaceae family. (Succulent
xerophyte plants).
Eg. are - Kalanchoe, Bryophyllum, Sedum, Kleinia , Opuntia, Crassula, Agave, Aloe, Euphorbia spa,
Pineapple, Welwitschia (Gymnosperm) etc.
Primary acceptor of CO2 is PEP (Phosphoenol pyruvate) and OAA (oxaloacetic acid) is the first
product of carboxylation reaction.
In CAM plants stomata are of Scotoactive type (open in night, closed during day) so initial CO2
fixation occurs in night but light reactions and Final CO2 fixation (C3 cycle) occurs in day time.
PEPcase induces carboxylation reaction in night, RuBisCO catalyses the final CO2 fixation in day
time.
PEP carboxylase (PEP case) & RuBisCO both are present in mesophyll cells. (No Kranz-Anatomy)
Final CO2 fixation i.e. calvin cycle occurs during daytime as ATP and NADPH2 are synthesized
during day time.
In CAM plants total 30 ATP and 12 NADPH2 are required for 1 Hexose synthesis.
CAM pathway helps in water conservation.
CAM Pathway
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FACTORS AFFECTING PHOTOSYNTHESIS
Law of Limiting Factors (by Blackman) :- When a process is affected by a number of factors,
then the rate of process at a time is limited by the factor which is at suboptimal level(less than
optimum value).
CO2 is limiting in clear sky but light becomes limiting in cloudy days and in dense forest or for
plants growing in shade.
EXTERNAL FACTORS (1)
Light With increasing light intensity, rate of photosynthesis also increases but upto a limit, after which other
factors becomes limiting and rate does not increase further. At higher light intensity photooxidation
(solarization) of pigments may occur.
Intensity of light at which rate of photosynthesis becomes equal (or compensate) to rate of
respiration in plants (True respiration + photorespiration) is known as light compensation point.
(Net photosyn-thesis or Net Primary productivity at this point is zero). No exchanges of gases
with atmosphere. This happens at the time of evening & morning.
(2)
Temperature The dark reactions being enzymatic are temperature controlled. Through the light reactions are also
temperature sensitive they are affected to a much lesser extent.
The C4 plants respond to higher temperature (30o - 40oC) and show higher rate of photosynthesis
while C3 plants have a much lower temperature optimum (20o - 25oC).
CO2 Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO2 is very low
in the atmosphere (between 0.03 and 0.04 percent). Increase in concentration upto 0.05 percent can
cause an increase in CO2 fixation rate, beyond this the levels can become damaging over longer
periods.
The C3 and C4 plants respond differently to CO2 concentrations. At low light conditions neither group
responds to high CO2 conditions. At high light intensities, both C3 and C4 plants show increase in the
rates of photosynthesis
C4- Plants show saturation at about 360lL–1 (0.036% or 360 ppm) while C3 responds to increased
CO2 concentration and saturation is seen only beyond 450lL–1 (0.045% or 450 ppm). Thus current
availability of CO2 levels is limiting to the C3 plants not for C4 plants.
CO2 concn at which CO2 fixation in photosynthesis becomes equal to volume of CO2 released in
respiration is CO2 compensation point.
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CO2 compensation point of C4 plants is 0-10 ppm, for C3 is 25-100 ppm and for CAM is 0-5 ppm.
The fact that C3 plants respond to higher CO2 concentration by showning increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes
and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher
yields (CO2 fertilizing effect).
Atmospheric CO2 is not limiting factor for C4 plants and Submerged hydrophytes.
(3)
Water Less availability of water reduces the rate of photosynthesis (stomata get closed). Besides, water
stress also makes leaves wilt, thus reducing the surface area of the leaves and their metabolic
activity as well.
INTERNAL (PLANT) FACTORS (i)
Amount of Chlorophyll
(ii)
Leaf factor :Various leaf factors like number size, leaf age and leaf orientation (leaf angle) affects the photosynthesis.
Internal factors are dependent on genetic predisposition and growth of the plant.
INHIBITORS DCMU (Diuron/Dichlorophenyl Dimethyl Urea), CMU (Monuron), PAN, Atrazine, Simazime,
Bromocil, Isocil- inhibits the photosynthesis by blocking PS-II. These stop e– flow between P-680
& PQ.
Diquat, Paraquat (Viologen dyes) inhibit PS-I by stopping e– flow between P-700 & Fd.
All these chemicals are used as herbicides (weedicides) which mostly block ETS.
RED DROP AND EMERSON ENHANCEMENT EFFECT :Emerson & Arnold worked on Chlorella and gave the concept of two photosystem or two
pigment systems
When they gave only monochromatic red light, longer than 680 nm wavelength, then quantum
yield suddenly dropped down , this phenomenon was called red drop.
When Emerson gave light, shorter and greater than 680 nm (combined light) then
photosynthetic activity increases, it became more than the sum total of the photosynthetic yield
which was obtained separately in the light of more than 680 nm and less than 680 nm. This was
called Emerson enhance-ment effect.
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Photosynthesis in Higher Plants # 18
FULL FORMS
Pheo.
PQ.
PC
LHC
OEC
Cyto.
Fd
FRS
PS
PGA
PGAL
RUBP
RuBisco
PEP
OAA
PEPcase
PPDK
CAM
NADP/TPN
-
NAD+/DPN
FAD
-
Pheophytin
Plastoquinone
Plastocyanin
Light Harvesting Complex
Oxygen Evolving COmplex
Cytochrome
Ferredoxin
Ferredoxin
Pigment System
Phosphoglyceric Acid/Phosphoglycerate
Phosphoglyceraldehyde
Ribulose-1, 5-Bis Phosphate
Ribulose-1, 5-Bis Phosphate Carboxylase-Oxygenase
Phosphoenol Pyruvate
Oxaloacetic Acid
Phosphoenol Pyruvate Carboxylase
Pyruvate Phosphate Dikinase
Crassulacean Acid Metabolism
Nicotinamide Adenine Dinucleotide Phosphate/Triphospho pyridine
nucleotide
Nicotinamide Adenine Dinucleotide/ Diphospho pyridine nucleotide
Flavin Adenine Dinucleotide
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