Photosynthesis in plants.pptx

note:
 oxygen evolved
comes from the water
not from the co2

PHOTOSYNTHESIS
REDOX
ENDOTHERMIC
ANABOLIC
TRICK RENA

RESPIRATION
CATABOLIC
OXIDATION
EXOTHERMIC
TRICK COAX

MOLL’S HALF LEAF
EXPERIMENT
 SHOWED THAT CO2 IS REQUIRED
FOR PHOTOSYNTHESIS

Joseph Priestley
 He revealed the essential role of air in
the growth of green plants.
 He discovered oxygen

3.JAN INGENHOUSZ
 He showed that sunlight is essential
for the plants

4.JULIUS VON SACHS
 Found that glucose is made in the
green plants.

5.T.W.ENGELMANN
 Described the first action spectrum of
photosynthesis by using cladophora.

6.CORNELIUS VAN NIEL
 Inferred that oxygen evolved by the
green plants comes from the water
and not from the co2.

7.RUBEN, KAMEN ET AL
 Proved that oxygen evolve during light
reaction comes from the H2O not from
CO2.

 Photosynthesis takes place in
chloroplasts.
 Pigments are substances that have an
ability to abborb light , at specific
wavelengths.

Chlorophyll
pigments
Many types of chlorophylls are
:
 Chlorophyll a
 Chlorophyll b
 Chlorophyll c
 Chlorophyll d
 Chlorophyll e
 Bacteriochlorophyll a and b etc…..

 But the chromatographic separation of
the leaf pigments shows that the color
of leaves is due to four pigments:
 Chlorophyll a bright or
blue green
 Chlorophyll b yellow
green
 Xanthophylls yellow
 Carotene yellow orange

 Chlorophyll a is the primary
photosynthetic pigment.

EMERSON’S EXPERIMENTS
1.RED DROP OR EMERSON’S FIRST EFFECT
2.EMERSON’ ENHANCEMENT EFFECT

Emerson effect
 Rate of photosynthesis depends
directly on two main factors
 Wavelength of light
 Quantum yield
 Quantum yield = amount of O2
release
amount of light
absorbed

Quantum yield = amount of O2 release
amount of light absorbed

RED DROP OR EMERSON’S
FIRST EFFECT
 Emerson conducted experiment in
chorella using only one wavelength of
light (monochromatic light) at a time
and he measured quantum yield.

 He plotted a graph of the quantum
yield in terms of O2 evolution at
various wavelengths of light.

 His focus was to determine at which
wavelengths of light the
photochemical yield of oxygen was
maximum.

Observations:
 He found that in the wavelength of 600
to 680 the yield was constant
 But suddenly dropped in the region
above 680 nm (red region)

Inference
 The fall in the photosynthetic yield
beyond red region of the spectrum is
referred as red drop or Emerson’s first
effect.

Emerson’s enhancement
effect

PS I
 Location :
 Stromal lamella + thylakoid membrane

Chemiosmotic hypothesis
 Was first explained by Peter Mitchell.
 This mechanism explains how ATP is
synthesised in the chloroplast.
 In respiration it is called oxidative
phosphorylation
 In photosynthesis it is called
photophosphorylation .

ATP SYNTHESIS
 ATP synthesis is linked to the
development of a proton gradient
across the membrane of the thylakoid
 the proton accumulation is towards the
inside of the membrane ie,. the lumen.

Processes involved in
chemiosmotic hypothesis
 Photolysis of water towards thylakoid
lumen
 Transfer of H+ from stroma to lumen
as electrons move through
photosystem
 NADPH reductase reaction occur
towards stroma.

1.PHOTOLYSIS OF WATER
TOWARDS THYLAKOID LUMEN

 THE SPLITTING OF WATER
MOLECULE TAKES PLACE ON THE
INNER SIDE OF THE MEMBRANE
AND SO THE HYDROGEN IONS
THAT ARE PRODUCED , THEY
ACCUMULATE WITHIN THE LUMEN
OF THE THYLAKOID.

2.TRANSFER OF H+ FROM STROMA
TO LUMEN AS THE ELECTRONS
MOVE THROUGH PHOTOSYTEMS
 The primary acceptor of electron
located towards the outer side of the
membrane transfers its electron to a
H+ carrier and this molecule then
removes a proton from the stroma
while transporting an electron.

 When this H+ carrier molecule passes
on its electron to an electron carrier
present on the innner side of the
membrane , the H+ is released into
the lumen of the membrane.

3.NADPH reductase reaction
occurs towards stroma
 The NADP reductase enzyme is
located on the stroma side of the
membrane.
 Protons are necessary for the
reduction of NADP+ to NADPH + H+
and protons are removed from the
stroma.

 So, within the chloroplasts, protons in
the stroma decrease while in lumen
there is increase in H+.
 This causes a decrease in PH in the
lumen and creates a gradient across
the thylakoid membrane.

 The gradient is important because the
breakdown of the gradient leads to
synthesis of ATP.

 The gradient is broken down by the
movement of protons across the
membrane to the stroma through the
transmembrane channel of the F0 of
the ATP synthetase.

The ATP synthetase consists of
two parts:
 CF0 is embedded in the membrane
and forms the transmembrane
channel that carries out facilitated
diffusion of protons across the
membrane.
 CF1 protudes on the

Melvin Calvin used radioactive
C14
in algal photosynthesis studies.

 This led to the discovery that the first
CO2 fixation product was a three
carbon organic acid.
 He also helped to mark the complete
biosynthetic pathway.
 Hence it is called calvin cycle.
 The first stable product identified was
3-phosphoglyceric acid.(PGA)

 Calvin cycle occurs in all
photosynthetic plants whether they are
C3 or C4 pathway.

1.The primary acceptor molecule during the
C3 cycle is a 5 C ketose sugar RuBP (ribulose
bisphosphate)
2.The enzyme for CO2 FIXATION IS
RuBisCO (Ribulose bisphosphate carboxylase
oxygenase)

 Before this discovery it was believed
that since the first product was a C3
acid ,the primary acceptor would be a
2C compound.

 It is the most abundant enzyme on
earth.
 It is characterised by the fact that its
active site can bind both CO2 and O2.

 RuBisCO has a much greater affinity
for CO2 than O2 and the binding is
competitive.
 It is the relative concentration that of
O2 and CO2 that determines which of
the two will bind to the enzyme.

1Q
 The assimilatory powers produced in
cyclic photophosphorylation is/are
1. ATP only
2. NADPH only
3. Both ATP and NADPH
4. ATP and NADH

Stages of calvin cycle
 1.carboxylation.
 2.reduction
 3.regeneration

1.Carboxylation or carbon
fixation
 It is the fixation of CO2 into a stable
organic intermediate.
 In this,CO2 is utilised for carboxylation
of RuBP.

 This reaction is catalysed by RuBisCO
 RESULTS :
 Formation of 2 molecules of 3-PGA (3-
phosphoglyceric acid).

2.Reduction
 This reaction leads to the formation of
glucose.
 The steps involve utilization of two
molecules of ATP for phosphorylation
and two of NADPH for reduction, per
molecule of CO2 fixed .

 The fixation of six molecules of co2
and six turns of cycle are required for
the removal of 1 molecule of glucose
from the pathway.

3.Regeneration
 For the cycle to continue
uninterrupted, regeneration of the
CO2 acceptor molecule is crucial.
 This step requires one ATP for
phosphorylation to form RuBP.

 To make 1 molecule of glucose six
turns of the cycle is required.
 18 ATP and12 NADPH molecules are
required to make a glucose.

 It is to meet this differrence in number
of ATP and NADPH that the cyclic
phosphorylation takes place.
 RuBisCO and many other enzymes of
calvin cycle are regulated by light.

IN OUT
6 CO2 I GLUCOSE
18 ATP 18 ADP
12 NADPH 12 NADP

C4 PATHWAY (HATCH AND
SLACK PATHWAY)
 Most of the plants adapted to dry
tropical regions have the C4 pathway.
TRICK : SAMS

 In these plants double fixation of CO2
occurs.

 The initial or the first product of this
pathway is a 4C compound OAA
(oxaloacetic acid) and hence the
name.

 Two Australian botanists HATCH AND
SLACK discovered that tropical plants
are more efficient in CO2 utilization.

C4 PLANTS
 C4 plants have a special type of leaf
anatomy , they can tolerate higher
temperature.
 They show a higher response to high
intensities of light.
 They lack a wasteful process called
photorespiration.
 Hence, they show a greater
productivity and higher yield
compared to C3 plants.

 C4 pathway requires two types of cells
:
 Mesophyll cells
 Bundle sheath cells

 The particularly large cells around the
vascular bundles of C4 plants are
called bundle sheath cells.

 These cells form several layers
around the vascular bundles.
 They are characterised by :
 Large no of chloroplasts
 Grana are absent
 Thick walls impervious to gaseous
exchange.
 No intercellular spaces.

 Q) In C4 plants the bundle sheath
cells
a. Have thin walls to facilitate gaseous
exchange .
b. Have large intercellular spaces
c. Are rich in PEP carboxylase.
d. Have a high density of chloroplasts.

KRANZ ANATOMY
 This special anatomy of leaves of the
C4 plants is called KRANZ ANATOMY.
 KRANZ means wreath and is a
reflection of the arrangement of cells.

C4 PATHWAY (HATCH AND
SLACK PATHWAY)
 The primary CO2 acceptor is a 3C
compound PEP ( phosphoenol
pyruvate)
 It is present in mesophyll cells.

PEP carboxylase or PEPcase
 The enzyme that catalyses this CO2
fixation is PEP carboxylase or
PEPcase.
 The mesophyll cells of C4 plants lack
RuBisCO.
 So the 4C compound OAA is formed
in the mesophyll cells.

 It is then converted into other 4C compounds like
maleic acid and aspartic acid in the mesophyll cells
itself and then transferred into bundle sheath cells.

Bundle sheath cells
 In the bundle sheath cells these C4
acids are broken down into CO2 and
3C compounds.
 The CO2 released enters C3 cycle .

Bundle sheath cells
 The bundle sheath cells are rich in the
enzymes RuBisCO , but lacks
PEPcase.

MESOPHYLL CELLS
 The 3C molecule is transported back
into the mesophyll cells and converted
into PEP again with the help of cold
sensitive enzyme PEP synthetase.
 Thus completing the cycle.

 Thus the basic pathway that results in
the formation of the sugars , calvin
pathway is common in both C3 and
C4 plants.

Regeneration
1.Regeneration of PEP from
C3 acid requires 2 ATP
equivalent.
2.However there is no net
gain or loss in NADPH in C4
cycle.

C4 PLANTS
 Has both C3 and C4 cycle.
 ATP consumed in C4 plants :
 C4 cycle = 2 ATP per CO2 fixed.
 C3 cycle = 3 ATP per CO2 fixed.
 Total = 5 ATP per CO2 fixed.

C3 CYCLE C4 CYCLE
It is a slower process of
CO2 fixation.
It is a faster process
of CO2 fixation.

Importance of C4 plants
 They can tolerate saline conditions
due to abundant occurrence of organic
acids (maleic and OAA) which lowers
their water potential than that of soil.

 Can perform photosynthesis even
when their stomata are closed due to
the presence of strong CO2 fixing
enzyme PEPcase.

 Concentric arrangement of cells in leaf
produces smaller area in relation to
volume for better water utilization.

CAM PATHWAY
CRUSSULACEAN ACID
METABOLISM

 This metabolism was first of all
reported in BRYOPHYLLUM a
member of family Crassulaceae and
hence it is called Crassulacean acid
metabolism.

CAM (CRASSULACEAN ACID
METABOLISM) OR DIURNAL ACID
CYCLE
 Certain plants have scotoactive
stomata are called CAM plants.

 These plants fix CO2 during night but
form sugars only during day ( when
RuBisCO is active.

CAM
 CO2 is fixed during night (dark) to
OAA using PEPcase.
 Step1 :

 This CO2 comes from respiration
(breakdown of starch) also from the
atmosphere.

Maleic acid gets stored in the
vacuole.

 The CAM also contains enzymes of
calvin cycle.
 During day time maleic acid
breakdowns to form pyruvate and
CO2.

PYRUVATE
 Pyruvate is used up to regenerate
PEP.

CO2
 CO2 enters the calvin cycle.

 The succulents , therefore synthesise :
 Plenty of organic acid (maleic acid)
during night (when stomata are open)

 Plenty of carbohydrates during the day
(when stomata are closed).

Important note
 Like calvin cycle ,CAM cycle also
operates in the mesophyll cells only.
 None of these has shown chloroplast
dimorphism as is found in C4 plants.

 It should be remembered slow
growing desert succulents exhibiting
CAM have the slowest photosynthetic
rate while C4 plants show highest
rates.

 Thus CAM plants are although not
efficient as C4 plants , they definitely
better suited to adverse conditions (
ie,. Conditions of extreme desiccation)

PHOTORESPIRATION OR C2
CYCLE
 It is a process which involves loss of
fixed CO2 in plants in the presence of
light.
 It is initiated in chloroplasts.
 This process does not produce ATP or
NADPH and is wasteful process.

CONDITIONS
 Photorespiration usually occurs when
there is high concentration of O2.

 Under such circumstances RuBisCO
functions as an oxygenase.
 Some O2 binds to the RuBisCO and
hence CO2 fixation is reduced.

 The RuBP binds with O2 to form 1
molecule of PGA and
phosphoglycolate.

WHY IT IS A WASTEFUL
PROCESS ?
 There is neither synthesis of sugar nor
ATP.
 Rather, it results in the release of
CO2, with the utilization of ATP.

LOSS
 It leads to a 25 % loss of the fixed
CO2.
 O2 is first utilised in chloroplats and
then in peroxisomes.

ORGANELLES INVOLVED:
 Chloroplast
 Peroxisomes
 Mitochondria (loss of CO2 occurs
here)

NOTE:
 In C4 plants , photorespiration does
not occur.
 Becoz they have a mechanism that
increases the concentration of CO2 at
the enzyme site.

Mechanism
 During the C4 pathway, when the C4
acid ( maleic acid ) in the mesophyll
cells is broken down in the bundle
sheath cells it releases CO2.

 Thus increasing the intercellular conc.
of CO2.
 Thus ensures that RuBisBP functions
as a carboxylase minimising the
oxygenase activity.

WHY C4 PLANTS ARE MORE
EFFICIENT?
Thus the productivity and yields are
better in C4 plants compared to C3
plants.
In addition , C4 plants show tolerance to
higher temperature also.

FACTORS AFFECTING
PHOTOSYNTHESIS
 External factors
 Plant factors or internal factors

EXTERNAL FACTORS
 Availability of sunlight
 Temperature
 CO2 concentration
 Water

PLANT FACTORS
 LEAVES (NASO)
1. Number
2. Age
3. Size
4. Orientation
 Mesophyll cells
 Chloroplasts
 INTERNAL CO2 CONCENTRATION
 AMOUNT OF CHLOROPHYLL

 The plant factors are dependent on
the :
 Genetic predisposition
 Growth of the plant

1905 BLACKMAN gave LAW
OF LIMITING FACTORS

 When several factors affect any
biochemical process then this law
comes into effect

Law of limiting factors
 “If a chemical process is affected by
more than one factor , then its rate will
be determined by the factor which
directly affects the process if its
quantity is changed”

SOLARISATION
 The intensity beyond light saturation
point causes chlorophyll destruction
and decrease in photosynthetic rate,
this is called solarisation.

 Q) Very strong light has a direct
inhibiting effect on photosynthesis
which is known as
a. Solarisation .
b. Etiolation
c. Chlorosis .
d. Defoliation .

Oxygen
 Small quantity of oxygen is essential
for photosynthesis except in some
anaerobic bacteria.

WARBURG EFFECT
 At a very high oxygen
concentration , the rate of
photosynthesis declines in all
green plants. This phenomenon is
called warburg effect.

Photosynthesis in plants.pptx

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