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Photosynthesis in Higher Plants
Photosynthesis
Early Discoveries
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Joseph Priestley: Concluded that air is necessary for the growth of a plant
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Jan Ingenhousz: Concluded that sunlight is essential for plant processes that purify the air
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Julius Von Sachs: Discovered that the green substance in plants (chlorophyll) is located in
special bodies (chloroplast)
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T
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Engelmann: Discovered the first action spectrum of photosynthesis using green
alga Cladophora
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Cornelius van Niel : Demonstrated that photosynthesis is a light-dependent reaction in
which hydrogen from an oxidisable compound reduces carbon dioxide to carbohydrates
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When he worked on sulphur bacteria, he found that sulphur is the oxidation product in
them, and not oxygen, since H2S is the hydrogen donor there
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Finally, the correct equation for photosynthesis was discovered
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They are known as autotrophs and
those organisms that cannot produce their own food are called heterotrophs
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Chemoautotrophs: they are organisms that use other chemicals to make their own food by
some chemical reactions
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They are the sites where the synthesis of food
occurs in plants
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This pigment captures the sun’s energy, which is used to prepare food from
carbon dioxide and water
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Since solar energy is essential for plants to prepare food, we can say that sun is the ultimate
source of energy for plants and otherwise
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This process can be represented in the form of the following equation:

Carbohydrates, which are produced during photosynthesis, are ultimately converted into
starch to be stored in plants
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They too contain chlorophyll and can prepare their own food
through the process of photosynthesis
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Without sunlight, photosynthesis cannot take
place in plants, and can even lead to the death of plants
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Let us perform the following activity to test the production of food in plants
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How are raw materials supplied to plants?

The tiny pores found on the underside of leaves are called stomata
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Water and minerals enter plants through the roots
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Therefore, the carbohydrates produced in
leaves are converted into starch, while the oxygen produced in the process is released into
the atmosphere through the stomata
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They are present in the mesophyll cells of the
leaves
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The chlorophyll pigments trap the photons of light and get excited and thus initiate the
process of photosynthesis
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All heterotrophs depend on plants for their energy requirements
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The byproduct of photosynthesis, oxygen is the main factor responsible for the
maintenance of life on earth
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Chloroplast- The site of Photosynthesis
Site of Photosynthesis
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Mesophyll cells in green leaves have large number of chloroplasts, which are the site of
photosynthesis
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Membrane system of chloroplast − traps light, and synthesises ATP and NADPH (site of
light-dependent reaction of photosynthesis)
Stroma − CO2 is incorporated into the plant by enzymatic reactions, leading to the synthesis
of sugar (site of light-independent reaction of photosynthesis)
Chloroplasts are aligned along the walls of mesophyll cells so as to get optimum light
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An action spectrum is the rate of a physiological activity plotted against the wavelength of
light
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(A) Absorption spectrum of chlorophylls a, b, and carotenoids
(B) Action spectrum of photosynthesis
(C) Action spectrum superimposed on absorption spectrum of chlorophyll a
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4 types of pigments may be present in leaves:
Chlorophyll a (blue-green)
Chlorophyll b (yellow-green)
Xanthophylls (yellow)
Carotenoids (yellow to yellow-orange)
Chlorophyll a is the main pigment in photosynthesis
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Accessory pigments: Chlorophyll b, xanthophylls and carotenoids
Absorb a wider range of light, and transfer the energy to chlorophyll a
Protect chlorophyll a from photo-oxidation
Light Reaction of Photosynthesis and Cyclic and Non-Cyclic Photophosphorylation
Light Reaction (Photochemical Phase)

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This phase directly depends on light
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Includes:

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Light absorption

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Water splitting

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Oxygen release

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Formation of ATP and NADPH, which is then used in the biosynthetic phase

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Pigment molecules bound to the proteins form LHC (light harvesting complexes)
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Photo-Phosphorylation

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The process of formation of ATP in chloroplast in the presence of sunlight

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Photo-phosphorylation is of two types:

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Non-cyclic photo-phosphorylation

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Cyclic photo-phosphorylation
Non-Cyclic Photo-Phosphorylation

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PSII absorbs 680 nm wavelength of red light, causing electrons to become excited and jump
into an orbit farther from the nucleus
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Electron transport system transfers the electrons to PSI
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These
electrons are again transferred to another electron acceptor having a greater redox
potential
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Addition of these electrons reduces the NADP+ to NADPH+ H+
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e
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The flow of electrons assumes the shape of the letter ‘Z’ when all carriers are placed
according to their redox potential
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Since the electrons lost by PSII do not come back to it, this process of formation of ATP is
called non-cyclic photo-phosphorylation
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Hence, the electrons are circulated within the
photosystem
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This scheme could possibly be occurring in stroma lamellae because it lacks both PSII and
NADP reductase enzyme
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Splitting Of Water

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The electrons being transferred in the photo-phosphorylation reactions are generated by
the splitting of water
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Manganese, chlorine, etc
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The light-dependent splitting of water is called photolysis
2H2O → 4H+ + O2 + 4e−

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Electrons formed are used for replacing the electrons lost from P680
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Protons are used for the formation of reducing power NADP to NADPH+
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Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase
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So H+ produced in
the reaction accumulates within the lumen
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NADP reductase enzyme is located on the stroma side of the membrane
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As a result, the protons in the stroma decrease in number, while the protons in the lumen,
increase in number, thus creating a proton gradient across the thylakoid membrane
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ATPase: This enzyme has two parts

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F0 − embedded in the membrane and carry out facilitated diffusion of H+

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F1 − protrudes towards the stroma

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Conformational changes are induced in the F1 particle of the ATPase
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Dark Reaction- C3 Pathway
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Products of light reaction: ATP, NADPH, CO2 and H2O

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Next stage is the biosynthetic phase
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CO2 is fixed to form CO2 fixation product
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The reaction involved in the biosynthetic phase takes place in the stroma of chloroplasts
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These plants are said to adopt the C3 pathway
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These plants are said to adopt the C4 pathway
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Calvin pathway occurs in all photosynthetic plants, irrespective of whether they have C3 or
C4 pathway
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This was surprising as it was believed that for the formation of a 3carbon product, CO2 would have to be accepted by a 2-carbon compound
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3 stages of Calvin cycle: Carboxylation, Reduction and Regeneration

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Carboxylation

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Ribulose 1, 5-bisphosphate combines with CO2, and fixes it to a stable organic intermediate
(3-phosphoglycerate − 2 molecules)
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Reduction

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Here, 2 molecules each of ATP and NADPH are required for fixing 1 molecule of CO2
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Glucose is formed as a result of this series of reactions
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1 ATP molecule is required
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ATP required:
For fixing 1 molecule of CO2 − 3 (2 for reduction and 1 for regeneration)
For fixing 6 molecules of CO2 − 3 × 6 = 18 ATP

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NADPH required:
For fixing 1 molecule of CO2 − 2 (for reduction)
For fixing 6 molecules of CO2 − 2 × 6 = 12 NADPH

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Thus, the synthesis of 1 molecule of glucose requires 18 ATP and 12 NADPH
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They have special large cells around their
vascular bundles called bundle sheath cells
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First CO2 fixation product is a 4-carbon compound called oxaloacetic acid, but C3 cycle is
used as the main biosynthetic phase
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Primary CO2 acceptor: Phosphoenol pyruvate (PEP) − a 3-carbon molecule

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PEPcase (PEP Carboxylase) fixes CO2 in the mesophyll cells
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These compounds are transported to the bundle sheath cells
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C3 acid, so formed, is again transported to the mesophyll cells and regenerated back into
PEP
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They lack PEPcase
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It is also considered a wasteful process
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Do C4 plants have better yield than C3 plants?
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To understand this, let us first recall the first step of Calvin pathway
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The active site of RuBisCO can bind to
both O2 and CO2, though it has a greater affinity for CO2
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There is no production of sugar or ATP
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In C4 plants, photorespiration does not occur since they have mechanism to increase the
CO2 concentration at enzyme site
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This is the reason why C4 plants are better yielding as compared to C3 plants
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The binding to
the enzyme is determined by the relative concentration of oxygen and carbon dioxide
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Internal factors: Number, size and orientation of leaves, mesophyll cells and chloroplasts,
internal carbon dioxide concentration and the amount of chlorophyll
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Law of Limiting Factors − If a chemical process is affected by more than one factor, then
its rate will be determined by factor which is nearest to its minimal value (factor which
directly affects the process if its quantity is changed)
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Light

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Incident light ∝ CO2 fixation rate; but at higher light intensities, the rate does not increase
further as other factors become limiting

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Light is rarely a limiting factor (with exception of the shade plants or plants of dense
forest) because light saturation occurs at 10% of the full sunlight
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CO2 Concentration

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Major limiting factor
Usually low in atmosphere (0
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04%)
Up to 0
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05% − damaging effect
Though both C3 and C4 show increase in rate of photosynthesis at high light intensities
accompanied by high CO2 concentration
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Therefore, CO2 concentration is more of a limiting factor
for C3 plants
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Temperature

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Dark reactions are more sensitive to an increase in temperature
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Adaptations according to habitat also affect temperature optimum for photosynthesis
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Water

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Water stress causes stomata to close and hence, less CO2 is available
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