Photosynthesis is a biological process used by many cellular organisms to convert light energy into chemical energy, which is stored in organic compounds that can later be metabolized through cellular respiration to fuel the organism's activities.

1. Importance in Bio Productivity
Photochemical Process,
Photochemical Reactions
Vikas Kumar
M.Sc (G&PB)
Chaudhary Charan Singh University

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Introduction
Definition of Photosynthesis
Steps of Photosynthesis
Site of Photosynthesis
Chloroplast as Solar Harvesting Unit
Pigments and Its types in Plants
Mechanism of Photosynthesis
Light Reaction (Photosystems, Cyclic and Non Cyclic Phosphorylation)
Dark Reaction
Importance in Bio-productivity
References

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Introduction
• Photosynthesis by far the most significant biological process on
the planet earth.
• It is through this phenomenon that all the useful organic matter
is available on the earth has been produced.
• The leaves in the higher plants may be regarded as machinery
engineered to undertake photosynthesis efficiently even under
very hostile environments.
• Photosynthesis is not a monopoly of green plants but is also
occurs in cyanobacteria and photosynthetic bacteria.

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What is Photosynthesis?
“ Photosynthesis can be defined as the anabolic process
during which complex energy rich organic molecules/ compounds are
synthesized by organisms from CO2 and H2O using solar energy.”
In higher and other non-flowering plants the
photosynthetic reactions occur in the chloroplast – “An Incredible
Thermodynamic Machine.”

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Steps of Photosynthesis:
 During photosynthesis, light energy from the sun is absorbed by
chlorophyll.
 Water from the soil is absorbed by the root hairs and is transported to
the leaves.
 Carbon dioxide from the air enters through the stomata of the leaves.
 In the meantime, water molecules are split into hydrogen and oxygen.
 This hydrogen and carbon dioxide then combine to form carbohydrates
(glucose).
 Glucose is the source of energy that provides growth and development.
 Later, it is converted into starch and is stored in different plant organs.

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Site of Photosynthesis
 Photosynthesis occurs in chloroplasts of the plants cell.
 All green parts of plants are having chloroplasts and carry out
photosynthesis.
 Majorly chloroplasts are found in leaves.
 The green color comes from the chlorophyll in the chloroplasts.
 The pigments absorbs light energy according to their
absorption capacity.
 Chlorophyll is the green pigment which captures light for
photosynthesis.

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Chloroplast: A Solar Harvesting Unit
 Chloroplast was first described by a
German Botanist Von Mohl in 1837.
 A single leaf cell may contain as many
as 50 chloroplast, each with a 4-6 µm
diameter and surrounded by a
double membrane.
 The inner membranes of chloroplast
are folded into sacs or vesicles known
as thylakoids and often many
thylakoids are stacked one above the
other, in flattened columns to form
Granum.

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Chloroplast: A Solar Harvesting Unit
 Surrounding the thylakoid membrane
system is a semi fluid substance called
“Stroma.”
 Within the thylakoid membranes,
photosynthetic pigments are
organized n a network called the
“Photosystem” capable of capturing
photons of light.
 The photosystem functions a huge
antenna, amplifying the power of
individual pigment molecules to
capture light.

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A plant pigment is any type of colored substance produced by a plant.
These colouring substance are also called Biological Pigments or
Biochromes.
Plant pigments are following types:-
1. Chlorophyll:
Primary pigment found within the plants cells of all green plants.
Most significant and essential for photosynthesis.
Chlorophyll is further classified in following:
Chlorophyll a – Found in algae, cyanobacteria & all higher plants.
Chlorophyll b – Found only in Green algae and in higher plants.
Chlorophyll c – Found in some marine algae.
Chlorophyll d – Found only in Red algae
Chlorophyll e – Found only in algae.
Among all these pigments, Chl. a and Chl. b are considered as the primary
photosynthetic pigments.
Pigments and their types in plants:

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2. Carotene:
Carotenoids are the pigments in the form of orange, red and yellow
colours.
Insoluble in water and attached to the membranes of the cell bodies.
3. Anthocyanins:
A type of flavonoid pigment naturally found in all the tissues of higher
plants.
This pigment provide colour mainly to fruits and flowers.
4. Flavonoids:
Yellow coloured pigments found in lemons and oranges.
Largely found in the plastids and cytoplasm of the plant cell.
Pigments and their types in plants:

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Photosynthesis comprises two phases:
•The first phase is the photochemical phase or light-dependent process. This
phase is commonly known as the light reaction.
•The second phase is the biosynthetic phase of the dark reaction of
photosynthesis. This phase is the light-independent process.
The whole process of photosynthesis takes place within the chloroplast.
Mechanism of Photosynthesis:

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1. Light Reaction
 The light reaction is a light-dependent process which includes a series of events such as
light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH.
 The light reaction of photosynthesis initiates only when it is supplied with light energy.
 The photosystem is the arrangement of pigments, including chlorophyll within thylakoids.
 There are two photosystems in plants:
a. Photosystem I (PS-I)
b. Photosystem II (PS-II)
Mechanism of Photosynthesis:
1. Light Reaction
 The light reaction is a light-dependent process which includes a series of events such as
light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH.
 The light reaction of photosynthesis initiates only when it is supplied with light energy.
 The photosystem is the arrangement of pigments, including chlorophyll within thylakoids.
 There are two photosystems in plants:
a. Photosystem I (PS-I)
b. Photosystem II (PS-II)
Mechanism of Photosynthesis:

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 The light reaction occurs in the
thylakoids of the chloroplast.
 When the light hits, chlorophyll-
a gets excited to a higher energy
state followed by a series of
reactions.
 This energy is converted into energy
molecules ATP and NADPH by using
PS I and PS II. Also, hydrolysis
occurs and releases oxygen.
Mechanism of Photosynthesis:

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 Photosystems are the functional units for photosynthesis,
defined by a particular pigment organization and association
patterns, whose work is the absorption and transfer of light
energy, which implies transfer of electrons.
 Physically, photosystems are found in
the thylakoid membranes.
 There are two kinds of photosystems: photosystem I (PSI)
and photosystem II (PSII).
 PSII acts first during the light transformation process in
photosynthesis, but it was named PSII because it was
discovered second.
Photosystem
s:

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 Each photosystem consists of two closely linked components: the
first is the antenna complex formed by hundreds of pigment
molecules that capture photons and transfer the harvested light
energy to the second component named the reaction center, which
possesses Chl a molecules in a matrix of protein.
 When excitation energy reaches chlorophyll a at the reaction
center, electron transfer is initiated through an electron transport
chain.
 PSI is located at the outer surface of the thylakoid membrane, and
contains chlorophyll b; chlorophyll a and carotenoids.
 PSII is located at the inner surface of the thylakoid membrane, and
contains chlorophyll b; chlorophyll a, phycobillins and xanthophylls.

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Photosystems are the functional and structural elements of photosynthetic
protein complexes. They work together to carry out light absorption, energy and
electron transport.
Components of photosystem:
1. Antenna Complex: light-harvesting complex made up of proteins and
numerous pigments.
2. Reaction centers: Molecules of chl. A & principal electron acceptor.

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Photophosphorylation
“Formation of ATP form ADP and
iP (Inorganic Phosphate) in the
presence of light.”
Cyclic Photophosphorylation
 Performed by PS I independently.
 External source of electrons is not
required. Same electrons are
recycled.
 Synthesizes only ATP.
 Operates under low light intensity.
 Occurs in stromal thylakoid.

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Non-Cyclic Photophosphorylation
 Performed by PS I and PS II.
 Electrons are supplied by
photolysis of water.
 Synthesizes ATP and NADPH.
 Operates under optimum light.
 Occurs in granal thylakoids

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2. Dark Reaction
 Dark reaction is also called carbon-fixing reaction.
 It is a light-independent process in which sugar molecules are formed from the carbon
dioxide and water molecules.
 The dark reaction occurs in the stroma of the chloroplast, where they utilize the products of
the light reaction.
 Plants capture the carbon dioxide from the atmosphere through stomata and proceed to
the Calvin cycle.
 In the Calvin cycle, the ATP and NADPH formed during light reaction drives the reaction and
convert six molecules of carbon dioxide into one sugar molecule, i.e. glucose.
Mechanism of Photosynthesis:

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Role of Photosynthesis in Bio-productivity:
Photosynthesis is a crucial process for the productivity of the
biosphere. It is the primary mechanism through which energy
from the sun is captured and converted into organic
compounds that sustain life. Photosynthesis occurs in green
plants, algae, and some bacteria, and it is responsible for
producing the vast majority of the oxygen in the Earth's
atmosphere.
The importance of photosynthesis in bio productivity can be
understood through the following points:
1. Energy Source: Photosynthesis is the primary source of
energy for most living organisms on the planet. Plants,
algae, and some bacteria use photosynthesis to produce
organic compounds that are consumed by other organisms,
including animals and humans.

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2. Oxygen Production: Photosynthesis also
plays a critical role in producing the oxygen
that all living organisms require for
respiration. Without photosynthesis, the levels
of oxygen in the atmosphere would be
significantly lower, making it difficult for most
organisms to survive.
3. Carbon Fixation: Through photosynthesis,
plants and algae capture carbon dioxide from
the atmosphere and use it to build organic
molecules. This process is known as carbon
fixation and is essential for reducing the
amount of carbon dioxide in the atmosphere,
which is a greenhouse gas that contributes to
climate change.

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4. Food Production: Photosynthesis is the
foundation of most food chains and is the basis
for the production of crops and other food
sources. The carbohydrates, proteins, and fats
produced by plants through photosynthesis
serve as the primary source of nutrients for
animals and humans.
In summary, photosynthesis is essential for
maintaining the productivity of the biosphere. It
provides the energy and organic compounds
necessary for most living organisms to survive,
produces oxygen for respiration, fixes carbon
dioxide, and forms the basis of food chains and
agriculture.

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References
 Plant Physiology: Theory and Applications, SL Kochar and
Sukhbir Kaur Gujral (Book)
 Matthew P. Johnson, Photosynthesis (article)
 Encyclopedia.com
 Sciencedirect.com
 Biologyonline.com
 Byjus.com
 Picture are taken from various internet sources.

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