Photosynthesis is the process by which plants, algae and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two phases - the light-dependent reactions and the light-independent reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH through the process of photophosphorylation. These products are then used in the light-independent reactions, also known as the Calvin cycle, to fix carbon from carbon dioxide into organic carbon compounds like glucose.

1. Photosynthesis in Plants
By
Siba Prasad Pidika
Lecturer in Botany
Rayagada Autonomous College Rayagada

2. Photosynthesis
• Photosynthesis (Photon = Light, Synthesis = Putting
together) is an anabolic, endergonic process by
which green plant synthesize carbohydrates
(initially glucose) by the help of carbon dioxide,
water, pigments and sunlight and produce Oxygen
as its by product.
• In other words, we can say that photosynthesis is
transformation of solar energy/radiant energy/light
energy into chemical energy.

4. • Simple general equation of photo synthesis is
as follows:
• According to Van Neil and Robert Hill, oxygen
liberated during photosynthesis comes from
water and not from carbon dioxide.
• Thus, the overall correct biochemical reaction
for photosynthesis can be written as:

5. Site of photosynthesis
• Chloroplast

6. Mechanism of Photosynthesis:
• Photosynthesis is an oxidation reduction
process in which water is oxidized and carbon
dioxide is reduced to carbohydrate.
• Blackmann (1905) pointed out that the process
of photosynthesis consists of two phases:
(1) Light reaction or Light phase or Light-
dependent phase or Photochemical phase
(2) Dark reaction or Dark phase or Light
independent phase or Biochemical phase.

7. • During light reaction, oxygen is evolved and
assimilatory power (ATP and NADPH2) are
formed.
• During dark reaction assimilatory power is
utilized to synthesize glucose.

8. Components of photosynthesis
• Light
• Water
• Carbon dioxide
• Pigments

9. Light
• The source of light for photosynthesis is sunlight.
• Sun Light is a form of energy (solar energy) that travels as
a stream of tiny particles.
• Discrete particles present in light are called photons.
• They carry energy and the energy contained in a photon
is termed as quantum.
• The energy content of a quantum is related to its wave
length.
• Depending upon the wave length electro magnetic
spectrum comprises cosmic rays, gamma rays, X-rays,-UV
rays, visible spectrum, infra red rays, electric rays and
radio waves.
• The visible spectrum ranges from 390 nm to 760 nm

11. Absorption Spectrum:
• All photosynthetic organisms contain one or more
organic pigments capable of absorbing visible radiation
which will initiate the photochemical reactions of
photosynthesis. When the amount of light absorbed by a
pigment is plotted as a function of wave length, we
obtain absorption spectrum.

12. Action Spectrum:
• It represents the extent of response to different wave lengths
of light in photosynthesis. It can also be defined as a measure
of the process of photosynthesis when a light of different
wave lengths is supplied but the intensity is the same. For
photochemical reactions involving single pigment, the action
spectrum has same general shape as the absorption spectrum
of that pigment, otherwise both are quite distinct

13. Quantum Requirement and Quantum
Yield:
• The number of photons or quanta required by
a plant or leaf to release one molecule of
oxygen during photosynthesis is called
quantum requirement. It has been observed
that in most of the cases the quantum
requirement is 8.
• If the quantum requirement is 8 then
quantum yield will be 0.125 (1/8).

14. Emerson Red Drop Effect and
Enhancement Effect:
Emerson’s first experiment
• R. Emerson and Lewis (1943) while determining the
quantum yield of photosynthesis in Chlorella by
using monochromatic light of different wave
lengths.
• He noticed a sharp decrease in quantum yield at
wave length greater than 680 mμ.
• This decline in photosynthesis is called Red drop
effect.

15. Emerson’s second experiment
• Emerson and his co-workers (1957) found that the
inefficient far red light in Chlorella beyond 680nm
could be made fully efficient if supplemented with
light of short wave length.
• The quantum yield from the two combined beams
was found to be greater than the effect of both
beams when used separately.
• This enhancement of photosynthesis is called
Emerson Enhancement Effect.

17. Photosynthetic Pigments:
• Photosynthetic pigments are grouped into 3
categories:
(i) Chlorophyll:
(ii) Carotenoids:
(iii) Phycobilins (Biliproteins):

18. Chlorophyll:

19. (i)Chlorophyll:
• These are green coloured most abundant
photosynthetic pigments that play a major role
during photosynthesis.
• Types: Chlorophyll a, b, c, d and e,
Bacteriochlorophyll a, b and g
(ii) Carotenoids:
• These are yellow, red or orange colour pigments
embedded in thylakoid membrane
• These are of two of types viz., Carotene and
Xanthophyll (Carotenol/Xanthol).
(iii) Phycobilins (Biliproteins):
• These are water soluble pigments
• There are two important types of phycobilins-
Phycoerythrin (Red) and Phycocyanin (Blue).

20. Photosystems/Pigment systems
• The discovery of red drop effect and the
Emerson’s enhancement effect concluded in a
new concept about the role played by
chlorophyll-a and accessory pigments in
photosynthesis that photosynthesis involves two
distinct photochemical processes.
• These processes are associated with two groups
of photosynthetic pigments called as
• Pigment system I (Photoact I or Photosystem I) and
• Pigment system II (Photoact II or Photosystem II).

21. • Each pigment system consists of a central core
complex and light harvesting complex (LHC).
• LHC comprises antenna pigments
• Their main function is to harvest light energy and
transfer it to their respective reaction centre.
• The core complex consists of reaction centre
associated with proteins and also electon donors and
acceptors.

22. PS-I: Chlorophyll a 700 or P700 is the reaction centre of PS I.
PSI is found in thylakoid membrane and stroma lamella.
PS II: It is found in thylakoid membrane
P680 is the reaction centre of PS II.

23. Light Reaction (Photochemical
Phase):
• Light reaction or photochemical reaction takes
place in thylakoid membrane or granum and it
is completely dependent upon the light. The
raw materials for this reactions are pigments,
water and sunlight.
It takes place in the following steps
1. Excitation of chlorophyll
2. Photolysis of water
3. Photophosphorylation

24. 1. Excitation of Chlorophyll:
• It is the first step of light reaction. When
P680 or P700 (special type of chlorophyll a) of
two pigment systems receives quantum of
light then it becomes excited and releases
electrons.

25. 2. Photolysis of Water and Oxygen
Evolution (Hill Reaction):
• Robert Hill observed that the chloroplasts
extracted from leaves of Stellaria media and
Lamium album when suspended in a test tube
containing suitable electron acceptors (Potassium
feroxalate or Potassium fericyanide), Oxygen
evolution took place due to photochemical
splitting of water.

26. Photolysis of Water
• The splitting of water during photosynthesis is called
Photolysis of water.
• Mn, Ca, and CI ions play prominent role in the
photolysis of water.
• This reaction is also known as Hill reaction.
• To release one molecule of oxygen, two molecules of
water are required.

27. 3. Photophosphorylation:
• Synthesis of ATP from ADP and inorganic
phosphate (pi) in presence of light in chloroplast
is known as photophosphorylation. It was
discovered by Arnon et al (1954).
• Photophosphorylation is of two types.
(a) Cyclic photophosphorylation
(b) Non-cyclic photophosphorylation.

28. (a) Cyclic Photophosphorylation

29. Noncyclic Photophosphorylation (Z-Scheme)

32. Dark Reaction (Biosynthetic Phase)
• Also known as carbon fixation or photosynthetic carbon
reduction (PCR.)
• It is completely light independent reaction but it
depends on the products of light reaction(NADPH2 and
ATP).
• The carbon dioxide fixation takes place in the stroma of
chloroplasts because it has enzymes essential for
fixation of CO2.
• This is accomplished through a series of complex steps
involving different enzymatic actions.

33. Calvin or C3 Cycle or PCR
(Photosynthetic Carbon Reduction
Cycle):
• It is the basic mechanism by which CO2 is fixed
(reduced) to form carbohydrates. It was proposed
by Melvin Calvin. Calvin along with A.A. Benson, J.
Bassham used radioactive isotope of carbon (C14)
to determine the sequences of dark reaction. For
this work Calvin was awarded Nobel prize in 1961.

34. Calvin cycle completes in 4 major
phases:
• 1. Carboxylation phase
• 2. Reductive phase
• 3. Glycolytic reversal phase (sugar formation
phase)
• 4. Regeneration phase

35. 1. Carboxylation phase:
2. Reductive Phase:

36. 3. Glycolytic Reversal (Formation of sugar) Phase:

37. 4. Regeneration Phase:

39. C-4 Cycle (Hatch and Slack Cycle)
• C-4 cycle or C4 carbon biosynthesis seen in C-4 plants only.
• The C4 -Cycle was discovered by Hatch and Slack (Australia)
in 1966, so this cycle was named after the discoverers as
Hatch and Slack Cycle .
• The first stable compound of Hatch and Slack Cycle is 4-
carbon oxaloacetic acid. Therefore, it is called C4 Cycle.
• Such plants which possess C4 Cycle are called C4 plants.
• C4 Cycle is prominent in Gramineae (Corn, sugarcane),
Chenopodiaceae(Atriplex) and Cyperaceae family.

40. Leaf Anatomy of C4 Plants (Kranz
Anatomy).
• The leaves of C4 Plants are unique in
possessing two types of photosynthetic cell.
• The vascular bundles are surrounded by
• Bundle sheath cells (non stacked thyllakoid cells)
• Mesophyll cells ( stacked thyllakoid cells)
They form a crown (wreath) like structure known as kranz
anatomy.

42. The Basic C4 Cycle consists of four
stages :-
• 1. Fixation of CO2 by the carboxylation of
phosphoenolpyruvate in the mesophyll cells to form a
C4 acid (malate or aspartate)
• 2. Transport of the C4 acids to the bundles sheath cells.
• 3. Decarboxylation of the C4 acids within the bundle
sheath cells and generation of CO2, which is the
reduced to carbohydrate via the Calvin cycle.
• 4. Transport of the C3 acid ( pyruvate or alanine) that is
formed by the decarboxylation step back to the
mesophyll cell and regeneration of the CO2 acceptor
phosphoenol- pyruvate.