Importance of photosynthesis. Light reaction of photosynthesis, Dark reaction of photosynthesis. Hill, and Blackman reaction or C3 cycle or Calvin Cycle

1. Plant Physiology
BOT-232
PHOTOSYNTHESIS
Lecture- 8th & 9th
By
Prof. SD Tidke
Modern CABT, Pune

3. Photosynthesis
 It is the process by which green plants use sunlight to
manufacture food from simple molecules like CO₂ and H₂O.
 Is the process by which autotrophic organisms use light energy to
make sugar and oxygen gas from carbon dioxide and water.
 Occurs in plants, algae and some prokaryotes.
 Anabolic (small molecules combined) & Endergonic (stores energy)
 Photosynthesis is sometimes called carbon assimilation and is
represented by following equation;

4. Importance of Photosynthesis
Photosynthesis and Sun Energy:
• Harnesses the sun's energy into utilizable
forms of energy on earth.
• A process that most biological organisms are
unable to perform.
• ATPis used to power these processes.
• Converts light energy into chemical energy in
the form of glucose.
• Then the process of cellular respiration
converts energy in glucose to energy in the
form of ATP which is used to power
biological processes.

5. Importance of Photosynthesis
Photosynthesis and Carbon
Dioxide Removal:
• Converts carbon dioxide into
oxygen.
• During photosynthesis, carbon
dioxide leaves the atmosphere
and enters the plant and leaves
as oxygen.
• A process which is ecologically
and environmentally important
in nature.

6. Importance of Photosynthesis
Photosynthesis and the Ecosystem:
• The energy produced by photosynthesis
forms the basis of virtually all terrestrial
and aquatic food chains.
• As a result, photosynthesis is the
ultimate source of carbon in the organic
molecules found in most organisms.
• The high oxygen concentration in the
atmosphere is derived directly from the
light reactions of photosynthesis.
• Prior to the evolution of photosynthesis
on earth, the atmosphere was anoxic.

7. Importance of Photosynthesis
Photosynthesis and side products
• Solar energy, transformed by photosynthesis, is
the source of countless vegetable, animal and
organic side products.
• Wood is a very important material used as fuel
and in many fields including construction.
• Paper, cotton and other natural fibres consist of
cellulose produced almost entirely by
photosynthesizing plants.
• Wool depends on the energy sheep obtain from
grass, and photosynthesis.

8. THE BASICS OF PHOTOSYNTHESIS
 Almost all plants are photosynthetic autotrophs, as are some bacteria
and protists
– Autotrophs generate their own organic matter through
photosynthesis
– Sunlight energy is transformed to energy stored in the form of
chemical bonds
(a) Mosses, ferns, and
flowering plants
(b) Kelp
(c) Euglena (d) Cyanobacteria

9. THE BASICS OF PHOTOSYNTHESIS
Sunlight
 Form of energy (photons)
 Measured in wavelengths
 Sun radiates full spectrum,
atmosphere screens out most,
allows primarily only visible light
through (380-750 nm)
 Amount of energy is inversely
related to the wavelength of light

10. Electromagnetic Spectrum and Visible Light
Wavelength (nm)
Gamma
rays X-rays UV
Infrared &
Microwaves Radio waves

11. Absorption of Light by plant pigments
Wavelength
A
B
S
O
R
B
A
N
C
E

12. Sites of Photosynthesis
• Photosynthesis occurs in chloroplasts, organelles in
certain plants
• All green plant parts have chloroplasts and carry out
photosynthesis
• The leaves have the most chloroplasts
• The green colour comes from chlorophyll in the
chloroplasts
• The pigments absorb light energy
• A chloroplast contains:
 stroma, a fluid
 Grana, stacks of thylakoids
• The thylakoids contain chlorophyll
 Chlorophyll is the green pigment that captures
light for photosynthesis

13. What are Pigments?
 Pigments are defined as the set of compounds that have an intense
colour and are used in the colouring of other materials.
 These colouring substances are also called Biological Pigments or
the Biochromes, which mainly refers to the true pigments.
 These biological pigments are insoluble in water and are applied as
ground particles in solid form with the liquids.
 We can find various type of Biological pigments in both plants as
well as animals.
 The pigments are produced by the living organisms and have a
colour, which results from the colour absorption techniques.
 There are two different types of pigments an are classified base on
their sources.

14. Types of Pigment in Plants
 Plants have a unique feature of capturing the light energy
and convert it into sugars through the process
called photosynthesis.
 The process begins with the absorption of light energy by
some specialized form of organic molecules, called the
pigments.
 The photosynthesis, a biological process requires green
coloured pigment called chlorophyll along with other forms
of yellow and red coloured pigments.
 Other essential plant pigments include anthocyanins,
betalains, carotenoids, porphyrins and much more.
 All these pigments stimulate the process of chemical
reactions by reflecting the wavelengths.

15. 1. Chlorophyll:
 Chlorophyll is one of the primary pigment found within the plant cells of all green
plants.
 The green colouring of the plant leaves and the tender part of the stem is due to the
presence of a pigment called chlorophyll.
 The chlorophyll pigment is the most significant and essential pigments, as it plays a
vital role in the biological process of photosynthesis.
 There are different types of chlorophyll pigments and are classified mainly based on
their structure, functions and other features. The different types of chlorophyll pigments
are:
 Chlorophyll a– found in algae, cyanobacteria and in all higher plants.
Chlorophyll b– found only in green algae and in higher plants.
Chlorophyll c– found in certain photosynthetic Chromista and in some marine algae.
Chlorophyll d– found only in red algae.
Chlorophyll e– found only in algae.
 Among all these five types of chlorophyll pigments, chlorophyll a and b are considered
as the primary photosynthetic pigments.

16. 2. Carotenoids
 Carotenes (Orange) and Xanthophyll (yellow)
 Carotenoids are the pigments in the form of orange,
red, yellow colours. These compounds are insoluble in
water and are attached to the membranes of the cell
bodies. These Biomolecules are antioxidants which
promote a good eyesight in humans.
3. Anthocyanins
 Anthocyanins are a type of flavonoid pigments found
naturally in all the tissues of the higher group of
plants. This pigment functions by providing colour to
the stem, leaves, roots, fruits, and flowers. Based on
their pH, these type of pigments appears red, blue,
purple and other dark colours.

17. 4. Flavonoids
 Flavonoids are a type of yellow coloured pigments, which are abundantly found
in lemons, grapefruit, oranges and in some ark and yellow coloured flowers.
 This type of pigments is largely found in the plastids and cytoplasm of the plant
cell.
 Flavonoids are the chemicals with the antioxidant properties and help in
lowering the cholesterol levels.
 Other applications of flavonoids are: They are extracted and utilized as dyes.
 These pigments such as lycopene and astaxanthin are used as a dietary
supplement in most of the food products.
5. Phycobillins:
 It absorbs maximum absorption in green parts of the spectrum. Phycocyanin
absorbs in the orange part of the spectrum.

18. Photosystems
 Pigment molecules organized into photosystems capture sunlight in the chloroplast.
 Photosystems are clusters of light-absorbing pigments with some associated
molecules—proton (hydrogen ion) pumps, enzymes, coenzymes, and cytochromes.
 Each photosystem contains about 200 molecules of a green pigment
called chlorophyll and about 50 molecules of another family of pigments
called carotenoids.
 In the reaction center of the photosystem, the energy of sunlight is converted to
chemical energy.
 The center is sometimes called a light-harvesting antenna.
 There are two photosystems within the thylakoid membranes,
designated photosystem I and photosystem II.
 The reaction centers of these photosystems are P700 and P680, respectively.
 The energy captured in these reaction centers drives chemiosmosis, and the energy of
chemiosmosis stimulates ATP production in the chloroplasts.

20. Electron Excitation

21. Excitation of chlorophyll in a chloroplast

22. Reaction of Photosynthesis
Photosynthesis
Light dependent
Stage
Cyclic
photophosphorylation
Non- cyclic
photophosphorylation
Light
independent
Stage

23. Light or Hill Reaction:
 Photo-phosphorylation is the process in which the light energy is converted into chemical
energy in the form of ATP.
 Light reaction is the first step in photosynthesis occurring in grana of chloroplast and
needs the utilization of light energy.
It consists of following three phases:
(a) Photolysis of water:
 The light energy trapped by chlorophyll molecule and decomposes water into its
constituent elements, called photolysis of water.
2H2O → 4H+ + 4e- + O2
(b) Electron transfer:
 The electrons produced during the photolysis of water pass via 2 photosystems (PS –I and
II). Each photo system has its own trap center and a primary pigment molecule.
(C) Photo-phosphorylation:
 It is the process of synthesis of ATP from ADP using light energy.
ADP + ip → (light) ATP

24. Types of Light
reaction:
 Non-cyclic photo-
phosphorylation:
 Cyclic photo-
phosphorylation:
Two types of photosystems cooperate in the
light reactions

25. Non-cyclic photo-phosphorylation
 Involves two photosystems (P680, P 700 nm)
 These photosystems contains chlorophyll a and chlorophyll b as well as
accessory pigments.
 Flow of electron is non cyclic.
 Uses the photolysis of water
H2O 2H+ + 2e- + ½ O2
 End products are reduced NADP, ATP and O2
 High energy electrons released from P680 of PS-II are accepted by
primary electron acceptor. The electrons pass via a series of electron
acceptor i.e. PQ- cytochorome complex- PC and finally to P700 of PSI.
 Again, the electrons given out by P700 of PS-I are taken up by primary
pigment molecule and are ultimately passed to NADP through Fd. The
electrons combine with ions and reduce NADP to NADP H2.

27. Cyclic photo-phosphorylation
 Involves only photosystem 1 (at 700 nm)
 End product is ATP only
 High energy electrons expelled from P700 of PS-I are
taken up by primary pigment molecule, when the
pass through series of electron acceptors i.e. Fd-PQ-
Cytochorome complex-PC and finally to the same
pigment molecule from which they have been
originated.

29. Difference between Non-cyclic and cyclic photo- phosphorylation:
Non-cyclic photo- phosphorylation Cyclic photo- phosphorylation
The electrons do not come back to
the same molecules.
The electrons come back to the
same molecule.
Both photosystems are involved. Only PS I is involved.
First electron donor is H₂O. First electron donor is PS I (P700)
Last electron acceptor is NADP
Last electron acceptor is PS I
(P700)
O₂ is evolved. O₂ is not evolved.
Net products are ATP, NADH₂ and
O₂.
Net product is ATP only.

30. Dark reaction or Blackman’s reaction:
 This is the second step in the mechanism of photosynthesis.
 The chemical processes of photosynthesis occurring independent of light is called
dark reaction.
 It takes place in the stroma of chloroplast.
 The dark reaction is purely enzymatic and it is slower than the light reaction.
 The dark reactions occur also in the presence of light.
 In dark reaction, the sugars are synthesized from CO2.
 The energy poor CO2 is fixed to energy rich carbohydrates using the energy rich
compound, ATP and the assimilatory power, NADPH2 of light reaction.
 The process is called carbon fixation or carbon assimilation. Since Blackman
demonstrated the existence of dark reaction, the reaction is also called as
Blackman’s reaction.

31. Calvin cycle or C₃ cycle:
 It is a cyclic reaction occurring in the dark phase of photosynthesis.
 In this reaction, CO₂ is converted into sugars and hence it is a process of
carbon fixation.
 The Calvin cycle was first observed by Melvin Calvin in chlorella,
unicellular green algae.
 Calvin was awarded Nobel Prize for this work in 1961.
 Since the first stable compound in Calvin cycle is a 3 carbon compound
(3 phosphoglyceric acid), the cycle is also called as C₃ cycle.
 The reactions of Calvin’s cycle occur in three phases.
1. Carboxylative phase
2. Reductive phase
3. Regenerative phase

32. 1. Carboxylative phase:
 Three molecules of CO2 are accepted by 3 molecules of 5C compound ,
ribulose diphosphate to form three molecules of an unstable intermediate
6C compound. This reaction is catalyzed by the enzyme, carboxy dismutase
3 CO₂ + 3 Ribulose diphosphate → 3 unstable intermediate 6 carbon compound
 The three molecules of the unstable 6 carbon compound are converted by
the addition of 3 molecules of water into six molecules of 3 phosphoglyceric
acid. This reaction is also catalyzed by the enzyme carboxy mutase.
3 unstable intermediate 6 C compound + 3 H₂O → Carboxy dismutase3 phosphoglyceric
acid
 3 phosphoglyceric acid (PGA) is the first stable product of dark reaction of
photosynthesis and since it is a 3 carbon compound, this cycle is known as
C3 cycle.

33. 2. Reductive phase
 Six molecules of 3PGA are phosphorylated by 6 molecules of ATP
(produced in the light reaction) to yield 6 molecules of 1-3 diphospho
glyceric acid and 6 molecules of ADP. This reaction is catalyzed by the
enzyme, Kinase
3 Phosphoglyceric acid + ATP → 1, 3 diphosphoglyceric acid +
ADP
 Six molecules of 1, 3 diphosphoglyceric acid are reduced with the use
of 6 molecules of NADPH2 (produced in light reaction) to form 6
molecules of 3 phospho glyceraldehyde. This reaction is catalysed by
the enzyme, triose phosphate dehydrogenase.
1,3 diphosphoglyceric acid + NADPH₂ → 3 phosphoglyceraldehyde + NADP +
H₃PO₄

34. 3. Regenerative phase
 In the regenerative phase, the ribose diphosphate is
regenerated. The regenerative phase is called as pentose
phosphate pathway or hexose monophophate shunt.

36. Factors affecting photosynthesis:
I. External factors
 Light:
 It is the most important factor of photosynthesis. Any kind of artificial light such as
electric light can induce photosynthesis. Out of the total solar energy, only 1-2 % is
used for photosynthesis and the rest is used for other metabolic activities. The effect
of light on photosynthesis can be studied under three categories.
 Light intensity: The rate of photosynthesis is greater in intense light than in diffused
light.The rate of photosynthesis is directly proportional to light intensity.
 Light quality (wavelength):Photosynthesis occurs only in the visible part of the light
spectrum i.e., between 400 and 700 nm. The maximum rate of photosynthesis occurs
at red light followed by blue light. The green light has minimum effect and
photosynthesis cannot take place either in the infrared or in the ultraviolet light.
 Light duration: In general tropical plants get 10-12 hours of light per day and this
longer period of light favours photosynthesis.

37. Carbon dioxide:
 CO2 is one of the raw materials required for photosynthesis. If the CO2
concentration is increased at optimum temperature and light intensity, the rate of
photosynthesis increases. But, it is also reported that very high concentration of
CO2 is toxic to plants inhibiting photosynthesis.
Temperature:
 The rate of photosynthesis increases by increase in temperature up to 40 ºC and
after this, there is reduction in photosynthesis. High temperature results in the
denaturation of enzymes and thus, the dark reaction is affected.
Water:
 Water has indirect effect on the rate of photosynthesis although it is one of the raw
materials for the process. Water rarely acts as a limiting factor for photosynthesis.
During water scarcity, the cells become flaccid and the rate of photosynthesis might
go down.
Oxygen:
 Oxygen is a byproduct of photosynthesis and an increase in the O2 concentration in
many plants results in a decrease in the rate of photosynthesis.

38. II Internal factors:
Leaf:
 The leaf characters such as leaf size, chlorophyll content, number of
stomata. Leaf orientation and leaf age are some of the factors that are
responsible for photosynthesis.
Chlorophyll content:
 It is very much essential to tarp the light energy. In 1929, Emerson
found direct relationship between the chlorophyll content and rate of
photosynthesis. In general, the chlorophyll sufficient plants are green
in colour showing efficient photosynthesis. The chlorotic leaves due to
irregular synthesis of chlorophyll or breakdown of chlorophyll pigment
exhibit inefficient photosynthesis.