Photosynthesis is the process by which plants and some bacteria convert sunlight into chemical energy in the form of sugar, using water and carbon dioxide while releasing oxygen. It is critical for life on Earth, sustaining ecosystems and providing energy for various biological functions, and is facilitated by structures such as chloroplasts and specialized leaf anatomy. The process involves light-dependent reactions occurring in the thylakoids and light-independent reactions (Calvin cycle) occurring in the stroma, and various factors like light intensity, carbon dioxide concentration, and temperature affect its rate.

1. Prepared By: Vipin Kr Shukla
Assistant Lecturer
Photosynthesis

2. What is Photosynthesis?
 Photosynthesis is the process by which plants, some bacteria, use
the energy from sunlight to produce sugar, which cellular
respiration converts into ATP, the "fuel" used by all living things.
 The conversion of unusable sunlight energy into usable chemical
energy, is associated with the actions of the green pigment
chlorophyll.
 Most of the time, the photosynthetic process uses water and
releases the oxygen that we absolutely must have to stay alive.
 Oh yes, we need the food as well!
 We can write the overall reaction of this process as:
 Sun light
 6CO2 + 12H2O -------------> C6H12O6+ 6O2+6H2O

3. six molecules of water plus six molecules of carbon dioxide produce one molecule
of sugar plus six molecules of oxygen
• Diagram of a typical plant, showing the inputs and outputs of the photosynthetic
process.

4. Importance of Photosynthesis:
 Photosynthesis is the only process which produces enormous
quantities of organic matter for sustaining the life on this globe.
 Photosynthetic products not only build up the bodies of organisms
but also provide energy for carrying out metabolic activities and
different types of movements.
 The chemical energy present in the organic food is the converted
form of radiant or solar energy.
 Coal, petroleum and natural gas represent the photosynthetic
capital of the past geological ages.
 Several materials derived from the organic world (and hence
photosynthesis) are in our daily use. Examples: Natural fibers,
drugs, vitamins, gum, tannins, turpentine, furniture, etc.
 By photosynthesis, green plants keep the concentration of the two
gases almost constant by absorbing carbon dioxide and evolving
oxygen during photosynthesis.

5. Structural feature of leaf advantage for photosynthesis:
 Upper epidermis and cuticle are transparent to light, thus allows most
light to pass to photosynthetic mesophyll tissues.
 Palisade mesophyll cells are closely packed and contain many
chloroplasts to carry out photosynthesis more efficiently.
 Extensive vein system allow sufficient water to reach the cells in the leaf
and to carry food away from them to other parts of the plant .
 Spongy mesophyll cells are loosely packed with numerous large air
spaces to allow rapid diffusion of gases throughout the leaf.
 Numerous stomata on lower epidermis allow rapid gaseous exchange
with the atmosphere .

6. Contd….
 Upper epidermis and cuticle are transparent to light, thus allows most light to
pass to photosynthetic mesophyll tissues.
 Palisade mesophyll cells are closely packed and contain many chloroplasts to
Carry out photosynthesis more efficiently.
 Extensive vein system allow sufficient water to reach the cells in the leaf and to
carry food away from them to other parts of the plant .
 Spongy mesophyll cells are loosely packed with numerous large air spaces to
allow rapid diffusion of gases throughout the leaf.
 Numerous stomata on lower epidermis allow rapid gaseous exchange with the
atmosphere .

7. Leaves and Leaf Structure:
 Plants are the only photosynthetic organisms to have leaves (and
not all plants have leaves).
 A leaf may be viewed as a solar collector.
 The raw materials of photosynthesis, water and carbon dioxide,
enter the cells of the leaf, and the products of photosynthesis,
sugar and oxygen.

8. Leaf anatomy of C3 and C4 plants:

9. Contd….
 Water enters the root and is transported up to the leaves through
specialized plant cells known as xylem.
 Stomata is the structure to allow gas to enter the leaf. Carbon
dioxide cannot pass through the protective waxy layer covering the
leaf (cuticle), but it can enter the leaf through an opening of stoma.
 Likewise, oxygen produced during photosynthesis can only pass
out of the leaf through the opened stomata.
 Unfortunately for the plant, while these gases are moving between
the inside and outside of the leaf, a great deal water is also lost.

10. Contd….
 In the electromagnetic spectrum the wavelengths between
400-700nm is called as PHOTOSYNTHETICALLY ACTIVE
RADIATION (PAR).
 Light or visible light is electromagnetic radiation that is
visible to the human eye, and is responsible for the sense
of sight.
 Visible light has wavelength in a range from about 380
nanometers to about 740 nm.
 while its speed in a vacuum, 299,792,458 meters per second
(about 300,000 kilometers per second.
 Light, which is emitted and absorbed in tiny "packets“ called
photons, exhibits properties of both waves and particles.
 This property is referred to as the wave–particle duality.

11. Contd…
.
 The order of colors is determined by the
wavelength of light. Visible light is one
small part of the electromagnetic
spectrum.
 The longer the wavelength of visible
light, the more red the color. Likewise
the shorter wavelengths are towards the
violet side of the spectrum.
 Wavelengths longer than red are referred
to as infrared, while those shorter than
violet are ultraviolet.

12. Contd….
 Chlorophyll and Accessory Pigments. A pigment is any substance that
absorbs light.
 The color of the pigment comes from the wavelengths of light reflected (
those not absorbed). Chlorophyll, the green pigment common to all
photosynthetic cells, absorbs all wavelengths of visible light except
green, which it reflects to be detected by our eyes.
 Pigments have their own characteristic absorption spectra, the
absorption pattern of a given pigment.

13. Chlorophyll:
 Chlorophyll is a complex molecule. Several modifications of
chlorophyll occur among plants and other photosynthetic
organisms.
 All photosynthetic organisms have chlorophyll a Accessory
pigments absorb energy that chlorophyll a does not absorb.
Accessory pigments include chlorophyll b (also c, d, and e in
algae), xanthophylls, and carotenoids.
 Chlorophyll a absorbs its energy from the Violet-Blue and
Reddish orange- Red wavelengths, and little from the
intermediate (Green- Yellow-Orange) wavelengths.

14. The molecular structure of chlorophylls:
 Chlorophyll b (C55H70O6N4Mg) differs from
chlorophyll a (C55H72O5N4Mg) by the
substitution of a CHO group for the CH3 marked
at position R.

15. Contd….
 If a pigment absorbs light energy, one of
three things will occur.
 Energy is dissipated as heat.
 The energy may be emitted immediately
as a longer wavelength, a phenomenon
known as fluorescence.
 Energy may trigger a chemical reaction, as
in photosynthesis. Chlorophyll only
triggers a chemical reaction when it is
associated with proteins embedded in a
membrane (as in a chloroplast) or the
membrane infoldings found in
photosynthetic prokaryotes such as
cyanobacteria and prochlorobacteria.

16. Structure of chloroplast:
 The thylakoid is the structural unit of photosynthesis. Both
photosynthetic prokaryotes and eukaryotes have these flattened
sacs/vesicles containing photosynthetic chemicals.
 Only eukaryotes have chloroplasts with a surrounding membrane.
 Thylakoid are stacked like pancakes in stacks known collectively as
grana. The areas between grana are referred to as Stroma.
 While the mitochondrion has two membrane systems, the chloroplast
has three, forming three compartments.

17. Functions of Chloroplast:
 These cellular organs are the main sites of photosynthesis in
which both light and dark reactions are found. Light reaction
takes place in the grana region whereas dark reaction takes place
in the Stroma region.
 Chloroplast is capable for some amount of protein synthesis
which is used in the structural organizations and some
photosynthetic enzymes also synthesize in chloroplast.
 Chloroplast has its own genetic system (DNA) and participates
in Cytoplasmic inheritance.

18. Stages of Photosynthesis:
 Photosynthesis is a two stage process. The first process is the Light
Dependent Process (Light Reactions), requires the direct energy of light
to make energy carrier molecules.
 The Light Independent Process (or Dark Reactions) occurs when the
products of the Light Reaction are used to form C-C covalent bonds of
carbohydrates. The Dark Reactions can usually occur in the dark.
 Recent evidence suggests that a major enzyme of the Dark Reaction is
indirectly stimulated by light.
 The Light Reactions occur in the grana and the Dark Reactions take
place in the stroma of the chloroplasts.

19. Light Reactions:
 In the Light Dependent Processes (Light Reactions) light strikes
chlorophyll a in such a way as to excite electrons to a higher
energy state.
 In a series of reactions the energy is converted (along an electron
transport process) into ATP and NADPH. Water is split in the
process, releasing oxygen as a by-product of the reaction.
 Photosynthetic unit:
 A photosynthetic unit is the smallest group of pigment molecules
which combined together to cause a photochemical act. i.e.
absorption and migration of a light quantum to a trapping center
where it brings about the release of an electron.

20. Contd….
 Photons:- Light behaves as it is consists of tiny particles called
photons.
 Quantum:- A photon has a certain energy value which is called
quantum.
 About 2500 chlorophyll molecules ( under photosynthetic unit)
combined together to evolve one molecule of O2 and that for 10
quanta of light are used.
 Red drop:- Emerson observed that photosynthesis decreased when
a plant was shifted from short wave length (650 nm) to wave
length longer than 680 nm. This decline in the rate of
photosynthesis under long wave length of red is described as red
drop.
 Emerson effect:- Emerson also observed that when both short
 and long wave lengths of red light acted on at the same time,
 the rate of photosynthesis was higher than the sum of
 photosynthetic rates obtained by using short and long wave
 lengths respectively. This effect described as Emerson effect.

21. PHOTOSYSTEM I & II:
 The reaction centre chlorophyll of photo system I absorbs
maximally at 700 nm in its reduced state. Accordingly, it is
named P 700 (the P stands for Pigments), photo system I, its
associated antenna pigments, ATP synthase enzymes are found
exclusively in the stroma lamella and at the edges of the grana
lamellae.
 The reaction centre chlorophyll of photo system II absorbs
maximum at 680 nm (P 680 ) so its reaction centre chlorophylls
and associated electron transport proteins is located predominantly
in the stacked regions of the grana lamellae. At a wavelength grater
than 680 nm PSII cannot operate. Therefore, quantum yield
decreased beyond 680 nm. This is what red drop is observed. Photo
system I contains large amount of chlorophyll a, a small amount of
chlorophyll b and some B carotene. Photo system II also contains
chlorophyll a, B carotene but a large amount of chlorophyll b.

22. The light reaction:
 Light reactions or the „Photochemical phase include light ‟ absorption, water
splitting, oxygen release, and the formation of high-energy chemical
intermediates, ATP and NADPH. In PS I the reaction centre chlorophyll a has an
absorption peak at 700 nm, hence is called P700, while in PS II it has absorption
maxima at 680 nm, and is called P680.
 Water is oxidized to oxygen by Photo system II:
 The chemical reaction by which water is oxidized is given by the following
equation.
 2H2O O2 + 4H+ + 4e-
 This equation indicates that four electrons are removed from two water
molecules, generating an oxygen molecule and four hydrogen ions.
 Robert Hill demonstrated that isolated chloroplasts evolved Oxygen when they
were illuminated in the presence of suitable electron acceptor, such as
ferricyanide.
 The ferricyanide is reduced to ferrocyanide by photolysis of water .This reaction
is now called as HILL REACTION and it explains that water is used as a
source of electrons for CO2 fixation and Oxygen is evolved as a by-product.

23. Cyclic photophosphorylation & Non Cyclic
Photophosporylation:
 It functions in a closed
circle. Electron free
from chlorophyll to
acceptor for return to
chlorophyll.
 NADPH2 is not
formed and so
assimilation of CO2 is
retarded.
 O2 is not evolved.
 Bacteria have only
this.
 It is insensitive to
 Independent electron
donor is essential.
H2O is ultimate
source of Electrons
and NADP is final
electron acceptor.
 NADPH2 is formed
and it is used in CO2
assimilation.
 O2 is evolved.
 In green plants this
system.
 DCMU stop it.

24. MECHANISM OF ATP SYNTHESIS:
 The phenomenon of synthesis of ATP in light reactions of
photosynthesis is known as photophosphorylation. The
mechanism of ATP synthesis is explained by the chemi-
osmotic mechanism first proposed in 1960 by Peter Mitchell.
 There is one difference though, here the proton accumulation
is towards the inside of the membrane, i.e., in the lumen. The
basic principle of chemi-osmosis is that in concentration
differences and electrical potential differences across the
membranes are a source of free energy that can be utilized by
the cell for the synthesis of ATP.
 In the light reactions electron flow is coupled to proton
translocation, creating transmembrane proton motive force.
The energy in the proton motive force is then used for
synthesis of ATP by the enzyme called ATP synthase.

25. Schematic presentation of light reaction:

26. Dark reaction:
 Dark reaction:-Carbon-Fixing Reactions are also known as the Dark
Reactions (or Light Independent Reactions). Stomata to allow gas to
enter and leave the leaf.
 The Calvin Cycle occurs in the stroma of chloroplasts. Carbon dioxide
is captured by the chemical ribulose biphosphate (RuBP-5C).
 Six molecules of carbon dioxide enter the Calvin Cycle, eventually
producing one molecule of glucose.

27. Crassulacean:
 Xerophytes, such as cacti and most succulents, also use PEP carboxylase to
capture carbon dioxide in a process called Crassulacean acid metabolism
(CAM).
 In contrast to C4 metabolism, which physically separates the CO2 fixation to
PEP from the Calvin cycle, CAM only temporally separates these two
processes.
 CAM plants have a different leaf anatomy from C4 plants, and fix the CO2 at
night, when their stomata are open.
 CAM plants store the CO2 mostly in the form of malic acid via carboxylation of
phosphoenolpyruvate to oxaloacetate, which is then reduced to malate.
Decarboxylation of malate during the day releases CO2 inside the leaves, thus
allowing carbon fixation to 3-phosphoglycerate by rubisco.

28. CRASSULACEAN ACID METABOLISM (CAM):
 CAM is an acronym for Crassulacean acid metabolism. The CAM
mechanism is similar in many respects to the C4 cycle, but differs from
it in two important features.
 1) In C4 plants the formation of C4 acids is spatially, but not temporally,
separated from the Decarboxylation of the C4 acids and re fixation of
the resulting CO2 by the Calvin cycle. CAM plants lack the specialized
leaf anatomy kranz leaf anatomy typical of C4 plants.
 2) CAM plants open their stomata during the cool, desert nights and
closed during the hot, dry days.
 This minimizes water loss.
 The CO2 is assimilated at night.

29. Light reaction & DARK REACTION:
LIGHT REACTION
DARK REACTION
 Also called as Hillman
reaction.
 Occurs in presence of
light.
 It takes place in
grana.
 Photolysis of water
takes place and
energy rich compound
generated ATP &
NADPH2.
 Photo-chemical
 Also called as
Blackman reaction.
 Occurs in absence of
light.
 It takes place in
stroma.
 CO2 is reduced to
form hexose sugar
with utilization of
energy produced
during light reaction.
 Bio-chemical reaction.

30. C3 Plants & C4 PLANTS:
C3 PLANTS C4 PLANTS
 Plant which have Calvin cycle.
 Only one CO2 acceptor- RuBP.
 First stable product- 3 carbon
compound, phosphoglyceric acid.
 Kranz type anatomy is absent.
 The optimum temp. range between
10-25 Oc.
 C3 plants are less efficient. Rate of
photosynthesis- 15-35 mg/dm2 of leaf
area per hour
 Plant which have Hatch-Slack
cycle.
 Two CO2 acceptors, in
mesophyll- PEP and in bundle
sheath- RuBP.
 First stable product- 4 carbon
compound, oxaloacetic acid.
 Kranz type anatomy is present.
 The optimum temp. range
between 30-45 oC.
 C4 plants are more efficient.
Rate
 of photosynthesis: 40-80
mg/dm2
 of leaf area per hour

31. Factors affecting rate of photosynthesis:
 External factors:-
 (1) Light: (a) Reflection (b) Absorption (c) Intensity (d) Quality
(e) Duration (f) Destructive effect of light.
 Light is green, most of light reflected in green colour and
absorbed in blue and red region.
 (2) CO2: 0.036 % in nature by volume. High conc. Of CO2
reduced rate of photosynthesis.
 (3) Temperature: Light reaction is independent of temp.
 Cold temp retard the rate by affecting enzyme activities.
 Ice formation affect the rate and injurious.
 20-30 oC is optimum, higher than 45 oC adversely affect by
denaturing the enzymes.
 (4) O2: It favors respiration rate, compete for certain common
intermediate in the process of respiration and
photosynthesis.
 (5) Water: In water stress photosynthesis rate reduced due to
the close stomata.

32. Internal factors::
 Chlorophyll content: Amount of chlorophyll has little influence on rate
of photosynthesis.
 Hydration of protoplasm: It is absolutely essential for almost in all
metabolic processes.
 Decrease hydration of the protoplasm reduced the rate of
photosynthesis.
 Protoplasmic factors: It includes factors most probably enzymatic in
nature and is called the “time factor”.
 Accumulation of carbohydrates: Accumulation of photosynthate in
plant cells, if not translocated, slow down and finally stop the process.