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 ATP and NADPH. It takes place in two stages - the light reactions where light energy is captured to make ATP and NADPH, and the dark reactions where ATP and NADPH are used to incorporate carbon from CO2 into organic compounds like glucose. The chloroplast is the organelle where photosynthesis takes place, containing chlorophyll pigments in the thylakoid membranes which absorb light energy to drive the light-dependent reactions.

1. PHOTOSYNTHESIS

2. THE SUN: MAIN SOURCE OF
ENERGY FOR LIFE ON EARTH

3. Photosynthesis
• An anabolic, endergonic, carbon dioxide
(CO2) requiring process that uses light energy
(photons) and water (H2O) to produce organic
macromolecules (glucose).
SUN
photons
6CO2 + 6H2O  C6H12O6 + 6O2
glucose

4. WHAT IS PHOTOSYNTHESIS?
 Photosynthesis is the process by which organisms
convert light energy into chemical energy in the form
of reducing power (as NADPH or NADH) and ATP,
and use these chemicals to drive carbon dioxide
fixation and reduction to produce sugars.
 2 + 22 → 2 + 2 + 2
 It is estimated that photosynthesis annually fixes
~1011 tons of carbon, which represents the storage of
over 1018 kJ of energy.
 Photosynthesis, over the eons, has produced 2 the
in Earth’s atmosphere.

5. 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
(c) Euglena (d) Cyanobacteria
(b) Kelp
(a) Mosses, ferns, and
flowering plants

6. Light Energy Harvested by Plants &
Other Photosynthetic Autotrophs
6 CO2 + 6 H2O + light energy → C6H12O6 + 6
O2

7. WHY ARE PLANTS GREEN?
Different wavelengths of visible light are seen by
the human eye as different colors.
Gamma Micro- Radio
X-rays UV Infrared
rays waves waves
Visible light
Wavelength (nm)

8. WHY ARE PLANTS GREEN?
Sunlight minus absorbed
wavelengths or colors
equals the apparent color of
an object.
Transmitted light

9. WHY ARE PLANTS GREEN?
Plant Cells
have Green
Chloroplasts
The thylakoid
membrane of the
chloroplast is
impregnated with
photosynthetic
pigments (i.e.,
chlorophylls,
carotenoids).

10. PHOTOSYNTHESIS: A BRIEF
HISTORY OF INVENTIONS

11. BRIEF HISTORY:PRIESTLEY’S EXPERIMENT
Finding that candles burn very well in air in
which plants had grown a long time, and
having some reason to think, that there was
something attending vegetation, which
restored air that had been injured by
respiration, I thought it was possible that the
same process might also restore the air that
had been injured by the burning of candles.
Accordingly, on the 17th of August, 1771,I
put a sprig of mint into a quantity of air, in
which a wax candle had burned out, and
found that, on the 27th of the same month,
another candle burned perfectly well in it.
--Joseph Priestley--

12. BRIEF HISTORY OF INVENTIONS
 Priestley later discovered oxygen, which he named “dephlogisticated
air”
 Antoine Lavoisier elucidated its role in combustion and respiration.
 Dutch physician Jan Ingenhousz ,in 1779 demonstrated that the
“purifying” power of plants resides in the influence of sunlight on
their green parts.
 In 1782,the Swiss pastor Jean Senebier showed that CO2, which he
called “fixed air” , is taken up during photosynthesis.
 Nicolas-Théodore de Saussure in 1804, found that the combined
weights of the organic matter produced by plants and the oxygen
they evolve is greater than the weight of the CO2 they consume. He
concluded that water, the only other substance he added to his
system, was also necessary for photosynthesis.
 The final ingredient in the overall photosynthetic recipe was
established in 1842 by the German physiologist Robert Mayer, one
of the formulators of the first law of thermodynamics, who
concluded that plants convert light energy to chemical energy.

13. SITE OF
PHOTOSYNTHESIS

14. Structure of Plant Cell and Chloroplast

15. PLASTIDS
 major organelles found in the cells of plants and algae.
 site of manufacture and storage of important chemical
compounds used by the cell.
 contain pigments used in photosynthesis, and the types of
pigments present can change or determine the cell's color.
 Plastids are responsible for photosynthesis, storage of
products like starch and for the synthesis have the ability to
differentiate, or redifferentiate, between these and other
forms.
 All plastids are derived from proplastids (formerly "eoplasts",
), which are present in the meristematic regions of the plant.
 Proplastids and young chloroplasts commonly divide, but
more mature chloroplasts also have this capacity.

16. In plants, plastids may differentiate into several forms,
depending upon which function they need to play in the
cell. Undifferentiated plastids (proplastids) may develop
into any of the following plastids:
 Chloroplasts: for photosynthesis
 Chromoplasts: for pigment synthesis and storage
 Gerontoplasts: control the dismantling of the photosynthetic
apparatus during senescence
 Leucoplasts: for monoterpene synthesis; leucoplasts
sometimes differentiate into more specialized plastids:
 Amyloplasts: for starch storage and detecting gravity
 Elaioplasts: for storing fat
 Proteinoplasts: for storing and modifying protein

17. The location and structure of chloroplasts
Chloroplast
LEAF CROSS SECTION MESOPHYLL CELL
LEAF
Mesophyll
CHLOROPLAST Intermembrane space
Outer
membrane
Granum Inner
membrane
Grana Stroma Thylakoid
Stroma Thylakoid compartment

18. CHLOROPLAST
 The chloroplast is made up of 3 types of
membrane:
◦ A smooth outer membrane which is freely
permeable to molecules.
◦ A smooth inner membrane which contains
many transporters: integral membrane
proteins that regulate the passage in an out of
the chloroplast of small molecules like sugars
proteins synthesized in the cytoplasm of the
cell but used within the chloroplast.
◦ A system of thylakoid membranes

20. Photosynthesis occurs in two distinct
phases:
 1. The light reactions, which use light
energy to generate NADPH and ATP.
 2. The dark reactions, actually light-
independent reactions, which use NADPH
and ATP to drive the synthesis of
carbohydrate from C2 and H2O.

21. Thylakoids
 The thylakoid membranes enclose a lumen: a system
of vesicles (that may all be interconnected).
 At various places within the chloroplast these are
stacked in arrays called grana (resembling a stack of
coins).
 Four types of protein assemblies are embedded in
the thylakoid membranes:
◦ Photosystem I which includes chlorophyll and carotenoid
molecules
◦ Photosystem II which also contains chlorophyll and
carotenoid molecules
◦ Cytochromes b and f
◦ ATP synthase
These carry out the light reactions of photosynthesis

22. Stroma
The thylakoid membranes are surrounded
by a fluid stroma
 The stroma contains:
◦ all the enzymes, e.g., RUBISCO, needed to
carry out the "dark" reactions of
photosynthesis
◦ A number of identical molecules of DNA,
each of which carries the complete
chloroplast genome.

23. LIGHT REACTIONS

24. LIGHT REACTIONS (NIEL
HYPOTHESIS)
 Americanmicrobiologist Van Niel studied
photosynthesis in purple sulfur bacteria.
 The chemical similarity between 2 S and
2 O led van to propose that the general
photosynthetic reaction is
where 2 A is 2 O in green plants and
cyanobacteria and 2 S in photosynthetic
sulfur bacteria

25.  photosynthesis is a two-stage process in which light
energy is harnessed to oxidize 2 A (the light
reactions):
 and the resulting reducing agent [H] subsequently
reduces C2 (the dark reactions):

26. VALIDITY OF NEIL HYPOTHESIS
1. Hill reaction :
 In 1937,Robert Hill discovered that when isolated chloroplasts that
lack CO2 are illuminated in the presence of an artificial electron
acceptor such as ferricyanide, O2 is evolved with concomitant
reduction of the acceptor [to ferrocyanide]. This demonstrates that
CO2 does not participate directly in the O2 -producing reaction.
 It was discovered eventually that the natural photosynthetic
electron acceptor is NADP, whose reduction product, NADPH, is
utilized in the dark reactions to reduce CO2 to carbohydrate.
2. Radioactive O :
In 1941,when the oxygen isotope 18 O became
available,Samuel Ruben and Martin Kamen directly
demonstrated that the source of the O2 formed in
photosynthesis is H2O

27. ABSORPTION OF LIGHT: CHLOROPHYLL
 The principal photoreceptor in photosynthesis is
chlorophyll. This cyclic is derived biosynthetically
from protoporphyrin IX.
 Has a central metal ion Mg 2+
 It has a cyclopentenone ring, Ring V, fused to pyrrole
Ring III.
 Pyrrole Ring IV is partially reduced in chlorophyll a
(Chl a) and chlorophyll b (Chl b), the two major
chlorophyll varieties in eukaryotes and cyanobacteria,
whereas in bacteriochlorophyll a (BChl a) and
bacteriochlorophyll b (BChl b), the principal
chlorophylls of photosynthetic bacteria, Rings II and
IV are partially reduced.

29.  The propionyl side chain of Ring IV is esterified to a
tetraisoprenoid alcohol. In Chl a and b as well as in
BChl b it is phytol but in BChl a it is either phytol or
geranylgeraniol, depending on the bacterial species.
 In addition, Chl b has a formyl group in place of the
methyl substituent to atom C3 of Ring II of Chl a.
Similarly, BChl a and BChl b have different
substituents to atom C4.

30. QUANTUM PHYSICS OF LIGHT ABSORPTION
 Electromagnetic radiation is propagated as discrete quanta
(photons) whose energy E is given by Planck’s law:
ℎ
= ℎ =
where h is Planck’s constant (6.626 x 1034 J.s),
c is the speed of light (2.998 x 108 m.s-1 in vacuum),
is the frequency of the radiation
and is its wavelength (visible light ranges in
wavelength from 400 to 700 nm).
 Thus red light with = 680 nm has an energy of 176
kJ.einstein-1 (an einstein is a mole of photons)

31.  Molecules have numerous electronic quantum states of
differing energies. As molecules contain more than one
nucleus, each of their electronic states has an associated
series of vibrational and rotational sub-states that are closely
spaced in energy
 Absorption of light by a molecule usually occurs through the
promotion of an electron from its ground state molecular
orbital to one of higher energy
 But, a given molecule can only absorb photons of certain
wavelengths because, the energy difference between the two
states must exactly match the energy of the absorbed photon
(by law of conservation of energy).
 The peak molar extinction coefficients of the various
chlorophylls, > 105 M-1cm-1 ,are among the highest known for
organic molecules.

32. An electronically excited molecule can dissipate its excitation energy in
many ways:
1. Internal conversion:
 a common mode of decay in which electronic energy is converted to the
kinetic energy of molecular motion, i.e., to heat.
 process occurs very rapidly, being complete in <10-11 s.
 Many molecules relax in this manner to their ground states but
Chlorophyll molecules usually relax only to their lowest excited states.
 Therefore, the photosynthetically applicable excitation energy of a
chlorophyll molecule that has absorbed a photon in its short wavelength
band, which corresponds to its second excited state, is no different than
if it had absorbed a photon in its less energetic long wavelength band.
2. Fluorescence:
 electronically excited molecule decays to its ground state by emitting a
photon.
 Process is much more slower than internal conversion and requires ~10-8
s.
 A fluorescently emitted photon generally has a longer wavelength (lower
energy) than that initially absorbed.
 Fluorescence accounts or the dissipation of only 3 to 6% of the light
energy absorbed by living plants.
 However, chlorophyll in solution, where of course the photosynthetic
uptake of this energy cannot occur, has an intense red fluorescence.

33. 3. Exciton transfer:
 also known as resonance energy transfer
 an excited molecule directly transfers its excitation energy to nearby unexcited
molecules with similar electronic properties
 process occurs through interactions between the molecular orbitals of the
participating molecules in a manner analogous to the interactions between
mechanically coupled pendulums of similar frequencies.
 An exciton (excitation) may be serially transferred between members of a group of
molecules or, if their electronic coupling is strong enough, the entire group may act
as a single excited “supermolecule.”
 Exciton transfer is of particular importance in funneling light energy to
photosynthetic reaction centers
4 . Photooxidation
 a light-excited donor molecule is oxidized by transferring an electron to an acceptor
molecule, which is thereby reduced.
 process occurs because the transferred electron is less tightly bound to the donor in
its excited state than it is in the ground state.
 In photosynthesis, excited chlorophyll (Chl*) is such a donor.
 The energy of the absorbed photon is thereby chemically transferred to the
photosynthetic reaction system.
 Photooxidized chlorophyll, Chl +, a cationic free radical, eventually returns to its
ground state by oxidizing some other molecule.

35. DIFFERENT PIGMENTS ABSORB LIGHT
DIFFERENTLY

36. The Light Reactions
(light dependent)
• Photosystem I…cyclic
photophosphorylation
• Photosystem II…noncyclic
photophosphorylation
• Photolysis

39. The Z scheme (Light Reactions)

40. Cyclic Photophosphorylation
 Process for ATP generation associated with
some Photosynthetic Bacteria
 Reaction Center => 700 nm

41. CYCLIC PHOTOPHOSPHORYLATION
 In cyclic electron flow, the electron begins in a pigment
complex called photosystem I, passes from the primary
acceptor to plastoquinone, then to cytochrome b6f (a similar
complex to that found in mitochondria), and then to
plastocyanin before returning to chlorophyll.
 This transport chain produces a proton-motive force,
pumping H+ ions across the membrane; this produces a
concentration gradient that can be used to power ATP
synthase during chemiosmosis.
 This pathway is known as cyclic photophosphorylation, and it
produces neither O2 nor NADPH. Unlike non-cyclic
photophosphorylation, NADP+ does not accept the electrons,
but they are sent back to photosystem I. NADPH is not
produced in cyclic photophosphorylation. In bacterial
photosynthesis, a single photosystem is used, and therefore is
involved in cyclic photophosphorylation.
 It is favoured in anaerobic conditions and conditions of high
irradiance and CO2 compensation point.

42. Noncyclic Photophosphorylation
 Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product
Primary
electron acceptor
Primary
electron acceptor
Photons
Energy for
synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II by chemiosmosis

43. PLANTS PRODUCE O2 GAS BY SPLITTING
H2O
 The O2 liberated by photosynthesis is made from the
oxygen in water (H+ and e-)

44. Noncyclic Photophosphorylation
 Noncyclic photophosphorylation, is a two-stage process
involving two different chlorophyll photosystems. Being a
light reaction, Noncyclic photophosphorylation occurs on
thylakoid membranes inside chloroplasts
 First, a water molecule is broken down into 2H+ + 1/2 O2 +
2e- by a process called photolysis (or light-splitting). The two
electrons from the water molecule are kept in photosystem II,
while the 2H+ and 1/2O2 are left out for further use.
 Then a photon is absorbed by chlorophyll pigments on
surrounding the reaction core center of the photosystem. The
light excites the electrons of each pigment, causing a chain
reaction that eventually transfers energy to the core of
photosystem II, exciting the two electrons that are
transferred to the primary electron acceptor, pheophytin. The
deficit of electrons is replenished by taking electrons from
another molecule of water. .

45.  The electrons transfer from pheophytin to plastoquinone,
then to plastocyanin, providing the energy for hydrogen ions
(H+) to be pumped into the thylakoid space. This creates a
gradient, making H+ ions flow back into the stroma of the
chloroplast, providing the energy for the regeneration of ATP.
 The still-excited electrons are transferred to a photosystem I
complex, which boosts their energy level to a higher level
using a second solar photon. The highly excited electrons are
transferred to the acceptor molecule, but this time are
passed on to an enzyme called Ferredoxin- NADP
reductase|NADP+ reductase(FNR) which uses them to
catalyse the reaction : NADP+ + 2H+ + 2e- → NADPH + H+
 This consumes the H+ ions produced by the splitting of water,
leading to a net production of 1/2O2, ATP, and NADPH+H+
with the consumption of solar photons and water.
 The concentration of NADPH in the chloroplast may help
regulate which pathway electrons take through the light
reactions. When the chloroplast runs low on ATP for the
Calvin cycle, NADPH will accumulate and the plant may shift
from noncyclic to cyclic electron flow

46. Concept of Light Reaction
• Two types of
photosystems
cooperate in the
light reactions
ATP
mill
Water-splitting NADPH-producing
photosystem photosystem

47. HOW THE LIGHT REACTIONS GENERATE ATP AND
NADPH?
Primary NADP
electron
acceptor
Energy
Primary to make 3
electron
acceptor 2
Light
Light
Primary
electron
acceptor
Reaction-
1 center NADPH-producing
chlorophyll photosystem
Water-splitting
photosystem
2 H + 1/2

48. IN THE LIGHT REACTIONS, ELECTRON
TRANSPORT CHAINS GENERATE ATP,
NADPH, & O2
 Two connected photosystems collect photons of light and
transfer the energy to chlorophyll electrons
 The excited electrons are passed from the primary
electron acceptor to electron transport chains
 Their energy ends up in ATP and NADPH

49. Chemiosmosis powers ATP
synthesis in the light reactions

50. CHEMIOSMOSIS POWERS ATP SYNTHESIS
IN THE LIGHT REACTIONS
 The electron transport chains are arranged with the
photosystems in the thylakoid membranes and pump H+
through that membrane
 The flow of H+ back through the membrane is harnessed by
ATP synthase to make ATP
 In the stroma, the H+ ions combine with NADP+ to form
NADPH

51.  The production of ATP by chemiosmosis in
photosynthesis
Thylakoid
compartment
(high H+) Light Light
Thylakoid
membrane
Antenna
molecules
Stroma ELECTRON TRANSPORT
(low H+) CHAIN
PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE