Photosynthesis
Photosynthesis is a process by which green
plants and other organisms such as algae
and some bacteria synthesize their own
food in the presence of light
Historical perspective
Jan Baptista Van
Helmont
Concluded that all the substance of the plant was
produced from water and none from the soil
Joseph Priestley
Showed that plants have the ability to take up CO2
from the atmosphere and release O2
Jan Ingenhousz
Confirmed Priestley’s work; showed that sunlight is
essential for photosynthesis, O2 is evolved during
photosynthesis – this was demonstrated using
aquatic plants;
Julius Von Sachs
Provided evidence for the production of glucose
Theodore De
Saussure
Showed that water is essential for photosynthesis
T.W. Engelmann
First action spectrum (blue and red region) of
photosynthesis was described
C.B. Van Niel
Gave a simplifies equation for photosynthesis
n(CO2 + 2H2O)  (CH2O)n + nH2O + nO2
T.W. Engelmann Experiment
C.B. Van Niel
 H2 from a suitable oxidisable compound reduces CO2 to
carbohydrates. H2O is the H2 donor and is oxidized to O2. In
purple ad green sulphur bacteria H2S is the H2 donor and the
oxidation product is sulphur or sulphate.
 Inferred that O2 evolved by green plants comes from water not
from CO2. This was proved by radio isotopic technique
 6CO2 + 12H2O  C6H12O6 + 6H2O + 6O2
Site for photosynthesis
 Green plants – mostly in leaves to a
lesser extends in green stems and
floral parts
 Specilized cells in leaves called
mesophyll cells – these cells contain
chloroplasts which are located at the
outer margin with their broad
surface parallel to the cell wall of
mesophyll cells. This helps in easy
diffusion of CO2
Chloroplasts in Onion Root Cells
Chloroplast
Which part of the
chloroplast contain the
machinery for
photochemical reactions
of photosynthesis?
Thylakoids contain pigment require for capturing solar
energy to initiate photosynthesis
Pigment is a substance that absorbs light of different
wavelength
Photosynthesis is a two stage process
1. Light Reactions
Light Dependent Process, requires the direct
energy of light to make energy carrier molecules
that are used in the second process. The grana, the
stroma lamellae trap light energy synthesis ATP
and NADPH
2. Dark Reactions
The Light Independent Process, reactions are not
directly light driven but are dependent on products
of light reaction (ATP, NADPH) to form C-C covalent
bonds of carbohydrates. This does not mean it
occurs in darkness.
Light
reactions
Dark
reactions
Light absorption properties of chlorophyll
Chlorophyll absorbs light in the violet and blue wavelength and also in
red region of the visible spectrum. This portion of the spectrum between
400 nm and 700 nm is referred to as PAR (photsynthetically active
radiation)
Spectrum of sun light
Chlorophyll reflect the green light, hence, impart green colour to leaves
Structure of chlorophyll
It is a large molecule composed of four 5 membered rings
called pyrrole rings and a central core of magnesium. A side
chain called phytol chain extends from one of the pyrrole
ring. The long side chain is made of insoluble carbon and
hydrogen atom which help to anchor the chlorophyll
molecules with thylakoids
Phyrrol
Phytol
Mg
Molecular model of chlorophyll
Types of chlorophyll
In plants mostly there are two kinds – chlorophyll a and b. They are
similar in their molecular structure except that the CH3 group in
chlorophyll a is replaced by CHO group in chlorophyll b
Different pigments in leaf
Paper chromatography is used to separate leaf
pigments
Four important pigments are:
1. Chlorophyll a (blue-green)
2. Chlorophyll b (yellow-green)
3. Xanthophyll (yellow)
4. Carotenoids (yellow-orange)
Absorption spectrum of pigments
Absorption
spectrum
A curve obtained by plotting the amount of
absorption of different wavelengths of light by a
particular pigment
Action
spectrum
A curve showing the rate of photosynthesis at
different wavelength of light
What is light reaction?
1. Photochemical phase – light absorption
2. Water splitting
3. Oxygen release
4. Formation of ATP and NADPH
Photosystem
 Photosystem are arrangements of chlorophyll and other
pigments packed into thylakoids.
 Many Prokaryotes have only one photosystem, Photosystem II
(so numbered because, while it was most likely the first to
evolve, it was the second one discovered).
 Eukaryotes have Photosystem II plus Photosystem I.
 Photosystem I uses chlorophyll a, in the form referred to as
P700.
 Photosystem II uses a form of chlorophyll a known as P680.
Both "active" forms of chlorophyll a function in photosynthesis
due to their association with proteins in the thylakoid membrane.
 Photosystem is LHC – light harvesting complex
(antennae)
Reaction centres
PS1 – The reaction centre, chlorophyll a, has absorption
peak at 700 nm called P700
PS2 – The reaction centre, chlorophyll a, has absorption
peak at 680 nm called P680
 Lamellae of the grana have both PSI and PSII
 The Stroma lamellae lack PSII and NADP
reductase enzyme
 Cyclic photophosphorylation occurs only when
light of wave length above 680 nm is available for
excitation
Non-Cyclic Photo
phosphorylation
Cyclic Photophosphorylation
Chemiosmotic hypothesis
1. Spliting of water molecules takes place on inner side of membrane, the
H+ produced during this process accumulate within lumen of thylakoid
Chemiosmotic hypothesis
2. Electrons move through photosystems, protons are transported across
the membrane (into lumen) by cytochrome complex which is a H+ carrier.
Electrons are transported to the electron carrier present on the inner side
of the membrane. The protons are released into the lumen.
H+ carrier
Chemiosmotic hypothesis
3. The NADP reductase enzyme is located on the stroma side of
membrane. H+ are required for reduction of NADP+. The protons are
removed from the stroma. Electrons are also required. They come from
PSI.
Chemiosmotic hypothesis
4. These processes result in increased H+ concentration in lumen and
decreases concentration in stroma. This creates a proton gradient.
Proton gradient is important because the breakdown of this gradient
leads to release of energy
Chemiosmotic hypothesis
5. The H+ move through the trans membrane Channel of ATPase to stroma.
ATPase enzyme has two parts – F0 and F1. As the H+ pass through F0
and F1 complex, it releases enough energy to produce ATP. There is a
conformation change in F1 which activates the enzyme.
F0
F1
Dark Reaction - Biosynthetic phase
Melvin Calvin, Ernest Orlando Lawrence Berkeley National Laboratory
Using carbon-14, and the new techniques of ion exchange, paper
chromatography, and radio autography, Calvin and his many associates
mapped the complete path of carbon in photosynthesis. The
accomplishment brought him the Nobel prize in chemistry in 1961.
Phase 1: Carbon Fixation
CO2 comes into the stroma
of the chloroplast. Rubisco
catalyzes the bonding of
CO2 to RuBP to create an
unstable 6-carbon
molecule that instantly
splits into two 3-carbon
molecules of 3-PG.
Phase 2: Reduction
ATP phosphorylates each 3PG molecule and creates 1,3bisphosphoglycerate. This in
turn results in the loss of the
terminal phosphate group
from ATP thus making ADP.
NADPH reduces 1,3bisphosphoglycerate which
causes the phosphate group
to break off once again. The
molecule then picks up a
proton (H+) from the medium
to become glyceraldehyde-3phosphate. The broken off
phosphate group also gains
a proton to become
H3PO4. NADPH is oxidized
by this process and
becomes NADP+.
Phase 3: Regeneration
For every six molecules of
G3P created five molecules
continue on to phase 3 while
one leaves to be used for
organic compounds.
ATP is once again
needed. However, this time
it phosphorylates G3P to
regenerate RuBP after some
rearrangement.
Photo respiration
When carbon dioxide levels decline below the threshold for
RuBP carboxylase, RuBP is catalyzed with oxygen instead of
carbon dioxide. The product of that reaction forms glycolic acid,
a chemical that can be broken down by photorespiration,
producing neither NADH nor ATP, in effect dismantling the
Calvin Cycle.
O2
2-Phosphoglycolate
RuBP
Rubisco
+
3-Phosphoglycerate
Calvin cycle
CO2
C4-Pathway
The C4 pathway is designed to efficiently fix CO2 at low
concentrations and plants that use this pathway are known as C4
plants. These plants fix CO2 into a four carbon compound (C4)
called oxaloacetate. This occurs in cells called mesophyll cells.
Examples: Maize, Sugar cane, Pearl millet, Amaranth
C4-Plants
 C4 plants require presence of two types of photosynthetic cells
- Mesophyll cells and bundle sheath cells.
 It contains dimorphic chloroplast.
 Chloroplast in mesophyll cells are granal and in bundle sheath
cells are agranal.
 Rubisco is present only in bundle sheath cells
C3 plant
C4 plant (Kranz anatomy)
C4-Pathway
The C4 pathway is
designed to efficiently
fix CO2 at low
concentrations and
plants that use this
pathway are known as
C4 plants. These plants
fix CO2 into a four
carbon compound (C4)
called oxaloacetate.
This occurs in cells
called mesophyll cells.
C4-Pathway
1. CO2 is fixed to a threecarbon compound called
phosphoenolpyruvate to
produce the four-carbon
compound oxaloacetate.
The enzyme catalyzing
this reaction, PEP
carboxylase, fixes CO2
very efficiently so the C4
plants don't need to to
have their stomata open
as much. The oxaloacetate
is then converted to
another four-carbon
compound called malate
in a step requiring the
reducing power of
NADPH.
C4-Pathway
2. The malate then exits the
mesophyll cells and enters
the chloroplasts of
specialized cells called
bundle sheath cells. Here
the four-carbon malate is
decarboxylated to produce
CO2, a three-carbon
compound called pyruvate,
and NADPH. The CO2
combines with ribulose
bisphosphate and goes
through the Calvin cycle.
C4-Pathway
4. The pyruvate re-enters
the mesophyll cells, reacts
with ATP, and is converted
back to
phosphoenolpyruvate, the
starting compound of the
C4 cycle.