Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018 Plant physiology INTRODUCTION TO BIOENERGETCS The study of energy flow, energy transformation and energy exchange within the living system and in between the living and surrounding environment is called bioenergetics. The energy present in living organisms and their product is called bioenergy or biomass energy. Sunlight is the primary source of energy for all living systems (plants, animals and microbes). However some organisms are capable to utilize solar energy are called phototrophs (Autotrophs). The total energy reaching the earth is about 170000X10 12 Watts, of this only reaching 40X10 12 (0.0236%) is used in photosynthesis. The total range of wavelength of radiations from the shortest to the longest is called electromagnetic spectrum. Introduction to bioenergetics and Laws of thermodynamics. Photosynthesis: SEMR energy, Pigments and pigments systems, Absorption spectrum and Action spectrum. Mechanism: Light reaction, Electron flow through cyclic and non-cyclic pathway and dark reaction, C 3 and C 4 plants. CAM pathway. Photorespiration (C 2 cycle). Factors affecting the photosynthesis. Cellular respiration: Types: Aerobic and Anaerobic respiration, Energy utilization, cell fuels. Mechanism: EMP path way, Krebs cycle and ETS. Fermentation: Alcoholic and lactic acid fermentation. Applications of fermentation. Respiratory Quotient (RQ). Factors affecting the respiration.
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Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Plant physiology
INTRODUCTION TO BIOENERGETCS
The study of energy flow, energy transformation and energy exchange within the
living system and in between the living and surrounding environment is called
bioenergetics. The energy present in living organisms and their product is called bioenergy
or biomass energy. Sunlight is the primary source of energy for all living systems (plants,
animals and microbes). However some organisms are capable to utilize solar energy are
called phototrophs (Autotrophs). The total energy reaching the earth is about
170000X1012 Watts, of this only reaching 40X1012 (0.0236%) is used in photosynthesis.
The total range of wavelength of radiations from the shortest to the longest is called
electromagnetic spectrum.
Introduction to bioenergetics and Laws of thermodynamics.
Photosynthesis:
SEMR energy, Pigments and pigments systems, Absorption spectrum and Action spectrum. Mechanism:
Light reaction, Electron flow through cyclic and non-cyclic pathway and dark reaction, C3 and C4 plants.
CAM pathway. Photorespiration (C2 cycle). Factors affecting the photosynthesis.
Cellular respiration:
Types: Aerobic and Anaerobic respiration, Energy utilization, cell fuels. Mechanism: EMP path way,
Krebs cycle and ETS. Fermentation: Alcoholic and lactic acid fermentation. Applications of
fermentation. Respiratory Quotient (RQ). Factors affecting the respiration.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Visible spectrum
Light radiations are emitted from natural source sun by thermonuclear reactions. About 25% of
light reflects back. 25% is absorbed by atmosphere and 50% reach the earth. Very small amount
of energy (SEMR) is used in the photosynthesis. Blue light (450 nm) have more energy than red
light (650 nm). SEM radiations consist of small particles of energy called ‘photons’. Energy
present in a photon is called ‘quantum’. The photosynthetic pigments present in plants absorb
light energy and get excited. The excited molecules release energy through their electrons.
Electrons pass through the electron transport system and release the energy, which is coupled with
the synthesis of ATP and NADPH+H+ molecules. These are utilized to synthesize organic
substances like starch, cellulose, proteins, that constitute biomass. The food molecules are utilized
in metabolic activities for the growth, development and other vital activities. It is made available
by cellular respiration. The organic molecules possessing bond energy are called cell fuels. E.g.,
Carbohydrates, proteins, lipids, etc.
LAWS OF THERMODYNAMICS:
Laws of thermodynamics are applicable bioenergetics of living organisms. Law of
conservation of energy is the first law states that “energy can neither be created nor destroyed,
but it can be converted from one form to another”.
The second law states that “during the transformation of energy large amount of energy is
degraded or lost in the form heat”.
Concept of free energy:
The capacity to do work is called energy. The energy which is readily available to do work
in isothermal condition is called free energy. Free energy is represented as ‘G” in honour of J. W.
Gibbs who proposed the concept of energy.
Thus,
G = H – TS (H= enthalpy, T = temperature & S = entropy).
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
When molecules undergo changes in chemical reactions there will be a difference in the
free energy values. This difference is represented as ΔG means free energy change.
Enthalpy: the total heat content of a system (H).
Entropy: A randomized, disordered or dissipated state of energy that is unavailable to do work
(S).
ATP: Energy Currency of Cells.
Adenosine triphosphate is called a biological currency. ATP is a derivative of a molecule
of ribonucleotide. It is composed of a molecule of ribonucleotide. It is composed of adenine,
nitrogen base, a ribose sugar and three phosphates attached in sequence to the 5’ carbon of ribose
moiety.
ATP = Adenine + Ribose sugar + 3H3PO4
ATP = A - ~ ~
OH OH OH OH - p ~ O - p ~ O – p – O – CH2 O Adenine
O O 0
OH 0H Ribose sugar
The first phosphate group is linked to adenosine by a phosphoester bond, the second and
third is linked by a phosophoanhydride bonds. The bonds are unstable. Generally ATPs are
hydrolysed, when cell requires energy. The third phosphate is removed to release 7.3 K. cals.
ATP + H2O ADP + Pi ΔG = -7.3 K.cals.
Second phosphate is removed from ADP, 7.3 K. cals of energy is released and leaves an
AMP.
ADP + H2O AMP + Pi ΔG = -7.3 K.cals.
P P P NH2
N
N
N
N
Phosphoanhydride
bonds Phosphoester bond
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
The first phosphate is removed and only 3.3 K. cals. of energy is released and leaving
adenosine molecule.
ADP + H2O Adenosine + Pi ΔG = -3.3 K.cals.
Karl Lohman discovered ATP in 1929. Fritz Lipman showed that ATP is universal
energy carrier or energy currency of living system. Alexander Todd clarified the structure of
ATP and awarded Nobel Prize in 1957.
ATP SYNTHESIS (PHOSPHORYLATION):
The synthesis of ATP from ADP and inorganic phosphate with an input of 7.3 K. cals. of
energy is called phosphorylation.
ADP + Pi + energy ATP + H2O
Types of phosphorylation:
1. Substrate level phosphorylation: The synthesis of ATP by coupling with the hydrolysis of
energy rich compounds such as phosphoenolpyruvate (PEP) is called substrate level
phosphorylation.
(3C) phosphoenolpyruvate Pyruvic acid (3C)
ADP + Pi ATP
2. Oxidative phosphorylation: The synthesis of ATP from ADP and Pi directly during electrons
transport pathway in presence of molecular oxygen in aerobic respiration is called oxidative
phosphorylation.
3. Photophosphorylation: The synthesis of ATP from ADP and a free phosphate group directly
during electron transport in presence of solar energy with the help of chlorophyll in Racker’s
particle or ATP synthetase or CF0-CF1 particles is called photophosphorylation.
ADP ATP ADP ATP
SEMR Chlorophyll Electron carriers Electron carriers
Mechanism of ATP synthesis:
Enzymes Enzymes
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Peter Mitchell (1961) proposed chemisosmotic hypothesis for the synthesis of ATP. He awarded
the Nobel Prize in 1978. Racker’s particles or ATP synthetase or CF0-CF1 particles or F0-F1
particles are the sites of ATP synthesis. ATP synthesis is coupled with electron transfer and proton
motive force (PMF).
Structure of ATP Synthetase/FoF1 or CFoCF1 particle
1) Energy released during electron transfer in redox reactions is used in pumping protons across a
membrane into lumen (perimtiochondrial space or thylakoid lumen in chloroplasts).
2) Because of the accumulation of protons in lumen create a proton gradient. This represents
reservoir of potential energy called proton motive force (PMF), like water collected behind the
dam.
3) The protons flow back to original site through the channels of Racker’s particles. As the
protons move the potential energy of protons is captured to synthesize ATP by the enzymatic
activity of F0F1 particles. Paul D. Boyer and John E. Walker (Nobel Laureates) provided
experimental proof for this hypothesis.
Electron carriers e- e-
ADP + Pi
Lumen H+
H+ H+ H+
ATP
F1 or CF1
Stalk
Base or
Stalk or
CF0
Membrane
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
F0F1 particle
Inner membrane of mitochondria
or thylakoid membrane of chloroplast
Reduced compounds are energy stores:
Cells used reduced biological molecules as energy reserves. During cellular respiration
they are oxidized to release readily available form of energy (ATP). However before reaching
ATP it produces short time energy carrier molecules like NADH+H+ and FADH+H+. In
photosynthesis NADPH+H+ is energy reserves used to reduce phosphoglyceraldehyde.
Organism Energy reserves
1) Plants Starch
2) Beetroot, sugarcane Disaccharides
(sucrose)
3) Mammals Glycogen
4) Fungi Glycogen
5) Sunflower and castor
seeds
Fats (lipids)
Hydrolysis Glycolysis
Starch glucose NADH+H+and FADH+H+
ETS
F0F1 ETS
ATP Proton gradient other electron carriers
CF0CF1
There are two main energy transducing mechanisms namely, photosynthesis and cellular
respiration. Photosynthesis produce ATP in chloroplasts and respiration produce ATPs in
mitochondria. Hence chloroplasts and mitochondria are called as energy transducers.
Coupling of reactions: In living cells the exorganic and endorganic reactions are coupled together
to minimize the loss of energy (in the form heat) is called coupling of reactions.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Definition: The manufacture of complex organic food substances like carbohydrate by utilizing
simple inorganic substances like carbon dioxide and water in presence of sunlight (SEMR-Solar
Electromagnetic Magnetic Radiant energy) energy with the help of chlorophyll (pigments) and
releasing oxygen as a byproduct is called photosynthesis.
Photosynthesis is a dye sensitized-redox and biochemical series of reactions taking place in
all the plants like blue green algae (Cyanobacteria), bacteria, bryophytes, Pteridophyta,
gymnosperms and angiosperms. Photosynthesis is sometimes called as carbon assimilation and is
represented by the following reaction.
Light
6CO2 + 12H2O C6H12O6 + 6O2 ↑ + 6H2O Green plants
About 90% of the total photosynthesis in world is carried out by algae growing in oceans
and also in fresh water. All green plants are Autotrophs because they can synthesize their own
food by photosynthesis. Photosynthesis is the most important physico-biological process of the
world on which the existence of life on the earth depends. It is only process in which solar energy
is trapped by Autotrophs and converted into potential energy in food for the rest of organisms.
Much of our understanding of photosynthesis in higher plant is derived from simpler organisms
like Chlorella vulgaris and Scenedesmus obliques.
History of Photosynthesis:
1. At the time of Aristotle (17the centaury) believed that plants derive all their nutrition from the
soil.
2. Van Helmont (1577-1644), a Belgian conducted an experiment with a willow (Salix) twig
and concluded that it was water and soil which contributed to the growth of the plant.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
3. Woodward, 1699 stated that the plant is made up of some peculiar terrestrial material and not
of soil and water.
4. Stephen Hales (1727), an English clergyman showed that plants obtain a part of their
nutrition from air and also suggested that sunlight may play a role in it. He is often referred as
‘father of plant physiology’.
5. In 1772, Joseph Priestly showed that the plants might restore the injured air (polluted air). He
discovered that oxygen was produced by green plants. He did not recognize the role of CO2
and role of light.
6. 1n 1779, Ingenhousz noticed that only the green parts of plants were able to purify the air and
that too in presence of sunlight. He recognized the role of the participation of chlorophyll and
light in the photosynthetic process.
7. Jean Senebier (1782) was recognized that fixed air (CO2) was essential for photosynthesis.
He thought that the oxygen liberated during photosynthesis is come directly from the carbon
dioxide of air. He also discovered the effect of red wave length on the rate of the
photosynthesis.
8. Nicolas Theodore de Saussure (1804) confirmed the gas exchange during photosynthesis in
presence of light (photosynthesis) and in darkness (respiration).
9. In 1845, Meyer recognized the role of light as source of energy and conversion of water, CO2
and light energy into organic matter and O2by the green plants.
10. Dutrochet (1837) confirmed that the green part (chlorophyll) was essential
11. In 1864, Julius Sachs shoed that the process of photosynthesis takes place in chloroplast and
results in the synthesis of starch (organic matter).
Chloroplast (Photosynthetic Apparatus):
Chloroplasts are the site of photosynthesis. They are known as photosynthetic apparatus. They are
self duplicating cellular organelles where the photosynthesis occurs.
They occur in the cytoplasm of all the green cells of the plants. Usually they found in mesophyll
cells of the leaf I angiosperms, gymnosperm, pteridophytes and vegetative cells of lower plants.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Chloroplasts have different size and shape. In algal members, they are spiral in Spirogyra, Collar
shape in Ulothrix, star shaped in Zygnema, reticulate in Oedogonium and bell shaped in Chlorella.
They are discoid or biconvex lens shaped and usually measure 4-10µm in diameter and 1-3µm in
thickness.
Ultra structure:
Mature chloroplasts of higher plants have complex structure. Electron microscopic studies of the
sections of chloroplasts show the following structural details.
1. Membrane: Each chloroplast enclosed by two unit membranes (outer and inner). Each
membrane is lipoproteinaeous trilamellar and about 50 -70Aº thick. These membranes are
smooth continuous and differential permeable. The outer and inner membranes enclose a space
called periplstidial space of 80-90Aº thickness. The membranes separate the chloroplast matrix
from cytoplasm.
2. Matrix or Stroma: The double membrane boundary of chloroplast encloses a thick granular,
proteinaceous, transparent fluid called matrix or stroma. The tertiary membranous sac
like/coin like structures arranged in the form of stalks or racks called grana embedded in the
stroma. In addition to these, 70S ribosmes, granules, lipid droplets, starch grains, soluble
proteins are present in the matrix. There are on or few double stranded circular DNA molecules
are present. The several enzymes required for photosynthesis are present in matrix.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
3. The Grana (lamellar system): The disc or discoid shaped membranous structure called
thylakoids are present or placed one above the other like a pile of coins to form grana
(granum-sing.). The size of the grana may range from 0.3-0.7µm and the number of
grana per chloroplast may be 40-60. The number of thylakoids per granum may be 2-
100. Each thylakoid is a plate like sac, approximately 5000Aº in diameter and 160Aº
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
thick. The thylakoid membrane has unit membrane structure. The grana are
interconnected by membranous lamellae called stroma lamellae or intergranal
lamellae or frets. The space enclosed by thylakoid is called loculus. The end portion of
each thylakoid facing the stroma is called margin. The photosynthetic pigments and
other molecules of light reaction are found on the membrane of the thylakoids between
the proteins and phospholipids layer as a monomolecular layer.
4. Photosynthetic pigments: Chlorophylls, caretenoids and phycobillins (bliproteins)
are photo synthetically active pigments found in chloroplasts and chromatophores.
I. Chlorophylls: the chlorophylls are basically chelate salts of magnesium. These are
eight major types of chlorophylls are found in plat kingdom. They are chlorophyll a,
b, c, d and e; bacteriochlorophyll a and b and Chlorobium chlorophyll.
1. Chlorophyll a: All oxygen evolving photosynthetic organisms possess chlorophyll
a. the Chl a has a molecular formula as C55H72O5N4Mg with mol wt. 893. The
molecule is distinguished into a head (15AºX15Aº) and a tail (20Aº). The head is
made up of “porphyrin” a tetrapyrole closed ring derivative and tail of phytol.
There is a 5th Isocyclic ring of cyclopentanone. A non-ionic Mg atom is held within
tetrapyrole ring by to covalent and two co-ordinate bonds. There is a vinyl group at
carbon 2 position; methyl at carbon-3, whereas the pyrrole rings III and IV are
esterified methyl and phytol esters. The Chl a absorbs blue, yellow ad red wave
length of the spectrum at 430, 578 and 662 nm respectively. It is found in all
photosynthetic organisms except bacteria.
2. Chlorophyll b: it is found in all green plants except blue green algae and bacteria.
Its molecular formula is C55H70O6N4Mg and the molecular wt. is 907. It is similar
to Chl a except in having a formyl (CHO) group instead of methyl (CH) group at
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
carbon-3 position of the pyrrole ring. It absorbs ble and orange wavelength o about
430, 595 and 644 nm.
3. Chlorophyll c: it is found in brown algae, diatoms, Pyrrophyta and Cryptophyta. Its
molecular formula is C35H32O5N4Mg and Mol. Wt is 712. It lacks phytyl
esterification. It absorbs blue and orange wavelength of the spectrum at 447 and
579nm wavelength.
4. Chlorophyll d: it is reported in red algae. Its molecular formula is C54H70O6N4Mg
and molecular wt is 895. It absorbs blue, yellow and red wavelength of light at 447,
548 and 688nm respectively.
5. Chlorophyll e: it is reported in Xanthophyta members like Vaucheria and
Tribonema. Its molecular formula and molecular wt is not well known. It absorbs
blue and red wavelength of light at 415 and 654nm respectively.
6. Bacteriochlorophylls: It is found in all photosynthetic bacteria. There are two types
a and b. The molecular formula is C55H74O6N4Mg with mol. Wt 911. It absorbs Uv,
violet, yellow and red lights at 358, 391, 577 and773 nm. The bacteriochlorophyll b
is found in Rhodopseudomonas. Its structure is not yet known.
7. Chlorobium chlorophyll: (old name bacterioviridin) it has hydroxymethyl group at
carbon-2 position in tetrapyrole nucleus. The molecular formula is not yet known.
II. Carotenoids: Carotenoids are present in close association of chlorophyll in all
photosynthetic cells of higher plants. They are sometimes called lipochromes due to heir
fat soluble nature. They are found in non-green parts of plant. Light is not necessary for
their photosynthesis. Most of the Carotenoids are yellow or orange in colour present as
chromaprotein in thylakoid. They are soluble in organic solvents. The Carotenoids are
unsaturated polyhydrocarbons being composed of eight isoprene (C55H8) units. There
are two groups of Carotenoids namely carotenes and xanthophylls.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Carotenes: Since they were isolated in roots of carrot by Wakenroder in 1891, hence the
name carotene. The chemical formula is C40H56 and mol. Wt. 536. They are found in all
groups of plants i.e. from algae to angiosperms. The carotenes are yellowish orange in
color, absorb blue and green color and transmit yellow and red. They occur in several
isomeric forms like α, β, γ and δ carotene; Phytotene, lycopene, neurosporene etc. Carotene
is provitamin A on hydrolysis the β–carotene gives vitamin A.
Carotenase
C40H56 + 2H2O 2C20H29OH (Vitamin A)
Xanthophylls or carotenols: these are yellow colored oxygen containing Carotenoids are more
abundant in nature. Most common xanthophyll in green plants is lutein (C40H56O2). In brown algae
the brown pigment is fucoxanthin (C40H60O2). The yellow autumn coloration of leave is due to
zeaxanthin (isomer of lutein). Other common xanthophylls are cryptoxanthin, violaxanthin,
neoxathin etc. in bacteria spirilloxanthin is similar to xanthophylls.
The Carotenoids mainly absorbs violet-indigo and blue wavelength of the spectrum ad to some
extent the green wavelength too, ranging between 400 to 505nm. The most important function of
these Carotenoids is to protect chlorophyll molecules from photo oxidation.
III. Phycobillins (biliprotenis): phycobilins are major group of photosynthetic pigments occurring
in blue green algae and red algae. The phycobillins comprise a bile pigment or phycobilin
attached to a protein. There are three groups namely phycoerythrin (red), phycocyanin (blue)
and allophycocyanin. The phycoerythrin occurs in Rhodophyceae (red algae), phycocyanin
occurs in cyanophyceae (blue green algae) and allophycocyanin. Occurs in both theses classes.
The phycobillins are water soluble pigments occur in the matrix of chloroplasts of
red algae and attached to photosynthetic lamellae of the blue green algae. Phycocyanin
absorbs orange and green, phycoerythrin absorbs yellow and green; and allophycocyanin
absorbs orange and red wavelength.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Role of Photosynthetic pigments in photosynthesis:
Functional components of chloroplasts/ Photo systems:
A photo system is a complex of pigments and proteins arranged on the thylakoid membrane
as functional unit or set. It is composed of three components.
a. Photochemical reaction centre: the Chl a 700 or Chl a. 680 combined to as specific protein
at as a photochemical reaction centre these molecules expel energized electrons (e-).
b. Light harvesting complex (LHC) or accessory molecules or Antenna molecules. It is c
composed of 220 to 300 molecules of Chl a, b, Carotenoids and phycobillins. The LHC
molecules surround the centre. They form a complex.
c. Electron carriers: there are molecules present in the membrane of thylakoids in between
photo systems and electron accepting molecules are called electron carries.
There are two photo systems present in thylakoids, namely PS-I and PS-II.
i. Photo system I (PS-I): these are smaller units measure about 85Aº in diameter and made
up of 220-250 molecules. They are exclusively located in the stroma lamellae (frets) and
non-stacked grana lamellae i.e., the region of grana that face the stroma. The reaction
centre of PS-I is P-700 [Chlorophyll a-700] absorbs red light of 700 nm most efficiently.
Its antenna molecules called light harvesting complex I or LHC-I. The electron carriers
associated with PS-I are A0 (chlorophyll a). A (Phylloquinone), Fx (Fe4-4S) protein and
Chl a (PRC)
ENERGY
LHC
e-
SEMR
Chlorophylls
Carotenoids
Electron
absorbing molecule
The photosynthetic pigments in thylakoid
membranes are functionally present as many
photosynthetic units called ‘quantasomes’.
According to Park and Biggins (1964)
quantasomes are small spherical single membrane
bounded particles made up of 230-300 pigments
along with granular structure called cytochromes.
These quantasomes have different chlorophylls
and other accessory pigments. They are
distributed within the granal and intergranal
membranes of chloroplasts.
The pigments absorb the solar
electromagnetic Radiant Energy (SEMR) and
immediately transferred to some common pool
called photoreaction centre. The electrons of this
centre chlorophyll molecule are photo exited and
transfer through electron transport system.
During his process they release the energy is used
to phosphorylate ADP into ATP and reduce
NADPH+H+.
These pigments are also responsible for
photolysis of water and produce H+ (protons) and
oxygen.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Fab (4Fe-4S) protein. PS-I transfer its electron to soluble Fe-S protein called Ferrodoxin
(Fd), Cytochrome b/f complex and plastocyanin (PC-containing copper) molecules.
ii. Photo system II (PS-I): These are larger units made up of about 250 – 300 molecules and
have 110 Aº diameters. The photoreaction centre is P-680 (Chl-680) absorb wavelength of
680 nm. Its antenna system is LHC-II. PSII is associated with oxygen evolving complex
(OEC) made up of protein, Z (Mn+2-Z- Mn+2) associated with 4 manganese ions. The
electron carrier associated with PS-II is Phaeophytin, plastoquinones (QA and QB) and
cytochrome b/f complex.
PS-I PS-II
Differences between PS-I and PS-II
Sl. No. Photo system-I Photo system-II
1. PS-I is smaller unit (85Aºin diameter) PS-II is larger unit (110Aºin diameter)
2. PS-I is located on the unstacked
membrane of grana and frets
PS-II is located on the stacked membrane of
grana
3. Photoreaction centre is P-700
(Chlorophyll-700)
Photoreaction centre is P-700 (Chlorophyll-
800)
4. Higher ratio of Chl-a to Chl-b Lower ratio of Chl-a to Chl-b
5. Involved in both cyclic and non-cyclic
photophosphorylation
Involved in only non-cyclic
photophosphorylation
6. Function independent of PS-II Function only in association with PS-I
7. Not associated with OEC, hence does
not produce oxygen.
Associated with OEC, hence produce
oxygen.
8. Associated with electron carriers like
A0, A1, Fx and Fab
Associated with two electron carriers
Phaeophytin and plastoquinone
9. First electron acceptor is Ferrodoxin
(Stable) First electron acceptor is Phaeophytin
Chl b
Chl a
Carotenoids
Chl a
700
Primary
electron
Acceptor
Fd
Photoreaction
Centre
Chl b
Chl a
Carotenoids
Chl a
700
Primary
electron
Acceptor
Fd
SEMR
Photoreaction
Centre
Chl b
Chl a
Carotenoids
Chl a
700
Primary
electron
Acceptor
Fd
Photoreaction
Centre
Chl b
Chl a
Carotenoids
Chl a
700
Primary
electron
Acceptor
Fd
SEMR
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Mechanism of Photosynthesis:
Photosynthesis is a series of biochemical reactions takes place in subsequent phase’s viz., light
reaction and dark reaction.
1) LIGHT REACTION: Light reaction is a light dependent series of reactions occur in grana and
frets of chloroplasts. This is first step of photosynthesis synthesize ATP molecules and reducing
power molecules (NADPH+H+) by utilizing solar electromagnetic radiation (SEMR) energy.
Hence it is also called photophosphorylation. Light reactions were discovered by Robert Hill in
1937; hence it is also called as Hill’s Reactions. During light reactions the following events takes
place.
Photo excitation of chlorophyll molecule of photoreaction centre
Photo ionization of water or photolysis of water
Production of reducing power molecules (NADPH+H+)
Evolution of oxygen from water molecules
Formation of ATP molecules
During day time SEMR energy is absorbed by the chlorophyll molecules and electron is excited.
This charged (excited) electron moves across the electron transport chain by reduction and
oxidation (Redox) process. During this create Proton Motive Force (PMF) by which ATPs are
synthesized in CF0-CF1 particles or Racker’s particles or ATP synthetases. Based on electron
movement light reaction takes place in two pathways namely cyclic and non-cyclic
photophosphorylation.
A) Cyclic photophosphorylation: The synthesis of ATP through cyclic pathway of electron flow
in grana of the chloroplast during light reaction is called cyclic photophosphorylation. This
pathway involves only one photo system, i.e. PS-I. The electron released from PS-I from its
photoreaction centre P-700 (Chl a.700) is passing through electron transport chain of thylakoid
membrane and return back to the same chlorophyll. The electron flow is coupled to proton
transport and synthesizes ATP by Chemi-osmosis at CF0-CF1 particles.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Schematic representation of Cyclic Photophosphorylation
Non-cyclic photophosphorylation:
The synthesis of ATP through non-cyclic pathway of electron during light reaction in grana
of chloroplast is called non-cyclic photophosphorylation. It involves both PS-I and PS-II. The
electron emitted from PS-II pass through electron acceptors and PS-I and do not return back to PS-
II. Hence this is non-cyclic. This pathway of electron by redox process from PS-II (P-680) to PS-I
(P-700) looks like zig-zag or Z like hence it is called Z-scheme electron pathway. This process is
mainly associated with photolysis of water, phosphorylation of ADP, evolution of oxygen and
reduction of NADP+ simultaneously.
Photolysis of water or photoionisation of water: The process in which the breaking of water into
oxygen, protons and electrons by the help of light energy and chlorophyll is called photolysis of
water. Water splits in the manganese containing oxygen evolving complex (OEC) under the
influence of LHC-II of PSII in presence of sunlight. It produces protons (H+), electrons (e-) and
oxygen (O2). Protons are released into thylakoid lumen to create proton motive force (PMF).
Electrons released are transferred to P-680 of PS-II. The oxygen (O2) gas is a byproduct released
out of chloroplast. Hence the oxygen comes from water (therefore, water is source of oxygen) but
not from CO2.
PSI-Chl a-700
A0
A1
Fx
Fab Fd
SEMR
Q
Cyt b
Cyt f
PC PSI-Chl a-700
A0
A1
Fx
Fab Fd
SEMR
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Mn+2 to reduce NADP into NADP+H+
2H2O Z 4H+ + 4e- + O2
Mn+2 to generate P-680 of PS – II
The light quanta absorbed by LHC-II are transferred to photoreaction centre P-680. The
chlorophyll a 680 boosts its electron of last orbit to a higher energy level. This electron is accepted
by Phaeophytin, an intermediate stable compound. Electron is then passed to plstoquinone, QA.
The QB accepts a second electron and using the energy transfer protons from stroma into thylakoid
lumen to create proton motive force (PMF). Further electron move to Cytochrome b + f complex.
The complex pumps the protons (H+) from stroma into lumen by using energy of electrons. Then
electron move to plastocyanin. From plastocyanin electron is transferred to PS -I (P-700). P-700
harvests the light energy and transfers the electron through A0, A1, Fx, Fab into Ferrodoxin.
Electrons from Ferrodoxin are transferred to coenzyme NADP+ which pick up 2H+ from stroma
and reduced into NADPH+H+. It is called as photo reduction. Here electron transport is
unidirectional. During non-cyclic pathway of electron, to form one molecule of oxygen, it requires
the transfer of 4e- from 2H2O to NADPH molecules. Thus total 8 photons (quanta), four by each
photo systems are required.
2H2O 4 photons 4 photons 2NADP+
Mn+2 --- Z ---Mn+2
4H+ + 4e- + O2 PS - I PS - II 2NADPH+H+
Schematic representation of Z-scheme pathway of electron/Non-cyclic photophosphorylation
PS-II
Chla-680
PS-I
Chla-700
Phaeophytin
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
The protons released from water pass through electron transport system. During this
plastoquinone B (QB) and Cyt b/f complex pump the protons into lumen from stroma. It produces
Proton Motive Force (PMF). The 3 protons (3H+) passed through CF0-CF1 particles (ATP
synthetase) from lumen into stroma, create enough energy to synthesize one ATP. Here 4 ATP
molecules per O2 molecule are produced by utilizing 8 photons of energy. Hence to produce 1
ATP, 2 photons of energy is required.
Products of Non-cyclic photophosphorylation are
1. 2 ATP for 1 molecule of H2O
2. 1 NADPH+H+ per molecule of H2O split
3. Oxygen is evolved from water (one oxygen for 2H2O split)
4. H2O is released during the formation of ATP from ADP
Differences between Cyclic and Non-cyclic photophosphorylation
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
3. It provides organic substances, which are used in the production of fats, proteins,
nucleoproteins, pigments, enzymes, vitamins, cellulose, organic acids, etc. Some
of them become structural parts of the organisms.
4. It makes use of simple raw materials such as CO2, H2O and inexhaustible light
energy for the synthesis of energetic organic compounds.
5. It is significant because it provides energy in terms of fossil fuels like coal and
petrol obtained from plants, which lived millions and millions of years ago.
6. Plants, from great trees to microscopic algae, are engaged in converting light
energy into chemical energy, while man with all his knowledge in chemistry and
physics cannot imitate them.
7. Plants purify air and maintain the ratio of O2 and CO2 of the atmosphere along with
another vital process called respiration. They maintain both CO2 and O2 cycle.
8. Biomass of the biosphere is the direct or indirect product of photosynthesis.
9. The firewood is the fuel for domestic use in rural area is a photosynthetic product. Acacia arabica is called as Indian Firewood tree.
10. Feces of grazing animals (dung) are cellulose rich organic matter produced by the plants is used to produce the biogas (methane). The biodiesel produced from
Jatropha curcas plants is also a product of photosynthesis.
11. All the photosynthetic plants are autotrophs supply the food to all the trophic levels of the ecosystem and keep the entire biosphere as a dynamic system.
12. All the 2, 3, 4, 5, 6 and 7 carbon organic compounds produced during
photosynthesis are the raw materials for all the biochemical reactions of all the
living cells.
ABSORPTION SPECTRUM
A graphic representation of various wavelength of light absorbed by photosynthetic
pigments is called Absorption spectrum. The absorption of radiation by a substance
can be quantified with an instrument called a spectrophotometer. The portion of
electromagnetic spectrum which participates in photosynthesis is from 300-900 nm. In
green plants only the visible spectrum (400-750nm) is effective in photosynthesis. In
bacteria it is 375-950nm. The chlorophyll pigments absorb chiefly violet blue and red
parts of the spectrum. T. W. Engelmann (1882) studies in Spirogyra. The chlorophyll
‘a’ absorbs 430nm and 66nm. Sometime the variations due to environmental changes
absorption peaks at 660, 670, 680, 685 and 690nm. The absorption peaks of Chlb are
453 and 642nm.
The graph shows the absorption spectrum of a mixture of chlorophyll a and
chlorophyll b in the range of visible light. Note that both chlorophylls absorb light
most strongly in the red and violet portions of the spectrum. Green light is poorly
absorbed so when white light (which contains the entire visible spectrum) shines on
leaves, green rays are transmitted and reflected giving leaves their green color.