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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
Cyclic photophosphorylation Non-cyclic photophosphorylation
1. It occurs in frets/stroma lamella or
intergranal lamella of chloroplast
It occurs in grana of chloroplast
2. It involves PS-I It involves PS-I and PS-II
3. Oxygen evolving complex is absent Oxygen evolving complex is associated with this
4. Photo excited electron pass through ETS
and return back to the same photo
system- I
Photo excited electron from PS-II pass through
ETS and enter into PS-I. It do not return back to
the same photo system- II
5. During this reaction only ATP’s are
produced
During this reaction only ATPs & NADPH+H+
are produced
6. Oxygen is not evolved Oxygen is evolved
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
B) DARK REACTION OR CALVIN CYCLE OR C3 CYCLE
Definition: The series of reactions takes in stroma of chloroplast produce organic
molecules like glucose by utilizing the products of light reactions like ATP and
NADPH+H+ is called dark reaction. It is an independent of light hence the name is
dark reaction. It was discovered by F. F. Blackman (1905). The pathway of reactions
was traced by Melvin Calvin, A. A. Benson and F. A. Bessham in unicellular alga called
Chlorella, using a carbon radioisotope 14CO2 in lollipop like apparatus. He was
awarded Nobel Prize in 1961. Hence it is known as Calvin cycle or Calvin-Benson-
Bessham cycle. The first stable intermediate product is 3-carbon compound
phosphoglyceric acid (PGA), hence the name is C3 cycle. The plants showing this
cycle are called C3 plants. The cycle is also called as Photosynthetic Carbon
Reaction cycle or RPP (Reductive Pentose Phosphate) cycle. The entire reactions
are summarized as follows.
RubisCO 6RuMP + 6CO2+18ATP +12NADPH+H+ 6RuMP+ C6H12O6 + 18ADP+18Pi +12NADP+ + 6H2O
Mechanism of Dark Reaction: the reactions are taking place in the stroma of the
chloroplast in four main steps namely
1. Carboxylation 2. Reduction 3. Formation of glucose and 4. Regeneration
a) Carboxylation: The Rubilose mono phosphate a 5 carbon compound (RuMP) present in the
stroma of the chloroplast is phosphorylated into Rubilose bis phosphate (RuBP) by using 6 ATP
molecules. 6 molecules of RuBP assimilate the 6 CO2 molecules to produce 12 molecules of
Phosphoglyceric acid (PGA) in presence of enzyme RubisCO (Rubilose bis phosphate
carboxylase).
6ATP 6ADP 6CO2 RubisCO
6RuMP(5C) 6RuBP (5C) 12 PGA (3C)
b) Reduction: The phosphoglyceric acid molecules utilize ATPs and NADPH++H+ and
reduced to form 12 molecules of phosphoglyceraldehyde (PGLD).
12 ATP 12 ADP 12 NADPH++H+ 12NADP
12 PGA (3C) 12 DPGA (3C) 12 PGLD (3C)
Enzymes Enzymes
c) Formation of Glucose: The 2 Phosphoglyceraldehyde (PGLD) molecules are
transported from chloroplast into cytoplasm through antiport. In cytoplasm 2 PGLD
undergoes some reactions to produce glucose molecule (C6H12O6) and polymerized to
starch and stored in the cells.
2PGLD (3C) Gl-1, 6-di PO4 (6C) Gl-6-PO4 (6C) Glucose Starch
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Schematic representation of Calvin cycle
6RuMP (5C)
RuBP (5C)
12 PGA (3C)
12DPGA (3C)
(3C)12PGLD 2PGLD Fructose-1, 6-di PO4(6C)
(3C)5PGLD (3C)5DHAP Glucose-1, 6-di PO4(6C)
(6C)2 Fructose -1, 6-di PO4 Glucose-6-PO4(6C)
2 Erythrose-4-PO4 2 Xylulose-5-PO4 Glucose
2 Sedoheptulose-1, 7-di PO4 Starch (stored)
2 Sedoheptulose-7-PO4
2 Ribulose-5-PO4 2 Xylulose-5- PO4
2 RuMP 2RuMP 2RuMP
d) Regeneration: The remaining phosphoglyceraldehydes are regenerated from
10PGLD molecules through a series of reactions.
5PGLD (3C) + 5 DHAP (3C) 6RuMP (5C)
Carboxylation
Reduction
F
o
r
m
a
t
i
o
n
of
g
l
u
c
o
s
e
Regeneration
of 6RuMP
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Differences between light reaction and Dark reaction
Light Reaction Dark Reaction
1. Occur in grana and frets of
chloroplast
Occur in stroma of chloroplast
2. Dependent on sunlight Independent of sunlight
3. Both PSI & PSII are involved Photosystems are not necessary
4. CO2 is not used CO2 is used
5. Photolysis of water takes place No photolysis of water
6. Release O2 as a product Do not release O2
7. Products are ATP and NADPH++H+ Products are glucose and intermediate
products like PGA, DPGA, PGLD, DHAP,
Sedoheptulose-7- phosphate, etc
8. Both photochemical and
biochemical reactions are involved
Only biochemical reactions are involved
C4 Pathway or Hatch –Slack pathway
The C4 pathway is alternate pathway of CO2 fixation. In 1965 it was
discovered by Australian physiologists M. D. Hatch and C. R. Slack in monocots like
sugar cane (Saccharum officinarum), Maize, (Zea mays), Jawar (Sorghum vulgare) and
few dicots like Amaranthus, Euphorbia etc. The first stable intermediate product is 4-
carbon compound, oxaloacetic acid, malic acid, aspartate) rather than 3C compound
phosphoglyceric acid. Hence the name C4 cycle and plants performing this reactions
care called C4 plants. The leaves of C4 plants have two rings of cells around the
vascular bundle, the inner bundle sheath cells and outer mesophyll cells. The
chloroplasts of bundle sheath cells are large and rudimentary (agranal) and
mesophyll cells are normal i.e., dimorphic chloroplasts. Such unique feature of C4
plants leaves (look like halo) is called Kranz Anatomy. The chloroplasts in mesophyll
cells perform C4 cycle and bundle sheath cells perform C3 cycle. The primary
acceptor of CO2 in C4 plants is Phsophoenol pyruvate (PEP).
Mechanism:
1. The C4 pathway starts in mesophyll cells, where the CO2 fixed by phosphoenol
pyruvate and produce 4C compound called Oxaloacetic acid by the help of
enzyme Pep Carboxylase.
2. Oxaloacetic acid is reduced to malic acid.
3. Malate enters the bundle sheath cells through plasmodesmata and decarboxylated
to produce pyruvate and CO2 by the help of enzyme decarboxylase.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
4. The CO2 enters the Calvin cycle to produce glucose.
5. The puruvate return to mesophyll cells to form PEP again and repeat the process.
Kranz Anatomy C4 Cycle or Hatch –Slack Pathway
Advantages of C4 cycle:
1. C4 plants have higher affinity for CO2.
2. They can photosynthesize even in low concentrations of CO2 in air.
3. C4 cycle takes place in even in high temperature.
4. C4 plants avoid photorespiration and increase photosynthetic yield.
(In photorespiration CO2 is released without ATP synthesis which is loss in yield).
CAM PATHWAY (Crassulacean Acid Metabolism)
CAM was first suspected by de Saussure in 1804. It was confirmed and refined by
Aubert, E. in 1892. CAM pathway is another alternative method of CO2 fixation found
in succulent plants like Bryophyllum, Kalanchoe and Sedum belong to the family
Crassulaceae and some other plants like orchids & pineapple. Such plants are called
CAM plants. In CAM plants the stomata are scotoactive i.e., stomata are closed
during day time and open during night. These plants fix CO2 at night. In night starch is
converted into phosphoenol pyruvate (PEP). The PEP molecules accept CO2 to form
Oxaloacetic acid (OAA) by the help of enzyme PEP Carboxylase. The OAA are
converted into malic acid. During day time malic acid converted into pyruvic acid
and release CO2 through the malic acid by CAM pathway.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
CAM Pathway
Night time
Oxaloacetate (4C) Malic Acid (4C)
CO2
Phosphoenol Pyruvate (3C)
Day time
Pyruvic Acid (3C)
CO2
Starch
Advantages of CAM Pathway:
1. Due to the closure of stomata during day time the water loss is prevented and
photorespiration is also avoided. So that the unnecessary loss of energy is
minimized.
2. In night time when stomata are open, CO2 is efficiently taken by the plant in low
temperature. Such CO2 is fixed into Oxaloacetate and release the CO2 during
day time and assimilated by Calvin Cycle to produce starch. So that the yield is
enhanced in these plants.
Comparison of CAM plants with C4 plants:
The C4 pathway bears resemblance to CAM; both act to concentrate
CO2 around RuBisCO, thereby increasing its efficiency. CAM concentrates it
temporally, providing CO2 during the day, and not at night, when respiration is the
dominant reaction. C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO
reaction centre in a "bundle sheath cell" being inundated with CO2. Due to the
inactivity required by the CAM mechanism, C4 carbon fixation has a greater
efficiency in terms of PGA synthesis.
Calvin Cycle PEP Carboxylase
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Sl. No C3 plants C4 Plants
1. Examples of these plants are
wheat, oats, barley, rice cotton,
beans, spinach, sunflower,
Chlorella etc.
Example of these plants are sugarcane,
Maize, Sorghum, Atriplex, Amaranthus
etc.
2. Carbon pathway in
photosynthesis is C3 pathway i.e.
Calvin cycle only.
Carbon pathway in photosynthesis is
C4—di- carboxylic acid pathway
(Hatch-Slack pathway).
3. First stable product of above
carbon pathway is 3-C
compound phosphoglyceric acid
(PGA).
First stable product of above carbon
pathway is 4C compound Oxaloacetic
acid (OAA).
4. The leaves have diffused
mesophyll and only one type of
chloroplasts.
The leaves have ‘cane type’ of anatomy
(Krantz anatomy) with compact
mesophyll around the bundle sheath of
vascular bundles and dimorphic
chloroplasts. Those of bundle sheath
are large and lack grana, while those of
mesophyll are smaller and contain
grana.
5. Optimum temp, for
photosynthesis is low to high.
Optimum temperature for
photosynthesis is high.
6. Photorespiration occurs. No photorespiration (or very little
photorespiration). 7. Photosynthetically less efficient. Photosynthetically more efficient.
8. Carbon dioxide compensation
point is high, about 50 ppm.
Carbon dioxide compensation point is
low, 2 to 5 or even 0 ppm.
9. Rate of CO2 evolution in light is
higher.
Rate of CO2 evolution in light is
apparently none.
10. Carbonic anhydrase activity is
higher.
Carbonic anhydrase activity is low.
11. Rate of translocation of end
products of photosynthesis is
low.
Rate of translocation of end products of
photosynthesis is high.
12. Optimum temperature for growth is
low to high. Optimum temperature for growth is
high.
Blackman’s Law of Limiting Factors5M
Frederick Frost Blackman (1866–1947) was a British plant physiologist. He studied
medicine at St. Bartholomew's Hospital, graduating MA. In the subsequent years, he
studied natural sciences at the University of Cambridge and was awarded DSc.
F.F. Blackman who in 1905 enunciated the law of limiting factors. He states that
“When a process is conditioned as to its rapidity by a number of separate factors,
the rate of the process is limited by the pace of the ‘slowest’ factor”. To explain this
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
principle, Blackman gave the following illustration which is also shown
diagrammatically in Fig.
Suppose a leaf is exposed to a certain light intensity which can utilize 5 mg. of
CO2 per hour in photosynthesis. If only 1 mg. of CO2 enters the leaf in an hour, the rate
of photosynthesis is limited due to CO2 factor. But as the concentration of the
CO2 increases from 1 to 5 mg./hour the rate of photosynthesis is also increased along
the line AB.
Any further increase in the CO2 concentration will have no effect on the rate of
photosynthesis which has become constant along the line BC. It is because the low
light intensity has become a limiting factor. Now the rate of photosynthesis will
increase further along the line BD only if the intensity of light is also increased from
low to a medium. At point D, the medium light intensity again becomes limiting factor
and the rate of photosynthesis will again become constant along the line DE.
In the same way, at still higher light intensity an increase in CO2 will bring
about an increase in the rate of photosynthesis along the line DF. And after the point F
when the higher light intensity also becomes a limiting factor, further increase in
CO2 concentration will have no favourable effect on the rate of photosynthesis which
becomes constant along the line FG.
Thus, it is quite evident from the above illustration that the rate of
photosynthesis cannot be increased by increasing only one factor. The Other factors
should also be increased in proper proportion for favourable effect. Besides CO2 and
light, other factors which affect rate of photosynthesis such as temperature, water etc.
may also become limiting factors under certain conditions.
Significance of photosynthesis (5M)
1. Photosynthesis is a source of all our food and fuel. It is the only biological process
that provide vital force for the whole animal kingdom and for the non-
photosynthetic organism.
2. It drives all other processes of biological and abiological world. It is responsible
for the growth and sustenance of our biosphere.
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.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
The spectra are not identical, though, because carotenoids, which absorb strongly in
the blue, play a role as well. The carotenoids help fill in the absorption gaps of
chlorophyll so that a larger part of the sun's spectrum can be used. The energy
absorbed by these "antenna pigments" is passed to chlorophyll a where it drives the
light reactions of photosynthesis.
Action Spectrum:
A graphical representation stating the effect of different wavelength of sunlight on the
rate of photosynthesis is called Action spectrum. An action spectrum is the rate of a
physiological activity plotted against wavelength of light. In 1881, the German plant
physiologist T. W. Engelmann placed a filamentous green alga under the
microscope and illuminated it with a tiny spectrum of visible light. In the medium
surrounding the strands were motile, aerobic bacteria.
After a few minutes, the bacteria had congregated around the portions of the filament
illuminated by red and blue light because the oxygen being evolved
in photosynthesis, Engelmann concluded that red and blue light are the most effective
colors for photosynthesis. With modern instruments, a plot of the rate of
photosynthesis as a function of wavelength of light produces a graph like this.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
RED DROP AND EMERSON EFFECT
The number of Oxygen Molecules released per light quanta absorbed is called
“Quantum yield”. The late Robert Emerson concluded that 8 quanta of light energy
are required for the reduction of 1 molecule of CO2 and producing one molecule of
O2. The quantum yield is thus 1/8 or 12%. The transfer of 4 electrons reduces one
CO2. Hence 2 quanta of light are required for one CO2. Emerson and Lewis found that
when a monochromatic light i.e. red light having more than 680nm (far red) in the red
zone suddenly decrease in the rate of photosynthesis is called “Red Drop Effect”.
Emerson and Chalmer (1951) found that simultaneously giving two wavelengths of
light enhance the rate of photosynthesis is called Emerson Effect or Emerson
Enhancement Effect.
Factors affecting the rate of Photosynthesis (5/10 marks)
The two types of factors are affecting the rate of photosynthesis are external and
internal factors.
1. External factors: Sunlight, CO2 supply, Water, Oxygen & temperature.
a) Light: is the source of energy and a direct factor. Three properties of light like
quality, quantity and duration are affecting on the rate of photosynthesis.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
i) Quality of light: the different pigments absorb different wavelength of light. A
graphic representation of various wave length of light absorbed by photosynthetic
pigments is called absorption spectrum. A graphic representation of the effect of
different wavelength of light on the rate of photosynthesis is called action spectrum.
Photosynthesis is best in Red (655nm) and blue (440nm) and poor in green light. If a
monochromatic light of wavelength above 680 nm (far red) is provided to the plant the
photosynthetic rate is suddenly fall down is called Red drop effect. To such a plant
red light of shorter wave length is given the photosynthetic yield is increased is called
Enhancement effect or Emerson effect.
ii) Intensity of light: about 2000 to 2500 ft Candela of light intensity is optimum for
photosynthesis. If more than 3000 ft Candela is supplied the photoxidation of
chlorophyll and destruction of chlorophylls and chloroplast takes place is called
solarization.
iii) Duration: The number of hours of sunlight available for photosynthesis
determines the rate of the process. Warburg has noticed that, by interrupting the light
with short period of darkness to C3 plants increase the rate of photosynthesis.
b) CO2 supply: An increase in the concentration of CO2 supply the rate of
photosynthesis is increase by 30 to 60%. If it is more than 0.9% of CO2 concentration it
becomes toxic to the plants.
c) Oxygen: in normal O2 concentration (20%) photosynthesis is maximum. If the
concentration is increased more than 21% it decreases the rate of photosynthesis is
called Warburg Effect. (it is noticed in an experiment on Chlorella).
d) Temperature: Since the photosynthesis is an enzymatic process is very sensitive to
temperature variations. The optimum temperature to mesophytes is 30°C to 35°C.
Alpine plants perform photosynthesis even in 0°C. Xerophytes fix the CO2 even in
50°C.
Vont Hoff’s Law states that an increase in the temperature by 10°C doubles the rate of
the process (2.2 to 2.6 times more) up to a maximum temperature of 35°C. Beyond this
the rate of the process is decreasing.
e) Water: Water is the medium for the biochemical reactions of photosynthesis. It is
the source of hydrogen (H+), Oxygen and electrons. In presence of water the
temperature and pH are maintained. Water maintains the turgidity of guard cells by
that they are open and exchange the gases. Dehydration of cells results into the
collapse of the cell’s vital activities.
2. Internal factors: Stomatal frequency, index, protoplasmic factors, antitranspiranats
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Definition: Respiration a catabolic process can be defined as the biological oxidation
of food molecules (complex organic substances) like carbohydrates, proteins, lipids
into simple inorganic substances like CO2 & H2O and produce the utilizable energy in
the form of ATP molecules.
Respiration is a cellular process occurs in the cells of all organisms since the birth to
the death. It is a biochemical process takes place in a series of reactions. In
eukaryotic cells some reactions occur in cytosol (cytoplasm) and major part occurs in
mitochondria. The mitochondria generate the energy in the form ATP molecules;
hence they are called power houses of the cell. Prokaryotic cells do not possess
mitochondria.
Cell fuels or respiratory substrates or food molecules: The organic substances or
compounds rich in bond energy are oxidized in living cells to produce utilizable
energy in the form of ATP molecules are called cell fuels. E.g., carbohydrates,
proteins, lipids etc.
Types of respiration: Living organisms shows two types of respiration namely aerobic
and anaerobic. Both the reactions generate the energy rich ATP molecules in as series
of steps.
1. AEROBIC RESPIRATION:
Definition: A stepwise, complete biological oxidation of complex organic food
molecules like glucose into simple inorganic molecules like CO2 & H2O in the
presence of molecular oxygen to produce ATP molecules is called aerobic
respiration. The organisms showing aerobic respiration are called aerobes. Aerobic
respiration occurs in cytosol and mitochondria of the cells in a series of enzymatic
reactions. It can be represented as
C6H12O6+602 Enzymes 6C02 + 6H20 +686 K.cals (278 k.cal in the form of 36 ATP)
MITOCHONDRION (POWER HOUSE OF THE CELL):
Mitochondria are granules or threads like cell organelle found in eukaryotic cells are
also called as chondriosomes. First time they were observed and studies by Kollikar
in 1850 in muscle cells of insects. Altman (1892) called them as bioplasts and Benda
named them as mitochondria. They vary in number. Generally they are more in
young and active cells like meristematic cells and less in old cells. In oocytes 1000 of
mitochondria are present. The sperm cell possesses 20 to 24 mitochondria. They are
spherical, rod shaped or thread like or club shaped measure about 3-5 μm long and
0.5 μm in diameter.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Ultra structure of Mitochondrion:
Electron microscopic study reveals that the mitochondrion is a double membranous
cell organelle occurs in the cytoplasm of eukaryotic cells. Mitochondrion consists of
envelope and matrix.
1. Mitochondrial envelope: It is made up of two membranes similar to the plasma
membrane. The outer membrane is about 60 A° thick and smooth, made up of
phospholipids bilayer. They possess specific protein called porins for transport of
solutes. The inner membrane is 60-80 A° thick and selectively permeable. It forms
number of tubular in folding called cristae. The inner membrane contains the
complex of molecules like cytochrome b, c1, c, a and a3 constitute the electron
transport chain or respiratory chain. The inner membranes including the cristae
possess the Lolly pop like structures called ATP synthetase or F0 F1 particles or
Racker’s particles.
2) Mitochondrial matrix: the inner membrane encloses a fluid called matrix. It
contains highly concentrated mixture of several enzymes associated with the Kreb’s
cycle. Matrix also has circular double stranded DNA molecules, different RNAs
(mRNA, tRNA and rRNA) and mitoribosomes (70S ribosomes).
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
Mechanism of Aerobic respiration: The aerobic respiration involves three important
steps at different sites of the cells. They are glycolysis, Kreb’s cycle and terminal
oxidation.
1. Glycolysis: A series of enzymatic reactions which convert molecule of glucose into
two molecules of pyruvate without utilization of oxygen in cytoplasm of the cells is
called glycolysis. It is also called as EMP Pathway named after three Germans
Embden, Meyerhoff and Paranas who traced the pathway. This is anaerobic phase
of respiration common to both aerobic and anaerobic respiration. Glycolysis occurs in
two stages. In first stage energy is used and second stage energy is released.
Schematic representation of glycolysis:
Glucose
Glucose-6-Phosphate (6C)
Fructose-6-Phosophate (6C)
Fructose 1, 6-diPhosophate (6C)
Dihydroxy acetone phosphate (3C) Phosphoglyceraldyhide (3C)
1, 3-diphosphoglyceric acid (1,3-DPGA) (3C)
3-Phosphoglyciric acid (3-PGA) (3C)
2-Phosphoglyciric acid (2-PGA) (3C)
Phsophoenol pyruvate (PEP) (3C)
Pyruvic acid (3C)
ATP ADP Hexokinase
Isomerase
Phsophofructokinase
Aldolase
Dehydrogenase
Phsophoglycerokinase
Mutase
Dehydrolase
NAD+
Phosphopyruvate kinase
NADH+H+
ATP
ATP
ADP
ADP
ADP ATP
Net products of glycolysis are
1) 2 Pyruvic acid molecules
2) 2 ATP molecules
3) 2NADH+H+
During first stage glucose is phosphorylated by using ATP molecules to produce fructose 1, 6-diphosphate. In
2nd stage Fructose 1, 6-diPhosophate split into two molecules of 3-carbons PGA and DHAP. They are both
isomers. Generally DHAP also convert into 3-PGLD and follow the same pathway. The 2 molecules of 3PGLD are
undergoing series of reactions to produce 2 molecules of 3 carbon pyruvic acid molecules. During these
reactions energy is released is used to synthesize 4 ATP by substrate level phosphorylation and protons
released are used to reduce 2NAD+ into 2NADH+H+ molecules. The 2ATP molecules are used in preparatory
phase. Hence net profit is only 2ATP molecules.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
2) Krebs cycle or TCA cycle or Citric acid cycle:
The second phase of aerobic respiration takes place in the mitochondrial matrix in a
series of reactions. The reactions were studies by Sir. Hans A. Kreb, a British
Biochemist. He received the Nobel Prize in 1953 for this work. In these reactions the
products of glycolysis (pyruvate) are oxidized to produce NADH+H+, ATPs and CO2
molecules. It takes place in two steps.
a) Preparatory reactions: Once the pyruvic acids produced form glycolysis in
cytoplasm are enter into mitochondrial matrix where they are converted into 2-
carbons acetyl –CoA molecules by decarboxylation and dehydrogenation by the
help of enzymes dehydrogenase & decarboxylase and CoASH (together called
PDC) respectively. Further acetyl-CoA is reacting with 4 carbon compound called
Oxaloacetic acid to produce 6 carbon citric acid by the help of enzyme citrate
synthatase. Hence the name citric acid cycle. The cycle contains three carboxylic
acid groups; hence the name is TCA or tricarboxylic acid cycle. The citric acid
undergoes the following series of reactions and releases the protons and energy to
produce NADH+H+, GTPs, FADH+H+ and CO2 molecules.
b) Krebs cycle: 2 pyruvic acid moles produced from 1 glucose molecule pass through
the reactions & produce 8 NADH+H+, 2GTPs, 2FADH+H+ and 6CO2 molecules.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
3) Terminal oxidation or Electron Transport Pathway:
Definition: the process in which the synthesis of ATP as a result of transfer of electrons
from reduced coenzymes or compounds like NADH+H+ and FADH+H+ up to oxygen
through a series of electron carriers is called terminal oxidation. During this process
NADH+H+ and FADH+H+ are oxidized through ETS and produce 3 ATP and 2 ATP
respectively.
NADH+H+ + ½O2 NAD+ + H2O + 3ATP
FADH+H++ ½O2 FAD+ + H2O + 3ATP
There are 4 types of complexes made up of electron carriers constitute the electron
transport system or ETS are present repeatedly in the inner membrane of
mitochondrial cristae.
1. Complex-I (NADH-Dehydrogenase complex): it is made up of FMN and Fe-S
cluster.
2. Complex-II (Succinate-Dehydrogenase complex): associated with Krebs cycle
and not with terminal oxidation.
3. Complex-III (Cytochrome b/c1 complex): it contains cytochrome b and c1 and
one Fe-S cluster.
4. Complex-IV (Cytochrome C Oxidase complex): it contains cytochrome a+a3 (Cu
containing molecules).
There are mobile or soluble electron carriers between complex I and II called
Ubiquinone or Coenzyme A (CoQ) and between III and IV is called cytochrome C.
Mechanism of ETP or ETS or Terminal oxidation:
Each NADH+H+ transfers its proton to FMN of complex I and oxidized to NAD+
FMN is reduced FMNH2 and oxidized to transfer its electrons to Fe-S cluster. The
complex pumps the protons into perimitochondrial space to create a proton motive
force (PMF)
Reduced Fe-S transfer electrons to carrier molecule CoQ. The CoQ also accepts
electrons and proton from FADH2 and reduce into CoQH2
The CoQH2 is oxidized to transfers electrons to complex III. The complex III pumps
the proton from matrix into perimitochondrial space to create a PMF
The electron flow from Cyt b to Cyt c1 of same complex III
The cytochrome c transfers the electron from Cyt c1 to Cyt a +a3 of complex IV.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
The Cyt a + a3 accepts electrons, protons and one atom of oxygen ½O2 to produce
a molecule of water. In this process oxygen is terminal acceptor of electrons
hence the name is terminal oxidation
(Red) Cyt a + a3 + ½O2 +2H+
+2e-
Complex II also pumps the protons (H+) to perimitochondrial space to create PMF
The protons accumulated in the perimitochondrial space create PMF and protons
are pumped back to the matrix through the channels to F0F1 particles and release
the energy. This process is associated with synthesis of ATP. About 40% of energy
of glucose is use to synthesize 36 ATP molecules.
The balance sheet showing the energy released from Glucose
Total amount of energy released from one glucose molecule is 673 K. cals.
Amount of energy trapped in the form of ATPs is 262.8. Hence the efficiency of
aerobic respiration is approximately 40%. About 60% of energy lost in the form of
heat is utilized to maintain the body temperature which help for the all the
biochemical reactions.
Sl.
No.
Reactions Number of
ATP or
GTP
Number of
NADH+H+
Number of
FADH+H+
Energy in
term of ATPs
1. Glycolysis 02
02 (2X3=06 ATP)
- 06
2. Preparatory
of phase of
Krebs cycle
- 02
(2X3=06 ATP) - 06
3. Krebs cycle 02
06 (6X3=06 ATP)
02 (2X2=04 ATP)
24
4. Total number of ATPs synthesized from 1 glucose
molecules during aerobic respiration 36
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
ANAEROBIC REPSIRATION
Definition: An incomplete oxidation of organic food molecules (cell fuels) like
carbohydrates into organic products like ethyl alcohol or lactic acid, CO2 and release
energy to form ATP in absence of molecular oxygen is called anaerobic respiration.
The organisms performing such a respiration are called anaerobes. E.g.
Microorganisms like Yeast, bacteria, may be higher plant cells and animal cells in
special condition. There are two types of anaerobes.
1. Facultative anaerobes: They can live in either in presence or absence of
oxygen depending upon the environmental conditions and requirement of
energy they can perform respiration. E.g. Yeast, butyric acid and lactic acid
bacteria.
2. Obligatory anaerobes: These are strictly anaerobes can live only in absence of
oxygen and cannot survive in presence of oxygen. E.g. Clostridium
perfringens, C. tetani and C. botulinum.
Mechanism: During anaerobic respiration glucose undergoes glycolysis and
produce a pyruvate and 2NADH++H+ and 2 ATP molecules. Two pyruvate molecules
further undergo in some reactions to produce ethyl alcohol or lactic acid depending
upon the organism involves. Depending upon the organic products released the
anaerobic respiration is classified into several types, such as alcoholic, lactic acid,
butyric acid, propionic acid fermentation. Fermentation is a type of anaerobic
respiration occurs outside the cell by extracellular enzymes secreted by the
organisms.
Alcoholic fermentation: It was first studies by Louis Pasteur (1857). It is a type of
anaerobic respiration occurs in yeast, Saccharomyces cerviceae and some bacteria
where glucose break into ethyl alcohol and CO2 in absence of molecular oxygen and
produce 2 ATP molecules. It can be summarized as,
C6H12O6 Enzymes 2C2H5OH + 2CO2 + 2 ATP
(Glucose) (Ethyl alcohol)
1. Glucose is first break into 2 Pyruvic acid molecules during glycolysis.
C6H12O6 + 2 NAD+ + 2 ADP Enzymes 2 CH3COCOOH + 2 ATP + 2NADH+H+
(Glucose) (Pyruvic Acid)
2. Pyruvic acid molecules undergo decarboxylation to form acetaldehyde and CO2
2 CH3COCOOH Enzymes 2 CH3CHO + 2CO2
(Pyruvic acid) (Acetaldehyde)
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
3. Two acetaldehyde molecules react with 2 molecules of 2 NADH + H+ produced
during glycolysis to form ethyl alcohol or ethanol.
2 CH3CHO + 2NADH+2H+ Enzymes 2C2H5OH + 2NAD+
(Acetaldehyde) (Ethyl alcohol)
Now it has been confirmed that fermentation takes place as extra cellular method
(outside the cells). E.g. Yeast cells secrete the enzymes collectively called zymase.
Zymase is iron containing enzyme complex composed of glycolase and
Carboxylase that ferment the glucose into ethyl alcohol and carbon dioxide. This
property of yeasts used in brewery industries to produce different grades of alcohols.
E.g., Grape sugar Wine; Malt (Sprouting barley grains) beer Lactic acid fermentation:
A type of anaerobic respiration takes place in certain bacteria, fungi and
muscle cells of higher animals (in special conditions) where the glucose is converted
into lactic acid. Lactic acid bacteria ferment the milk sugar (lactose) into lactic acid.
During this process lactose sugar is hydrolysed to form glucose and galactose and
then hexose sugars are enter into fermentation.
C12H22O11 + H2O Enzymes C6H12 O6 + C6H12 O6
(Lactose) (Glucose) (Galactose)
1. Glucose enters into glycolysis to produce 2 pyruvic acid molecules and
2NADH+2H+ molecules and 2 ATP.
C6H12O6 + 2 NAD+ + 2 ADP Enzymes 2 CH3COCOOH + 2 ATP + 2NADH+H+
(Glucose) (Pyruvic Acid)
2. In the next step the hydrogen atoms from 2NADH+H+ are transferred to pyruvic
acid and which is reduced to lactic acid.
2 CH3COCOOH + 2NADH+H+ Enzymes 2C3H6O3 + 2NAD+
(Lactic Acid)
The dissociation of lactic acid into lactate and H+ lowers the pH; denaturing the milk
protein causing them to precipitate to form curd is called curdling.
Bacteria ferment the cabbage into sauerkraut. The oxygen delivered to muscle cells of
human and higher animal is not enough during strenuous conditions like running. In
this situation muscle cells shift temporarily from aerobic to anaerobic respiration that
is lactic acid fermentation. As lactate accumulates in muscle cells it contributes to
muscle fatigue. During anaerobic respiration (fermentation) fuel molecules are
partially oxidized and produce only 2 ATP molecules compared to 38 ATP during
aerobic respiration. Therefore anaerobic respiration is inefficient compared to
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
aerobic. Anaerobic respiration requires large supply respiratory substrates of cell
fuels, i.e., about 20 times more than what the aerobic respiration require.
Differences between alcoholic and lactic acid fermentation
Alcoholic fermentation Lactic acid fermentation
It occurs in microbes like yeast, bacteria
and roots of water logged soil (Paddy)
It occurs in microbes like bacteria and
muscle cells of animal in strenuous
condition
Pyruvate is broken into ethyl alcohol &
CO2
Pyruvate is broken into lactic acid & CO2
It is represented as C6H12O6 Enzymes 2C2H5OH + 2CO2 + 2 ATP
(Glucose) (Ethyl alcohol)
It is represented as C12H22O11 + H2O Enzymes C6H12 O6 + C6H12 O6
(Lactose) (Glucose) (Galactose)
Differences between aerobic and anaerobic respiration
Aerobic respiration Anaerobic respiration
1. Occurs in majority of the plants and
animals
1. Occurs in microbes and tissues of higher
plants and animals under special
condition
2. Takes place in presence of oxygen 2. Takes place in absence of oxygen
3. The fuel molecules are completely
oxidized
3. The fuel molecules are partially oxidized
4. The end products are CO2 and H2O 4. End products are ethyl alcohol & lactic
acid
5. The reactions occur in both cytoplasm and
mitochondria
5. Reactions occur in cytoplasm or outside
the cell (extracellular)
6. 36 ATPs are produced per glucose 6. Only 2 ATP are produced per glucose
7. A small amount of fuel is sufficient 7. Require large amount of fuel, nearly 20
times more than aerobic respiration
Differences between anaerobic respiration and Fermentation
Anaerobic respiration Fermentation
Here the respiratory substrate is
found inside the cell (Intracellular)
Here the respiratory substrate is found outside
the cell (extracellular)
The reactions takes place in the
cytoplasm
The reactions takes place outside the cells and
enzymes are released from the ells into the
surrounding liquid. E.g., Zymase secreted from
yeasts ferment the glucose into ethyl alcohol
Commercial applications of fermentation
Fermentation in typically is the conversion of carbohydrates to alcohols and carbon
dioxide or organic acids using yeasts, bacteria, or a combination thereof, under
anaerobic conditions. Fermentation usually implies that the action of microorganisms
is desirable, and the process is used to produce alcoholic beverages such as wine,
beer, and cider. Fermentation is also employed in the leavening of bread, and for
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
preservation techniques to create lactic acid in sour foods such as sauerkraut, dry
sausages and yogurt, or vinegar (acetic acid) for use in pickling foods.
Food fermentation has been said to serve five main purposes
1. Enrichment of the diet through development of a diversity of flavors, aromas, and
textures in food substrates
2. Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid
and alkaline fermentations
3. Biological enrichment of food substrates with protein, essential amino acids,
essential fatty acids, and vitamins
4. Elimination of antinutrients
5. A decrease in cooking times and fuel requirements
FERMENTATION PRODUCTS
Since olden times people are using fermented products in their daily food
consumption. There are many advantages of fermented foods such as they easily
digestible and have improved flavour, texture and nutritive value. Some of the most
commonly used fermented products are cheese, bread, yoghurt, sausages, soy sauce
etc. With the advances made in microbiology and biotechnology, food and beverage
fermentation and production has become a major industry. The food biotechnology
has helped in improving the quality, nutrition value, safety and preservation of foods
which in turn has helped in making these foods available through out the year.
YOGHURT
Two species of bacteria Lactobacillus bulgaricus and Lactococcus thermophilus in
approximately equal proportions, are used to make yoghurt. Commercial producers
pasteurize and homogenize the milk before adding the starter. After stirring, the
mixture is then incubated for 3-6 hours at 40-450C. At this temperature the two
bacteria have a mutually stimulating effect on one another. Proteolytic enzymes from
L. bulgaricus break down milk proteins into peptides. These stimulate the growth of L.
thermophilus which, in turn, produce formic acid and carbon dioxide, growth
stimulants for L. bulgaricus. As the incubation proceeds, L. bulgaricus converts the
lactose to lactic acid and the pH falls to 4.2-4.4 which leads to the coagulation of
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
proteins by lactic acid and the thickening of the yoghurt. Further processing involves
the addition of flavour, colour, fruit pulp and heat treatment to kill off any bacteria.
Typical products
Sweetened and flavored yoghurt
To offset its natural sourness, yoghurt can be sold sweetened, flavored or in
containers with fruit or fruit jam
Strained yoghurts
Strained yoghurts are types of yoghurt which are strained through a paper or
cloth filter, traditionally made of muslin, to remove the whey, giving a much
thicker consistency and a distinctive, slightly tangy taste.
Vinegar
In the form of vinegar, acetic acid solutions (typically 4% to 18% acetic acid, with the
percentage usually calculated by mass) are used directly as a condiment( A
condiment is sauce, or seasoning added to food to impart a particular flavor or to
complement the dish. Often pungent in flavour and therefore added in fairly small
quantities), and also in the pickling of vegetables and other foods. Table vinegar
tends to be more diluted (4% to 8% acetic acid), while commercial food pickling, in
general, employs more concentrated solutions. The amount of acetic acid used as
vinegar on a worldwide scale is not large, but is by far the oldest and best-known
application.
BEER
The basic ingredients of beer are water; a starch source, such as malted barley, able
to be fermented (converted into alcohol); a brewer's yeast to produce the
fermentation; and a flavouring such as hops. A mixture of starch sources may be used,
with a secondary starch source, such as maize (corn), rice or sugar, often being
termed an adjunct, especially when used as a lower-cost substitute for malted barley.
Less widely used starch sources include millet, sorghum and cassava root. Cheese is
made from the casein of milk that is produced after separating the whey –the liquid
portion of the milk. The bacteria used in cheese making are either gas producers or
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
acid producers. Gas producers release carbon dioxide, while the acid producers form
lactic acid from lactose. It is the gas producers that determine the texture of a cheese
and the acid producers determine the flavour.
The cheese production involves the following steps:
a) Acidification of milk
b) Coagulum formation
c) Separation of curd from whey
d) Ripening of cheese.
Cheddar cheese is made from milk sterilized at 720C for 15 secs. A starter consisting
of Streptococcus lactis is added and the milk is left to ripen for an hour. During the
“ripening” the lactic acid content rises after which the milk is subjected to ‘renneting’.
Rennet is a mixture of chymosin and pepsin from the stomach of a calf which
coagulates the casein, the principal milk protein. There are several sources of rennet
for cheese production. These include calves, adult cows, pigs, and fungal sources.
Using genetic engineering, some workers have cloned the genes of animal chymosin
and transfer the same into microorganisms. After renneting, a semi solid mass or
‘coagulum’ is formed consisting of water, fat and solutes trapped in a casein matrix.
The coagulum is cut into pea-sized pieces to separate it into small, creamy particles of
curd suspended in a watery whey. ‘Scalding’ the mixture at 30-390C for 45 minutes is
done to expel more whey and to change the texture of the curd. After the scalding, the
curd is allowed to settle under gravity or ‘pitch’ and the whey is run off. After the
formation of blocks of curd, the blocks are cut, stacked, drained and turned in a
process called ‘cheddaring’. Following the cheddaring, the pH falls to 5.2 and the
curd is ‘milled’ in to small pieces. In the final stages of preparation, salt is added
which helps to preserve the finished cheese and bring out it’s flavour. ‘Ripening’
consists of storing the cheese under appropriate conditions so that bacteria and other
microorganisms can cause chemical changes in the curd, improving and enhancing its
flavour.
Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018
CITRIC ACID
Citric acid is the product of fermentation of Aspegillus niger. The acid is one of the
principal organic acids produced in the citric acid cycle. During the production of CA,
the activity of the condensing enzyme (operating in the condensation of acetyl CoA
and oxaloacetic acid to citric acid) is increased, while the activities of the isocitrate
dehydrogense and acotinase disappear. The enzyme acotinase is responsible for the
control of biosynthesis of isocitric acid from citric acid and in turn isocitrate
dehydrogenase mediates in the hydrogen removal which yield axalosuccinic acid
from isocitric acid. Inactivity of these enzymes is the reason for CA accumulation.
-0O0-
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