Plant Metabolism

Plant metabolism
Welcome to Carbon Assimilation: Historical
Background and Plant Pigments System
By
N.Sannigrahi, Associate Professor,
Dep't. Of Botany, Nistarini College, Purulia (W.B)
India

PHOTOSYNTHESIS
 The sun, center of our planetary system and star of the Milky
way galaxy, is the source of the energy that is processed and
consumed by the living beings. Every day, we are drinking a
glass of sunlight as far as energy source and the quantum of
energy consumed. Photosynthesis is defined as the process of
carbon assimilation by the reduction of carbon in which the
atmospheric carbon dioxide is reduced to carbohydrate by the
green cells at the expense of radiant energy via number of
enzyme catalyzed biochemical reactions. It is very complex
process and for the sake of convenience and ease of
understanding, plant biologists divide in into two stages-
 Photochemical reactions –Light dependent mechanisms
 Biochemical reactions-light independent mechanisms called
dark reactions. The two stages reflect the literal meaning of
photosynthesis.

CARBON ASSIMILATION
 The path of Carbon in photosynthesis includes those reactions
which incorporate carbon into more reduced or more energetic
compounds. There are three major and one minor pathways by
which atmospheric CO2 can be assimilated in photosynthesis.
The first is the Calvin cycle or C3 cycle since the early product
in this pathway is a C3 compound 3-phosphoglyceric acid
(PGA).
 It is also named as reductive pentose phosphate pathway or
photosynthetic carbon-reduction cycle (PCR cycle). The
second is called the C4 cycle because the early products are C4
acids, malate and aspartate, while the terminal steps include
reactions of the Calvin cycle.

TYPES OF CARBON ASSIMILATION
 In the third group, much of the CO2 is fixed in the process known as
Crassulacean acid metabolism (CAM), a specialized pattern of
photosynthesis in which CO2 is absorbed and stored at night as
malic acid and released during the day by decarboxylation inside the
tissue in which it is fixed by the Calvin cycle. This permits water
conservation because stomata can remain closed during daytime
when there is almost no CO2 assimilation directly from the air.
 Intimately connected with and dependent on the C3 cycle, there is
another minor cycle named the C2 photo respiratory carbon
oxidation cycle or the C2 cycle. The enzyme responsible for
initiating the C3 cycle is ribulose-1, 5-bisphosphate carboxylase
(RuBPcase) which has the property for fixing not only CO2 but also
O2 leading to the formation of phosphoglycolate and then to
glycolate, a C2 compound which is the source of photo respiratory
CO2.

STALWARTS IN PHOTOSYNTHESIS
 Aristotle (384 – 322 B.C.
 Jan van Helmont (1580 – 1644)
 Robert Boyle (1627 – 1691)
 Nehemiah Grew (1641 – 1712)
 S. Hales (1677 – 1761)
 Joseph Priestley (1733 – 1804)
 Jan Ingenhousz (1730 – 1799)
 Antoine Lavoisier (1743 – 1794)
 Julius Robert Mayer (1814 – 1878)
 J. v. Sachs (1832 – 1897)
 George Washington Carver (1860 – 1943)
 Melvin Calvin (1911 – 1997)
 Norman Borlaug (1914 – )

HISTORY OF CARBON ASSIMILATION
 Van Helmont showed that plants got nutrients from water &
somewhere,
 Joseph Priestly showed that plants shoots produce oxygen,
 Jan Ingenhouz demonstrated the requirement of light in
producing oxygen.
 Jean Senebier discovered the need of carbon dioxide in
photosynthetic growth,
 Nicolas-Theodore de Saussare showed that water is required
for photosynthesis
 Blackman showed the two steps of photosynthesis
 Melvin Calvin showed the carbon fixation called Calvin cycle,
 Hill & Scarisbrick demonstrated the O2 evolution in absence
of CO@ in illuminated chloroplast.

PHOTOCHEMICAL REACTIONS
 The light reaction phase of photosynthesis can be discussed
under the following headings:
 1. Photosynthetic unit
 2. Red drop and Emerson Effect/ Two pigment systems
 3. Production of assimilatory powers
 A. Electron transport system.:
 i. Non-cyclic transport ( LEF)
 Ii. Cyclic transport ( CEF)
 Iii. Pseudo-cyclic transport ( WWC)
 B. Photophosphorylation
 The entire process involves the active participation of different
enzymes and other factors to drive the process in sustainable
patheway.

PIGMENTS & PHOTOSYNTHESIS
 A pigment is a generic term for a molecule that absorbs light
and has a color. Plants contain many pigments, giving rise to
the various colors we see. Flowers and fruits obviously contain
a large number of organic molecules that absorb light. Leaves,
stems and roots also contain a variety of pigments. Such
pigment molecules include anthocyanins, flavanoids, flavines,
quinones and cytochromes, just to name a few. However, none
of these should be considered a photosynthetic pigment.
Photosynthetic pigments are the only pigments that have the
ability to absorb energy from sunlight and make it available to
the photosynthetic apparatus. In land plants, there are two
classes of these photosynthetic pigments, the chlorophylls and
the carotenoids.

PHOTOSYNTHETIC PIGMENTS
 The ability of chlorophyll and carotenoid molecules to absorb
the energy of light and use it effectively is related to their
molecular structure and to their organization within the cell.
The pigments absorb the energy from photons through systems
of conjugated double bonds.
 The main photosynthetic pigments are-
 Chlorophyll- a, b
 Carotenoids-Combination of carotenes and Xanthophylls
 Phycobilins- mostly distributed in different algae of lower
group of plants.
 In addition to the aforesaid pigments, the other pigments play
either direct or indirect role in photosynthesis as stated below.

PHOTOSYNTHETIS UNIT
 Photosynthetic unit (PSU) is defined as the smallest group of
collaborating pigment molecules necessary for a
photochemical act i.e., the absorption and migration of a light
quantum to a trapping centre where it promotes the release of
electron. A photosynthetic unit is presently conceived as an
assembly of about 600 chlorophyll molecules ( in two systems
with 300 chlorophylls per reaction centre) together with an
electron transport chain that is able to harvest light
independently leading to oxygen evolution and NADP+
reduction to NADPH2. This is very convincing after the
experiment conducted by Emerson and Arnold(1932) to
establish the existence of two pigment systems to continue the
process of light dependent reactions and O2 as byproduct of
this mechanism.

DISTRIBUTION OF PIGMENTS IN PLANTS

LIGHT & PHOTOSYNTHESIS
 Chief source of light energy for photosynthesis is sun.
 ii. The earth receives only about 40% (or about 5 × 1020
k.cal.) of the total solar energy. The rest is either absorbed
by the atmosphere or is scattered into space.
 iii. All the incident light energy falling on green parts of
the plants is not absorbed and utilized by pigments. Some
of the incident light is reflected, some is transmitted
through them while only a small portion is absorbed by
the pigments.
 iv. Photosynthetic pigments absorb light energy only in
the visible part of the spectrum ranging usually between
400-700 mµ (nm). Such radiations are called as photo
synthetically active radiations (PAR). However, certain
photosynthetic bacteria use infra-red light of compara-
tively shorter wavelengths.

LIGHT & PHOTOSYNTHESIS
 In modern scientific literature, some plant physiologists equate
PAR with visible part of spectrum of radiant energy which is
erroneous. This is because such scientists working on
photobiology use commercially available instruments that are
limited to that portion of spectrum between 400-700 nm only,
thus excluding visible light in the 700-760 and 390-400 nm
range.)
 vi. Only about 1% of the total solar energy received by the
earth is absorbed by the pigments and is utilized in
photosynthesis.
 vii. There is very weak absorption by pigments in green part of
the spectrum and hence, the chloroplasts appear green in green
plants.

ABSORPTION SPECTRUM
 Absorption Spectra of Chlorophylls:
 They chiefly absorb in the violet-blue and red parts of the
spectrum. The absorption band shown by the chlorophylls in
violet-blue region is also called as soret band. Characteristic
absorption peaks shown by different chlorophylls both in vivo
(i.e., intact cell) and in vitro (i.e., in solvents).
 Absorption spectra of different chlorophylls differ in vivo and
in vitro.
 ii. It is quite evident that there are several forms of the
chlorophyll-a in vivo showing a number of absorption peaks in
the red part of the spectrum.
 Absorption spectra of chlorophyll-a and chlorophyll-b (in-
vitro).

ABSORPTION SPECTRA OF Chlorophyll-a

ACTION & ABSORPTION SPECTRUM
 The mechanisms of solar energy transduction depends on the
chlorophylls which are added in this function by the
carotenoids and phycobillins.The spectrum of the radiant
energy whose intensity at each wavelength is a measure of the
amount of energy at that wavelength that has passed through a
selectively absorbing substance. As a result of the absorption
of energy , a physiological activity is observed by the
constituent molecule reflected by its action called action
spectrum. Chlorophylls II mainly absorbs the blue violet and
red regions of the visible spectrum. The absorption band of the
chlorophyll II in blue violet region of the spectra is called soret
band. Although, the absorption spectra may vary with respect
to their in vitro and in vivo conditions. So, the action spectrum
is the relative effectiveness of light quanta of different
energies.

DIFFERENCE BETWEEN
Action Spectrum Absorption Spectrum
It is the rate of a physiological activity
plotted against the wavelength of
light.
It is a spectrum of radiant energy
whose intensity at each wavelength is
a measure of the amount of energy at
the wavelength that has passed
through a selectively absorbing
substance.
The action spectrum shows what
wavelengths of light actually be used
to make photosynthesis work.
In photosynthesis, the absorption
spectrum describes the range of a
pigments' ability to absorb various
wavelengths of light.
T.W . Engelmann( 1881) discovered
the action spectrum in photosynthesis.
Fraunhofer ( 1814) discovered
absorption spectrum in chemicals.
It shows which wavelength of light is
most effectively used in specific
chemical reaction.
The absorption spectrum is unique
and can be used to identify the
material

QUANTUM YEILD IN PHOTOSYNTHESIS
 The number of photons (or quanta) required to release one
molecule of oxygen in photosynthesis is called as quantum
requirement. On the other hand, the number of oxygen
molecule released per photon of light in photosynthesis is
called as quantum yield. The quantum yield is always in
fraction of one.
 Warburg found minimum quantum requirement for
photosynthesis to be 4. It is because the reduction of one
molecule of CO2 by two molecules of H2O requires the
transfer of 4 H atoms. The transfer of each H atoms from H2O
to CO2 requiring one photon or quantum of light.
 The quantum yield of photosynthesis is a very significant
indicator of the light harvestation mechanism of plants .

EFFICIENCY OF PHOTON
 (CH2O) in the above equation represent 1/6 of the
carbohydrate molecule such as glucose. One molecule of
glucose molecule contains about 686 k. cal. of energy,
therefore, 1/6 glucose molecule will contain 686/6 = app. 112
k. cal. of energy. We also know that the rate of photosynthesis
is maximum in red light and each photon of red light contains
about 40 k. cal. of energy. There is a considerable loss of light
energy absorbed during photosynthesis, therefore, the
minimum quantum requirement for photosynthesis as
suggested by them is 8—10 which is widely accepted at
present. Considering that the quantum requirement for
photosynthesis is 8-10, the quantum yield would accordingly
be 1/8 = 0.125 to 1/10 = 0.10. y.

ANTENNAE MOLECULES
 The light-harvesting complex (or antenna complex; LH or LHC)
is an array of protein and chlorophyll molecules embedded in the
thylakoid membrane of plants and cyanobacteria, which transfer
light energy to one chlorophyll a molecule at the reaction centre.
 Chlorophyll or bacteriochlorophyll molecules are associated with
proteins to form complexes consisting of anywhere from 50 to 300
molecules. Only a very small number of these pigment molecules
participate directly in the conversion of light energy to chemical
energy (ATP), and are called reaction centre chlorophylls or
bacteriochlorophylls .
 The latter are surrounded by the more numerous light-harvesting or
antenna chlorophylls or bacteriochlorophylls. The antenna pigments
capture light and transfer the energy of light to the reaction centre.

PHOTOSYNTHETIC UNIT
 Photosynthetic unit is defined as the smallest group of
collaborating pigment molecules necessary for a
photochemical act i.e the absorption and migration of a light
quantum to a trapping centre where it promotes the release of
electron. Emerson & Arnold (1932) calculated that the number
of chlorophyll molecules associated with the evolution of 1
molecule of oxygen was 2500.It has been further established
that at least 8 quanta light are required for the evolution of 1
molecule of oxygen. Thus , a fundamental association of
2500/8=300 chlorophyll molecules is termed photosynthetic
unit or quantasome. Thus, only a pair of special chlorophyll is
concerned with processing of light energy while the rest act to
absorb and transfer light energy to the special chlorophyll
.Practically, at least 600 chlorophyll molecules with two photo
systems altogether required for the same.

LIGHT ENERGY & PHOTOSYSTEM
 Chlorophyll absorbs both blue and red light, and this raises
electrons to Sbπ or Saπ states. The return of an electron from
the Sbπ state to the Saπ state is extremely fast (about 10-12
seconds) and does not afford an opportunity for the energy to
be lost by fluorescence or by transfer to another molecule.
Consequently, the energy is lost as heat.
 The decay of an electron from the Saπ state does permit the
transfer of energy to another molecule, and this is the event
that initiates photosynthesis. Consequently, a photon of red
light is just as effective in initiating photosynthesis as a photon
of blue light, even though the former is much less energetic.
 Thus, the system plays a very crucial role in this regard for the
transduction of energy from the physical system toi biological
systems.

PATH OF PHOTOCHEMICAL REACTIONS
 The chloroplast contains many different pigment molecules
(other chlorophylls, carotenoids, phycobilins, etc.) and the
electrons of these molecules may be excited to various energy
states by the absorption of light. As these excited accessory
pigment molecules return to the ground state, the resulting
energy is transferred to chlorophyll a molecules, causing their
excitation. Because the chlorophyll a molecules present in a
thylakoid vary in their absorption maxima, varying quantities
of energy are required to raise their electrons to the Saπ state.
The molecule that requires the least amount of energy is
believed to be a pigment that absorbs long, red wavelengths,
namely, the P700 molecule. (Pigments may be identified by a
letter [e.g., “P”] followed by a numerical subscript.

PATHWAY-------------
 The subscript indicates the wavelength of the light absorbed
by the pigment.) It seems reasonable that light energy captured
by the accessory pigments and transferred to chlorophyll a is
in turn transferred from the latter to P700. Each accessory
pigment or chlorophyll a molecule can only pass its energy on
to pigments having absorption maxima of longer wavelength
because these require less energy to be activated to the Saπ
state. Because P700 has its absorption maximum at the longest
wavelength, it serves as the final energy trap in a part of the
photosynthetic unit called a reaction center. There are two
types of reaction centers, each with an ultimate energy-
trapping chlorophyll a molecule. One is P700 and the other is
P680.

PATHWAY-----------------------------
 When some plants are exposed to light containing only
wavelengths of 690 nm or longer, photosynthetic efficiency
decreases. The effect is called the red drop (Fig. 17-12).
Because the absorbed energy is funneled to P700, it would be
expected that the absorption of light by the accessory
pigments, chlorophyll a, or even P700 should be equally
efficient. The efficiency can be increased through the addition
of shorter-wavelength radiations.
 This enhancement phenomenon can increase the
photosynthetic rate 30 to 40% above the rate obtained by
either the short wavelength or long wavelength alone. The
synergistic effect of the two different wavelengths led early
investigators to conclude that two distinct, photochemical
reactions exist.

TWO PIGMENTS SYSTEMS
 In one of these (called photo system I, PS-I), P700 serves as the
ultimate energy trap or reaction center, in the other (called photo
system II, PS-II), the ultimate reaction center is a chlorophyll that
has its absorption maximum at 680 nm and is called P680. Within
photo systems I and II, the organization of auxiliary pigments and
chlorophylls form antenna complexes. These complexes act to direct
light energy to the reaction center molecules P700 and P680.
 In higher plants, photo system I is a unit containing several hundred
molecules of chlorophyll (mostly chlorophyll a), about 50
carotenoids, one cytochrome f, one plastocyanin, two cytochrome
6564 molecules, one or two ferredoxin molecules, and one molecule
or a dimer of chlorophyll P700. Photo system II has about 200
molecules of chlorophylls a and b absorbing light at a wavelength
that is less than 680 nm, 50 carotenoids, one P680 molecule or dimer
(the primary electron donor), a primary electron acceptor that is a
quinone, four plastoquinones, six manganese atoms, and two
cytochrome 6559 molecules.

RED DROP & EMERSON EFFECT
 Robert Emerson(1950) observed that the red wavelength of
light longer than 680 nm is inefficient in exciting
photosynthesis, even though much of it is absorbed by
chlorophyll in vitro. The deficiency of the ability of red light
to carry on photosynthesis was termed as Red drop. Later on it
was found by Emerson et al. that the inefficient far red light
(beyond 680 nm) could made fully efficient by the
simultaneous illumination with the second beam of red light of
shorter wavelength at 650 nm. Thus, photosynthesis obtained
from the two combined beams of light was found to be much
more higher than the sum total of the production under the
separate beams of light. This synergism or enhancement
known as Emerson enhancement Effect.
 E= ∆O2 combined-∆O2 short wave length alone/∆O2 long
wave length alone

WHAT IS Q CYCLE ?
 The cytochrome b6f complex functions as a plastoquinol-
plastocyanin oxidoreductase, transferring electrons from
plastquinol to plastocyanin. The electron transfer is
accompanied by the translocation of protons across the
membrane , from stroma to lumen. An unusual feature of the
cytochrome complex mechanism permits the translocation of
two protons for every electron transferred to plastocyanin and
thereby facilitates formation of proton gradient that drives the
ATPO synthesis.
 The most accepted mechanism to describe the reactions of the
cytochrome b/f complex is the cycle popularly called Q cycle.
Here the cytochrome b/f complex contains one quinol binding
site and one quinone-binding site present on the opposite sides
of the membrane.

Q CYCLE
 The Q cycle (named for quinol) describes a series of reactions
that describe how the sequential oxidation and reduction of the
lipophilic electron carrier, Coenzyme Q10 (CoQ10), between
the ubiquinol and ubiquinone forms, can result in the net
movement of protons across a lipid bilayer (in the case of the
mitochondria, the inner mitochondrial membrane).
 The Q cycle was first proposed by Peter D. Mitchell, though a
modified version of Mitchell's original scheme is now
accepted as the mechanism by which Complex III moves
protons (i.e. how complex III
 contributes to the biochemical generation of the proton or pH,
gradient, which is used for the biochemical generation of
ATP).
 To summarize, the first reaction of Q cycle is:

Q-CYCLE
 CoQH2 + cytochrome c1 (Fe3+) → CoQ−• + cytochrome c1
(Fe2+) + 2 H+ (intermembrane) Then the second reaction of the
cycle involves the reduction of the transient semiquinone by
another electron to give CoQH2:
 CoQH2 + CoQ−• + cytochrome c1 (Fe3+) + 2 H+ (matrix) →
CoQ + CoQH2 + cytochrome c1 (Fe2+) + 2 H+ (intermembrane)
Combining the two equations, we have the overall reaction of
Q cycle:
 CoQH2 + 2 cytochrome c1 (Fe3+) + 2 H+ (matrix) → CoQ + 2
cytochrome c1 (Fe2+) + 4 H+ (intermembrane) In chloroplasts,
a similar reaction is done with plastoquinone by cytochrome
b6f complex.

SUMMARY
 Photosynthesis is the unique property of plants involving an
energy transduction reaction by which the light energy is
converted into chemical energy necessary for the vital
functions of living organisms. By enzymatic reactions, light
energy is conserved as phosphate bond energy of ATP as
reducing power in the form of NADPH, which are then
utilized by green plant cells for the reduction of CO2 to form
carbohydrate. All the light mediated reactions forming the
reducing power and the assimilatory power occur in the
thylakoid membranes. Electron flow between the two systems
serves as an energy source of ATP formation . In the meantime
, the Antennae plays an important role as unit of light
harvesting function along with the protection against active
oxygen species and the regulation of the light utilization.

THANKS FOR YOUR PLEASURE
 The author highly acknowledges the use of Google for
different images and WebPages for content. I convey my
thanks to different authors for their books for illustrations and
contents.
 Disclaimer: This presentation has been made as free online
study material for academic domain without any financial
interest.

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