1
Unit III (Botany)
B.Sc. III Year
Study material
Photosynthesis
The process in which electromagnetic radiations are converted into chemical energy
by the green plants is called photosynthesis. Or it can be defined as the process by which
green plants are able to synthesize energy rich sugar from the simple inorganic material like
CO2 and H2O in the presence of sunlight and chlorophyll pigment.
Light
12H2O + 6CO2
C6 H12O6 + 6O2 + 6H2O
Chlorophyll
Site of photosynthesis (Chloroplast):- Structurally chloroplast has three main components
the envelope, the stroma and the thylakoids.
Envelope is made of a double limiting membrane which does not contain chlorophyll
and do not participate directly in photosynthesis. Isolate membranes have a yellow colour due
to presence of small amounts of carotenoids. The outer membrane of the chloroplast contains
porins and thus is permeable to metabolites of smaller molecular weight. The inner
membrane forms a permeability barrier that contains transport proteins for regulating the
movement of metabolites into and out of the organelle. These membranes contain 1-2 % of
total proteins of the chloroplast.
The stroma fills most of the volume of the chloroplast and is kind of a gel fluid that
surrounds the thylakoids. This component contains about 50% of the chloroplast proteins and
most of these are of soluble type. It has ribosomes and also DNA both of which play role in
the synthesis of some of the structural proteins of the chloroplast and a component of Rubisco
enzyme. The stroma is where CO2 fixation occurs and where the synthesis of sugars starch,
fatty acids and some proteins take place.
The Chloroplast contains a third membrane – the thylakoid membrane on which light
reaction of the photosynthesis occurs. The chloroplast thylakoid membrane is believed to
constitute a single sheath that forms numerous small interconnected vesicles, the thylakoids
which commonly are arranged in stacks termed grana. The space within all the thylakoids
contains a single continuous compartment the thylakoid lumen. The granal thylakoids are
connected to each other by unstacked thylakoid membranes called stroma thylakoids or frets
or integranal membranes. The thylakoid membranes contain a number of integral membrane
proteins to which are bound several important prosthetic groups and light absorbing
pigments. The number of thylakoids per granum may vary from a few to 50 or more.
Chloroplast
Photosynthetic pigments:- In higher plants the chlorophyll consists of two closely
related pigments Chlorophyll a and chlorophyll b. Chlorophyll a is the principal pigment
and chrophyll b the accessory pigment other accessory pigments include carotenoids.
2
Structure of Chlorophyll :- The chlorophylls have a porphyrin like ring structure of a
tetrapyrole nucleus with a Mg atom coordinated in the centre and a long hydrophobic
hydrocarbon tail called phytol tail that anchors them in the photosynthetic membrane. An
isocyclic ring called cyclopentanon is attached to third pyrole ring. The empirical formula of
chlorophyll a is C55H72O5N4Mg. Chlorophyll a is blue green microcrystalline solid.
Chlorophyll b has empirical formula C55H70O6N4Mg. It is a green black microcrystaline
solid. It differs from the chlorophyll a in having an aldehyde (CHO) group attached to carbon
atom 3 instead of methyl (CH3) group.
Carotenoids:- These are a group of yellow , brown to reddish pigments which are
associated with
the chlorophylls inside the chloroplast and alone inside chromoplasts. These are of two
types.
1. Carotenes:- They are hydrocarbons with a
general formula of C40H56 .The most common carotene is
B- carotene.
2. Xanthophyll (carotenols):- These are O2
containing
derivatives
of
carotenes
eg.C40H56O
(Cryptoxanthin) C40H56 O2 (Lutein, Zeaxanthin).
The unique feature of both carotenoids and chlorophylls is
the presence of a system of alternating double bonds with
resonating electrons which are rather easily excited by
photons of the visible light, especially at the blue and red
ends.
Molecular structure of Chlorophyll
Carotene
Xanthophyll
Light:-Light is the visible part of electromagnetic radiations. Electromagnetic radiations are a
form of energy that consists of a stream of tiny particles which travel in wave. Depending
upon wavelength electromagnetic spectrum consists of eight types of radiations. (1). Cosmic
rays (2). Gamma rays (3). X rays. (4). ultraviolet radiations. (5). light spectrum. (6). infra red
rays. (7). electric rays. (8). radio waves. Visible light consists of radiations having a wave
length between 390 – 760nm. . It can be resolved into light of different colours. Red light
above 700nm is called infra red. Radiations shorter than those of violet are called ultraviolet.
Radiations which reach to earth have wave lengths between 300nm – 2600nm in the infra red
range.
Discrete particles believed to be present in light are called photons which carry
energy. The energy contained in a photon is termed as quantum (hv). The energy content of a
3
quantum is related to its wave length. The shorter the wave length the greater is the energy
present in quantum. It is measured in Einsteins.
Absorption and Action spectrum of Chlorophyll:- All the photosynthetic pigments do
not absorb all the wavelengths of the visible spectrum of light. If the amount of light
absorbed by a pigment say Chl.a is plotted against the different wavelengths of light,
it will represent its absorption spectrum . If the actual rate of O2 evolved or CO2
consumed is plotted against the different wavelengths of light absorbed by the same
pigment, it will represent the action spectrum
Absorption spectrum
of photosynthesis. From absorption spectrum it
becomes clear that chlorophylls absorb more
of blue and red light. If action and absorption
spectrum is compared, It is observed that
those wavelengths of light which are chiefly
absorbed by the chlorophyll pigments also
stimulate the higher or maximum rate of
photosynthesis. Experimental observations reveal
that the absorption and action spectra of
chlorophyll run almost parallel to each other there
by indicate that it is the most efficient
photosynthetic pigment. Pigments are often
named after the wavelength which is absorbed to
the maximum e.g Chl.a 700, Chl.a 680.
Actiontion and action spectrum of Chlorophyll a
Mechanism of Photosynthesis:- Photosynthesis is completed in two phases. (1). Light phase
(2).
Dark phase
1. Light phase:- It occurs in the thylakoid and intergranal membranes . This phase
results in the generation of energy rich molecules coenzyme NADPH and ATP called
assimilatory power which is utilized in the fixation of CO2 in the dark phase which occurs in
the stroma of the chloroplast. This phase is also celled Hill reaction after the name of its
discoverer. This phase can be discussed under the following headings
In chloroplasts light energy is changed into chemical energy by he help of two
functional units called photosystems. Light energy promotes the transfer of electrons through
a series of compounds that act as electron donors and electron acceptors. The majority of
electrons ultimately reduces NADP to NADPH and oxidizes H2O into O2. Light is also used
to generate a proton motive force across the thylakoid membrane which is used to synthesize
ATP. Light phase can be discussed under the following headings.
(a). Photosystems:- A photosystem is the
collection of different pigment molecules consisting of
two closely linked components one is the reaction
centre where the principal reactions of photosynthesis
occurs. Other component is the antenna complex
termed light harvesting complexes (LHC S) which
capture light energy and transmit it to reaction centre.
Both reaction centre s and antennas contain
tightly bound light absorbing pigments molecules. Chla
is the principal pigment involved in photosynthesis
being present in both reaction centre and antenna
complex. . Antenna complex in addition contains other
pigments like Chl b and carotenoids. The presences of various antenna pigments which
absorb light at different wavelengths greatly extend the range of light that can be absorbed
4
and used for photosynthesis. The size of antenna system varies considerably in different
organisms .It is generally 200-300 chlorophylls per reaction centre in higher plants.
In bright sunlight a chlorophyll molecule absorbs only a few photons / sec. If every
chlorophyll has a complete reaction centre associated with the enzymes that make up this
system would be idle for most of the time, only occasionally being activated by photo
absorption. However if many pigments can sent energy into a common reaction centre the
system is kept active a large fraction of the time.
When chlorophyll a or any other pigment absorbs visible light, energy raises the
chlorophyll to a higher energy (exited). State. In the higher exited state chlorophyll is
extremely unstable, very rapidly gives some of its energy to the surroundings as heat and
enters the lowest exited state where it can be stable for maximum of several nano seconds
(10-9sec.)
The exited chlorophylls have four alternative pathways for disposing of its available
energy.
1. It can remit photons (Phosphorescence).
2. It can convert its excitation energy into heat energy.
3. It can participate in energy transfer i.e. transfer its energy to another molecule.
4. The exited state causes chemical reaction to occur (Photochemistry).
The chemical reactions of photosynthesis are among the fastest known chemical
reactions. The physical phenomenon by which the excitation energy is conveyed from the
chlorophyll that absorbs the light to the reaction centre is thought to be resonance
(photosensitized resonance) transfer.
Types of photosystems. In late 1950s R. Emerson a biophysicist performed an
experiment for measuring the quantum yield of photosynthesis and revealed an effect known
as red drop It was found that any photon absorbed by chlorophyll or other pigments is as
effective as any other photon in driving photosynthesis. However the yield drops dramatically
in the far red region of chlorophyll absorption (greater than 680nm) called red drop. Thus
light with wavelength greater than 680 nm is much less efficient than light of shorter
wavelength. In another experiment Emerson measured the rate of photosynthesis separately
with light of two different wavelengths. When red and far red light were given together the
rate of photosynthesis was greater than the sum of their individual rates. This has been called
as Emerson enhancement effect. These observations led to the discovery that two
photochemical complexes now called Photosystem I and photosystem II (PSI and PSII )
operate in series to carry out the early energy storage reactions of photosynthesis.
Photosystem I:- It is driven by the light of wavelength 700nms. It is primarily
present in the unstacked thylakoids. It transfers electrons to the final electron acceptor NADP
when it works in coordination with PSII. It can work independently or play role in cyclic
electron transfer pathway but supports only ATP synthesis. It has reducing agent X, Fe-S
protein called ferredoxin, Plastoquinone, cytochrome complex and plastocyanin.
Photosystem II:- It is driven by the light of wavelength 680nms. It is
primarily present in the stacked region (grana). It splits water to form oxygen. In combination
with PSI it plays role in linear electron transfer and supports ATP synthesis, formation of
NADPH for CO2 fixation. It contains Mn, Cl, Quencher Q, Plastoquinone cytochrome
complex and plastocyanin.
The relative quantiies of the two photosystems PSII and PSI in the chloroplasts is
about 1.5 :1 respectively.
(b). Photolysis of water (Photocatalytic splitting of water):- Water is oxidized
according to following chemical reaction.
2H2O
4H+ + 4e- + 2O2
5
This equation indicates that four electrons are removed from two water molecules
generating an oxygen molecule and four hydrogen ions.
Water is very stable molecule . Oxidation of water to form molecular oxygen is very
difficult and the photosynthetic oxygen evolving complex is the only known biochemical
system that carries out this reaction.
The photochemically oxidized reaction centre chlorophyll of PSII. (P680+) is the
strongest biological oxidant known. The reduction potential of P680+ is more positive than
that of water and thus it can oxidize water to generate O2 + H+ ions.
The splitting of water which provides the electrons for reduction of P680+ in PSII is
catalyzed by a three protein complex The oxygen evolving complex located on the laminar
surface of the thylakoid membrane. The oxygen evolving complex contains four Mn ions
bound Cl and Ca ions.
The oxidation of two molecules of water to form O2 requires the removal of 4
electrons but absorption of each photon by PSII results in the transfer of just one electron.
PSII must loose an electron and then oxidize the O2 evolving complex four times in a row for
an O2 molecule to be formed. The electrons released from water are transferred once at a time
via the Mn ions and a nearby tyrosine side chain on the D1 subunit to the reaction centre
P680+ where they regenerate the reduced chlorophyll P680. The protons released from H2O
remain in the thylakoid lumen and develop proton motive force across the thylakoid
membranes.
(c).Photoposphorylation:- It is the synthesis of ATP molecules in presence of light.
It is of two types cyclic and non cyclic
Non cyclic photophosphorylation ( linear electron flow):- It involves PSII
and PSI in an obligate series in which electrons are transferred from water to NADP. This
process begins with absorption of a photon by PSII causing an electron to move from a P680
Chla to an acceptor plastoquinone (QB) on the stromal surface. The resulting oxidized
P680+strips one electron from the water after oxidizing it to molecular oxygen protons and
electrons. Protons which move in thylakoid lumen contribute to proton motive force. After
P680 absorbs a second photon of light the semiquinone Q- accepts a second electron and picks
up two protons from the stroma generating QH2. After diffusing QH2 in the membrane binds
to the Q0 site on the cytochrome bf complex.
Cytochrome bf complex transfers electrons on, at a time to the CU2+ form of of
plastocyanin reducing it to Cu+ form. Reduced plastocyanin then diffuses in the thylakoid
lumen carrying electrons to P700+ in PSI, which has already got oxidized after receiving the
photons of light. The electrons excited in PSI can be transferred from ferridoxin via the
electron carrier FAD to NADP+ forming together with one proton picked up from the stroma,
the reduced molecule NADPH.
Protons are also transported into the lumen by the action of cytochrome bf complex
and contribute to the proton motive force. These protons must then diffuse to the ATP
synthetase enzyme where their diffusion down their electrochemical potential gradient is used
to synthesize ATP in the stroma.
Cyclic photophosphorylation:- Reduced ferredoxin can donate two electrons
to a Quinone (Plastoquinone) bound to a site on stromal surface of PSI, the quinone then
picks up two protons from the stroma to form QH2. The QH2 then diffuses through the
thylakoid membrane to Q0 binding site on the luminal surface of the cytochrome bf complex.
There it releases two electrons to the cytochrome bf complex and two protons to the
thylakoid lumen generating proton motive force. As in linear electron flow these electrons
return to PSI via plastocyanin. A Q cycle operates in the cytochrome bf complex during
cyclic electron flow, leading to transport of two additional protons into lumen for each pair of
electron transported and a greater proton motive force.
6
The proton motive force generated during cyclic electron flow in chloroplasts powers
ATP synthesis by F0 to F1 complexes in the thylakoid membrane. This process however
generates no NADPH and no O2 is evolved.
The proton gradient developed will not allow continuing the electron transport chain.
According to law of thermodynamics the difference in charged particles between two points
is the source of energy. Thus this energy is known as proton motive force. Overall gradient is
known as electrochemical gradient. It is utilized to synthesize ATP from ADP and Pi via a
special H+ channel in thylakoid membrane known as ATP synthetase. ATP synthetase is
composed of two parts a hydrophobic membrane protein CF0 and the protein that sticks out
into the satroma called CF1. The CF0 contains proton channel, when the protons are extruded
from theCF0 component it rotates the CF1 component and during this step ATP is
synthesized from ADP + Pi in the stroma portion. The movement of charged particles from
higher concentration to lower concentration has been named as chemiosmosis by Robert
Mitchell.
Linear electron flow in plants which requires both chloroplast photosystems PSI and PSII
NonCyclic Photophosphorylation
Cyclic photophosphorylation
7
Dark phase or reaction
It is the phase of CO2 fixation. It does not require light but requires assimilatory
power ATP and NADPH produced during photochemical phase for fixation and reduction of
CO2. The enzymes required for the process are present in the matrix or stroma of the
chloroplast. There are two main pathways for dark phase. 1. C3 or Calvin cycle 2. C4 or Hatch
and Slack cycle.
Calvin cycle or C3 cycle:
It is divided into three distinct phases. 1.Caboxylation 2.Glycolytic reversal 3. Regeneration
of RUBP. The various reactions occurring during the cycle are as under.
(1). To balance the overall reaction of the cycle let us start with 6 molecules of CO2
combines with the 6 molecules of ribulose 1,5 biphosphate and 6 molecules of an unstable
intermediate compound 2 carboxy 3 keto ribitol 1, 5 biphosphate (B- keto acid) is formed
which splits into 12 molecules of 3 phosphoglyceric acid. This reaction is catalysed by the
enzyme rubisco (ribulose biphosphate carboxylase). This is the most abundant protein on
earth comprise about 16% of the chloroplast protein .
(2). 12 molecules of 3 phosphoglyceric acid are phosphorylated in presence of 12
molecules of ATP and enzyme phosphoglycerokinase to form 12 molecules of 1, 3
diphosphoglyceric acid.
(3). 12 molecules of 1,3 diphosphoglyceric acid are now reduced in presence of 12
molecules of NADPH and enzyme triosephosphate dehydrogenase to form 12 molecules of 3
phosphoglyceraldehyde.
(4). 5 molecules of 3 phosphoglyceraldehyde are isomerised to 5 molecules of its
isomer dihydroxy acetone phosphate in presence of enzyme triosephosphate isomerase.
(5). 3 molecules of 3 phosphoglyceraldehyde undergo condensation with 3 molecules
of dihydroxy acetone phosphate to form 3 molecules of fructose 1, 6 diphosphate.
(6). 3 molecules of fructose 1, 6 diphosphate are dephosphorylated in presence of
enzyme phosphatase to form 3 molecules of fructose 6 phosphate.
(7). one of the molecules of fructose is converted to its isomer glucose 6 phosphate
then dephosporylated to glucoser the continuous operation of this cycle ADP + Pi and NADP
formed will be again utilized in the light reaction for the generation of ATP and NADPH
respectively. The CO2 needed in the cycle will be absorbed from the atmosphere and Ribulose
1,5 biphosphate will be regenerate from the 4 molecules of 3 phosphoglyceraldehyde, 2
molecules of dihydroxy acetone phosphate and 2 molecules of fructose 6 phosphate left
behind
in
C3
cycle.
8
The various reactions involved in the regeneration of RUBP are as under.
(1). 2 molecules of 3 phosphoglyceraldehyde combine with the 2 molecules of
fructose 6 phosphate in presence of enzyme transketolase to form 2 molecules of xylulose 5
phosphate and 2 molecules of erythrose 4 phosphate.
(2). 2 molecules of erythrose 4 phosphate combine with 2 molecules dihydroxy
acetone phosphate in presence of enzyme aldolase to form 2 molecules of sedoheptulose 1,7
diphosphate.
(3). 2 molecules of sedoheptulose 1,7 diphosphate are dephosphorylated to form 2
molecules of sedoheptulose 7 phosphate in presence of enzyme phosphatase.
(4). 2 molecules of sedoheptulose 7 phosphate condence with 2 molecules of 3
phosphoglyceraldehyde to form 2 molecules of xylulose 5 phosphate and 2 molecules of
ribose 5 phosphate.
(5). All the 4 molecules of xylulose 5 phosphate are isomerised to 4 molecules of
ribulose 5 phosphate
(6). 2 molecules of ribose 5 phosphate are isomerised to 2 molecules of ribulose 5
phosphate.
(7). All the 6 molecules of ribulose 5 phosphate are phosphorylated in presence of 6
ATP molecules to get converted into 6 molecules of ribulose 1,5 biphosphate.
Overall reaction
6CO2 + 12 NADPH + 18 ATP + 11 H2O
Fructose 6 phosphate + 12 NADP +
12 ADP.
This is called C3 cycle because first stable product formed is the carbon 3 compound
3 phosphoglycericacid. It is called calvin cycle after name of its discoverer Melvin calvin.
Energy of 6,86,000 calories per molecule glucose is stored . This energy is provided by a
total of 18 ATP and 12 NDPH molecules, which represent 7,50,000 calories. The efficiency
reached by the dark cycle is thus as high as 90 %.
Photorespiration and glycolic acid metabolism.
Photosynthesis is believed to have evolved in an atmosphere much richer in CO2 than it is
today and in relatively little O2 probably about 0.02% oxygen compared with 21 %
today.Since 1920 it has been known O2generally inhibits photosynthesis and the reason for
this was discovered in 1971. It was shown that the CO2 fixing enzyme rubisco will accept not
only CO2 but also O2 as a substrate. The two gases compete infact for the same active site. If
O2 is accepted by Rubisco the following reaction is catalysed.
RUBP
(1). O2 + RUBP
Phospoglycolate + 3
phosphoglyceric acid
(5c)
Oxygenase
(2c)
(3c)
If CO2 is accepted the followingreaction is catalysed
RUBP
RUBP
3 phosphoglyceric acid
(5c) Carboxylase
(3c)
First reaction is called Oxygenation the same enzyme is therefore called RUPB
oxygenase. Second reaction is carboxylation and the enzyme is called RUBP carboxylase.
The enzyme is
always called ribulose biphosphate carboxylase – oxygenase or
RUBISCO. In reaction (1) one molecule of
each 3 phosphoglyceric acid and 2
phosphoglycolate are formed Instead of two GP molecules as in reaction(2).
Phosphoglycolatae (phophoglycolic acid) is converted immediately to glycolate (glycolic
acid) by removal of phosphate group in presence of enzyme phosphatase.
(2). CO2 +
9
●The plants have the problem of what to do with the Glycolate and the pathway
which deals with it called photorespiration which is defined as a light dependent uptake of
O2 and giving out of CO2. The function of photorespiration is to recover some of the carbon
from the excess glycolate. It was discovered by Decker and Tio in 1959. It is exhibited by
plants like wheat, rice, legumes, sugar cane and maize.
Glycolate now leaves the chloroplast and moves into peroxisomes where it is oxidized
into glyoxylate in presence of enzyme glycolate oxidase and then aminated to amino acid
glycin in presence of enzyme aminotransferase.H2O2 (Hydrogen peroxide) formed is
converted back into water and O2 by enzyme RUBP + O
2Phosphoglycolate + PGA
2
catalase.
Two molecules of glycin interact PGA
Glycolate
inside the mitochondria to form a molecule of
serine, CO2 and ammonia is released in this Glyceric Acid
Chloroplast
process. The amino acid serine now enters
peroxisomes, where it is again deaminated to
form glyceric acid which is again converted to Glyceric Acid
Glycolate
phosphoglyceric acid in chloroplast.
●The pathway obviously requires close
Glyoxylate+ H2O2
cooperation of biochemical activities among
three organelles, the chloroplast, the
Serine
Glycin
peroxisomes
and
the
mitochondria.
Peroxisome
Remarkably electron micrograph does show
these three organelles very closely appressed
to each other indicating that there is indeed Serine
Glycin
some important functional relationship among
NH3
them.
C
O
2
●The pathway serves to recycle three
NADH
NAD+
Mitochondria
carbon atoms (entering up as PGA) out of the
4 carbon atoms i.e. 2 molecules of glycolate.
There is loss of one of them as CO2. It reduces the potential yield of C3 plants by 30% - 40%.
The photorespiration occurs only in the C3 plants.
However C4 plants have overcome the problem of photorespiration by performing
calvin cycle in the interior of leaves (bundle sheath cells) where both temperature & O2 are
lower. They have further ensured high CO2 supply to cells performing Calvin cycle.
C4 cycle or Hatch and Slack Cycle
Kortschak Harth and Burr (1965) demonstrated with the
use of 14CO2 that in sugarcane leaves the chief labeled
synthesized products are C4 dicarboxylic acids like
mallate, aspartate Oxaloacetate. This observation was
confirmed by M.D Hatch and C. R. Slack (1966) Later
on these observations have been confirmed in
monocotyledonous plants like Zea mays, Sorghum and
a dicot
Amaranthus etc. It is also called BCarboxylation
pathway
and
Cooperative
photosynthesis. The first stable compound of Hatch and
Slack cycle is 4 carbon oxaloacetic acid Therefore it is
called C4 cycle and the plants are called C4 plants. Hatch
and Slack cycle is completed in the chloroplast of
mesophyll cells and bundle sheath cells. Following
reactions occur during this cycle.
10
In the mesophyll cells the CO2 acceptor is phosphoenol pyruvic acid (PEP) instead of
RUBP & the enzyme is PEP carboxylase instead of RUBP carboxylase. PEP carboxylase has
two advantages over RUBP carboxylase.
C4 cycle
1. It has much affinity for CO2.
2. It does not accept O2 & hence does not contribute to photorespiration.
Reactions occurring in the chloroplasts of mesophyll and bundle sheath cells are.
1. Phosphoenol pyruvic acid combines with CO2 in presence of PEP Carboxylase &
forms 4 Carbon acid, oxaloacetic acid.
2. Oxalo acetic acid is quite unstable and is converted into mallic acid with the help of
NADPH and in presence of enzyme mallic dehydrogenase.
3. Mallic acid is now transported to bundle sheath cells it is decarboxylated to form
pyruvic acid and CO2. This reaction is aided by mallic enzyme. Here the conc. Of CO2 is
increased so calvin cycle will start.
4. Pyruvic acid is then transported to mesophyll cells here it gets converted to PEP on
the expenditure of ATP.
Characters of C4 Plants:
1. The leaves of C4 plants possess special anatomy called Kranz anatomy. The leaves
of C4 plant vascular bundles remain surrounded by bundle sheath containing chloroplasts in
abundance. The bundle sheath is surrounded by 1-3 layers of mosophyll cells which posses
very small intercellular spaces.
2. The chloroplasts of C4 plants are dimorphic. The chloroplasts of mesophyll
cells of normal type, but the chloroplasts of bundle sheath are comparatively quite
larger in size without grana and PSII.
3. C4 cycle is performed in mesophyll cells while C3 in the bundle sheath cells.
4. Two types of carboxylase:- PEP carboxylase in mesophyll cells and RUBISCO
in bundle sheath cells.
5. C4 plants are found in tropical and sub tropical regions.
6. They grow fast at high temperature and in more light intensities so called efficient
plants. 7. The optimum temperature required for their growth varies from 30-400C.
Sigificance of C4 cycle:
1. In C4plants it increases the photosynthetic yield two to three times more than C3
plants.
2. In C4 plants, it performs a high rate of photosynthesis even when the stomata are
nearly closed.
3. It increases the adaptability of C4 plants to high temperature and high intensities.
4. It increases the rate of CO2 fixation at 25-300c as compared to C3plants
5. In C4 plants the O2 cannot have inhibitory effect. They lack photorespiration.
Factors affecting the rate of photosynthesis
The rate of photosynthesis is affected by several factors which have been divided
chiefly into two main groups:
(A). External factors :- 1. Light
2. CO2
3.Temperature
4.Water
(B). Internal factors 1 . Chlorophyll 2. Protoplasm 3. Accumulation of end products 4.
leaf age
1. Light :- Effect of light on the process of photosynthesis can be discussed in three
ways.
Quality of light :- Light between the wave length of 390nm and 700nm is most
effective for photosynthesis. It does not take place in ultra violet, green and infra red light.
The maximum photosynthesis occurs in red light and slightly less to it in blue light.
11
Light intensity :- The intensity of light has favorable effect on the rate of
photosynthesis. The rate of photosynthesis increases with the increase in light intensity until
some other factors become limiting. It has been observed through various experiments that
the rate of photosynthesis increases if the light intensity is increased gradually from 2500 foot
candles to 3000 foot candles and other factors are available in sufficient amount.
At very high light intensity beyond a certain point the temperature of cell
increases resulting into photoxidation of its constituents. The phenomenon is called
solarization. It shows a direct inhibitory effect on photosynthetic rate. Green plants are
adapted to various light intensities. Those plants which are shade loving and require low
intensity of light for optimum photosynthesis are called sciophytes.
On the other hand those plants which grow in sunny places & require high
intensity of light for optimum photosynthesis are called heliophytes.
The light intensity at which the amount of CO2 used in photosynthesis and
amount of CO2 liberated is respiration becomes volumetrically equal is known as
compensation point. The intensity of light at which further increase is not followed by
increase in CO2 intake is called light saturation point.
Duration of light :- Even a brief flash of light is enough for photosynthesis to occur.
However the rate of photosynthesis is greater in intermittent light than in continuous light
because in continuous light the assimilatory power accumulates and is not consumed in the
dark reaction at the same rate at which it is produced in light reaction. Longer duration of
light period favours photosynthesis in leaves without being damaged. Good photosynthesis
yield occurs if the plant gets 10 – 12 hours light per day.
2. Effect of CO2 concentration:- The atmospheric air contains only 0.03% CO2
concentration by volume. As the CO2 concentration in the atmosphere increases, the rate of
photosynthesis also increases but after a definite concentration 0.9% of CO2 the rate of
photosynthesis does not increase. In this case the light acts as limiting factor. It has been
experimentally demonstrated that the rate of photosynthesis varies in different plants and
depend upon the CO2 concentration e.g. in hydrophytes, the rate of photosynthesis increases
up to 1.1% CO2 concentration. While in Triticum aestivum maximum photosynthesis occurs
at 0.15% CO2
3. Temperature:- Temperature shows a little effect on photosynthetic rate as
compared to other process. The variation in temperature effects only calvin cycle of
photosynthesis and not light reaction. In certain experiments in most of the plants the rate of
photosynthesis increases from 100C- 300C. An increase of each 100C temperature up to 300C
initially increases the rate of photosynthesis but after some times reduces the rate of
photosynthesis. The photosynthesis will stop in many plants at about freezing point but in
certain conifers it takes place even at - 350C. In certain species of algae indigenous to hot
spring it takes place even at 750C. Usually the temperatures beyond 400C – 500C retard
photosynthesis in most of the plants because most of the enzymes present in the chloroplast
becomes inactive and stop functioning.
4. Water:- Water is an essential raw material in photosynthesis. This rarely acts as
limiting factor, because less than 1% of water absorbed by plants is used in photosynthesis.
However the rate of photosynthesis may decrease if the plants are inadequately supplied with
water. According to some scientists the rate of photosynthesis decreases up to 87% in water
deficient soil. The reasons behind this are 1. Closure of stomata. 2. Stopage of CO2
absorption. 3. Reduction in the activity of photosynthetic enymes.
B. Internal factors
1. Chlorophyll :- Chlorophyll is necessary for photosynthesis. Due to this reason
the photosynthesis does not take place in etiolated & achlorophillous plants. Willstater used
12
the term assimilation power to find out the importance of chlorophyll. The quantity of
chlorophyll present in the cell is directly related with the photosynthetic rate .
2. Protoplasm:- In cells there are certain unknown factors which are catalytic in
nature & effect the rate of photosynthesis called protoplasmic factors. Proper hydration
reduces their effect.
3. Accumulation of end products of photosynthesis: If the photosynthetic products
are not translocated, there is a retarding effect upon photosynthesis. Quick translocation of
the carbohydrates or the end product of photosynthesis will have therefore favourable effect
on the photosynthetic rate.
4. Leaf age:- As leaves grow, their ability to photosynthesize increases for a time
then declines. Old senescent leaves eventually become yellow and are unable to
photosynthesize because of chlorophyll break down & loss of functional chloroplasts,
however even apparently healthy leaves of conifers that persist several years usually show
gradually decreasing photosynthetic rates during successive summers.
Significance of photosynthesis.
The phenomenon of photosynthesis is a boon to the nature and human beings. It is the only
natural process by which the solar energy is trapped. Without this the life is impossible on the
earth. It helps in.
Food production:- Photosyntheesis is the only process that links the physical and the
biological world. It helps in conversion of the solar energy into organic matter which makes
the bulk of the dry matter of any organism. The plant biomass or dry matter derived through
photosynthesis supports men and all other heterotrophic organisms living in the biosphere.
Atmospheric control and air purification:- Each living organism produces CO2
and energy as a result of oxidation of carbohydrates, fats and proteins during respiration, CO2
is also added to the atmosphere by burning coal, petrol and diesel etc. Green plants fix CO2 of
the atmosphere and thus maintaining its level in the atmosphere. This CO2 is utilized in the
biosynthetic phase of photosynthesis and atmosphere is made free from excess of CO2.
Evolution of O2:- In the process of photosynthesis O2 is evolved which is helpful to
human in two ways.
1. Oxidation of carbohydrates formed in the process of photosynthesis.
2. In making ozone, which helps in stopping the harmful radiations like violet
rays?
The carbohydrates produced during photosynthesis are used by plants and animals to
synthesize organic acids, proteins, fats, nucleic acids, hormones, pigments, vitamins,
alkaloids, and other metabolites.
In addition to organic food plants yield food, timber, fiber, fire wood, rubber, resins,
gums, oils are the products of photosynthesis. Fossil fuels (Coal petroleum and natural gas)
are also products of photosynthetic organisms which lived in the remote past.