Chapter 19 Photosynthesis in Higher Plants by Teaching Care online coaching classes
All living organisms require continuous use of energy to carry out their different activities. This energy directly or indirectly comes from sun.
Photosynthesis is the only process on earth by which solar energy is trapped by autotrophic organisms and converted into food for the rest of organisms.
In photosynthesis process, ‘energy rich compounds like carbohydrates are synthesized from simple inorganic compounds like carbon dioxide and water in the presence of chlorophyll and sunlight with liberation of oxygen’. The process of photosynthesis can also be defined as “transformation of photonic energy (i.e. light or radiant energy) into chemical energy“.
Earlier, photosynthesis was considered to be reverse of respiration, i.e.,
6CO2 + 6H 2 O ¾¾Lig¾ht ® C6 H12 O6 + 6O2
Above reaction gives an idea that O2 comes from CO2. But Ruben and Kamen (1941) experimentally verified that source of liberated O2 in photosynthesis is H2O, not CO2.
Thus, overall reaction can be corrected as given below :
6CO2 + 12H 2 O ¾¾Sun¾lig¾ht ® C6 H12 O6 + 6O2 + 6H 2 O
About 90% of total photosynthesis in world is done by algae in oceans and in freshwater. More than 170 billion tonnes of dry matter are produced annually by this process. Further CO2 fixed annually through photosynthesis is about 7.0 × 1013kg. Photosynthesis is an anabolic and endothermic reaction. Photosynthesis helps to maintain the equilibrium position of O2 and CO2 in the atmosphere.
Before seventeenth century it was considered that plants take their food from the soil.
- Van Helmont (1648) concluded that all food of the plant is derived from water and not from soi
- Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition from air and light may also play a role in this
- Joseph Priestley (1772) demonstrated that green plants purify the foul air (e., Phlogiston), produced by burning of candle, and convert it into pure air (i.e., Dephlogiston).
- Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air (Having O2) during sunlight and phlogisticated air (Having CO2) during
- Jean Senebier (1782) proved that plants absorb CO2 and release O2 in presence of light. He also showed that the rate of O2 evolution depends upon the rate of CO2
- Lavoisier (1783) identified the pure air (e., dephlogiston) as oxygen (O2) and noxious air (i.e., Phlogiston) produced by the burning of candle as carbon dioxide (CO2).
- Nicolus de Saussure (1804) showed the importance of water in the process of photosynthesis. He further showed that the amount of CO2 absorbed is equal to the amount of O2
- Pelletier and Caventou (1818) discovered chlorophyll. It could be separated from leaf by boiling in
- Dutrochet (1837) showed the importance of green pigment chlorophyll in
- Julius Robert Mayer (1845) proposed that light has radiant energy and this radiant energy is converted to chemical energy by plants, which serves to maintain life of the plants and also
- Liebig (1845) indicated that main source of carbon in plants is CO2.
- Bousingault (1860) reported that the volume of CO2 absorbed is equal to volume of O2 evolved and that CO2 absorption and O2 evolution get start immediately after the plant was exposed to
- Julius Von Sachs (1862) demonstrated that first visible product of photosynthesis is starch. He also showed that chlorophyll is confined to the
- C. Maxwell (1864) developed ‘wave model of light’, leading to recognition that light is source of energy in photosynthesis.
- Theodore Engelmann (1884, 88) showed that chloroplast as the site of photosynthesis in the cell and also discovered the role of different wave lengths of light on photosynthesis and plotted the action spectrum.
- F. Blackmann (1905) proposed the ‘law of limiting factor’ and also discovered two steps of photosynthesis i.e., light dependent and temperature independent steps and a light independent and temperature dependent step.
He proved that photosynthesis is a photochemical and biochemical reaction. Photochemical reaction is light reaction and biochemical reaction is dark reaction or carbon dioxide fixation.
- Willstatter and Stoll (1912) studied structure of photosynthetic pigments.
- Warburg (1919) performed flashing light experiment using green alga-Chlorella as a suitable material for the study of
- Van Niel (1931) demonstrated that some bacteria use H2S instead of H2O in the process of
- Emerson and Arnold (1932) proved the existance of light and dark reactions by flashing of light experiment in
- Robert Hill (1937) demonstrated photolysis of water by isolated chloroplast in the presence of suitable electron
- Ruben and M. Kamen (1941) used heavy isotope 18O and confirmed that oxygen evolved in photosynthesis comes from water and not from CO2.
- Melvin Calvin (1954) traced the path of carbon in photosynthesis (Associated with dark reactions) and gave the C3 cycle (Now named Calvin cycle). He was awarded Nobel prize in 1961 for the technique to trace metabolic pathway by using radioactive
- Emerson, Chalmers and Cederstrand (1957) discovered Emerson
- Hill and Bendall (1960) proposed Z scheme and suggested that two photosystems operate in
- Arnon (1961) discovered photophosphorylation and gave the term ‘assimilatory powers‘.
- Peter Mitchell (1961) proposed chemi-osmotic coupling hypothesis.
- Kortschak (1965) discovered the formation of C4 dicarboxylic acid in sugarcane
- Hatch and Slack (1966) reported the C4 pathway for CO2 fixation in certain tropical
- Huber, Michel and Deisenhofer (1985) crystallised the photosynthetic reaction center from the purple photosynthetic bacterium, Rhodopseudomonas viridis. They analysed its structure by X-ray diffraction technique. In 1988 they were awarded Nobel prize in chemistry for this
Photosynthesis in higher plants.
- Chloroplast-The site of photosynthesis : The most active photosynthetic tissue in higher plants is the mesophyll of Mesophyll cells have many chloroplast. Chloroplast are present in all the green parts of plants
and leaves. There may be over half a million chloroplasts per square millimetre of leaf surface. In higher plants, the chloroplasts are discoid or lens-shaped. They are usually 4-10mm in diameter and 1-3mm in thickness.
These are double membrane-bound organelles in the cytoplasm of green plant cells. Chloroplast has two unit membranes made up of lipoprotein. Outer membrane of
Granum Grana lamellae
chloroplast is permeable and an inner one impermeable to
protons. Inside the membranes is the proteinaceous ground
membrane Osmiophilic droplets
substance called stroma, which contain a variety of particles, osmiophilic droplets, dissolved salts, small double stranded
Fig : Internal structure of a typical chloroplast (Diagrammatic representation of sectional view)
circular DNA molecules and 70S type ribosomes along with various enzymes. Inside the stroma is found a system of chlorophyll bearing double-membraned sacs thylakoids or lamellae.
Thylakoids are flattened sacs arranged like the stacks of coins. One stack of thylakoids is called granum. Different grana are connected with the help of tubular connections called stroma lamellae or frets. Grana are the sites for light reaction of photosynthesis and consist of photosynthetic unit ‘quantasomes’ (Found in surface of thylakoids). Photosynthetic unit can be defined as number of pigment molecules required to affect a photochemical act, that is the release of a molecule of oxygen. Park and Biggins (1964) gave the term quantasome for photosynthetic units is equivalent to 230 chlorophyll molecules.
- Chloroplast pigments : Pigments are the organic molecules that absorb light of specific wavelengths in the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat soluble and are located in the lipid part of the thylakoid membranes. There is a wide range of chloroplastic pigments which constitute more than 5% of the total dry weight of the chloroplast. They are grouped under two main categories : (i) Chlorophylls and (ii) Carotenoids
The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.
- Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven types e., chlorophyll a, b,
c, d, e, bacteriochlorophyll and bacterioviridin.
Of all, only two types i.e., chlorophyll a and chlorophyll b are widely distributed in green algae and higher plants.
Chlorophyll ‘a‘ is found in all the oxygen evolving
C H CH3
A B C2H5
photosynthetic plants except photosynthetic bacteria.
H H E
Reaction centre of photosynthesis is formed of
CH2 H O
chlorophyll a. It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (e.g., Chl a680 or P680, Chl a700 or P700, etc.). It directly takes part in photochemical reaction. Hence, it is termed as primary photosynthetic pigment. Other photosynthetic pigments including chlorophyll b, c, d
and e ; carotenoids and phycobilins are called accessory pigments because they do not directly take part in photochemical act. They absorb specific wavelengths of light and transfer energy finally to chlorophyll a through electron spin resonance.
Chlorophyll a is blue black while chlorophyll b is green black. Both are soluble in organic solvents like alcohol, acetone etc. chlorophyll a appears red in reflected light and bright green in transmitted light as compared to chlorophyll b which looks brownish red in reflected light and yellow green in transmitted light.
Fig : Chemical structure of chlorophyll a and b molecules
Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll a possesses — CH3 (methyl group), which is replaced by — CHO (an aldehyde) group in chlorophyll b. Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head is linked together by methine (CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are indicative of various types of chlorophylls.
Phytol tail is made up of 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid ester bond. The basic structure of all chlorophyll comprises of porphyrin system.
When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called ‘cytochrome’ which is used in photosynthesis (Photophosphorylation) and respiration both.
Chlorophyll synthesis is a reduction process occurring in light. In gymnosperm seedlings, chlorophyll synthesis takes place in darkness in presence of enzyme called ‘chlorophyllase’. The precursor of chlorophyll is chlorophyllide.
|Chlorophyll a||C55H72O5N4Mg||All photosynthetic organisms except photosynthetic bacteria.|
|Chlorophyll b||C55H70O6N4Mg||Chlorophyta, Euglenophyta and in all
|Chlorophyll c||C35H32O5N4Mg||Brown algae (Phaeophyta), Diatoms
|Chlorophyll d||C54H70O6N4Mg||Red algae (Rhodophyta).|
|Chlorophyll e||Not fully known||Xanthophyta.|
|Bacteriochlorophyll||C55H74O6N4Mg||Purple photosynthetic bacteria.|
|Green sulphur bacteria.|
- Carotenoids : The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8) They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments. They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.
- Carotenes : They are orange red in colour and have general formula C40H56. They are isolated from
They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are a,
b, g and d carotene; phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. The b-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. b-carotene is black yellow pigment of carrot roots.
C40 H56 + 2H 2 O ¾¾Car¾oten¾a¾se ® 2 C20 H 29 OH
Carotene vitamin A
- Xanthophylls : They are yellow coloured carotenoid also called xanthols or carotenols. They contains oxygen also along with carbon and hydrogen and have general formula C40H56O2.
Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).
- Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol
Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyanin and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae. Like carotenoids, phycobilins are accessory pigments i.e. they absorb light and transfer it to chlorophyll a.
- Nature of light : Sunlight is a type of energy called radiant energy or electromagnetic energy. This energy, according to electromagnetic wave theory (Proposed by James Clark Maxwell, 1960), travels in space as The distance between the crest of two adjacent waves is called a wavelength (l). Shorter the wavelength greater the energy.
The unit quantity of light energy in the quantum theory is called quantum (hn), whereas the same of the electromagnetic field is called photon. Solar radiation can be divided on the basis of wavelengths. Radiation of
shortest wavelength belongs to cosmic rays whereas that of longest wavelength belong to radio waves. Light represents only one part of electromagnetic radiation. Other parts include cosmic rays, X-rays, UV rays, infra- red radiation and radio waves. A visible light has seven
separated groups of more or less complete absorption. In
10–14 10–12 10–10 10–8 10–6 10–4 10–2 1 102 104 106 cm
|Cosmic rays||X-rays||Ultra violet||Infrared||Radio waves||
a spectrum of sunlight, bands of blending colours are seen i.e., dark red at one end running through red, orange, yellow, green, blue, indigo, violet and ending in
700 800 nm
darkest violet. Wavelengths in the violet portion of spectrum are about 400 millimicrons (mm) in length and at other end of spectrum — the red portion — are much
Blue Green Yellow Orange Red
Fig : Electromagnetic spectrum of light
longer about 730mm. In other words, visible light lies between wavelengths of ultra-violet and infra-red. The visible spectrum of solar radiations are primarily absorbed by carotenoids of the higher plants are violet and blue. However, art of blue and red wavelengths, blue light carry more energy.
Shortest wavelength ¾
¾® Longest wavelength
Visible light : 390nm (3900Å) to 760nm (7600Å). Violet (390–430nm), blue (430–470nm), blue-green (470–500nm), green (500–580nm), yellow (580–600nm), orange (600–650nm), orange-red (650–660nm) and red (660–760nm) Far-red (700–760nm). Infra-red 760nm – 100mm. Ultraviolet 100–390nm. Solar Radiations 300nm (ultraviolet) to 2600nm (infra-red). Photosynthetically active radiation (PAR) is 400–700nm. Leaves appear green because chlorophylls do not absorb green light. The same is reflected and transmitted through leaves.
Absorption and action spectra : The curve representing the light absorbed at each wavelength by pigment is called absorption spectrum. Curve showing rate of photosynthesis at different wavelengths of light is called action spectrum.
Absorption spectrum is studied with the help of spectrophotometer. The absorption spectrum of chlorophyll a and chlorophyll b indicate that these pigments mainly absorb blue and red lights. Action spectrum shows that maximum photosynthesis takes place in blue and red regions of spectrum. The first action spectrum of photosynthesis was studied by T.W. Engelmann (1882) using green alga Spirogyra and oxygen seeking bacteria.
In this case actual rate of photosynthesis in terms of oxygen
380 420 460 500 540 580 620 660
Wavelength, m m
Fig : Absorption spectra of chlorophylls a and b
evolution or carbon dioxide utilisation is measured as a function of wavelength.
Mechanism of photosynthesis.
Before 1930 it was considered by physiologists that one molecule each of CO2 and H2O form a molecule of formaldehyde (HCHO), of which 6 mols are polymerized to one molecule of glucose (a hexose sugar).
CO2 + H 2 O ¾¾Lig¾ht ® HCHO + O2
6CH 2 O(or 6HCHO) ¾¾Poly¾me¾risat¾i¾on ® C6 H12 O6
(Formaldehyde) (Hexose sugar)
However formaldehyde is a toxic substance which may kill the plants. Hence, formaldehyde hypothesis could not be accepted.
On the basis of discovery of Nicolas de Saussure that “The amount of O2 released from plants is equal to the amount of CO2 absorbed by plants”, it was considered that O2 released in photosynthesis comes from CO2, but Ruben proved that this concept is wrong.
In 1930, C.B. Van Niel proved that, sulphur bacteria use H2S (in place of water) and CO2 to synthesize carbohydrates as follows :
6CO2 + 12H 2 S ¾¾® C6 H12 O6 + 6H 2 O + 12S
This led Van Niel to the postulation that in green plants, water (H2O) is utilized in place of H2S and O2 is evolved in place of sulphur (S). He indicated that water is electron donar in photosynthesis.
6CO2 + 12H 2 O ¾¾® C6 H12 O6 + 6H 2 O + 6O2
This was confirmed by Ruben and Kamen in 1941 using Chlorella a green alga.
They used isotopes of oxygen in water, i.e., H218O instead of H2O (normal) and noticed that liberated oxygen contains 18O of water and not of CO2. The overall reaction can be given as under :
6CO2 + 12H 218 O ¾¾Lig¾ht ® C6 H12 O6 + 618 O2 + 6H 2 O
The fate of different molecules can be summarised as follows :
6CO2 + 12H 2 O ¾¾chlo¾rop¾h¾yll ® C6 H12 O6 + 6H 2 O + 6O2
Fig : Fat of different molecules
Modern concept of photosynthesis.
Photosynthesis is an oxidation reduction process in which water is oxidised to release O2 and CO2 is reduced to form starch and sugars.
Scientist have shown that photosynthesis is completed in two phases.
- Light phase or Photochemical reactions or Light dependent reactions or Hill’s reactions : During this stage energy from sunlight is absorbed and converted to chemical energy which is stored in ATP and NADPH + H+.
- Dark phase or Chemical dark reactions or Light independent reactions or Blackman reaction or Biosynthetic phase : During this stage carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP and NADPH formed in the light dependent
- Evidence for light and dark reactions in photosynthesis : Evidences in favour of light and dark phases in photosynthesis are :
Physical separation of chloroplast into grana and stroma fractions : It is now possible to separate grana and stroma fractions of chloroplast. If light is given to grana fraction in presence of suitable H-acceptor and in complete absence of CO2, then ATP and NADPH2 are produced (i.e., assimilatory powers). If these assimilatory powers (ATP and NADPH2) are given to stroma fraction in presence of CO2 and absence of light, then carbohydrates are formed.
Experiments with intermittent light or Discontinuous light : Rate of photosynthesis is faster in intermittent light (Alternate light and dark periods) than in continuous light. It is because light reaction is much faster than dark reaction, so in continuous light, there is accumulation of ATP and NADPH2 and hence reduction in rate of photosynthesis but in discontinuous light, ATP and NADPH2 formed in light are fully consumed during dark in reduction of CO2 to carbohydrates. Accumulation of NADPH2 and ATP is prevented because they are not produced during dark periods.
Temperature coefficient studies : The temperature coefficient (Q10) is defined as the ratio of the velocity of a reaction at a particular temperature to that at a temperature 10°C lower. For a physical process the value of Q10 is slightly greater than one. In photochemical reaction the energy source is light and any increase in temperature is not sufficient to cause an increase in the rate. Thus here also the value of Q10 is one. However, in case of chemical reactions the value of Q10 is two or more i.e., with the rise of 10°C temperature, the rate of chemical reaction is doubled. If the process of photosynthesis includes a hidden chemical reaction in addition to usual photochemical reaction, its value of Q10 should be two or more.
Blackman found that Q10 was greater than 2 in experiment when photosynthesis was rapid and that Q10 dropped from 2 often reaching unity, i.e., 1 when the rate of photosynthesis was low. These results show that in photosynthesis there is a dark reaction (Q10 more than 2) and a photochemical or light reaction (with Q10 being unity).
= Reaction rate of (t + 10)°C Reaction at t°C
- Light phase (Photochemical reactions) : Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on Assimilatory powers (ATP and NADPH2) are mainly produced in this light reaction.
Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptors, e.g., Potassium ferroxalate (Some plants require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of water. Under these conditions, no CO2 was consumed and no carbohydrate was produced, but light-driven reduction of the electron acceptors was accompained, by O2 evolution.
The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is known as Hill reaction.
Hill reaction proves that
- In photosynthesis oxygen is released from water.
- Electrons for the reduction of CO2 are obtained from water [e., a reduced substance (hydrogen donor) is produced which later reduces CO2].
Dichlorophenol indophenol is the dye used by Hill for his famous Hill reaction.
According to Arnon (1961), in this process light energy is converted to chemical energy. This energy is stored in ATP (this process of ATP formation in chloroplasts is known as photophosphorylation) and from electron acceptor NADP+, a substance which found in all living beings NADP*H is formed as hydrogen donor. Formation of hydrogen donor NADPH from electron acceptor NADP+ is known as photoreduction or production of reducing power NADPH.
Light phase can be explained under the following headings :
(i) Transfer of energy (ii) Quantum yield (iii) Emerson effect (iv) Two pigment systems
(v) Z-scheme (vi) Cyclic and non-cyclic photophosphorylation
- Transfer of energy : When photon of light energy falls on chlorophyll molecule, one of the electrons pair from ground or singlet state passes into higher energy level called excited singlet It comes back to hole of chlorophyll molecule within 10–9 seconds.
This light energy absorbed by chlorophyll molecule before coming back to ground state appears as radiation energy, while that coming back from excited singlet state is called fluorescence and is temperature independent. Sometimes the electron at excited singlet state gets its spin reversed because two electrons at the same energy level cannot stay; for some time it fails to return to its partner electron. As a result it gets trapped at a high energy level.
of light Original orbit
Ground state Excited state
Fig : Photoexcitation of chlorophyll molecule i.e. of its atoms
Excited second singlet state Heat
Excited first singlet state
Due to little loss of energy, it stays at comparatively lower energy level (Triplet state) from excited singlet state. Now at this moment, it can change its spin and from this triplet state, it comes back to ground state again losing excess of energy in the form of radiation. This type of loss of energy is
called as phosphorescence.
When electron is raised to higher energy level, it is called at second singlet state. It can lose its energy in the form of heat also. Migration of electron from excited singlet state to ground state along with the release of excess energy into radiation energy is of no importance to this process.
Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.
(ii) Quantum yield
Fig : Movement of electron due to photoexcitation of pigment molecule
- Rate or yield of photosynthesis is measured in terms of quantum yield or O2 evolution, which may be defined as, “Number of O2 mols evolved per quantum of light absorbed in “
On the other hand quantum requirement is defined as, “Number of quanta of light required for evolution of one mol of O2 in photosynthesis.”
- Quantum requirement in photosynthesis = 8, e., 8 quanta of light are required to evolve one mol. of O2.
- Hence quantum yield = 1 / 8 = 125 (i.e., a fraction of 1) as 12%.
- Emerson effect and Red drop : Emerson and C.M. Lewis (1943) observed that the quantum yield of photosynthesis decreased towards the far red end of the spectrum (680nm or longer). Quantum yield is the number of oxygen molecules evolved per light quantum absorbed. Since this decrease in quantum yield is observed at the far region or beyond red region of spectrum is called red drop.
Emerson et al. (1957) further observed that photosynthetic efficiency of light of 680nm or longer is increased if light of shorter wavelengths (Less than 680nm) is supplied simultaneously.
When both short and long wavelengths were given together the quantum-yield of photosynthesis was greater than the total effect when both the wavelengths were given separately. This increase in photosynthetic efficiency (or quantum yield) is known as Emerson effect or Emerson enhancement effect.
E = Quantum yield in combined beam – Quantum yield in red beam
Quantum yield in far red beam
400 480 560 640 720
Fig : Red drop
- Two pigment systems : The discovery of Emerson effect has clearly shown the existence of two distinct
photochemical processes, which are believed to be associated with two different specific group of pigments. One group of pigments absorbs light of both shorter and longer wavelengths (More than 680nm) and another
group of pigments absorbs light of only
shorter wavelengths (Less than 680nm).
These two groups of pigments are known as
pigment systems or photosystems.
Pigment system I or Photosystem I : The important pigments of this system are chlorophyll a 670, chlorophyll a 683, chlorophyll a 695, P700. Some physiologist also include carotenes and chlorophyll b in pigment system I. P700 acts as the reaction centre. Thus, this system absorbs both wavelengths shorter and longer than 680nm.
Pigment system II or photosystem II :
chl a690–700 chl a705–710
chl a670–680 chl a678–687 chl a685–695
The main pigments of this system are chlorophyll a 673, P680, chlorophyll b and
Fig : Distribution of pigments in the two photosystems or pigment systems
phycobilins. This pigment system absorbs wavelengths shorter than 680nm only. P680 acts as the reaction centre.
Pigment systems I and II are involved in non-cyclic electron transport, while pigment system I is involved only in cyclic electron transport. Photosystem I generates strong reductant NADPH. Photosystem II produces a strong oxidant that forms oxygen from water.
Comparison of photosystem I and photosystem II
|S.No.||Photosystem I||Photosystem II|
|(1)||PS I lies on the outer surface of the thylakoids||PS II lies on the inner surface of the thylakoid.|
|(2)||In this system molecular oxygen is not evolved.||As the result of photolysis of water molecular oxygen is evolved.|
|(3)||Its reaction center is P700.||Its reaction center is P680.|
|(4)||NADPH is formed in this reaction.||NADPH is not formed in this reaction.|
|(5)||It participate both in cyclic and noncyclic photophosphorylation.||It participate only in noncyclic photophosphorylation.|
|(6)||It receives electrons from photosystem II.||It receives electrons from photolytic dissociation of water.|
|(7)||It is not related with photolysis of water.||It is related with photolysis of water.|
- Z-Scheme of light reactions : When sunlight strikes the thylakoid membrane, the energy is absorbed
simultaneously by the
antenna pigments of both PS I and PS II and passed on to the reaction centers of both photosystems. Electrons of both reaction center pigments are boosted to an outer
orbital and each
Excited state P680+
photoexcited electron is transferred to a primary electron acceptor. The transfer of electrons out of the photosystems leaves the two reaction center pigments
missing an electron and thus,
positively charged. After losing their electrons, the
Water splitting center
Cyt b6-f complex
reaction centers of PS I and
PS II can be denoted as P700+ and P680+
Released into atmosphere
charged reaction centers act
as attractants for electrons, which sets the stage for the
Antenna chlorophyll molecules
Fig : The Z-scheme of photosynthesis simplified diagram of the electron flow from
flow of electrons between carriers.
In oxygenic photosynthesis, in which two photosystems act in series, electron flow occurs along three legs- between water and PS II, between PS II and PS I and between PS I and NADP+ an arrangement which is described as the Z scheme. The Z scheme as originally proposed by Hill and Bendall, 1960.
- Photophosphorylation : Light phase includes the interaction of two pigment systems. PS I and PS II constitute various type of pigments. Arnon showed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.
ADP + Pi ¾¾Lig¾ht ® ATP .
(Where ADP = Adenosine diphosphate, Pi = Inorganic phosphate and ATP = Adenosine triphosphate).
When the light quantum is absorbed by various types of pigments (Like chlorophylls, phycobilins, carotenoids etc.), it is transferred to reaction centre i.e. P700 in PS I and P680 in PS II. Electrons excite from reaction centres due to funneling of energy. P700 gets photo excited and comes under first excited singlet state. As a result electron is lost, which is accepted by an electron, acceptor in the way. After absorbing light, excited electron liberated from reaction centre interacts with water.
4 H 2 O ¾¾Lig¾ht ® 4 H + + 4OH –
4OH – + 4e –
® 2H 2 O + O2
4 H + + 2A + 4e – ® 2AH 2
Another important aspect of light reactions is the formation of ATP and NADPH2 (Assimilatory power). H+ from water and electron from chlorophyll are made available to NADP to form NADPH2. The electrons are accepted by NADP after passing through electron carriers. The carriers in the way undergo oxidation and reduction and are arranged in accordance with their redox potential value.
Photophosphorylation is of two types
- Cyclic photophosphorylation : It involves only PS Flow of electron is cyclic. When NADP is not available then this process will occurs. When the photons activate PS I, a pair of electrons are raised to a higher energy level. They are captured by primary acceptor which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS I i.e. P700. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pump H+ across the thylakoid membrane. The
proton gradient, thus established is responsible for
Fig : Cyclic photophosphorylation
forming (2 molecules) ATP. No reduction of NADP to NADPH+ H+. ATP is synthesized at two steps.
- Non cyclic photophosphorylation : It involves both PS-I and PS-II. Flow of electron is unidirectional. Here electrons are not cycled back and are used in the reduction of NADP to NADPH2. Here H2O is utilized and O2 evolution occurs. In this chain high energy electrons released from ‘P-680’ do not return to ‘P-680’ but pass through pheophytin, plastoquinone, cytochrome b6–f complex, plastocyanin and then enter P-700. In this transfer of electrons from plastoquinone (PQ) to cytochrome b6–f complex, ATP is Because in this process high energy electrons released from ‘P-680’ do not return to ‘P-680’ and ATP (1 molecules) is formed, this is called Noncyclic photophosphorylation. ATP is synthesized at only one step.
P 680 Antenna
Fig : Non cyclic photophosphorylation
Fig : Final products of light reactions
Comparison of cyclic and noncyclic photophosphorylation
|S.No.||Cyclic photophosphorylation||Noncyclic photophosphorylation|
|(1)||No oxygen is given off (Anoxygenic).||Oxygen is given off (Oxygenic).|
|(2)||No water is consumed.||Water is used up.|
|(3)||Only one light-trapping system (Photosystem I) is involved.||Two light-trapping systems (Photosystem I and II) are involved.|
|(4)||No NADPH synthesized.||NADPH synthesized|
|(5)||Last electron acceptor is P700||Last electron acceptor is NADP.|
|(6)||The system is found dominantly in bacteria.||The system is dominant in green plants.|
|(7)||The process is not inhibited by DCMU.||The process is stopped by use of DCMU|
- Pseudocyclic photophosphorylation : Arnon and his coworker (1954) demonstrated yet another kind of photophosphorylation. They observed that even in absence of CO2 and NADP, if chlorophyll molecules are illuminated, it can produce ATP from ADP and Pi (Inorganic phosphate) in presence of FMN or K and oxygen. The process is thus very simple and requires no net chemical change but for the formation of ATP and water. Arnon called this oxygen dependent FMN catalysed photophosphorylation or pseudocyclic photophosphorylation which involves the reduction of FMN with the production of oxygen. FMN is an auto-oxidisable hydrogen acceptor with the effect that the reduced FMN is reoxidised by oxygen. Thus the process can continue repeatedly to produce ATP.
Since this process can be continuously self repeated, it appears that a single molecule of water should be sufficient to operate pseudocyclic photophosphorylation to meet the requirement of ATP.
FMN + H2O FMNH2 + ½ O2
- Dark phase : The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR) cycle or dark reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, e. NADP .2H and ATP. The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of CO2 and synthesis of sugar. The techniques used for studying different steps were Radioactive tracer technique using 14C (Half life – 5720 years), Chromatography and Autoradiography and the material used was Chlorella (Cloacal alga) and Scenedesmus (these are microscopic, unicellular algae and can be easily maintained in laboratory).
The assimilation and reduction of CO2 takes place in this reaction by which carbohydrate is synthesized through following three pathways :
- Calvin cycle (C3) (ii) Hatch and Slack cycle (C4) (iii) Crassulacean acid metabolism (CAM plants)
(i) Calvin cycle : Calvin and Benson discovered the path of carbon in this process. This is known as C3 cycle because CO2 reduction is cyclic process and first stable product in this cycle is a 3-C compound (i.e., 3- Phosphoglyceric acid or 3-PGA).
Calvin cycle is divided into three distinct phases : Carboxylation, Glycolytic reversal, Regeneration of RuBP.
- Carboxylation : CO2 reduction starts with a 5-carbon sugar, ribulose-5-phosphate. 6 molecules of this sugar react with 6 molecules of ATP (Produced in light reactions) forming 6 molecules of ribulose-1, 5-biphosphate (RuBP) and 6 molecules of (equation i).
Ribulose-5 – phosphate + 6ATP
¾¾Pho¾sph¾ope¾ntok¾ina¾se ®Ribulose -1, 5 – biphosphate +6ADP .…. (i)
The reaction is catalysed by the enzyme ribulose biphosphate carboxylase (RUBISCO). Ribulose-1,5- biphosphate (RuBP) (=Ribulose diphosphate) acts as CO2 acceptor and 6 mols of RuBP react with 6 mols of CO2 and 6 mols of water giving rise to 12 mols of 3-phosphoglyceric acid (a 3 carbon compound) (equation ii).
Ribulose -1, 5 – biphosphate + 6CO2 + 6H 2O ¾¾¾C¾arbo¾xyd¾ism¾utas¾e ¾ ®
3 – phosphoglyceric acid
- Glycolytic reversal : Carboxylation is followed by reactions that involve reversal of glycolysis part of
12 mols of 3-phosphoglyceric acid react with 12 mols of ATP (Produced in light reactions) giving rise to 12 mols each of 1, 3-diphosphoglyceric acid + ADP (equation iii).
3 – phosphoglyceric acid + 12ATP ¾¾Pho¾sph¾ogly¾ceri¾c kin¾a¾se ® 1, 3 – diphosphoglyceric acid + 12 ADP
(12 mols) (12 mols)
12 mols of NADP.2H formed in light reactions are used to reduce 12 mols of 1,3-diposphoglyceric acid leading to the formation of 12 mols of 3-phosphoglyceraldehyde, 12 moles of NADP and 12 moles of phosphoric acid (equation iv).
1,3 – diphosphoglyceric acid + 12 NADP.2H ¾¾Trio¾se p¾hos¾pha¾tede¾hyd¾rog¾ena¾se ® (12 mols)
3 – phosphoglyceraldehyde + 12NADP + + 12H3 PO4 …. (iv)
In this way by the reduction of CO2, 12 molecules of 3-phosphoglyceraldehyde are formed. Out of these 12 molecules, 2 molecules go to synthesize sugar, starch and other carbohydrates and remaining 10 molecules are recycled to regenerate 6 molecules of ribulose 5 phosphate.
Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate (equation v).
3 – phosphoglyceraldehyde ¾¾Pho¾sph¾otrio¾se ® 3 – dihydroxyacetone phosphate
One mol of 3-dihydroxyacetone phosphate react with 1 mol of 3-phosphoglyceraldehyde to form one molecule of fructose-1,6-biphosphate (equation vi).
Phosphoglyceraldehyde + Dihydroxyacetone phosphate ¾¾Ald¾ola¾se ®
Fructose -1,6 – biphosphate
One mol of fructose-6-phosphate and one mol of phosphoric acid is released from one mol of fructose-1,6- biphosphate with the help of the enzyme phosphatase with utilizations of one mol of H2O (equation vii).
Fructose -1,6 – biphosphate ¾¾Pho¾sph¾ata¾se ® Fructose- 6 – phosphate + H3 PO4
Fructose-6-phosphate can be converted to other sugars (viz., glucose, sucrose, starch, etc.). In this way, the atmospheric CO2 is used in the synthesis of carbohydrates.
- Regeneration of RuBP : Both triose phosphates, e., 3-phosphoglyceraldehyde and dihydroxy acetone phosphate, actively participate in the regeneration of CO2-acceptor ribulose-1,5-diphosphate. The sequence of reactions are as follows :
- 3 – phosphoglyceraldehyde ¾¾Trio¾se p¾hos¾pha¾te ®Dihydroxyacetone phosphate
- Dihydroxyacetone phosphate + 3 – phosphoglyceraldehyde ¾¾Ald¾ola¾se ® Fructose – 1,6 – diphosphate
- Fructose -1,6 – diphosphate + 2H 2 O ¾¾Pho¾sph¾ata¾se ®Fructose – 6 – phosphate + 2H3 PO4
- Fructose – 6 –
phosphate + 3 – phosphoglyceraldehyde ¾¾Tra¾nske¾tola¾se ®
Xylulose- 5 –
phosphate + Erythrose- 4 –
- Erythrose- 4 –
phosphate+ Dihydroxyacetone phosphate ¾¾Tra¾nsal¾dola¾se ®
(Produced in reaction (iv))
of the 4 mols produced in reaction (i)
Sedoheptulose-1,7 – diphosphate
(one mol) (one mol)
(one mol) (vi)
(one mol) (vii)
- Phosphoglyceric kinase,
(one mol) Glucose-1-phosphate
(one mol) (v1ii)6
- Triose phosphate dehydrogenase,
- Phosphotriose isomerase,
Fig : Simplified diagram of Calvin cycle
Sedoheptulose-1,7 – diphosphate ® Sedoheptulose- 7 –
phosphate + 2H3 PO4
Sedoheptulose- 7 –
phosphate + 3 – p hosphoglyceraldehyde
Ribose- 5 –
phosphate + Xylulose- 5 –
- Ribose- 5 –
phosphate ®Ribulose- 5 –
(Produced in reaction (vii))
Ribulose- 5 –
2 + 4 = 6 molecules of Ribulose 5 phosphate are formed during the changes from equation (viii) and (ix) these molecule changed in Ribulose 1, 5 diphosphate (RuDP) by the consumption of 6 ATP. These RuDP again ready for reduction of new molecules of CO2. Hence in this way regeneration of RuDP is going on. They are used in next calvin cycle. In the overall reactions 18 ATP molecules and 12 NADPH2 molecules consumed and one molecule of glucose (Hexose) is obtained (1 NADPH2 = 3ATP \ Total ATP consumed = 54 ATP). The whole photosynthesis can be summarized in terms of equation which is as follows :
Light reaction :
Dark reaction :
Final equation :
- Hatch and Slack cycle (C4 cycle) : Kortschak and Hart supplied CO2 to the leaves of sugarcane, they found that the first stable product is a four carbon (C4) compound oxalo acetic acid instead of 3-carbon atom The detailed study of this cycle has introduced by M.D. Hatch and C.R. Slack (1966). So it is called as “Hatch and Slack cycle”. The stable product in C4 plant is dicarboxylic group. Hence it is called dicarboxylic acid
cycle or DCA-cycle. C4 plants are true xerophytic plants.
The plants that perform C4 cycle are found in tropical (Dry and hot regions) and sub-tropical regions. There are more than 900 known species in which C4 cycle occurs. Among them, more than 300 species belong to dicots and the rest belong to monocots. The
important among them are sugarcane, maize, Sorghum, Cyperus rotundus, Digitaria brownii, Amaranthus, etc. These plants have
“Kranz” (German term meaning halo or wreath) type of leaf anatomy. The vascular bundles, in C4 leaves are surrounded by a
Cells of bundle sheath with special types of chloroplast
layer of bundle sheath cells that contain large number of chloroplasts. The chloroplasts in C4 leaves are dimorphic (Two
Fig : Cross section of leaf showing “krantz” type of anatomy
morphologically distinct types). The chloroplasts of bundle sheath cells are larger in size and arranged centripetally. They contain starch grains but lack grana. The mesophyll cells, on the other hand, contain normal types of chloroplasts. The mesophyll cells perform C4 cycle and the cells of bundle sheath perform C3 cycle.
CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA). This acid is converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells and there undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) and CO2. CO2 released in bundle sheath cells reacts with Ribulose-1,5-biphosphate (RuBP) already present in the chloroplasts of bundle sheath cells and thus Calvin cycle starts from here. Pyruvic acid re-enters mesophyll cells and regenerates phosphoenol pyruvic acid. CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP carboxylase and pyruvate orthophosphate dikinase enzyme while the bundle sheath cells have decarboxylase and complete enzymes of Calvin cycle. In C4 plants, there are 2 carboxylation reactions, first in mesophyll chloroplast and second in bundle sheath chloroplast.
Mesophyll cell Bundle sheath cell
Fig : Hatch-slack pathway (cycle)
Enzymes : (i) Phosphoenol pyruvate carboxylase, (ii) Malate dehydrogenase,
- Decarboxylase, (iv) Pyruvate orthophosphate dikinase
C4 plants are better photosynthesizers. There is no photorespiration in these plants. In C4 plants, for formation of one mole of hexose (glucose) 30 ATP and 12 NADPH2 are required. There is difference in different C4 plants in mechenism of C4 mode of photosynthesis. The main difference is in the way the 4C dicarboxylic acid is decarboxylated in the bundle sheath cells. The three categories of C4 pathways in C4 plants are recognised such as :
- Some C4 plants g., Zea mays, Saccharum officinarum, Sorghum utilise NADP+ specific malic enzyme for decarboxylation. This mechanism of C4 pathway in these C4 plants is said to be of NADP+ –Me Type.
- Some C4 plants g., Atriplex, Portulaca, Amaranthus utilise NAD+ specific malic enzyme for decarboxylation. This mechanism of C4 pathways in these C4 plants is said to be of NAD+ –Me Type.
- Some C4 plants g., Panicum, Chloris utilise PEP-carboxykinase enzyme. The mechanism of C4 pathway in these plants is called as PCK-me-Type.
Characteristics of C4 cycle
- C4 species have greater rate of CO2 assimilation than C3 species. This is on account of the fact that
- PEP carboxylase has great affinity for CO2.
- C4 plants show little photorespiration as compared to C3 plants, resulting in higher production of dry matter.
- C4 plants are more adapted to environmental stresses than C3
- CO2 fixation by C4 plants require more ATP than that by C3 This additional ATP is needed for conversion of pyruvic acid to phosphoenol pyruvic acid and its transport.
- CO2 acceptor molecule in C4 plants is Further, PEP-carboxylase (PEPCO) is the key enzyme (RuBP- carboxylase enzyme is negligible or absent in mesophyll chloroplast, but is present in bundle sheath chloroplast).
Differences between C3 and C4 plants
|S.No.||Characters||C3 plants||C4 plants|
|(1)||CO2 acceptor||The CO2 acceptor is Ribulose 1,5 diphosphate.||The CO2 acceptor is phosphoenol-pyruvate.|
|(2)||First stable product||The first stable product is phosphoglyceric acid.||Oxaloacetate is the first stable product.|
|(3)||Type of chloroplast||All cells participating in photosynthesis have one type of chloroplast.||The chloroplast of parenchymatous bundle sheath is different from that of mesophyll cells. Leaves have ‘Kranz’ type of anatomy. The bundle sheath chloroplasts lack grana. Mesophyll cells have normal chloroplasts.|
|(4)||Cycles||Only reductive pentose phosphate cycle is found.||Both C4-dicarboxylic acid and reductive pentose
phosphate cycles are found.
|(5)||Optimum temperature||The optimum temperature for the process is 10-25°C.||In C4 plants, it is 30-45°C.|
|(6)||Oxygen inhibition||Oxygen present in air (=21% O2) markedly inhibit the photosynthetic process as compared to an external atmosphere containing no oxygen.||The process of photosynthesis is not inhibited in air as compared to an external atmosphere containing no oxygen.|
|(7)||PS I and PS II||In each chloroplast, photosystems I and II are present. Thus, the Calvin cycle occurs.||In the chloroplasts of bundle sheath cells, the photosystem II is absent. Therefore, these are dependent to mesophyll chloroplast for the supply of NADPH + H+|
|(8)||Enzymes||The Calvin cycle enzymes are present in mesophyll chloroplast.||Calvin cycle enzymes are absent in mesophyll chloroplasts. The cycle occurs only in the chloroplasts of sheath cells.|
|(9)||Compensation point||The CO2 compensation point is 50-150ppm.||CO2 compensation point is 0-10ppm.|
|(10)||Photorespiration||Photorespiration is present and easily detectable.||Photorespiration is present only to a slight degree
and difficult to detect.
|(11)||Net rate||Net rate of photosynthesis in full sunlight (10,000- 12,000 ft.c) is 15-35mg. of CO2 per dm2 of leaf area per h.||It is 40-80mg. of CO2 per dm2 of leaf area per h. That is photosynthetic rate is quite high. The plants are efficient.|
|(12)||Saturation intensity||The saturation intensity reached in the range of 100-
|It is difficult to reach saturation even in full
(iii) Crassulacean acid metabolism plants (CAM plants) : This dark CO2 fixation pathway proposed by Ting (1971). It operates in succulent or fleshy plants e.g. Cactus, Sedum, Kalanchose, Opuntia, Agave, orchid, pine apple and Bryophyllum helping them to continue photosynthesis under extremely dry condition.
The stomata of succulent plants remain closed during day and open during night to avoid water loss (Scotactive stomata). They store CO2 during night in the form of malic acid in presence of enzyme PEP carboxylase. The CO2 stored during night is used in Calvin cycle during day time. Succulents refix CO2 released during respiration and use it during photosynthesis.
This diurnal change in acidity was first discovered in crassulacean plants e.g. Bryophyllum. So it is called as crassulacean acid metabolism. The metabolic pathways are –
- Acidification : In dark, stored carbohydrates are converted to phosphoenol pyruvic acid (PEP) by the process of The opening of stomata in CAM plants in dark causes entry of CO2 in leaf. So, phosphoenol pyruvic acid in presence of PEP carboxylase is converted to oxaloacetic acid (OAA). OAA is then reduced to malic acid in presence of enzyme malic dehydrogenase with the help of NADH2. This malic acid (Produced by acidification) is stored in vacuole.
Phosphoenol pyruvic acid
Calvin RuDP cycle
Fig : CAM synthesis
- Deacidification : In light the malic acid is decarboxylated to produce pyruvic acid and evolve CO2. This process is called deacidification.
The malate may be decarboxylated in two ways –
- In some CAM plants the malate is directly decarboxylated in the presence of NADP+ malic enzyme into
CO2 and pyruvate (ME-CAM plants).
- In other CAM plants, the malate is first oxidised to oxaloacetic acid by enzyme malate dehydrogenase which is then converted into CO2 and phosphoenol pyruvate with the utilization of ATP by enzyme PEP carboxykinase (PEPCK-CAM plants).
The CO2 produced by any above process is then consumed in normal photosynthetic process to produce carbohydrate.
Characteristics of CAM pathway
- CO2 assimilation and malic acid assimilation take place during the
- There is decrease in pH during the night and increase in pH during the day.
- Malic acid is stored in the vacuoles during the night which is decarboxylated to release CO2 during the
- CAM plants have enzymes of both C3 and C4 cycle in mesophyll This metabolism enable CAM plants to survive under xeric habitats. These plants have also the capability of fixing the CO2 lost in respiration.
Photorespiration (Photosynthetic carbon oxidation cycle).
Decker and Tio (1959) reported that light induces oxidation of photosynthetic intermediates with the help of oxygen in tobacco. It is called as photorespiration. The photorespiration is defined by Krotkov (1963) as an extra input of O2 and extra release of CO2 by green plants is light.
Photorespiration is the uptake of O2 and release of CO2 in light and results from the biosynthesis of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the same leaf cell. Biochemical mechanism for photorespiration is also called glycolate metabolism. Loss of energy occurs during this process. The process of photorespiration involves the involvement of chloroplasts, peroxisomes and mitochondria. RuBP carboxylase also catalyses another reaction which interferes with the successful functioning of Calvin cycle.
- Ribulose bisphosphate carboxylase
(RUBISCO), the main enzyme of Calvin cycle that fixes CO2, acts as ribulose bisphosphate oxygenase
under low atmospheric concentration of CO2
Fig : The biochemical pathway of photorespiration
(i.e., below 1%) and increased concentration of O2. In presence of high concentration of O2 the enzyme RuBP oxygenase splits a molecule of Ribulose-1, 5-bisphosphate into one molecule each of 3-phosphoglyceric acid and 2- phosphoglycolic acid.
Ribulose-1, 5- bisphosphate
2 Phosphoglycolic acid +3 Phosphoglyceric acid
- The 2-phosphoglycolic acid loses its phosphate group in presence of enzyme phosphatase and converted into glycolic acid –
2 Phosphoglycolic acid + H2O
Glycolic acid + Phosphoric acid.
- The glycolic acid, synthesized in chloroplast as an early product of photosynthesis, is then transported to the The glycolic acid reacts with O2 and oxidizes to glyoxylic acid and hydrogen peroxide with the help of enzyme glycolic acid oxidase.
Glycolic acid + O2
Glyoxylic acid + H2O2
The hydrogen peroxide is destroyed by enzyme catalase as follows :
- The glyoxylic acid is then converted to an amino acid-glycine by transamination reaction catalyzed by enzyme glutamate-glyoxylate
Glyoxylic acid + Glutamic acid
Glycine + a-keto glutaric acid
- The glycine is transported out of peroxisomes into mitochondria, where two molecules of glycine interact to form one molecule each of serine, CO2 and NH3 –
2 Glycine + H2O + NAD+
Serine + CO2 + NH3 + NADH
The CO2 is then released in photorespiration from mitochondria. The NH3 released during glycine decarboxylation is transported to cytoplasm or chloroplast, where it is incorporated into synthesis of glutamic acid.
- The amino acid serine returns to peroxisome where it is deaminated and reduced to hydroxypyruvic acid and finally to glyceric acid –
Serine + Glyoxylic acid
Hydroxypyruvic acid + Glycine Hydroxypyruvic acid
- The glyceric acid finally enters the chloroplast where it is phosphorylated to 3-phosphoglyceric acid, which enters into C3 cycle –
Glyceric acid + ATP
- Phosphoglyceric acid + ADP +
Importance of photorespiration : The process of photorespiration interferes with the successful functioning of Calvin cycle. Photorespiration is quite different from respiration as no ATP or NADH are produced. Moreover, the process is harmful to plants because as much as half the photosynthetically fixed carbon dioxide (in the form of RuBP) may be lost into the atmosphere through this process.
Any increase in O2 concentration would favour the uptake of O2 rather than CO2 and thus, inhibit photosynthesis for this rubisco functions as RuBP oxygenase. Photorespiration is closely related to CO2 compensation point and occurs only in those plants which have high CO2 compensation point such as C3 plants.
It is absent in plants which have very low CO2 compensation point such as maize, sugarcane (C4 plants). Photorespiration generally occurs in temperate plants. Few photorespiring plants are : Rice, bean, wheat, barley, rice etc. Inhibitors of glycolic acid oxidase such as hydroxy sulphonates inhibit the process of photorespiration. Unlike usual mitochondria respiration neither reduced coenzymes are generated in photorespiration nor the oxidation of glycolate is coupled with the formation of ATP molecules. Photorespiration (C2 cycle) is enhanced by bright light, high temperature, high oxygen and low CO2 concentration.
Differences between photorespiration, photosynthesis and true respiration
|(1)||Occurs in green plants in light.||Occurs in green plants in light.||Occurs in all living organisms in light and dark.|
|(2)||The primary substrate is
glycolate formed from RuBP.
|Substrate is CO2 and H2O.||Substrates are carbohydrates, fat and proteins.|
|(3)||Occurs in most of the C3 plants.||Occurs in all green plants.||Occurs in all living organisms.|
|(4)||Intracellularly, the process occurs in peroxisomes in association with
chloroplasts and mitochondria.
|Occurs in chloroplasts.||Occurs in cytosol and mitochondria.|
|(5)||The process increases with
increasing concentration of O2 and decreasing concentration of CO2.
|The process is inhibited with increasing concentration of O2.||The process saturates at 2-3% O2 in the
atmosphere and beyond this conc, virtually no increase occurs.
|(6)||Hydrogen peroxide is formed
during this process.
|H2O2 is not formed.||H2O2 is not formed.|
|(7)||Phosphorylation does not occur.||Photophosphorylation occurs.||Oxidative phosphorylation occurs.|
CO2 compensation point : In photosynthesis, CO2 is utilized in presence of light to release O2 whereas in respiration, O2 is taken and CO2 is released. If light factor is saturating, there will be certain CO2 concentration at which rate of photosynthesis is just equal to rate of respiration or photosynthesis just compensates respiration or apparent photosynthesis is nil. It is called CO2 compensation point. Rate of photosynthesis is higher than that of respiration during day time and ratio of O2 produced to that consumed is 10 : 1.
CO2 compensation point is very low in C-4 plants, i.e., 0 to 5 ppm whereas high CO2 compensation point is found in C-3 plants, i.e. 25 to 100 ppm.
During compensation point there is no evolution of any gas.
Adenosine triphosphate (ATP).
A molecule of Adenosine is formed by reaction between a molecule of adenine (A nitrogenous base) and sugar D-ribose (A pentose sugar). Adenosine is a nucleoside. Adenosine monophosphate (AMP = Adenylic acid) is formed by condensation of a phosphate group at CH2OH site of 5th carbon atom of deoxyribose sugar.
With the formation of this bond (represented by –) between sugar and phosphate energy of 1500-1800 cal./mol is stored. This is low energy bond. When next group of phosphate is attached to AMP, Adenosine diphosphate (ADP) is formed. In this bond 7300 cal./mol of energy is stored and this bond is represented by wavy lines (~). This is high energy bond. In the same way when third phosphate group is attached to ADP, ATP is formed. This third bond is also represented by wavy line (~) and the energy stored is equal to the second bond.
Adenine Ribose Three phosphate radicals
In photochemical reactions of photosynthesis 18 ATP molecules are synthesized. Out of these 18 molecules of ATP, 6 react with ribulose monophosphate to form ribulose-1,5-biphosphate and the remaining 12 molecules react with 12 mols of 3-phosphoglyceric acid to form 12 mols of 1,3-diphosphoglyceric acid. ATP synthesized in cyclic and noncyclic photophosphorylation is utilized in dark reaction of photosynthesis.
Functions of ATP : In living cells energy yielding and energy consuming reactions take place continuously. By release of energy from one substance (e.g., glucose) another substance, e.g., protein is synthesized. By release of energy from proteins other activities of plants can be carried out. There is a mechanism of temporary storage of energy in the cells. This is ATP. This chemical is extremely important for all living cells. Energy released as a result of oxidation of carbohydrates, proteins and fats is utilized in the synthesis of ATP (from ADP and inorganic phosphate). This method of synthesis of ATP in respiration is called oxidative phosphorylation which is essential for various other synthetic activities, e.g., synthesis of carbohydrates, fats, proteins and osmosis, active absorption, translocation of foods, streaming of protoplasm, growth, etc. In this way by taking out energy from one compound and transferring it to another, ATP, functions as an intermediary compound of energy transfer. This is why ATP is called as monetary system of energy exchange in living organisms.
Like green plants, some purple and green sulphur bacteria are capable of synthesizing their organic food in presence of light and in absence of O2, which is known as bacterial photosynthesis.
Van Niel was the first to point out these similarities. Oxygen is liberated in bacteria during process of photosynthesis. Their photosynthesis is non-oxygenic. Because bacteria use H2S in place of water (H2O) as hydrogen donor. Photosynthetic bacteria are anaerobic. Only one type of pigment system (PSI) is found in bacteria except cyanobacteria which possess both PSI and PSII. Bacteria has two type of photosynthetic pigments.
- Bacterial chlorophyll
- Bacterio viridin
The photosynthetic bacteria fall under three categories :
- Green sulphur bacteria : They are The hydrogen donor is H2S and the pigment involved in the process is chlorobium chlorophyll (Bacterioviridin) e.g. Chlorobium.
- Purple sulphur bacteria : They are also The hydrogen donor is thiosulphate and the pigment involved in photosynthesis is bacteriochlorophyll a. e.g., Chromatium.
- Purple non-sulphur bacteria : They are heterotrophic utilizing succinate or malate or alcoho
e.g., Rhodospirillum, Rhodopseudomonas.
Characteristics of bacterial photosynthesis are :
- No definite chloroplasts but contain simple structures having pigments called chromatophores (term coined by Schmitz).
- Contain chlorobium chlorophyll or bacterio-chlorophyll.
- Use longer wavelengths of light (720-950nm).
- No utilization of H2O (but use H2S or other reduced organic and inorganic substances).
- No evolution of O2.
- Photoreductant is NADH2 (Not NADPH2).
- Only one photoact and hence one pigment system and thus one reaction centre, e., P890.
- Cyclic photophosphorylation is
- It occurs in presence of light and in absence of O2.
Some forms of bacteria obtain energy by chemosynthesis. This process of carbohydrate formation in which organisms use chemical reactions to obtain energy from inorganic compounds is called chemosynthesis. Such chemoautotrophic bacteria do not require light and synthesize all organic cell requirements from CO2 and H2O and salts at the expense of oxidation of inorganic substances like (H2, NO3–, SO4 or carbonate). Some examples of chemosynthesis are :
- Nitrifying bacteria : These bacteria oxidises ammonia to nitrites and release chemical energy. g. Nitrosomonas, Nitrococcus etc.
- Sulphur bacteria : Convert H2S to e.g, Beggiatoa, Thiothrix and Thiobacillus.
- Iron bacteria : Oxidises ferrous to ferric g. Ferrobacillus, Leptothrix and Cladothrix.
Fe 2+ (Ferrous)
- Hydrogen bacteria : g. Bacillus pentotrophus
- Carbon bacteria : Convert carbon monoxide to carbon e.g., Carboxydomonas, Bacillus oligocarbophilus.
Factors affecting photosynthesis.
l Blackmann’s law of limiting factors
F.F. Blackmann (1905) proposed the law of limiting factors according to which ‘when process is conditioned to its rapidity by a number of factors, the rate of process is limited by the pace of the slowest factor’. Blackmann’s law of limiting factor is modification of Leibig’s law of minimum, which states that rate of process controlled by several factors is only as rapid as the slowest factor permits. Theory of three cardinal points was given by Sachs in 1860. According to this concept, there is minimum, optimum and maximum for each factor. For every factor, there is a minimum value when no photosynthesis occurs, an optimum value showing highest rate and a maximum value, above which photosynthesis fails to take place. The law can be explained best by the following illustration :
Fig : The concept of three cardinal points
A CO2 concentration
Fig : Blackman’s law of limiting factor
Light intensity provided to a leaf is just sufficient to permit it to utilize 5 mg of CO2. At ‘A’ no photosynthesis occurs due to non-availability of CO2. If concentration is increased from 0 to 1 mg, rate of photosynthesis will increase from ‘A’ to ‘C’. Now even if the CO2 concentration is further increased to 5 mg rate becomes constant. Further increase from ‘C’ to ‘E’ is possible only when light intensity is increased, which is at this time working as limiting factor. Because the factor which is quantitatively smaller may not be limiting one, while a factor which is relatively less than the amount actually required will act as limiting factor. That is why many modifications in name have been suggested e.g. ‘Law of relatively limiting factor’ or ‘Law of most significant factor’.
Factors : The rate of photosynthetic process is affected by several external (Environmental) and internal factors.
- External factors : These include light, temperature, CO2, water and
- Light : The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth about 2% is used in photosynthesis and about 10% is used in other metabolic Light varies in intensity, quality (Wavelength) and duration. The effect of light on photosynthesis can be studied under these three headings.
- Light intensity : The total light perceived by a plant depends on its general form (viz., height, size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is
In general, rate of photosynthesis is more in intense light than diffused light. (Upto 10% light is utilized in
sugarcane, i.e., Most efficient converter).
Another photosynthetic superstar of field growing plants is Oenothera claviformis (Winter evening-primrose), which utilizes about 8% light.
However, this light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except very high light intensity where ‘Solarization’ phenomenon occurs, i.e., photo-oxidation of different cellular components including chlorophyll occurs.
It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO2 uptake is called light saturation point.
- Light quality : Photosynthetic pigments absorb visible part of the radiation e., 380mm to 760mm. For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light. The green light has minimum effect. On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.
- Duration of light : Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12hrs light per day it favours good Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.
- Temperature : The optimum temperature for photosynthesis is 20 to 35°C. If the temperature is increased too high, the rate of photosynthesis is also reduced by time factor which is due to denaturation of enzymes involved in the Photosynthesis occurs in conifers at high altitudes at 35°C. Some algae in hot springs can undergo photosynthesis even at 75°C.
- Carbon dioxide : Carbon dioxide present in the atmosphere is about 032% by volume and it is really a low concentration which acts as limiting factor in nature. If we increase the amount of CO2 under laboratory conditions and if the light and temperature are not the limiting factors, the rate of photosynthesis increases. This increase is observed upto 1% of CO2 concentration. At the same time very high concentration of CO2 becomes toxic to plants and inhibit photosynthesis.
- Water : Water is an essential raw material in photosynthesis. This rarely, acts as a limiting factor because less than 1% of the water absorbed by a plant is used in photosynthesis. However, lowering of photosynthesis has been observed if the plants are inadequately supplied with
- Oxygen : Excess of O2 may become inhibitory for the Enhanced supply of O2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O2 is not a limiting factor of photosynthesis. An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. (Reported by German scientist Warburg (1920) in Chlorella algae).
This is due to competitive inhibition of RuBP-carboxylase by increased O2 levels, i.e., O2 competes for active sites of RuBP-carboxylase enzyme with CO2. The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C3 cycle and no change in C4 cycle.
- Pollutants and Inhibitors : The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill reaction. Diquat and Paraquat (Commonly called as Viologens) block the transfer of electrons between Q and PQ in II. Other inhibitors of photosynthesis are monouron or CMU (Chlorophenyl dimethyl urea) diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil and atrazine etc. which have the same mechanism of action as that of viologens.
At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.
- Minerals : Presence of Mn++ and Cl– is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) Mg++, Cu++ and Fe++ ions are important for synthesis of
(2) Internal factors
- Protoplasmic factors : There is some unknown factor which affect the rate of
These factors effect the dark reactions. The decline in the rate of photosynthesis at temperature above 30°C or at strong light intensities in many plants suggests the enzymatic nature of this unknown factor.
- Chlorophyll content : Chlorophyll is an essential internal factor for photosynthesis. The amount of CO2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it The assimilation number of variegated variety of a species was found to be higher than the green leaved variety. Emerson (1929) also found a direct relationship between chlorophyll contents and photosynthetic rate in Chlorella.
- Accumulation of products : The food is largely prepared in the mesophyll cells of the leaf from where it is translocated to storage If the rate of translocation becomes slower than the rate of manufacture, the former declines due to accumulation of end products.
- Structure of leaves : The amount of CO2 that reaches the chloroplast depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching in the chloroplast.
Significance of photosynthesis.
- Synthesis of food : Body of all living organism and their survival is dependent upon foods (Carbohydrates, fats and proteins). They need energy for different life activities which is derived from foods. Green plants are unique in the character that they are able to synthesize foods for all living
- Purification of atmosphere : By oxidation of carbohydrates, fats and proteins CO2 is released along with energy. Coal, petrol and many other type of oils release CO2 when they are used in different industries. CO2 so released is added to the atmosphere and would have proved harmful to living organisms, but in photosynthesis green plants take in CO2 and release O2 thus purifying the
- Conversion of radiant energy : It changes radiant energy into chemical energy. All organisms use chemical energy for their
- Plant products : A number of useful products are obtained from plants, they are synthesized by plants through Important plant products are fire wood, timber, oils, gums resins, rubber, cork, tannins, alkaloids or drugs, fibres, etc.
- Productivity : Rising of photosynthetic capacity will reduce the effect of excess carbon dioxide It will increase crop productivity for feeding the rising human and cattle population. Therefore, methods of photosynthetic enhancement are being studied.
Experiment : 1
Ganong’s light screen : This simple experiment confirms that light is necessary for photosynthesis. It is a metallic structure with a specific cut out. When a destarched leaf is covered by the screen and placed in sunlight, photosynthesis occurs. The leaf is then taken out, treated with ethanol and then with iodine. Only the exposed parts of the leaf turn blue. The covered parts remain unstained as no starch could be formed there due to non- availiability of light.
Experiment : 2
Moll’s half leaf experiment : This experiment is designed
Fig : Ganong’s light screen to study the effect of light on the photosynthesis
to prove that CO2 is necessary for photosynthesis. A plant is destarched by keeping in dark. A leaf of this plant is half inserted in a vial or bottle containing some KOH solution through a split cork. If the leaf is detached, its petiole should be dipped in a petridish containing water. The apparatus is kept in sunlight. After a few hours the leaf is taken out and put in ethanol for removing chlorophyll. It is, then treated with iodine. The part of the leaf lying outside the cork is stained blue confirming the occurrence of photosynthesis in that region. The part of the leaf that lies inside the vial remains unstained because no photosynthesis occurs in that part due to non-availability of CO2. The part which lies in between the cork pieces also remains unstained because it neither gets light nor CO2.
Green or chlorophyllous portion
Non-green or colourless portion
Fig : Experiment to show that chlorophyll is essential for photosynthesis (A) Variegated leaf before experiment, (B) Variegated leaf after treatment with iodine solution
Experiment : 3
To show that chlorophyll is necessary for photosynthesis : Select a potted Croton or Coleus plant having variegated leaves. Select a few young leaves and sketch the extent of the green as well as other colours of these leaves on a piece of paper. Place the pot in the sunlight for a few hours and then take starch test. Only chlorophyll containing cells give positive starch test.
Experiment : 4
To show that oxygen is evolved in photosynthesis by green plants : Water-weeds like Hydrilla or
Ceratophyllum are best for this experiment. Take some water-weeds and cut the bases of the plant and tie them with a thread. Put them in a beaker containing water and invert a funnel over them as shown in the figure. Fill a test tube with water and invert it over the nozzle of the funnel so that no air-bubble gets in. Expose the whole apparatus to light.
It is seen that some bubbles come out continuously and are collected at the top of test tube by displacing the water.
On testing the gas it is found to be oxygen. This evolved oxygen is produced by the green aquatic plant in the process of photosynthesis.
Experiment : 5
To show that starch is formed in photosynthesis : Detach a destarched green leaf from a plant. Boil the leaf in water and then boil in 70% ethyl alcohol for 15 minutes (In a water bath). The leaf becomes colourless as chlorophyll gets dissolved in alcohol. Wash the
Fig : Liberation of oxygen in photosynthesis by an aquatic plant
leaf with water and test it with iodine solution. It gives negative starch test. Now keep the plant in light for 8 hours and again test it for starch. The leaf becomes bluish black or bluish-purple indicating the presence of starch.
- Photosynthetic Materials : 264 gm of CO2 and 216 gm of water give rise to 108 gm of water, 192 gm of O2 and 180 gm of
- Rubisco : Rubisco constitutes 16% of chloroplast It is the most abundant protein on this planet.
- Actual reduction of CO2 to carbohydrates is independent of light, e., occurs in presence or absence of light, but production of assimilatory powers (ATP and NADPH2) needs light and is light dependent.
- Willmott’s bubbler is used to measure rate of O2 evolution or rate of photosynthesis.
- W. Engelmann (1882) experimentally verified that in monochromatic lights, photosynthesis is maximum in red light.
- Cyclic photophosphorylation is the most effective anaerobic phosphorylation
- NADP (Nicotinamide adenine dinucleotide phosphate) was earlier called as TPN (Triphosphopyridine nucleotide),
- In green plants the hydrogen acceptor is NADP, but in bacteria it is
- No Emerson effect is seen in
- NAD is considered to be the “Universal hydrogen acceptor”.
- Non-cyclic photophosphorylation or Z-scheme is inhibited by CMU and DCMU.
- As Calvin cycle takes in only one carbon (as CO2) at a time, so it takes six turns of the cycle to produce a net gain of six carbons (i.e., hexose or glucose).
- Cytochromes : The terms was coined by Keilin (1925) though the biochemicals were discovered by Mac Munn (1866).
- Intensity of light can be measured by Luxmeter.
- Isolated chlorophyll ‘a’ in pure form emits red It is called fluoresence.
- Phytochrome is a proteinaceous pigment found in low concentrations in most plant Which absorbs red (PR or P660) and far red (PfR or P730) light.
- Anthoxanthins and Anthocyanin pigments are also soluble in water and found in cell sap, due to which white, yellow and orange colour produce in