Chapter 19 Photosynthesis in Higher Plants
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
Chlorophyll
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
Chlorophyll
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.
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.
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
Stroma
Double
Stroma lamellae
chloroplast is permeable and an inner one impermeable to
protons. Inside the membranes is the proteinaceous ground
membrane Osmiophilic droplets
Fret channel
Starch
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.
The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.
Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven types i.e., chlorophyll a, b,
c, d, e, bacteriochlorophyll and bacterioviridin.
H CH2
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
H3C
H H3C
C H CH3
A B C2H5
N N
Mg H
N N
Chlorophyll b
photosynthetic plants except photosynthetic bacteria.
D C
H H E
CH3
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
| CH2
| O=C
|
O
| CH2
| CH
|
COOCH3
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.
C– CH3
| (CH2)3
| HC–CH3
| (CH2)3
| HC–CH3
| (CH2)3
| CH
CH3 CH3
Chlorophyll a
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.
|
Pigments |
Chemical Formula |
Distribution |
|
Chlorophyll a |
C55H72O5N4Mg |
All photosynthetic organisms except photosynthetic bacteria. |
|
Chlorophyll b |
C55H70O6N4Mg |
Chlorophyta, Euglenophyta and in all higher plants. |
|
Chlorophyll c |
C35H32O5N4Mg |
Brown algae (Phaeophyta), Diatoms and Pyrrophyta. |
|
Chlorophyll d |
C54H70O6N4Mg |
Red algae (Rhodophyta). |
|
Chlorophyll e |
Not fully known |
Xanthophyta. |
|
Bacteriochlorophyll |
C55H74O6N4Mg |
Purple photosynthetic bacteria. |
|
Chlorobiumchlorophyll (Bacterioviridin) |
|
Green sulphur bacteria. |
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
Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).
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.
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 |
Sound |
|
|
||||||
Solar rays
Gamma rays
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
l=400
500
600
Visible light
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
Violet
Blue Green Yellow Orange Red
Fig : Electromagnetic spectrum of light
Infrared
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
Maximum energy
¾® Longest wavelength
Minimum energy
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
180
160
140
120
100
80
60
40
20
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.
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
Chlorophyll (Formaldehyde)
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
Chlorophyll
The fate of different molecules can be summarised as follows :
Light
6CO2 + 12H 2 O ¾¾chlo¾rop¾h¾yll ® C6 H12 O6 + 6H 2 O + 6O2
Fig : Fat of different molecules
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.
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).
Q10
= Reaction rate of (t + 10)°C Reaction at t°C
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.
4 Fe 3+ + 2H 2 O ¬ ¾® 4 Fe 2+ + 4 H + + O2
Electron acceptor
Electron donor
Reduced Product
The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is known as Hill reaction.
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.
(i) Transfer of energy (ii) Quantum yield (iii) Emerson effect (iv) Two pigment systems
(v) Z-scheme (vi) Cyclic and non-cyclic photophosphorylation
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.
Photon
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
Chemical
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
Internal
conversion
Heat/radiation
¯
(Fluorescence)
Triplet state
reaction
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.
e–
Ground state
Heat/radiation
¯
(Phosphorescence)
Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.
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 photosynthesis."
On the other hand quantum requirement is defined as, "Number of quanta of light required for evolution of one mol of O2 in photosynthesis."
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.
0.10
0.08
0.06
0.04
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