Theory of Photosynthesis

In photosynthesis process, 𠆎nergy 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”.

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.

Historical background

Before seventeenth century it was considered that plants take their food from the soil.

$\bullet$ Van Helmont (1648) concluded that all food of the plant is derived from water and not from soil.

$\bullet$ 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 process.

$\bullet$ Joseph Priestley (1772) demonstrated that green plants (mint plant) purify the foul air (i.e., Phlogiston), produced by burning of candle, and convert it into pure air (i.e., Dephlogiston).

$\bullet$ 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 dark.

$\bullet$ 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 consumption.

$\bullet$ 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 released.

$\bullet$ 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 animals.

$\bullet$ Liebig (1845) indicated that main source of carbon in plants is CO₂.

$\bullet$ 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 sunlight.

$\bullet$ Julius Von Sachs (1862) demonstrated that first visible product of photosynthesis is starch. He also showed that chlorophyll is confined to the chloroplasts.

$\bullet$ 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 isotope.

$\bullet$ 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 work.

Photosynthesis in higher plants

Chloroplast (The site of photosynthesis) : Chloroplast are green plastids which function as the site of photosynthesis in eukaryotic photoautotrops.

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 :

(1) Chlorophylls : Chlorophyll ‘a’ is found in all the oxygen evolving photosynthetic plants except photosynthetic bacteria. Reaction centre of photosynthesis is formed of 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 bluish-green while chlorophyll b is olive-green. Both are soluble in organic solvents like alcohol, acetone etc. Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll a (C₅₅H₇₂O₅N₄Mg) possesses - CH₃ (methyl group), which is replaced by - CHO (an aldehyde) group in chlorophyll b (C₅₅H₇₀O₆N₄Mg). 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 are linked together by methine (CH =) groups forming a ring system.

When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 𠆌ytochrome’ which is used in photosynthesis (Photophosphorylation) and respiration both.

(2) Carotenoids : 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.

(i) Carotenes : They are orange red in colour and have general formula C₄₀H₅₆. They are isolated from carrot.

They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are $\alpha$, $\beta$, $\gamma$ and $\delta$ carotene phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. The $\beta$-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. $\beta$-carotene is black yellow pigment of carrot roots.

(ii) 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 (C₄₀H₅₆O₂) a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin (C₄₀H₅₆O₆) is another important xanthophyll present in Phaeophyceae (Brown algae).

(3) 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 chain.

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 waves. 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. 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, out of blue and red wavelengths, blue light carry more energy.

$\underset{\text{Maximum}\text{energy}}{\mathrm{Shortestwavelength}}\to \underset{\text{Minimum}\text{energy}}{\mathrm{Longestwavelength}}$

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 evolution or carbon dioxide utilisation is measured as a function of wavelength.

Mechanism of photosynthesis

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:

$6C{O}_2+12H_{2}S\to C_6{H}_{12}{O}_6+6H_{2}O+12S$

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.

$6C{O}_2+12H_{2}O\to C_6{H}_{12}{O}_6+6H_{2}O+6O_2$

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 :

$6C{O}_2+12{H_2}^{18}O\underset{\text{Chlorophyll}}{\stackrel{\text{Light}}{\to }}{C}_6{H}_{12}{O}_6+6^{18}{O}_2+6H_{2}O$

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.

Scientists have shown that photosynthesis is completed in two phases.

(1) 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⁺.

(2) 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 reactions.

Evidence for light and dark reactions in photosynthesis :

(1) 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.

(2) 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.

(3) Temperature coefficient studies : 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).

$Q_{10}=\frac{\text{Reaction}\text{rate}\text{of}(t+10{)}^{\circ }C}{\text{Reaction}\text{at}{t}^{\circ }C}$