DEFINITION
Photosynthesis (Gk.photon = light; Synthesis putting together) is the most important anabolic = process on earth by which green plants (autotrophic organisms) synthesize complex carbohydrates from simple substances like carbon dioxide and water with the help of light energy and purify the atmospheric air by consuming carbon dioxide and evolving oxygen. The process of photosynthesis can also be defined as the transformation of photonic energy (i.e., light or radient energy) into chemical energy (locked in high energy bonds of carbohydrate molecules) by the green parts of the plants
EQUATION: The simple overall equation usually given to represent photosynthesis is as follows: LightChlorophyll 6 CO2 + 6 H2O———————–>C6H12O6+6O2
The above equation represents early model of photosynthesis when it was presumed that molecular oxygen came from carbon dioxide and not from water. It was Van Niel (1931), a graduate student, who observed that during bacterial photosynthesis hydrogen sulphide was broken down to hydrogen and sulphur. The hydrogen reduces carbon dioxide to synthesize carbohydrates and sulphur is accumulated as a waste product
12 H2S + 6 CO₂———–>C6H12O6+12S+6 H₂O
This led to the postulation that in higher plants water is utilized in the place of H2S and O2 is evolved in the place of sulphur. Nearly 10 years later, this hypothesis was confirmed by isotopic studies made by Ruben and Kamen (1941). They used the radioactive isotope of oxygen in water and found that the O₂ liberated in the process was isotopically labelled. On the contrary, when 180 in CO2 was used, the evolved O₂ did not show radioactivity. Thus, the overall equation of photosynthesis is currently accepte
MAGNITUDE OF PHOTOSYNTHESIS:
The concentration of gaseous carbon dioxide (CO2) in the atmosphere is about 0.03% by volume. At this rate, the atmosphere of our earth contains about 2200 billion tonnes of CO2 which is sufficient to support photosynthesis for a few hundred years, even if no further amount is added. The CO2 present in the oceans in the form of dissolved gas or carbonates, is estimated about 11.2 x 1014 tonnes which is more than 50 times of the amount present in the atmosphere.
Green plants fix about 70-80 billion tonnes of carbon dioxide annually by the process of photosynthesis. Out of total CO2 fixed, about 90% is fixed by water plants (mainly in oceans) and less than 10% is fixed by terrestrial plants. Curiously, only 0.2% of the total solar energy falling on earth’s surface is utilised by photosynthetic organisms, yet this amount of trapped energy is just sufficient to meet the entire food requirement of all living or- ganisms on earth.
The Chloroplast
– (The Site for Photosynthesis)
Chloroplasts (Gk. Chloros = green, plastos = moulded) are the green plastids which occur in all the green parts of the plants. They are the actual sites of photosynthesis (Fig. 3.1). The maximum number of chloroplasts occur in leaves with over half a million per square millimetre. They occur mostly in the chlorenchymatous cells (particularly in mesophylls) of leaves and young stems.
Eachmesophyll cell may contain as many as 300 chloroplasts in its peripheral cytoplasm. In algae and bryophytes, they occur in almost all the green cells. Algal chloroplasts vary in shape and size. They are spiral in Spirogyra, collar shaped in Ulothrix, star-shaped in Zygnema, cup-shaped in Chlamydomonas, bell-shaped in Chlorel- la and reticulate in Oedogonium. Chloroplasts of higher plants are usually discoid, ellipsoidal or biconvex lens-shaped.
They are usually 4-10 um in diameter and 1-3 μm in thickness. Each chloroplast of higher plant. is an organelle with an outer envelope consisting of a double membrane which acts as a selective barrier to the movement of cellular metabolites into or out of the chloroplast. The chloroplast envelope encloses liquidy a proteinaceous matrix called stroma.
The whole liquidy medium of stroma is hyaline, slightly electron-dense, granular matrix and contains all the necessary enzymes of photosynthesis. It also contains 70S ribosomes, circular DNA (ct-DNA) and RNA. The chloroplastic DNA is called plastidome. The stroma is the site of dark reac- tion of photosynthesis.The lamellar system within the stroma forms flattened sac like lamellae called thylakoids (Gk.thylakos = suc, oid = like). These thylakoids are stacked in some places to form grana.
The grana stacks are interconnected by membranous lamellae called stroma lamellae or Fret channels. The space within each thylakoid (loculus) is connected to the space within every other thylakoid forming a common thylakoid space. The major function of thylakoids is to perform photosynthetic light reaction (photochemical reaction). The pigments and other factors of light reaction are usually located in thylakoid membranes.
Cyanobacteria and other photosynthetic bacteria do not possess chloroplasts However, the photo- synthetic pigments of these organisms are located on thylakoids which lie freely in the ments are also different of cytoplasm, Their photosynthetic pig-from those eukaryotes.Location of photosynthetic pigments in chloroplast: The major photosynthetic pigments are chlorophyll chlorophyll, and carotenoids (carotenes and xanthophylls). It has been shown that chlorophyll molecules are non-covalently as-sociated with proteins forming pigment-protein complexes.
They are located in the thylakoid membranes of chloroplast (Fig. 3.3). There are four types of complexes – (i) Photosystem I (PSI) complex; (ii) Photosystem II (PS II) complex; (iii) Cyt. b-f complex and (iv) ATPase complex. The pigment molecules associated with PSI complex are located in the non-appressed parts as well as stroma thylakoids whereas those associated with PSII complex are located more towards the appressed regions of the grana thylakoids.
The Chloroplast Pigments
Pigments are the organic molecules that ab- sorb 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 aigae and cyanobacteria are phycobilins.
(1) Chlorophylls .
: Chlorophylls (Gk. Chlor green, phyllo = leaf) are the green photosynthetic pigments present in all photosynthetic autotrophic organisms, except bacteria. There are about ten chlorophylls-Chlorophylls a, b, c, d and e; Bac- types of teriochlorophyll a, b, c and d ; and Bacterioviridin, Of all, only two types i.e., chlorophyll a and chlorophyll b are widely distributed in green algae and higher plants. A chart of their distribution is given in the next page. Chlorophyll ‘a’ is found in all the oxygen evolving photosynthetic plants.
It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (eg, Chl a680 or P680. Chl #700 or P700 etc.). It directly takes part in photochemical reactions. Hence, it is termed as primary photosynthetic pigmen 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 the photochemical act.
They absorb specific wavelengths of light and transfer energy finally to chlorophyll a through electron spin resonance. Chlorophyll a is bluish-green whereas chlorophyll b is olive green in pure state. They are soluble in organic solvents. Chlorophyll a is more soluble in petrolium ether whereas chlorophyll b is more soluble in 92% methanol. All the chlorophyll molecules absorb light near both ends of the visible spectrum. They transmit or reflect green light and, therefore, appear green in colour.
The structure of chlorophyll molecule was first studied by Wilstatter, Stoll and Fischer in 1912. Each chlorophyll molecule shows a tadpole-like configuration consisting of a porphyrin head and a phytol tail. The porphyrin head is made up of por- phyrin system in which four pyrrole (tetrapyrrole) rings, linked together by methane groups, form a ring system.
The centre of tetrapyrrole is occupied by divalent magnesium (Mg++) which is complexed with the nitrogen atoms of the four pyrrole rings. Each chlorophyll has a fifth ring containing only carbon atoms. The porphyrin ring has several side groups which specify the properties of the pigment molecule.The phytol tail is made up of 20 carbon alcohol and bound in ester linkage to the 4th pyrrole ring (Chle being an exception).
The phytol chain is responsible for lipoidal solubility of the chlorophylls.The chlorophyll b differs from chlorophyll a in having an aldehyde (-CHO) group instead of a methyl (CH3) group on ring II of the porphyrin head.
(2) Carotenoids:
Carotenoids are yellow, brown, orange or red-dish pigments usually found in close association of chlorophylls in all photosynthesizing cells. They occur in thylakoids and act as accessory pigments of photosynthesis. They absorb light energy in the mid-region of visible spectrum and transfer their absorbed energy to chlorophyll molecules. They pick up nascent oxygen, released during photooxidation of water, and change them into the molecular state.
Thus, they protect chlorophyll molecules from photooxidation. The carotenoids also occur inside the chromoplasts in flowers and fruits and make them conspicuous to animals for pollination and dispersal. In general, there are two major groups of carotenoids: the carotenes and the xanthophylls. Both of them are soluble in organic solvents.
(i) Carotenes:
Carotenes are unsaturated hydrocarbons with a general formula of C40H56 They absorb blue and green lights and appear yellow and red in colour. The most common carotene is B-carotene (Fig. 3.5) which is converted to vitamin A by animals and human beings- C40H56+2H2O2C19H27CH₂OH10B-carotene vitamin A lot
(ii) Xanthophylls
(= Carotenols) : xanthophylls (e.g., lutein, zeaxanthin, etc.) are oxygen derivatives of carotenes containing 1-8 oxygen atoms. Lutein is responsible for yellow colour in autumn foliage.
(3) Phycobilins:
The phycobilins constitute a major group of photosynthetic pigments occurring in blue-green algae (Cyanobacteria), cryptomonads and red algae. They are protein-linked pigments (ie of biliproteins) destroyed by heat. These water-soluble pigments are thought to be localized in small granules, called phycobilisomes, attached to thylakoids.
They are useful in chromatic adaptations. Like chlorophyll, these pigments are open tetrapyrrole but do not contain magnesium and phytol chain . They absorb light and help in photosynthesis as accessory pigments. There are two types of phycobilins- (i) Phycocyanin and (ii) phycoerythrin. Blue-green algae (cyanobacteria) have more quantity of phycocyanin whereas red algae have more phycoerythrin.
Activity-. To extract the chloroplast pigments and separate them by paper chromatography.
Requirements:
Spinach leaves, Pestle and mortar, 80% acetone, calcium carbonate, Buchner funnel, beaker, measuring cylinder, glass jar with a tight cork, Whatman No. 1 filter paper, petroleum ether, acetone, hook, micropipette..
Method:
Take 50 gms of fresh spinach leaves in a Pestle and morter. Crush them with 20 ml of 80% acetone. Add a pinch of calcium carbonate and again crush. Filter the extract on a Buchner filter. The deep green-coloured filtrate containing chloro- phylls and carotenoids is obtained. Evaporate the extract to concentrate.Take a glass jar (about 45 cm high) with a tight cork fitted in it. The cork should have a hole in the centre. Fit a small glass rod, having a small hook, in the hole of cork. Now prepare the solvent by mixing 25 ml petroleum ether and 3 ml acetone.
Pour the solvent into the jar and allow the jar to become saturated. Cut a strip of filter paper of the size which can easily be hanged on the hook. Apply a circular spot of pigment extract about 3 cm from the base of strip with the help of a micropipette. Now hang the strip inside the jar to the hook of cork and close the cork. Care should be taken that the spot is not dipped in the solvent. Make the apparatus airtight and observe.
Observation:
The solvent will run on the filter paper. After few hours, the chloroplast pigments will be separated in the form of different spots on the paper (Fig. 3.7). Take out the paper when the solvent reaches upto upper level. After drying the paper, identify the different pigments with the help of their specific colours. Carotene is yellow, xanthophyll is yellow-brown, chlorophyll a is blue-green and chlorophyll b is olive green in colour.
