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Photosynthesis

The term "photosynthesis" means synthesis of food with the help of light energy. It is defined as the conversion of light energy into chemical energy in the presence of carbon dioxide, water and photosynthetic pigments.

The overall equation of photosynthesis is

The term "photosynthesis" means synthesis of food with the help of light energy. It is defined as the conversion of light energy into chemical energy in the presence of carbon dioxide, water and photosynthetic pigments.

The overall equation of photosynthesis is

PHOTOSYNTHESIS

Green plants are said to be autotrophic in their mode of nutrition since they are capable of manufacturing their own food materials. Plants like fungi are described as heterotrophs since they directly or indirectly depend upon the autotrophs for their nutritional requirements. It must be noted that some of the bacteria also come under autotrophs. They are classified into photosynthetic and chemosynthetic autotrophs. You have already learnt about bacterial photosynthesis.

Types of photosynthesis

1. Oxygenic photosynthesis: The organisms release oxygen during photosynthesis. Water is the electron donor, e.g. higher plants.

2. Non-oxygenic photosynthesis: The organisms release compounds other than oxygen. Hydrogen sulphide is the electron donor, e.g. bacteria.

History of Photosynthesis

1845 Von Mayer recognized that green plants convert solar energy into Chemical energy.

1932 Emerson and Arnold discovered the light reactions of Photosynthesis.

1937 Hill demonstrated photolysis of water.

1954 Melvin Calvin explained the C3 cycle of carbon fixation.

1965 Hatch and Slack explained the C4 cycle of carbon fixation.

Raw materials for photosynthesis

Solar energy, photosynthetic pigments, carbon di oxide and water are the raw materials for photosynthesis. Solar energy is obtained from sun light. Less than one percent of solar energy is only used by plants for photosynthesis.

The photosynthetic pigments are present in the chloroplast The green terrestrial plants obtain water from the soil through the roots and carbon di oxide from the air through the stomata. Aquatic plants obtain carbon dioxide as bicarbonates from the water.

Site of Photosynthesis

Leaves are the main photosynthetic organs of the plant and the chloroplasts is the photosynthetic apparatus. The Chloroplasts are disc shaped organelles of eukaryotic plant cells. Each chloroplast measures 4-10 microns in diameter and 2-3 microns in thickness. It has a double membranous envelope. The matrix enclosed by the membranes is called stroma. It is traversed by a number of flattened membranous sacs called thylakoids. A stack of thylakoids appears like a pile of coins and is called granum. The adjacent grana are connected by fret membranes or stromal lamellae. The pigment molecules are located in the grana. Light reaction of photosynthesis takes place in the grana. The stroma of the chloroplast contains enzymes and substrates necessary for the dark reaction of photosynthesis. The stroma also contains circular DNA, RNA and ribosomes.

PHOTOSYNTHESIS

Pigments involved in Photosynthesis

The chloroplasts contain two kinds of pigments, primary pigments and accessory pigments. Chlorophylls are the primary pigments. They occur in different types - chlorophyll a, b, c, d and e. Chlorophyll is an universal pigment found in all photosynthetic organisms except bacteria. Hence they are called primary photosynthetic pigments. The other pigments are called accessory pigments, e.g., Carotenoids (carotenes and xanthophylls). Carotenes are orange pigments and xanthophylls are yellow pigments. Primary pigments are directly involved in photosynthesis whereas accessory pigments absorb the light energy and transfer it to the primary pigments for photo chemical reaction. They do not directly carry out the photo chemical reactions.


Mechanism of Photosynthesis

The process of photosynthesis occurs in two phases, the photochemical (light) reaction and carbon assimilation (dark) reaction. The light reaction or photochemical phase takes place in the grana of the chloroplast. The dark reaction takes place in the stroma of the chloroplast. This is also called photosynthetic carbon reduction pathway (PCR-pathway).

Light reaction

This is a light induced reaction. Photolysis and Photophosphorylation occur during this phase. Based on the involvement of primary and accessory pigments, Emerson (1957) proposed two pigment systems, which participate in the light reaction. They are PS I and PS II (Photosystem I and Photosystem II).

Photosystem I

It consists of more primary pigments - chlorophylls and less of accessory pigments. The reaction center is a special chlorophyll a molecule called P700. It is fed with energy harvested by the other molecules of the system.

Photosystem II

This is mainly composed of more accessory pigments and less primary pigments. The reaction center of PS II is P680, which consist of a special type of chlorophyll a molecule.

Photophosphorylation

Synthesis of ATP in the chloroplast utilizing light energy is called Photophosphorylation. The pigment molecules absorb photons when light energy is incident on them. They emit energized electrons. These electrons pass through a number of electron carriers. During this transport they lose energy and reach the ground state. This energy loss is channelised for the synthesis of ATP and reduction of NADP. Two types of photophosphorylation have been identified by Arnon(1954)

Cyclic Photophosphorylation

This involves only PS I. The pigment molecules of PS I (Photosystem I) absorb light energy and finally transfer it to the reaction center P700. It emits energized electrons, which is accepted by Ferredoxin (FD). The electrons are cycled back to PS I through carriers called cytochromes. The energy loss during this process is linked to the production of ATP and is called photophosphorylation. It is described as cyclic photophosphorylation because the electron lost by PS I is cycled back to it. Two molecules of ATP are synthesized for transport of each pair of electrons during Cyclic Photophosphorylation. Cyclic photophosphorylation occurs during specific conditions only.

Non-Cyclic Photophosphorylation

PS I and PS II (Photosystem I & II) take part in non-cyclic photophosphorylation. The pigment molecules absorb light energy. The reaction centers P700 and P680 of PS I and PS II respectively emit energized electrons. The electron from PS I is accepted by Ferredoxin and used to reduce NADP to NADPH_2.

PHOTOSYNTHESIS

The electrons liberated from PS II and successively transported through the electron carriers, reaches PS I and compensates for the electron loss of PS I. During this transfer of electrons ATP molecules are synthesized (photophosphorylation ). PS II gets back its lost electron by photolysis of water.

This is light dependent splitting of water molecules. Protons released during photolysis are used in NADPH2 formation. Oxygen is evolved as a by-product. Since the electrons travel in a non-cyclic manner, the process is described as noncyclic photophosphorylation. For each pair of electrons transported during Non-cyclic photophosphorylation one ATP and one NADPH2 molecule is formed. ATP and NADPH2 are utilized in the dark reaction to reduce carbon di oxide to synthesize carbohydrates.

Photosynthetic Carbon Reduction - (PCR) pathway (Dark Reaction)

Light is not essential for this cycle of chemical reactions. It was discovered by Blackmann (1954). The reactions involve fixation of atmospheric carbon di oxide to form carbohydrates by using ATP and NADPH2 generated in the light reaction. Prof. Melvin Calvin and his associates traced the path of carbon di oxide and hence it is also known as Calvin's cycle. The cycle occurs in three stages.

PHOTOSYNTHESIS

Stage I - One molecule of carbon dioxide is accepted by a 5-carbon molecule. Ribulose - 1, 5-Bisphosphate (RUBP).to form an unstable six carbon compound. The enzyme catalyzing this reaction is RUBP carboxylase (RUBISCO). The six carbon compound splits into two molecules of a 3-carbon compound i.e. phospho glyceric acid. This the first stable product. Hence Calvin cycle is also known as C3 cycle. The plants having this mode of PCR pathway are called C3 plants, e.g.. bean, potato, wheat.

Stage II - During this stage the ATP and NADPtL, formed during the light reaction is utilised. Phosphoglyceric acid gets reduced to form Glyceraldehyde 3-phosphate. It takes place in two reactions. In the first reaction phosphoglyceric acid is converted into 1 -3 Bisphosphoglyceric acid with the help of the enzyme PGA kinase. In the second step, 1-3 Bisphosphoglyceric acid is reduced to form 3 phosphoglyceraldehyde with the help of the enzyme 3 Phosphoglyceraldehyde dehydrogenase.

Stage III - The molecules of Glyceraldehyde - 3-phosphate are converted into RUBP (the initial carbon dioxide accepter molecule). A series of reactions occur which generate 4c, 6c and 7c phosphorylated compounds as intermediates. A three-carbon compound is used for sugar synthesis. A continuous supply of ATP, NADPH2 and RUBP is required for continuation of calvin's cycle.

Significance of Photosynthesis

? Photosynthesis is the only process by which autotrophic organisms trap solar energy and convert it to food for heterotrophs. Only a few bacteria can synthesize food materials by chemosynthesis.

? Plant produce like timber, fibres, firewood, etc. are the products of photosynthesis in a plant.

? The process adds oxygen to the atmosphere, which compensates for the oxygen used in the respiration of living organisms.

? It provides energy in terms of fossil fuels like coal and petroleum. Thus all life on earth directly or indirectly depends on photosynthesis.

Mohl's Half Leaf Experiment

Aim: To prove that carbon di oxide is essential for photosynthesis.

A destarched potted plant is taken. The plant is destarched by keeping it in darkness for few days. A wide mouthed bottle containing a small quantity of potassium hydroxide solution is taken. A split cork is fitted to the mouth of the bottle. One of the leaves of the potted plant is inserted through the split cork such that half of the leaf is inside the bottle. The whole apparatus is kept in sunlight. The leaf is tested for starch after few hours.

PHOTOSYNTHESIS

Test for Starch

The leaf to be tested for starch is dipped in boiling water for 5 minutes. It is then dipped in 90% alcohol to decolorize it. It is washed with water. Few drops of Iodine solution are added to it. The starch containing part of the leaf will turn blue in colour. It is observed that the part of the leaf inside the bottle does not answer the starch test. This is because the c.abon di oxide inside the bottle was absorbed by potassium hydroxide and not available to the leaf for photosynthesis. The part of the leaf outside the bottle could make use of atmospheric carbon di oxide for photosynthesis. It turns blue in colour during the starch test. This proves that carbon di oxide is essential for photosynthesis.

Test Tube and Funnel Experiment

Aim: To prove that oxygen is evolved during photosynthesis.

A beaker is taken and filled with water. A few branches of Hydrilla (aquatic plant) are taken and inserted inside a glass funnel. The funnel is kept inverted inside the beaker. A test tube full of water is inverted over the stem of the funnel. The apparatus is kept in sunlight for few hours. Air bubbles are seen to collect in the test tube by the downward displacement of water. The test tube is slowly removed by closing the mouth with the thumb. The gas is tested for oxygen using a burning splinter. The splinter glows brightly proving that the gas is oxygen. Thus the gas evolved during photosynthesis is proved to be oxygen.

PHOTOSYNTHESIS

Points to Remember

• Chloroplasts function as sites of photosynthetic activity.

• Light reaction takes place in the grana and dark reaction takes place in the Stroma of chloroplast. Synthesis of ATP in the chloroplast utilizing light energy is called Photophosphorylation.

• Cyclic Photophosphorylation involves only PS I.

• Non-cyclic Photophosphorylation involves PS I and PS II.

• Splitting of water molecules during light reaction is called photolysis.

• Dark reaction was discovered by Melvin Calvin.

• Ribulose -1,5- Bisphosphate is the acceptor of atmospheric carbon di oxide.

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