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Detailed Table of the Chapter 13 Notes – Photosynthesis in Higher Plants Class 11 Notes PDF

Photosynthesis in Higher Plants Class 11 Notes pdf

Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is an enzyme-regulated anabolic process.

• Photosynthesis is the basis of life on earth because it is the primary source of all food on earth and it is responsible for release of  in the atmosphere.
• Chlorophyll, light and  is required for photosynthesis. It occurs only in the green part of leaves and in presence of light.

Early Experiments

• Joseph Priestley in 1770, on the basis of his experiments showed the essential role of air in growth of green plants. A mouse kept in closed space could get suffocated and die but if a mint plant is kept in bell jar neither candle will extinguish nor will the mouse die. He concluded that foul air produced by animal is converted into pure air by plants. Priestley discovered Oxygen gas in 1774.
• Julius Von Sachs in 1854 shows that green part in plants produces glucose which is stored as starch. Starch is the first visible product of photosynthesis.
• T.W.Engelmann (1843-1909) used prism to split light into its components and then illuminated Cladophora (an algae) placed in a suspension of aerobic bacteria. He found that bacteria accumulated in blue and red light of the split spectrum. He thus discovered the effect of different wavelength of light on photosynthesis (action spectrum).
• Cornelius Van Neil (1897-1985) on the basis of studies with purple and green sulphur bacteria showed that photosynthesis is a light dependent reaction in which hydrogen from an oxidisable compound reduces  to form sugar.

In green sulphur bacteria, when  , instead of  was used as hydrogen donor, no  was evolved. He inferred that  evolved by green plants comes from  but not from  as thought earlier.

Where Does Photosynthesis Takes Place?

• Chloroplasts are green plastids which function as the site of photosynthesis in eukaryotic photoautotrophs. Inside the leaves, chloroplast is generally present in mesophyll cells along their walls.
• Within the chloroplast there is a membranous system consisting of grana, the stroma lamellae and the fluid stroma.

• The membrane system is responsible for synthesizing light energy for the synthesis of ATP and NADPH. In stroma enzymatic reactions incorporate  in plants leading to synthesis of sugar.
• The reaction in which light energy is absorbed by grana to synthesis ATP and NADPH is called light reaction. The later part of photosynthesis in which  is reduced to sugar, light is not necessary and is called dark reaction.

Pigments involved in Photosynthesis – Chromatographic separation of leaf pigments are as follows-Maximum absorption by chlorophyll a occurs in blue and red regions having higher rate of photosynthesis. So, chlorophyll a is the chief pigment.

• Other thylakoid pigments like chlorophyll b, xanthophyll and carotenoids are called accessary pigments that absorb light and transfer energy to chlorophyll a and protect them from photo-oxidation.

Light reaction

• Light reaction(photochemical phase) includes:

1. Light absorption
2. Water splitting
3. Oxygen release
4. Formation of high energy chemical intermediates (ATP and NADPH).

• The pigments are organized into two discrete LHC( light harvesting complex) within photosystem I and photosystem II.
• LHC are made up of hundreds of pigments molecules containing all pigments except single chlorophyll a molecules in each PS.

• The pigments in photosystem I and photosystem II absorbs the lights of different wavelength. Single chlorophyll a molecule makes the reaction centre. In PS I reaction centre has highest peak at 700nm, hence called P700. And PS II reaction centre has highest peak at 680 nm, so called P680.

Photosynthesis in Higher Plants Notes PDF – Exam Oriented

The Electron Transport System

• Reaction centre of photosystem II absorbs light of 680 nm in red region and causing electron to become excited. These electrons are picked by an electron acceptor which passes to electron transport system consisting of cytochromes.

• Electrons are passed down the electron transport chain and then to the pigment of PS I.
• Electron in the PSI also get excited due to light of wavelength 700nm and are transferred to another accepter molecule having a greater redox potential.
• When electron passes in downhill direction, energy is released. This is used to reduce the ADP to ATP and NADP+ to NADPH. The whole scheme of transfer of electron is called Z-scheme due to its shape.
• Photolysis of water release electrons that provide electron to PS II. Oxygen is also released during this process.

Difference between cyclic and non-cyclic photophosphorylation

Chemiosmotic Hypothesis of ATP FORMATION

This hypothesis was proposed by Mitchell in 1961. ATP synthesis is linked to development of proton gradient across the membrane of thylakoid and mitochondria.

The process that causes development of proton gradient across the membrane is-

1. Splitting of water molecules occurs inside the thylakoid to produce hydrogen ion or proton.
2. As electron passes through the photosystems, protons are transported across the membrane because primary acceptor of electron is located towards the outer side the membrane.
3. TheNADP reductase enzyme is located in the stroma side of membrane. Electrons come out from the acceptor of electron of PSI, protons are necessary for reduction of NADP+ to NADP + H+. These protons are also removed from the stroma. This creates proton gradient across the thylakoids membrane along with pH in the lumen.
4. ATP and NADPH produced due to movement of electron is used immediately to fix CO2 to form sugar.

• The product of light reaction used to drive the process leading to synthesis of sugar are called biosynthetic phase of photosynthesis.

Calvin Cycle/C3 cycle/Reductive Pentose Sugar Phosphate Pathway

Malvin Calvin, Benson and their colleagues used radioactive 14C and Chlorealla and Scenedesmus algae to discover that first  fixation product is 3-carbon organic compound (3-phosphoglyceric acid) or PGA. Later on a new compound was discovered which contain 4-carbon called Oxaloacetic Acid (AAO). On the basis of number of carbon atoms in first stable product they are named C3 and C4 pathway.

Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

• Carboxylation is the fixation of  into 3-phosphoglyceric acid (3-PGA). Carboxylation of RuBP occurs in presence of enzyme RuBP carboxylase (RuBisCO) which results in the formation of two molecules of 3-PGA.
• Reduction is series of reaction that leads to formation of glucose. Two molecules of ATP and two molecules of NADPH are required for reduction of one molecules of . Six turn of this cycle are required for removal of one molecule of Glucose molecules from pathway.
• Regeneration is the generation of RuBP molecules for the continuation of cycle. This process require one molecules of ATP.

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