NEET Biology Notes Photosynthesis
Photosynthesis
Concept of Photosynthesis
Photosynthesis (a means of autotrophic nutrition) is the formation of carbohydrates from C02 and H20 with the help of sunlight in the chlorophyll containing green parts of the plants. It is an anabolic process.
Photosynthesis (a means of autotrophic nutrition) is the formation of carbohydrates from C02 and H20 with the help of sunlight in the chlorophyll containing green parts of the plants. It is an anabolic process.
Ruben and Kamen (1941) proved that the source of liberated oxygen (02) in the photosynthesis is water (H20) and not the carbon dioxide (C02).
The source of oxygen in carbohydrate produced through photosynthesis is carbon dioxide (C02). In green plants, water (H20) is the hydrogen donor and is oxidised to i oxygen (02), whereas in purple and green sulphur bacteria, H2S is the hydrogen donor and sulphur or sulphate is the oxidation product. Cyanobacteria are first ; photosynthetic organisms evolved during evolution.
Bacterial Photosynthesis
It occurs without evolution of oxygen hence, is anoxygenic. In bacterial photosynthesis, H2, H2S and other compounds are employed as hydrogen donor i instead of water. Bacteria have only one pigment system which is similar to photosystem-I (PS-I).
Thylakoids are not organised into grana in cyanobacteria (blue-green algae) and [ photosynthetic bacteria (prokaryotes) and are present scattered in cytoplasm. In these organisms, photosynthetic pigments are distributed uniformly on or in the lamella (unilamellar thylakoids).
About 90% of total photosynthesis is carried out by algae in oceans and in freshwater. Only about 0.2% of the sunlight energy falling on earth is utilised by photosynthetic organisms.
Site of Photosynthesis
In eukaryotes, photosynthesis takes place in chloroplasts present in cytoplasm, varying in numbers from one (e.g. Chlorella) to about 100 (e.g. palisade mesophyll cells). Chloroplasts are visible under light microscope (3-10 pm diameter). They are covered by two membranes. They contain chlorophyll and other photosynthetic pigments in thylakoid membranes. Thylakoids form granum. A number of grana are suspended in the matrix which is called stroma. Thylakoids are the site of light reaction, whereas the stroma is the site of dark reaction.
Pigments Involved in Photosynthesis
The most common photosynthetic pigments in higher plants and green algae are as follows :
- Chlorophyll-a is essential pigment in photosynthesis because it can convert light energy intd chemical energy (ATP). It is also called as universal photosynthetic pigment or primary photosynthetic pigment. The basic structure of all chlorophyll molecules is a porphyrin system, in which four pyrrole (tetrapyrrole) rings linked together by methane groups form a ring system.
- Carotenoid is a group of accessory photosynthetic pigments of yellowish or reddish colour. They are of two types, i.e. carotenes (e.g. p-carotene) and xanthophylls (lutein and zeaxanthin). Carotenoids absorb light radiations in the mid-region of light spectrum. They convert nascent oxygen to molecular oxygen and protect various chloroplast constituents from nascent oxygen.
- Phycobilins are water soluble, open tetrapyrrole pigments found in red algae and blue-green algae. These are of three types, i.e. phycocyanin (blue), allophycocyanin (blue) and phycoerythrin (red).
- Other pigments such as chlorophyll-b, carotenoid, etc., act as accessory pigments because they collect and transfer light energy to chlorophyll-a for photosynthesis.Spectrum of Light
Light is a form of radiant energy. It is a narrow band of energy within the continuous electromagnetic spectrum of radiation emitted by the sun. Visible light varies from 390-760 nm.
Absorption Spectrum
The graphic representation of the curve showing the various wavelengths of light absorbed by a substance is known as absorption spectrum.
Chlorophylls absorb light radiations in blue and red parts of light spectrum (430 nm and 662 nm for chlorophyll-a, 455 nm and 644 nm for chlorophyll-b). It is a graphic representation or curve depicting the rate of photosynthesis in various wavelengths of light.
Action Spectrum
It is the actual rate of photosynthesis with respect to the wavelength of light absorbed. It is closely related to the absorption spectra of chlorophyll-a and b. It is measured in terms of 02 evolved at different light wavelengths.
Emerson-Enhancement Effect
- In 1950, Robert Emerson and co-workers found that if light of shorter wavelengths was provided at the same time as the longer red wavelengths, photosynthesis was even faster than the sum of two rates with either colour alone. This synergism or enhancement is known as Emerson-Enhancement Effect.
- Emerson’s experiments gave conclusive idea that process of photosynthesis involves two light reactions, one carried by short wavelength absorbing form of chlorophyll-a and other by pigments including a long wavelength absorbing form of chlorophyll-a. This led to the idea of two photosystems.
Photosystems
Photosystems are functional and structural units of protein complexes involved in photosynthesis. Two photosystems have been founds in plants. They are:
- Photosystem-I (PS-I)
Photosystem-I (pigment system-I) is present in stroma thylakoids and non-appressed part of granal thylakoids. Its reaction centre is P700. It can perform cyclic photophosphorylation independently. PS-I is active both in red and far red light. It carries out reduction of NADP.
PS-I is having pigments, chl-a 660, chl-a 670, chl-a 680, chl-a 690, chl-a 700 and carotenoids.
It consists of photocentre, Light Harvesting Complex [LHC-I] and some electron carriers.
- Photosystem-II (PS-II)
Phofosystem-II (pigment system-II) is located in the appressed part of the grana thylakoids. The PS-II is inactive in far red light (beyond 680 nm). Its reaction centre is P680. It picks up electrons emitted during photolysis of water and performs non-cyclic photophosphorylation. *
PS-II is having pigments chl-b 650, chl-a 660, chl-a 670, chl-a 678, chl-a 680 and phycobilins.
It consists of photocentre, oxygen evolving complex, Light Harvesting Complex (LHC-II) and some electron carriers.
Mechanism of Photosynthesis
The two major steps of photosynthesis are as follows:
- Light Reaction or Photochemical Phase
It takes place only in the presence of light in the grana portion of the chloroplast. The function of this phase is to produce assimilatory power consisting of reduced co-enzyme NADPH and energy rich ATP molecules.
It comprises of following steps:
Photolysis of Water
This phenomenon is associated with pigment system-II and are catalysed by the presence of Mn2+ and Cl- ions. When pigment system-II is active, the water molecules split into OH- and H+ ions. The OH ions unite to form the water molecules again and release 02 and electron.
Production of Assimilatory Power
(NADPH and ATP)
It has already been stated that when chlorophyll-a molecule receives a photon of light, the photocentre expels an electron with the gain of energy (23 kcal/mol). This is also known as quantum conversion because the light energy gets converts into chemical form.
The expelled electrons after travelling through a series of carriers are either cycled back or consumed in reducing NADP to NADPH + H+.
Electron Transport and Acceptors
- The whole scheme of transfer of electrons, starting from the PS-II uphill to the acceptor, down the electron transport chain to PS-I, excitation of electrons, transfer to another acceptor and finally down hill to NADP+ causing it to be reduced to NADPH2 is called the Z-scheme. In Z-scheme, the main electron acceptors are cytochromes, plastoquinones and plastocyanin.
- Cytochromes These are small intrinsic membrane proteins that contain a co-factor haeme, which holds an iron atom. The iron carries electrons and cycles between the +2 and +3 oxidation states.
- Consequently, they carry electrons only between sites that are extremely close together within a membrane rather than diffusing throughout the stroma as NADPH does.
- Plastoquinones These like cytochromes, transport electrons over short distance within a membrane. After they pick up two electrons, they also bind two protons. Their long hydrocarbon tail causes them to be hydrophobic, so they dissolve easily into the lipid component of the chloroplast membranes.
- Plastocyanin Like cytochromes, plastocyanin is a small protein that carries electrons on a metal atom, in this case copper. When oxidised, the copper ion is in the +2 oxidation state, but as it picks up the electron, it goes to the +1 oxidation state. It is reduced one level. Plastocyanin is loosely associated with chloroplast membranes.
