- Photosynthesis: Photosynthesis is an enzyme regulated anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and water with the help of sunlight as a source of energy.
Historical Perspective :
- Joseph Priestley (1770) : Showed that plants have the ability to take up CO2 from atmosphere and release O2.
- Jan Ingenhousz (1779) : Release of O2 by plants was possible only in sunlight and only by the green parts of plants.
- Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis to occur.
- Julius Von Sachs (1854) : Green parts in plant produce glucose which is stored as starch.
- T. W. Engelmann (1888) : The effect of different wavelength of light on photosynthesis and plotted the first action spectrum of photosynthesis.
- C. B. Van Niel (1931) : Photosynthesis is essentially a light dependent reaction in which hydrogen from an oxidisable compound reduces CO2 to form sugar. He gave a simplified chemical equation of photosynthesis.
- Hill (1937) : Evolution of oxygen occurs in light reaction.
- Calvin (1954-55) : Traced the pathway of carbon fixation.
- Hatch and Slack (1965) : Discovered C4 pathway of CO2 fixation.
Site for photosynthesis :
- Photosynthesis takes place only in green parts of the plant, mostly in leaves.
- Within a leaf, photosynthesis occurs in mesophyll cells which contain the chloroplasts.
- Chloroplasts are the actual sites for photosynthesis.
- The thylakoids in chloroplast contain most of pigments required for capturing solar energy to initiate photosynthesis.
- The membrane system (grana) is responsible for trapping the light energy and for the synthesis of ATP and NADPH. Biosynthetic phase (dark reaction) is carried in stroma.
Pigments involved in photosynthesis:
- Chlorophyll a : (Bright or blue green in chromatograph). Major pigment, act as reaction centre, involved in trapping and converting light into chemical energy.
- Chlorophyll b : (Yellow green)
- Xanthophylls : (Yellow)
- Carotenoid : (Yellow to yellow-orange)
- In the blue and red regions of spectrum shows higher rate of photosynthesis.
What is light reaction?
- Light reactions or the ‘Photochemical ‘phase includes light absorption, splitting of water, evolution of oxygen and formation of high energy compound like ATP and NADPH.
- Light Harvesting Complexes (LHC) : The light harvesting complexes are made up of hundreds of pigment molecules bound to protein within the photosystem I (PSI) and photosystem II (PSII).
- Each photosystem has all the pigments except one molecule of chlorophyll ‘a’ forming a light harvesting system (antennae).
- The reaction centre (chlorophyll a) is different in both the photosystems.
- Photosystem I (PSI) : Chlorophyll ‘a’ has an absorption peak at 700 nm (P700).
- Photosystem II (PSII) : Chlorophyll ‘a’ has absorption peak at 680 nm (P680).
Process of photosynthesis :
- It includes two phases - Photochemical phase and biosynthetic phase.
- Photochemical phase (Light reaction) : This phase includes - light absorption, splitting of water, oxygen release and formation of ATP and NADPH.
- Biosynthetic phase (Dark reaction) : It is light independent phase, synthesis of food material (sugars).
The electron transport :
- In photosystem centre chlorophyll a absorbs 680 nm wavelength of red light causing electrons to become excited and release two electrons from the atomic nucleus.
- These electrons are accepted by primary electron acceptor i.e. ferredoxin.
- The electron from the ferredoxin passed to electron transport system consisting cytochromes.
- The electron moved in down hill in terms of redox potential by oxidation-reduction reactions.
- Finally the electron reached photosystem-I.
- Simultaneously electron released from photosystem-I is accepted by electron acceptor.
- Electron hole created in PS-I is filled up by the electron from PS-II.
- Electron from PS-I passed down hill and reduce NADP into NADPH+ + H+.
Photolysis of water :
- PS-II loose electrons continuously, filled up by electrons released due to photolysis of water.
- Water is split into H+, (O) and electrons in presence of light and Mn2+ and Cl-.
- This also creates O2 the bi-product of photosynthesis.
- Photolysis takes place in the vicinity of the PS-II.
- 2H2O → 4H+ + O2 + 4e-.
Photophosphorylation :
- The process of formation of high-energy chemicals (ATP and NADPH).
Non Cyclic photophosphorylation :
- Two photosystems work in series – First PSII and then PSI.
- These two photosystems are connected through an electron transport chain (Z. Scheme).
- ATP and NADPH + H+ are synthesized by this process. PSI and PSII are found in lamellae of grana, hence this process is carried here.
