CLASS 11TH BIOLOGY CHAPTER- 13 PHOTOSYNTHESIS IN HIGHER PLANTS NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM
PHOTOSYNTHESIS IN HIGHER PLANTS
HISTORICAL PERSPECTIVE:
¶ Josheph Priestley (1770): Showed that plants have the ability to take up CO₂ from atmosphere and release O₂. (Candle with belljar and mouse expt.)
¶ Jan Ingenhousz (1779): Release of O, by plants was possible only in sun light and only by the green parts of plants. (Expt. with aquatic plant in light & dark)
¶ 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 CO, to form sugar. He gave a simplified chemical equation of photosynthesis.
¶ 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 thy lakoids 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.
Importance of Photosynthesis:
(1) Synthesis of organic compounds.
(2) Change of radiant energy into chemical energy.
(3) Useful products are obtained from plants gums, oils timber fire wood, resins rubber, fibers and drugs, etc.
(4) Balance the percentage of O2 and CO2 in atmosphere.
(5) Fossil fuels like coal, natural gas and petroleum have been formed inside the earth indirectly as a product of photosynthesis.
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. It is called universal photo-synthetic pigment.
(i) Chlorophyll b: (Yellow green)
(ii) Xahthophylls: (Yellow)
(iii) Carotenoids: (Yellow to yellow-orange)
¶ In the blue and red regions of spectrum shows higher rate of photosynthesis.
¶ Light Harvesting Complexes (LHC): The light harvesting complexes are made up of hundreds of pigment molecules bound to protein within the photosystem I (PS-I) and photosystem II (PS-II). 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 II (PS-II): Chlorophyll 'a' has absorption peak at 680 nm (P680),
¶ Process of photosynthesis: It includes two phases-Photochemical phase and biosynthetic phase. (Formerly known as Light reaction and dark reaction)
(i) Photochemical phase (Light reaction): This phase includes-light absorption, splitting of water, oxygen release and formation of ATP and NADPH. It occurs in grana region of chloroplast.
(ii) Biosynthetic phase (Dark reaction): It is light independent phase, synthesis of food material (sugars). (Calvin cycle). It occurs in stroma region of chloroplast.
¶ Photophosphorylation: The process of formation of high-energy chemicals (ATP and NADPH) in presence of light.
¶ 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). Both ATP and NADPH + H+ are synthesised by this process. PSI and PSII are found in lamellae of grana, hence this process is carried here.
¶ Cyclic photophosphorylation : Only PS-I works, the electron circulates within the photosystem. It happens in the stroma lamellae (possible location) because in this region PSII and NADP reductase enzyme are absent. Hence only ATP molecules are synthesised. It occurs when only light of wavelengths beyond 680 nm are available for excitation.
¶ The electron transport (Z-Scheme): In PS II, reaction centre (chlorophyll a) absorbs 680 nm wavelength of red light which make the electrons to become excited. These electrons are taken up by the electron acceptor that passes them to an electron transport system (ETS) consisting of cytochromes. The movement of electron is down hill. Then, the electron pass to PS I and move down hill further.
¶ The splitting of water: It is linked to PS II. Water splits into H+ [0] and electrons.
2H₂O → 4H+ + O₂ + 4e-
Chemiosmotic Hypothesis: Chemiosmotic hypothesis explain the mechanism of ATP synthesis in chloroplast. In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane. The protons are accumulated inside of membrane of thylakoids (in lumen). ATPase enzyme has a channel of that allow diffusion of protons back across the membrane. This release energy to activate ATPase enzyme that catalyses the formation of ATP.
ATP and NADPH, the products of light reaction are used in synthesis of food. The first CO₂ fixation product in C3 plant is 3-phosphoglyceric acid or PGA. The CO₂ acceptor molecule is RuBP (ribulose bisphosphate). The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discover of this pathway. Calvin cycle proceeds in three stages.
(1) Carboxylation : CO₂ combines with ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme (present in stroma)
(2) Reduction : Carbohydrate is formed at the expense of ATP and NADPH. It involves 2ATP for phsophorylation and 2NADH2 for reduction per CO₂ molecule fixed.
(3) Regeneration: The CO₂ acceptor ribulose 1, 5-bisphosphate is formed again.
6 turns of Calvin cycles and 18 ATP molecules are required to synthesize one molecule of glucose.
Input Output
6CO2 - one glucose
18ATP - 18ADP
18NADPH - 12NADPH
THE C4 PATHWAY:
C4 plants such as maize, sorghum, sugarcane have special type, of leaf anatomy, they tolerate higher temperatures. In this pathway, oxaloacetic acid (OAA) is the first stable product formed. It is 4 carbon atoms compound, hence called C4 pathway (Hatch and Slack Cycle). The leaf has two types of cells: mesophyll cells and Bundle sheath cells (Kranz anatomy). Initially CO₂ is' taken up by phosphoenol pyruvate (PEP) in mesophyll cell and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase. Oxaloacetate is reduced to maltate/asparate that reach into bundle sheath cells.
The decarboxylation of maltate/asparate occurs with the release of CO₂ and formation of pyruvate (3C). In high CO₂ concentration RuBisCo carboxylase and not as oxygenase, hence the photosynthetic losses are prevented. RuBP operates now under Calvin cycle and pyruvate transported back to mesophyll cells and changed into phosphoenol pyruvate to keep the cycle continue.
Photorespiration : The light induced respiration in green plants is called photorespiration. In C3 plants some O2 binds with RuBisCo and hence CO₂ fixation is decreased. In this process RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule of PGA and phosphoglycerate.
RuBP + O₂ → PGA + Phosphoglycolate
There is neither synthesis of ATP nor NADPH, or sugar. There is 25% loss of fixed CO₂ so it is wastefulprocess.
C4 PLANTS:
(1) Lack Photorespiration.
(2) Show response to high light intensities.
(3) Have greater productivity of biomass.
ADAPTATIONS IN C₁ PLANTS:
(i) Kranz Anatomy.
(ii) Occurrence of two types of cells.
(iii) Dimorphic chloroplast.
(iv) Presence of RuBisco in Bundle Sheath cells and PEPase in mesophyll cells.
(v) Mechanism to increase CO₂ concentration near RuBisco in Bundle Sheath cells.
CAM (/Crassulacean Acid Metabolism) Plants-Stomata open at night. e.g., Cacti (Bryophyllum), Pineapple.
Law of Limiting Factors: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed Factors affecting photosynthesis:
1. Light : Rate of photo synthesis increases at low light intensities. At high intensities of light beyond a point the rate of CO₂ fixation decreases. Longer hours of light duration favour more photosynthesis rate.
2. Carbon dioxide: Increase in CO₂ concentration causes increases in CO₂ fixation. It is the major limiting factor for photosynthesis.
3. Temperature: The rate of photosynthesis at optimum temperature is, high. It is 20°C-25°C For C, plants and 30-45°C for C₁ plants.
4. Water: Water is one of the reactant in photosynthesis, but it effects the rate of CO₂ fixation. Low water content causes the stomata to close and reduces the CO₂ availability.
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