· Photosynthesis is the synthesis of carbohydrates by green plants in presence of light using CO2 and H2O.
· Anabolic (Constructive process or Increase in dry weight)
· Oxidation of H2O into O2.
· Source of 'H' in Glucose is H2O
· Source of oxygen in Glucose is CO2.
· Reduction of CO2 into glucose (C6H12O6).
· Volume of CO2 utilized is equal to the volume of Oxygen released.
· By the process of photosynthesis solar energy (ultimate source of energy for all living beings) is trapped by autotrophic organisms and stored in the form of chemical energy.
· Only 0.2% of the light falling on the earth is utilized by photosynthesis organisms.

Photosynthetic Pigments
1. Chlorophyll- Chl.a, Chl.b, Chl.c etc.
2. Carotenoids
    (a) Carotene- Orange coloured
    (b) Xanthophylls- Yellow coloured
3. Phycobilins
    (a) Phycoerythrin - red pigment
    (b) Phycocyanin- blue pigment.
· Primary photosynthetic pigment – Chlorophyll a
· Chlorophyll a is found in all green plants except bacteria where bacteriochlorophyll is found instead of Chlorophyll a
· Carotenoids prevent plants from photodynamic damage of chloroplast.
· Quantasomes (photosynthetic unit PSU is a group of photosynthetic pigment present in the membrane of thylakoids or lamellae.
· Each quantasome contains 230 pigments.

Photosynthetic units
· Two types: PSI & PSII
a. PS I
· Reaction centre P700.
· Located at both stroma and grana lamellae.
· More Chlorophyll a than Chlorophyll b.
b. PS II
· Reaction centre P680.
· Located at grana lamellae.
· More Chlorophyll b than Chlorophyll a

Mechanism of Photosynthesis

1. Light reaction.
2. Dark reaction or Blackman's reaction.

· Light reaction or Grana reaction uses sunlight to synthesize NADPH2 and ATP (assimilatory power).
· It comprises of following steps:
(a) Absorption of light energy by photosynthetic pigment.
(b) Transfer of light energy from accessory pigment to chlorophyll.
(c) Excitation of chlorophyll a molecule by proton.
(d) Donation of electron energy to the water molecule
(e) Photosynthesis of water.

· Also called Hill's reaction or Primary Photochemical reaction.
· Light is essential.
· Photolysis of water and evolution of O2 takes place.
· Formation of assimilatory powers (ATP and NADPH2)
· Source of energy is light.
· Occurs in grana lamellae.
· Formation of the energy-rich organic compound.
· 1st step of the light reaction is an activation of Chlorophyll-a molecule with the help of sunlight.

· Photolysis of water is associated with Pigment system II (PS II) or P680.
· i.e. 4H2O → 4H+ + 4OH
· 4OH– → O2 + 2H2O + 4e
· 2H→ O2 + 4H+ + 4e
· NADP + 2 H+ + 2e → NADPH2 (reducing process)
· Given by Robert Hill.
· Source of O2 is water was proved by Ruben and Kamen by using H2O18 in tracer technique or Radioactive isotope technique.
· Bacterial photosynthesis uses H2S instead of H2O. Therefore, sulphur is produced instead of O2.

· Formation of ATP from ADP in the membrane of thylakoids in presence of light.
2 types are:
i. Cyclic photophosphorylation
· 1st electron donor and last electron acceptor are PSI.
· Involves PSI (P700 only)
· Photophosphorylation at two places.
· Product of cyclic photophosphorylation is only ATP.
· There is no photolysis and ATP formation.

ii. Non-cyclic photophosphorylation
· Involves both PS I (P700) and PS II (P680).
· 1st electron donor is water (H2O) and the last electron acceptor is NADPH.
· Involves photolysis of water, so O2 is released.
· Manganese (Mn) and chlorine (Cl) helps in photolysis.
· Photophosphorylation takes place only in one place.
· Products of non-cyclic photophosphorylation are O2, ATP and NADPH2.

Inhibitor of PS II
· CMU (Chlorophenyl dimethyl urea), DCMU, DDT, blocks e-transport from PS II to PS I.

Fig: Showing Light Reaction

· Dark reaction often called Black man's reaction or carbon assimilation or CO2 fixation doesn't depend upon the presence of light and is catalyzed by several enzymes present in the stroma of the chloroplast.
· Fixation of CO2 and glucose is produced.
· Utilization of assimilatory powers (ATP and NADPH2)
· Occurs in the stroma region of the chloroplast.
· Formation of energy-rich compounds like glucose.

Two types are:
i. C3 cycle

· Also called Calvin cycle or Reductive pentose phosphate cycle.
· Discovered by Calvin, Benson, Bassham and Company.
· First observed in chlorella and scenedesmus by using 14CO2 by tracer technique.
· Occurs in the chloroplast of mesophyll tissue.
· It can be summarized as:

· 1st CO2 acceptor is Ribulose 1, 5 biphosphates (RUBP), a 5 carbon-containing compound.
· 1st stable product is Phosphoglyceric acid (PGA) a 3 'C' containing compound.
· 1st carbohydrate/sugar is a 3 'C' sugar called Phosphoglyceraldehyde (PGAL)
· 6 turns of C3 cycles are essential to produce one glucose molecule.
· For the synthesis of one molecule of glucose or fixation of 6CO2 molecule 18 ATP and 12NADPH2 are required.
· Rubisco (the most abundant protein of the biosphere) or RUBP carboxylase helps in carboxylation.

Fig: Showing Calvin Cycle.

ii. C4 Cycle (Hatch-slack pathway)
· It is also called the Hatch and Slack cycle which is an alternative pathway of CO2 fixation in tropical plants like Sugarcane, Maize, Atriplex etc.
· The C4 plants are characterized by the presence of Kranz anatomy.
· C4 plants are supposed to be more efficient in fixing CO2 than C3 plants.
· 1st observed in Sugarcane.
· Common in some tropical plants like Sugarcane, Maize etc.
· Dimorphic chloroplast is seen.
· Chloroplast of mesophyll tissue contain grana but lacks Rubisco.
· Chloroplast of bundle sheath contains Rubisco but lacks grana (agranal chloroplast in bundle sheath).
· Presence of Krantz anatomy in the leaf of a C4 plant.
· 1st CO2 acceptor is PEP (Phosphoenol pyruvic acid)
· PEP Carboxylase changes (PEP + CO2) into oxaloacetic acid.
· It has two advantages over RUBP carboxylase.
i. It does not accept oxygen and hence there is no photorespiration in C4 plant.
ii. It has much more affinities for CO2.
· First stable product is Oxaloacetic acid.
· 30 ATP and 12 NADPH2 are utilized for the formation of one glucose molecule or fixation of 6CO2.
· C4 pathway is more energy consuming than C3 pathway.
· Twice CO2 cycle at different places at the same time (i.e. in mesophyll and bundle sheath).
· Twice CO2 cycle in the same place at different time-CAM plants.
· Due to negligible photorespiration loss C4 plants are photosynthetically most effective than C3 plants.

Fg: Showing Hatch-slack pathway

iii. Photorespiration (C2 Cycle)
· Photorespiration is the respiration in green cells in presence of light and results in excess evolution of CO2.
· No energy release.
· Light-dependent uptake of oxygen and output of carbon dioxide without releasing energy i.e. ATP and NADPH2.
· Completed in chloroplast, peroxisome and mitochondria.
· Photorespiration occurs when CO2 concentration is low, the oxygen concentration is high and high light intensity.
· It is not a path of carbon fixation.
· During photorespiration, O2 is used and CO2 is produced inside mitochondria.
· It is a wasteful loss of ATP and NADPH2.
· It is common in C3 plants because RUBP accepts oxygen in some condition

Fig: Showing Photorespiration

iv. CAM (Crassulacean Acid Metabolism) Cycle
· It is an alternative to the C3 and C4 pathways of CO2 fixation found in plants living in dry hot climates.
· Common in succulents and xerophytic plants.
· First observed in Bryophyllum.
· Scoto-active stomata (stomata that open during the night) is common in CAM plants.
· Respiration quotient is zero at night.
· In CAM Plants, there is no release of CO2, So, RQ = O
· In anaerobic respiration, the Volume of O2 consumed = zero. So, RQ = (infinity)
· RQ for carbohydrates like glucose, sugar is = 1
· RQ is more than one for organic acids.
· RQ is less than one for proteins, fats, and mixed diet.
· CAM pathway is the natural pathway for supplying CO2 during daytime even though the stomata are closed.
· Twice CO2 cycle in the same place at different times.
· Compensation point – It is the light intensity at which the rate of photosynthesis is equal to the rate of respiration.
· It usually occurs during the early morning and evening hours.
· Willmott's experiment is used to measure the rate of O2 evolution or rate of photosynthesis.
· The yield of photosynthesis is measured in terms of Quantum yield.

Fig: Showing CAM cycle.

· High light intensity disintegrates chlorophyll and decreases the rate of photosynthesis, this phenomenon is called solarisation.

· High wavelength of light beyond 680nm inhibits photosynthesis.
· The two distinct photosystems were discovered due to the red drop effect.
· Warburg's effect - Higher O2 concentration acts as an inhibitor of RUBP carboxylase (Rubisco) and decreases the rate of photosynthesis.
· Rate of Photosynthesis is maximum in white light than in red light and blue light.
i.e. white > red light > blue light.

· Greater in intermittent light than continuous light.
· Greater in intense light than dim light.
· Maximum photosynthesis occurs in IR (infrared rays) in bacteria
· Limiting factor of photosynthesis is CO2.
· On cloudy days, the limiting factor is light.
· Rate of photosynthesis is more in matured leaves than young leaves.
· More in turgid cells than flaccid cells.
· The temperature coefficient (Q10) for photosynthesis is 2. i.e. rate of photosynthesis doubles every 10°C rises in temperature.
· At compensation point rate of photosynthesis is equal to the rate of respiration.
· Moll's half leaf experiment to show that CO2 is essential for photosynthesis.

· Occurs through phloem in all directions.
· Occurs in the form of sucrose (disaccharide) with the help of Boron.
· Most convincing theory of translocation of food is Munch's mass flow or pressure flow theory.
· Translocation is from higher solute concentration to lower solute concentration means higher turgor pressure to lower turgor pressure.

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