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Two-phases i.e., light reaction and dark reaction.

(A) Photochemical phase (light reaction or Hill reaction)

i. Takes place in thylakoids of grana.

ii. Solar energy is trapped by chlorophyll and is stored in form of chemical energy (ATP) and as reducing power (NADPH)

Photosystems:

Groupings of photosynthetic units which possess photocentres and are surrounded by specific light harvesting pigment molecules. 2 types.

(a) Photosystem-I (PS I):

Present in stroma thylakoid and non-appressed parts of granal thylakoid.

Photocentre P₇₀₀ along with accessory pigments which absorb at or below 700 nm constitute the PS-I.

(b) Photosystem-II (PS II):

Present in appressed part of granal thylakoids.

i. Possess chlorophyll a, chlorophyll b and carotenoids.

ii. The cluster of pigment molecules which transfer their energy to P₆₈₀ absorb light at or below the wavelegnth of 680 nm.

iii. Light reaction : in which light energy is transformed by photophosphorylation into chemical energy in the form of the energy source ATP and NADPH, which is used to reduce CO,, Takes place in following steps.

1. Excitation of pigments by light so that excited pigments lose electrons and become oxidised.

2. The electron is captured by an electron acceptor. This represents the conversion of light energy into chemical energy. Depending upon whether the electron lost by chlorophyll molecule is returned to it or not, the further events may be cyclic or non-cyclic.

Non-cyclic photophosphorylation (Hill and Bendall proposed Z-scheme for electron transport):

i. Light energised ATP synthesis in which the electron expelled by excited photocentres does not return to them.

ii. Both PS I and PS II are involved.

iii. Coordinate their activities to produce assimilatory power (ATP and NADPH).

iv. Oxygen is released during photolysis of water.

v. Takes place at one place only.

Cyclic photophosphorylation:

i. Light energised ATP synthesis in which the electron expelled by excited photo centre is returned to it after passing over a chain of electron carriers.

ii. It is related to photo act I (PS I) only.

iii. Electron released from chlorophyll return back.

iv. There is no photolysis of water and release of oxygen.

v. NADP is not oxidised.

vi. Takes place at two places.

vii.It occurs under low light conditions, wavelength longer than 680 nm and when CO, fixation is being inhabited.

(B) Biosynthetic phase (Dark phase or Blackman’s reaction):

Independent of light and studied by Blackman in following steps:

(a) Carboxylation i.e., acceptance of CO₂ by RuBP – CO, receptor, carboxylation of RuBP is catalysed by the enzyme RuBP carboxylase or Rubisco (most abundant protein on earth).

(b) Glycolytic reversal:

The 6 molecules of PGA (3 phosphoglyceric acid) utilises 6 ATP molecules to form 6 molecules of 1, 3-diphosphoglyceric acid. These, in turn, utilizes 6 NADPH supplied by the light reactions and get converted to a 3-C glyceraldehyde phosphate.

(c) Regeneration of RuBP:

RuBP is regenerated through the formation of 4-C, 5-C and 7-C intermediates. Five molecules of triose phosphate (TP) and three molecules of ATP are involved in the regeneration of RuBP.

Dark reaction takes place in stroma. It comprises a series of reactions controlled by enzymes. The sequence of these reactions was determined in Chlorella and Scendesmus by Calvin, Benson and Bassham using radioactive carbon ¹⁴C and techniques like chromatography and autoradiography. This is also known as Calvin Cycle or Calvin-Benson Cycle or C, Cycle or Reductive Pentose Pathway or Photosynthetic Carbon Reduction or PCR.

Photorespiration (Dicker and TIO, 1959)

i. Respiration which is initiated in chloroplasts and takes place in light only is called Photo respiration.

ii. The enzyme Rubisco catalyses the carboxylation reaction where CO, combines with RuBP, called oxygenation.

iii. Also called Photosynthetic Carbon Oxidation Cycle and involves an interaction of three organelles i.e., chloroplast, peroxisome and mitochondria.

iv. The function of photorespiration is to recover some of the carbon from the excess glycolate. However, there is a wasteful loss of carbon as C0₂ and loss of energy.

v. It is stimulated by high light intensity, high temperature, ageing of leaf and high oxygen concentration.

vi. Two molecules of phosphoglycolate (2C) produced by oxygenation of RuBP are changed into one molecule of phosphoglycerate (3C) and one molecule of CO₂ .

vii. 75% of carbon lost to oxygenation is recovered by PCO cycle.