Tag: introduction to photosynthesis

Chloroplasts contain an internal system of membranes called thylakoids. The thylakoids may be arranged in stacks called as grana. The thylakoid membranes not arranged in stacks and are in direct contact with chloroplast stroma are called as stroma lamellae. During photosynthesis, light reaction occurs in grana.
Hill conducted experiments on isolated chloroplasts, having some oxidants like ferricyanide and benzoquinone (Hill reagents). By placing them in sunlight, he noticed evolution of oxygen from water. Hill concluded that if there was hydrogen acceptor in the medium, it would have been reduced. Arnon in 1951, 1954, repeated the experiment with isolated chloroplast and concluded that -
(i) The hydrogen acceptor present in plants is NADP. It is reduced to $NADPH _2$.
(II) An ADP molecule is converted to ATP in light by utilizing inorganic phosphorous. This phenomenon is called as photophosphorylation.

Robert Mayer, 1942, gave a very important concept in which he stated that plants convert light power (light energy) into chemical difference (chemical energy).

Jagadish Chandra Bose, made pioneering contributions in many fields, like maths, physics, biology, botany, archaeology etc. He invented crescograph for measuring plant growth. He is generally regarded as 'father of indian plant physiology'.

Photophosphorylation is synthesis of ATP from ADP in presence of light in the chloroplast. It is also called as photosynthetic phosphorylation. It was discovered by Arnon in 1954. Arnon gave difference between two types of photophosphorylation, cyclic and non-cyclic photophosphorylation. He gave resemblance of photophosphorylation with oxidative phosphorylation. The difference between the two is in photophosphorylation ATP is synthesized using light energy whereas in oxidative phosphorylation, ATP is synthesized using chemical energy. 

Photosynthesis occurs in two stages in a cell. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage and transport molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals that feed on it). This set of reactions is also called as carbon fixation. The key enzyme of the cycle is called as RuBisCO. The Calvin cycle or reductive pentose phosphate cycle or $C _3$ cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. 

Carbon fixation pathway in C$ _{3}$ plants was discovered by Calvin, Benson and Basham in unicellular alga Chlorella using C-14 labelled carbon dioxide. They fed radiolabelled carbon dioxide to alga and killed algal cells and isolated the organic compounds at various time intervals, through co-chromatography technique in order to decipher the various compounds into which carbon radio label travels.

Photorespiration is the process of uptake of oxygen and production of carbon dioxide in the light by the photosynthesising tissue. It was discovered by Dicker and Tio (1959), when they observed that the rate of respiration of green leaves is much higher in light than in the dark.

Melvin Ellis Calvin was an American chemist most famed for discovering the Calvin cycle along with Andrew Benson and James Bassham, for which he was awarded the 1961 Nobel Prize in Chemistry.

In 1930 C.B Van Niel proved that, sulphur bacteria use $H _{2}S$ and $CO _{2}$ to synthesize carbohydrates as follows:

$6CO _{2}+12H _{2}S\rightarrow C _{6}H _{12}O _{6}+6H _{2}O+12S$

This led Van Niel to the postulation that in green plants, water $(H _{2}O)$ is utilized in place of $H _{2}S$ and $O _{2}$ is evolved in place of sulphur. He indicated that water is electron donor in photosynthesis.

$6CO _{2}+12H _{2}O\rightarrow C _{6}H _{12}O _{6}+6H _{2}O+6O _{2}$