Photosynthesis in Plants

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).

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.

Hatch and Slack pathway is a metabolic pathway for carbon-dioxide translocation occurring in $C _4$ plants. The overall effect of the pathway is to carry $CO _2$ from the mesophyll cells, which are in contact with air, to the bundle-sheath cells, which are the major sites of photosynthesis.

The Emerson effect is the increase in rate of photosynthesis after exposure to light of wavelength 670 nm far red light and 700 nm, red light respectively. When simultaneously exposed to light of both wavelengths, the rate of photosynthesis is far higher than the sum of red light and far red light photosynthesis rates. The effect was early evidence, that photosystems processing different wavelengths cooperate in photosynthesis. 

Chlorophyll is the liquid substance in a plant cell, which is responsible for its colour. Plants make food when sunlight strikes chlorophyll. Pelletier and Caventou discovered chlorophyll in 1818.

Jan Ingenhousz demonstrated that sunlight is essential to the plant process by showing that aquatic plants produce bubbles in the presence of light but not in the dark.

T. W. Engelmann experimented on Cladophora. Using a prism, he split light into its spectral components and then he illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of oxygen evolution. He found that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. Thus, action spectrum of photosynthesis was first studied by Engelmann.

Theodor Wilhelm Engelmann was a German scientist. In 1883, he performed his famous action spectrum experiment to learn which wavelengths or colour of light were the most effective in carrying out photosynthesis in the green alga.

A) Joseph Priestley first discovered oxygen and discovered that it is released by plants.
B) Jan Ingenhousz proved that sunlight is essential for photosynthesis.

Boussingault was a  French chemist and he made a very significant contribution to the fields of agricultural science and metallurgy.

Photosynthesis takes place in the green leaves of plants and other green parts of plants like stem etc. The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialized light absorbing pigments. These pigments are known as photosynthetic pigments. Chlorophyll and carotenoids are the types of photosynthetic pigments. Chlorophyll is the primary pigment and carotenoids are the accessory pigments. Chemistry of photosynthetic pigments was first studied by Willstatter and Stoll in 1912. They discovered that chlorophyll has a tadpole-like configuration. It has a head called porphyrin and a tail made up of long chain alcohol called phytol. Porphyrin head is made up of four pyrrole rings which are linked by methine bridges. 5 atoms— 4 carbon and one nitrogen form the skeleton of each pyrrole ring. Nitrogen lies towards the centre. Thus, the correct answer is option A.

• F.F. Blackman, a British plant physiologist discovered that photosynthesis is a two-step process and only one of which uses light directly.
• Hill discovered that green particles obtained from broken cells could produce oxygen from water in the presence of light and a chemical compound, such as ferric oxalate, able to serve as an electron acceptor. This process is known as Hill reaction.
• Benson was the co-researcher of Calvin who discovered the Calvin cycle in photosynthesis.
• The process of photosynthetic photophosphorylation was discovered by Arnon. Photosynthetic photophosphorylation is the process of phosphate group transfer into ADP to synthesize energy-rich ATP molecules making use of light as an external energy source. So, this option is correct.

The first photosynthetic organisms to appear on Earth were likely anaerobic bacteria, which evolved the ability to use light energy before the development of oxygenic photosynthesis in cyanobacteria.