All forms of living organisms on the planet earth depend upon building materials and energy for their growth, development, reproduction, metabolism, movement and all other activities associated with life.
They derive these building materials and energy from the complex organic compounds used by them as food which is obtained from plants or from organisms which ultimately derive their food from plants. Green chlorophyll containing plants, those we see all around, are endowed with the capacity of absorbing light energy from the sun and utilizing this energy to drive the synthesis of organic compounds from simple inorganic compounds. Cot only the green plants, but also algae, cyanobafcteria and various types of other bacteria are capable of using light energy for synthesis of organic compounds.
These organic compounds are used by all other organisms as the source of energy and building materials for their growth, development and all other activities associated with life. The process whereby green plants, algae, cyanobacteria and certain other bacteria use light energy to drive the synthesis of organic compounds is known as photosynthesis and the organisms capable of performing photosynthesis are called photosynthetic organisms.
The pigments which absorb light energy and then convert it into chemical energy are known as photosynthetic pigments. These photosynthetic pigments are localized in the internal membrane (thylakoid) of small cellular organelles known as chloroplasts. In this chapter the photosynthesis process of green plants is described. The major differences in this process with other photosynthetic organisms are also indicated.
ADVERTISEMENTS:
Early History of Photosynthesis
In 1771, Joseph Priestley, an English chemist, performed experiments showing that plants release a type of air that allows a candle to burn. He demonstrated this by putting a sprig of mint into a transparent closed vessel with a burning candle until the flame went out. After several days he relit the extinguished candle again and it burned perfectly well in the air that previously would not support it.
In another experiment, conducted in 1772, Priestley observed that a mouse kept in a transparent jar of air collapsed while a mouse kept with a plant in the same transparent jar survived.
These observations led Priestley to offer an interesting hypothesis that plants restore to the air whatever breathing animals or burning candles remove. He named this as “dephlogisticated air” which was later named as ‘oxygen’. Priestley’s work thus showed that plants release oxygen into the atmosphere. Jan Ingenhousz, a Dutch physician, took Priestley’s work further and demonstrated in 1779 that sunlight was necessary for photosynthesis and that only the green parts of plants could release oxygen.
ADVERTISEMENTS:
During this period Jean Senebier, a Swiss botanist and naturalist, discovered that C03‘is required for photosynthesis. Nicolas Theodore de Saussure, a Swiss chemist and pla physiologist, showed that water is also required for photosynthesis. Julius Robert von Mayer, German physician and physicist, in 1845 proposed that photosynthetic organisms convert li energy into chemical free energy.
By the end of the nineteenth century the key features of plant photosynthesis were know Plants could use light energy to make carbohydrates from C02 and water. Because glucose, and I carbon sugar, is often considered as an intermediate product of photosynthesis, the net equation photosynthesis is frequently written as:
6C02 + 12H20 + Light Energy
Early scientists studying photosynthesis concluded that the O, released by plants came from C02, which was thought to be split by light energy. In the 1930s, V. B. van Xiel was aware that some photosynthetic bacteria could use hydrogen sulfide (H2S) instead of water for photosynthesis and that these organisms released sulfur instead of oxygen. Van Xiel, among others, concluded that photosynthesis is a redox (reduction-oxidation) process depending on electron donor and acceptor reactions and that the 02 released during photosynthesis comes from the photooxidation of water. The generalized equation for photosynthesis, thus, can be written as:
ADVERTISEMENTS:
6C02 + 12H2A + Light Energy
In oxygenic photosynthesis, 12A is 602, whereas in anoxygenic photosynthesis, which occurs I in some photosynthetic bacteria, the electron donor can be an inorganic hydrogen donor, such as1 H2S (in which case A is elemental sulfur) or an organic hydrogen donor such as succinate (in which case, A is fumarate).
Experimental evidence that molecular oxygen came from water and not from C02 was provided by Robert Hill in 1937. He demonstrated light dependent 02 evolution in the absence of C02 in isolated chloroplast thylakoids using artificial electron acceptors such as iron salts. This has since been known as Hill reaction and the artificial electron acceptors as Hill oxidants. Hill reaction, with iron salt as Hill oxidant, is usually written as:
More direct evidence for the evolution of molecular oxygen from water during photosynthesis was provided by Ruben et al (1941) who used enriched water for their study on photosynthesis and demonstrated the evolution.