Test: What is Photosysnthesis (June 15) - NEET MCQ

Photosynthesis is an enzyme regulated anabolic process which involves the synthesis of organic compounds, inside the chlorophyll containing cells, from CO₂ and H₂O with the help of sunlight as a source of energy. Oxygen evolved during photosynthesis comes from water. The equation for photosynthesis can be summarised as follows.

Joseph Priestley (1770) observed that a candle burning in a closed space a bell jar, soon gets extinguished. Similarly. A mouse kept in a closed space would soon get suffocated and die However, when he placed a mint plant and candle continued to burn. Priestley hypotesised that four air or phologiston produced during burning of candles or animal (mice) respiration could be converted into pure air or dephlogiston by plants (mint). In 1774, Priestley discovered oxygen.

Jan Ingenhousz (1730-1799), In his experiment with an aquatic plant, showed that in bright sunlight, small bubble were formed around the green parts while in the dark no such bubbles formed He found out these bubbles to be of oxygen He thus, cnfirmed that purification of air or formation of dephlogiston is carried out by green plants only in presents of sunlight.

Sachs (1854) provided the evidence that green plant parts produce glucose which is stored in the form of starch. Starch is the first visible product of photosynthesis.

Engelmann (1843-1909) split light into components by the prism and then illuminated Cladophora (a green alga) was placed in a suspension of aerobic bacteria. He found that bacteria accumulated in the region of blue and red light of the split spectrum. He thus discovered the effect of different wavelengths of light on-photosynthesis and plotted the first action spectrum of photosynthesis. 

Van Niel (1897-1885), on the basic of his studies with puple and green sulphur bacteria, demonstrated that phoosynthesis is a light dependent reaction in which hydrogen from a oxidisable compound reduces CO₂ to form sugar.

In green sulphur bacteria, when H₂S instead of H₂O was used as hydrogen donor, no O₂ was evolved.

6CO₂+12H₂S→C6H₁₂O₆+6H₂O+125

From the above observation and conclusions, van Niel proposed that oxygenic photosyntheis of all organisms is actually an oxidation-reduction reaction involving action of hydrogen of water over carbon dioxide to form organic ompounds. He also propounded that oxygen is evolved from water.

Chlorophyll a is the primary photosynthetic pigment, which is bluish green in the pure state. It has an empirical formula C_55​H_72​O₅​N₄​ Mgand molecular weight of 893. It absorbs more red wavelength of light than violet blue . It is soluble in a number of organic solvents such as petroleum ether. 

The graphical curve showing the amount of energy of different wavelengths of light absorbed by a pgment is called absorption spectrum,. It is studies with the help of spectrophotometer. The absorption spectra of chorophylls a and b show that they absorb maximum light in the blue-violet and red wavelengths. The pgments are often known after the wvelength which is absorbed to the maximum e.g., chl a₆₇₃ Chl a₆₈₃(P₆₈₀), Chl a₇₀₃(P₇₀₀).

Xanthophylls (=Carotenois) are oxygen containing derivatives of carotenes, e.g., C₄₀H₅₆O (cryptoxanthin), C₄₀H₅₆O₂ (lutein, zeaxanthin). Yellowish colour of autumnal follage is due to lutein. The characteristic xanthophyll of brown algae is fucoxathin (C₄₀H₅₆O₆)