WATSON AND CRICKS MODEL OF DNA

PHOTOSYNTHESIS

Def- Photosynthesis is a process in which green plants manufacture food (carbohydrate) from simple inorganic materials like CO₂ and H₂O with the help of sunlight and chloroplast with the evolution of O₂ as a byproduct.

Overall equation of Photosynthesis –

                                                                              Light

                                6CO₂ + 12H₂O    -------------------> C₆H₁₂O₆ + 6H₂O + 6O₂

                                                                       Chlorophyll

1. Entire process of photosynthesis is completed within chloroplast.
2. CO₂ and H₂O are the raw materials for photosynthesis.
3. Mechanism of photosynthesis is completed in two major steps i.e. light reaction and dark reaction.
4.  During light reaction solar energy is trapped and covered into chemical energy as well as water is oxidized to release oxygen.
5. While in dark reaction CO₂ is reduced to sugar using chemical energy. Thus photosynthesis is called redox reaction.

Autotrophic nutrition –

Def.- The organisms which synthesize their own food material from simple inorganic substances the called autotrophs. Types of autotrophic nutrition-

      1. Photo autotrophs -

The autotrophic which prepare organic food with the help of light energy by the process of photosynthesis is known as photo autotrophs or photosynthetic autotrophs.

Exa.-      1. All green plants.

2. Prokaryotes like Cynobacteria (blue green algae), Diatoms, Euglena.

3. Few species of bacteria like purple sulphur bacteria(Chromatium), green                      sulphur bacteria, (Chlorobium lamicala), Rhodospirillium.

Except bacteria, all photosynthetic autotrophs utilized H2O as raw material for food synthesis thus it is oxygen process. Bacteria used H₂S instead of H₂O thus bacterial photosynthesis is anoxygenic process.

     2. Chemosynthetic autotrophs –

             The autotrophs which synthesize organic food by using energy that obtained from chemical reactions are called chemosynthetic autotrophs and the process is called as chemosynthesis. Chemosynthetic autotrophs oxidized inorganic substance into simple substances. Energy liberated during this oxidation utilized for synthesis of organic compound.

Exa.- Nitrosomonas (Oxidized NH₃ to nitrite),Colorless sulphur bacteria(Thiobacillus) (Oxidized H₂S), Nitrobacteria (Oxidized nitrite),Iron bacteria (Ferrobacillus) etc.

Van Niel Experiment: -

             Van Niel (1930) studied photosynthesis in green sulphur bacteria & observed that H₂S act as hydrogen donar & sulphur is liberated. In green plant, instead of H₂S, H₂O is used. Thus he suggested that, in green plant, H₂O act as hydrogen donar & O₂ is liberated.

nCO₂ + 2n H₂S ----------------->(CH₂O) n + H₂O +2S.

During the process of photosynthesis, the evolution of oxygen takes place from water molecules and not from the CO₂, it was proved by :- 1. Hill reaction 2. Ruben and Kamen‟s experiment.

Hill reaction-

In 1939, Robert Hill studied light reaction in green plants & observed that by using light energy in the presence of Chlo-a, H₂O splits into H₂ & O₂. This is called photolysis of water. Since photolysis is first observed by Hill, it is called as Hill reaction.

                                                2H₂O  ---------------------> 2H₂ + O₂

 Robert hill first time proved experimentally that in photosynthesis, O₂ comes from H₂O & not from CO₂. In his experiment, he isolated chloroplast from spinach leaves. The isolated chloroplast is suspended in CO₂ free water containing ferric (Fe+++) salts and haemoglobin. He kept the mixture in sunlight & observed that by using light energy & 8in the presence of Chl-a, water molecule splits into H₂ & O₂. H₂ was accepted by ferric salt (Fe+++) which get reduced to ferrous salt (Fe++) while O₂ combined with haemoglobin to form oxyhaemoglobin. Thus he concluded that, during photosynthesis by using light energy in the presence of Chlorophyll, H₂O splits into H₂ & O₂ (H+ & OH--). Thus he suggested that O₂ liberated from H₂O & not from CO₂.

       sunlight

    2H₂O + 2A   ------------------>   2AH₂ + O₂

                                                               Chlorophyll-a

                                                                            [A = unknown hydrogen acceptor]

Prof. Daniel Arnon (1954) discovered that coenzyme NADP (Nicotinamide adenine dinucleotide phosphate) present in green leaves acts as H₂ acceptor during photosynthesis.

Thus, Hill reaction can be corrected as-

                                                           sunlight

2H₂O + 2NADP  ---------------> 2 NADPH₂ + O₂

                              Chlorophyll-a

Thus it was proved that O₂ evolved during photosynthesis comes from H₂O & not from CO₂.