Question

With the help of a suitable diagram, describe Ultra structure of the cell organalle, which is essential for photosynthesis.

                                        (OR)

During photosynthesis “O2 is evolved from water molecule and not from CO2”. Give the experimental proof given by Robert Hill.

Answer 1

(1) Chloroplast is a double membrane bound structure and an outer and inner membrane is collectively called peristromium.
(2) It encloses colourless, colloidal matrix called stroma, which contains enzymes for reduction of CO₂ into glucose (dark reaction), 70S ribosomes and DNA.
(3) DNA is circular, closed, naked ring and is called plastidome.
(4) As DNA is present, Chloroplast is self-replicating and semi-autonomous cell organelle.

(5) Embedded in the stroma, there are approximately 40 to 60 green coloured structures called grana. Each granum consists of small, dis-like lamellae or thylakoids.
(6) Photosynthetic pigments are embedded in the thylakoids. ATP synthesis (light reactions) takes place in grana.
(7) Grana are interconnected by mambranes called stroma – lamellae or intergrana or fret membranes, which help in rapid transport of materials. (In prokaryotes, chloroplasts are absent and pigments are located in lamellae i.e. thylakoids).

                                           (OR)

Robert Hill (1937) performed an expriment in which he suspended isolated chloroplasts from spinach leaves in water which was without CO₂. He then added ferric salts as hydrogen acceptor and the suspension was exposed to sunlight. He observed that ferric salts are reduced to ferrous and O₂ bubbles evolve. Thus is called photolysis of water of Hill Reaction and is represented as follows;

         

Here, ‘A’ is unknown hydrogen acceptor present in the chloroplasts.