Question

Explain what is photosynthesis in 250 words

Answer 1

Photosynthesis is one of the most fundamental biological reactions.

The chlorophyll bearing plants trap the free energy of sunlight as photons and transform and store it as chemical potential energy by combining CO2 and water.

The end products of photosynthesis are carbohydrates with loss of oxygen. These directly or indirectly serve as the source of energy for all living beings, except chemosynthetic bacteria.

Photosynthesis
Image Courtesy : co2crc.com.au/Photosynthesis_media.jpg
Photosynthesis as an oxidation-reduction reaction:

In 1931, С. B. Niel suggested that water is the hydrogen donor in the oxidation-reduction that occurs in photosynthesis. The ratio of oxygen evolved to carbon dioxide consumed is one. The over all reaction of photosynthesis is —

nН2О + nСО2 → light / chlorophyll nO2+ (CH2O)n

Thus, water is the donor of H2. The O2 that is liberated also comes from water and not from CO2 as has been confirmed by experiments using 18 ‘heavy oxygen – 18O’.

Thus, in this reaction, CO2 acts as an electron acceptor (A) and H2O as an electron donor (D). The energy contained in the visible light portion between 400 and 700 nm is transmitted in discrete particles called photons. A photon contains one quantum of light energy. Photons with shorter wave lengths have higher energy. The quantasomes are thought to be the units which absorb light quanta and contain chlorophyll in association with lipid.Photosynthesis involves a complex series of reactions, some of which take place only in the presence of light, while others can also be carried out in the dark.

(1) Light or photochemical reactions.

(2) Dark reactions.

In 1937, R. Hill (British Botanist) was the first to provide evidence that the photochemical reactions of plant cells took place in chloroplasts. Later it was discovered that the oxidized form of the coenzyme NADP+ was the normal hydrogen acceptor of the photochemical reaction, producing NADPH.Arnon (1954) showed that in the presence of light, chloroplasts could make ATP from ADP and Pi (this is the process called photophosphorylation). In photosynthesis, the electrons flow from H2O to NADPH. The photochemical reaction takes place in the thylakoid membranes.

1. Light or photochemical reactions:

Robert Hill found that when isolated chloroplasts illuminated, they produced oxygen and acquired reducing properties. Carbon dioxide was not involved and water had been split into hydrogen and oxygen.
This is now called the Hill reaction. It is the main event in light reactions of photosynthesis.function of light reactions is two fold —

(1) The photochemical splitting of water provides hydrogen atoms for the reduction of CO2, and

(2) Producing of ATP which provides energy for the subsequent synthesis of carbohydrates.

When light strikes the chlorophyll molecule, the energy level of an electron is raised. In this ‘excited’ state the electron is emitted and taken up by an electron acceptor. The electron is then passed through a series of carriers, the last one handing it back to the chlorophyll molecule.

In losing an electron the chlorophyll becomes positively charged and unstable, but neutrality and stability are restored when the electron is returned. Thus the electrons emitted from chlorophyll are returned to it unchanged and energy being released as heat and fluorescence.

This cyclic process is known as cyclic photophosphorylation. It is concerned with synthesizing ATP molecules but the source of energy is different. In photophosphorylation energy comes from sunlight via chlorophyll.

Splitting of water:

We have already seen that when light strikes chlorophyll an electron is emitted. Now not all the electrons are returned via the electron carrier system to chlorophyll. Sometimes an electron, together with a hydrogen ion, is taken up by a hydrogen acceptor, nicotinamide adenine dinucleotide phosphate (NADP), which is thus reduced.

The reduced NADP (i.e., NADPH2) now enters the dark reactions of photosynthesis, handing on the hydrogen which is then used in the reduction of CO2. This hydrogen comes from the dissociation of water into hydrogen (H+) and hydroxyl (OH–) ions in light reactions.

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When NADPH2 loses its hydrogen, the reduced NADP becomes reoxidised and can be used again.

The chlorophyll molecule which has lost an electron is in an unstable state. The hydroxyl ion donates an electron to chlorophyll and the OH resulting from this forms water and oxygen.

OH– + chlorophyll+ chlorophyll + OH

4(OH) → 2H2O + O2

By acting as an electron donor the hydroxyl ion restores the stability of the chlorophyll molecule. The oxygen is given off in photosynthesis.

The electron derived from the hydroxyl ion is conveyed from one chlorophyll molecule to another via an electron carrier system. This results in the formation of ATP. This pathway is non-cyclic phosphorylation.

Answer 2

Photosynthesis is the process by which green plants prepare their own food with the help of CO2 and H2O, in presence of chlorophyll and sunlight. Photosynthesis is divided into two phases-light dependent and light independent .
Light depentdent reactions takes place in the walls of the the thyllakoids.The main events in this phase is activation of chlorophyll and the photolysis of water.There is a nees of four photons for the activation of chlorophyll.Futher O2 is released from water and photophosphorylation takes place.
Light independent or dark reaction takes place in the stroma of the chloroplast where CO2 is taken in with the help of RUBUSCO and the further process of formation of glucose carry on.
The glucose prepared by the plants in this process is converted to starch, a polymer of glucose,and is stored for the futher use by the plant.
Photosynthesis is the process by which all the animals get their food directly or indirectly as only the green plants can prepare food on their own and all the other organisms are depended on them for the purpose of nutrition.