Differentiate between photosystem I and II.

PS I

PS II

1. The system is located in both the non-appressed part of grana thylakoids as well as stroma thylakoids.

1. Photosystem II is present in the appressed part of grana thylakoids.

2. Its photocentre is P700.

2. Its photocentre is P680.

3. Has an absorption peak at wavelength of 700nm. 

3. Has an absorption peak at the wavelength of 680nm.

4. It receives electrons from photosystem II.

4. Electrons are received from photolytic reaction.

5. Molecular oxygen is not evolved in this system.

5. Molecular oxygen is evolved by break down of water.

 
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What led to the evolution of C4 plants or C4 pathway of photosynthesis ? Describe in detail.

C4 pathway is an adaptation to tolerate strong light, high temperature ; high quantity of oxygen etc. Evolution of C4 plants from C3 plants can be described as follows :

1. C3 plants undergo photorespiration in strong light or at high temperature which is a wasteful process in which no energy is produced.

2. In C3 plants RUBP carboxylase get denatured in strong light, but in C4 plants it does not.

3. These C4 plants have Kranz anatomy. In this case mesophyll cells have grana thylakoid and bundle sheath cells have agranal thylakoids present in stroma of chloroplast.

4. In C4 plant, light reaction occurs in mesophyll cells and RuBisCo is found in bundle sheath cells in which CO2 fixation occurs. Thus, in C4 plants RuBisCo remains protected from sunlight and is also protected from oxygenation, because in bundle sheeth cells only dark reaction occurs.

Thus, to avoid photorespiration, oxygenation of RuBisCo etc. C4 plants were evolved from C3plants. The C4 plants are maize, sugar cane, grasses etc.

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What is photorespiration ? Describe the process in detail and link it with the Calvin Cycle.

Photorespiration is the process where oxygen is consumed, food is oxidized and carbondioxide is liberated, but no energy is produced. It is a highly wasteful process which occurs in the presence of light. 

Mechanism of photorespiration:

It takes place in chloroplasts and completes with the assistance of peroxysomes and mitochondria.

1. At high temperature RUBP caThe enzyme has great affinity for oxygen than for carbon dioxide. It combines oxygen and RUBP to form PGA and phosphoglycolate.

2. Phosphoglycolate forms phosphoric acid and glycolate in the presence of hydrolase. It passes into peroxysome.

3. Glycolate molecule combines with oxygen to form glyoxylate and H2O2 which is very toxic. It occurs in peroxysome.

4. Glycoxylate undergoes amination to form glycine. The glycine molecules undergo condensation, decarboxylation, deamination to form serine.

5. Serine undergoes deamination to form PGA which enters into chloroplast and takes part in C3 cycle. Thus, it is linked with C3 cycle. Moreover RUBP carboxylase changes its behaviour at high temperature and acts RUBPoxygenase and thus it delinks from C3 cycle.

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Describe carbon reactions of the Cpathway. Does this pathway also operate in Cplants ?


Calvin Cycle (C3 cycle).
The steps involved in C3 cycle are discussed below:

(a) Carboxylation : It is the fixation of CO2 into a stable compound. the reaction is catalysed by RuBP carboxylase enzyme. In this the ribulose 1, 5 -bisphosphate reacts with carbon dioxide to give 3-PGA (3 phosphoglyceric acid).

(b) Reduction : In this step carbohydrate is formed at the expense of ATP and NADPH. Two molecules of ATP are utilized for phosphorylation and two NAdph for the reduction of COmolecule fixed. 6 molecules of CO2 and 6 turns of cycle are required for removal of one molecule of glucose from pathway. 

(c) Regenration - In this step the CO2 acceptor molecule RuBP is regenerated.





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What is photosystem ? Which is the pigment that acts as reaction centre ? Describe the interaction of photosystem I and photosystem II.

 


Photosystem : It is a group of pigment molecules and electron acceptors which take part in oxidation and reduction reaction in photosynthesis for producing ATP and NADPH.

Reaction Centre : Chla molecules  act as reaction centres. The Chla of speacial types form different reaction centres like P680 in PS II and P700 in PS I.

Interaction of PS I and PS II :

PS I and PS II are interlinked in following two ways :

Ist way : P680 reaction centre of PS II takes energy from sunlight. Then, it emits electron which pass through number of electron acceptors and then go to P700, a reaction centre of PSI.

IInd way : The P700 takes energy from sunlight via harvesting molecule and its electron is emitted. This electron passes through primary acceptor x and then to ferrodoxin and after this to NADP reductase.

During photolysis of H2O, the H2O splits into H+ and OH. The OH changes into H2O, O2 and electrons are released which go to P680 molecule. The H+ goes to NADP reductase which combines NADP, H+ and e form assimilatory power called NADPH

What is photosystem ? Which is the pigment that acts as reaction centre ? Describe the interaction of photosystem I and photosystem II.
Or
Discuss the mechanism of oxidation-reduction occurring in photosynthesis to generate ATP and NADPH.

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