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3 Biology -- Photosynthesis

Explain the C3 pathway of photosynthesis.

Explain the C3 pathway of photosynthesis.

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It is the reaction which occurs in the stroma of chloroplast in absence of light or in dark condition. In this ATP and NADPH2 produced during light reaction are utilized as source of energy. It is studied in the following steps:- 

1. Assimilation of carbondioxide or carboxylation:- It is the fixation  of carbondioxide by RuBP in presence of RuBISCO enzyme. When RuBP reacts with carbondioxide, 6-carbon compound is formed which is unstable and converted into 3-carbin compound PGA phosphoglyceric acid. PGA is 3-carbin first stable compound. So, this cycle is known as C3 cycle.

 RuBP + CO→ 6- carbon intermediate  compound(RuBISCO)



6-carbon intermediate → 3-PGA(carboxymutase)

2. Glycocytic reversal :- It is the conversion of 3-PGA into glucose molecule in many steps. Such steps are reversal. So, it is called glycotic reversal.

I. 3-PGA + ATP→ 1,3-diphosphoglyceric acid + ADP (triosephosphate kinase)

II. 1,3dPGA→ 3-phosphoglyceraldehyde+ H3PO4 ( triosephosphate dehydrogenase /NADPH)

III. 3-PGal →Dihydroxyacetone-3-posphate (phosphotriose isomerase)

IV. 3-PGal + Dihydroxyacetone-3-phosphate →Fructose (aldolase)

V. Fructose-1,6-phosphate +H2O → Fructose-6-phosphate +H3PO4 (phophatase)

VI. Fructose-6-phosphate →Glucose-6-phosphate (Isomerase)

VII. Glucose-6-phosphate→ Glucose→→ starch (phophatase)

3. Regeneration of RuBP :- Fructose 6- phosphate and glyceraldehyde-3-phosphate react to form erythrose-4-phosphate and Xylulose-5 phosphate. The erythrose-4 phosphate and dihydroxyacetone phosphate combine to produce sedoheptulose-7 phosphate. Ribulose biphosphate is regenerated from sedoheptulose- 7 phosphate reacting with other intermediates in many reactions.

Fructose-6-phosphate + Glyceraldehyde-3-phosphate →Erythrose-4-phosphate + xylulose-5phosphate

 Erythrose-4-phosphate + Dihydroxyacetone →Sedoheptulose-7 phosphates

edoheptulose-7 phosphate →→→→RuBP


More questions on Photosynthesis

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The synthesis of complex organic compounds from simple organic compounds like water and carbondioxide by green plants in the presence of chlorophyll using sunlight is called photosynthesis.

6CO2+ 6H2O CHO+6O

Write the importance of photosynthesis.

The importance's of photosynthesis are as follows:-

  1. It provides food for the plants and oxygen which makes environment clean
  2. It utilizes carbondioxide from the atmosphere which helps to reduce global warming to some extent.
  3. Food produced by plants by the process of photosynthesis is utilized by many organisms. So, it serves as a source of food.
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What is dark reaction? Where does it occur?

Dark reaction, also called calvin cycle is an anabolic process in which 3-Carbon sugar called glyceraldehyde-3-phospate is produced. This is a light independent process.

Differentiate between cyclic and non-cyclic photophosphorylation.

Differences between cyclic and non-cyclic photophosphorylation are given below:-


cyclic photophosphorylation non-cyclic photophosphorylation

1. In this process, electron released from the reaction centre passes through different different ways and return back to the same centre.

2. Only PSI system is involved in it.


3. Last electron acceptor in it is P (PSI) itself.

4. It produces ATP only.

5. First electron donor in it is P.

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1. In this...

What is photorespiration?

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It proceed in the following mechanism:-

1. During high temperature, high concentration of O2and low concentration of CO2, the affinity of Ribulose bi-phosphate (RuBP) carboxylase for CO decreases and for O increases and form oxyɡenase which catalyse the oxidation of RuBP to form 3-PGA(3-phospgoglyceric acid) and 2-phosphoglycolate.

RuBP +O...

Differentiate between C3 and C4 cycle.

Differences between C3 cycle and C4 cycle are as follows:-

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1. It's CO2acceptor molecule is phosphoenol pyruvic acid.
2.The first stable compound produced in it is phosphoglyceric acid which is 3 carbon compound 2. The first stable compound produced in it is oxalo acetic acid which is 4 carbon compound.
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It is completed in the following ways:-

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List down the significance of C4 plants 

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