Page 23 - EM BIOLOGY NOTES 10TH CLASS PSR DIGITAL BOOKS

2. Dark Reactions / Calvin cycle / Light independent reactions / Bio synthetic Phase :
 The term Dark reaction does not mean that they occur when it is dark at night. It only
means that the reactions are not depend on light. Hence, it is better to call the dark phase
as a light independent phase.
 Dark reactions are discovered by Melvin Calvin. So, the cycle of reactions is called Calvin

Cycle.
 Dark reactions occur in stroma of the chloroplast.

 During Dark reactions CO is converted into Glucose, by utilising ATP and NADPH which
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produced in light reactions, with the help of RuBP and enzymes.
 Finally Glucose is converted into starch.
Light reactions Dark reactions
1. It is the first phase in the photosynthesis. 1. It is the second phase in the photosynthesis.
2. Occurs in the grana of the chloroplast. 2. Occurs in the stroma of the chloroplast.
3. This reactions occur only in the 3. This reactions occur in the presence (or)
presence of light. absence of light.
4. Chlorophyll plays key-role. 4. RuBP plays key role.
5. Light energy converted into chemical energy 5. CO converted into glucose.
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 ATP and NADPH are the connecting substances between light reactions and dark reactions
because ....
During Dark reactions CO is converted into Glucose by the utilising of ATP and NADPH
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which are produced in light reactions. So, ATP and NADPH are the connecting substances
between light reactions and dark reactions.
Importance of Photosynthesis :-
 All living organisms need energy to be alive. They get energy in the form of food.
 Photosynthesis is the only process on earth to prepare food by utilising light energy and
releasing O into atmosphere.
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 Hence, all living organisms depend directly or indirectly on photosynthesis for their food

requirements and O .
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 Hence photosynthesis is considered as the basic energy source for most of living world.
 So, without green plants, there could be no life on earth.
 For that, I appreciated the photosynthesis for its amazing mechanism.
Joseph Priestly’s Experiment
1. Priestly took two bell jars.
2. One jar inverted over a burning candle and another one on a mouse.
3. Then he observed that the burning candle gets extinguished. Similarly mouse would soon
suffocated.
4. After that he placed a mint plant in the same bell jars and did the same experiment.
5. Then we observed that the candle did not extinguish and the mouse did not suffocate,
stayed alive.
6. Priestly hypothesized as, plants restore to the air whatever breathing animals and burning
candles remove.
7. This experiment says that plants were giving out a gas that supported burning and was
essential for the survival of animals.

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