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Explain mechanism of photosynthesis by giving the explanation of light and dark reaction..
Answers
Photosynthesis is the process by which green plants absorb light energy from the sun with the assistance of water and carbon dioxide, and transform it into chemical energy to make (synthesize) carbohydrate (specifically glucose) and oxygen. Photosynthesis can be summarized with this formula:
6CO2 + 6H2O + sunlight (light energy) → C6H12O6 + 6O2
If you need a longer review, check out our lesson on photosynthesis before reading on.
The "light-independent" or dark reactions happen in the stroma of the chloroplasts. This is also known as the Calvin Cycle. Since these processes can only happen in the chloroplast (a chlorophyll filled plastid in green plants), photosynthesis can only happen in green plants!
The first overall principle of photosynthesis is that the light energy from the sun is transformed into chemical energy and stored in the bonds of glucose (the sugar carbohydrate) for later use by the plant and/or organism that eats the plant.
The second overall principle of photosynthesis is that carbon, oxygen, and hydrogen atoms are taken from carbon dioxide and water molecules and are broken up and rearranged into new substances: carbohydrate (specifically glucose) and oxygen gas (so we can breathe, whew!). This reaction represents the transfer of matter: carbon dioxide from the atmosphere, water from the soil or atmosphere, into sugar in the plant and oxygen back into the atmosphere.
Light-Dependent Reactions
The first part of the process happens in the thylakoids of the chloroplasts and are the "light-dependent" reactions: The photosystems I and II absorb the photons from the sunlight and process them through the membranes of the thylakoids simultaneously. The photons excite electrons in the chlorphyll which then move through the electron transport chain and causes NADP- to combine with H+ forming NADPH. At the same time, ADP (adenosine diphosphate) has come from the dark reaction and a third phosphate chain is bonded forming ATP (adenosine triphosphate) to feed the Calvin Cycle next. Remember that ATP is the important source of all cellular energy.
We now believe that all the oxygen released in photosynthesis comes from the water molecules and all oxygen atoms that form the carbohydrates come from the carbon dioxide molecules. So, in other words during the light-dependent reaction a water molecule is broken down producing two H+ ions and half an oxygen molecule. We get the rest of the oxygen molecule when another water molecule is broken down.
Dark Reactions
Dark reactions are also known as the Calvin Cycle, the Calvin-Benson cycle, and light-independent reactions. The point is that they do not require sunlight to complete their process.
After ATP is formed in the first part of photosynthesis, for living things to grow, reproduce and repair themselves, the inorganic form of CO2 must be transformed into carbohydrate. This happens during the Calvin Cycle in the stroma (the fluid filled interior of the chloroplast). ATP and NADPH combine with CO2 and water to make the end product of glucose. The ADP and NADPH+ are recycled to the light-dependent side to start the process over.
Remember that during hours of darkness, plants cannot perform photosynthesis so they do cellular respiration in the mitochondria just as all living organisms do.
Mechanism of Photosynthesis
The process of photosynthesis can be divided into two main phases:
1.Light reaction (or light dependent reaction)
2.Dark reaction (or light independent reaction)
1.Light reaction :
It occurs in the grana present in chloroplast. During light reaction, chlorophyll absorbs light energy in the form of photons. Actually, light is emitted in the form of ‘packets of energy’ called photons. The light energy absorbed by chlorophyll is used to do following reactions:
a. Formation of ATP
ATP is the main energy source of a cell. Its full form is Adenosine Tri Phosphate. A molecule of ATP is formed by combining inorganic phosphate with ADP (Adenosine DiPhosphate) in the presence of energy.
ADP + phosphate + energy Formation of ATP ATP
This ATP is used as a source of energy during dark reaction.
b. Photolysis of water
The process of decomposition of water into hydrogen and oxygen by light energy absorbed by chlorophyll is called photolysis. The oxygen produced by the photolysis of water is released into the air as a byproduct of photosynthesis.
H2O Photolysis of water 2H+ + 1/2O2 + 2e-
It should be noted that oxygen released during photosynthesis comes from water.
3. Formation of NADPH (reduced Nicotinamide Adenine Dinucleotide Phosphate)
The hydrogen ions (H+) released during second reaction reduces the NADP (Nicotinamide Adenine Dinucleotide Phosphate) into NADPH in the presence of energy.
NADP + Hydrogen + Energy Formation of NADPH NADPH
NADPH is also used during dark reaction.
2.Dark reactions (light independent reactions)
In dark reactions, the ATP molecule and NADPH produced during light reaction, are used in the matrix (or stroma) of chloroplast to make carbohydrates from carbon dioxide. In this process, carbon dioxide enters into a cycle of reactions in the presence of a substance called RuDP (Ribulose DiPhosphate) and enzymes. At the end of the cycle, carbohydrate is synthesized and ribulose diphosphate is regenerated.
RuDP + CO2 + NADPH +ATP ⇒ RuDP + Carbohydrate + NADP
+ ADP + phosphate
This cycle of reactions is called Calvin-Benson cycle because it was discovered by Melvin Calvin and Andy Benson.
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