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ATP stands for adenosine triphosphate. It is called the molecular currency of energy. ATP is a nucleotide composed of the nitrogenous base adenine, the pentose sugar ribose and three phosphate radicals. The phosphate bond contains about 12,000 calories of energy per mole of ATP under the physical conditions of the body. When ATP releases its energy, a phosphoric acid radical is split away and adenosine diphosphate (ADP) is formed, and the vice versa reaction takes place when ATP is formed.
Function of ATP in Photosynthesis
In photosynthesis, ATP is synthesized from the thylakoid membrane (sites of the photochemical reactions of photosynthesis) of chloroplast cells of plants. The photon particles from the sunlight excite the chloroplast thylakoid membrane, which in turn converts this excitation into ATP's chemical energy. Photosynthesis consists of two phases--the light reaction and the dark reaction.
Chlorophyll pigment contained in chloroplasts of leaves is the organelle where the light reaction takes place. When sunlight falls on the pigment, due to chemical reaction in it, ATPs are formed. ATP is then used to provide the chemical energy necessary to power other metabolic reactions in cell.
Dark reaction is also called Calvin cycle. It is a light independent process in which the products of the light reaction (i.e., ATPs) are used to form carbon-carbon covalent bonds of carbohydrates. During this process ATP formed during light reaction are utilized. Hence the ATP energy is converted into carbohydrates, which are considered the energy-rich food.
Function of ATP in Respiration
According to Merriam-Webster Dictionary, "Respiration is the physical and chemical processes by which an organism supplies its cells and tissues with the oxygen needed for metabolism and relieves them of the carbon dioxide formed in energy-producing reactions." Hence the carbohydrates formed in the process of photosynthesis are used at cellular level to form ATP, as this is the chemical moiety used for energy at cellular level.
The ATP is formed from the carbohydrate (sugars) through a few steps:
Glycolysis--Glucose is broken down to pyruvic acid, yielding 2 ATP of energy.
Link reaction--Certain link reactions takes place in which pyruvate is converted into acetyl-CoA (an important molecule in metabolism), which is utilized inside the mitochondria of cell.
Krebs cycle--This is an important step in use of acetyl-CoA, releasing high energy products nicotinamide adenine dinucleotide phosphate (NADPH).
Electron transport chain--The NADPH formed is converted into ATPs in the electron transport chain. Hence complete breakdown of sugars into ATP occurs at cellular respiration
The ATP formed at cellular level are used by cell for various functions--mechanical functions of cells like beating of cilia, transport work like pumping substances across membranes and chemical reactions like formation of new substances or their breakdown.