Page 26 - 84 human physiolofy part-1

Chapter 2

Energy Rich Molecules

Adenosine Triphosphate (ATP)

ATP is the currency of the cell. When the
cell needs to use energy such as when it needs
to move substances across the cell membrane
via the active transport system, it "pays" with
molecules of ATP. The total quantity of ATP
in the human body at any one time is about 0.1
Mole. The energy used by human cells
requires the hydrolysis of 200 to 300 moles of
ATP daily. This means that each ATP
molecule is recycled 2000 to 3000 times
during a single day. ATP cannot be stored, Chemical diagram of an ATP molecule.
hence its consumption must closely follow its
synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body.
Looking at it another way, a single cell uses about 10 million ATP molecules per second to meet its
metabolic needs, and recycles all of its ATP molecules about every 20-30 seconds.

Flavin Adenine Dinucleotide (FAD)

When two hydrogen atoms are bonded, FAD is reduced to FADH and is turned into an energy-
2
carrying molecule. FAD accommodates two equivalents of Hydrogen; both the hydride and the proton
ions. This is used by organisms to carry out energy requiring processes. FAD is reduced in the citric
acid cycle during aerobic respiration

Nicotinamide Adenine Dinucleotide (NADH)

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Nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate
+
+
(NADP) are two important cofactors found in cells. NADH is the reduced form of NAD , and NAD is
the oxidized form of NADH. It forms NADP with the addition of a phosphate group to the 2' position
of the adenosyl nucleotide through an ester linkage.

Space-filling model of NADHNAD is used extensively in glycolysis and the citric acid cycle of
cellular respiration. The reducing potential stored in NADH can be converted to ATP through the
electron transport chain or used for anabolic metabolism. ATP "energy" is necessary for an organism to
live. Green plants obtain ATP through photosynthesis, while other organisms obtain it by cellular
respiration.

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Nicotinamide adenine dinucleotide phosphate (NADP )NADP is used in anabolic reactions, such
as fat acid and nucleic acid synthesis, that require NADPH as a reducing agent. In chloroplasts, NADP
is an oxidising agent important in the preliminary reactions of photosynthesis. The NADPH produced
by photosynthesis is then used as reducing power for the biosynthetic reactions in the Calvin cycle of
photosynthesis.

26 | Human Physiology