Page 27 - 84 human physiolofy part-1

Cell Physiology

+
+
MH + NAD → NADH + H + M: + energy,
2
where M is a metabolite. Two hydrogen ions (a
+
hydride ion and an H ion) are transferred from the
metabolite. One electron is transferred to the
positively-charged nitrogen, and one hydrogen
attaches to the carbon atom opposite to the nitrogen.

The change upon nicotinamide group when
+
NAD is reduced The human body synthesizes
NAD from the vitamin niacin in the form of
nicotinic acid or nicotinamide. Chemical diagram of an NADH molecule.

Cellular Respiration

Cellular respiration is the energy releasing process by which sugar molecules are broken down by
a series of reactions and the chemical energy gets converted to energy stored in ATP molecules. The
reactions that convert the fuel (glucose) to usable energy (ATP) are glycolysis, the Krebs cycle
(sometimes called the citric acid cycle), and the electron transport chain. Altogether these reactions are
referred to as "cellular respiration" or "aerobic respiration." Oxygen is needed as the final electron
acceptor, and carrying out cellular respiration is the very reason we breathe and the reason we eat.

Glycolysis

The glycolytic pathway (glycolysis) is where glucose, the smallest molecule that a carbohydrate
can be broken into during digestion, gets oxidized and broken into two 3-carbon molecules (pyruvates),
which are then fed into the Kreb's Cycle. Glycolysis is the beginning of cellular respiration and takes
place in the cytoplasm. Two molecules of ATP are required for glycolysis, but four are produced so
there is a net gain of two ATP per glucose molecule. Two NADH molecules transfer electrons (in the
form of hydrogen ions) to the electron transport chain in the mitochondria, where they will be used to
generate additional ATP. During physical exertion when the mitochondria are already producing the
maximum ATP possible with the amount of oxygen available, glycolysis can continue to produce an
additional 2 ATP per glucose molecule without sending the electrons to the mitochondria. However,
during this anaerobic respiration lactic acid is produced, which may accumulate and lead to temporary
muscle cramping.

Krebs Cycle

The Krebs cycle was named after Sir Hans Krebs (1900-1981), who proposed the key elements of
this pathway in 1937 and was awarded the Nobel Prize in Medicine for its discovery in 1953.

Two molecules of pyruvate enter the Krebs cycle, which is called the aerobic pathway because it
requires the presence of oxygen in order to occur. This cycle is a major biological pathway that occurs
in humans and every plant and animal.

After glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the

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