Metabolism
(from the Greek metabole, which means “change”) is the word for the myriad
chemical reactions that happen in the body, particularly as they relate to
generating, storing, and expending energy. All metabolic reactions are either catabolic
or anabolic. Catabolic reactions break food down into energy (memory tip: it
can be catastrophic when things break down). Anabolic reactions require the
expenditure of energy to build up compounds that the body needs. The chemical
alteration of molecules in the cell is referred to as cellular metabolism.
Enzymes can be used as catalysts, accelerating chemical reactions without being
changed by the reactions. The molecules that enzymes react with are called
substrates.
Adenosine
triphosphate (ATP) is a molecule that stores energy in a cell until the cell needs
it. As the tri– prefix implies, a single molecule of ATP is composed of three
phosphate groups attached to a nitrogenous base of adenine. ATP’s energy is
stored in high energy bonds that attach the second and third phosphate groups.
(The high-energy bond is symbolized by a wavy line.) When a cell needs energy,
it removes one or two of those phosphate groups, releasing energy and
converting ATP into either the two phosphate molecule adenosine diphosphate
(ADP) or the one-phosphate molecule adenosine monophosphate (AMP).
Later,
through additional metabolic reactions, the second and third phosphate groups are
reattached to adenosine, reforming an ATP molecule until energy is needed
again. Oxidation-reduction reactions are an important pair of reactions that
occur in carbohydrate, lipid, and protein metabolism. When a substance is
oxidized, it loses electrons and hydrogen ions, removing a hydrogen atom from
each molecule.
When
a substance is reduced, it gains electrons and hydrogen ions, adding a hydrogen
atom to each molecule. Oxidation and reduction occur together, so whenever one
substance is oxidized, another is reduced. The body uses this chemical-reaction
pairing to transport energy in a process known as the respiratory chain, or the
electron transport chain.
Carbohydrate
metabolism involves a series of cellular respiration. All food carbohydrates
are eventually broken down into glucose; therefore, carbohydrate metabolism is
really glucose metabolism. Glucose metabolism produces energy that is then
stored in ATP molecules. The oxidation process in which energy is released from
molecules, such as glucose, and transferred to other molecules is called
cellular respiration. It occurs in every cell in the body and it is the cell’s
source of energy. The complete oxidation of one molecule of glucose will produce
38 molecules of ATP. It occurs in three stages: glycolysis, the Krebs cycle,
and the electron transport chain:
1.
Glycolysis
From
the Greek glyco (sugar) and lysis (breakdown), this is the first stage of both aerobic
(with oxygen) and anaerobic (without oxygen) respiration. Using energy from
two molecules of ATP and two molecules of NAD+ (nicotinamide adenine di-nucleotide),
glycolysis uses a process called phosphorylation to convert a molecule of
six-carbon glucose — the smallest molecule that the digestive system can
produce during the breakdown of a carbohydrate — into two molecules of three-carbon
pyruvic acid or pyruvate, as well as four ATP molecules and two molecules of
NADH (nicotinamide adenine dinucleotide). Taking place in the cell’s cytoplasm,
glycolysis doesn’t require oxygen to occur. The pyruvate and NADH move into the
cell’s mitochondria, where an aerobic (with oxygen) process converts them into
ATP.
2.
Krebs cycle
Also
known as the tricarboxylic acid cycle or citric acid cycle, this series of energy
producing chemical reactions begins in the mitochondria after pyruvate arrives from
glycolysis. Before the Krebs cycle can begin, the pyruvate loses a carbon dioxide
group to form acetyl coenzyme A (acetyl CoA). Then acetyl CoA combines with a
four-carbon molecule (oxaloacetic acid, or OAA) to form a six carbon citric
acid molecule that then enters the Krebs cycle. The CoA is released intact to
bind with another acetyl group. During the conversion, two carbon atoms are
lost as carbon dioxide and energy is released. One ATP molecule is produced
each time an acetyl CoA molecule is split. The cycle goes through eight steps,
rearranging the atoms of citric acid to produce different intermediate molecules
called keto acids. The acetic acid is broken apart by carbon (or decarboxylated)
and oxidized, generating three molecules of NADH, one molecule of FADH2 (flavin
adenine dinucleotide), and one molecule of ATP. The energy can be transported
to the electron transport chain and used to produce more molecules of ATP. OAA
is regenerated to get the next cycle going, and carbon dioxide produced during
this cycle is exhaled from the lungs.
3.
Electron transport chain
The
electron transport chain is a series of energy compounds attached to the inner
mitochondrial membrane. The electron molecules in the chain are called cytochromes.
These electron-transferring proteins contain a heme, or iron, group. Hydrogen
from oxidized food sources attaches to coenzymes that in turn combine with molecular
oxygen. The energy released during these reactions is used to attach inorganic
phosphate groups to ADP and form ATP molecules.
Pairs
of electrons transferred to NAD+ go through the electron transport process and
produce three molecules of ATP by oxidative phosphorylation. Pairs of electrons
transferred to FAD enter the electron transport after the first phosphorylation
and yield only two molecules of ATP. Oxidative phosphorylation is important
because it makes energy available in a form the cells can use.
At
the end of the chain, two positively charged hydrogen molecules combine with
two electrons and an atom of oxygen to form water. The final molecule to which
electrons are passed is oxygen. Electrons are transferred from one molecule to
the next, producing ATP molecules.
Lipid
metabolism only requires portions of the processes involved in carbohydrate metabolism.
Lipids contain about 99 percent of the body’s stored energy and can be digested
at mealtime, but as people who complain about fats going “straight to their hips”
can attest, lipids are more inclined to be stored in adipose tissue — the stuff
generally identified with body fat. When the body is ready to metabolize
lipids, a series of catabolic reactions breaks apart two carbon atoms from the
end of a fatty acid chain to form acetyl CoA, which then enters the Krebs cycle
to produce ATP. Those reactions continue to strip two carbon atoms at a time
until the entire fatty acid chain is converted into acetyl CoA.
Protein
metabolism focuses on producing the amino acids needed for synthesis of protein
molecules within the body. But in addition to the energy released into the
electron transport chain during protein metabolism, the process also produces byproducts,
such as ammonia and keto acid. Energy is released entering the electron
transport chain. The liver converts the ammonia into urea, which the blood
carries to the kidneys for elimination. The keto acid enters the Krebs cycle
and is converted into pyruvic acids to produce ATP.
One
last thing: That severe soreness and fatigue you feel in your muscles after
strenuous exercise is the result of lactic acid buildup during anaerobic
respiration. Glycolysis continues because it doesn’t need oxygen to take place.
But glycolysis does need a steady supply of NAD+, which usually comes from the
oxygen-dependent electron transport chain converting NADH back into NAD+. In
its absence, the body begins a process called lactic acid fermentation, in
which one molecule of pyruvate combines with one molecule of NADH to produce a
molecule of NAD+ plus a molecule of the toxic byproduct lactic acid.
