Energy is the ability to do work.
Cells use oxygen to transform the breakdown products of the foods we eat into
the energy needed for muscle contraction; the transport of ions and other
molecules across cell membranes; and the synthesis of enzymes, hormones, and
other macromolecules. Energy metabolism refers to the processes by which
fats, proteins, and carbohydrates from the foods we eat are converted into
energy or complex energy sources in the cell. Catabolism and anabolism are the
two phases of metabolism. Catabolism consists of breaking down stored nutrients
and body tissues to produce energy. Anabolism is a constructive process in
which more complex molecules are formed from simpler ones.
The special carrier for cellular
energy is ATP. ATP molecules consist of adenosine, a nitrogenous base; ribose,
a five-carbon sugar; and three phosphate groups (Fig. 4.13). The phosphate
groups are attached by two high-energy bonds. Large amounts of free energy are
released when ATP is hydrolyzed to form adenosine diphosphate (ADP), an
adenosine molecule that contains two phosphate groups. The free energy
liberated from the hydrolysis of ATP is used to drive reactions that require
free energy. Energy from foodstuffs is used to convert ADP back to ATP. Because
energy can be “saved or spent” using ATP, ATP is often called the energy
currency of the cell.
Energy transformation takes place
within the cell through two types of energy production the anaerobic (i.e., without
oxygen) glycolytic pathway, occurring in the cytoplasm, and the aerobic (i.e.,
with oxygen) pathway, occurring in the mitochondria. The anaerobic
glycolytic pathway serves as an important prelude to the aerobic pathway. Both
pathways involve oxidation–reduction reactions involving an electron donor,
which is oxidized in the reaction, and an electron acceptor, which is reduced
in the reaction. In energy metabolism, the breakdown products of carbohydrate,
fat, and protein metabolism donate electrons and are oxidized, and the
coenzymes nicotinamide adenine dinucleotide (NAD+) and flavin adenine
dinucleotide (FAD) accept electrons and are reduced.
Anaerobic Metabolism
Glycolysis is the process by which
energy is liberated from glucose. It is an important energy provider for cells
that lack mitochondria, the cell organelle in which aerobic metabolism occurs.
This process also provides energy in situations when delivery of oxygen to the
cell is delayed or impaired. Glycolysis involves a sequence of reactions that
convert glucose to pyruvate, with the concomitant production of ATP from ADP.
The net gain of energy from the glycolysis of one molecule of glucose is two
ATP molecules. Although comparatively inefficient as to energy yield, the
glycolytic pathway is important during periods of decreased oxygen delivery, as
occurs in skeletal muscle during the first few minutes of exercise.
Glycolysis requires the presence of
NAD+. Important end products of glycolysis are pyruvate and NADH
(the reduced form of NAD+) plus H+. When oxygen is
present, pyruvate moves into the aerobic mitochondrial pathway, and NADH + H+
delivers its electron and proton (H+) to the oxi-dative electron transport
system. Transfer of electrons from NADH + H+ to the electron
transport system allows the glycolytic process to continue by facilitating the
regeneration of NAD+. Under anaerobic conditions, such as cardiac
arrest or circulatory shock, pyruvate is converted to lactic acid, which
diffuses out of the cells into the extracellular fluid. Conversion of pyruvate
to lactic acid is reversible, and after the oxygen supply has been restored,
lactic acid is converted back to pyruvate and used directly for energy or to
synthesize glucose.
Much of the conversion of lactic
acid occurs in the liver, but a small amount can occur in other tissues. The
liver removes lactic acid from the bloodstream and converts it to glucose in a
process called gluconeogenesis. This glucose is released into the bloodstream
to be used again by the muscles or by the central nervous system (CNS). Heart
muscle is also efficient in converting lactic acid to pyruvic acid and then
using the pyruvic acid for fuel. Pyruvic acid is a particularly important
source of fuel for the heart during heavy exercise when the skeletal muscles
are producing large amounts of lactic acid and releasing it into the
bloodstream.
Aerobic Metabolism
Aerobic metabolism occurs in the
cell’s mitochondria and involves the citric acid cycle and the electron
transport chain. It is here that the carbon compounds from the fats, proteins,
and carbohydrates in our diet are broken down and their electrons combined with
molecular oxygen to form carbon dioxide and water as energy is released. Unlike
lactic acid, which is an end product of anaerobic metabolism, carbon dioxide
and water are generally harmless and easily eliminated from the body. In a
24-hour period, oxidative metabolism produces 300 to 500 mL of water.
The citric acid cycle, sometimes
called the tricarboxylic acid (TCA) or Krebs cycle, provides the
final common pathway for the metabolism of nutrients. In the citric acid
cycle, which takes place in the matrix of the mitochondria, an activated
two-carbon molecule of acetyl-coenzyme A (acetyl- CoA) condenses with a
four-carbon molecule of oxaloacetic acid and moves through a series of
enzyme-mediated steps. This process produces hydrogen atoms and carbon dioxide.
As hydrogen is generated, it combines with NAD+ or FAD for transfer
to the electron transport system. In the citric acid cycle, each of the two
pyruvate molecules formed in the cytoplasm from one molecule of glucose yields
another molecule of ATP along with two molecules of carbon dioxide and eight
electrons that end up in three molecules of NADH + H+ and one of
FADH2 . Besides pyruvate from the glycolysis of glucose, products of amino
acid and fatty acid degradation enter the citric acid cycle and contribute to
the generation of ATP
Oxidative metabolism, which supplies
90% of the body’s energy needs,
takes place in the electron transport chain in the mitochondria. The electron
transport chain oxidizes NADH + H+ and FADH and donates the electrons to oxygen, which is reduced to water. Energy from reduction of oxygen is used for
phosphorylation of ADP to ATP. Because the formation of ATP involves the
addition of a high-energy phosphate bond to ADP, the process is sometimes
called oxidative phosphorylation. Among the members of the electron
transport chain are several iron-containing molecules called cytochromes.
Each cytochrome is a protein that contains a heme structure similar to that of
hemoglobin. The last cytochrome complex is cytochrome oxidase, which passes
electrons from cytochrome c to oxygen. Cytochrome oxidase has a lower binding
affinity for oxygen than myoglobin (the intracellular heme-containing oxygen
carrier) or hemoglobin (the heme-containing oxygen transporter in erythrocytes
in the blood). Thus, oxygen is pulled from erythrocytes to myoglobin and from
myoglobin to cytochrome oxidase, where it is reduced to H2O. Although
iron-deficiency anemia is characterized by decreased levels of hemoglobin, the
iron-containing cytochromes in the electron transport chain in tissues such as
skeletal muscle are affected as well. Thus, the fatigue that develops in
iron-deficiency anemia results, in part, from impaired function of the electron
transport chain.
