Cell Respiration and Metabolism 1075.1 Glycolysis and the Lactic Acid Pathway
In cellular respiration, energy is released by the stepwise
breakdown of glucose and other molecules, and some of
this energy is used to produce ATP. The complete combus-
tion of glucose requires the presence of oxygen and yields
about 30 ATP for each molecule of glucose. However, some
energy can be obtained in the absence of oxygen by the
pathway that leads to the production of lactic acid.This energy transfer involves oxidation-reduction reac-
tions. Oxidation of a molecule occurs when the molecule
loses electrons (chapter 4, section 4.3). This must be coupled
to the reduction of another atom or molecule, which accepts
the electrons. In the breakdown of glucose and other molecules
for energy, some of the electrons initially present in these
molecules are transferred to intermediate carriers and then
to a final electron acceptor. When a molecule is completely
broken down to carbon dioxide and water within an animal
cell, the final electron acceptor is always an atom of oxygen.
Because of the involvement of oxygen, the metabolic pathway
that converts molecules such as glucose or fatty acid to carbon
dioxide and water (transferring some of the energy to ATP) is
called aerobic cell respiration. The oxygen for this process
is obtained from the blood. The blood, in turn, obtains oxygen
from air in the lungs through the process of breathing, or ven-
tilation, as described in chapter 16. Ventilation also serves the
important function of eliminating the carbon dioxide produced
by aerobic cell respiration.
Unlike the process of burning, or combustion, which
quickly releases the energy content of molecules as heat (and
which can be measured as kilocalories—see chapter 4), the
conversion of glucose to carbon dioxide and water within the
cells occurs in small, enzymatically catalyzed steps. Oxygen
is used only at the last step. Because a small amount of the
chemical-bond energy of glucose is released at early steps in
the metabolic pathway, some tissue cells can obtain energy for
ATP production in the temporary absence of oxygen. This pro-
cess is described in the next two sections.
Figure 5.1 presents an overview of the processes by which
glucose can be obtained by the body cells and used for energy.
Plasma glucose is derived from the digestion of food or from
the breakdown of liver glycogen. The aerobic cell respira-
tion of glucose occurs in three successive steps: (1) glycolysis
(a metabolic pathway that takes place in the cytoplasm); (2) the
citric acid cycle (a metabolic pathway that occurs in the mito-
chondrial matrix); and (3) electron transport (a process that
occurs in the mitochondrial cristae). When glucose is metab-
olized anaerobically (without oxygen), glycolysis produces
pyruvic acid that is converted into lactic acid. Although anaer-
obic metabolism is important (it provides energy for exercising
skeletal muscles at certain times, for example), most body cells
obtain energy by aerobic cell respiration.Glycolysis
The breakdown of glucose for energy begins with a metabolic
pathway in the cytoplasm known as glycolysis. This term is
derived from the Greek ( glykys 5 sweet, lysis 5 a loosen-
ing), and it refers to the cleavage of sugar. Glycolysis is the
metabolic pathway by which glucose—a six-carbon (hexose)
sugar (see fig. 2.14)—is converted into 2 molecules of pyruvic
acid, or pyruvate. Even though each pyruvic acid molecule is
roughly half the size of a glucose, glycolysis is not simply the
breaking in half of glucose. Glycolysis is a metabolic pathway
involving many enzymatically controlled steps.Andrea was told by her physician that she could develop
metabolic syndrome if she didn’t lose weight. She started
a diet and checked her urine for ketone bodies to see if
she was breaking down fat. She went on a high-protein
diet, assured that she could still maintain a normal blood
glucose level while eating few carbohydrates. Exercis-
ing caused muscle fatigue and pain, which her trainer
said was normal, but a pain developed in her chest that
required medical attention.
Some of the new terms and concepts you will
encounter include:- Anaerobic pathway and aerobic respiration;
myocardial ischemia - Glycogenesis, glycogenolysis, lipogenesis, lipolysis,
and ketogenesis
Clinical Investigation
LEARNING OUTCOMES
After studying this section, you should be able to:- Describe the metabolic pathway of glycolysis,
how lactic acid is produced, and the physiological
significance of the lactic acid pathway.
All of the reactions in the body that involve energy transformation
are collectively termed metabolism. Metabolism may be divided
into two categories: anabolism and catabolism. Catabolic reac-
tions release energy, usually by the breakdown of larger organic
molecules into smaller molecules. Anabolic reactions require the
input of energy and include the synthesis of large energy-storage
molecules, including glycogen, fat, and protein.
The catabolic reactions that break down glucose, fatty
acids, and amino acids serve as the primary sources of energy
for the synthesis of ATP. For example, some of the chemical-
bond energy in glucose is transferred to the chemical-bond
energy in ATP. Because energy transfers can never be 100%
efficient (according to the second law of thermodynamics, dis-
cussed in chapter 4), some of the chemical-bond energy from
glucose is lost as heat.