 |  Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University
Margaret W. Kessel,
Ohio State University
Metabolism
Chapter 4 Summary- ATP is the major form of energy used for cellular metabolism. As ATP breaks
down to ADP plus Pi, energy is released from the broken bond. This energy
is used to pump ions, promote enzyme activity, and contract and later relax
muscles. All energy available to humans ultimately comes from the sun as solar
energy. Plants capture solar energy by way of photosynthesis. In humans, metabolic
pathways make it possible to extract energy from food and transform it into
ATP; in the process, some energy is lost as heat.
- In glycolysis, glucose is degraded into two pyruvate molecules, yielding
NADH + H+ (a form of potential energy) and ATP. Pyruvate can proceed through
other aerobic pathways to form carbon dioxide and water. Pyruvate also can
react with NADH + H+ in an anaerobic pathway to form lactate. Both pathways
allow NADH + H+ to eventually be re-formed into NAD, which is needed for glycolysis
to continue.
- In the citric acid cycle, acetyl-CoA is formed from pyruvate. A carbon dioxide
molecule is released in the process. Acetyl-CoA then undergoes many metabolic
conversions, eventually yielding two more carbon dioxide molecules. In this
way, the citric acid cycle accepts two carbons from acetyl-CoA and yields
two carbons as carbon dioxide. In the process, NADH + H+, FADH2, and a form
of energy that can yield ATP directly (GTP) are formed. The NADH + H+ and
FADH2 then enter the electron transport chain to yield numerous ATP molecules.
Water forms as oxygen combines with the electrons and hydrogen ions (released
from NADH + H+ and FADH2) in the electron transport chain.
- In fatty acid oxidation, two-carbon fragments are cleaved from a fatty acid,
producing multiple acetyl-CoA molecules. These enter the citric acid cycle
and electron transport chain, as did the acetyl-CoA that arose from carbohydrate
breakdown, to yield ATP, carbon dioxide, and water. In fat synthesis, acetate
molecules in effect are combined to yield a fatty acid, primarily the 16-carbon
palmitic acid. These fatty acids can then react with a form of glycerol to
produce a triglyceride.
- During starvation and uncontrolled diabetes, more acetyl-CoA is produced
in the liver than can be metabolized to carbon dioxide and water. This excess
acetyl-CoA is synthesized into ketone bodies, which flood into the bloodstream
and are metabolized by other tissues, such as nervous tissue.
- Amino acids lose their amino group and become carbon skeletons. These can
be metabolized to other compounds that enter the citric acid cycle, eventually
yielding energy for ATP synthesis. Some carbon skeletons can be formed into
oxaloacetate, an intermediate found in the citric acid cycle, which in turn
can be used to form glucose. Converting the carbon skeletons of amino acids
to glucose is part of a process known as gluconeogenesis. Acetyl-CoA molecules,
and thus fatty acids in general, cannot participate in gluconeogenesis.
- Glycolysis takes place in the cytosol of a cell, whereas the citric acid
cycle and the electron transport chain take place in the mitochondria. Fatty
acid oxidation takes place in the mitochondria, and fatty acids for the most
part are synthesized in the cytosol. The synthesis of urea and the pathway
for gluconeogenesis both take place partly in the cytosol and partly in the
mitochondria. Urea is made in the liver, while glucose is made in the liver
and kidneys.
- Acetyl-CoA is pivotal in cell metabolism because carbohydrates, proteins,
amino acids, fatty acids, and alcohol all can yield acetyl-CoA during their
metabolism. The coordination of various metabolic pathways for food fuels
allows the carbons of glucose to become the carbons of fatty acids and the
carbons of some amino acids to become the carbons of glucose.
- The vitamins thiamin, niacin, riboflavin, biotin, pantothenic acid, and
vitamin B-6 and the minerals magnesium, iron, and copper play important roles
in the metabolic pathways.
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2002 McGraw-Hill Higher Education