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Chapter 8
Metabolism
Key Terms
1.
metabolism: All chemical reactions within organisms that enable them to maintain life.
The two main categories are catabolism and anabolism.
2.
acetyl CoA: A key intermediate product in the metabolic breakdown of carbohydrates,
fatty acids, and amino acids. It consists of a two-carbon acetate group linked to coenzyme
A, which is derived from pantothenic acid.
3.
cells: The basic structural units of all living tissues. They have two major parts: the
nucleus and cytoplasm.
4.
ketogenesis: The process in which excess acetyl CoA from fatty acid oxidation is
converted into ketone bodies.
5.
aerobic: Referring to the presence of or need for oxygen.
6.
catabolism: Any metabolic process whereby cells break down complex substances into
simpler, smaller ones.
7.
photosynthesis: The process by which green plants use light energy from the sun to
produce carbohydrates from carbon dioxide and water.
8.
mitochondria: The sites of aerobic production of ATP, where most of the energy from
carbohydrate, protein, and fat is captured. Called the “power plants” of the cell, they are
where the citric acid cycle and electron transport chain are located. A human cell contains
about 2,000 of these.
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9.
citric acid cycle: The aerobic metabolic pathway in mitochondria that breaks down acetyl
CoA to yield two molecules of carbon dioxide, one molecule of GTP, and pairs of highenergy electrons. It transfers the electrons to three molecules of NAD+ (yielding three
NADH) and one molecule of FAD (yielding one FADH2). Also known as the Krebs cycle
and the tricarboxylic acid cycle.
10.
cytosol: The semifluid inside the cell membrane, excluding organelles. It is the site of
glycolysis and fatty acid synthesis.
11.
organelles: Various membrane-bound structures that form part of the cytoplasm. They
perform specialized metabolic functions.
12.
metabolites: Substances produced during metabolism.
13.
chemical energy: Energy contained in the bonds between atoms of a molecule.
14.
biosynthesis: Chemical reactions in which complex biomolecules, especially
carbohydrates, lipids, and proteins, are formed from simple molecules.
15.
anabolism: Any metabolic process whereby cells build complex substances from simple,
smaller units.
16.
cytoplasm: The material of the cell, excluding the cell nucleus and cell membranes. It
includes the semifluid cytosol, the organelles, and other particles.
17.
gluconeogenesis: Synthesis of glucose within the body from noncarbohydrate precursors
such as amino acids, lactic acid, and glycerol. Fatty acids cannot be converted to glucose.
18.
GTP (guanosine triphosphate): A high-energy compound similar to ATP, but with three
phosphate groups linked to guanosine instead of adenosine.
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19.
glycolysis: The anaerobic pathway that breaks down a glucose molecule into two
molecules of pyruvate and yields two molecules of ATP and two molecules of NADH.
This occurs in the cytosol of a cell.
20.
coenzymes: Organic compounds, often derived from B vitamins, that combine with
inactive enzymes to form active enzymes.
21.
alcohol dehydrogenase (ADH): The enzyme that catalyzes the oxidation of ethanol and
other alcohols.
22.
microsomal ethanol-oxidizing system (MEOS): An energy-requiring enzyme system in
the liver that normally metabolizes drugs and other foreign substances.
Fill-in-the-Blank
1.
The breakdown of a fatty acid into numerous molecules of the two-carbon compound
acetyl coenzyme A (acetyl CoA) is called beta-oxidation.
2.
ATP (adenosine triphosphate) is the main direct fuel that cells use to synthesize
molecules, contract muscles, transport substances, and perform other tasks.
3.
Nicotinamide adenine dinucleotide (NAD+), a coenzyme derived from the B vitamin
niacin, becomes NADH as it accepts a pair of high-energy electrons for transport in cells.
4.
The three-carbon compound that results from glycolysis is pyruvate.
5.
Cofactors are compounds required for an enzyme to be active. They include coenzymes
and metal ions such as iron, copper, and magnesium.
6.
Lipogenesis is the synthesis of fatty acids from acetyl CoA derived from the metabolism
of fats, alcohol, and some amino acids.
7.
Oxaloacetate is a four-carbon intermediate compound in the citric acid cycle.
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8.
The primary site of genetic information in the cell, enclosed in a double-layered
membrane, is the nucleus.
9.
Electron transport chain is an organized series of protein carrier molecules located in
mitochondrial membranes.
10.
Anaerobic refers to the absence of oxygen or the ability of a process to occur in the
absence of oxygen.
11.
Ketones are organic compounds that contain a chemical group consisting of C=O (a
carbon-oxygen double bond) bound to two hydrocarbons. Pyruvate and fructose are
examples of these.
12.
ADP (adenosine diphosphate) is a molecule composed of adenosine and two phosphate
groups.
13.
Coenzyme A is a coenzyme derived from the vitamin pantothenic acid.
14.
A metabolic pathway is a series of chemical reactions that either break down a large
compound into smaller units or synthesize more complex molecules from smaller ones.
15.
Carnitine is a compound that transports fatty acids from the cytosol into the
mitochondria, where they undergo beta-oxidation.
16.
A ketogenic amino acid breaks down to acetyl CoA.
17.
FAD is a coenzyme derived from the B vitamin riboflavin, which becomes FADH2 as it
accepts a pair of high-energy electrons for transport in cells.
18.
Acidification of the blood caused by a buildup of ketone bodies is called ketoacidosis. It
is primarily a consequence of uncontrolled type 1 diabetes mellitus and can be life
threatening.
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19.
The term glucogenic describes an amino acid whose carbon skeleton can be used in
gluconeogenesis to form glucose.
20.
Lactate is a three-carbon compound that is produced when insufficient oxygen is present
in cells to break down pyruvate to acetyl CoA.
Fill-in-the-Blank Summaries
Energy: Fuel for Work
Our cells get their energy from chemical energy held in molecular bonds of carbohydrates, fats,
and proteins. Green plants use light energy from the sun to make carbohydrate in a
process called photosynthesis. Our bodies extract energy from food in three stages. Stage
1 consists of digestion, absorption, and transportation. Stage 2 includes the breakdown
of many small molecules into a few key metabolites. In stage 3, the complete breakdown
of metabolites to water and carbon dioxide liberates large amounts of energy.
The term metabolic pathway describes a series of chemical reactions that either break down a
large compound (catabolism) or build more complex molecules (anabolism). Cells are
known as the “work centers” of metabolism. The basic animal cell is divided into two
parts—the nucleus and the cytoplasm, which is filled with a fluid called cytosol. The
mitochondria are power generators that contain many important energy-producing
pathways. Enzymes and their cofactors speed up chemical reactions in metabolic
pathways.
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Breakdown and Release of Energy
Although each energy-yielding nutrient initially follows a different metabolic pathway, they all
follow the Krebs cycle or citric acid cycle, and the electron transport chain. During
glycolysis one molecule of glucose yields two NADH, a net of two ATP and two
pyruvate. In the next step of carbohydrate metabolism, pyruvate is converted to acetyl
CoA. Without oxygen, pyruvate cannot be converted to this substance. In this case, it is
rerouted to form lactate. The citric acid cycle begins when acetyl CoA combines with
oxaloacetate to yield citrate. This cycle produces most of the energy-rich molecules that
ultimately generate ATP. It is also an important source of building blocks for fatty acids
and amino acids. The electron transport chain is the last step in glucose breakdown and
occurs in the inner mitochondrial membrane. Molecules of NADH produced in the citric
acid cycle deliver their high-energy electrons to the beginning of the chain. At the end of
the chain, oxygen accepts the energy-depleted electrons and then reacts with hydrogen to
form water. The three end products of glucose catabolism are water, carbon dioxide, and
ATP.
Special States: Fasting
When your body is faced with starvation, it must deal with several dilemmas. The first priority is
to preserve glucose-dependent tissue: red blood cells, brain cells, and the central nervous
system. After carbohydrate reserves are depleted within a few hours, circulating amino
acids are used to make glucose and ATP. The second priority is to maintain muscle
mass. Because little glucose can be made from triglycerides stored in the adipose tissue,
the brain cells adapt so that they can use ketone bodies for fuel.
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When glucose levels drop to baseline levels after several hours of fasting, the hormone glucagon
stimulates the breakdown of liver glycogen to glucose. During the first few days of
starvation, glucogenic amino acids, especially alanine, furnish 90 percent of the brain’s
glucose supply, and glycerol provide(s) the remaining 10 percent. Eventually protein
breakdown must slow down or the body will not survive for more than three weeks
without food. Fat catabolism rates double to supply fatty acids for fuel and glycerol for
glucose. The average person has three weeks of fat stores; then the body must turn back
to protein for energy. Protein breakdown slows drastically and gluconeogenesis drops by
two-thirds or more. A starving person can survive until 50 percent of his or her proteins
are broken down.
The Metabolism of Glucose to Yield Energy
Glycolysis breaks the six-carbon glucose molecule into two pyruvate molecules. Each
pyruvate loses a carbon and combines with coenzyme A to form acetyl CoA,
which then enters the citric acid cycle. Two carbons enter the cycle as part of
acetyl CoA, and two carbons leave as part of two carbon dioxide molecules.
Because two acetyl CoA molecules are formed from a single glucose molecule,
the citric acid cycle operates twice. Finally, the NADH and FADH2 formed in
these pathways carry pairs of high-energy electrons to the electron transport
chain, where ATP and water are produced. When completely oxidized, each
glucose molecule yields carbon dioxide, water, and ATP.
Extracting Energy from Fat
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First, triglycerides are separated into glycerol and three fatty acids. Glycerol forms pyruvate
and can be broken down to yield a small amount of energy. Beta-oxidation breaks down
fatty acid chains to two-carbon links that form acetyl CoA, which enters the citric acid
cycle. Beta-oxidation and the citric acid cycle form NADH and FADH2, which carry
pairs of high-energy electrons to the electron transport chain, where ATP and water
are made. The complete breakdown of one triglyceride molecule yields water, carbon
dioxide, and substantially more ATP than the complete breakdown of one carbon
dioxide, and substantially more ATP than the complete breakdown of one glucose
molecule.
Extracting Energy from Amino Acids
To extract energy from amino acids, first deamination removes the amino groups, leaving
behind carbon skeletons. The liver quickly converts these amino groups to urea and
sends them to the kidneys for excretion. The carbon skeleton structure determines where
it enters the catabolic pathways. Some carbon skeletons become pyruvate, others
become acetyl CoA, and still others become intermediate compounds of the citric acid
cycle. Complete breakdown of amino acids yields water, carbon dioxide, urea, and
ATP.
Short Answer
1.
The metabolism of glucose to yield energy occurs in several steps. When completely
oxidized, each glucose molecule yields which three components?
Carbon dioxide, water, and ATP
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2.
Define metabolism, anabolism, and catabolism, and give an example of each.
Metabolism encompasses the many reactions that take place in cells to build tissue, produce
energy, break down compounds, and do other cellular work. Anabolism refers to reactions
that build compounds, such as protein or glycogen. Catabolism is the breakdown of
compounds to yield energy.
3.
What is the fundamental goal of metabolism’s energy-producing pathways?
Production of ATP
4.
What are the names of the hydrogen and electron carriers that shuttle energy to ATP
production sites?
NADH and FADH2
5.
What are the four main pathways in which cells are able to extract usable energy from
carbohydrate?
The four main pathways: glycolysis, conversion of pyruvate to acetyl CoA, the citric acid
cycle, and the electron transport chain
6.
What is produced as the end result of the glycolysis for glucose, fructose, or galactose?
Although fructose and galactose enter glycolysis at intermediate points, the end result is the
same as for glucose. One molecule of glucose, fructose, and galactose produces two NADH,
a net of two ATP, and two pyruvates.
7.
If your estimated energy needs are 2,000 Kcalories/day, and you overeat to 2,300
Kcalories, but 90 percent of your diet intake includes carbohydrate-rich, low-fat foods,
what does your body do with the Kcalories it does not use for energy production, and can
this result in body weight gain?
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When the diet supplies an excess of energy (regardless of the macronutrient source), your
cells make fatty acids and triglycerides. Glycerol and fatty acids are assembled into
triglycerids on the endoplasmic reticulum. Although excess dietary carbohydrate is not
readily converted to fat, it does shift the body’s selection of fuel and encourages the
accumulation of dietary fat in body fat stores, ultimately resulting in weight gain.
8.
If you eat a very high-fat, high-protein, low-carbohydrate diet, how does your body
obtain the glucose fuel it needs for essential energy production?
A lack of available carbohydrate accelerates ketone body production. Ketone bodies
become the important fuel source.
9.
During a fast, when glycogen stores have been depleted, what does the body synthesize
glucose from?
Amino acids
Labeling
Cells use metabolic reactions to extract energy from food and to form building blocks for
biosynthesis. Label the missing components in the following image.
Extraction of Energy: Protein (amino acids); Carbohydrates (sugars); Fats (fatty acids);
Molecular building blocks: Amino acids & body protein; Glucose and glycogen; Fatty acids
and lipids
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