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[ Pobierz całość w formacie PDF ]Metabolism and Energetics
25
Did you know...?
Basal metabolic rate represents the calories that
we use when at rest. This energy supports
ongoing maintenance, repair, and other vital
functions.
Learning Outcomes
After completing this chapter, you should be able to do the following:
25-1
Define metabolism, and explain why cells must synthesize new
organic components.
25-2
Describe the basic steps in glycolysis, the TCA cycle, and the
electron transport system, and summarize the energy yields of
glycolysis and cellular respiration.
25-3
Describe the pathways involved in lipid metabolism, and
summarize the mechanisms of lipid transport and distribution.
25-4
Summarize the main processes of protein metabolism, and
discuss the use of proteins as an energy source.
25-5
Differentiate between the absorptive and postabsorptive
metabolic states, and summarize the characteristics of each.
25-6
Explain what constitutes a balanced diet and why such a diet is
important.
25-7
Define metabolic rate, discuss the factors involved in
determining an individual’s BMR, and discuss the homeostatic
mechanisms that maintain a constant body temperature.
Clinical Notes
Carbohydrate Loading p. 940
Dietary Fats and Cholesterol p. 943
Vitamins p. 954
Alcohol—A Risky Diversion p. 956
Induced Hypothermia p. 960
Thermoregulatory Disorders p. 961
930
Unit 5
Environmental Exchange
An Introduction to Metabolism
and Energetics
sion; and (3) special processes, such as secretion, contraction,
and the propagation of action potentials.
Figure 25–1
provides a broad overview of the processes
involved in cellular metabolism. The cell absorbs organic
molecules from the surrounding interstitial fluids. Amino
acids, lipids, and simple sugars cross the plasma membrane
and join nutrients already in the cytoplasm. All the cell’s or-
ganic building blocks collectively form a
nutrient pool
that
the cell relies on to provide energy and to create new intra-
cellular components.
The breakdown of organic substrates is called
catabolism
.
This process releases energy that can be used to synthesize ATP
or other high-energy compounds. Catabolism proceeds in a
series of steps. In general, the initial steps occur in the cytosol,
where enzymes break down large organic molecules previ-
ously assembled by the cell (such as glycogen, triglycerides, or
proteins) into smaller fragments that join the nutrient pool.
For example, carbohydrates are broken down into simple sug-
ars, triacylglycerols are split into fatty acids and glycerol, and
proteins are broken down to individual amino acids.
Relatively little ATP is produced during these prepara-
tory steps. However, further catabolic activity produces
smaller organic molecules that can be absorbed and
processed by mitochondria. It is that mitochondrial activity
that releases significant amounts of energy. As mitochondrial
enzymes break the covalent bonds that hold these molecules
together, they capture roughly 40 percent of the energy re-
leased and use it to convert ADP to ATP. The other 60 percent
escapes as heat that warms the interior of the cell and the sur-
rounding tissues.
The ATP produced by mitochondria provides energy to
support both
anabolism
—the synthesis of new organic mol-
ecules—and other cell functions. Those additional functions,
such as ciliary or cell movement, contraction, active trans-
port, and cell division, vary among cell types. For example,
muscle fibers need ATP to provide energy for contraction,
whereas gland cells need ATP to synthesize and transport
their secretions. We have considered such specialized func-
tions in other chapters, so here we will restrict our focus to
anabolic processes.
In terms of energy, anabolism is an “uphill” process that
involves the formation of new chemical bonds. Cells synthe-
size new organic components for four basic reasons:
The amount and type of nutrients you obtain in meals can
vary widely. Your body stores energy reserves when nutrients
are abundant, and mobilizes them when nutrients are in short
supply. The neuroendocrine system adjusts and coordinates
the metabolic activities of the body’s tissues and controls the
storage and mobilization of these energy reserves. This chap-
ter examines how the body obtains energy released by the
breakdown of organic molecules, stores it as ATP, and uses it
to support intracellular operations such as the construction
of new organic molecules.
25-1
Metabolism refers to all the
chemical reactions that occur in
the body
Cells are chemical factories that break down organic mol-
ecules to obtain energy, which can then be used to generate
ATP. Reactions within mitochondria provide most of the en-
ergy a typical cell needs.
l
p. 80
To carry out these meta-
bolic reactions, cells must have a reliable supply of oxygen
and nutrients, including water, vitamins, mineral ions, and
organic substrates (the reactants in enzymatic reactions).
Oxygen is absorbed at the lungs; the other substances are ob-
tained through absorption by the digestive tract. The cardio-
vascular system then carries these substances throughout the
body. They diffuse from the bloodstream into the tissues,
where they can be absorbed and used by our cells.
Mitochondria break down the organic nutrients to provide
energy for cell growth, cell division, contraction, secretion, and
other functions. Each tissue contains a unique mixture of var-
ious kinds of cells. As a result, the energy and nutrient require-
ments of any two tissues—loose connective tissue and cardiac
muscle, for instance—can be quite different. Moreover, activity
levels can change rapidly within a tissue, and such changes af-
fect the metabolic requirements of the body. For example,
when skeletal muscles start contracting, the tissue demand for
oxygen skyrockets. Thus, the energy and nutrient require-
ments of the body vary from moment to moment (resting ver-
sus exercising), hour to hour (asleep versus awake), and year
to year (growing child versus adult).
The term
metabolism
(me-TAB-o-lizm) refers to all the
chemical reactions that occur in an organism. Chemical reac-
tions within cells, collectively known as
cellular metabolism
,
provide the energy needed to maintain homeostasis and to
perform essential functions. Such functions include (1)
metabolic turnover
, the periodic breakdown and replacement
of the organic components of a cell; (2) growth and cell divi-
1.
To Perform Structural Maintenance or Repairs.
All cells
must expend energy to perform ongoing maintenance and
repairs, because most structures in the cell are temporary
rather than permanent. Their removal and replacement
are part of the process of
metabolic turnover
.
l
p. 61
2.
To Support Growth.
Cells preparing to divide increase in
size and synthesize extra proteins and organelles.
3.
To Produce Secretions.
Secretory cells must synthesize
their products and deliver them to the interstitial fluid.
931
Chapter 25
Metabolism and Energetics
INTERSTITIAL
FLUID
Plasma membrane
Results of
Anabolism
CYTOPLASM
Maintenance
and repairs
Growth
Secretion
Stored
reserves
CATABOLISM
ANABOLISM
Organic
molecules
Amino acids
Lipids
Simple sugars
NUTRIENT
POOL
Other ATP
Expenses
Locomotion
Contraction
Intracellular
transport
Cytokinesis
Endocytosis
Exocytosis
CATABOLISM
(in mitochondria)
HEAT
60%
40%
ATP
Figure 25–1
An Introduction to Cellular Metabolism.
Cells obtain organic molecules from the interstitial fluid and break them down to
produce ATP. Only about 40 percent of the energy released by catabolism is captured in the form of ATP; the rest is lost as heat. The ATP
generated by catabolism provides energy for all vital cellular activities, including anabolism.
4.
To Store Nutrient Reserves.
Most cells “prepare for a rainy
day”—a period of emergency, an interval of extreme activ-
ity, or a time when the supply of nutrients in the blood-
stream is inadequate. Cells prepare for such times by
building up reserves—nutrients stored in a form that can be
mobilized as needed. The most abundant storage form of
carbohydrate is glycogen, a branched chain of glucose mol-
ecules; the most abundant storage lipids are triglycerides,
consisting primarily of fatty acids. Thus, muscle cells and
liver cells, for example, store glucose in the form of glyco-
gen, whereas adipocytes and liver cells store triglycerides.
Proteins, the most abundant organic components in the
body, perform a variety of vital functions for the cell, and
when energy is available, cells synthesize additional pro-
teins. However, when glucose or fatty acids are unavailable,
proteins can be broken down into their component amino
acids, and the amino acids catabolized as an energy source.
So, although their primary function is not to serve as an en-
ergy source, proteins are so abundant and accessible that
they represent an important “last-ditch” nutrient reserve.
conserve the materials needed to build new compounds and
break down the rest. Cells continuously replace membranes,
organelles, enzymes, and structural proteins. These anabolic
activities require more amino acids than lipids, and few car-
bohydrates. In general, when a cell with excess carbohy-
drates, lipids, and amino acids needs energy, it will break
down carbohydrates first. Lipids are the second choice, and
amino acids are seldom broken down if other energy sources
are available.
Mitochondria provide the energy that supports cellular
operations. The cell feeds its mitochondria from its nutrient
pool, and in return, the cell gets the ATP it needs. However,
mitochondria are picky eaters: They will accept only specific
organic molecules for processing and energy production.
Thus, chemical reactions in the cytoplasm take whichever or-
ganic nutrients are available and break them down into
smaller fragments that the mitochondria can process. The mi-
tochondria then break the fragments down further, generat-
ing carbon dioxide, water, and ATP (
Figure 25–2
). This
mitochondrial activity involves two pathways: the
TCA cycle
and the
electron transport system
. We will describe these im-
portant catabolic and anabolic cellular reactions in the next
section.
The nutrient pool is the source of the substrates for both
catabolism and anabolism. As you might expect, cells tend to
932
Unit 5
Environmental Exchange
25-2
Carbohydrate
metabolism involves
glycolysis, ATP
production, and
gluconeogenesis
Structural,
functional,
and storage
components
Triglycerides
Glycogen
Proteins
Nutrient
pool
Fatty acids
Glucose
Amino acids
Most cells generate ATP and other high-energy
compounds by breaking down carbohy-
drates—especially glucose. The complete reac-
tion sequence can be summarized as follows:
Small
carbon
chains
ATP
C
6
H
12
O
6
6 O
2
¡
6 CO
2
6 H
2
O
glucose
oxygen
carbon dioxide water
Electron
transport
system
Coenzymes
TCA
cycle
The breakdown occurs in a series of
small steps, several of which release suffi-
cient energy to support the conversion of
ADP to ATP. The complete catabolism of one
molecule of glucose provides a typical body
cell a net gain of 36 molecules of ATP.
Although most ATP production occurs
inside mitochondria, the first steps take place
in the cytosol. The process of
glycolysis
, which
breaks down glucose in the cytosol and gener-
ates smaller molecules that can be absorbed
and utilized by mitochondria, was introduced
in Chapter 10.
l
p. 319
Because glycolysis
does not require oxygen, the reactions are said
to be
anaerobic
. The subsequent reactions,
which occur in mitochondria, consume oxy-
gen and are considered
aerobic
. The mito-
chondrial activity responsible for ATP production is called
aerobic metabolism
, or
cellular respiration
.
H
2
O
MITOCHONDRIA
CO
2
NAVIGATOR FIGURE
Figure 25–2
Nutrient Use in Cellular Metabolism.
Cells use the contents of the
nutrient pool to build up reserves and to synthesize cellular structures. Catabolism within
mitochondria provides the ATP needed to sustain cell functions. Mitochondria are “fed”
small carbon chains produced by the breakdown of carbohydrates (primarily glucose, stored
as glycogen), lipids (especially fatty acids from triglycerides), and proteins (amino acids). The
mitochondria absorb these breakdown products for further catabolism by means of the
tricarboxylic acid (TCA) cycle and the electron transport system. This figure will be repeated,
in reduced and simplified form as Navigator icons, as the text changes topics.
The
A
&
P Top 100
#87
There is an energy cost to staying alive, even at rest. All
cells must expend ATP to perform routine maintenance, like
removing and replacing intracellular and extracellular
structures and components. In addition, cells must spend
additional energy performing other vital functions, such as
growth, secretion, and contraction.
Glycolysis
Glycolysis
(gli-KOL-i-sis;
glykus
, sweet
lysis
, dissolution)
is the breakdown of glucose to
pyruvic acid
. In this process,
a series of enzymatic steps breaks the six-carbon glucose mol-
ecule (C
6
H
12
O
6
) into two three-carbon molecules of pyruvic
acid (CH
3
COOH). At the normal pH inside cells,
each pyruvic acid molecule loses a hydrogen ion and exists as
the negatively charged ion CH
3
CO
COO
. This ionized
form is usually called
pyruvate
, rather than pyruvic acid.
Glycolysis requires (1) glucose molecules, (2) appropriate
cytoplasmic enzymes, (3) ATP and ADP, (4) inorganic phos-
phates, and (5)
NAD
(
n
icotinamide
a
denine
d
inucleotide
), a coen-
zyme that removes hydrogen atoms during one of the enzymatic
reactions. (Recall from Chapter 2 that coenzymes are organic
molecules that are essential to enzyme function.
l
p. 57
) If any
of these participants is missing, glycolysis cannot occur.
CO
CHECKPOINT
1. Define metabolism.
2. Define catabolism.
3. Define anabolism.
See the blue Answers tab at the end of the book.
933
Chapter 25
Metabolism and Energetics
Figure 25–3
provides an overview of the steps in glycoly-
sis. Glycolysis begins when an enzyme
phosphorylates
—that
is, attaches a phosphate group—to the last (sixth) carbon
atom of a glucose molecule, creating
glucose-6-phosphate
.
This step, which “costs” the cell one ATP molecule, has two
important results: (1) It traps the glucose molecule within the
cell, because phosphorylated glucose cannot cross the plasma
membrane; and (2) it prepares the glucose molecule for fur-
ther biochemical reactions.
A second phosphorylation occurs in the cytosol before
the six-carbon chain is broken into two three-carbon frag-
ments. Energy benefits begin to appear as these fragments are
converted to pyruvic acid. Two of the steps release enough en-
ergy to generate ATP from ADP and inorganic phosphate
(PO
4
3
or P
i
). In addition, two molecules of NAD are con-
verted to NADH. The net reaction looks like this:
Glucose
2 NAD
2 ADP
2 P
i
¡
2 Pyruvic acid
2 NADH
2 ATP
This anaerobic reaction sequence provides the cell a net
gain of two molecules of ATP for each glucose molecule con-
verted to two pyruvic acid molecules. A few highly specialized
INTERSTITIAL
FLUID
Glucose
STEPS IN GLYCOLYSIS
1
ATP
CYTOPLASM
ADP
1
P
Glucose-6-phosphate
As soon as a glucose molecule
enters the cytoplasm, a phosphate
group is attached to the molecule.
ATP
2
ADP
2
P
P
Fructose-1,6-bisphosphate
A second phosphate group is
attached. Together, steps 1 and 2
cost the cell 2 ATP.
3
Dihydroxyacetone
phosphate
3
P
P
P
Glyceraldehyde
3-phosphate
The six-carbon chain is split
into two three-carbon molecules,
each of which then follows the
rest of this pathway.
2 NAD
From mitochondria
2
P
4
To mitochondria
2 NADH
4
P
P
Another phosphate group is
attached to each molecule, and
NADH is generated from NAD.
1,3-Bisphosphoglyceric acid
P
P
2 ADP
2
5
ATP
5
P
P
3-Phosphoglyceric acid
One ATP molecule is formed for
each molecule processed.
6
2
H
2
O
6
P
The atoms in each molecule are
rearranged, releasing a
molecule of water.
Phosphoenolpyruvic acid
P
2 ADP
2
ENERGY SUMMARY
Steps 1 & 2:
Step 5:
Step 7:
7
7
ATP
–2 ATP
+2 ATP
+2 ATP
A second ATP molecule is formed
for each molecule processed.
Step 7 produces 2 ATP molecules.
Pyruvic acid
+2 ATP
NET GAIN:
To mitochondria
Figure 25–3
Glycolysis.
Glycolysis breaks down a six-carbon glucose molecule into two three-carbon molecules of pyruvic acid through a
series of enzymatic steps. The further catabolism of pyruvic acid begins with its entry into a mitochondrion.
(See Figure 25–4.)
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