Transcript Chapter_027

Chapter 27
Nutrition and Metabolism
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Slide 1
Overview of
Nutrition and Metabolism

Nutrition refers to the food (nutrients) humans eat


Malnutrition—a deficiency in the consumption of food, vitamins,
and minerals
Categories of nutrients
• Macronutrients—nutrients needed in large amounts (bulk nutrients)



Macromolecules such as carbohydrates, fats (lipids), and proteins
Water
Macrominerals—minerals needed in large quantity; for example, sodium,
chloride, and calcium
• Micronutrients—nutrients needed in very small amounts



Vitamins
Microminerals (trace elements)—minerals such as iron, iodine, zinc, that
are needed only in very small quantities
Balance of nutrients is required for good health
(Figure 27-1 and 27-2)
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Slide 2
Overview of
Nutrition and Metabolism

Metabolism—the use of nutrients—a process made
up of many chemical processes (Figure 27-23)
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Catabolism breaks food down into smaller molecular
compounds and releases two forms of energy—heat and
chemical energy

Anabolism—a synthesis process
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Both processes take place inside of cells continuously and
concurrently

Chemical energy released by catabolism must be transferred
to ATP, which supplies energy directly to the energy-using
reactions of all cells (Figure 27-3)
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Slide 3
Carbohydrates

Dietary sources of carbohydrates

Complex carbohydrates
• Polysaccharides—starches; found in vegetables and grains;
glycogen is found in meat
• Cellulose—a component of most plant tissue; passes through
the digestive system without being broken down
• Disaccharides—found in refined sugar; must be broken down
before they can be absorbed
• Monosaccharides—found in fruits; move directly into the
internal environment without being processed directly

Glucose—carbohydrate most useful to the human cell;
can be converted from other monosaccharides (Figure 27-4)
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Slide 4
Carbohydrates

Carbohydrate metabolism—human cells catabolize most
of the carbohydrate absorbed and anabolize a small
portion of it

Glucose transport and phosphorylation—glucose reacts with ATP to
form glucose-6-phosphate; this step prepares glucose for further
metabolic reactions
• This step is irreversible except in the intestinal mucosa, the liver, and
the kidney tubules

Glycolysis—the first process of carbohydrate catabolism; consists
of a series of chemical reactions (Figure 27-5)
• Glycolysis occurs in the cytoplasm of all human cells
• An anaerobic process—the only process that provides cells with energy
under conditions of inadequate oxygen
• It breaks down chemical bonds in glucose molecules and releases
about 5% of the energy stored in them
• It prepares glucose for the second step in catabolism—the citric
acid cycle
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Slide 5
Carbohydrates

Carbohydrate metabolism (cont.)
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Citric acid cycle
• Two pyruvic acid molecules from glycolysis are converted to
two acetyl molecules in a transition reaction, losing one carbon
dioxide molecule per pyruvic acid molecule converted
• By end of transition reaction and citric acid cycle, two pyruvic
acids have been broken down to six carbon dioxide and six
water molecules (Figures 27-6 and 27-7)
• Citric acid cycle also called tricarboxylic acid (TCA) cycle
because citric acid is also called tricarboxylic acid
• Citric acid cycle formerly called Krebs cycle after Sir Hans
Krebs, who discovered this process
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Slide 6
Carbohydrates

Carbohydrate metabolism (cont.)
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Electron transport system (Figure 27-8)
• High-energy electrons (along with their protons) removed during citric
acid cycle enter a chain of molecules that are embedded in the inner
membrane of the mitochondria
• As electrons move down the chain, they release small bursts of energy
to pump protons between the inner and the outer membrane of the
mitochondrion
• Protons move down their concentration gradient and across the inner
membrane, driving ATP-synthase (Figure 27-9)
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Oxidative phosphorylation—the joining of a phosphate group to
ADP to form ATP by the action of ATP synthase
(Figure 27-10 and 27-11)
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Slide 7
Carbohydrates
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Carbohydrate metabolism (cont.)
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The anaerobic pathway—a pathway for the catabolism of glucose;
transfers energy to ATP using only glycolysis; ultimately ends with
the oxidative phosphorylation of ATP (paying the “oxygen debt”)
(Figure 27-12))
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Glycogenesis—a series of chemical reactions in which glucose
molecules are joined to form a strand of glucose beads; a process
that operates when the blood glucose level increases above the
midpoint of its normal range (Figures 26-13 and 26-14)
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Glycogenolysis (Figure 27-15)—the reversal of glycogenesis;
means different things in different cells
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Gluconeogenesis (Figure 27-16)—the formation of new glucose;
occurs chiefly in liver
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Slide 8
Carbohydrates
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Carbohydrate metabolism (cont.)
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Control of glucose metabolism—hormonal and neural devices
maintain homeostasis of blood glucose concentration
(Figures 27-17 and 27-18)
• Insulin—secreted by beta cells to decrease blood glucose level
•
•
•
•
•
•
(Figure 27-14)
Glucagon increases blood glucose level by increasing activity of enzyme
phosphorylase
Epinephrine—hormone secreted in times of stress; increases
phosphorylase activity
Adrenocorticotropic hormone stimulates adrenal cortex to increase its
secretion of glucocorticoids
Glucocorticoids accelerate gluconeogenesis
Growth hormone increases blood glucose level by shifting from
carbohydrate to fat catabolism
Thyroid-stimulating hormone has complex effects on metabolism
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Slide 9
Carbohydrates
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Carbohydrate metabolism (cont.)
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Hormones that cause the blood glucose level to
rise are called hyperglycemic
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Insulin is hypoglycemic because it causes the
blood glucose level to decrease
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Slide 10
Lipids
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Dietary sources of lipids
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Triglycerides—the most common lipids—composed
of a glycerol subunit that is attached to three fatty acids
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Phospholipids—an important lipid found in all foods
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Cholesterol—an important lipid found only in
animal foods
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Dietary fats
• Saturated fats contain fatty acid chains in which there are no
double bonds
• Unsaturated fats contain fatty acid chains in which there are
some double bonds
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Slide 11
Lipids
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Transport of lipids—they are transported in blood
as chylomicrons, lipoproteins, and fatty acids
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In the absorptive state, many chylomicrons are present
in the blood
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Postabsorptive state—95% of lipids are in the form
of lipoproteins
• Lipoproteins consist of lipids and protein and are formed
in the liver
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Blood contains three types of lipoproteins: very low density,
low density, and high density
Cholesterol lipoproteins associated with heart disease (Figure 27-19)
• Fatty acids are transported from the cells of one tissue to the cells
of another in the form of free fatty acids
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Slide 12
Lipids
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Lipid metabolism
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Lipid catabolism—triglycerides are hydrolyzed to yield fatty acids
and glycerol; glycerol is converted to glyceraldehyde-3-phosphate,
which enters the glycolysis pathway; fatty acids are broken down by
beta- oxidation and are then catabolized through the citric acid cycle
(Figure 27-20)
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Lipid anabolism consists of the synthesis of triglycerides, cholesterol,
phospholipids, and prostaglandins
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Control of lipid metabolism is through the following hormones:
•
•
•
•
Insulin
Growth hormone
ACTH
Glucocorticoids
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Slide 13
Proteins
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Sources of proteins
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Proteins are assembled from a pool of 20 different
amino acids
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The body synthesizes amino acids from other
compounds in the body
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Only about half of the necessary types of amino
acids can be produced by the body; the rest are
supplied through diet—found in both meat and
vegetables
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Slide 14
Proteins
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Protein metabolism—anabolism is primary, and catabolism
is secondary
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Protein anabolism—process by which proteins are synthesized by
ribosomes of cells (Figure 27-21)
 Protein catabolism—deamination takes place in liver cells and forms an
ammonia molecule, which is converted to urea and excreted in urine, and
a keto acid molecule, which is oxidized or converted to glucose or fat
(Figure 27-22)
 Protein balance—rate of protein anabolism balances rate of protein
catabolism
 Nitrogen balance—amount of nitrogen taken in equals nitrogen in protein
catabolic waste
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Two kinds of protein or nitrogen imbalance:
• Negative nitrogen balance—protein catabolism exceeds protein anabolism;
more tissue proteins are catabolized than are replaced by protein synthesis
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• Positive nitrogen balance—protein anabolism exceeds protein catabolism
Control of protein metabolism—achieved by hormones
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Slide 15
Vitamins and Minerals
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Vitamins (Table 27-3)—organic molecules
necessary for normal metabolism; many attach
to enzymes and help them work or have other
important biochemical roles (Figures 27-24
and 27-25)
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The body does not make most of the necessary
vitamins; they must be obtained through diet
• The body stores fat-soluble vitamins and does not store
water-soluble vitamins
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Slide 16
Vitamins and Minerals
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Minerals (Table 27-4)—inorganic elements or salts
found in the earth
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Attach to enzymes and help them work and function
in chemical reactions
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Essential to the fluid/ion balance of internal fluid environment
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Are involved in many processes in the body such as muscle
contraction, nerve function, hardening of bone, etc.
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Too large or too small an amount of some minerals
may be harmful

Recommended mineral intakes may vary over the lifespan
(Figures 27-26 and 27-27)
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Slide 17
Metabolic Rates
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Metabolic rate means the amount of energy released
by catabolism
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Metabolic rates are expressed in two ways:
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Number of kilocalories of heat energy expended per hour
or per day
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As normal or as a percentage above or below normal
Basal metabolic rate—rate of energy expended under
basal conditions
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Factors: size, body composition, sex, age, thyroid hormone, body
temperature, drugs, other factors (Figures 27-28 through 27-31)
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Slide 18
Metabolic Rates
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Total metabolic rate (Figure 27-28)—the amount of energy
used in a given time

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Main determinates:
• Basal metabolic rate
• Energy used to do skeletal muscle work
• Thermic effect of foods
Energy balance and weight—the body maintains a state
of energy balance
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Body maintains weight when the total calories in the food ingested equals
the total metabolic rate
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Body weight increases when energy input exceeds energy output
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Body weight decreases when energy output exceeds energy input

In starvation, carbohydrates are used up first, then fats, then proteins
(Figure 27-32)
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Slide 19
Mechanisms for Regulating
Food Intake (Table 27-6)
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Hypothalamus plays a part in food intake
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Feeding centers in hypothalamus exert primary
control over appetite
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Appetite center
• Cluster of neurons in lateral hypothalamus that, if stimulated,
brings about increased appetite
• Orexigenic effects—factors that trigger appetite
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Satiety center
• Group of neurons in ventral medial nucleus of hypothalamus
that, if stimulated, brings about decreased appetite
• Anorexigenic effects—factors that suppress appetite (anorexia
is loss of appetite)
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Slide 20
The Big Picture: Nutrition,
Metabolism, and the Whole Body
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Every cell in the body needs the maintenance
of the metabolic pathways to stay alive
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Anabolic pathways build the various structural
and functional components of the cells
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Catabolic pathways convert energy to a
usable form and degrade large molecules into
subunits used in anabolic pathways
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Slide 21
The Big Picture: Nutrition,
Metabolism, and the Whole Body
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Cells require appropriate amounts of vitamins
and minerals to produce structural and
functional components necessary for cellular
metabolism
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Other body mechanisms operate to ensure
that nutrients reach the cells
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Slide 22