Transcript CARBOHYDRATES: METABOLISM (cont.)

Chapter 27:
Nutrition and Metabolism
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OVERVIEW
• Nutrition refers to the food (nutrients) we eat
– Malnutrition: a deficiency in the consumption of food,
vitamins, and minerals
– Categories of nutrients
• Macronutrients: nutrients that the body needs in large amounts
(bulk nutrients)
– Macromolecules such as carbohydrates, fats (lipids),
proteins
– Water
– Macrominerals: minerals needed in large quantity (e.g.,
sodium, chloride, calcium)
• Micronutrients: nutrients needed in very small amounts
– Vitamins
– Microminerals (trace elements): minerals needed only in
very small quantities (e.g., iron, iodine, zinc)
– Balance of nutrients is required for good health (Figures
27-1 and 27-2)
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Organic Chemistry: Biochemicals
• Carbohydrates: composed of carbon, hydrogen, oxygen.
– Divided into monosaccharides, disaccharides, polysaccharides
– Example: glucose
– Energy sources and structure
• Lipids: composed mostly of carbon, hydrogen, oxygen.
– Relatively insoluble in water.
– Example: anabolic steroids
– Functions: protection, insulation, physiological regulation, component
of cell membranes, energy source
• Proteins: composed of carbon, hydrogen, oxygen, nitrogen,
sometimes iodine.
– Example: insulin
– Functions: regulate processes, aid transport, protection, muscle
contraction, structure, energy
• Nucleic Acids: composed of carbon, hydrogen, oxygen,
nitrogen, phosphorus.
– Examples: ATP, DNA, RNA
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Nutrition Overview
• Nutrition is used for two things
– Energy (ATP synthesis)
– Building blocks for healing and continual
homeostasis
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OVERVIEW
• Metabolism: the use of nutrients through many
chemical processes (Figure 27-23)
– Catabolism breaks food down into smaller molecular
compounds and releases two forms of energy: heat
and chemical
– Anabolism: a synthesis process
– Both processes take place inside cells continuously
and concurrently
– Chemical energy released by catabolism must be
transferred to adenosine triphosphate (ATP), which
supplies energy toward the reactions of all cells
(Figure 27-3)
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pathWays to make ATP
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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 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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CARBOHYDRATES (cont.)
• Carbohydrate metabolism: human cells
catabolize most of the carbohydrate
absorbed and anabolize a small portion of
it
– Glucose transport and phosphorylation: Once
glucose is inside the cell, it 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, liver, and kidney tubules
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CARBOHYDRATES: METABOLISM
– 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
• Breaks down chemical bonds in glucose molecules and
releases approximately 5% of the energy stored in them
• Prepares glucose for the second step in catabolism—the
citric acid cycle
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CARBOHYDRATES: METABOLISM
(cont.)
– Citric acid cycle (Krebs cycle)
• Pyruvic acid (from glycolysis) is converted into
acetyl coenzyme A (CoA) and enters the citric acid
cycle after losing carbon dioxide (CO2) and
transferring some energy to NADH
• Citric acid cycle is a repeating (cyclic) sequence of
reactions that occurs inside the inner chamber of a
mitochondrion; acetyl splits from CoA and is
broken down to yield waste CO2 and energy (in the
form of energized electrons), which is transferred
to ATP, NADH, and reduced flavin adenine
dinucleotide (FADH2)
CARBOHYDRATES: METABOLISM (cont.)
– Citric acid cycle
• 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 is also called the tricarboxylic acid cycle
(TCA) because citric acid is also called tricarboxylic acid
• Citric acid cycle is also called the Krebs cycle after Sir
Hans Krebs, who discovered this process
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CELL METABOLISM: CATABOLISM
(cont.)
– Electron transport system (ETS)
• Energized electrons are carried by NADH and FADH2 from
glycolysis and the citric acid cycle to electron acceptors embedded
in the cristae of the mitochondrion
• As electrons are shuttled along a chain of electron-accepting
molecules in the cristae, their energy is used to pump
accompanying protons (H+) into the space between mitochondrial
membranes
• Protons flow back into the inner chamber through ATP synthasein
the cristae, and their energy of movement is transferred to ATP
• Low-energy electrons coming off the ETS bind to oxygen and rejoin
their protons to form water
CARBOHYDRATES: METABOLISM (cont.)
– Electron transport system (Figure 27-8)
• The ETC is where the MOST ATP is produced in the body
• High-energy electrons (along with their protons) removed during the
citric acid cycle enter a chain of molecules 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 outer membrane of the
mitochondrion
• Protons move down their concentration gradient, across the inner
membrane, driving ATP synthase (Figure 27-9)
– Oxidative phosphorylation: the joining of a phosphate group to adenosine
diphosphate to form ATP by the action of ATP synthase (Figures 27-10
and 27-11) (vs. substrate level phosphorylation)
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Electron Transport Chain
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Overview of Cell Metabolism
• Production of ATP
necessary for life
• ATP production takes
place in the cytosol
(anaerobic) and
mitochondria (aerobic)
– Anaerobic does not
require oxygen. Results in
lactic acid formation and
very little ATP production.
– Aerobic requires oxygen.
Results in large amount of
ATP.
CARBOHYDRATES: METABOLISM (cont.)
– Cori cycle: circular pathway in which lactic
acid produced by anaerobic respiration in
skeletal muscles is carried to liver cells,
where it is converted back to glucose and
stored as liver glycogen or returned to the
bloodstream, where the glucose may be
taken up by muscle cells and used for
respiration or stored as muscle glycogen
(Figure 27-13)
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CARBOHYDRATES: METABOLISM (cont.)
– Glycogenesis: a series of chemical reactions in which
glucose molecules are joined to form a strand of glucose
beads (glycogen); a process that operates when the
blood glucose level increases above the midpoint of its
normal range (Figures 27-14 and 27-15)
• Done by all cells of the body but liver and muscle cells store the
most glycogen; astrocytes in the BBB also store some glycogen
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CARBOHYDRATES: METABOLISM (cont.)
– Glycogenolysis: the reversal of glycogenesis; it
means different things in different cells (Figure 2716)
• Enzyme phospatase allows glucose monomers (from
catabolized polymers (glycogen) made by glycogenesis) to
be exported from the cell and into circulation. Phospatase is
only found in hepatic, kidney and intestinal mucosa cells.
• Muscle cells don’t have phosphatase and can only break
glycogen down into G6P for glycolysis
– Gluconeogenesis: the formation of new glucose from
protein or fats, which occurs chiefly in the liver
(Figure 27-17)
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CARBOHYDRATES:
METABOLISM (cont.)
– Control of glucose metabolism: hormonal and neural
devices maintain homeostasis of blood glucose
concentration (Figures 27-18 and 27-19)
• Insulin: secreted by beta cells to decrease blood glucose
level
• Glucagon increases the blood glucose level by increasing the
activity of the enzyme phosphorylase promoting
glycogenolysis
• Incretins: GI hormones that, in the presence of glucose in the
gut, stimulate insulin release from the pancreas, thereby
decreasing blood glucose levels (e.g., GLP-1, GIP)
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CARBOHYDRATES: METABOLISM (cont.)
– Epinephrine: hormone secreted in times of stress; increases
phosphorylase activity, accelerating glycogenolysis of both liver and
muscle cells
– Adrenocorticotropic hormone stimulates the adrenal cortex to
increase its secretion of glucocorticoids
• Glucocorticoids accelerate gluconeogenesis by breaking down proteins
– Growth hormone increases blood glucose level by shifting from
carbohydrate to fat catabolism
– Thyroid-stimulating hormone and thyroid hormones have complex
effects on metabolism
• Some raise and some lower blood glucose
– Hormones that cause the blood glucose level to rise are called
hyperglycemic
– Insulin is hypoglycemic because it causes the blood glucose level to
decrease
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Lipids
•
Triglycerides (95%): used for energy to produce ATP or
stored in adipose tissue, liver; most common lipids in the
diet
– Saturated fats: FA with a full compliment of
Hydrodens attached
– meat fats, whole milk, cheese, eggs
– Unsaturated fats:FA’s with double bonds
–
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olive and peanut oil
Cholesterol: steroid found in liver, egg yolks but not found
in plants
Phospholipids: major components of plasma membranes
Linoleic acids: essential fatty acids. Found in seeds, nuts,
legumes, grains and green leaves
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Uses of Lipids in the Body
• Triglycerides: used to produce ATP, Excess stored in
adipose tissue or liver.
• Cholesterol: can be eaten or manufactured in the body.
Component of plasma membranes, can be modified to
form bile salts and steroids
• Eicosanoids derived from fatty acids. Involved in
inflammation, blood clotting, tissue repair, smooth
muscle contraction.
• Phospholipids: part of plasma membrane and used to
construct the myelin sheath. Part of bile
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LIPIDS (cont.)
• Transport of lipids: transported in blood as chylomicrons,
lipoproteins, and fatty acids
– In the absorptive state, many chylomicrons are present in
the blood
– Postabsorptive state: 95% of lipids are in the form of
lipoproteins
• Lipoproteins consist of lipids and protein and are
formed in the liver
– Blood contains three types of lipoproteins: very low
density, low density, and high density (VLDL, LDL,
HDL)
• Fatty acids are transported from the cells of one tissue
to the cells of another in the form of free fatty acids
– There is an increase in blood levels of fatty acids
during diabetes or starvation
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LIPIDS (cont.)
• Lipid metabolism
– 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 catabolized through the citric acid cycle (Figure 27-21)
• When fat catabolism occurs at an accelerated rate,
ketone bodies are formed (ketogenesis)
• Ketones can be used by the liver or transported to
other tissues to enter the CA cycle
– Lipid anabolism consists of the synthesis of triglycerides,
cholesterol, phospholipids, and prostaglandins
• Made from glycerol and FA or excess glucose or aa
• Most FA can be made by the body, but some must be
provided by the diet (essential fatty acids)
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LIPIDS
– Control of lipid metabolism is through the following
hormones
• Insulin
• Growth hormone, Adrenocorticotropic hormone,
Glucocorticoids increase fat catabolism when
blood glucose is low
• Fat is stored in adipose tissue when blood glucose
levels are sufficient
– Carbohydrates have a fat storing effect
• Leptin – secreted by fat storing cells to regulate
satiety and how fat is metabolized
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Proteins
• Chains of amino acids
– Types
• Essential: must be obtained in diet
• Nonessential: body can synthesize
• Complete proteins: contain all necessary amino
acids (meat, fish, poultry, milk, cheese, eggs)
• Functions
– Protection (antibodies), regulation
(enzymes, hormones), structure (collagen),
muscle contraction (actin, myosin),
transportation (hemoglobin, ion channels)
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PROTEINS
• Sources of proteins
– Proteins are assembled from a pool of 20 different amino
acids
– If one aa is absent then certain proteins cannot be
synthesized
– The body synthesizes amino acids from other compounds in
the body (nonessential Amino Acids)
– Only about half the necessary types of amino acids can
be produced by the body; the rest are supplied through
diet; found in both meat and vegetables (essential
Amino Acids)
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PROTEINS
• Protein metabolism: anabolism is primary and
catabolism is secondary
– Protein anabolism: process by which proteins are
synthesized by ribosomes of the cells
• Important because it constitutes major growth, reproduction,
tissue repair and cell replacements
– Protein catabolism: deamination takes place
in the liver cells and forms an ammonia
molecule, which is converted to urea and
excreted in urine, and a keto acid molecule,
which is oxidized by citric acid cycle or
converted to glucose or fat (Figure 27-22)
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PROTEINS (cont.)
– Protein balance: the rate of protein anabolism balances the rate of
protein catabolism
– Nitrogen balance: the amount of nitrogen taken in equals the
nitrogen in protein catabolic waste
• A body in protein balance will also be in nitrogen balance
– 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 (protein poor diet, starvation)
• Positive nitrogen balance: protein anabolism exceeds protein
catabolism (large amounts of tissue being made in growth or
pregnancy)
– Control of protein metabolism: achieved by hormones
• GH and testosterone = anabolic
• Glucocorticoids = catabolic
• Thyroid hormones can be either, depending on the situation
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Vitamins
• Organic molecules that exist in minute quantities in food
– Essential vitamins must be obtained by diet
• Function as coenzymes or parts of coenzymes
(combine with enzymes and make the enzyme
functional)
• Classifications
– Fat soluble: A, D, E, K. Can be stored in fatty tissues to the
point of toxicity.
– Water-soluble: B, C, and all others. Remain short time then are
excreted.
• Antioxidants: prevent formation of free radicals. Free
radicals are chemicals produced by metabolism that are
missing electrons. They take electrons from chemicals
in cells, damaging the cells.
• Table 27-3 Major Vitamins
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MINERALS
• Minerals: inorganic elements or salts found in
the earth (Table 27-4)
– Attach to enzymes and help them work and function
in chemical reactions
– Essential to the fluid/ion balance of the internal fluid
environment
– Involved in many processes in the body, such as
muscle contraction, nerve function, hardening of bone
– Too large or too small an amount of some minerals
may be harmful
– Recommended mineral intakes may vary over the life
span (Figures 27-25 and 27-26)
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METABOLIC RATES
• Metabolic rate is the amount of energy released by catabolism
• Metabolic rates are expressed in two ways
– Number of kilocalories of heat energy expended per hour or
day
– As normal or as a percentage above or below normal
• Basal metabolic rate: the rate of energy expended under basal
conditions:
– The person is awake but at rest
– 12-18 hours after a meal
– In a comfortably warm environment
– Not the minimum metabolic rate, but the smallest amount of
energy necessary to sustain life and maintain a normal
waking state
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METABOLIC RATES
• Factors influencing BMR
1. Size: larger individuals have greater BMRs
2. Body Tissue: higher ratio of lean to fat mass increases
BMR
3. Sex: men have greater BMR than women (size and lean
mass ratio)
4. Age: younger individuals have greater BMRs
5. Thyroid hormone: stimulates metabolism; hypo vs.
hyperthyroid
6. Body temperature: BMR increases with increasing
temperature
7. Drugs: certain drugs increase BMR (caffeine,
amphetamine)
8. Emotions, pregnancy and lactation increase BMR
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METABOLIC RATES (cont.)
• Total metabolic rate: amount of energy used in a
given time (Figure 27-27)
– Main determinants
• Factor 1: basal metabolic rate
• Factor 2: energy used to do skeletal muscle
work
• Factor 3: thermic effect of foods or energy
needed to metabolize foods
–Proteins have a greater thermic effect
than carbs and therefore require more
energy to process
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METABOLIC RATES (cont.)
• Energy balance and body weight: the body maintains a state of
energy balance
– The body maintains a weight when the total calories in the food
ingested equals the total metabolic rate
– Body weight increases when energy input exceeds energy
output
– Body weight decreases when energy output exceeds energy
input
– In starvation, the carbohydrates are used first, then fats, then
proteins (Figure 27-31)
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MECHANISMS FOR
REGULATING FOOD INTAKE
• The hypothalamus plays a part in food intake (Table 27-6)
• Feeding centers in the hypothalamus exert primary control over
appetite
– Appetite center
• Cluster of neurons in the lateral hypothalamus that, if
stimulated, increases appetite
• Orexigenic effects: factors that trigger appetite
– Satiety center
• Group of neurons in the ventral medial nucleus of the
hypothalamus that, if stimulated, decreases appetite
• Anorexigenic effects: factors that suppress appetite (anorexia
is loss of appetite)
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THE BIG PICTURE:
NUTRITION, METABOLISM, AND THE WHOLE BODY
• Every cell in the body needs the maintenance of the
metabolic pathways to stay alive
• Anabolic pathways build the various structural and
functional components of the cells
• Catabolic pathways convert energy to a usable form and
degrade large molecules into subunits used in anabolic
pathways
• Cells require appropriate amounts of vitamins and
minerals to produce structural and functional
components necessary for cellular metabolism
• Other body mechanisms operate to ensure that nutrients
reach the cells
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Also…
• Mechanisms of Disease
• pp.937-939
• Boxes
• pp. 929 MAJOR VITAMINS
• pp. 930 MAJOR MINERALS
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