chapter 27: nutrition and metabolism

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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

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)

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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

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

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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.

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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 27-16)

• 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– 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)

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)

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