25 Metabolism and Energetics

An Introduction to Metabolism and Energetics Metabolic Activity Cells break down organic molecules to obtain energy Used to generate ATP Most energy production takes place in mitochondria

25-1 Metabolism Metabolism Cellular Metabolism Refers to all chemical reactions in an organism Cellular Metabolism Includes all chemical reactions within cells Provides energy to maintain homeostasis and perform essential functions

25-1 Metabolism Essential Functions Metabolic turnover Periodic replacement of cell’s organic components Growth and cell division Special processes, such as secretion, contraction, and the propagation of action potentials

Figure 25-1 An Introduction to Cellular Metabolism INTERSTITIAL FLUID Plasma membrane Results of Anabolism • Maintenance and repairs • Growth • Secretion • Stored nutrient reserves CATABOLISM ANABOLISM Organic Molecules • Amino acids • Lipids • Simple sugars NUTRIENT POOL Anaerobic catabolism in the cytosol releases small amounts of ATP that are significant only under unusual conditions. ATP Other ATP EXpenses Aerobic Metabolism (in mitochondria) • Locomotion • Contraction • Intracellular transport • Cytokinesis • Endocytosis • Exocytosis HEAT 60% 40% ATP CYTOPLASM 5

25-1 Metabolism Catabolism Anabolism Is the breakdown of organic substrates Releases energy used to synthesize high-energy compounds (e.g., ATP) Anabolism Is the synthesis of new organic molecules In energy terms Anabolism is an “uphill” process that forms new chemical bonds

Figure 25-2 Nutrient Use in Cellular Metabolism Structural, functional, and storage components Triglycerides Glycogen Proteins Organic compounds that can be absorbed by cells are distributed to cells throughout the body by the bloodstream. Nutrient pool Fatty acids Glucose Amino acids Three-carbon chains Two-carbon chains MITOCHONDRIA Citric acid cycle Electron transport system Coenzymes KEY  Catabolic pathway  Anabolic pathway 7

25-2 Carbohydrate Metabolism Generates ATP and other high-energy compounds by breaking down carbohydrates Glucose + Oxygen  Carbon dioxide + Water 

25-2 Carbohydrate Metabolism Glucose Breakdown Occurs in small steps Which release energy to convert ADP to ATP One molecule of glucose nets 36 molecules of ATP Glycolysis Breaks down glucose in cytosol into smaller molecules used by mitochondria Does not require oxygen (anaerobic reaction)

25-2 Carbohydrate Metabolism Glucose Breakdown Aerobic Reactions Also called aerobic metabolism or cellular respiration Occur in mitochondria, consume oxygen, and produce ATP

25-2 Carbohydrate Metabolism Glycolysis Breaks 6-carbon glucose Into two 3-carbon pyruvic acid Pyruvate Ionized form of pyruvic acid

25-2 Carbohydrate Metabolism Glycolysis Factors Glucose molecules Cytoplasmic enzymes ATP and ADP Inorganic phosphates NAD (coenzyme)

Steps in Glycolysis Figure 25-3 Glycolysis INTERSTITIAL FLUID Glucose CYTOSOL Steps in Glycolysis Glucose-6-phosphate As soon as a glucose molecule enters the cytosol, a phosphate group is attached to the molecule. Fructose-1,6-bisphosphate A second phosphate group is attached. Together, steps 1 and 2 cost the cell 2 ATP. Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate The six-carbon chain is split into two three-carbon molecules, each of which then follows the rest of this pathway. 13

Steps in Glycolysis Figure 25-3 Glycolysis Energy Summary From mitochondria Steps in Glycolysis To mitochondria Another phosphate group is attached to each molecule, and NADH is generated from NAD. Energy Summary 1,3-Bisphosphoglycerate Steps 1 & 2: 2 One ATP molecule is formed for each molecule processed. Step 5: 2 3-Phosphoglycerate Step 7: 2 The atoms in each molecule are rearranged, releasing a molecule of water. NET GAIN: 2 Phosphoenolpyruvate A second ATP molecule is formed for each molecule processed. Step 7 produces 2 ATP molecules Pyruvate To mitochondria 14

25-2 Carbohydrate Metabolism Mitochondrial ATP Production If oxygen supplies are adequate, mitochondria absorb and break down pyruvic acid molecules H atoms of pyruvic acid are removed by coenzymes and are primary source of energy gain C and O atoms are removed and released as CO2 in the process of decarboxylation

25-2 Carbohydrate Metabolism Mitochondrial Membranes Outer membrane Contains large-diameter pores Permeable to ions and small organic molecules (pyruvic acid) Inner membrane Contains carrier protein Moves pyruvic acid into mitochondrial matrix Intermembrane space Separates outer and inner membranes

25-2 Carbohydrate Metabolism The Citric Acid Cycle The function of the citric acid cycle is: To remove hydrogen atoms from organic molecules and transfer them to coenzymes In the mitochondrion Pyruvic acid reacts with NAD and coenzyme A (CoA) Producing 1 CO2, 1 NADH, 1 acetyl-CoA

25-2 Carbohydrate Metabolism The Citric Acid Cycle Acetyl group transfers: From acetyl-CoA to oxaloacetic acid Produces citric acid CoA is released to bind another acetyl group One citric acid cycle removes two carbon atoms Regenerating 4-carbon chain Several steps involve more than one reaction or enzyme H2O molecules are tied up in two steps

25-2 Carbohydrate Metabolism The Citric Acid Cycle CO2 is a waste product Summary of the Citric Acid Cycle CH3CO  CoA + 3 NAD + FAD + GDP + Pi + 2 H2O  CoA + 2 CO2 + 3 NADH + FADH2 + 2 H+ + GTP

Figure 25-4a The Citric Acid Cycle Pyruvate Coenzyme A Acetyl-CoA Coenzyme A 4-carbon Citric acid 6-carbon CITRIC ACID CYCLE ELECTRON TRANSPORT SYSTEM 5-carbon 4-carbon (via GTP) An overview of the citric acid cycle, showing the distribution of carbon, hydrogen, and oxygen atoms 20

Figure 25-4b The Citric Acid Cycle Pyruvate Coenzyme A Acetyl-CoA Citric acid Oxaloacetic acid Isocitric acid Malic acid CITRIC ACID CYCLE -Ketoglutaric acid Fumaric acid Coenzyme A Coenzyme A Succinic acid Succinyl-CoA A detailed view of the citric acid cycle, showing the changes in the carbon chain 21

25-2 Carbohydrate Metabolism Oxidative Phosphorylation and the ETS Oxidative Phosphorylation Is the generation of ATP Within mitochondria In a reaction requiring coenzymes and oxygen Produces more than 90% of ATP used by body Results in 2 H2 + O2 2 H2O

25-2 Carbohydrate Metabolism Oxidative Phosphorylation and the ETS Electron Transport System (ETS) Is the key reaction in oxidative phosphorylation Is in inner mitochondrial membrane Electrons carry chemical energy Within a series of integral and peripheral proteins

25-2 Carbohydrate Metabolism Oxidation, Reduction, and Energy Transfer Oxidation (loss of electrons) Electron donor is oxidized Reduction (gain of electrons) Electron recipient is reduced The two reactions are always paired

25-2 Carbohydrate Metabolism Oxidation, Reduction, and Energy Transfer Energy Transfer Electrons transfer energy Energy performs physical or chemical work (ATP formation) Electrons Travel through series of oxidation–reduction reactions Ultimately combine with oxygen to form water

25-2 Carbohydrate Metabolism Coenzymes Play key role in oxidation–reduction reactions Act as intermediaries Accept electrons from one molecule Transfer them to another molecule In citric acid cycle: Are NAD and FAD Remove hydrogen atoms from organic substrates Each hydrogen atom consists of an electron and a proton

25-2 Carbohydrate Metabolism Oxidation–Reduction Reactions Coenzyme Accepts hydrogen atoms Is reduced Gains energy Donor molecule Gives up hydrogen atoms Is oxidized Loses energy

25-2 Carbohydrate Metabolism Oxidation–Reduction Reactions Protons and electrons are released Electrons Enter electron transport system Transfer to oxygen H2O is formed Energy is released Synthesize ATP from ADP

25-2 Carbohydrate Metabolism Coenzyme FAD Accepts two hydrogen atoms from citric acid cycle Gaining two electrons Coenzyme NAD Accepts two hydrogen atoms Gains two electrons Releases one proton Forms NADH + H+

25-2 Carbohydrate Metabolism The Electron Transport System (ETS) Also called respiratory chain Is a sequence of proteins (cytochromes) Protein Embedded in inner membrane of mitochondrion Surrounds pigment complex Pigment complex Contains a metal ion (iron or copper)

25-2 Carbohydrate Metabolism General Path of Electrons Captured and Delivered by Coenzymes Five Steps ETS Step 1 Coenzyme strips two hydrogens from substrate molecule Glycolysis occurs in cytoplasm NAD is reduced to NADH In mitochondria: NAD and FAD in citric acid cycle

25-2 Carbohydrate Metabolism ETS Step 2 NADH and FADH2 deliver H atoms to coenzymes in inner mitochondrial membrane Protons are released Electrons are transferred to ETS Electron Carriers NADH sends electrons to FMN (flavin mononucleotide) FADH2 proceeds directly to coenzyme Q (CoQ; ubiquinone) FMN and CoQ bind to inner mitochondrial membrane

25-2 Carbohydrate Metabolism ETS Step 3 CoQ releases protons and passes electrons to cytochrome b ETS Step 4 Electrons pass along electron transport system Losing energy in a series of small steps ETS Step 5 At the end of ETS Oxygen accepts electrons and combines with H+ to form H2O

Figure 25-5a Oxidative Phosphorylation Steps in Oxidative Phosphorylation Substrate-H2 Substrate-H2 A coenzyme strips 2 hydrogen atoms from a substrate molecule. NADH and FADH2 deliver hydrogen atoms to coenzymes embedded in the inner membrane of a mitochondrion. Coenzyme Q releases hydrogen ions and passes electrons to cytochrome b. Cytochrome b Electrons are passed along the Electron Transport System, losing energy in a series of small steps. Cytochrome c Oxygen accepts the low- energy electrons, and with hydrogen ions, forms water. Cytochrome a Cytochrome a3 The sequence of oxidation-reduction reactions involved in oxidative phosphorylation. 34

Figure 25-5b Oxidative Phosphorylation CYTOSOL Outer membrane Hydrogen ion channel Intermembrane space Inner membrane ATP synthase Reduced substrate molecule Oxidized substrate molecule MITOCHONDRIAL MATRIX The locations of the coenzymes and the electron transport system. 35

25-2 Carbohydrate Metabolism ATP Generation and the ETS Does not produce ATP directly Creates steep concentration gradient across inner mitochondrial membrane Electrons along ETS release energy As they pass from coenzyme to cytochrome And from cytochrome to cytochrome Energy released drives H ion (H+) pumps That move H+ from mitochondrial matrix Into intermembrane space

25-2 Carbohydrate Metabolism Ion Pumps Create concentration gradient for H+ across inner membrane Concentration gradient provides energy to convert ADP to ATP Ion Channels In inner membrane permit diffusion of H+ into matrix

25-2 Carbohydrate Metabolism The Importance of Oxidative Phosphorylation Oxidative Phosphorylation Is the most important mechanism for generation of ATP Requires oxygen and electrons Rate of ATP generation is limited by oxygen or electrons Cells obtain oxygen by diffusion from extracellular fluid

25-2 Carbohydrate Metabolism Energy Yield of Glycolysis and Cellular Respiration For most cells, reaction pathway: Begins with glucose Ends with carbon dioxide and water Is main method of generating ATP

25-2 Carbohydrate Metabolism Glycolysis One glucose molecule is broken down anaerobically to two pyruvic acid Cell gains a net two molecules of ATP Transition Phase Two molecules NADH pass electrons to FAD Producing an additional 4 ATP molecules

25-2 Carbohydrate Metabolism Citric Acid Cycle Breaks down 2 pyruvic acid molecules Produces 2 ATP by way of GTP Transfers H atoms to NADH and FADH2 Coenzymes provide electrons to ETS

25-2 Carbohydrate Metabolism ETS Each of eight NADH molecules Produces 3 ATP + 1 water molecule Each of two FADH2 molecules Produces 2 ATP + 1 water molecule Total yield from citric acid cycle to ETS 28 ATP

25-2 Carbohydrate Metabolism Summary of ATP Production For one glucose molecule processed, cell gains 36 molecules of ATP 2 from glycolysis 4 from NADH generated in glycolysis 2 from citric acid cycle (through GTP) 28 from ETS

Figure 25-6 A Summary of the Energy Yield of Aerobic Metabolism Glucose (6-carbon) CYTOSOL Energy Summary Glycolysis (Anaerobic): produced during enzymatic reactions in the cytosol GLYCOLYSIS used to initiate glycolysis Pyruvate (3-carbon) net gain to cell The Electron Transport System and Citric Acid Cycle (Aerobic): Via intermediates from NADH produced in glycolysis Coenzyme A Acetyl CoA from NADH generated in citric acid cycle ELECTRON TRANSPORT SYSTEM from FADH2 generated in citric acid cycle Citric Acid Cycle (2 turns) via GTP produced during enzymatic reactions net gain to cell from complete catabolism of one glucose molecule MITOCHONDRIA 44

25-2 Carbohydrate Metabolism Gluconeogenesis Is the synthesis of glucose from noncarbohydrate precursors Lactic acid Glycerol Amino acids Stores glucose as glycogen in liver and skeletal muscle

25-2 Carbohydrate Metabolism Glycogenesis Is the formation of glycogen from glucose Occurs slowly Requires high-energy compound uridine triphosphate (UTP) Glycogenolysis Is the breakdown of glycogen Occurs quickly Involves a single enzymatic step

Figure 25-7 Carbohydrate Breakdown and Synthesis CYTOSOL Glycogen 3 enzymatic steps GLYCOGENOLYSIS GLYCOGENESIS Glucose G L U C O N E S I G L Y C O S I Glucose-6-phosphate Fructose-6-phosphate Other carbohydrates Fructose-1,6-bisphosphate 3-carbon intermediates Glycerol (Several steps) 47

25-3 Lipid Metabolism Lipids Lipid molecules contain carbon, hydrogen, and oxygen In different proportions than carbohydrates Triglycerides are the most abundant lipid in the body

25-3 Lipid Metabolism Lipid Catabolism (Lipolysis) Breaks lipids down into pieces that can be: Converted to pyruvic acid Channeled directly into citric acid cycle Hydrolysis splits triglyceride into component parts One molecule of glycerol Three fatty acid molecules

25-3 Lipid Metabolism Lipid Catabolism Enzymes in cytosol convert glycerol to pyruvic acid Pyruvic acid enters citric acid cycle Different enzymes convert fatty acids to acetyl-CoA (beta-oxidation)

25-3 Lipid Metabolism Beta-Oxidation A series of reactions Breaks fatty acid molecules into 2-carbon fragments Occurs inside mitochondria Each step: Generates molecules of acetyl-CoA and NADH Leaves a shorter carbon chain bound to coenzyme A

Figure 25-8 Beta-Oxidation Triglyceride Absorption through endocytosis CYTOSOL Lysosomal enzymes break down triglyceride molecules into one glycerol molecule and 3 fatty acids. Fatty acid (18-carbon) Glycerol MITOCHONDRIA Fatty acids are absorbed into the mitochondria. In the cytosol, the glycerol is converted to pyruvate through the glycolysis pathway. Fatty acid (18-carbon) Pyruvate Coenzyme A Coenzyme A Fatty acid (18-carbon) – CoA Coenzyme A Fatty acid (16-carbon) – CoA Acetyl- CoA Acetyl CoA Citric acid cycle Coenzymes Electron transport system An enzymatic reaction then breaks off the first two carbons as acetyl-CoA while leaving a shorter fatty acid bound to the second molecule of coenzyme A. 52

25-3 Lipid Metabolism Lipids and Energy Production For each 2-carbon fragment removed from fatty acid, cell gains: 12 ATP from acetyl-CoA in citric acid cycle 5 ATP from NADH Cell can gain 144 ATP molecules from breakdown of one 18-carbon fatty acid molecule Fatty acid breakdown yields about 1.5 times the energy of glucose breakdown

25-3 Lipid Metabolism Lipid Storage Is important as energy reserves Can provide large amounts of ATP, but slowly Saves space, but hard for water-soluble enzymes to reach

25-3 Lipid Metabolism Lipid Synthesis (Lipogenesis) Can use almost any organic substrate Because lipids, amino acids, and carbohydrates can be converted to acetyl-CoA Glycerol Is synthesized from dihydroxyacetone phosphate (intermediate product of glycolysis)

25-3 Lipid Metabolism Lipid Synthesis (Lipogenesis) Other lipids Nonessential fatty acids and steroids are examples Are synthesized from acetyl-CoA Essential fatty acids Cannot be produced by the body, must be consumed

25-3 Lipid Metabolism Lipid Transport and Distribution Cells require lipids To maintain plasma membranes Steroid hormones must reach target cells in many different tissues

25-3 Lipid Metabolism Solubility Circulating Lipids Most lipids are not soluble in water Special transport mechanisms carry lipids from one region of body to another Circulating Lipids Most lipids circulate through bloodstream as lipoproteins Free fatty acids are a small percentage of total circulating lipids

25-3 Lipid Metabolism Free Fatty Acids Are an important energy source During periods of starvation When glucose supplies are limited Liver cells, cardiac muscle cells, skeletal muscle fibers, and so forth Metabolize free fatty acids

25-3 Lipid Metabolism Lipoproteins Are lipid–protein complexes Contain large insoluble glycerides and cholesterol Five classes of lipoproteins Chylomicrons Very low-density lipoproteins (VLDLs) Intermediate-density lipoproteins (IDLs) Low-density lipoproteins (LDLs) High-density lipoproteins (HDLs)

25-3 Lipid Metabolism Chylomicrons Are produced in intestinal tract Are too large to diffuse across capillary wall Enter lymphatic capillaries Travel through thoracic duct To venous circulation and systemic arteries

Figure 25-9 Lipid Transport and Utilization Micelles are absorbed into intestinal mucosa where they are converted into chylomicrons. Intestinal cells secrete chylomicrons, which are absorbed into lacteals. From the lacteals, the chylomicrons proceed within the lymphatic vessels and into the thoracic duct, and from there are distributed throughout the body. Micelle Lacteal Monoglycerides, fatty acids Chylomicron Triglycerides  other lipids and proteins Exocytosis 62

Figure 25-9 Lipid Transport and Utilization Lipoprotein lipase in the capillary endothelium breaks down the chylomicrons and releases fatty acids and monoglycerides into the interstitial fluid. Resting skeletal muscles absorb fatty acids and break them down for ATP production or storage as glycogen. Adipocytes absorb fatty acids and use them to synthesize triglycerides for storage. Skeletal muscle Adipocytes 63

Figure 25-9 Lipid Transport and Utilization Lipoproteins and Lipid Transport and Distribution The liver absorbs chylomicrons from the bloodstream and recycles the cholesterol producing LDL. Chylomicrons Triglycerides removed LDL leaves the liver. Cholesterol extracted HDL delivers cholesterol back to the liver for recycling. LDL is absorbed. Excess cholesterol is excreted with the bile salts PERIPHERAL CELLS High cholesterol Low cholesterol Lysosomal breakdown Used in synthesis of membranes, hormones, other materials Cholesterol release Cell extract cholesterol from LDL and use it. Unused cholesterol leaves the cells and binds HDL. 64

25-4 Protein Metabolism Protein Metabolism The body synthesizes 100,000 to 140,000 proteins Each with different form, function, and structure All proteins are built from the 20 amino acids Cellular proteins are recycled in cytosol Peptide bonds are broken Free amino acids are used in new proteins

25-4 Protein Metabolism Protein Metabolism If other energy sources are inadequate Mitochondria generate ATP by breaking down amino acids in citric acid cycle Not all amino acids enter cycle at same point, so ATP benefits vary

25-4 Protein Metabolism Amino Acid Catabolism Removal of amino group by transamination or deamination

25-4 Protein Metabolism Transamination Deamination Attaches amino group of amino acid To keto acid Converts keto acid into amino acid That leaves mitochondrion and enters cytosol Available for protein synthesis Deamination Prepares amino acid for breakdown in citric acid cycle Removes amino group and hydrogen atom Reaction generates ammonium ion

Figure 25-10a Amino Acid Catabolism and Synthesis Transamination In a transamination, the amino group (–NH2) of one amino acid gets transferred to a keto acid. Transaminase Glutamic acid Keto acid 1 Keto acid 2 Tyrosine 69

Figure 25-10b Amino Acid Catabolism and Synthesis Deamination In deamination, the amino group is removed and an ammonium ion is released. Deaminase Ammonium ion Glutamic acid Organic acid 70

25-4 Protein Metabolism Ammonium Ions Urea Cycle Are highly toxic, even in low concentrations Liver cells (primary sites of deamination) have enzymes that use ammonium ions to synthesize urea (water-soluble compound excreted in urine) Urea Cycle Is the reaction sequence that produces urea

Figure 25-10c Amino Acid Catabolism and Synthesis Urea cycle. The urea cycle takes two metabolic waste products—ammonium ions and carbon dioxide—and produces urea, a relatively harmless, soluble compound that is excreted in the urine. Ammonium ion Carbon dioxide Urea cycle Urea 72

25-4 Protein Metabolism Proteins and ATP Production When glucose and lipid reserves are inadequate, liver cells: Break down internal proteins Absorb additional amino acids from blood Amino acids are deaminated Carbon chains broken down to provide ATP

25-4 Protein Metabolism Three Factors against Protein Catabolism Proteins are more difficult to break apart than complex carbohydrates or lipids A by-product, ammonium ion, is toxic to cells Proteins form the most important structural and functional components of cells

25-4 Protein Metabolism Protein Synthesis The body synthesizes half of the amino acids needed to build proteins Nonessential amino acids Amino acids made by the body on demand Requires process called amination

Figure 25-10d Amino Acid Catabolism and Synthesis Amino Acid Synthesis Amination In an amination reaction, an ammonium ion (NH4+) is used to form an amino group that is attached to a molecule, yielding an amino acid. –Ketoglutarate Glutamic acid 76

25-4 Protein Metabolism Protein Synthesis Ten essential amino acids Eight not synthesized Isoleucine, leucine, lysine, threonine, tryptophan, phenylalanine, valine, and methionine Two insufficiently synthesized Arginine and histidine

Figure 25-11 Absorptive and Postabsorptive States 8 am 9 am 10 am 11 am 12 noon 1 pm 2 pm 3 pm 4 pm 5 pm 6 pm Glucose Levels Lipid Levels Amino Acid Levels MEALS Breakfast Lunch Dinner 78

Figure 25-11 Absorptive and Postabsorptive States 8 pm 9 pm 10 pm 11 pm 12 midnight 1 am 2 am 3 am 4 am 5 am 6 am 7 am 8 am Glucose Levels Lipid Levels Amino Acid Levels 79

Figure 25-11 Absorptive and Postabsorptive States Blood glucose levels elevated Insulin LIPIDS CARBOHYDRATES PROTEINS Triglycerides Glycogen Proteins Insulin Glucose Insulin G l y c o s i Insulin Androgens Estrogens Growth hormone Blood amino acids elevated Blood lipid levels elevated Fatty acids Glycerol Amino acids Insulin, Growth hormone Pyruvate Insulin KEY Catabolic pathway Acetyl-CoA Anabolic pathway Citric acid cycle Coenzymes Electron transport system Stimulation MITOCHONDRIA 80

Figure 25-11 Absorptive and Postabsorptive States Glucose released into the bloodstream by the liver KEY LIPIDS CARBOHYDRATES PROTEINS Catabolic pathway Triglycerides Glycogen Proteins Anabolic pathway Glucagon Epinephrine Stimulation Glucagon, Epinephrine, Glucocorticoids, Growth hormone Glucose G l u c o n e g s i Glucocorticoids, Growth hormone Fatty acids released into the bloodstream by adipocytes Amino acids released into the bloodstream by the liver Fatty acids Glycerol Amino acids Glucagon Glucocorticoids, Growth hormone Glucocorticoids stimulate lipid utilization in all tissues except neural tissue. Therefore, glucose is more available for the brain. Pyruvate Acetyl-CoA Active liver cells produce so much acetyl-CoA that it is converted to ketone bodies. Ketone bodies can be converted back to acetyl-CoA by other cells. Citric acid cycle Electron transport system Coenzymes MITOCHONDRIA Ketone bodies 81

25-5 Absorptive and Postabsorptive States Nutrient Requirements Of each tissue vary with types and quantities of enzymes present in cell Five Metabolic Tissues Liver Adipose tissue Skeletal muscle Neural tissue Other peripheral tissues

25-5 Absorptive and Postabsorptive States The Liver Is focal point of metabolic regulation and control Contains great diversity of enzymes that break down or synthesize carbohydrates, lipids, and amino acids Hepatocytes Have an extensive blood supply Monitor and adjust nutrient composition of circulating blood Contain significant energy reserves (glycogen deposits)

25-5 Absorptive and Postabsorptive States Adipose Tissue Stores lipids, primarily as triglycerides Is located in: Areolar tissue Mesenteries Red and yellow marrows Epicardium Around eyes and kidneys

25-5 Absorptive and Postabsorptive States Skeletal Muscle Maintains substantial glycogen reserves Contractile proteins can be broken down Amino acids used as energy source

25-5 Absorptive and Postabsorptive States Neural Tissue Does not maintain reserves of carbohydrates, lipids, or proteins Requires reliable supply of glucose Cannot metabolize other molecules In CNS, cannot function in low-glucose conditions Individual becomes unconscious

25-5 Absorptive and Postabsorptive States Other Peripheral Tissues Do not maintain large metabolic reserves Can metabolize glucose, fatty acids, and other substrates Preferred energy source varies According to instructions from endocrine system

25-5 Absorptive and Postabsorptive States Metabolic Interactions Relationships among five components change over 24-hour period Body has two patterns of daily metabolic activity Absorptive state Postabsorptive state

25-5 Absorptive and Postabsorptive States The Absorptive State Is the period following a meal when nutrient absorption is under way The Postabsorptive State Is the period when nutrient absorption is not under way Body relies on internal energy reserves for energy demands Liver cells conserve glucose Break down lipids and amino acids

25-6 Nutrition Minerals Are inorganic ions released through dissociation of electrolytes Are important for three reasons Ions such as sodium, chloride, and potassium determine osmotic concentrations of body fluids Ions play a major role in physiologic processes Ions are essential cofactors in many enzymatic reactions

25-6 Nutrition Metals Mineral Reserves Each component of ETS requires an iron atom Final cytochrome of ETS requires a copper ion Mineral Reserves The body contains significant mineral reserves That help reduce effects of variations in diet

Table 25-2-1 Minerals and Mineral Reserves 92

Table 25-2-2 Minerals and Mineral Reserves 93

25-6 Nutrition Vitamins A vitamin is an essential organic nutrient that functions as a coenzyme in vital enzymatic reactions Vitamins are assigned to either of two groups based on their chemical structure and characteristics Fat-soluble vitamins Water-soluble vitamins

25-6 Nutrition Fat-Soluble Vitamins Vitamins A, D, E, and K Are absorbed primarily from the digestive tract along with lipids of micelles Normally diffuse into plasma membranes and lipids in liver and adipose tissue

25-6 Nutrition Vitamin A Vitamin D Vitamin E Vitamin K A structural component of visual pigment retinal Vitamin D Is converted to calcitriol, which increases rate of intestinal calcium and phosphorus absorption Vitamin E Stabilizes intracellular membranes Vitamin K Helps synthesize several proteins, including three clotting factors

Table 25-3 The Fat-Soluble Vitamins 97

25-6 Nutrition Vitamin Reserves The body contains significant reserves of fat-soluble vitamins Avitaminosis (vitamin deficiency disease) Rare in fat-soluble vitamins Hypervitaminosis more likely Normal metabolism can continue several months without dietary sources

25-6 Nutrition Water-Soluble Vitamins Are components of coenzymes Are rapidly exchanged between fluid in digestive tract and circulating blood Excesses are excreted in urine

Table 25-4 The Water-Soluble Vitamins 100

25-6 Nutrition Vitamins and Bacteria Vitamin B12 Bacterial inhabitants of intestines produce small amounts of: Fat-soluble vitamin K Five water-soluble vitamins Vitamin B12 Intestinal epithelium absorbs all water-soluble vitamins except B12 B12 molecule is too large Must bind to intrinsic factor before absorption