Chapter
8 Metabolism
Energy: Fuel for Work • Energy source • Chemical energy in carbohydrates, fat, protein
• Transferring food energy to cellular energy • Stage 1: digestion, absorption, transport • Stage 2: breakdown of molecules • Stage 3: transfer of energy to a form cells can use
What Is Metabolism? • Catabolism • Reactions that break down compounds into small units
• Anabolism • Reactions that build complex molecules from smaller ones
What Is Metabolism? • Cell is the metabolic processing center • Nucleus • Cytoplasm • Cytosol + organelles
• ATP is the body’s energy currency • ATP = adenosine triphosphate • Form of energy cells use
What Is Metabolism? • NADH and FADH2: The Body’s Energy Shuttles • Electron acceptors
• NADPH: An Energy Shuttle for Biosynthesis
Breakdown and Release of Energy • Extracting energy from carbohydrate • Glycolysis • Pathway splits glucose into 2 pyruvates • Transfers electrons to NAD • Produces some ATP
• Pyruvate to acetyl CoA • Releases CO2 • Transfers electrons to NAD
Breakdown and Release of Energy • Extracting energy from carbohydrate • Citric acid cycle • Releases CO2 • Produces GTP (like ATP) • Transfers electrons to NAD and FAD
• Electron transport chain • Accepts electrons from NAD and FAD • Produces large amounts of ATP • Produces water
• End products of glucose catabolism • ATP, H2O, CO2
Breakdown and Release of Energy • Extracting energy from fat • Carnitine shuttle • Beta-oxidation • Breaks apart fatty acids into acetyl CoA • Transfers electrons to NAD and FAD
• Citric acid cycle and electron transport chain • Complete fatty acid breakdown • Acetyl CoA from beta-oxidation enters cycle
Breakdown and Release of Energy • Fat Burns in a Flame of Carbohydrate • End products of fat breakdown • ATP, H2O, CO2
Breakdown and Release of Energy • Extracting energy from protein • Split protein into amino acids • Split off amino group • Converted to urea for excretion
• Carbon skeleton enters pathways at different points • End products of Amino Acid Catabolism • ATP, H2O, CO2, urea
Alcohol Metabolism • Small amount of alcohol • Alcohol dehydrogenase • Alcohol to acetaldehyde
• Aldehyde dehydrogenase • Acetaldehyde to acetate
• Metabolites to acetyl CoA to fat
Alcohol Metabolism • Large amount of alcohol • Overwhelms alcohol dehydrogenase system • Uses microsomal ethanoloxidizing system (MEOS)
Biosynthesis and Storage • Making carbohydrate (glucose) • Gluconeogenesis: Pathways to glucose • Uses pyruvate, lactate, glycerol, certain amino acids
• Storage: Glucose to glycogen • Liver, muscle make glycogen from glucose
Breakdown and Release of Energy
Biosynthesis and Storage • Making fat (fatty acids) • Lipogenesis: Pathways to Fatty Acids • Uses acetyl CoA from fat, amino acids, glucose
• Storage: Dietary Energy to Stored Triglyceride • Stored in adipose tissue
Biosynthesis and Storage • Making ketone bodies • Ketogenesis: Pathways to Ketone Bodies • Made from acetyl CoA • Inadequate glucose in cells
• Making protein (amino acids) • Biosynthesis: Making Amino Acids • Amino acid pool supplied from: diet, protein breakdown, cell synthesis
Regulation of Metabolism • May favor either anabolic or catabolic functions • Hormones of metabolism • • • •
Insulin Glucagon Cortisol Epinephrine
Special States • Feasting • Excess energy intake from carbohydrate, fat, protein • Promotes storage
• The Return to Normal
Special States • Fasting • Survival priorities and potential energy sources
Special States • Fasting • The prolonged fast: in the beginning • Protects body protein as long as possible
• • • •
The first few days The early weeks Several weeks of fasting The end is near
The ADP–ATP Cycle • When extracting energy from nutrients, the formation of ATP from ADP + P captures energy. • Breaking a phosphate bond in ATP to ADP + P, releases energy for biosynthesis and work.
When Glycolysis Goes Awry • Red blood cells do not have mitochondria, so they rely on glycolysis as their only source of ATP. • They use ATP to maintain the integrity and shape of their cell membranes. • A defect in red blood cell glycolysis can cause a shortage of ATP, which leads to deformed red blood cells. • Destruction of these cells by the spleen leads to a type of anemia called hemolytic anemia.
Electron Transport Chain • This pathway produces most of the ATP available from glucose. NADH molecules deliver pairs of highenergy electrons to the beginning of the chain. • The pairs of high-energy electrons carried by FADH2 enter this pathway farther along and produce fewer ATP than electron pairs carried by NADH. • Water is the final product of the electron transport chain.
Carnitine • Without assistance, activated fatty acid cannot get inside the mitochondria where fatty acid oxidation and the citric acid cycle operate. • This entry problem is solved by carnitine, a compound formed from the amino acid lysine.
• Carnitine has the unique task of ferrying activated fatty acids across the mitochondrial membrane, from the cytosol to the interior of the mitochondrion.
Deamination • A deamination reaction strips the amino group from an amino acid.
Ketones • Organic compounds that contain a chemical group consisting of C=O (a carbon–oxygen double bond) bound to two hydrocarbons. • Pyruvate and fructose are two examples of ketones. • Acetone and acetoacetate are both ketones and tetone bodies. • While betahydroxybutyrate is not a ketone, it is a ketone body.
Cholesterol • Your body can make cholesterol from acetyl CoA by way of ketones. In fact, all 27 carbons in synthesized cholesterol come from acetyl CoA. • The rate of cholesterol formation is highly responsive to cholesterol levels in cells. If levels are low, the liver makes more. If levels are high, synthesis decreases. • That is why dietary cholesterol in the absence of dietary fat often has little effect on blood cholesterol levels.
Transamination • A transamination reaction transfers the amino group from one amino acid to form a different amino acid.
Indispensable and Dispensable Amino Acids • Proteins are made from combinations of indispensable and dispensable amino acids. • The body synthesizes dispensable amino acids from pyruvate, other glycolytic intermediates, and compounds from the citric acid cycle. • To form amino acids, transamination reactions transfer amino groups to carbon skeletons.