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Metabolism
Chapter
8
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 ethanol-
oxidizing 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 high-energy 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.