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.