Metabolism

Chapter

7

Metabolism

• Metabolism: All chemical reactions within organisms that enable them to sustain life. The two main categories are catabolism and anabolism.

Metabolism

• Catabolism: Any metabolic process whereby cells break down complex substances into simpler, smaller ones.

• Anabolism: Any metabolic process whereby cells convert simple substances into more complex ones.

Metabolism

• Thousands of chemical reactions occur every moment in cells throughout the body.

• The most active metabolic sites are the liver, muscle, and brain cells.

Energy: Fuel for Work

• Energy source– Chemical energy (stored in molecular bonds) in

carbohydrates, fat, protein• Food energy to cellular energy– Stage 1: digestion, absorption, transport– Stage 2: breakdown of molecules to a few key

metabolites– Stage 3: transfer of energy to a form cells can use

What Is Metabolism?• Catabolism– Reactions that

breakdown 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

• NAD and FAD: transport shuttles– Accept high-energy electrons for use in

ATP production

Breakdown and Release of Energy

• Extracting energy from carbohydrate– Glycolysis

• Pathway splits glucose into 2 pyruvates

• Transfers electrons to NAD• Produces 2 ATP• anaerobic

– 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 NADand FAD

• Produces large amounts of ATP• Produces water

– End products of glucose breakdown• ATP, H2O, CO2

Breakdown and Release of Energy

• Extracting energy from fat– Split triglycerides into glycerol and fatty acids– Beta-oxidation

• Breaks apart fatty acids into acetyl CoA• Transfers electrons to NAD and FAD

– Citric acid cycle• Acetyl CoA from beta-oxidation enters cycle

– Electron transport chain– 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 breakdown pathways– End products• ATP, H2O, CO2, urea

Breakdown and

Release of Energy

Cellular Respiration

• Cellular respiration is the controlled release of chemical-bond energy from large, organic molecules.

• This energy is utilized for many activities to sustain life.

• Both autotrophs and heterotrophs carry out cellular respiration.

Aerobic Vs. Anaerobic

• Aerobic respiration requires oxygen.• Anaerobic respiration does not require

oxygen.

Aerobic Respiration

• Aerobic cellular respiration is a specific series of enzyme controlled chemical reactions in which oxygen is involved in the breakdown of glucose into carbon-dioxide and water.

• The chemical-bond energy is released in the form of ATP.

• Sugar + Oxygen carbon dioxide + water + energy (ATP)

Aerobic Respiration

• Simplified Reaction:• C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) ΔHc -

2880 kJ• Covalent bonds in glucose contain large

amounts of chemical potential energy.• The potential energy is released and utilized

to create ATP.

Glycolysis

• Glycolysis is a series of enzyme controlled anaerobic reactions that result in the breakdown of glucose and the formation of ATP.

• A 6-carbon sugar glucose molecule is split into two smaller 3-carbon molecules which are further broken down into pyruvic acid or pyruvate.

Glycolysis

• 2 ATP molecules are created during glycolysis and electrons are released during the process.

Krebs Cycle

• The Krebs cycle is a series of enzyme-controlled reactions that take place inside the mitochondrion.

• Pyruvic acid formed during glycolysis is broken down further.

• Carbon dioxide, electrons, and 2 molecules of ATP are produced in this reaction.

Electron Transport System

• The electrons released from glycolysis and the Krebs cycle are carried to the electron-transport system (ETS) by NADH and FADH2.

• The electrons are transferred through a series of oxidation-reduction reactions until they are ultimately accepted by oxygen atoms forming oxygen ions.

• 32 molecules of ATP are produced.

Aerobic Respiration Summary

• Glucose enters glycolysis.– Broken down into pyruvic acid.

• Pyruvic acid enters the Krebs cycle.– Pyruvic acid is further broken down and carbon-dioxide is

released.• Electrons and hydrogen ions from glycolysis and the

Krebs cycle are transferred by NADH and FADH2 to the ETS.– Electrons are transferred to oxygen to form oxygen ions.– Hydrogen ions and oxygen ions combine to form water.

Anaerobic Cellular Respiration

• Anaerobic respiration does not require oxygen as the final electron acceptor.

• Some organisms do not have the necessary enzymes to carry out the Krebs cycle and ETS.

• Many prokaryotic organisms fall into this category.

• Yeast is a eukaryotic organism that performs anaerobic respiration.

Fat Respiration

• A triglyceride (neutral fat) consists of a glycerol molecule with 3 fatty acids attached to it.

• A molecule of fat stores several times the amount of energy as a molecule of glucose.

• Fat is an excellent long-term energy storage material.

• Other molecules such as glucose can be converted to fat for storage.

Protein Respiration

• Protein molecules must first be broken down into amino acids.

• The amino acids must then have their amino group (-NH2) removed (deamination).

• The amino group is then converted to ammonia. In the human body ammonia is converted to urea or uric acid which can then be excreted.

Biosynthesis and Storage

• Making carbohydrate (glucose)– Gluconeogenesis

• Uses pyruvate, lactate, glycerol, certain amino acids

• Storing carbohydrate (glycogen)– Liver, muscle make glycogen from glucose

• Making fat (fatty acids)– Lipogenesis

• Uses acetyl CoA from fat, amino acids, glucose

• Storing fat (triglyceride)– Stored in adipose tissue

Biosynthesis and Storage

• Making ketone bodies (ketogenesis)– Made from acetyl CoA• Inadequate glucose in cells

• Making protein (amino acids)– Amino acid pool supplied from • Diet, protein breakdown, cell synthesis

Regulation of Metabolism

• May favor either anabolic or catabolic functions

• Regulating hormones– Insulin– Glucagon– Cortisol– Epinephrine

Special States• Feasting– Excess energy

intake from carbohydrate, fat, protein• Promotes storage

Special States• Fasting– Inadequate

energy intake• Promotes

breakdown– Prolonged

fasting• Protects body

protein as long as possible

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.

Fuel for Distance Walking• A recent study sought to explore

whether or not humans naturally select a preferred walking speed (PWS), and that the body’s fuel selection can be critical to the total distance traveled. The hypothesis maintained that humans select a preferred walking speed that primarily uses fat as fuel and does not deplete carbohydrate (CHO) stores.

• The major finding of this study was that able-bodied subjects naturally selected a walking speed just below the speed preceding an abrupt rise in CHO oxidation that would deplete the body’s small stores of CHO quickly.

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.