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 CO 2 Transfers electrons to NAD

Breakdown and Release of Energy Extracting energy from carbohydrate – Citric acid cycle Releases CO 2 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 breakdown ATP, H 2 O, CO 2

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, H 2 O, CO 2

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, H 2 O, CO 2, 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: C 6 H 12 O 6 (aq) + 6O 2 (g) → 6CO 2 (g) + 6H 2 O (l) ΔH c 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 FADH 2. 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 FADH 2 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 FADH 2 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.