Obtaining Energy from Food Cellular Respiration Obtaining Energy from Food

Role of ATP in linking anabolic and catabolic reactions

Energy can be transported by Electron carriers NADH & FADH2.

Four Stages of Cellular Respiration Glycolysis: Enzymes break a 6-carbon molecule of glucose into two 3-carbon molecules of pyruvate. Some ATP is synthesized by substrate-level phosphorylation. substrate-level phosphorylation: an enzyme-catalyzed reaction that transfers a phosphate group from a substrate to ADP. Pyruvate Oxidation: Enzymes convert the 3-carbon pyruvate into a 2-carbon acetyl group. Citric Acid Cycle: which enters the citric acid cycle and is completely oxidized to carbon dioxide. Some ATP is synthesized during the citric acid cycle. Electron Transfer Chain: High-energy electrons are delivered to oxygen by a sequence of electron carriers in the electron transfer system Free energy released by electron flow generates an H+ gradient by chemiosmosis. ATP synthase uses the H+ gradient as the energy source to make ATP.

Road Map

Glycolysis Occurs in the cytosol Is anaerobic (does not use oxygen) No CO2 released Requires 2 ATP activation energy 4 ATP are produced (net gain = 2ATP) 2 (electron) NADH are formed 2 pyruvate produced

Glycolysis

10 Steps in Glycolysis Figure 8.7: Reactions of glycolysis, which occur in the cytosol. Because two molecules of G3P are produced in reaction 5, all the reactions from 6 to 10 are doubled (not shown). The names of the enzymes that catalyze each reaction are in rust.

Pyruvate Oxidation and the Citric Acid Cycle Active transport moves pyruvate into the mitochondrial matrix where pyruvate oxidation and the citric acid cycle take place Oxidation of pyruvate generates CO2, acetyl-coenzyme A (acetyl-CoA), and NADH The acetyl group of acetyl-CoA enters the citric acid cycle

Reactions of Pyruvate Oxidation

Mitochondria Structure found inside all eukaryotes Contains a membrane within a membrane Site of Citric Acid Cycle (inner matrix) and oxidative phosphorylation (inner membrane) The eight reactions of the citric acid cycle (tricarboxylic acid cycle or Krebs cycle) oxidize acetyl groups completely to CO2 , generate 3 NADH and 1 FADH2, and synthesize 1 ATP by substrate-level phosphorylation.

Citric Acid Cycle (Krebs Cycle) Oxaloacetate (4C) 1 8 Citrate (6C) Citrate synthase Malate dehydrogenase Malate (4C) 2 Aconitase Fumarase Isocitrate (6C) 7 Citric Acid Cycle (Krebs Cycle) Fumarate (4C) Isocitrate dehydrogenase 3 Succinate dehydrogenase α -Ketoglutarate dehydrogenase Succinyl–CoA synthetase 6 Succinate (4C) α -Ketoglutarate (5C) 5 4 Succinyl–CoA (4C)

Citric Acid Cycle

Inside the Mitochondria Pyruvic acid is converted to Acetyl CoA 1 CO2 released Electrons transferred to NADH Acetyl CoA enters Citric Acid Cycle 2 CO2 molecules are released (from each Acetyl CoA) 1 ATP molecule is generated Electrons transferred to 3 NADH and 1 FADH2

Electron Transport Chain NADH and FADH2 carry electrons to Electron Transport Chain. E.T.C. moves protons into space between inner and outer membrane Build up of protons causes high ion gradient. Oxygen picks up electrons and forms water

High to Low Free Energy

ATP synthase Complex III Complex I Complex IV Cytosol Outer mitochondrial membrane Intermembrane compartment ATP synthase Stator Basal unit Inner mitochondrial membrane Complex III Stalk Complex I Complex IV Head-piece Complex II Mitochondrial matrix Oxidative phosphorylation

Making ATP Protons packed between inner and outer membranes Protons move into inner matrix through protein channels (ATP synthase) The H+ gradient powers ATP synthesis by ATP synthase by chemiosmosis ATP synthase binds ADP with Pi

Total Energy - summary Where Steps ATP NADH FADH2 Cytosol 1. Glycolysis 2 (net gain) 2 - Mitochondria 2. Pyruvate Oxidation 3. Krebs Cycle 2 (1 in each cycle) 6 (3 in each cycle) Total 4 10 = 30 (10x3) 2 = 4 ( 2X2) Oxidative Phosphorylation Substrate level phosphorylation The total number of ATP produced from one Glucose molecule is 38. In some cases, NADH are produced in the cytosol during glycolysis, resulting in 2 ATP per NADH rather than 3. So the total ATP in those cases would be 34 instead of 38 Note: about 3 ATP per NADH and 2 ATP per FADH2 molecules are produced

Anaerobic Respiration (no oxygen) Without Oxygen the E.T.C. cannot function Citric Acid Cycle shuts down Pyruvate is still produced from glycolysis So what do we do with pyruvic acid?

Fermentation When oxygen is absent or limited, electrons carried by the 2 NADH produced by glycolysis may be used in fermentation fermentation Electrons carried by NADH are transferred to an organic acceptor molecule (converts NADH to NAD+) Glycolysis continues to supply ATP by substrate-level phosphorylation Lactate fermentation Converts pyruvate into lactate Occurs in some bacteria, plant tissues, skeletal muscle Used to make buttermilk, yogurt, dill pickles Alcoholic fermentation Converts pyruvate into ethyl alcohol and CO2 Occurs in some plant tissues, invertebrates, protists, bacteria, and single-celled fungi such as yeasts Used to make bread and alcoholic beverages

A. Lactate fermentation Glucose Glycolysis Pyruvate Lactate

B. Alcoholic fermentation Glucose Glycolysis Pyruvate Acetaldehyde Ethyl alcohol

The Versatility of Cellular Respiration

Lipid Metabolism Lipid molecules contain carbon, hydrogen, and oxygen In different proportions than carbohydrates Triglycerides are the most abundant lipid in the body Lipid Catabolism (also called lipolysis) Breaks lipids down into pieces that can be Converted to pyruvic acid Channeled directly into TCA cycle Hydrolysis splits triglyceride into component parts One molecule of glycerol Three fatty acid molecules

Lipid Metabolism Lipid Catabolism Enzymes in cytosol convert glycerol to pyruvic acid Pyruvic acid enters TCA cycle Different enzymes convert fatty acids to acetyl-CoA (beta-oxidation)

Lipid Metabolism Beta-Oxidation A series of reactions Breaks fatty acid molecules into 2-carbon fragments Occurs inside mitochondria Each step Generates molecules of acetyl-CoA and NADH Leaves a shorter carbon chain bound to coenzyme A

Lipid Metabolism

Lipid Metabolism Lipids and Energy Production For each 2-carbon fragment removed from fatty acid, cell gains: 12 ATP from acetyl-CoA in TCA cycle 5 ATP from NADH Cell can gain 144 ATP molecules from breakdown of one 18-carbon fatty acid molecule Fatty acid breakdown yields about 1.5 times the energy of glucose breakdown

Lipid Metabolism Free Fatty Acids (FFAs) Are lipids Can diffuse easily across plasma membranes In blood, are generally bound to albumin (most abundant plasma protein) Sources of FFAs in blood Fatty acids not used in synthesis of triglycerides diffuse out of intestinal epithelium into blood Fatty acids diffuse out of lipid stores (in liver and adipose tissue) when triglycerides are broken down

Protein Metabolism Amino Acid Catabolism Removal of amino group by transamination or deamination

Protein Metabolism Transamination Attaches amino group of amino acid To keto acid Converts keto acid into amino acid That leaves mitochondrion and enters cytosol Available for protein synthesis

Protein Metabolism Deamination Prepares amino acid for breakdown in TCA cycle Removes amino group and hydrogen atom Reaction generates ammonium ion

Protein Metabolism Ammonium Ions Are highly toxic, even in low concentrations Liver cells (primary sites of deamination) have enzymes that use ammonium ions to synthesize urea (water-soluble compound excreted in urine)

Protein Metabolism

Protein Metabolism Proteins and ATP Production When glucose and lipid reserves are inadequate, liver cells Break down internal proteins Absorb additional amino acids from blood Amino acids are deaminated Carbon chains broken down to provide ATP

Protein Metabolism Three Factors Against Protein Catabolism Proteins are more difficult to break apart than complex carbohydrates or lipids A byproduct, ammonium ion, is toxic to cells Proteins form the most important structural and functional components of cells