Overview of carbohydrate breakdown pathways All organisms (including photoautotrophs) convert – chemical energy organic compounds to chemical energy of ATP ATP – common energy currency that drives metabolic reactions in cells

Pathways of carbohydrate breakdown glycolysis start in cytoplasm – Convert glucose and other sugars to pyruvate Fermentation pathways – End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose Aerobic respiration – Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule you are here

Overview of aerobic respiration Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C 6 H 12 O 6 + 6O 2 → 6CO H 2 O

Glycolysis – glucose breakdown starts Enzymes of glycolysis use two ATP convert one molecule of glucose two molecules of three- carbon pyruvate transfer electrons and hydrogen atoms to NADH 4 ATP formed

Products of glycolysis Net yield of glycolysis: – 2 pyruvate – 2 ATP – 2 NADH Pyruvate may: – Enter fermentation pathways in cytoplasm – Enter mitochondria and be broken down further in aerobic respiration

Second stage of aerobic respiration within inner compartment of mitochondria pyruvates – converted to acetyl-CoA CO 2 leave cell Acetyl-CoA – enters Krebs cycle – carbon converts molecules of CO 2 (waste)

Energy products Reactions transfer electrons and hydrogen atoms to NAD + and FAD – Reduced to NADH and FADH 2 ATP forms by substrate- level phosphorylation – Direct transfer of a phosphate group from a reaction intermediate to ADP

Net results from second stage! Second stage of aerobic respiration results in – Six CO 2, two ATP, eight NADH, and two FADH 2 for every two pyruvates Adding the yield from glycolysis, the total is – Twelve reduced coenzymes and four ATP for each glucose molecule Coenzymes deliver electrons and hydrogen to the third stage of reactions

Third stage: aerobic respiration’s big energy payoff Coenzymes deliver electrons & hydrogen ions – H + ions accumulate in the outer compartment, forming a gradient across the inner membrane – H + ions flow by concentration gradient back to the inner compartment through ATP synthases release energy – electron transfer chains transport proteins that drive ATP synthesis electron transfer chains

The aerobic part of aerobic respiration Oxygen combines with electrons and H + at the end of the transfer chains, forming water Overall, aerobic respiration yields up to 36 ATP for each glucose molecule

Anaerobic energy-releasing pathways Different fermentation pathways begin with glycolysis and end in the cytoplasm – Do not use oxygen or electron transfer chains – do not produce Lactate fermentation – End product: Lactate Alcoholic fermentation – End product: Ethyl alcohol (or ethanol) Both pathways have a net yield of 2 ATP per glucose (from glycolysis)

The twitchers Slow-twitch and fast- twitch skeletal muscle fibers can support different activity levels Aerobic respiration and lactate fermentation proceed in different fibers of muscles