 Fermentation: a catabolic process that makes a limited amount of ATP from glucose without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid

 Fermentation provides a mechanism by which some cells can oxidize organic fuel and generate ATP without using oxygen.  Oxidation refers to the loss of electrons to any electron acceptor, not just oxygen.

 Glycolysis… › Is an exergonic process › Has an oxidizing agent that is NAD+ › Produces 2 molecules of pyruvate › 2 ATP molecules result from substrate-level phosphorylation  Can be aerobic or anaerobic › Aerobic: containing oxygen › Anaerobic: lacking oxygen (an- means without)  Electrons from NADH are passed to pyruvate, regenerating the NAD+ required to keep cycle running.  Cycle shuts down if lacking an oxidizing agent

 Alcohol Fermentation: the conversion of pyruvate to acetaldehyde, releasing carbon dioxide, and then reduced to ethyl alcohol *Ex: yeast used for brewing beer, baking bread  Lactic Acid Fermentation: the conversion of pyruvate to lactate with no release of carbon dioxide *Ex: fungi cultured commerically for yogurt and cheese -lactic acid in muscles after strenuous exercise

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 Similarities › Both use glycolysis to oxidize glucose › NAD+ is the oxidizing agent › Both are catabolic reactions to harvest energy  Differences › Contrasting mechanisms for oxidizing NADH to NAD+ › In fermentation, the final electron acceptor is an organic molecule (like pyruvate or acetaldehyde). › In cellular respiration, the final electron acceptor is oxygen.  Respiration yields as much as 19 times more ATP per glucose than fermentation.

 Facultative anaerobes: an organism that makes ATP by aerobic respiration if oxygen is present but that switches to fermentation under anaerobic conditions › Example: our muscle cells › Pyruvate is a fork in the road… ›  pyruvate converts to acetyl CoA ›  pyruvate is diverted from the citric acid cycle and serves as an electron acceptor to recycle NAD+

Glycolysis occurs in nearly all organisms and most likely evolved in ancient prokaryotes before there was oxygen in the atmosphere.

 Free glucose molecules are not common in the diets of humans or animals.  Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration.  Glycolysis can accept a wide range of carbohydrates for catabolism.  Examples: › Starch is hydrolyzed to glucose, which can be broken down by glycolysis and the citric acid cycle. › Glycogen can also be hydrolyzed to glucose between meals as fuel for respiration.

 Beta oxidation: a metabolic sequence that breaks fatty acids down to two- carbon fragments that enter the citric acid cycle as acetyl CoA  Fats make excellent fuel. › A gram of fat oxidized by respiration produces more than twice as much ATP as a gram of carbohydrate.

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 Cells need substance as well as energy.  In addition to calories, food must also provide the carbon skeletons that cells require to make their own molecules  The body can use smaller molecules from food directly or use them to build other substances through glycolysis or the citric acid cycle.  Glycolysis and the citric acid cycle function as metabolic interchanges that enable cells to convert some kinds of molecules to others as we need them.

 The cell doesn’t waste energy making more of a particular substance than it needs.  The most common mechanism for this control is feedback inhibition: the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway  Cellular respiration is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle.

 The cells control catabolism.  When there is plenty of ATP to meet demand, respiration slows down.  An important switch is phosphofructokinase, which is the enzyme that catalyzes step 3 of glycolysis.  Phosphofructokinase: an allosteric enzyme with receptors for specific inhibitors and activators Feedback Regulation of Respiration