Chapter 9.5 and 9.6

9.5-Fermentation enables some cells to produce ATP without the use of oxygen

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.

http://www.youtube.com/watch?v=AFLu6lhF1YI

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

Types of Fermentation 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

http://leavingbio.net/RESPIRATION-(higher%20level).htm

Located on page 175

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Fermentation and Cellular Respiration Compared

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.

Fermentation vs. Respiration

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+

Evolutionary Significance

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

9.6-Glycolysis and the citric acid cycle connect to many other metabolic pathways

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.

Alternate Metabolic Pathways

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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Biosynthesis (Anabolic Pathways)

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.

Regulation of Cellular Respiration via Feedback Mechanisms

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