6.13 Fermentation

• Fermentation is an anaerobic (without oxygen) energy-generating process – It takes advantage of glycolysis, producing two ATP

molecules and reducing NAD+ to NADH– The trick is to oxidize the NADH without passing its

electrons through the electron transport chain to oxygen

6.13 Fermentation

• Most cellular respiration requires O2 to produce ATP

• Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)

• In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

6.13 Lactic Acid Fermentation

• Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation– NADH is oxidized to NAD+ when pyruvate is reduced

to lactate– Pyruvate is serving as a place to dispose of the

electrons generated in glycolysis

– Products: 2 NAD+, lactic acid, 2ATP(from glycolysis)– Can be added to milk to make cheese…..how?

Glucose

NADH

NAD+

2

2

NADH2

NAD+2

2 ADPP

ATP2

2 Pyruvate

2 Lactate

GLY

CO

LYSI

S

Lactic acid fermentation

2

6.13 Fermentation enables cells to produce ATP without oxygen

• The baking and wine-making industry have used alcohol fermentation for thousands of years– Yeasts are single-celled fungi that not only can use

respiration for energy but can ferment under anaerobic conditions

– They convert pyruvate to CO2 and ethanol while oxidizing NADH back to NAD+

– Products: ethyl alcohol, 2CO2 , 2 NAD+, 2ATP– Can be added to grapes to make wine….how?

2 ADPP

ATP2 GLY

CO

LYSI

S

NADH

NAD+

2

2

NADH2

NAD+2

2 Pyruvate

2 Ethanol

Alcohol fermentation

Glucose

CO22released

2

• Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2

• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration

So there are 2 choices for facultative anaerobes…

Fig. 9-19Glucose

Glycolysis

Pyruvate

CYTOSOL

No O2 present:Fermentation

O2 present: Aerobic cellular respiration

MITOCHONDRIONAcetyl CoAEthanol

orlactate

Citricacidcycle

Regulation of Cellular Respiration via Feedback Mechanisms

• Feedback inhibition is the most common mechanism for control

• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down

• Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

You should now be able to:

1. Explain in general terms how redox reactions are involved in energy exchanges

2. Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result

3. In general terms, explain the role of the electron transport chain in cellular respiration

4. Explain where and how the respiratory electron transport chain creates a proton gradient

5. Distinguish between fermentation and anaerobic respiration

6. Distinguish between obligate and facultative anaerobes