Biology Now, 2e

Rock Eaters ■ 91

Strains of yeasts, a

type of single-celled

fungi, are used in the

brewing of alcoholic

beverages such as

beer. When oxygen

is excluded from the

fermentation tanks,

the yeasts resort to

fermentation of

sugars, producing

ethanol and CO 2 as

by-products of the

postglycolysis steps.

Glycolysis

Fermentation

Anaerobic organisms such as

yeasts extract energy from

sugars, producing ethanol, in

the absence of oxygen.

Glucose / sugar molecule

Cytoplasm

Alcoholic

Fermentation

Lactic Acid

Fermentation

Pyruvate molecules

Pyruvate

Lactic acid

H

H OH

H 3

ADP+P

NADH ATP

NADH

NAD+

NAD+

Pyruvate

CO 2

Ethanol

NADH

NAD+

When ATP demand is high, muscle cells

make more by increasing the rate of

glycolysis. The supplemental ATP can be

made only if enough NAD+ is regenerated

by turning pyruvate into lactic acid.

Glycolysis proceeds as

long as the cell’s supply

of NAD+ is replenished.

The fermentation reactions

regenerate NAD+ to sustain

high rates of glycolysis.

H

O–

OH

O

H 3

Lactic acid

fermentation occurs in

our muscles during

short bursts of

strenuous exercise. In

contrast with alcoholic

fermentation, no CO 2 is

made.

Figure 5.12

Fermentation produces energy in the absence of oxygen

When the oxygen supply is inadequate to support ATP production through cellular respiration, fermentation enables ATP

production through glycolysis alone.

Q1: Which product released by fermentation accounts for the bubbles in beer?

Q2: Bakers of yeast breads also rely on fermentation, allowing bread to “rise” before baking. Describe what is occurring

with the yeast as the bread rises.

Q3: Explain in your own words why lactic acid builds up in your muscles during strenuous physical activity.