Biology 12

80 MHR • Unit 1 Metabolic Processes

Alcohol Fermentation

In alcohol fermentation, two reactions of

fermentation convert pyruvate to ethanol. The first

reaction reduces pyruvate to CO 2 and acetaldehyde.

In the second reaction, NADH reduces the

acetaldehyde to ethanol (see Figure 3.15). Two NAD+

are formed and participate in glycolysis II. This

process ensures that glycolysis can continue. Yeast

cells, shown in Figure 3.16, are eukaryotes that

carry out alcohol fermentation. This process is

used in the manufacture of wine, for example. The

Figure 3.15Possible pathways of pyruvate metabolism

Figure 3.16These yeast cells can survive in an oxygen-free

environment. How do yeast cells produce ATP molecules?

formation of ethanol from pyruvate occurs naturally

if grapes are allowed to ferment on the vine or

ferment in winemaking vats. When the ethanol

concentration reaches a certain level, 12–16 percent

depending on the variety of yeast being used, the

toxic effects of the ethanol kill the yeast cells.

Lactic Acid Fermentation

Lactic acid fermentation occurs in certain fungi,

bacteria, and muscle cells. For example, when

human muscle cells undergo strenuous activity

they may experience a depletion of oxygen. When

there is insufficient oxygen for muscle cells to

undergo aerobic cellular respiration, the cells will

continue to produce ATP through fermentation.

During this process, no CO 2 is produced. Instead,

lactic acid is produced. During strenuous activity,

lactic acid levels increase in muscle cells. In the

short term, lactic acid buildup can cause muscle

fatigue and pain resulting from muscle cramps. The

excess lactic acid is eventually removed from the

cells by the circulatory system and muscle pain

begins to ease. The lactic acid is cleansed from the

bloodstream by the liver. There is a physiological

advantage to producing lactic acid when energy

output exceeds oxygen intake. Through lactic acid

fermentation, muscles can continue to function in

the absence of oxygen.

Table 3.2 compares the number of ATP

molecules produced through anerobic and aerobic

cellular respiration.

Anerobic

Cellular respiration

Lactic acid Ethanol Aerobic

Table 3.2

Lactic acid and ethanol fermentation produce the same

number of ATP molecules. These processes are far less

efficient in ATP synthesis than is aerobic cellular respiration.

glucose

pyruvate

glycolysis

2 lactic acid

2 ATP

+

fermentation

glucose

pyruvate

glycolysis

CO 2

+ CO 2

2 ethanol

2 ATP

+

fermentation

glucose

pyruvate

glycolysis

+

water

36 ATP

+

Krebs cycle

and electron

transport chain

4 ADP

2 ADP

4

2

2

2

ATP

2 NADH

2 NADH

PGAL

glucose

2

PGAP

pyruvate

2 lactic acid ( )

or

P P

P

ATP

2

C 6

2 NAD+

2 NAD+

C 3

C 3

C 3

C 3 2 ethanol ( )C 2

acetaldehyde

CO 2

Pi