What Happens When There Is No Oxygen Available at the End of Slow Glycolysis?

What Happens When There Is No Oxygen Available at the End of Slow Glycolysis
••• Bluberries/iStock/GettyImages

Glycolysis is the first step in cell respiration, and it requires no oxygen to proceed. Glycolysis converts a molecule of sugar into two molecules of pyruvate, also producing two molecules each of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH). When oxygen is absent, a cell can metabolize the pyruvates through the process of fermentation.

Energy Metabolism

ATP is the cell’s energy storage molecule, while NADH and its oxidized version, NAD+, take part in the cell reactions that involve the transfer of electrons, known as the redox reactions. If oxygen is present, the cell can extract substantial chemical energy by breaking down pyruvate through the citric acid cycle, which converts NADH back to NAD+. Without oxidation, the cell must use fermentation to oxidize NADH before it builds up to unhealthy levels.

Homolactic Fermentation

Pyruvate is a three-carbon molecule that the enzyme lactate dehydrogenase converts into lactate through the process known as homolactic fermentation. In the process, NADH is oxidized into NAD+ that is needed for glycolysis to proceed. In the absence of oxygen, homolactic fermentation prevents NADH from accumulating, which would halt glycolysis and rob the cell of its energy source. Fermentation doesn't yield any ATP molecules, but it does allow glycolysis to continue and producing a small trickle of ATPs. In homolactic fermentation, lactate is the sole product.

Heterolactic Fermentation

In the absence of oxygen, certain organisms such as yeast can convert pyruvate into carbon dioxide and ethanol. Brewers capitalize on this process to convert grain mash into beer. Heterolactic fermentation proceeds in two steps. First, the enzyme pyruvate dehydrogenase converts pyruvate to acetaldehyde. In the second step, the enzyme alcohol dehydrogenase transfers hydrogen from NADH to the acetaldehyde, converting it into ethanol and carbon dioxide. The process also regenerates NAD+, which enables glycolysis to continue.

Feeling the Burn

If you've ever felt your muscles burn during heavy physical activity, you are experiencing the effect of homolactic fermentation in your muscle cells. Strenuous exercise temporarily depletes a cell’s oxygen supply. Under these conditions, muscles metabolize pyruvate into lactic acid, which produces the familiar burning sensation. However, this is a stopgap reaction to low oxygen levels. Without oxygen, cells can quickly die.

Cabbage and Yogurt

Anaerobic fermentation is used to create several foodstuffs besides beer. For example, cabbage benefits from fermentation to yield delicacies such as kimchee and sauerkraut. Certain strains of bacteria, including Lactobacillus bulgaricus and Streptococcus thermophiles, convert milk to yogurt through homolactic fermentation. The process congeals the milk, gives the yogurt flavor and increases the milk’s acidity, which makes it unpalatable to many harmful bacteria.

Related Articles

What Is Needed for Glycolysis to Take Place?
Relationship Between Calories & Cellular Respiration
The Disadvantages of Lactic Acid Fermentation
5 Uses of Fermentation
What Are the Reactants in Fermentation?
How Does Glycolysis Occur?
Products Produced by Anaerobic Respiration
What Is Needed for Glycolysis to Take Place?
Equation for Glucose Metabolism
Where Is Starch Stored in Plant Cells?
What Are the Functions of Coenzymes?
What Can Stop Glycolysis?
How are Chemical Bonds Important in Metabolism
Why Do Plants Need Photosynthesis & Cellular Respiration?
Which Molecules Enter & Leave the Krebs Cycle?
How Do Humans Get Oxygen in Their Bodies?
How Does Temperature Affect Metabolism?
Which Two Ingredients Are Needed for Fermentation to...
What Are the Four Phases of Complete Glucose Breakdown?