Fermentation and anaerobic respiration

What is fermentation?

Fermentation is an anaerobic pathway for breaking down glucose that occurs when oxygen is not available, consisting of glycolysis plus extra reactions at the end to regenerate NAD+ from NADH, allowing glycolysis to continue.

How do yeast and muscle cells differ in fermentation products?

In yeast, fermentation produces alcohol (ethanol), while in human muscle cells, fermentation produces lactic acid (lactate).

Why do cells use fermentation or anaerobic respiration?

Cells use these pathways when oxygen is not present to act as the final electron acceptor in the electron transport chain, so ATP can still be produced via glycolysis.

What is anaerobic cellular respiration?

Anaerobic cellular respiration is a process in some bacteria and archaea where electrons from fuels are passed through an electron transport chain, but a molecule other than oxygen (like sulfate or nitrate) serves as the final electron acceptor.

Which organisms perform anaerobic cellular respiration?

Some prokaryotes, including certain bacteria and archaea living in low-oxygen environments, perform anaerobic respiration. Examples include methanogens and sulfate-reducing bacteria.

What do methanogens use as a terminal electron acceptor and what is produced?

Methanogens use carbon dioxide as the terminal electron acceptor, producing methane as a by-product.

Where are methanogens commonly found?

Methanogens are found in soil and in the digestive systems of ruminants, such as cows and sheep.

What do sulfate-reducing bacteria use as a terminal electron acceptor and what is produced?

Sulfate-reducing bacteria use sulfate as a terminal electron acceptor and produce hydrogen sulfide as a by-product.

How does fermentation start?

Fermentation begins with glycolysis, converting one glucose molecule into two pyruvate molecules, producing 2 net ATP and 2 NADH.

Why can pyruvate not continue through the citric acid cycle in fermentation?

Because the electron transport chain is not functional without oxygen, pyruvate cannot enter the citric acid cycle, and NADH cannot offload electrons to regenerate NAD+.

What is the main purpose of the extra reactions in fermentation?

The extra reactions regenerate NAD+ from NADH by transferring electrons to an organic molecule like pyruvate, allowing glycolysis to continue and ATP to keep being produced.

Describe lactic acid fermentation.

In lactic acid fermentation, NADH transfers its electrons directly to pyruvate, producing lactate and regenerating NAD+.

Where does lactic acid fermentation occur?

It occurs in bacteria that make yogurt, in red blood cells (which lack mitochondria), and in human muscle cells under low-oxygen conditions.

What happens to lactate produced in muscles?

Lactate is transported via the bloodstream to the liver, where it is converted back to pyruvate for processing in cellular respiration.

Does lactate accumulation cause muscle soreness?

Recent research suggests lactate accumulation probably does not cause muscle soreness after exercise.

Describe alcohol fermentation.

In alcohol fermentation, NADH donates electrons to a derivative of pyruvate, producing ethanol and regenerating NAD+.

What are the steps of alcohol fermentation?

Step 1: Pyruvate loses a carboxyl group, releasing CO2 and forming acetaldehyde. Step 2: NADH donates electrons to acetaldehyde, regenerating NAD+ and forming ethanol.

What is produced per glucose molecule during alcohol fermentation?

Two net ATP from glycolysis, two NADH regenerated, two CO2 molecules, and two ethanol molecules.

What limits ethanol production in yeast?

High ethanol concentrations are toxic to yeast, limiting ethanol production to about 12–15% depending on the yeast strain and environmental conditions.

What are facultative anaerobes?

Facultative anaerobes are organisms that can switch between aerobic respiration and anaerobic pathways depending on oxygen availability, allowing them to maximize ATP production when oxygen is present.

What are obligate anaerobes?

Obligate anaerobes can only live and grow in the absence of oxygen; oxygen is toxic to them and can injure or kill them.

Why is oxygen toxic to obligate anaerobes?

Obligate anaerobes lack detoxifying enzymes like superoxide dismutase, catalase, and peroxidase, so reactive oxygen species damage cellular components.

Give an example of an obligate anaerobe.

Clostridium bacteria, which can cause botulism, are obligate anaerobes.

How do some multicellular animals survive in oxygen-free environments?

Recent research has found some deep-sea multicellular animals living in sediments completely lacking oxygen, relying on anaerobic metabolism.

What gas is produced by yeast during alcohol fermentation in winemaking?

Yeast produce carbon dioxide gas, which is why wine fermentation tanks need pressure-release valves.

Why do fermentation pathways regenerate NAD+?

Regenerating NAD+ allows glycolysis to continue, ensuring a steady supply of ATP when oxygen is not available.

What are the inputs and outputs of lactic acid fermentation?

Input: NADH and pyruvate. Output: NAD+ and lactate (lactic acid).

What are the inputs and outputs of alcohol fermentation?

Input: NADH and pyruvate. Output: NAD+ and ethanol plus CO2.

Why can bacteria perform aerobic respiration without mitochondria?

Bacteria have plasma membrane-bound electron transport chains, allowing them to perform aerobic respiration even in the absence of mitochondria.