Fermentation

Overview.

Aerobic respiration.

Fermentation process.

Lactate fermentation.

Muscles.

Alcohol fermentation.

Overview.

Fermentation is a simpler and faster process, than cellular respiration,

for cells to create energy.

It is an anaerobic process, which converts chemical energy like glucose, into ATP.

An electron accepter, in the cell facilitates this process.

Fermentation takes place, in the cytoplasm of the cell.

It requires no special organelles, like mitochondria.

It is a catabolic process.

Harvesting energy from chemically energy rich molecules,

can be conceived as converting chemical energy to electrical energy.

Sugar and carbohydrates are a way to store electrical energy,

in a chemical form.

Electrical energy is the result, of the flow of electrons.

In catabolic processes electrons flow from high energy molecules to low energy molecules.

Electron carriers are molecules which play a role in this.

One such common electron carrier molecule is NADH.

NADH is a energy rich molecule.

When stripped electrons, it becomes NAD+.

NAD+ is an energy depleted molecule.

This can be compared to a charge and discharged battery.

Adding an electron is a process of reduction.

We can say that adding electrons to NAD+ reduces it to NADH.

Releasing electron is a process of oxidation.

When NADH is oxidised it becomes NAD+.

This adding and releasing electrons to NAD facilitates the electron flow between molecules.

In chemical reactions which occur in cycles, the alternate adding and releasing electrons,

from NAD completes an electrical circuit.

Cells have a limited supply of NAD.

They need to be constantly recycled.

Aerobic respiration.

Unlike fermentation, aerobic respiration requires the presence of oxygen.

Most animals including human beings, make extensive use of aerobic respiration, in their cells.

Aerobic respiration is a very efficient process.

Most of the chemical energy, in glucose or any carbohydrate,

is harvested by aerobic respiration.

The six carbon molecule like glucose, is fully broken down, in steps,

to harvest energy, eventually leading to the harvest of ATP molecules.

ATP is the currency of energy in metabolism.

Most of the energy in glucose, is harvested, in the end by product, is carbon dioxide.

In contrast the by-products of fermentation are high energy multi carbon molecules.

This is the reason that aerobic respiration is much more efficient than fermentation.

A single molecule of glucose will yield 36 ATP's in aerobic respiration.

The same molecule will yield only 2 ATP's in fermentation,

but it will leave behind energy rich molecules, as by-products.

In aerobic respiration, breaking down of carbon bonds in glucose or other molecules,

releases electrons.

These electrons flow in a chain, through other molecules.

The end recipient of the electrons, is oxygen.

Oxygen is converted to water.

This happens at the end of the electron transport chain, in aerobic respiration.

Oxygen being present for receiving electrons, is crucial for aerobic respiration.

In the Krebs cycle, NADH acts as the electron carrier.

During the electron transport chain process, the electrons from NADH, generate ATP.

Carbon dioxide and water are by-products of aerobic respiration.

Bacteria and microbes do not use oxygen for respiration.

They use anaerobic fermentation, to break down glucose and nutrition.

Fermentation process.

The process of fermentation, typically starts with the process of glycolysis.

Glucose is a 6 carbon molecule, and acts as a good example of a sugar or carbohydrate.

Carbohydrate and polysaccharides are eventually broken down into simple sugars like glucose.

Glucose is a good example, to discuss these basic processes.

Glycolysis breaks down 6 carbon sugar molecules, into two 3 carbon molecules, called pyruvate.

The breaking of the carbon bonds releases electrons.

NAD+ in the cell becomes energy rich NADH.

Some energy is harvested as ATP.

The chemical equation for this is :

Glucose + 2 ADP + 2 P_i results in 2 pyruvate + 2 NADH + 2 ATP.

So, glucose is converted to 2 pyruvate, plus 2 NADH molecules, plus 2 net ATP molecules.

NADH then releases electrons to the pyruvate molecule.

This converts NADH back to NAD+.

This regeneration of NAD+ helps to continue the process, of breaking down glucose in a cycle.

In each cycle one more glucose is broken down, and NAD+ is regenerated.

The by-product pyruvate molecule is a high energy molecule.

Fermentation harvests a limited amount of ATP.

Typically a net of two molecules of ATP, is harvested from one molecule of glucose, in glycolysis.

We note that no oxygen is present in this process.

Fermentation is anaerobic process.

Further processes in fermentation, can proceed in different metabolic processes.

The type of the cell, and the enzymes present, determine the metabolic pathway.

Two important types of fermentation are :

Lactate fermentation.

Alcohol fermentation.

Lactate fermentation.

Lactate or lactic acid fermentation, is the process that takes place,

when milk is converted to yogurt, cheese etc..

Milk has the sugar called lactose.

Lactose is a bi-saccharide, comprising of glucose and galactose.

By adding a starter dose of bacteria, we can induce the process,

of fermentation to convert milk into yogurt.

Usually milk is heated, to denature the proteins.

It is then cooled to a warm temperature, and some yogurt is added,

from an earlier batch.

This is left for several hours, to ferment into bacteria.

The bacteria lactobacillus, helps to ferment milk into yogurt.

Glycolysis is the first step in lactic acid fermentation.

Lactose in the milk is converted to 2 pyruvate,

plus 2 NADH molecules, plus 2 net ATP molecules.

The NADH molecules contribute electrons to pyruvate, to generate lactate.

Lactate is a high energy molecule.

It has the formula CH₃CH(OH)COOH, which is the conjugate of lactic acid.

The lactic acid acts on the proteins in the milk, to produce yogurt.

The sour taste in the yogurt comes from the lactic acid.

In the process NADH regenerates NAD+.

This as we discussed is crucial to continue glycolysis in a continuous cycle.

Muscles.

Muscles need a lot of energy to do work.

Most of this energy is derived from aerobic respiration,

which takes place, in the Krebs cycle.

Aerobic respiration is a very efficient way to produce energy.

About 36 molecules of ATP, can be generated from one molecule of glucose.

This is the preferred method for muscle cells to produce energy,

when abundant oxygen is present.

This is a reason why we breathe deeper and faster,

why doing strenuous work like running.

The muscle cells have an alternate metabolic pathway,

to produce energy, when oxygen is deficient.

This is a fermentation pathway.

The first step is the usual glycolysis.

Glucose is broken down to two pyruvate molecules.

When oxygen is deficient, pyruvate is converted to lactic acid.

NADH is converted to NAD⁺.

This keeps the fermentation process going in a cycle.

The fermentation process is not energy efficient.

Only 2 net ATP molecules are produced from one glucose molecule.

But fermentation is a faster process, compared to the aerobic respiration process.

The body uses this metabolic pathway to supplement or augment energy production,

specially when oxygen supply is insufficient.

Under these conditions more and more lactic acid is produced,

in the muscle cells.

Lactic acid is not friendly to muscle cells.

When we feel tiredness or pain in the muscle, it is due to lactic acid formation.

This is one of the reason why athletes warm up, before performing.

Lactic acid fermentation is a reversible process.

Lactic acid can be reconverted to pyruvate, in muscle cells.

The electrons go back to NAD⁺ to generate NADH.

When muscles are resting, and more oxygen is available, this process can take place.

Alcohol fermentation.

Alcohol fermentation is another type of fermentation.

It is used for making wine, beer, and baking products like bread, cake, etc..

The type of fermentation depends on the cell, the yeast, and the enzymes.

Grapes for example, when fermented becomes wine.

Barley can be used for making beer.

Wheat flour can be used for baking bread.

The first step is again glycolysis, glucose is broken down into 2 pyruvate molecules.

The pyruvate molecules are converted to ethanol and carbon dioxide .

The chemical formula for this is :

The 2 pyruvate molecules becomes 2 CH₃CH₂OH + 2CO₂ + 2 ATP.

Ethanol is a high energy molecule.

It can be used for fuelling cars.

The important difference in this type of fermentation is the release of carbon dioxide.

This released carbon dioxide, is what makes bread fluffy, and beer bubbly.

Alcohol fermentation is not reversible, like lactate fermentation,

because CO₂ is lost.

Fermentation processes was used by civilisation thousand of years ago.

Some of the earliest processes dates back to 8000 BCE.