Lesson Explainer: Anaerobic Respiration

In this explainer, we will learn how to recall the reactants and products of anaerobic respiration and compare this process to aerobic respiration.

Have your muscles ever cramped while running very fast? This is often thought to result from the buildup of lactic acid in the cells of your muscles, which is produced during the process of lactic acid fermentation, sometimes called anaerobic respiration. Lactic acid is also produced by bacteria to make yoghurt.

Cellular respiration is a process by which cells in living organisms release energy for their life processes, such as growth and cell division. Respiration involves breaking down carbon-containing organic compounds, such as glucose, into smaller molecules, such as carbon dioxide and water. This process releases energy in the form of a molecule called adenosine triphosphate, or ATP.

Definition: Cellular Respiration

Cellular respiration is a process in living organisms by which carbon-containing compounds are broken down to release energy in the form of ATP.

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration primarily occurs in mitochondria, and it requires oxygen and glucose and produces carbon dioxide and water. Anaerobic respiration, which takes place in the cytoplasm of a cell, can occur in the absence of oxygen, as it only requires glucose. This means that aerobic and anaerobic respiration are used at different times, depending on the availability of oxygen.

Definition: Aerobic Respiration

Aerobic respiration is the process by which energy is released in cells in the presence of oxygen.

Definition: Anaerobic Respiration

Anaerobic respiration is the process by which energy is released in cells in the absence of oxygen.

Example 1: Identifying the Site of Anaerobic Respiration in Animal Cells

The diagram shows a basic outline of an animal cell.

Which letter indicates where anaerobic respiration occurs in the cell?

Answer

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration occurs primarily in the mitochondria, which are kidney-shaped organelles with a highly folded inner membrane. Aerobic respiration requires oxygen and glucose and produces carbon dioxide, water, and ATP. Anaerobic respiration, which takes place in the cytoplasm of a cell, can occur even in the absence of oxygen, as it just requires glucose.

There are two other structures labelled on this diagram. One is the nucleus, which is a large round organelle that contains the genetic material of the cell, and the other is the cell membrane, which surrounds the cell and mainly functions to control which materials can move into and out of it.

Let’s label where these different structures are on the diagram below.

Therefore, the letter that indicates the site of anaerobic respiration in the cell is D.

When Usain Bolt sprinted the 100 m in the 2016 Olympic Games, his muscles demanded more energy than aerobic respiration can provide. Generally, when the amount of oxygen inhaled is insufficient to meet the demands of aerobic respiration, the muscle cells start carrying out anaerobic respiration. Sprinting requires energy to be released rapidly, compared to running for longer distances. When Mo Farah ran the 10‎ ‎000 m at the same Olympic Games, he had a longer time to inhale oxygen than Usain did in his sprint. As Mo had sufficient oxygen, his cells mostly used aerobic respiration to release the energy they needed.

Anaerobic respiration acts as the body’s backup energy source when the oxygen available is too low to meet our energy demands through aerobic respiration alone.

So, why does the human body carry out aerobic respiration at all? Anaerobic respiration can be beneficial to release energy when the amount of oxygen is low, but it has some disadvantages. In animal cells and in some bacterial cells, anaerobic respiration is called lactic acid fermentation, a process that produces two molecules of lactic acid by breaking down one molecule of glucose. Anaerobic respiration also requires some specific enzymes for the reaction to proceed. The word and balanced symbol equations for lactic acid fermentation are shown in the box below.

Reaction: Lactic Acid Fermentation

Anaerobic respiration begins in the same way as aerobic respiration. You may remember that this involves breaking down one molecule of glucose into two molecules of pyruvate. This is also the stage that releases two molecules of ATP and two molecules of NADH. In animal cells, in a low-oxygen environment, particularly in muscle cells, pyruvate is reduced via a reaction with NADH, which adds two hydrogen atoms to the pyruvate molecule, converting it into lactic acid, otherwise called lactate.

Example 2: Identifying the Reactants of Aerobic Respiration

What reactant is required for aerobic respiration to take place but is not required for anaerobic respiration?

Answer

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration requires oxygen and glucose and produces carbon dioxide, water, and ATP. Anaerobic respiration, which takes place in the cytoplasm of a cell, can occur even in the absence of oxygen, as it just requires glucose. The product of anaerobic respiration in animal cells is lactic acid, so this form of respiration is sometimes called lactic acid fermentation. Anaerobic respiration releases less energy than that released by aerobic respiration. You can see the equation for lactic acid fermentation below:

Therefore, the reactant required for aerobic respiration to take place but not for anaerobic respiration is oxygen.

Some scientists have suggested that the buildup of lactic acid in the cells is what causes muscle cramps to develop while exercising. Therefore, the lactic acid produced during anaerobic respiration must be removed from the cells. This is achieved by reacting the lactic acid with oxygen to produce carbon dioxide and water. You can see the word and balanced symbol equations for the breakdown of lactic acid in the box below.

Reaction: Lactic Acid Breakdown

As removing the lactic acid from the cells requires oxygen, lactic acid fermentation is said to create an oxygen debt. This means that although anaerobic respiration does not require oxygen itself, getting rid of this lactic acid by-product does. This is why people breathe so heavily after exercising, as they are trying to obtain as much oxygen as possible to pay off this oxygen debt.

Key Term: Oxygen Debt

Oxygen debt describes the heavy breathing that occurs after an intense exercise to break down the lactic acid produced during anaerobic respiration.

Anaerobic respiration also releases less energy than that released by aerobic respiration. While aerobic respiration can release up to 38 molecules of ATP per one molecule of glucose, anaerobic respiration only releases 2 molecules of ATP. This means that aerobic respiration is preferable to carry out whenever possible to release cellular energy as it is far more efficient.

Let’s compare the processes of aerobic and anaerobic respiration in animals using a Venn diagram in Figure 1. The left circle refers to the main characteristics of aerobic respiration, whereas the right circle refers to the characteristics of anaerobic respiration. The common aspects to both reactions are shown where the circles overlap.

**Figure 1**: Venn diagram showing the differences and similarities between aerobic and anaerobic respiration in animal cells.

Example 3: Determining the Method of Respiration in Different Scenarios

For the following situations, decide whether the muscle cells in the human body are likely to be relying mainly on aerobic or anaerobic respiration to supply their energy requirements.

A person is running a marathon distance of 26 miles over 4 hours.
A child sprints 50 metres down a hallway in 8 seconds.
A rabbit in a garden spots a fox and immediately jumps away.

Answer

Cellular respiration is a process by which cells in living organisms release energy for their life processes, such as growth and cell division. Respiration involves breaking down glucose into smaller molecules. This process releases energy in the form of a molecule called adenosine triphosphate, or ATP.

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration requires oxygen and glucose and produces carbon dioxide and water, whereas anaerobic respiration occurs in the absence of oxygen, as it only requires glucose.

Part 1

If the energy demands of an organism’s cells are spread over a long period of time, such as while running a marathon, aerobic respiration is more likely to occur rather than anaerobic respiration. This is because enough oxygen will be available for the more efficient aerobic respiration to fulfil the energy requirements.

Therefore, if a person is running a marathon, their body cells are more likely to be respiring aerobically.

Part 2

Anaerobic respiration releases less energy than that released by aerobic respiration. In animal cells, this process produces a by-product called lactic acid, which needs to be removed from the cells. Therefore, anaerobic respiration is only used when the amount of oxygen available is too low to meet high energy demands, such as while sprinting a race or running away from a predator.

Therefore, if a child is sprinting quickly down a hallway, the cells in their body are more likely to be respiring anaerobically.

Part 3

Foxes commonly prey on rabbits, as foxes are one of the apex, or top, predators in many food webs. It is in the best interest of the rabbit to, therefore, escape from the fox as quickly as possible in order to avoid being killed and eaten. As the rabbit will need to move fast, indicated in the question by the use of the word rapidly, its cells are most likely to require more energy than what the oxygen-fueled aerobic respiration can provide.

Therefore, the cells in the body of the rabbit escaping the fox are most likely to be respiring anaerobically.

Anaerobic respiration varies between organisms in the different kingdoms of life. While animal cells and some bacterial cells produce lactic acid, plant and fungal cells, such as yeast, also carry out anaerobic respiration, but instead they produce carbon dioxide and ethanol. This is called alcoholic fermentation, as ethanol is an alcohol. Interestingly, in flowering plants, which are also called angiosperms, even the germinating seeds have been observed to carry out anaerobic respiration. The word and balanced symbol equations for alcoholic fermentation are shown in the box below.

Reaction: Alcoholic Fermentation

Example 4: Identifying the Word Equation for Anaerobic Respiration in Animal Cells

Which of the following is the correct word equation for anaerobic respiration in animal cells?

Lactic acid glucose
Glucose + oxygen carbon dioxide + water
Glucose + oxygen lactic acid
Glucose lactic acid
Glucose carbon dioxide

Answer

Cellular respiration is a process by which cells in living organisms release energy for their life processes, such as growth and cell division. Respiration involves breaking down glucose into smaller molecules. This process releases energy in the form of a molecule called adenosine triphosphate, or ATP.

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration requires oxygen and glucose and produces carbon dioxide and water. Anaerobic respiration, which takes place in the cytoplasm of a cell, can occur in the absence of oxygen, as it only requires glucose. In animal cells and some bacterial cells, anaerobic respiration produces lactic acid. In yeast and plant cells, anaerobic respiration produces ethanol and carbon dioxide.

As glucose must be the reactant for any form of cellular respiration, option A is incorrect. Options B and C both include oxygen as a reactant for respiration. Oxygen is required only for aerobic respiration and not for anaerobic respiration, so options B and C are incorrect. Option B is actually the equation for aerobic respiration.

While yeast and bacterial cells produce ethanol and carbon dioxide when they carry out anaerobic respiration, animal cells produce lactic acid.

Therefore, the correct word equation for anaerobic respiration in animal cells is glucose lactic acid.

The products of anaerobic respiration in different microorganisms can be very helpful to humans.

Some bacteria produce lactic acid when they respire anaerobically. These bacteria can be cultured to carry out lactic acid fermentation on a massive scale. The large volumes of lactic acid they produce can be used to produce yoghurt.

Yeast is a fungus whose cells produce carbon dioxide and ethanol through anaerobic respiration. Yeast is often used in making bread, as the carbon dioxide that it releases is what causes the bread to rise. The other product of anaerobic respiration in yeast cells is ethanol, a type of alcohol. This process is called alcoholic fermentation. Yeast is also cultured on a large scale to carry out alcoholic fermentation for brewing beverages like beer and wine. When yeast cells respire anaerobically, they produce an alcohol, which is added to these drinks during their production.

Example 5: Identifying the Word Equation for Anaerobic Respiration in Yeast Cells

Without including energy/ATP, state the word equation for anaerobic respiration in yeast cells.

Glucose ethanol + carbon dioxide
Ethanol glucose + carbon dioxide
Glucose carbon dioxide
Glucose + carbon dioxide ethanol

Answer

Cellular respiration is a process by which cells in living organisms release energy for their life processes, such as growth and cell division. Respiration involves breaking down glucose into smaller molecules. This process releases energy in the form of a molecule called adenosine triphosphate, or ATP.

There are two different types of cellular respiration: aerobic and anaerobic. Aerobic respiration requires oxygen and glucose and produces carbon dioxide and water. Anaerobic respiration occurs in the absence of oxygen, as it only requires glucose. In animal cells and some bacterial cells, anaerobic respiration produces lactic acid. In yeast and plant cells, anaerobic respiration produces ethanol and carbon dioxide.

As glucose must be the reactant for any form of respiration, option B is incorrect, as ethanol is the reactant here. In anaerobic respiration, glucose is the only reactant, which also makes option D incorrect, as glucose and carbon dioxide are listed as reactants here. While animal cells produce lactic acid when they carry out anaerobic respiration, yeast cells produce ethanol and carbon dioxide.

Therefore, the word equation for anaerobic respiration in yeast cells is glucose ethanol + carbon dioxide.

There is a simple experiment that can be carried out to demonstrate anaerobic respiration in yeast. The apparatus used for this experiment is shown in Figure 2.

**Figure 2**: A diagram showing the apparatus used in an experiment to demonstrate anaerobic respiration in yeast.

First, glucose is added to the test tube containing the yeast cells. This glucose provides the yeast with a sugar that can be broken down to release energy through respiration. Liquid paraffin is then added on top of the yeast and glucose solution. This layer is impermeable to oxygen, which prevents oxygen from diffusing near the yeast and, hence, prevents aerobic respiration. This means that only anaerobic respiration should occur in the yeast. The screw clip at the top of this tube ensures that no atmospheric air enters either of the test tubes.

The other test tube, linked to the first via a delivery tube, contains limewater. Limewater is a substance that changes from clear to cloudy when it comes into contact with carbon dioxide. Atmospheric air contains small concentrations of carbon dioxide, so it is vital that the screw clip is closed at the start of the experiment. This is because we are trying to detect any carbon dioxide released when the yeast respires anaerobically, rather than detecting the presence of carbon dioxide in the atmospheric air. This production of gas bubbles in the yeast and glucose solution is indicative of the yeast cells producing carbon dioxide and, therefore, carrying out anaerobic respiration.

How To: Investigating Anaerobic Respiration in Yeast

Add limewater to a test tube.
Add glucose to a different test tube.
Boil the glucose solution to sterilize it and remove any oxygen.
Note: This is to ensure that any respiration is anaerobic and that no other microorganisms are present.
Let the glucose solution cool down before adding yeast cells.
Note: A very high temperature will kill the yeast cells.
Place a layer of paraffin oil above the glucose and yeast solution.
Note: This ensures that no oxygen enters the solution so that respiration is anaerobic.
Connect the two test tubes with a delivery tube and close the screw clip.
Note: This prevents atmospheric air containing carbon dioxide from entering the tubes.
Let the yeast anaerobically respire.
Note: It may take several hours for an observable change to occur. The yeast cells should produce ethanol and carbon dioxide and release heat. The test tube containing the yeast should heat up slightly as a result.
Observe any changes to the limewater.
Note: If the yeast cells are anaerobically respiring, the limewater should become white and cloudy.

Various other experiments can be used to determine the effects of temperature and different sugar reactants on the rate of anaerobic respiration in yeast. Most of these experiments include measuring the volume of carbon dioxide or ethanol produced over a set period of time.

Let’s recap some of the key points we have covered in this explainer.

Key Points

Aerobic respiration requires oxygen, whereas anaerobic respiration does not.
Anaerobic respiration in animal cells and some bacterial cells is called lactic acid fermentation, as glucose is broken down to produce lactic acid and some ATP.
Anaerobic respiration in yeast and plant cells is called alcoholic fermentation, as glucose is broken down to produce ethanol, carbon dioxide, and some ATP.
Anaerobic respiration in different microorganisms can be used commercially to produce yoghurt, bread, and alcoholic beverages.