Cellular Respiration

Cellular Respiration Definition

Cellular respiration is the process through which cells convert fuel into energy and nutrients.

To create ATP and other forms of energy that they can use to power their life functions, cells require both fuel, and an electron acceptor which drives the chemical process of turning energy from that fuel into a useable form.

Eukaryotes, including all multicellular organisms and some single-celled organisms, use aerobic respiration to produce energy. Aerobic respiration uses oxygen – the most powerful electron acceptor available in nature.

Aerobic respiration is an extremely efficient process allows eukaryotes to have complicated life functions and active lifestyles. However, it also means that they require a constant supply of oxygen, or they will be unable to obtain energy to stay alive.

Prokaryotic organisms such as bacteria and archaebacteria can use other forms of respiration, which are somewhat less efficient. This allows them to live in environments where eukaryotic organisms could not, because they do not require oxygen.

Examples of different pathways for how sugars are broken down by organisms are illustrated below:

Cellular respiration

More detailed articles an aerobic respiration and anaerobic respiration can be found on this site. Here we will give an overview of the different types of cellular respiration.

Purpose of Cellular Respiration

All cells need to be able to obtain and transport energy to power their life functions. For cells to continue living, they must be able to operate essential machinery, such as pumps in their cell membranes which maintain the cell’s internal environment in a way that’s suitable for life.

The most common “energy currency” of cells is ATP – a molecule which stores a lot of energy in its phosphate bonds. These bonds can be broken to release that energy and bring about changes to other molecules, such as those needed to power cell membrane pumps.

Because ATP is not stable over long periods of time, it is not used for long-term energy storage. Instead, sugars and fats are used as a long-term form of storage, and cells must constantly process those molecules to produce new ATP. This is the process of respiration.

The process of aerobic respiration produces a huge amount of ATP from each molecule of sugar. In fact, each molecule of sugar digested by a plant or animal cell yields 36 molecules of ATP!

Anaerobic respiration processes used by bacteria and archaebacteria yield smaller amounts of ATP, but they can take place without oxygen.

Below we’ll discuss how different types of organisms perform respiration to produce ATP.

Types of Cellular Respiration

Aerobic Respiration

Eukaryotic organisms perform cellular respiration in their mitochondria – organelles that are designed to break down sugars and produce ATP very efficiently. Mitochondria are often called “the powerhouse of the cell” because they are able to produce so much ATP!

Aerobic respiration is so efficient because oxygen is the most powerful electron acceptor found in nature. Oxygen “loves” electrons – and its love of electrons “pulls” them through the electron transport chain of the mitochondria.

The specialized anatomy of the mitochondria – which bring together all the necessary reactants for cellular respiration in a small, membrane-bound space within the cell – also contributes to the high efficiency of aerobic respiration.

The byproduct of aerobic respiration is carbon dioxide gas. This gas is created when sugars are completely broken down into carbon molecules with oxygen molecules attached.

Animals breath O2 in breathe CO2 out – meaning that we add an atom of carbon, derived from cellular fuels such as sugar and fat, to each molecule we breathe out.

This means that when we lose weight through diet and exercise, we are actually breathing that weight out! This also explains why activities that require us to burn more sugars and fats to produce energy require us to breathe hard to supply our cells with more oxygen – and accelerate weight loss!

In the absence of oxygen, most eukaryotic cells can also perform different types of anaerobic respiration, such as lactic acid fermentation. However, these processes do not produce enough ATP to maintain the cell’s life functions, and without oxygen cells will eventually die or cease to function.

Lactic acid fermentation is actually one reason why your muscles can be sore after vigorous exercise: when your muscle cells use up ATP faster than they take in the necessary oxygen to perform aerobic respiration, they start to use lactic acid fermentation instead! The buildup of lactic acid can cause pain in the muscles for days to come.


Fermentation is the name given to many different types of anaerobic respiration, which are performed by different species of bacteria and archaebacteria, and by some eukaryotic cells in the absence of oxygen.

These processes can use a variety of electron acceptors, and produce a variety of byproducts. A few types of fermentation are:

  • Alcoholic fermentation – This type of fermentation, performed by yeast cells and some other cells, metabolizes sugar and produces alcohol and carbon dioxide as byproducts.
    This is why beers are fizzy: during fermentation, their yeasts release both carbon dioxide gas, which forms bubbles, and ethyl alcohol.This is also why distillation is required to produce drinks with alcohol content higher than about 20% – at an alcohol content of 20%, the alcohol actually becomes toxic to the yeast, and they begin to die off instead of producing more alcohol.
  • Lactic acid fermentation – This type of fermentation is performed by human muscle cells in the absence of oxygen, and by some bacteria.Lactic acid fermentation is actually used by humans to make yogurt. To make yogurt, harmless bacteria are grown in milk. The lactic acid produced by these bacteria gives yogurt its distinctive sharp-sour taste, and also reacts with milk proteins to create a thick, creamy texture.
  • Proprionic acid fermentation – This type of fermentation is performed by some bacteria, and is used to make swiss cheese.Proprionic acid is responsible for the distinctive sharp, nutty flavor of swiss cheese, and gas bubbles created by these bacteria are responsible for the holes found in the cheese.
  • Acetogenesis – Acetogenesis is a type of fermentation performed by bacteria, which produces acetic acid as its byproduct. Acetic acid is the distinctive ingredient in vinegar which gives it its sharp, sour taste and smell.Interestingly, the bacteria that produce acetic acid use ethyl alcohol as their fuel. This means that to produce vinegar, a sugar-containing solution must be first fermented with yeast to produce alcohol, then fermented again with bacteria that turn the alcohol into acetic acid!


Methanogenesis is a unique type of anaerobic respiration that can only be performed by archaebacteria.

In methanogenesis, a fuel source carbohydrate is broken down to produce carbon dioxide and methane.

Methanogenesis is performed by some symbiotic bacteria in the digestive tracts of humans, cows, and some other animals.

Some of these bacteria are able to digest cellulose, a sugar found in plants that cannot be broken down through cellular respiration.

Symbiotic bacteria allow cows and other animals to obtain some energy from these otherwise undigestible sugars!

Cellular Respiration Steps

Steps of Anaerobic Fermentation

1. Glycolysis is the only step which is shared by all types of respiration. In glycolysis, a sugar molecule such as glucose is split in half, generating two molecules of ATP.

The equation for glycolysis is:

C6H12O6 (glucose) + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCOO− + 2 NADH + 2 ATP + 2 H2O + 2H+

The name “glycolysis” comes from the Greek “glyco,” for “sugar” and “lysis” for “to split.” This may help you to remember that glycolysis it the process of splitting a sugar.

In most pathways, glycolysis starts with glucose, which is then split into two molecules of pyruvic acid. These two molecules of pyruvic acid are then processed further to form different end products, such as ethyl alcohol or lactic acid.

2. Reduction. In chemical terms, to “reduce” a molecule means to add electrons to it.

In the case of lactic acid fermentation, NADH donates an electron to pyruvic acid, resulting in the end products of of lactic acid and NAD+. This is helpful to the cell because NAD+ is necessary for glycolysis.

In the case of alcoholic fermentation, pyruvic acid undergoes an additional step in which it loses an atom of carbon in the form of CO2.

The resulting intermediate molecule, called acetaldehyde, is then reduced to produce NAD+ plus ethyl alcohol.

Equations of Cellular Respiration

Equation of Aerobic Respiration

The equation for aerobic respiration shows glucose being combined with oxygen and ADP – which are backbones of ATP molecules whose energy has been used up – to produce carbon dioxide, water, and replenished ATP:

C6H12O6 (glucose)+ 6O2 + 36 ADP (depleted ATP) + 36 Pi (phosphate groups)→ 6CO2 + 6H2O + 36 ATP

You can see that once it is completely broken down, the carbon molecules of glucose are exhaled as six molecules of carbon dioxide.

Equation of Lactic Acid Fermentation

In lactic acid fermentation, one molecule of glucose is broken down into two molecules of lactic acid. The chemical energy that was stored in the broken glucose bonds is moved into bonds between ADP and a phosphate group.

The energy in those bonds can later be released when ATP binds to a biomolecule that needs energy, and releases its phosphate group to allow that energy to be used for other things:

C6H12O6 (glucose) + 2 ADP (depleted ATP) + 2 Pi (phosphate groups) → 2 CH3CHOHCOOH (lactic acid) + 2 ATP

Equation of Alcoholic Fermentation

C6H12O6 (glucose) + 2 ADP (depleted ATP) + 2 Pi (phosphate groups)→ 2 C2H5OH (ethyl alcohol) + 2 CO2 + 2 ATP

  • ATP – The energy currency of a cell. ATP can give chemical energy to any protein or other molecule it binds with, breaking one of its phosphate bonds to release the chemical energy.
  • Mitochondria – Specialized organelles that produce huge amounts of ATP in eukaryotic cells. Mitochondria might once have been symbiotic bacteria, which started living inside eukaryotic cells!


1. Which of the following is NOT necessary for cellular respiration?
A. A fuel source, such as a molecule of sugar.
B. An electron acceptor, such as oxygen.
C. A means of extracting energy from sunlight through photosynthesis.
D. None of the above.

Answer to Question #1
C is correct. Although all sugars are ultimately created by autotrophs through photosynthesis or chemosynthesis, organisms can perform cellular respiration without performing photosynthesis.

2. Which of the following types of cells CANNOT survive by using fermentation alone?
A. An archaebacteria cell
B. A bacterial cell
C. A yeast cell
D. A muscle cell

Answer to Question #2
D is correct. Muscle cells can use fermentation in addition to aerobic respiration, but they cannot survive without oxygen. Only very simple organisms, such as bacteria or single-celled yeasts, can survive without oxygen.

3. Which of the following is NOT a reason why multicellular organisms need oxygen to survive?
A. They cannot perform fermentation.
B. They have complex metabolisms that require large amounts of energy.
C. They perform high-energy actions such as locomotion.
D. None of the above.

Answer to Question #3
A is correct. Multicellular organisms can perform respiration – it just isn’t sufficient to meet their energy needs, for reasons such as those listed in B and C.
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