Callular Respiration

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This is a Lecture Note on Cellular Respiration for GCE and GCSE students.

Submitted: August 13, 2015

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Submitted: August 13, 2015



Cellular Respiration

This is the process by which a cell gets its energy by breaking down food molecules.

To be able to carry out all the processes needed for life, a cell needs a source of energy. It gets this by breaking down food molecules to release the stored chemical energy.

Many people think of respiration as meaning ‘breathing’, but although there are links between the two processes, the biological meaning of respiration is very different.

The process of respiration happens in all the cells of our body. Oxygen is used to oxidize food, and carbon dioxide (and water) are released as waste products. The main food oxidized is glucose (a sugar). Glucose contains stored chemical energy that can be converted into other forms of energy that the cell can use. It is rather like burning a fuel to get the energy out of it, except that burning releases all the energy as heat, whereas respiration releases some heat energy, but most is trapped as energy in other chemicals. This chemical energy can be used for a variety for purposes, such as:

  • Contraction of muscle cells, producing movement
  • Active transport of molecules and ions
  • Building large molecules, such as proteins
  • Cell division

The energy released as heat is also used to maintain a steady body temperature in animals such as mammals and birds.

In respiration, carbon passes from glucose out into the atmosphere as carbon dioxide. The carbon can be traced through this pathway using radioactive C14.

Types of Respiration

There are two (2) types, namely:

  1. Aerobic Respiration
  2. Anaerobic Respiration


  1. Aerobic Respiration

This is the type of respiration where cells can respire by using oxygen. Thus the name aerobic. It is also referred to as the overall reaction of respiration.

This process of respiration is summarized by the equation below:


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The process actually takes place gradually, as a sequence of small steps which release the energy of the glucose in small amounts. Each step in the process is catalysed by a different enzyme. The later steps in the process are the aerobic ones, and these release the most energy. They happen in the cell mitochondrion.

Aerobic respiration is not just carried out by human cells, but by all animals and plants and many other organisms.


  1. Anaerobic Respiration

This is the type of respiration where cells can respire without using oxygen. Here, glucose is not completely broken down, and less energy is released. However, the advantage of anaerobic respiration is that it can occur in situations where oxygen is in short supply. Two important examples of this are in Yeast Cells and Muscle Cells.


Yeasts are single-celled fungi. They are used in commercial processes such as making wine and beer, and baking bread. When yeast cells are prevented from getting enough oxygen, they stop respiring anaerobically, and start to respire aerobically instead. The glucose is partly broken down into ethanol and carbon dioxide, and is referred to as Alcoholic Fermentation.


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The ethanol from this respiration is the alcohol in wine and beer. The carbon dioxide is the gas that makes bread rise when it is baked.Think about the properties of ethanol – it makes a good fuel and will burn to produce a lot of heat, so it still has a lot of ‘stored’ chemical energy in it.


Muscle Cells can also respire anaerobically when they are short of oxygen. If muscles are overworked, the blood cannot reach them fast enough to deliver enough oxygen for aerobic respiration. This happens when a person does a ‘burst’ activity, such as sprint, or quickly lifting a heavy weight. This time the glucose is broken down into a substance called lactic acid. Thus is referred to as Lactic Acid Fermentation.


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Anaerobic respiration provides enough energy to keep the overworked muscles going for a short period, but continuing the ‘burst’ activity makes lactic acid build up in the bloodstream, producing muscle cramps. The person then has to rest, to oxidize the lactic acid fully. This uses oxygen. The volume of oxygen needed to completely oxidize the lactic acid that builds up in the body during anaerobic respiration is called the oxygen debt.


Anaerobic respiration has two (2) main disadvantages over aerobic respiration, these are:

  • It converts much less of the energy stored in food into a form of chemical energy that cells can use.
  • It also produces toxic waste products, such as lactic acid or ethanol.

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