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RESPIRATION
What is Respiration?
Respiration is the chemical process inside living cells that releases energy by breaking down
glucose. Because living things need a continuous supply of energy, the cells of all living things carry out
respiration continuously.
Unfortunately, the term respiration is also used to mean breathing. To avoid any confusion, the
respiration process that takes place inside cells is normally called cellular respiration. The two kinds of
respiration are related. Breathing brings oxygen to your lungs, and oxygen is needed for cellular
respiration to occur in most cells.
There are 2 types of cellular respiration: aerobic respiration and anaerobic respiration.
Aerobic Respiration
Aerobic respiration is a chemical process that breaks down glucose for energy using oxygen.
The term “aerobic” means with oxygen. Aerobic respiration occurs in the mitochondria of all living cells.
Aerobic respiration produces carbon dioxide and water as waste products.
The word and chemical equations for aerobic respiration are:
glucose + oxygen à carbon dioxide + water +
C6H12O6 +
6O2
à
6CO2
+ 6H2O
energy
+ 32 ATP
Aerobic respiration has several advantages. Firstly, it is a very efficient reaction. It releases
approximately 3000 kJ (kilojoules) of energy from each glucose molecule. Secondly, aerobic respiration
creates relatively harmless waste products (H2O and CO2) which are easily removed by the body.
However, aerobic respiration also has some disadvantages. It is a relatively
slow reaction. During very intense exercise, there comes a point when the body
cannot breathe any faster. This means that there is not enough oxygen entering the
muscle cells in the body and the cells become unable make enough energy through
aerobic respiration. When this happens, the muscle cells start producing energy through
a different process: anaerobic respiration.
Anaerobic Respiration
Anaerobic respiration is a chemical process that breaks down glucose for energy without using
oxygen. The term “anaerobic” means without oxygen. Unlike aerobic respiration, anaerobic respiration
takes place in the cytoplasm. It is also known as lactic acid fermentation or simply fermentation.
The word and chemical equations for anaerobic respiration are:
glucose à lactic acid
+
a little energy
C6H12O6 à
+
2 ATP
2 C3H6O 3
There is one major advantage of anaerobic respiration: it is a very fast reaction. It releases
energy from glucose much quicker than aerobic respiration. This means that it is used during very
intense exercise for short periods of time, such as sprinting, running up stairs, or lifting heavy weights.
However, there are two major disadvantage of anaerobic respiration. Firstly, it is a very
inefficient reaction. It releases only 120 kJ of energy for each glucose molecule. That is 25 times
LESS energy per glucose molecule compared to aerobic respiration! Secondly, the process of anaerobic
respiration creates lactic acid, which is a toxic waste that is harmful to your body’s muscle cells. When
lactic acid builds up in muscle cells, it causes them to fatigue and to cramp. Lactic acid buildup is why
your muscles ache after running up a large flight of stairs or running fast in a sprint.
Anaerobic Respiration = Oxygen Debt
As mentioned before, lactic acid is harmful to muscle cells. Because of this, it must be broken
down quickly after intense exercise. The body breaks lactic acid down by combining it with oxygen,
creating the harmless waste products of water and carbon dioxide, according to the word equation:
lactic acid + oxygen à
water + carbon dioxide
To do this, extra oxygen is needed, in addition to the oxygen you need to survive.
The extra oxygen needed to remove harmful lactic acid is called the oxygen debt.
To “pay back” the oxygen debt, your body must breathe deeply after the intense
exercise to get this extra oxygen. This is why you see sprinters breathing deeply after they
have run a race. Once all the lactic acid in the muscles has been broken down, the oxygen
debt has been “repaid” and normal aerobic respiration resumes. This usually takes a few
minutes and the time can be decreased with better fitness and training.
Review Questions: Cellular Respiration
Answer the following questions in your books, IN COMPLETE SENTENCES.
1. Define “respiration”.
2. Define “aerobic respiration”. What is it also called?
3. Write out the word and chemical equations for aerobic respiration.
4. What are two advantages of aerobic respiration? What is one major disadvantage?
5. Define “anaerobic respiration”. What is it also called?
6. Write out the word and chemical equations for anaerobic respiration.
7. What is one advantages of anaerobic respiration? What are two major disadvantages?
8. Under what conditions will the body undergo anaerobic respiration vs. aerobic respiration?
9. Lactic acid is a harmful product of anaerobic respiration. What does it do to the body?
10. Define “oxygen debt”.
11. Explain why you breathe deeply for a short time after very intense exercise?