Background

Respiration can be considered the opposite of the photosynthesis: the organism demolishes sugars to obtain the energy necessary for its biochemical processes. The energy organisms use is supplied by the sun in the form of light during photosynthesis. The process of oxidization of glucose happens through many stages and can be summarized in the following reaction: C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O + energy. The final stages occurs in the mitochondria as energy, carbon dioxide, and water are produced.

Demonstrations

How can you see carbon dioxide, coming from the mitochondria? When breathing, plants and animals emit carbon dioxide and water vapor.
              
1 - The presence of carbon dioxide in the breath of animals can be shown by having a boy or girl blow exhaled air into a solution of lime water Ca(OH)₂. They can do this by exhaling through a straw that is submerged in a beaker of lime water. In presence of carbon dioxide lime water becomes milky.            

2 - Place a little animal inside a plugged flask. As time passes, part of the         oxygen will be consumed by the respiration process, and the same number of molecules of CO2 will be produced. In this way, there will not be any change in the volume of the gas contained in the flask. To show the consumption of oxygen, it is necessary to subtract the carbon dioxide. To do this, you  can use a piece of cotton balls soaked in a solution of 10% of potassium hydroxide (KOH).
               
3 - To highlight the reduction of volume of the gas inside the flask as C0₂ is removed by chemical reaction with KOH, insert a pipette with a capillary tube on the end through the plug of the container. Add a drop of colored water with a trace amount of detergent. The movement of this drop, in reference to a ruler, will give you information on how much oxygen is consumed. The second tube which crosses the plug will be useful to you to set the stained drop, but during the experiment it has to be closed.

4 - Pay attention to not suffocate the animals. Unseal the container as soon as you see that they are having breathing problems. 

WARNING. Be very careful with the potassium hydroxide because it is caustic. Safer to buy the solution already made than mix it yourself. Avoid touching it with bare hands and particularly protect your eyes. Do not allow any animals you use for the tests to be come in contact with it either. When using this substance, children must be supervised by an adult.

Sources:

http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGlyc.html   Cellular Metabolism and Fermentation
       
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookTOC.html   Online Biology Book
 

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