Respiration is a chemical reaction that happens in all living cells. It is the way that energy is released from glucose, for our cells to use to keep us functioning.
Remember that respiration is not the same as breathing (which is properly called ventilation).
The glucose and oxygen react together in the cells to produce carbon dioxide and water. The reaction is called aerobic respiration because oxygen from the air is needed for it to work.
Here is the word equation for aerobic respiration:
glucose + oxygen → carbon dioxide + water (+ energy)
(Energy is released in the reaction. We show it in brackets in the equation because energy is not a substance.)
Now we will look at how glucose and oxygen get to the cells so that respiration can take place and how we get rid of the carbon dioxide.
Glucose is a type of carbohydrate, obtained through digestion of the food we eat. Digestion breaks food down into small molecules. These can be absorbed across the wall of the small intestine into the bloodstream.
Glucose is carried round the body dissolved in blood plasma, the pale yellow liquid part of our blood. The dissolved glucose can diffuse into the cells of the body from the capillaries. Once in the cell glucose can be used in respiration.
When we breathe in oxygen enters the small air sacs, called alveoli, in the lungs. Oxygen diffuses from there into the bloodstream.
Oxygen is not carried in the plasma, but is carried by the red blood cells. These contain a red substance called haemoglobin, which joins onto oxygen and carries it around the body in the blood, then lets it go when necessary. Like glucose, oxygen can diffuse into cells from the capillaries.
Red blood cells carry oxygen around the body
The carbon dioxide produced during respiration diffuses out of the cells and into the blood plasma. The blood carries it to the lungs. It then diffuses across the walls of the alveoli and into the air, ready to be exhaled.
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