Word Equations & Balanced Chemical Equations.

If you are doing the basic level biology you only need to know the word equations. If you are doing the higher level you really ought to know the balanced chemical equations given below.

Aerobic respiration:

In your tissues energy is needed for growth, repair, movement, excretion and so on. This energy is obtained from glucose supplied to the tissues by your blood. Aerobic respiration also requires Oxygen. The products of tissue respiration are Carbon Dioxide and water.

Glucose         +     Oxygen   = Carbon Dioxide + Water    +    Energy

C6H12O6  +     6O2     =     6CO2    +     6H2O  +  Energy

The first one is a word equation. All it says is glucose and Oxygen are turned into Carbon Dioxide and water releasing energy. If you still don’t understand this, just pretend that you are a parrot and memorise it.

The second one is a balanced chemical equation.

C6H12O6 … means one molecule of glucose, but we are much too lazy to put a figure 1 in front of the glucose molecule.

The C6 .means that the glucose molecule contains 6 atoms of Carbon.

The H12 .means that the glucose molecule contains 12 atoms of Hydrogen.

The O6 …. means that the glucose molecule contains 6 atoms of Oxygen.

6O2 … means 6 molecules of Oxygen, and each molecule contains 2 atoms of Oxygen.

The left-hand side of the equation shows 6 atoms of Carbon, 12 atoms of Hydrogen and a total of 18 atoms of Oxygen. So does the right-hand side. The two sides of the equation must balance.

6CO2 …means 6 molecules of Carbon Dioxide. The 6 multiplies the Carbon and the Oxygen.

The C …means one atom of Oxygen, but again we are ever so lazy and do not bother to put a little figure 1 after the Carbon atom. So all together there are 6 atoms of Carbon. ( 6 times 1). The same number of Carbon atoms on each side.

The O2 …means that every molecule of Carbon Dioxide contains 2 atoms of Oxygen. So all together there are 12 atoms of Oxygen. Plus the 6 atoms of Oxygen in the water makes a total of 18. Same on both sides again.

By now you should realise that there are six molecules of water and that each contains two atoms of Hydrogen and one atom of Oxygen. Now calculate the number of Hydrogen atoms on the right-hand side. You should find that 6 times 2 makes 12. So the Hydrogen atoms balance up as well.


Carbon Dioxide + Water + Light     =   Glucose      +   Oxygen

6CO2          +        6H2O + Light  =  C6H12O6  +   6O2

Of course this needs light and chlorophyll.

Here are the equations for photosynthesis. If you have understood the aerobic respiration equations, these are easy. If you have not understood, just learn them.

Anaerobic respiration:

Glucose = Ethanol + Carbon Dioxide + Energy

C6H12O6 = 2C2H5OH + 2CO2 + Energy

This is what yeast does when it runs out of Oxygen. It does not release so much energy and the ethanol (alcohol) is poisonous, so yeast will respire aerobically if it can. This is just a last resort.

When we respire anaerobically we produce lactic acid instead of ethanol. It is the lactic acid which gives us cramp. So lactic acid is also poisonous and we must try to avoid making too much.