During respiration, the cells move electrons around, using complex chemical reactions. In the process, they make and break atomic bonds, and so release energy, or store it as chemical potential energy in molecules like ATP. These are a bit like little rechargeable batteries for the body.
Fermentation results in far fewer ATP molecules being made per amount of sugar that is broken down. Sugar is the fuel for the yeast cells.
Fermentation involves the use of yeasts. Yeast can live in an environment where there is no oxygen, because yeasts can respire without oxygen. The respiration during fermentation is anaerobic, or ‘without oxygen’. When yeast respires without oxygen it produces alcohol, and this is really very important. The yeast makes the alcohol that makes alcoholic drinks alcoholic. This alcohol is actually a waste product of the cells, along with carbon dioxide. The carbon dioxide gas helps make the beer fizzy. Alcohol and carbon dioxide might be waste products for the yeast cells, but they certainly are not a waste product for the man down in the pub, with a nice head on his beer.
Here is a basic form of the fermentation equation, which you might find useful:
Fermentation breaks down sugars to form alcohol, carbon dioxide, and energy.
Sugar --- ---> alcohol + carbon dioxide + energy
C6H12O6 ---> 2C2H5OH + 2CO2 + E (2 ATP)
WINE PRODUCTION
The process of wine making involves turning the sugar in grapes into an alcohol (ethanol). This process is called fermentation. Rather than add enzymes we use a microbe called yeast. There are many different strains of yeast that have been developed for different uses, but grapes can be fermented naturally without people adding any yeasts, because yeast is a naturally occurring fungus found on the surface of grapes. As the grapes are trodden into a pulp, the sugary juices from inside the grape and the yeast from the surface are mixed up.
The yeast turns the sugar into alcohol, but it doesn’t do this because it likes us and wants to do us a favour. It does this because it is ‘feeding’ on the sugar and breaking the sugar down to get energy to live. We do something very similar with the sugar in our bodies. The process is called RESPIRATION. In humans we use oxygen to break the sugar down and so this is called AEROBIC RESPIRATION. Fermentation does not use oxygen so it is called ANAEROBIC RESPIRATION.
Glucose → ethanol + carbon dioxide + energy
To carry this out the yeast cells produce enzymes to break the glucose down.