To show just what happens if bacteria make it past the skin and nose the students look at the digestive system, the path that food takes when we eat. One of the scientists is given a special camera to swallow. They are normally used in hospitals to investigate the causes of many gastro-intestinal problems. Because the scientist hadn't eaten for 24 hours there is no food in his stomach but plenty of enzymes and digestive juices. And of course there is stomach acid, the same pH as car battery acid. It is this acid that is responsible for killing off unwanted pathogens in our food.

Exam Tip: Think about the journey of food through the digestive system as you would any other journey. There's only one beginning and only one end, but other things get added along the way. Describe the movement of food and what happens to it at each stage. Challenge yourself to memorise the functions of all the major parts. Try to remember that there are only three main digestive enzymes: carbohydrase for carbohydrates, protease for proteins and lipase for fats.

Example Exam Questions:

• Describe how surface area of the small intestine is increased by villi.
• What are the three main types of food molecules? And what are each broken down into by digestive enzymes?
• Describe how you would test for

(a) Starch,

(b) Protein

(c) Simple sugars

(d) Fats.