Digestion in the mouth
Chewing breaks down the food in your mouth, otherwise known as mechanical digestion. As you chew, the surface area of your food increases, making it easier for enzyme amylase in your saliva to break down the food even more into maltose, so it is small enough to pass into the oesophagus. Your food then passes through your oesophagus and into the gut. The food is moved through the gut by the peristalsis. The circular muscles contract as the longitudinal muscles relax, and this narrows the gut behind the food, pushing it forwards. In front of the food, the circular muscles relax and the longitudinal muscles contract, which widens the gut.
Digestion in the stomach
Food is held in the stomach for several hours while protein is digested. The walls of the stomach secrete hydrochloric acid. The function of the acid is to kill the bacteria the are taken into the gut along with the food, helping to protect us from food poisoning. The protease enzyme that is made in the stomach and digests protein has to be able to work in acidic conditions. This enzyme is called Pepsin, and has to work in acidic conditions of pH2. The food is held back in the stomach by a ring of muscle called the sphincter muscle, while the protein is digested and broken down into amino acids.
Digestion in the small intestine
The first part of the small intestine, the duodenum, is where enzymes made in the pancreas are added. These enzymes are enzyme amylase (which breaks down the starch into maltose), enzyme maltase (which breaks down the starch into glucose), and enzyme lipase (which breaks down the lipids into fatty acids). As well as this, bile is also added to the food in the duodenum, and it turns fats into an emulsion of tiny droplets which makes it easier for enzyme lipase to digest the lipids (fat). Bile is also alkaline and so it neutralises the acidic mixture of food and enzymes coming from the stomach.