IN THE MOUTH:
Food is mechanically cut by incisors and canines, chewed by molars and premolars, and mixed with saliva by the tongue. The saliva has been produced by salivary glands, which pour it into the mouth through salivary ducts. This process of introducing food into the mouth is called ingestion. Chewing breaks food into smaller particles so that chemical digestion can occur faster. This cutting and mixing is called mastication. Moreover, food is chemically digested by salivary amylase (carbohydrase) in saliva which is an enzyme (biological catalyst) which breaks down the insoluble polysaccharide starch to the soluble simpler sugar called maltose. In digestion, food molecules are broken down by hydrolysis reactions (breakdown with water). Saliva also contains mucus, which lubricates and helps hold together chewed food in a clump called a bolus. Hydrogencarbonate is also present in the mouth; this maintains an ideal pH for amylase to work in (pH6.5-7.5). Finally, as the tongue is muscular it can move food, so it therefore pushes food back to where it is swallowed. IN THE OESOPHAGUS:
The oesophagus or gut is a muscular tube which leads from the mouth to the stomach. The swallowing is accomplished by peristalsis (waves of muscular contraction pushing the bolus down towards the stomach). When swallowing, the trachea (windpipe) is covered to prevent food from entering. Fibre is important in our diet because without it the contents in the gut would be very liquid and peristalsis would not work. IN THE STOMACH:
The stomach is a muscular bag with a lining which contains digestive glands. The stomach may store up to 2 litres of food. The glands in its lining produce mucus (protects the walls of the stomach from HCl and pepsin), pepsin (a protease enzyme) and HCl(hydrochloric acid, which provides the acidic conditions for pepsin to digest, pH2). HCl also dissolves food and kills any microorganisms or bacteria; moreover it converts pepsinogen to...