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 pepsin which breaks down proteins to peptides. Inside the stomach, the food is churned up with the gastric juices as its muscular walls contract vigorously; this produces a creamy liquid called chyme. Once the food is liquid enough, it is squeezed through the pyloric sphincter (ring of muscle at the bottom of the stomach) in tiny spurts and enters the duodenum (1st part of the small intestine.
IN THE SMALL INTESTINE:
At this point, proteins and carbohydrates (starch) are only partially digested and lipid digestion has not begun. However, digestion of food molecules is completed in the small intestine. The pancreas produces pancreatic juices which empty into the small intestine via a duct. In the small intestine there are digestive juices which contain pancreatic enzymes, intestine-wall enzymes and bile from the liver. Pancreatic juice contains hydrogencarbonate which neutralizes the acidic pH brought by the stomach. Here, starch is completely broken down to maltose by pancreatic amylase and proteins are completely broken down to peptides by trypsin.
Also, fats are broken down to fatty acids and glycerol by lipase. Lipase is helped in its action by bile. The liver produces bile which is stored in the gall bladder and sent to the duodenum through a duct. Bile emulsifies (separates from large globules to smaller droplets) which gives it a larger surface area for the lipase to work on and so the smaller droplets are able to mix with water and be acted upon by enzymes. Bile also contains hydrogencarbonate to neutralise the acid from the stomach. Furthermore, enzymes on the lining of the small intestine include maltase, which completes the breakdown of maltose to glucose.
It is an extremely important substance for the body as digestive juices are largely made up of water. Also, water is the solvent for the biochemical reactions when digestion and it is also used in hydrolysis reactions which break down the large insoluble food molecules.
After these large, insoluble molecules have been digested to become small, soluble ones, they are transported across the lining of the gust into the bloodstream, and this is called absorption. After this, they will be distributed to different parts of the body where they are needed. The majority of absorption takes place in the ileum (lower part of the small intestine).