Human digestion

Key terms:

1. Mastication
2. Peristalsis
3. Absorption
4. Assimilation
5. Ingestion
6. Digestion
7. Egestion
8. Excretion

1. Mastication is the physical break-up of food in the mouth. It is the chewing and crushing of the food in order to obtain a higher surface area. A higher surface area enables enzymes to break down the food more efficiently.

2. Peristalsis is the muscular contractions that move food along the gut i.e. small intestine. Peristalsis occurs to ensure food is moved along the gut. It first occurs in the oesophagus yet it continues in the small intestine.

3. Absorption is the taking-in of small, soluble molecules into the bloodstream [at the end of the small intestine called the ileum]. It is performed efficiently as the ileum is lined with villi. These increase the surface area greatly, allowing the soluble molecules to be absorbed at a high rate. They are one cell thick and also contain blood capillaries to ensure the diffusion distance is small. The blood has a lower concentration of the food molecules therefore diffusion occurs quickly. The bloodstream transports the molecules all around the body to various tissues.

4. Assimilation is the taking in of molecules into the cells where they are then utilised.

5. Ingestion is the consumption of food by an organism.

6. Digestion is the breakdown of large food molecules into smaller, soluble ones.

7. Egestion is the removal of undigested semi-solid waste as faeces.

8. Excretion is the removal of all waste products from the body. Not necessarily as faeces.

Structure of the human alimentary canal

1. Mouth is where the physical breakdown of food occurs. Mastication increases the surface area greatly and this aids the enzymes. Amylase is an enzyme released with saliva. It breaks down starch into maltose. The saliva itself lubricates the food to help swallowing.

2. The oesophagus is situated next to the trachea and it connects the mouth to the stomach. Food never enters the trachea as it is covered with the epiglottis when food is swallowed. Therefore food only enters the oesophagus. Peristalsis occurs in the oesophagus to ensure food is moved along.

3. The stomach contains hydrochloric acid. This is a powerful acid with a pH of 1-2. As a result, it breaks down food even further to maximise the surface area more. The food here is now chyme, a liquid solution.

4. At the start of the small intestine, enzymes are released. These enzymes catalyse reactions to break down large food molecules into smaller, soluble ones. Each enzyme is specific to the reaction of a particular food molecule. Enzymes work most efficiently in the right temperature, pH and when there is a large surface area. There is more about enzymes further down the page.

5. The end of the small intestine is called the ileum. This is where absorption occurs. The ileum is lined with millions of small lumps called villi. These maximise the surface area hence they increase the rate of diffusion of soluble molecules into the bloodstream.

6. The large intestine is where water from chyme is absorbed and transported around the body.

7a. The pancreas produces enzymes essential to the chemical breakdown of food molecules such as amylase, lipase and protease. It is connected to the start of the small intestine [where enzymes are released to catalyse the breakdown of food] by the common bile duct.

7b. Bile is produced in the liver and stored in the gall bladder. It emulsifies lipids to increase the surface area. This makes their breakdown more efficient and occur at a greater rate. Bile is released with the pancreatic juices i.e. the enzymes through the common bile duct. Bile is also essential in creating alkaline conditions for the enzymes to work in. Note that it is not released specifically to neutralise stomach acid, it simply neutralises the acidic food molecules which have come out of the stomach. 

The digestive enzymes

These enzymes catalyse the reactions of the chemical breakdown of food molecules into smaller soluble molecules. They are made from proteins and are specific to the type of molecule they catalyse the reaction for. Amylase is produced in the salivary glands and is released in the mouth along with saliva. It breaks down starch into maltose. Maltose is then broken down by maltase into glucose at the start of the small intestine.

Protease breaks down proteins into amino acids. It is produced in the pancreas and released through the common bile duct along with the other enzymes at the start of the small intestine.

Lipase breaks down lipids into fatty acids and glycerol. It is produced in the pancreas and released along with bile from the common bile duct at the start of the small enzymes. Bile emulsifies the lipids, increasing the surface area so that the enzyme can work more efficiently.