I/GCSE Biology - The Digestive System

In I/GCSE Biology, a good definition would be: digestion is the chemical and mechanical breakdown of food. It converts large insoluble molecules into small soluble molecules, which can be absorbed into the blood.

Digestion begins with the mouth. Food is taken into the mouth to our body. This stage is known as ingestion.

• A slippery liquid called saliva helps moisten the food and contains the enzyme amylase, which starts the breakdown of starch.
• This is known as chemical digestion, and when chemicals such as enzymes aid the digestion process.
• The food is also chewed, cutting the food down into smaller pieces to increase surface area, so enzymes can act on the food more quickly. This is an example of mechanical digestion, the physical breakdown of food.

The lump of food, mixed with saliva, then passes along the oesophagus, to the stomach.

• The food is held in the stomach for several hours, while initial digestion of proteins takes place.
• The stomach wall secretes hydrochloric acid, so the stomach contents are strongly acidic. This is important as it kills bacteria that might have been taken into the gut along with the food, helping to protect us from pathogens.
• The protease enzyme that is made in the stomach, called pepsin, has to be able to work in these acidic conditions, and has an optimum pH of 2. 

Muscles are responsible for moving food along the gut by muscle contraction. The wall of the intestine contains two layers of muscles in I/GCSE Biology.

• One layer has fibres running rings around the gut. This is the circular muscle layer.
• The other has fibres running down the length of the gut, and is called the longitudinal muscle layer.

Together, the two layers push food along. When one layer contracts, the other relaxes. Waves of muscle contraction like this is what helps push food along. This is called peristalsis. It means that the movement of food in the gut doesn’t depend on gravity – we can eat while standing on our heads!

• Several digestive enzymes are added to the food in the duodenum. These are made by the pancreas, and digest starch, proteins and lipids.
• The liver makes a digestive juice called bile, which is stored in the gall bladder. Bile does not contain enzymes. Its function is to turn any large lipid globules in the food into an emulsion of tiny droplets. This increases the surface area of the lipid, so that lipase can break it down more easily.

Bile and pancreatic juice are both alkaline. It is also used to neutralize the semi-digested food (which gets acidic from the stomach) before continuing its way down the gut.

As food continues along the intestine, more enzymes are added, until the parts of the food that can be digested have been fully broken down into soluble end products, which can be absorbed. This is the role of the last part of the small intestine, the ileum.

The ileum is highly adapted to absorb digested food. The lining of the ileum has a very large surface area, which means that it can quickly and efficiently absorb the soluble products of digestion into the blood.

• The length of the intestine helps provide a large surface area, and this is aided by the folds in its lining.
• due to the tiny projections from the lining, known as the villi, there is the greatest increase in area. Each is only about 1-2mm long, but there are millions of them, totaling the area of the lining to be about 300m². This provides a massive area in contact with the digested food. As well as this, each of the villus has even more projections known as microvilli.

Each villus has a wall made of single cells and a network of blood capillaries.

• Most of the digested food enters these blood vessels, but the fat droplets enter a tube in the middle called the lacteal.
• The lacteal is part of the body’s lymphatic system, which transports liquid called lymph.
• The lymph eventually drains into the blood system too. Tiny food molecules enter the blood stream by a process known as diffusion (high concentration to low concentration). A rich blood supply in the capillaries maintains the concentration gradient.

The blood vessels from the ileum join up to form a large blood vessel called the hepatic portal vein, which leads to the liver. The liver breaks some molecules down, builds some and stores some.

The digested food molecules are distributed around the body by the bloodstream. The soluble food molecules are absorbed from the blood into cells of tissues, and are used to build new parts of cells. This is called assimilation.

This process is known as egestion (removal of faeces), NOT excretion. Excretion is the removal of the waste products of cells of the body. In I/GCSE Biology, once everything has been digested and absorbed into the bloodstream,

• The waste from the bloodstream, which consists mainly of cellulose fibre and other indigestible remains, water, dead and living bacteria and cells lost from the lining of the gut, will move on to the large intestine.
• The remaining water is absorbed by the first part of the intestine known as the colon, leaving a semi-solid waste material called faeces. This is stored in the rectum, until expelled out of the body through the anus.

That's the end of the topic!

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