#11 Digestion (English)

Digestion

 Digestion is the breakdown of large, insoluble food molecules into small, soluble food molecules so that they can be absorbed into the blood stream.

 

 

The food we eat travels down a single long tube, running from the mouth to the anus, called the alimentary canal. The food is moved along by peristalsis. Peristalsis is the involuntary constriction and relaxation of the muscles of the intestine or another canal, creating wave-like movements, which push the contents of the canal forward.

Food enters the mouth and then travels through the oesophagus. This takes it past a ‘gateway’ called the cardiac sphincter and into the stomach. It may stay here for a while before out through another ‘gateway,’ the pyloric sphincter, into the small intestine. It then enters the large intestine and the remains of the food eventually leave the alimentary canal through the anus.
 

Functions

Mouth	Food is chewed into smaller pieces and mixed with saliva.
Salivary glands	Produce amylase, which begins the digestion of starch; makes the food moist.
Pharynx	Swallows
Oesophagus	Transports food from your mouth to your stomach.
Stomach	Muscular bag that secretes acid.
Liver	Chemical factory – makes lots of chemicals including bile.
Gallbladder	Stores bile.
Pancreas	Produces lots of different enzymes.
Ileum	Also known as small intestine, place where food is absorbed into the blood.
Colon	Also known as the large intestine, place where water is absorbed from the waste food.
Rectum	Stores faeces.
Anus	Ring of muscle that controls faeces release.

Mechanical digestion (Physical breakdown)
The process of breaking down food into smaller molecules so that chemical digestion can occur more efficiently. 

Teeth

Teeth break down food by the process of mastication. This makes it easier to swallow the food, to taste it, and it increases the surface area so that enzymes can work better.

The part of the tooth above the gum is called the crown. It is covered by the enamel.

The part buried in the jawbone is called the root. It is covered by cement.

Enamel: Made of calcium salts, it is very strong.

Dentine: It is covered by the enamel and surrounds the pulp cavity.

The pulp cavity: It contains the nerves and blood vessels.

The tooth is held in place by fibres.

Tooth decay

Although enamel is very hard, it is quite easily dissolved by acids. This is how tooth decay happens.

We all have bacteria living in our mouths. These bacteria feed on food remains from when we chew. The mixture of bacteria and food in the teeth is called plaque.

These bacteria produce acid, especially if the food is sugary. The acid begins to dissolve the enamel on the teeth. Eventually, the acid may make a hole right through the enamel and into the dentine. This is called a cavity.

How can this be preventd?

1. Avoid sugary foods

2. Clean your teeth regularly
3. Use toothpaste containing fluoride. Fluoride is absorbed by the teeth and helps them to resist attack by acid. (High concentrations of fluoride in drinking water can cause black marks on the teeth and can also cause sickness and abdominal pain).
4. If possible, visit a dentist regularly

Chemical digestion (Enzyme breakdown)
The process of chemicals breaking down substances into smaller, soluble substances.

 Enzymes in the digestive system

Food material	Digestive enzyme	Source of enzyme	End products
Starch	Amylase	Salivary glands and pancreas	Glucose
Protein	Pepsin (protease)	Stomach	Amino acids
Fat	Lipase	Pancreas	Fatty acids and glycerol

1. The mouth

The three pairs of salivary glands secrete saliva into the mouth cavity. Saliva is mostly water with the enzyme amylase dissolved in it. It also contains mucus, a slippery substance, which helps the chewed food to slide easily through the alimentary canal. The amylase in the saliva begins to digest any starch in the food to maltose.

While the food is in your mouth, mastication takes place.

The tongue manipulates the food in the mouth, helping to form it into a ball called a bolus, which is then swallowed.

2.  The stomach

It has muscles and elastic tissue in its walls, which allow it to change size and shape.

When you swallow food, the cardiac sphincter muscle relaxes, allowing the food into the stomach, and then contracts, closing the tube so that the food cannot go backwards into the oesophagus again.

The pyloric sphincter muscle is at the other end of the stomach. When this muscle contracts, the food is trapped in the stomach, and when it relaxes, the food moves out into the small intestine.

While the food is in the stomach, the cells lining the stomach walls secrete gastric juice on to it. The fluid contains hydrochloric acid and a protease enzyme called pepsin. These two can be very dangerous. Therefore, a thick protective layer of mucus covers the whole inner surface of the stomach wall.

The function of the hydrochloric acid in the stomach is to kill bacteria in the food.

Pepsin is a protease, so it begins to break down protein molecules into polypeptides. Pepsin works best at a low pH due to the hydrochloric acid mixed with it.

3.      The small intestine

The first part of the small intestine is called the duodenum, and the rest is called the ileum.

Juices from the liver and pancreas are poured into the duodenum.

Bile, from the liver, is squirted along the bile duct. It contains bile salts that help to break down the insoluble fats. This is called emulsification.

Pancreatic juice, from the pancreas, contains sodium hydrogencarbonate as well as several enzymes. Sodium hydrogencarbonate helps to neutralize the acidic food that has entered the small intestine from the stomach. This is important because none of the enzymes in pancreatic juice can work in very acidic conditions.

The enzymes in pancreatic juice include all three types – carbohydrase, protease and lipase.

The carbohydrase is amylase, which breaks down starch into maltose. The protease is called trypsin, and it breaks down proteins into polypeptides. Lipase breaks down fats into fatty acids and glycerol.

The wall of the whole small intestine is covered with villi. The cells on the surface of the villi in the ileum produce enzymes that finish digesting the food. Any remaining complex sugars, such as maltose, are broken down into simple sugars, such as glucose. Polypeptides are broken down to amino acids.

Absorption

The small, soluble molecules that have been produced by digestion can now be absorbed into the blood. Simple sugars, amino acids, fatty acids and glycerol, vitamins, inorganic ions and water are all absorbed by the villi in the small intestine and into the blood or lymph.

The villi greatly increase the surface area of the small intestine and have thin walls. These factors speed up the rate at which absorption can take place. Inside the villi, there are blood capillaries and a lacteal. Everything is absorbed into the blood in the capillaries except for the fatty acids and glycerol, which are absorbed into the lacteal.

4.      The large intestine

After the food has been through the small intestine, it passes into the large intestine. All that is left is fibre and some water.

As the food is moved along the first part of the large intestine, called the colon, most of the remaining water and inorganic ions pass through its walls and into the blood.

The fibre carries on into the rectum and is later passed out through the anus as faeces. The removal of faeces from the body is known as egestion.

Assimilation

Assimilation means making use of the molecules that have been absorbed from the food. They are transported around the body in the blood or lymph, and then taken up by the cells that need them. The molecules become part of the cells.

When the small food molecules are first absorbed into the blood, they are first taken to the liver. One of the liver’s functions is to ‘sort out’ these molecules. Some of them will be allowed to continue in the blood, and be carried all around the body to cells that need them. Some will be stored in the liver, to be used later. Some will be changed into something else, and then either allowed into the blood, or stored.

Yet another role of the liver is to break down any dangerous substances – toxins – that may have been present in the food. For example, alcohol can be very damaging to cells. This is why the liver cells break down the alcohol changing it to harmless substances. Excess alcohol causing permanent damage done to the cells of the liver, may lead to a liver disease called cirrhosis.