Digestion

The break-down of large, insoluble food molecules into small, water-soluble molecules using mechanical and chemical processes.

Mechanical digestion: food is physically broken down into smaller pieces. Starts in the mouth as the teeth chew the food. Muscular contractions in the stomach continue this process.

Chemical digestion: the break-down of large food molecules into smaller molecules by enzymes. Occurs in the mouth, stomach and small intestine.

Digestion

Human teethEach is designed to do a different job.

premolar molar

canine

2nd premolar

latent incisor

1st molar

3rd molar

2nd molar

1st premolar

central incisor

incisorcanine

Tooth structure

Enamel◦ The hard outer layer (white colour)

Dentine◦ Below the enamel◦ Softer than enamel◦ Yellowy colour

Pulp cavity◦ Contains blood vessels and nerves

Tooth structure

Incisors◦ Chisel-shaped for biting and cutting◦ 8 in total

Canines◦ Pointed for piercing and tearing◦ 4 in total

Premolars◦ Have uneven ‘cusps’ for grinding and chewing◦ 8 in total

Molars◦ Also have cusps for chewing up food◦ 12 in total

Function of the teeth

Dental decay

Decay is caused by naturally occurring bacteria in your mouth

The bacteria mix with saliva to form plaque Bacteria feed on left over sugars in your mouth,

releasing an acidic product that wears down the enamel

When the protective enamel is worn away, the dentine is then broken down

If decay reaches the pulp cavity, this will be very painful

Dental decay

1. List some ways to help prevent tooth decay2. Read “extension” section on p81

Proper care of teeth

• What is fluoride?• How does it help prevent

tooth decay?• Where can it be found in

consumables?• Look at the graph.• What is the average

number of decayed teeth at 0.5 units of fluoride?

• What is the average number of decayed teeth at 1.0 units of fluoride?

• Write a conclusion for the trend shown in this graph.

Food and drink is ingested into the mouth.◦ Teeth grind the food into a bolus (ball)◦ Saliva contains amylase to break starch into maltose

Food passes from the mouth into the oesophagus. Peristalsis moves the food into the stomach.

The alimentary canal

The stomach wall has cells that produce gastric juice.

Gastric juice contains:◦ HCl to kill bacteria (pH 1.5-2.0)◦ The protease pepsin to start the breakdown of proteins

into smaller molecules After 2-3h of mixing, the food passes into the

duodenum of the small intestine.◦ Passes through the pyloric sphincter

The alimentary canal

The pancreas is connected to the duodenum by the pancreatic duct.

Pancreatic juice flows into the duodenum. It contains:◦ Amylase to continue breaking starch into maltose◦ The protease trypsin to breakdown proteins ◦ Lipase to break down fats into fatty acids and glycerol

Bile is made in the liver and stored in the gall bladder.◦ Neutralises acid from the stomach so enzymes can work◦ Emulsifies fats to smaller globules

The alimentary canal

The second part of the small intestine is the ileum.◦ Cells on the wall of the ileum secrete enzymes to finish

the breakdown of molecules When the food is fully digested into simple

sugars, amino acids, fatty acids and glycerol, it passes into the bloodstream.

The walls of the small intestine have a large surface area because they are lined with villi.

The alimentary canal

Absorption is the movement of food molecules through the wall of the intestine into the blood and lymph.

Assimilation is the movement of digested food molecules into the cells of the body where they become part of the cells.

Food wastes move into the large intestine where water is absorbed into the bloodstream.

Faeces are stored in the rectum and pass out of the body through the anus in a process called egestion.

The alimentary canal

20 questions