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

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  • 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.
human teeth

central incisor

latent incisor


1st premolar

2nd premolar

1st molar

2nd molar

3rd molar

Human teeth

Each is designed to do a different job.





tooth structure1
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
function of the teeth
Function of the teeth
  • 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
dental decay1
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
proper care of teeth
Proper care of teeth
  • List some ways to help prevent tooth decay
  • Read “extension” section on p81
  • 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.
the alimentary canal
The alimentary canal
  • 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 canal1
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 gallbladder.
    • Neutralises acid from the stomach so enzymes can work
    • Emulsifies fats to smaller globules
the alimentary canal2
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 canal3
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 anusin a process called egestion.