Download Biology Notes Chapter 4: Digestion The food an animal eats every

Biology Notes
Chapter 4: Digestion
The food an animal eats every day is called a Diet.
Important nutrients essential for animals diet are:
Carbohydrates, Proteins, Fats, Vitamins, Minerals, Water and Roughage.
Vitamin Foods containing
A Butter, Egg yolk,
Carrots
B Liver, wholemeal,
Brown rice
C Citrus Fruits
Orange, Lime
D Butter, Egg yolk
Skin in sunlight
Purpose
Healthy cells in the lining of gaseous exchange
Healthy rod cells in the retina of the eye
Involved in many chemical reactions in the body,
such as Respiration.
Healthy tissue in skin
Deficiency Disease
Infections of cells lining in gaseous exchange
Night blindness
Beri-Beri: Muscular weakness and paralysis.
Scurvy: Pain in joints and muscles,
bleeding from gums
Helps Calcium to be absorbed, for making bones Rickets: Bones become soft and deformed.
and teeth.

Digestion is the breakdown of large food molecules broken down into small molecules which
can be absorbed.

There are 2 types of Digestion: Mechanical and Chemical Digestion.
Mechanical Digestion is when large pieces of food are broken down with your teeth and the
churning movement of the alimentary canal.
Chemical Digestion is when large molecules are broken down into smaller ones which involve
chemical change via enzymes.

Teeth help with the ingestion and mechanical digestion of food. They can bite of, chop, crush or
grind food to give them a larger surface area for digestion.
Mammals have 4 different types of teeth:
Incisors- sharp-edge, chisel shaped teeth used for biting off food.
Canines- pointed teeth used for tearing off food.
Premolars and Molars- flat, edged shaped teeth used for grinding.

The alimentary canal is a long tube which runs from the mouth to the anus. The wall of the
alimentary canal contains muscles, which contract and relax to make food move along which is
called peristalsis.

Food is ingested using the teeth, lips and tongue. The tongue mixes the food with saliva and
forms it into a bolus which is then swallowed. Saliva is made in the salivary glands. It is a mixture
of water, mucus and the enzyme amylase. The water helps us to dissolve substances in the
food. Amylase begins to digest the starch in the food to maltose. The mucus helps the chewed
food to bind them together to form bolus and lubricates it so that it slides easily down the
oesophagus when it is swallowed.

There are 2 tubes leading down from the back of the mouth. The one in front is the trachea or
windpipe and the tube behind is the oesophagus, which takes the food down to the stomach.
When you swallow, a piece of cartilage covers the entrance to the trachea and it is called the
epiglottis which stops food going down into the lungs. The entrance to the stomach from the
oesophagus is guarded by a ring of muscles called a sphincter.

The stomach has strong, muscular walls. The muscles contract and relax to churn the food and
mix it with the enzyme and mucus. The mixture is called chyme. The stomach walls contain
goblet cells which secrete mucus. It also contains other cells which produce enzymes called
pepsin and rennin, and others which make hydrochloric acid. Pepsin is a protease. It begins to
digest proteins by breaking them down into polypeptides in acid conditions. The acid also helps
kill any bacteria in the food. Rennin is only produced in the stomach of young mammals. It
causes milk that they get from their mothers to clot. The milk proteins are then broken down by
pepsin. After one or two hours, the sphincter at the bottom of the stomach opens and lets the
chyme into the duodenum.

Several enzymes are secreted into the duodenum. They are made in the pancreas and called the
pancreatic fluid. This fluid contains amylase which breaks down starch to maltose. Trypsin
which is a protease and breaks down proteins to polypeptides. Another is lipase, which breaks
down lipids (fats) to fatty acids and glycerol. Pancreatic juices contain sodium
hydrogencarbonate which partially neutralizes the acidic chyme from the stomach.

Bile is made in the liver and stored in the gall bladder and it flows into the duodenum along the
bile duct. Bile does not contain any enzymes. It does, however help to digest fats by breaking up
the large drops of fat into very small ones, making it easier for the lipase in the pancreatic juice
to digest them. This is called emulsification and this is done by the salts in the bile called bile
salts.

As well as receiving enzymes made in the pancreas, the small intestine makes some enzymes
itself (maltase, sucrose, lactase, carbohydrase, proteases and lipase). The inner walls of the
small intestine are covered with tiny projections called villi. Absorption is when molecules pass
through the small intestine into the blood. The nutrients are taken into the liver via the hepatic
portal vein. It is broken down, converted into other substances, some stored and the remainder
left unchanged.

The colon and rectum are called the large intestine where food that isn’t digested or absorbed
found. In the colon, more water and salts are absorbed. All that remains is indigestible food
(roughage), bacteria and some dead cells from the inside of the alimentary canal. This mixture
forms the faeces, which are passed out at intervals through the anus. This process is called
egestion.