DIGESTION Chapter 3(Biology)

The body carries out digestion of food to convert large insoluble food molecules into smaller soluble ones.

carbohydrate molecule protein

molecule

fat molecule

Small food molecules can pass through the walls of the small intestine and then dissolve into the blood stream. Large food molecules cannot do this.

WHAT IS DIGESTION?

THE BREAKDOWN OF FOOD

Physical breakdown

Chemical breakdown

PHYSICAL BREAKDOWN OF FOOD - It starts in the mouth.

- Food is physically broken down from large pieces into small pieces.

- The teeth play a major part in the physical breakdown of food

4 TYPES OF TEETH

Teeth Shape FunctionIncisors Chisel

shapedBiting into soft food

Canines Sharp and pointed

Tearing into tough food (meat)

Premolars Cups with grooves

Form a crushing and grinding surface

Molars Cups with groves

Form a crushing and grinding surface

- We have two sets of teeth.

Milk teeth Permanent teeth- 20 teeth- Emerge through the

gums at about 6 months of age.

- This set is complete by about the age of 3 years.

- 32 teeth- Replaces milk teeth.- The replacement

starts at the age of 6 and is complete by the age of 17.

CHEMICAL BREAKDOWN OF FOOD

- Carbohydrates, fats and proteins are made from large molecules (insoluble).

- When the molecules are broken down into smaller molecules they become soluble and can pass through the digestive system.

ENZYMES

- Enzymes are made by the body from proteins and they speed up chemical reactions.

- Almost ALL reactions in living things involve enzymes.

- Enzymes belong to a group of chemical substance called CATALYSTS.

- CATALYSTS are substances that speeds up chemical reactions without being changed or used up in the reaction.

- Digestive enzymes speed up the breakdown of the large molecules into smaller ones.

WHEN YOUR MOUTH WATERS

Sub maxillary

- The water formed in your mouth is called saliva.

- We can make 1.5 L of saliva in 24 hrs.

- Saliva is made by 3 pairs of salivary glands (made up of saliva secreting cells and ducts).

- Saliva is 99% water.- It also contains mucin (slimy

substance) and amylase (breaks down starch molecules into sugar molecules)

WHEN YOU SWALLOW- When you have chewed your food it

is made into a pellet called the bolus.

- The bolus is pushed to the back of your mouth by the tongue.

- Swallowing causes the bolus to slide down your gullet (oesophagus).

- The oesophagus has 2 layers of muscles:

(a)Outer layer------ Longitudinal layer(b)Inner layer ------ Circular layer

- In the gullet when the circular muscles contract they push on the food.

- Then the longitudinal muscles contract to stretch (relax) the circular muscles.

- Circular muscles do not all contract at the same time.

- The wave of muscular contraction is called peristalsis.

- Peristaltic waves occur in other parts of the alimentary canal to push the food.

After food is swallowed it enters the stomach, which is basically a muscular bag filled with hydrochloric acid.

THE STOMACH

muscle

tissuefood

leavesthe

stomach

food enters from the

gullet

Two things happen to food in the stomach: the chemical breakdown of food begins; microbes are destroyed.

- The stomach wall is lined with glands which secret hydrochloric acid and a protein digesting enzyme called pepsin (protease).

- The hydrochloric acid kills many bacteria in the food and provides the pepsin with the pH conditions to work.

- The food is churned up by the action of the muscles as they send peristaltic waves down the stomach wall (3 waves/min.)

- When food is broken down into a creamy liquid the valve that stopped the food from leaving the stomach opens.

- This allows the food to pass to the next part of the digestive system, the DUODENUM

THE DUODENUM, LIVER AND PANCREAS

- The duodenum is part of the small intestine.

- It is connected to the stomach.

- 2 tubes (ducts) open into the duodenum.

- The 2 tubes leading into the duodenum are:

Bile duct Pancreatic ductFunction:- Carries a green liquid

called bile from the gall bladder.

- Bile is made in the liver.

- It contains chemicals that break down the fat into small droplets so that the fat-digesting enzymes can work easily.

Function:- Produces a juice

containing enzymes that digest proteins, fats and carbohydrates.

- The mixture of liquids from the stomach, liver and pancreas pass on into the small intestine.

THE FATE OF UNDIGESTED FOOD

Sub maxillary

- Indigestible parts of the food (ex. Cellulose) pass on through the small intestine to the large intestine.

- The 1st part of the large intestine is called the caecum.

- This is attached to a much longer part called the colon.

- Here water and some dissolved vitamins are absorbed and taken into the body. The remaining semi-solid substances form the faeces.

- The faeces are removed from the body through the anus. This is called egestion (defecation)

ENZYMES SUMMARYRegion of production

Type of enzyme

Notes

Salivary glands in the mouth

Carbohydrase Enzyme is called salivary amylase

Gastric glands in stomach

Protease - Enzyme is called pepsin- Hydrochloric acid is

made to make the enzyme work

Pancreas Protease, Carbohydrase, lipase

- Enzymes enter the duodenum and mix with food and bile.

EXCRETORY SYSTEM Biology

A CLOSER LOOK AT THE EXCRETORY SYSTEM

- The wastes produced by the body collect in the blood.- They are removed from the body by the excretory system.- As the blood passes through the kidneys, a waste called

urea is filtered from the blood with some water.- On a hot day, when the blood passes through the skin

carrying the water and the urea, they are released onto the skin to cool the skin.

- As the blood passes through the lungs the carbon dioxide is removed and passed into the air.