Human Digestive System – JSS2 Intermediate Science Lesson Note (Second Term)

Digestion is the breaking down of large food pieces into small, soluble pieces that the body can absorb into the blood.

Behavioural objectives

1. Define digestion and the alimentary canal.
2. Name the parts of the human digestive system and state the function of each.
3. Name the types of human teeth and state what each does.
4. Classify animals as herbivores, carnivores or omnivores, with examples.
5. Describe filter, fluid, saprophytic and parasitic feeding with examples.

Why do we eat?

We eat because food supplies the energy and materials the body needs to work, grow and repair itself.

• Fuel: food is to the body what petrol is to a car or batteries are to a torch.
• Problem: the body cannot use a whole piece of yam or a leg of chicken as it is.
• Solution: food must be broken down into tiny, liquid pieces that can travel in the blood.

What is digestion?

Digestion is the process that breaks big food into tiny bits the body can absorb.

• Physical digestion: food is broken into smaller pieces without changing what it is made of, for example chewing with teeth.
• Chemical digestion: enzymes change food into simpler substances, for example starch into sugar.
• Enzyme: a substance made by the body that speeds up the breakdown of food.

What is the alimentary canal?

The alimentary canal is the long tube that starts at the mouth and ends at the anus.

• Length: it is about 9 metres long in an adult.
• Other name: it is also called the gut or digestive tract.
• Job: every part along the tube has its own special work.

What are the parts of the human digestive system and their functions?

The main parts are the mouth, oesophagus, stomach, small intestine, large intestine and anus.

Follow a piece of bread through the body:

1. The mouth (the gate): digestion starts here.
   • Teeth chew the food (physical digestion).
   • Saliva softens the food.
   • Saliva contains an enzyme that starts changing starch into sugar.
2. The oesophagus (the food pipe): a muscular tube that squeezes food down to the stomach.
   • The wavy squeezing movement is called peristalsis.
3. The stomach (the mixer): a bag of strong muscles.
   • It mixes the food with gastric juice and acid.
   • The acid kills many germs.
   • The food becomes a thick porridge called chyme.
4. The small intestine (the main site of digestion and absorption): most of the work happens here.
   • It receives bile from the liver to break down fats.
   • It receives juice from the pancreas that digests food.
   • It absorbs nutrients from the food into the blood.
   • It is very long and coiled so that it fits into the belly.
5. The large intestine: only watery waste reaches it.
   • It absorbs water so that the body does not lose it.
6. The anus: the solid waste (faeces) leaves the body here when you go to the toilet.

Summary table of the parts

What are the organs that help digestion?

These organs help digestion but food does not pass through them.

• Salivary glands: make saliva in the mouth.
• Liver: makes bile, which helps to break down fats.
• Pancreas: makes a juice with enzymes that works in the small intestine.

What are the types of human teeth?

There are four types of teeth: incisors, canines, premolars and molars.

• Incisors: flat, sharp front teeth for cutting and biting food.
• Canines: pointed teeth for tearing food.
• Premolars: broad teeth for crushing and grinding.
• Molars: large back teeth for grinding food.

What are the types of animals based on diet?

There are three types of animals based on diet: herbivores, carnivores and omnivores.

• Herbivores: animals that eat only plants, such as goat, cow and grasshopper. They usually have flat teeth for grinding grass.
• Carnivores: animals that eat only flesh, such as lion, dog and hawk. They have sharp canine teeth or beaks for tearing flesh.
• Omnivores: animals that eat both plants and meat, such as humans, pigs and chimpanzees.

Comparison of the three groups

What are the feeding mechanisms of animals?

A feeding mechanism is the way an animal gets food into its body, and there are four main kinds: filter, fluid, saprophytic and parasitic feeding.

1. Filter feeding: the animal strains tiny food particles out of water like a sieve, as in baleen whales and oysters.
2. Fluid feeding: the animal drinks its food in liquid form.
   • Mosquitoes: females suck blood.
   • Butterflies and bees: suck nectar from flowers with a long straw called a proboscis.
3. Saprophytic feeding: the organism lives on dead and decaying matter, as in mushrooms and some bacteria.
   • It pours digestive juice on the food.
   • The food is digested outside its body.
   • It then soaks up the digested food.
4. Parasitic feeding: the organism lives on or inside another living thing (the host) and takes its food, as in tapeworms in the human gut and lice on hair.

Evaluation

1. What is the name of the wavy movement that pushes food down the food pipe?
2. Which part of the digestive system absorbs water from waste?
3. What is the difference between a carnivore and an omnivore?
4. How does a butterfly get its food?
5. Name the four types of human teeth.

Assignment

Draw the alimentary canal as a flow chart of six boxes, from the mouth to the anus. Write one function under each box.