Introduction to digestion
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Introduction to digestion. OM NOM NOM. Why do we eat?. To gain various nutrients needed for our life processes. Cellular respiration Tissue growth and repair Keep us strong and healthy . Nutrients in food. Proteins. Fats. Carbo -hydrates. Vitamins. Minerals. Complex Carbo -hydrates.

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Why do we eat
Why do we eat?

  • To gain various nutrients needed for our life processes.

    • Cellular respiration

    • Tissue growth and repair

    • Keep us strong and healthy










Why digestion
Why digestion?

  • Complex molecules such as starch, proteins and fats are too large

    • cannot be absorbed by the bloodstream

    • cannot diffuse through partially permeable cell membranes

Purpose of digestion:

Break down large food molecules into smaller ones so that they can be absorbed and used by our body cells

Walls of the small intestine


  • Digestion

  • Chemical digestion

Substrate does not fit this enzyme


Amino acids

Let us look at the types of digestive enzymes required to break down complex food molecules into smaller molecules.


  • Fats

  • Starch

  • Maltose

  • Fatty acids

  • Glucose




  • Proteins

  • Glycerol


The Alimentary Canal

The alimentary canal is a continuous tube in the body starting from the mouth and ending at the anus.

What is the alimentary canal made up of?

  • Mouth

  • Oesophagus

  • Stomach

  • Small intestine

  • Large intestine

  • Rectum

  • Anus

What is the digestive system made up of?

  • Mouth

  • Salivary glands

  • Oesophagus

  • Stomach

  • Liver

  • Small intestine

  • Gall bladder

  • Large intestine

  • Pancreas

  • Rectum

  • Anus

Gotta label em all
Gotta label ’em all


salivary gland

  • oesophagus



gall bladder

  • pancreas

  • small intestine

  • large intestine



Hold on tight…

It’s time for journey down the alimentary canal!

Yipee! Here we go!

1 st stop the mouth
1st stop: The Mouth

What type of digestion occurs when Cookie Monster is munching his cookies?

Physical / Mechanical Digestion

1 st stop the mouth1
1st stop: The Mouth

Purpose of physical digestion?

The teeth help to break largefood pieces into smallerpieces, resulting in an increase in surface areaexposed to digestive enzymes.

1 st stop the mouth2
1st stop: The Mouth

What happens concurrently with physical digestion?

Chemical Digestion

1 st stop the mouth3
1st stop: The Mouth

Chemical digestion:


  • Starch

  • Maltose

Food is mixed with saliva produced by salivary glands. Saliva contains amylase, which breaks down starch into maltose.

1 st stop the mouth4
1st stop: The Mouth

The tongue helps to mix the food with the saliva and rolls the partially digested food into a small ball (bolus) which is then swallowed into the oesophagus.

1 st stop the mouth5
1st stop: The Mouth

What’s next?

Down the oesophagus!

Down the Oesophagus…

2 nd stop oesophagus
2nd stop: Oesophagus

Also known as gullet.

A long and narrow tube joining the mouth and the stomach.

No enzymes = No digestion

2 nd stop oesophagus1
2nd stop: Oesophagus

Has strong muscles in its walls.

These muscles contract and relax to produce a wave-like movement that pushes the food into the stomach.

2 nd stop oesophagus2
2nd stop: Oesophagus

These muscles contract and relax to produce a wave-like movement that pushes the food into the stomach.

This involuntary wave-like pattern of contractions is called peristalsis.

3 rd stop stomach
3rd stop: Stomach

  • The stomach is a large muscular bag.

  • It canexpand to take in various amounts of food.

  • It can contract to churn the food to mix it with gastric juice, a type of digestive juice produced by the stomach lining.

3 rd stop stomach1
3rd stop: Stomach

Physical Digestion:

Peristalsis in the stomach wall churns and breaks up the food.

Peristalsis also mixes the food well with gastric juice

3 rd stop stomach2
3rd stop: Stomach

Gastric juice contains

Hydrochloric acid (HCl)


3 rd stop stomach3
3rd stop: Stomach

Functions of HCl:

Provides acidiccondition in the stomach for the proteases to work.

Kills bacteria which may have been swallowed with the food.

3 rd stop stomach4
3rd stop: Stomach


Chemical Digestion:

The proteases in the stomach digest the proteins in the food into shorter chains of amino acidscalled peptides.

Peptides are then digested to form amino acids.


amino acids


Thinking time
Thinking time…

Why does the stomach not “digest” itself?

Thinking time1
Thinking time…

Cells lining the stomach produce a thick layer of mucus which coats the inner wall of the stomach.

The mucus protects the stomach from digesting itself and from the corrosive action of the hydrochloric acid.

3 rd stop stomach5
3rd stop: Stomach

The food stays in the stomach for a few hours and is slowly moved by peristalsis to the small intestine.

By the time the food leaves the stomach, it would have turned into a thick semi-liquid called chyme.

4 th stop small intestine
4th stop: Small Intestine

Small intestine:

A long and narrow tube joining the stomach and the large intestine.

4 th stop small intestine1
4th stop: Small Intestine

As the chyme enters the small intestine from the stomach, it is mixed with secretions from



Small intestine

4 th stop small intestine2
4th stop: Small Intestine

The small intestine is where most of the chemical digestion and absorption of food takes place.

Note: liver, gall bladder and pancreas are not part of the alimentary canal, but they are parts of the digestive system.

4 th stop small intestine3
4th stop: Small Intestine


Produces a liquid called bile

Bile is stored in gall bladder and released into the small intestine through the bile duct.

Gall bladder

4 th stop small intestine4
4th stop: Small Intestine

  • Function of bile:

  • Emulsifies fats by breaking it into small droplets

  • Increases total surface area of fat droplets to speed up digestion by lipase


  • Develop when bile contains too much cholesterol and not enough bile salts

    • Due to diet, weight, medication

  • Removed surgically or through the use of medication to dissolve them slowly

4 th stop small intestine5
4th stop: Small Intestine

  • Pancreas

  • Produces pancreatic juices

  • Pancreatic juice

  • Contains amylase, proteases, lipase


4 th stop small intestine6
4th stop: Small Intestine

  • Small intestine

  • Produces intestinal juices

  • Intestinal juice

  • Contains maltase, proteases, lipase

4 th stop small intestine7
4th stop: Small Intestine

Main end products of digestion:

  • Glucose

  • Amino acids

  • Glycerol

  • Fatty acids

These products are now small enough to pass through the wall of the small intestine and into the bloodstream.

Walls of the small intestine


Last stop:

Large Intestine

The undigested food moves from the small intestine into the large intestine by peristalsis.

5 th stop large intestine
5thstop: Large Intestine

  • After absorption in the small intestine, the substances which pass into the large intestine include:

  • Water

  • Fibres

  • Vitamins

  • Mineral Salts

5 th stop large intestine1
5thstop: Large Intestine

Large intestine is the site of water, mineral salt and vitamins absorption.

Dietary fibres prevent constipation.

5 th stop large intestine2
5thstop: Large Intestine

After water, mineral salts and vitamins are absorbed, the undigested food (which include fibres) will be stored in the rectum.

Relaxation of the muscles in the rectum will discharge the undigested food as faeces through the anus. This is known as defaecation or egestion.

Digestion of carbohydrates
Digestion of carbohydrates

  • Starch is broken down into maltose by

    • amylase(mouth, pancreas)

    • Maltoseis broken down into glucose by

    • maltase(secreted by the small intestine)

Digestion of proteins
Digestion of proteins

Protein is broken down into amino acids by

- Proteases (found in gastric juice, pancreatic juice & intestinal juice)

Digestion of fats
Digestion of fats

Fat is broken down into tiny fat droplets by

  • Bile (secreted by the gall bladder)

    Fat is broken down into fatty acids and glycerol by

  • lipase (small intestine)

  • Overview of digestive process
    Overview of digestive process

    • Physical digestion

      • Chewing, bile

    • Chemical digestion

      • Enzymes

    • Small diffusable molecules enter bloodstream

    • Undigested matter removed as faeces

    Question one

    The breaking down of large, insoluble food molecules into small, soluble molecules in the body is called ___________.

    Answer: Digestion

    Question Two

    An __________is a special type of protein that speeds up chemical reactions in the body, but remains unchanged at the end of the reaction.

    Answer: Enzyme

    Question Three

    In our bodies, digestion takes place in the _______, ______ and ______.

    Answer: mouth, stomach and small intestine

    Question Four

    Saliva in the mouth contains enzymes that digest only ______.

    Answer: starch

    Question Five

    Proteins are made up of many small ___________ molecules chemically combined together. 

    Answer: amino acids

    Question six

    Fats are made up of many small  _____ and _____ _____ molecules chemically combined together.

    Answer: glycerol and fatty acids

    Question seven

    The chemical used to test for glucose is __________. 

    Answer: Benedict’s solution

    Teacher: What is the chemical formula for water?

    Student: H, I, J, K, L, M, N, O

    Teacher: That's not what I taught you.

    Student : But you said the chemical formula for water was... H to O.