The Human Digestive System

In this chapter you will learn:

  1. The Human Digestive System

  2. Enzymes

  3. The organs of the digestive system

Principles of organisation

Most animals and plants consist of different types of cells organised as tissues, organs and organ systems.

multicellular organisms are made of cells, tissues, organs and organ systems.

Cells are the building blocks of multicellular organisms.

A tissue is a group of cells with a similar structure and function.

Organs are made up of groups of different tissues performing specific functions.

An organ system is a group of organs that work together to perform certain functions within the body.

The Human Digestive System

The essential nutrients that people need for a healthy diet are carbohydrates, proteins and lipids.

When we take in food it contains many large molecules, like carbohydrates, fats and proteins.

The digestive system breaks down large molecules of food, to produce the useful smaller molecules to supply cells with necessary nutrients.

In the mouth, teeth chop food into small pieces and grind up your food into a pulp and moistened with saliva containing enzymes.

This allows the food to move more easily through the digestive system.

Carbohydrates in our diet include sugars and starches.

The glucose molecule is small enough so that it can be absorbed directly through the walls of the digestive system.

Lipase enzymes break down fats into glycerol and long chain fatty acids.

Protease enzymes break down proteins into amino acids.

Carbohydrase enzymes break down starch into sugars.


Enzymes are complex protein molecules that act as biological catalysts. they speeds up the rate of a chemical reaction in the cell.

Enzymes are involved in most of the chemical reactions that take place in organisms.

Enzymes Break down large molecules into smaller ones.

Enzymes have a complex 3D shape.

Each enzyme has a region that is called an active site.

Enzymes work best at a particular temperature.

High temperatures may harm the structure of biological molecules and will reduce its activity, or prevent it from working. The enzyme will have been denatured.

The molecules which fit into the active site are knows as the substrate.

The enzyme and the substrate fit together using a lock and key mechanism.

In the lock and key hypothesis, the shape of the active site matches the shape of its substrate molecules. This makes enzymes highly specific and only few molecules can bind them, this is called Lock and Key mechanism.

The enzyme, or the active site, is referred to as the 'lock', and in an analogy with door locks, the substrate molecules are referred to as the 'key'.

The action by which enzymes function is called the 'key and lock' mechanism.

Enzymes are also sensitive to pH. A change in pH causes a change in the shape of the active site.

Changing the pH outside of the optimal range will slow enzyme activity.

Most enzymes function within a working pH range of between 5 and 9, with neutral pH 7 being the optimum.

All enzymes have a specific pH value at which they work best.

Pepsin enzyme has an optimum of pH 2 and can operate in the very acid conditions.

pepsin enzyme breaks down proteins in the very acid conditions of the stomach.

Stomach enzymes have an optimum pH of about 2

Enzymes in the small intestine have an optimum pH of about 7.5

The optimum pH for an enzyme depends on where it normally works.

Enzymes work best when there is a plenty of substrate concentration for the reaction they catalyse.

As the concentration of the substrate increases, the rate of enzyme activity will also increase.

But the rate of enzyme activity can not increase forever, because enzymes become saturated at a certain point and no more substrates can fit even though there is plenty of substrate available.

The organs of the digestive system

Salivary glands in mouth moisten food and produce saliva that contain amylase enzyme.

Amylase enzyme breaks down carbohydrates like starch into maltose in the mouth and small intestine.

The stomach mixes and mashes up food using strong muscular walls.

The stomach produces hydrochloric acid to kill harmful microbes like bacteria and create right pH ~2 for enzymes to work properly.

The liver produces alkaline bile that neutralises stomach acid.

Bile is a substance produced by the liver and stored in the gall bladder.

Bile neutralises the acid and provids alkaline conditions needed in the small intestine.

Bile emulsifies fats turning large droplets of fat into smaller droplets and providing a larger surface area over which the lipase enzymes can work.

The gall bladder is a small bag where bile is stored before it is released into the small intestine.

Pancreas produces and releases enzymes into the small intestine.

Most indigestible food ends up with the rest of the body's waste and stored as faeces in the anus and rectum.

Egestion- the removal of undigested food materials happens when these faeces pass out of the body through the anus.