Analysing Cells
Cell Structure
Cell Transport
Gas Exchange



Extracellular Digestion

This means where digestion is done outside of the organism. This is seen in saprophytic fungus (saprophytic meaning decomposer). The fungi secretes the enzymes into their surrounding material and then absorb the products.

A way of measuring the activity of amylase is to have an agar plate with starch; make a small well and put in the amylase enzyme. Incubate for a time and then flood the area with iodine. It will show a clear zone where the starch has been digested and hence you can see which is the more effective (diagram below).

testing of amylase activity

Human Gut Wall

Below is a digram of the human gut wall, on the right are labelled the different layers that exist.

The lumen is not really a layer of the gut but the space which the food goes in. You have to imagine the structure below curled into a circle; as this is what it looks like in the body.

The first actual layer is the mucosa, it has a layer of epithelium, made of epithelial cells, which make the villi.

Below this is the muscle layer, known scientifically as the muscularis externa, it is reponsible for peristalsis which moves food through the digestive tract.

the human gut wall

In the diagram you should also notice the capillaries, part of the blood netowrk which take absorbed food away. And also the lacteals which are a special type of lymph gland which take fats away in the lymphatic system.


the digestive tract or alimentary canal

The diagram to the left is of the alimentary canal also known as the digestive tract and also includes other organs of the digestive system like the liver.

After being swallowed, the food travels down the Oesophagus or esophagus, this is continually being damaged by the friction of food, so the epithelium is a few cells thick and it secretes mucas to lubricate the food's passage. The muscularis externa is also much larger to give more force to peristalsis.

The next place it enters is the stomach this is a temporary store, mixes the contents up and also is the site for a bit of digestion. The mucosa are folded to allow for expansion as more food is added. The muscularis externa is larger still here to allow the food to be churned up.

Into the small intestine opens the pancreatic and bile ducts: opening into the duodenum. This then leads into the ileum. The bile salts neutralise the stomach acid as well as emulsify fats. Also here, the mucosa has thin projections of villi which greatly increase the surface area for maximum absorption.

Digestion of Triglycerides

To learn more about enzymes see the page enzymes.

Triglycerides are a type of lipid; here you will learn how the body breaks down this molecule.

Digestion begins in the duodenum where bile enters from the liver, bile salts make the big blobs of fat into small micelle droplets which massively increases the surface area and makes digestion much easier.

Also in the duodenum, pancreatic lipase this breaks the triglyceride into fatty acid and glycerol. When these substances get into the ileum they diffuse into the epithelial cells ... and then are put back together again!

These resynthesised lipids make proteins called chylomicrons, these enter the lacteals and travel through the lymphatic system, making it milky. They travel in this system until entering the blood stream at the vena cava where they are stored as fat tissue.