Digestive System


  • Feeding (or ingestion)
  • Digestion: breakdown of large food particles into soluble, diffusible molecules (so that the cells can absorb them)
  • Absorption: cells taking in digested food particles
  • Assimilation: food materials converted into new parts for the cell or used to provide energy

Five Stages of Human Nutrition

  • Ingestion: placing food into the alimentary canal at the mouth.
  • Digestion: breakdown of complex food into their simple soluble absorbable subunits.
  • Absorption: the passage of the products of digestion into the blood or lymph.
  • Assimilation: conversion of the absorbed nutrients into complex molecules for growth, repair and defence.
  • Egestion: expulsion of the undigested and unabsorbed material from the alimentary canal.

From Ingestion to Egestion – The Simplified Diagram

Now, about Digestion…

Major Functions of Digestive System

Digestion: breakdown of complex food into their simple, soluble and absorbable sub-units.

  • Physical processing: by breaking it up (chewing), mixing, adding fluid etc.
  • Chemical processing: by adding digestive enzymes to split large food molecules into smaller ones.

Absorption: the passage of the products of digestion into the blood or lymph.
Movement of food (Transportation): controlled by sphincter muscles, longitudinal and circular muscles in the gut wall.

Digestion is an enzyme-catalysed process

– Proteins called enzymes cause the rate of production of “products” (e.g. glucose, nutrients needed by body) to be much greater

– In other words, it catalyses chemical reactions. Read more here

Useful links for revision:

National Geographic


Animated digestive system

Group work:




The digestive process


Explain the length of the various parts of the digestive system
Esophagus → It has to squeeze food past our lungs and into the stomach, hence its length
Small intestine → It is the major site of chemical digestion which takes a long time to carry out, so it has the greatest length to allow complete digestion of the bolus before it leaves.
Large intestine → Its length maximises the amount of water and minerals absorbed into the bloodstream. As most of the nutrients have already been absorbed in the small intestine, the large intestine has an easier job and is comparatively shorter.

How is the digestive system adapted to suit its function of absorption?


  • Highly folded mucous membrane (mucosa). Allows for expansion during digestion.
  • Gastric pits secrete digestive juices to break down the bolus into chyme.
  • Muscular walls contract to mix bolus with the digestive juices.
  • Mucosa prevents the acidic digestive juices from destroying the stomach walls.
  • These maximise the amount of food digested and make digestion more efficient. This allows for better absorption of nutrients from the food.

Small intestine

Increased surface area -> rate of diffusion

  • Highly folded mucosa arranged in villi. Epithelial cells lining the small intestine have a folded cell membrane, microvilli. This increases surface area to volume ratio to allow for effective absorption of nutrients.
  • Long –> Maximises absorption of nutrients.

Increased rate of diffusion

  • Thin epithelial wall – The permeable villi lining its walls are only one cell thick,  allowing for quick diffusion of nutrients into the capillaries in the walls.
  • Dense network of thin-walled blood capillaries, creating a constant steep diffusion gradient. Allows for quick transportation throughout the body as well.
  • Circular and longitudinal muscles which allow bi-directional peristalsis. This improves the amount of food digested.

Why do we need food?

We need the substances in food to live and grow. (stated below)

Why do we need to digest food?

We need to break down the food into useable substances, such as:

  1. proteins needed for growth, bodily functions (e.g. cell repair, RBC production)
  2. nutrients, vitamins, minerals to keep our organs healthy, for growth and bodily functions (e.g. vitamins)
  3. glucose for respiration to provide energy for bodily functions (e.g. muscle movement)

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