Ch 2 · Digestive System · IGCSE Biology 0610
Topic 2 of 5 in Human Nutrition — IGCSE Biology 0610 — 38 lessons.
Introduction
The digestive system turns a meal into small soluble molecules that the blood can carry to every cell, and removes what cannot be used. It consists of the alimentary canal, a tube from mouth to anus, plus associated organs (salivary glands, liver, gall bladder and pancreas) that pour secretions into it. This lesson gives the Cambridge definitions you must quote precisely, the job of each organ, and the order in which food passes through.
Key ideas — The key definitions
Learn these word for word.
🧑🏫 Teacher's explanation
Think of your digestive system like a high-tech recycling plant. Imagine you have a large, bulky wooden crate (the food) that needs to be turned into tiny, useful Lego bricks (nutrients) to build a new house (your body cells). First, you have to smash the crate into smaller splinters (mechanical digestion). Then, you use a chemical solvent to turn those splinters into tiny individual plastic pieces (chemical digestion). Once they are tiny enough, they can pass through a fine mesh screen into the factory floor (absorption). Once they are inside the factory, they are actually clicked together to form new walls (assimilation). Finally, any wood scraps that couldn't be broken down are swept out the back door (egestion).
📖 Definitions
🔍 Worked example — Easy
A student is asked to identify the process where a large starch molecule is broken down into glucose by amylase. Is this mechanical or chemical digestion? Explain your answer.
🔍 Worked example — Standard
Compare the processes of absorption and assimilation.
🔍 Worked example — Exam-style
Calculate the percentage change in the length of the small intestine if it increases from 6 m to 7.5 m during development.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Define the term 'digestion'. [2] (b) Describe the difference between mechanical digestion and chemical digestion. [2] [4 marks]
Key ideas — Organs and their functions
🧑🏫 Teacher's explanation
Think of your digestive system as a highly specialized disassembly line in a factory. Instead of finished products, you are breaking down complex, large molecules into tiny, individual parts that the factory (your cells) can actually use.
📖 Definitions
🔍 Worked example — Easy
A student is tracing the path of a piece of bread. List the organs the bread passes through in the correct order, starting from the mouth.
🔍 Worked example — Standard
Calculate the percentage increase in the surface area of the small intestine if the villi increase the surface area from 2 m^2 to 15 m^2.
🔍 Worked example — Exam-style
Compare the functions of the liver and the pancreas in the digestive process.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Tracing a meal through the alimentary canal
The alimentary canal is the tube itself. In order, food passes:
🧑🏫 Teacher's explanation
Think of the alimentary canal as a long, continuous highway through which a vehicle (your meal) travels. The highway itself is the tube, but there are specialized service stations (the liver, pancreas, and salivary glands) located off the main road. These stations send specialized fluids through side-roads (ducts) to help process the cargo, but the cargo itself never actually enters the service stations.
📖 Definitions
🔍 Worked example — Easy
A student is asked to list the organs that food actually passes through in the human digestive tract, starting from the mouth.
🔍 Worked example — Standard
Compare the roles of the small intestine and the large intestine in the process of absorption.
🔍 Worked example — Exam-style
An experiment measures the length of a food bolus moving through a model alimentary canal. If the bolus moves 5 cm in the oesophagus and 25 cm in the stomach before entering the small intestine, calculate the total distance traveled through these two organs.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the organ where chemical digestion of carbohydrates begins. [1] (b) Describe the difference between mechanical and chemical digestion. [2] (c) Explain why the small intestine is highly adapted for the absorption of nutrients. [2] [5 marks]
Worked example
Question. A student eats a sandwich. Describe what happens to the sandwich as it passes from the mouth to the stomach, and name the process by which undigested food leaves the body. [5]
🔍 Worked example — Easy
Identify the correct order of the following organs as food travels through the alimentary canal: Stomach, Mouth, Large Intestine, Oesophagus, Small Intestine.
🔍 Worked example — Standard
A student consumes a meal containing 40g of starch. After digestion, 35g of glucose is absorbed into the blood. Calculate the percentage of starch that was successfully absorbed as glucose.
🔍 Worked example — Exam-style
Compare the functions of mechanical and chemical digestion in the mouth. [4]
⚠️ Common errors
Common mistakes
Practice
Answers (a) Taking substances (food and drink) into the body through the mouth [B1]; (b) passing out undigested or unabsorbed food as faeces through the anus [B1]. (a) Liver [B1]; (b) gall bladder [B1]; (c) pancreas [B1]. Duodenum, ileum [B1]; colon [B1]; rectum, anus [B1]. Peristalsis [B1]: circular muscles in the wall contract behind the bolus and relax in front of it, squeezing it along [B1]. Absorption is the movement of small food molecules and ions through the intestine wall into the blood [B1]; assimilation is the movement of those molecules into body cells where they are used and become part of the cells [B1].