Ch 4 · Chemical Digestion · IGCSE Biology 0610
Topic 4 of 5 in Human Nutrition — IGCSE Biology 0610 — 58 lessons.
Introduction
Starch, protein and fat molecules are far too large to pass through the wall of the intestine into the blood. Chemical digestion uses enzymes to cut them into small, soluble molecules that can be absorbed. This lesson links the enzyme ideas from earlier (specificity, optimum pH) to the digestive organs, so that you can state for each enzyme what it breaks down, where it is made and where it acts.
Key ideas — What chemical digestion achieves
Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules. It is carried out by digestive enzymes, each specific to one type of food molecule.
🧑🏫 Teacher's explanation
Imagine you are trying to fit a large, bulky piece of furniture through a very narrow doorway. No matter how hard you push, the furniture is too big to pass through. In your body, food molecules like starch or proteins are like that bulky furniture; they are too large to pass through the walls of your small intestine into your bloodstream.
📖 Definitions
🔍 Worked example — Easy
A student is given a sample of starch. Describe the change that occurs during chemical digestion and state the end product.
🔍 Worked example — Standard
Compare the role of hydrochloric acid in the stomach with the role of bile in the small intestine.
🔍 Worked example — Exam-style
A patient has a condition where their pancreas does not secrete enough lipase. Predict and explain the effect on their nutrition.
⚠️ Common errors
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🎯 Exam question
(a) State the name of the enzyme that digests proteins in the stomach. [1] (b) Explain why the pH in the stomach must be acidic for this enzyme to work effectively. [2] [3 marks]
Key ideas — The three groups of digestive enzymes
Distinguish where an enzyme is secreted from where it acts. Pancreatic enzymes are made in the pancreas but act in the duodenum, reaching it along the pancreatic duct. Salivary amylase stops working in the stomach because the pH there is far from its optimum, so starch digestion pauses until pancreatic amylase arrives.
🧑🏫 Teacher's explanation
Imagine you are trying to swallow a large, bulky crate of building supplies. You cannot fit the crate through a narrow door (the cell membrane). To get the supplies inside, you first have to break the crate down into small, individual bricks. Chemical digestion is exactly this: using biological catalysts called enzymes to break large, insoluble molecules into small, soluble ones that can pass through your intestinal walls into the blood.
📖 Definitions
🔍 Worked example — Easy
A student is testing for the presence of reducing sugars in a solution. They find that starch is present, but maltose is also present. Explain why this indicates that amylase has been active.
🔍 Worked example — Standard
Compare the role of the stomach and the small intestine in protein digestion.
🔍 Worked example — Exam-style
Calculate the percentage increase in the concentration of glucose in the small intestine if the initial concentration is 0.5 mmol/L and it rises to 2.5 mmol/L due to maltase activity.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the color change occurs. [2] (b) State the name of the product formed from the starch. [1] [3 marks]
Key ideas — Hydrochloric acid in the stomach
Gastric juice contains hydrochloric acid, giving the stomach contents a pH of about 2. The acid has two functions:
🧑🏫 Teacher's explanation
Imagine you are trying to dissolve a large, hard sugar cube in a glass of water. The sugar cube is too big to pass through a tiny straw, so you need to break it down into tiny grains first. Chemical digestion is exactly like that; it uses special biological 'scissors' called enzymes to chop up large, insoluble food molecules into tiny, soluble ones that can fit through the walls of your gut into your blood.
📖 Definitions
🔍 Worked example — Easy
State the two main functions of hydrochloric acid in the stomach.
🔍 Worked example — Standard
A student measures the pH of gastric juice to be 2.0. If the pH were increased to 7.0, explain the effect on the activity of the stomach protease pepsin.
🔍 Worked example — Exam-style
Explain why the stomach does not undergo self-digestion by the proteases it secretes.
⚠️ Common errors
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🎯 Exam question
Key ideas — Starch and protein digestion in detail
🧑🏫 Teacher's explanation
Imagine you are trying to get a large, bulky crowd of people through a very narrow revolving door. The people are the large, insoluble food molecules (like starch or protein). They simply cannot fit through the door to enter the building (the bloodstream). Chemical digestion is like a team of specialists breaking that crowd down into individual people (small, soluble molecules) who can easily walk through the door.
📖 Definitions
🔍 Worked example — Easy
A student measures the pH of the stomach and finds it is 2.0. Explain why this acidic environment is necessary for protein digestion in the stomach.
🔍 Worked example — Standard
Compare the digestion of starch in the mouth and the small intestine.
🔍 Worked example — Exam-style
A sample of food containing protein is moved from the stomach to the small intestine. Calculate the change in pH if the stomach pH is 2 and the small intestine pH is 8.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the pH changes from 2 to 8 as food moves from the stomach to the small intestine. [2] (b) Describe the effect of a pH of 7 on the activity of pepsin. [1] [3 marks]
Key ideas — Bile and pH in the small intestine
🧑🏫 Teacher's explanation
Imagine you are trying to clean a greasy pan. If you pour a strong acid on it, it might not do much for the grease. But if you add a specialized alkaline detergent, the grease breaks into tiny droplets that are easy to wash away. In your body, the stomach is like that acidic pan—it's very harsh and low in pH. However, the enzymes in your small intestine are like delicate soap molecules; they can't work in acid; they need a neutral or slightly alkaline environment to function.
📖 Definitions
🔍 Worked example — Easy
A student observes that the pH in the stomach is approximately 2.0, while the pH in the duodenum (start of small intestine) is approximately 8.0. Calculate the change in pH.
🔍 Worked example — Standard
Explain why the presence of bile is essential for the efficient digestion of lipids in the small intestine.
🔍 Worked example — Exam-style
An experiment shows that when gastric juice (pH 2.0) is mixed with pancreatic lipase, no fat is digested. However, when bile is added to the mixture, fat digestion occurs rapidly. Interpret this data.
⚠️ Common errors
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🎯 Exam question
(a) Predict which tube will show the highest rate of fat digestion. [1] (b) Explain your answer to part (a) using the concept of enzyme action. [2] [3 marks]
Worked example
Question. (Supplement) Describe the chemical digestion of a meal containing starch and protein as it passes through the alimentary canal. [6]
🔍 Worked example — Easy
Identify the enzyme and the substrate for the following reaction occurring in the mouth: Starch arrow Maltose.
🔍 Worked example — Standard
A student measures the pH of the stomach and finds it to be 2.0. Explain the role of hydrochloric acid in this environment regarding enzyme activity and protection.
🔍 Worked example — Exam-style
(Supplement) Describe the chemical digestion of a meal containing starch and protein as it passes through the alimentary canal. [6]
⚠️ Common errors
Common mistakes
Practice
Answers Breakdown of large, insoluble molecules into small, soluble molecules by enzymes [B1]; large molecules cannot pass through the intestine wall [B1]; small soluble molecules can be absorbed into the blood [B1]. Pancreas [B1]; fatty acids [B1]; glycerol [B1]; small intestine (duodenum) [B1]. Kills harmful microorganisms in food [B1]; provides the acidic pH that is optimum for protease (pepsin) [B1]. Amylase from the salivary glands acts in the mouth, breaking starch into maltose [B1]; pancreatic amylase continues this in the duodenum [B1]; maltase on the membranes of the small-intestine epithelial cells [B1] breaks maltose into glucose, which is absorbed [B1]. Bile neutralises the acid, making the duodenum alkaline [B1]; pepsin has an optimum pH of about 2 and is denatured in alkaline conditions [B1]; trypsin has an alkaline optimum pH, so it is active there [B1].