Biology • Year 11 • Module 2 • Lesson 11
The Mammalian Digestive System
Lock in the core vocabulary, the physical-vs-chemical digestion distinction, and the enzyme reference table for every region of the gut.
1. Complete the digestive pathway table
Fill in the missing information for each region of the digestive system. Use the lesson's digestive pathway diagram and organ descriptions. 16 marks
| Organ | Physical digestion | Enzyme(s) / secretion, product | pH range | Key function |
|---|---|---|---|---|
| Mouth | 6.5–7.0 | |||
| Oesophagus | ||||
| Stomach | ||||
| Small intestine | ||||
| Large intestine |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: digestion, physical digestion, chemical digestion, enzyme, substrate, hydrolysis, peristalsis, emulsification, pH optimum, chyme. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | The specific pH at which an enzyme functions at its maximum rate, determined by the shape of its active site. | |
| 2.2 | A biological catalyst that speeds up a specific chemical reaction without being consumed in the process. | |
| 2.3 | Rhythmic waves of muscular contraction in the wall of the digestive tract that propel food along the gut. | |
| 2.4 | A chemical reaction in which water is used to break a bond, splitting a large molecule into smaller components. | |
| 2.5 | The process of breaking down food into small, soluble molecules that can be absorbed into the body. | |
| 2.6 | The specific molecule that an enzyme binds to and acts upon at its active site. | |
| 2.7 | The semi-liquid paste formed when the stomach churns food together with gastric acid and pepsin. | |
| 2.8 | Mechanical breakdown of food into smaller pieces without changing the chemical structure of molecules. | |
| 2.9 | The physical breakdown of large fat globules into tiny droplets by bile salts, increasing surface area for lipase action. | |
| 2.10 | Enzyme-catalysed hydrolysis that breaks chemical bonds within large food molecules, converting polymers into monomers. |
3. True or false, with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 10 marks (1 for T/F, 1 for the correction where needed)
3.1 Bile is a digestive enzyme that chemically breaks down fat molecules in the small intestine. T / F
3.2 Salivary amylase begins the chemical digestion of starch in the mouth, producing maltose. T / F
3.3 The stomach secretes HCl, which directly digests proteins into amino acids. T / F
3.4 Fat digestion can begin in the mouth because salivary lipase is present in humans. T / F
3.5 The pancreas secretes sodium bicarbonate into the duodenum to neutralise acidic chyme from the stomach. T / F
4. Complete the enzyme reference table
Fill in the missing columns. All enzymes are from the lesson's Complete Enzyme Reference card. 16 marks
| Enzyme | Location active | Substrate | Product(s) | pH optimum |
|---|---|---|---|---|
| Salivary amylase | Starch | ~7.0 | ||
| Pepsin | Stomach | Polypeptides | ||
| Small intestine | Starch | Maltose | ~7.0 | |
| Pancreatic lipase | Fatty acids + glycerol | ~7.5 | ||
| Trypsin / chymotrypsin | Small intestine | Polypeptides | ||
| Maltase | Glucose + glucose | ~7.0 | ||
| Lactase | Small intestine | Lactose | ||
| Peptidases | Amino acids | ~7.5 |
5. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
5.1 What is the role of physical digestion in supporting chemical digestion? Refer to surface area in your answer.
5.2 Why is pepsin secreted in the inactive form (pepsinogen) rather than as active pepsin?
5.3 At what point in the digestive pathway does carbohydrate digestion start, and where is it completed?
5.4 Why does fat digestion only begin in the small intestine, unlike carbohydrate and protein digestion?
Q1, Digestive pathway table
Mouth: Physical, mastication (chewing) by teeth. Enzyme, salivary amylase: starch → maltose. Key function, initial physical and chemical processing; food moistened and lubricated for swallowing.
Oesophagus: Physical, peristalsis only. Enzyme, none. Key function, transport of bolus from mouth to stomach.
Stomach: Physical, churning (producing chyme). Enzyme, pepsin: proteins → polypeptides; HCl secreted to activate pepsin and kill bacteria. pH 1.5–3.5. Key function, protein digestion begins; acid denaturation.
Small intestine: Physical, bile emulsification of fats. Enzymes, pancreatic amylase (starch → maltose), pancreatic lipase (triglycerides → fatty acids + glycerol), trypsin/chymotrypsin (polypeptides → shorter peptides), maltase/sucrase/lactase (disaccharides → monosaccharides), peptidases (peptides → amino acids). pH 7.0–8.5. Key function, majority of chemical digestion completed; nutrient absorption site.
Large intestine: Physical, water and electrolyte reabsorption. Enzyme, none (bacterial fermentation only). Key function, water reabsorption; faeces formation; vitamin production by bacteria.
Q2, Term–definition matches
2.1 pH optimum • 2.2 enzyme • 2.3 peristalsis • 2.4 hydrolysis • 2.5 digestion • 2.6 substrate • 2.7 chyme • 2.8 physical digestion • 2.9 emulsification • 2.10 chemical digestion.
Q3, True / false with correction
3.1 False. Correction: bile is NOT an enzyme, bile salts are detergent-like molecules that physically emulsify fat globules into tiny droplets (emulsification). No chemical bonds in fat molecules are broken by bile. Pancreatic lipase is the enzyme that chemically digests fats.
3.2 True. Salivary amylase is produced by the salivary glands in the mouth and catalyses the hydrolysis of starch into maltose (a disaccharide).
3.3 False. Correction: HCl does not directly digest proteins. HCl creates the acidic environment (pH ~2) that activates pepsinogen to the active enzyme pepsin, which then digests proteins to polypeptides. HCl also kills pathogens but is not itself a protease.
3.4 False. Correction: according to this lesson's content, fat digestion occurs in the small intestine. Bile emulsification is required before lipase can act, and both bile (from the liver) and pancreatic lipase are delivered to the duodenum. (Note: trace salivary lipase exists in infants but is not the primary route in adult digestion and is not covered at this level.)
3.5 True. The pancreas secretes sodium bicarbonate (NaHCO3) into the duodenum, which neutralises acidic chyme and raises pH from ~2 to ~7.5, creating the alkaline environment needed for pancreatic enzyme function.
Q4, Enzyme reference table
Salivary amylase: Location, Mouth. Product, Maltose.
Pepsin: Substrate, Proteins. pH optimum, ~2.0.
Pancreatic amylase: (missing enzyme name). Location, Small intestine.
Pancreatic lipase: Location, Small intestine. Substrate, Triglycerides.
Trypsin / chymotrypsin: Product, Shorter peptides. pH optimum, ~8.0.
Maltase: Location, Small intestine. Substrate, Maltose.
Lactase: Product, Glucose + galactose. pH optimum, ~6.0.
Peptidases: Location, Small intestine. Substrate, Short peptides.
Q5.1, Physical digestion and surface area
Physical digestion breaks food into smaller pieces, dramatically increasing the total surface area of food exposed to digestive enzymes. Because enzyme reactions can only occur at the surface of substrate molecules, a greater surface area means more simultaneous enzyme-substrate contacts, increasing the rate of chemical digestion. A 1 cm cube of food has 6 cm² of surface area; cutting it into 8 equal pieces doubles this to 12 cm².
Q5.2, Why pepsin is secreted inactive
Pepsin is secreted as inactive pepsinogen to prevent it from digesting the gastric gland cells that produce it, those cells are themselves made of protein. If pepsin were secreted in its active form, it would digest the cells of the stomach wall. Activation to pepsin only occurs in the acidic stomach lumen when HCl is present, safely away from the secreting cells.
Q5.3, Carbohydrate digestion start and completion points
Carbohydrate digestion starts in the mouth, where salivary amylase cleaves starch into maltose. It is completed in the small intestine, where pancreatic amylase continues starch hydrolysis and intestinal maltase, sucrase, and lactase convert disaccharides (maltose, sucrose, lactose) into absorbable monosaccharides (glucose, fructose, galactose).
Q5.4, Why fat digestion only begins in the small intestine
Fat digestion requires two things that only occur in the small intestine: (1) bile emulsification, bile salts from the liver break large fat globules into tiny droplets, vastly increasing the surface area accessible to lipase; (2) an alkaline pH of ~7.5, the conditions under which pancreatic lipase is active. In the mouth there is no lipase of significance, and in the stomach the extremely low pH (~2) would denature lipase.