CIE iGCSE Co-ordinated Sciences-B7.3 Digestion- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-B7.3 Digestion – Study Notes
CIE iGCSE Co-ordinated Sciences-B7.3 Digestion – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe physical digestion as the breakdown of food into smaller pieces without chemical change to the food molecules
- State that physical digestion increases the surface area of food for the action of enzymes in chemical digestion
- Describe chemical digestion as the breakdown of large insoluble molecules into small soluble molecules
- State the role of chemical digestion in producing small soluble molecules that can be absorbed
- Describe the functions of enzymes as follows:
(a) amylase breaks down starch to simple reducing sugars
(b) proteases break down protein to amino acids
(c) lipase breaks down fats and oils to fatty acids and glycerol - State where, in the digestive system, amylase, protease and lipase are secreted and where they act
- Describe the functions of hydrochloric acid in gastric juice, limited to killing harmful microorganisms in food and providing an acidic pH for optimum enzyme activity of proteases in the stomach
- Explain that bile is an alkaline mixture that neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action in the small intestine
- Outline the role of bile in emulsifying fats and oils to increase the surface area for chemical digestion
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Physical Digestion
Definition
Physical (or mechanical) digestion is the breakdown of food into smaller pieces without any chemical change to the food molecules.
📝 Key Points
Purpose:
- Increases the surface area of food so that enzymes can act more efficiently during chemical digestion.
- Makes food easier to swallow and move along the alimentary canal.
Where it happens:
- Mouth
Teeth chew and grind food (mastication).
Tongue mixes food with saliva to form a bolus. - Stomach
Muscles contract and relax (peristalsis), churning food into chyme (semi-liquid). - Bile in duodenum
Breaks fat into tiny droplets (emulsification) – increases surface area for lipase to act.
⚡ Quick Tip
Physical = size change, no chemical change
Chemical = molecules broken down into smaller soluble units (like starch → glucose, protein → amino acids, fats → glycerol + fatty acids).
Physical Digestion and Its Role in Enzyme Action
📌 Definition
Physical (mechanical) digestion is the breaking down of food into smaller pieces without changing its chemical structure.
🌱 How It Works
- Mouth (Chewing / Mastication):
Teeth tear, crush, and grind food.
Food mixes with saliva, forming a soft bolus.
Increases surface area for enzymes later in the gut. - Stomach (Churning / Peristalsis):
Muscles in the stomach contract and relax, mixing food with gastric juices.
Produces a semi-liquid mixture called chyme.
Exposes more food surface to enzymes.
⚡ Importance for Chemical Digestion
- More surface area → faster enzyme action
- Enzymes like:
Amylase → starch → glucose
Protease → proteins → amino acids
Lipase → fats → fatty acids + glycerol - Ensures food is broken down efficiently into soluble molecules for absorption in the small intestine.
💡 Quick Tip
Physical digestion = teeth + stomach muscles
Purpose = increase surface area → faster chemical digestion
Chemical Digestion
📌 Introduction
Chemical digestion is the breakdown of large, insoluble food molecules into small, water-soluble molecules. This process allows nutrients to be absorbed through the gut wall and used by the body.
🌱 Key Points
- Definition: Chemical digestion = enzymatic breakdown of large insoluble molecules into small soluble molecules.
- Purpose: Makes food molecules small enough to be absorbed into the blood.
- Where it happens:
Mouth: Salivary amylase begins starch → maltose
Stomach: Pepsin breaks proteins → amino acids
Small intestine (duodenum & ileum):
Pancreatic enzymes digest starch, protein, and fats
Bile emulsifies fats → increases surface area for lipase
🍴 Process & Examples
| Food Type | Enzyme/Agent | Product (Soluble) | Location |
|---|---|---|---|
| Starch | Salivary amylase, Pancreatic amylase | Maltose → Glucose | Mouth, Small intestine |
| Protein | Pepsin, Trypsin | Amino acids | Stomach, Small intestine |
| Fats | Lipase (with bile salts) | Glycerol + Fatty acids | Small intestine |
Enzymes act on the large molecules to produce small, soluble molecules:
- Proteins → amino acids
- Starch → glucose
- Fats → glycerol + fatty acids
Soluble molecules can pass through the intestinal wall into the bloodstream for use by cells.
💡 Quick Tip
“Chemical digestion dissolves food into small molecules → ready for absorption!”
Role of Chemical Digestion
📌 Introduction
Chemical digestion is essential because most food is insoluble in its original form and cannot pass into the blood. Enzymes break these large molecules into small, soluble molecules that the body can absorb and use.
🌱 Key Role
- Breakdown of large molecules:
Proteins → amino acids
Carbohydrates (starch) → glucose
Fats (lipids) → glycerol + fatty acids - Solubility: Small molecules are water-soluble, allowing them to pass through the intestinal lining into the blood.
- Absorption: Only after chemical digestion are nutrients ready for absorption into the bloodstream and transport to cells.
💡 Quick Tip
“Chemical digestion → small soluble molecules → absorbed by blood → used by cells.”
Functions of Digestive Enzymes
📌 Introduction
Enzymes are biological catalysts that speed up chemical reactions. In digestion, they break large insoluble food molecules into small, soluble molecules that can be absorbed and used by the body.
🌱 Key Digestive Enzymes
| Enzyme | Substrate (Food) | Product | Function |
|---|---|---|---|
| Amylase | Starch | Simple reducing sugars (e.g., maltose, glucose) | Breaks down starch to sugars for absorption. Found in saliva and pancreatic juice. |
| Proteases (e.g., pepsin, trypsin) | Proteins | Amino acids | Breaks proteins into amino acids for building new cells and tissues. Found in stomach, pancreas, small intestine. |
| Lipase | Fats and oils (lipids) | Fatty acids + glycerol | Breaks fats into smaller soluble molecules for absorption. Works best after emulsification by bile. Found in pancreatic juice. |
💡 Quick Tip
Amylase → starch → sugars
Protease → proteins → amino acids
Lipase → fats → fatty acids + glycerol
All these enzymes ensure nutrients are small, soluble, and ready for absorption.
Digestive Enzymes: Site of Secretion and Action
📌 Introduction
Digestive enzymes are secreted by different glands and organs in the digestive system. Each enzyme has a specific location where it is produced and a specific place where it acts to break down food.
🌱 Enzymes, Secretion & Action
| Enzyme | Secreted By | Acts In | Substrate → Product |
|---|---|---|---|
| Amylase | – Salivary glands (saliva) – Pancreas (pancreatic juice) | – Mouth (saliva) – Duodenum (pancreatic juice) | Starch → Maltose / Glucose |
| Proteases (e.g., pepsin, trypsin) | – Stomach lining (gastric juice, pepsin) – Pancreas (pancreatic juice, trypsin) | – Stomach (pepsin) – Duodenum (trypsin) | Proteins → Amino acids |
| Lipase | Pancreas (pancreatic juice) | Duodenum (after emulsification by bile) | Fats & oils → Fatty acids + Glycerol |
💡 Notes for Easy Memorization
Amylase: Mouth → starch begins digestion, pancreas → continues in duodenum.
Protease: Stomach → starts protein digestion, pancreas → continues in small intestine.
Lipase: Only acts in small intestine, after bile emulsifies fat.
Key point: Enzymes act only in the location suitable for their optimal conditions – e.g., pepsin works best in acidic stomach, pancreatic enzymes in slightly alkaline duodenum.
Hydrochloric Acid (HCl) in Gastric Juice
📌 Introduction
Hydrochloric acid is a key component of gastric juice in the stomach. It is secreted by parietal cells in the stomach lining and plays a critical role in digestion and protection.
🌟 Functions of Hydrochloric Acid
- Kills Harmful Microorganisms:
HCl creates a strongly acidic environment (pH ~1.5–2) in the stomach.
Most bacteria and pathogens in food cannot survive this acidic condition.
This protects the body from infections and foodborne illnesses. - Provides Acidic pH for Protease Activity:
The enzyme pepsin (a protease) requires an acidic environment to work efficiently.
HCl activates pepsinogen (inactive form) into pepsin (active form).
Pepsin then breaks down proteins → amino acids, facilitating protein digestion.
💡 Quick Tip
HCl = “Microbe killer + Protease helper”
Without HCl:
– Food may carry harmful microbes into the intestines.
– Pepsin cannot digest proteins effectively.
Role of Bile in Digestion
📌 Introduction
Bile is a greenish, alkaline fluid secreted by the liver and stored in the gall bladder. It is released into the duodenum to aid digestion, particularly of fats.
🌟 Functions of Bile
- Neutralises Acidic Chyme:
Food leaving the stomach is mixed with gastric juices, making it highly acidic.
Bile contains sodium hydrogencarbonate, which neutralises the acidity.
This raises the pH to a level suitable for the enzymes in the small intestine to function properly. - Provides Suitable pH for Enzymes:
Enzymes in the duodenum and small intestine (like pancreatic amylase, protease, and lipase) require a slightly alkaline environment.
By neutralising acid, bile ensures enzymes work efficiently, allowing proper digestion of carbohydrates, proteins, and fats.
💡 Quick Tip
Think of bile as: “Body’s natural antacid” + enzyme helper
Without bile:
– Enzymes in the small intestine would be less effective.
– Digestion of food, especially fats, would be slower or incomplete.
Role of Bile in Emulsifying Fats
📌 Introduction
Bile is a green, alkaline fluid produced by the liver and stored in the gall bladder. One of its key roles in digestion is to emulsify fats.
🌟 Emulsification of Fats
- What is Emulsification?
Breaking large fat droplets into tiny droplets without chemical change.
This is a physical process, not chemical digestion. - How Bile Helps:
Bile contains bile salts, which surround large fat droplets.
They break the fat into smaller droplets, increasing the total surface area available. - Why Emulsification is Important:
Lipase (the enzyme that digests fats) works only at the surface of fat droplets.
By increasing surface area, bile speeds up fat digestion into fatty acids and glycerol.
💡 Quick Tip
Bile → Emulsifies fats → bigger surface area → faster digestion by lipase
Remember: Physical breakdown, not chemical digestion.