CIE iGCSE Biology-7.4 Chemical digestion- Study Notes- New Syllabus
CIE iGCSE Biology-7.4 Chemical digestion- Study Notes – New syllabus
CIE iGCSE Biology-7.4 Chemical digestion- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- 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 providing an acidic pH for optimum enzyme activity
Supplement
- Describe the digestion of starch in the digestive system:
(a) amylase breaks down starch to maltose
(b) maltase breaks down maltose to glucose on the membranes of the epithelium lining the small intestine - Describe the digestion of protein by proteases in the digestive system:
(a) pepsin breaks down protein in the acidic conditions of the stomach
(b) trypsin breaks down protein in the alkaline conditions of the small intestine - 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
Chemical Digestion: Definition and Importance
What is Chemical Digestion?
- Chemical digestion is the process of breaking down large, insoluble food molecules into small, soluble molecules using enzymes.
- These smaller molecules can then be absorbed into the blood and used by the body.
- It occurs after physical digestion, which only breaks food into smaller pieces without changing its chemical structure.
🧪 How Does It Work?
Special biological catalysts called enzymes are involved.
These enzymes target specific nutrients (like carbohydrates, proteins, and fats) and break their chemical bonds.
Why Is Chemical Digestion Important?
- Most food molecules are too large to pass through the wall of the small intestine.
- Chemical digestion converts them into smaller, soluble molecules, which can be absorbed into the bloodstream and used by body cells.
Examples of Chemical Digestion
| Nutrient | Enzyme | End Product | Soluble? |
|---|---|---|---|
| Starch | Amylase | Glucose (a sugar) | Yes |
| Proteins | Protease | Amino acids | Yes |
| Fats & Oils | Lipase + Bile | Fatty acids + Glycerol | Yes |
🔍 Key Points to Remember:
Chemical digestion changes the food molecules chemically.
It occurs mainly in the stomach and small intestine.
It makes nutrients small and soluble, ready for absorption into the blood.
🧠 Quick Recap:
Chemical digestion is the enzyme-driven process that breaks large, complex food molecules into smaller, soluble forms the body can use.
Role of Chemical Digestion in Producing Small Soluble Molecules
🧠 What is Chemical Digestion?
- Chemical digestion is the process in which enzymes break down large, complex, and insoluble food molecules into small, simple, and soluble molecules.
- This happens mostly in the mouth, stomach, and small intestine.
- It is essential because only small soluble molecules can be absorbed into the bloodstream and used by the body.
🔎 Why Are Small Soluble Molecules Needed?
| Original Molecule | Broken Down into | Use in the Body |
|---|---|---|
| Starch (complex carbohydrate) | Glucose (simple sugar) | Provides energy |
| Proteins | Amino acids | For growth and repair |
| Fats and oils | Fatty acids + Glycerol | Stored energy, insulation, cell membranes |
⚙️ How Does Chemical Digestion Happen?
Enzymes are biological catalysts that speed up reactions without being used up.
Each enzyme works on a specific type of food molecule:
- Amylase breaks down starch to glucose
- Protease breaks down proteins to amino acids
- Lipase breaks down fats to fatty acids and glycerol
Key Points Summary:
Chemical digestion happens using enzymes.
It breaks large insoluble molecules into small soluble ones.
These small molecules are absorbed through the small intestine into the blood.
Once absorbed, they are used for:
- Energy production
- Tissue growth and repair
- Storage (in some cases)
Functions of Enzymes in Digestion
🧪 What Are Enzymes?
- Enzymes are biological catalysts made by the body.
- They speed up chemical reactions, especially in digestion, without being used up themselves.
In digestion, enzymes help break down large, complex molecules into small, soluble molecules that can be absorbed into the blood.
(a) Amylase – Breaks Down Starch
- Function: Amylase breaks starch (a complex carbohydrate) into simple reducing sugars, like maltose and glucose.
- Why Important? The body needs glucose for energy.
- Where It Works: Starts in the mouth (salivary amylase), continues in the small intestine (pancreatic amylase).
- 🧠 Example: Starch → Amylase → Glucose
(b) Proteases – Break Down Proteins
- Function: Protease enzymes break proteins into amino acids.
- Why Important? Amino acids are used for:
- Building cells
- Repairing tissues
- Making enzymes and hormones
- Where It Works: In the stomach (pepsin), in the small intestine (trypsin, from pancreas).
- 🧠 Example: Protein → Protease → Amino Acids
(c) Lipase – Breaks Down Fats and Oils
- Function: Lipase breaks fats and oils (lipids) into fatty acids and glycerol.
- Why Important?
- Fatty acids = energy source
- Glycerol = used in cells and metabolism
- Where It Works: In the small intestine (secreted by pancreas).
- 🧠 Example: Fat/Oil → Lipase → Fatty Acids + Glycerol
🧾 Summary Table
| Enzyme | Substrate | End Product | Location of Action |
|---|---|---|---|
| Amylase | Starch | Simple sugars (e.g. glucose) | Mouth, Small Intestine |
| Protease | Protein | Amino acids | Stomach, Small Intestine |
| Lipase | Fat/oil | Fatty acids + Glycerol | Small Intestine |
Where Enzymes Are Secreted and Where They Act in the Digestive System
🍞 1. Amylase
Function: Breaks down starch into simple sugars (e.g. maltose, glucose)
- Where It Is Secreted:
- Salivary glands – produce salivary amylase
- Pancreas – produces pancreatic amylase
- Where It Acts:
- Mouth – salivary amylase begins starch digestion
- Small intestine – pancreatic amylase continues digestion
🍗 2. Protease
Function: Breaks down proteins into amino acids
- Where It Is Secreted:
- Stomach – produces pepsin
- Pancreas – produces trypsin and other proteases
- Where It Acts:
- Stomach – pepsin acts in acidic pH
- Small intestine – trypsin acts in slightly alkaline pH
🧈 3. Lipase
Function: Breaks down fats and oils into fatty acids and glycerol
- Where It Is Secreted:
- Pancreas
- Where It Acts:
- Small intestine – aided by bile from liver (which emulsifies fats)
| Enzyme | Secreted By | Acts In | Function |
|---|---|---|---|
| Amylase | Salivary glands, Pancreas | Mouth, Small Intestine | Starch → Simple sugars |
| Protease | Stomach (pepsin), Pancreas | Stomach, Small Intestine | Proteins → Amino acids |
| Lipase | Pancreas | Small Intestine | Fats/Oils → Fatty acids + Glycerol |
Functions of Hydrochloric Acid in Gastric Juice
Hydrochloric acid (HCl) is a strong acid found in the stomach, and it plays a very important role in digestion.
🔬 1. Provides an Acidic pH for Enzyme Activity
- HCl makes the stomach contents acidic (pH ~1.5 to 2).
- This acidic environment is necessary for the enzyme pepsin to work properly.
- Pepsin is responsible for breaking down proteins, but it only works well in acidic conditions.
- So, HCl helps enzymes digest food effectively.
🛡️ 2. Kills Harmful Microorganisms in Food
- Many germs and bacteria enter the stomach through the food we eat.
- The acidic environment created by HCl destroys most of these harmful microbes.
- This helps protect the body from infections and keeps the digestive system safe.
✅ Summary:
| Function of HCl | Why It’s Important |
|---|---|
| Makes stomach contents acidic | Helps digestive enzymes like pepsin work properly |
| Kills harmful microorganisms in food | Protects the body from infection |
Digestion of Starch in the Human Body
Starch is a large, insoluble carbohydrate found in foods like rice, bread, and potatoes. To be absorbed by the body, starch must be broken down into glucose, a small soluble sugar.
🧪 Step-by-Step Process of Starch Digestion:
(a) Amylase breaks down starch to maltose
- Where it starts: In the mouth and continues in the small intestine.
- Enzyme involved: Amylase
- Produced by:
- Salivary glands (in the mouth)
- Pancreas (released into the small intestine)
- What it does: Breaks long starch molecules into a simpler sugar called maltose (a disaccharide).
- Equation: Starch → (amylase) → Maltose
(b) Maltase breaks down maltose to glucose
- Where it happens: In the small intestine, specifically on the membranes of the epithelial cells lining the villi.
- Enzyme involved: Maltase
- This enzyme is found on: The surface of the villi, not secreted into the gut.
- What it does: Breaks down maltose into two glucose molecules.
- Equation: Maltose → (maltase) → Glucose
✅ Final Product: Glucose
Glucose is a small, soluble sugar. It can be absorbed into the blood through the walls of the small intestine. The body uses glucose for energy production through respiration.
🧠 Summary Table:
| Stage | Location | Enzyme | Substrate | Product |
|---|---|---|---|---|
| 1 | Mouth + small intestine | Amylase | Starch | Maltose |
| 2 | Small intestine (villi surface) | Maltase | Maltose | Glucose |
Digestion of Proteins in the Human Body
Proteins are large, complex molecules found in foods like meat, eggs, dairy, pulses, and nuts. To be absorbed, they must be broken down into amino acids — the building blocks of proteins.
🧪 Step-by-Step Process of Protein Digestion:
(a) Pepsin breaks down protein in the stomach
- Where: Stomach
- Enzyme: Pepsin
- Condition: Acidic pH (due to HCl)
- Action: Breaks proteins into smaller polypeptides
- Equation: Protein → (pepsin) → Polypeptides
(b) Trypsin breaks down protein in the small intestine
- Where: Small intestine (duodenum)
- Enzyme: Trypsin (from pancreas)
- Condition: Alkaline pH (neutralized by bile)
- Action: Converts polypeptides into amino acids
- Equation: Polypeptides → (trypsin) → Amino Acids
✅ Final Product: Amino Acids
Amino acids are small, soluble molecules absorbed through the small intestine into the blood. They are used for:
- Growth and tissue repair
- Making enzymes and hormones
🧠 Summary Table:
| Stage | Location | Enzyme | pH Condition | Substrate | Product |
|---|---|---|---|---|---|
| 1 | Stomach | Pepsin | Acidic | Protein | Polypeptides |
| 2 | Small Intestine | Trypsin | Alkaline | Polypeptides | Amino Acids |
Role of Bile in Neutralizing Stomach Acid
What is Bile?
- Bile is a yellow-green alkaline fluid made by the liver and stored in the gall bladder.
- It is released into the duodenum (first part of the small intestine) through the bile duct.
Why is Neutralization Important?
- Food from the stomach is acidic due to hydrochloric acid (HCl).
- Enzymes in the small intestine (e.g. trypsin and lipase) need an alkaline pH to work properly.
Function of Bile in Neutralization:
- Bile neutralizes the acidic mixture (chyme) entering the duodenum from the stomach.
- This raises the pH to around 7–8, creating ideal conditions for enzyme action in the small intestine.
Summary Points:
- Bile is alkaline.
- It neutralizes acidic stomach contents.
- It ensures a suitable pH for enzymes like lipase and trypsin.
Think of bile as a “buffering agent” – it cools down the strong acid from the stomach so the next set of enzymes can work comfortably.