CIE iGCSE Biology-3.1 Diffusion- Study Notes- New Syllabus
CIE iGCSE Biology-3.1 Diffusion- Study Notes – New syllabus
CIE iGCSE Biology-3.1 Diffusion- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.
- State that the energy for diffusion comes from the kinetic energy of random movement of molecules and ions.
- State that some substances move into and out of cells by diffusion through the cell membrane.
- Describe the importance of diffusion of gases and solutes in living organisms.
- Investigate the factors that influence diffusion, limited to: surface area, temperature, concentration gradient and distance.
Diffusion
🧠 What Is Diffusion?
Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, due to their random movement. This movement happens naturally and does not require energy from the cell.
Important Terms Explained:
- Net movement: Overall movement in one direction (more particles move one way than the other)
- Concentration: How many particles are packed in a space
- Concentration gradient: The difference in concentration between two areas
- Random movement: Particles move in all directions on their own
🧪 Where Does Diffusion Happen?
- In liquids and gases, where particles are free to move
- Across cell membranes in living organisms
🔄 Everyday & Biological Examples:
| Example | What’s Moving? | From → To |
|---|---|---|
| Perfume in a room | Scent particles | High (spray spot) → Low (rest of room) |
| Oxygen into blood (lungs) | Oxygen molecules (O₂) | Alveoli → Red blood cells |
| Carbon dioxide out of plant cells | CO₂ gas | Inside leaf → Outside air |
| Sugar spreading in tea | Sugar molecules | Concentrated spot → Evenly in liquid |
Key Features of Diffusion:
- Passive process – no ATP energy needed
- Happens due to kinetic energy (particles move randomly)
- Faster diffusion when:
- Temperature is higher (particles move faster)
- Concentration difference is bigger
- Distance is shorter (e.g., thin membranes)
- Surface area is larger
📌 Simple Definition to Remember:
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, due to their random movement.
Energy Source for Diffusion
🧠 Key Statement:
The energy for diffusion comes from the kinetic energy of the random movement of molecules and ions.
What Does This Mean?
- Diffusion is a passive process – it does not require energy from the cell (like ATP).
- Particles move on their own because they already have kinetic energy.
🔬 What Is Kinetic Energy?
Kinetic energy is the energy a particle has because it is moving.
In gases and liquids, all molecules and ions are constantly moving in random directions.
This motion is caused by thermal energy (heat) – particles naturally vibrate and collide with each other.
🔄 How Does Kinetic Energy Cause Diffusion?
- Random Movement: Molecules and ions move in all directions on their own, spreading out over time.
- Concentration Difference: More particles move from areas of high concentration to areas of low concentration – this is net movement.
- No Energy from the Cell: Cells don’t use ATP – diffusion happens naturally.
📌 Passive Process Reminder:
Because particles already move due to kinetic energy, no extra energy is needed from the cell.
📌 Key Terms Explained:
| Term | Meaning |
|---|---|
| Kinetic energy | Energy of motion – molecules and ions are always moving |
| Random movement | Particles move in all directions without a set path |
| Passive process | No energy (ATP) needed from the cell |
| Net movement | Overall flow of particles from more crowded to less crowded areas |
🧪 Real-Life Biological Examples:
| Where? | What’s Moving? | Driven by Kinetic Energy? |
|---|---|---|
| Lungs | Oxygen and carbon dioxide | Yes |
| Leaf of a plant | Carbon dioxide in and out | Yes |
| Kidney tubules | Urea out of cells | Yes |
| Air freshener in a room | Perfume particles | Yes |
Substances Moving In and Out of Cells by Diffusion
🧠 Key Statement:
- Some substances move into and out of cells by diffusion through the cell membrane.
- This means that small particles, like gases and small molecules, can pass freely across the membrane without energy, due to diffusion.
🔬 What Is the Cell Membrane?
The cell membrane is a thin barrier that surrounds the cell.
It is partially permeable (also called selectively permeable), meaning:
- It allows some substances to pass through, but not all.
- This helps control the movement of materials in and out of the cell.
🧪 Examples of Substances That Move by Diffusion:
| Substance | Direction | Why It Moves |
|---|---|---|
| Oxygen (O₂) | Into the cell | Needed for respiration |
| Carbon dioxide (CO₂) | Out of the cell | Waste gas produced by respiration |
| Urea | Out of the cell | A waste product from the breakdown of amino acids |
| Water (H₂O) | In or out (osmosis) | Moves to balance water concentration (special case of diffusion) |
📌 How It Works:
Particles outside or inside the cell are in constant motion.
- If there is a concentration gradient across the cell membrane:
- Substances move from high to low concentration.
- Small molecules like gases can pass directly through the membrane by diffusion.
This helps the cell to:
- Take in useful substances (e.g., oxygen)
- Get rid of waste (e.g., CO₂, urea)
Only small and non-charged molecules like O₂, CO₂, and urea can diffuse easily through the membrane.
🔎 Important Features of Diffusion Through Membranes:
| Feature | Explanation |
|---|---|
| Passive process | No energy needed from the cell (no ATP) |
| Small molecules | Only small particles (like O₂, CO₂, urea) can pass easily |
| Down concentration gradient | Movement is always from more to less concentrated area |
| Partially permeable | Membrane allows some molecules, blocks larger ones like proteins |
Summary Table:
| Key Point | Explanation |
|---|---|
| Cell membrane controls entry/exit | Acts like a gate – only some substances can pass |
| Diffusion moves particles in or out | Happens naturally, using kinetic energy |
| Only small substances diffuse easily | Gases like O₂ and CO₂, and small waste like urea |
| No energy required | It’s a passive process – cell doesn’t use ATP |
Importance of Diffusion of Gases and Solutes in Living Organisms
Diffusion is essential for life processes in both animals and plants. It ensures that gases and dissolved substances move where they are needed – without the use of energy simply by the natural movement of particles.
What Is Diffusion?
Diffusion is the net movement of particles (like molecules and ions) from a region of higher concentration to a region of lower concentration, due to their random movement. It is a passive process – it does not require energy.
Why Is Diffusion Important in Living Organisms?
- Take in essential substances (like oxygen and glucose)
- Remove waste products (like carbon dioxide and urea)
- Maintain internal balance (homeostasis)
🔬 Key Areas Where Diffusion Happens
1. Gas Exchange in Animals (e.g. Humans)
| Gas | Direction of Movement | Why It’s Important |
|---|---|---|
| Oxygen (O₂) | Lungs → Blood → Cells | Needed for aerobic respiration |
| Carbon Dioxide (CO₂) | Cells → Blood → Lungs | Waste gas – must be removed |
Oxygen diffuses into red blood cells across alveoli, while CO₂ diffuses out – both across thin membranes.
2. Gas Exchange in Plants
| Gas | Direction of Movement | Importance |
|---|---|---|
| Carbon Dioxide (CO₂) | Air → Stomata → Leaf Cells | Used in photosynthesis |
| Oxygen (O₂) | Leaf Cells → Stomata → Air | Waste product of photosynthesis |
This diffusion occurs through stomata, mainly on the underside of leaves.
3. Absorption of Solutes in the Digestive System
| Substance | Where It Moves | Importance |
|---|---|---|
| Glucose, Amino Acids | Small Intestine → Blood Capillaries | Used for energy and growth |
Nutrients diffuse into the bloodstream after digestion.
4. Removal of Waste Products (Excretion)
| Waste | Where It Diffuses | Why It Matters |
|---|---|---|
| Urea | Liver Cells → Blood → Kidneys | Toxic waste — must be removed in urine |
Diffusion helps safely remove urea from the body before it becomes harmful.
5. Osmosis – Special Type of Diffusion (Water)
Osmosis is the diffusion of water across a partially permeable membrane.
- Maintains water balance in cells
- Prevents animal cells from bursting or shrinking
- Keeps plant cells turgid and upright
📌 Summary Table: Why Diffusion Is Vital
| Biological Process | Substances Moved | Why It’s Important |
|---|---|---|
| Breathing / Gas Exchange | O₂ in, CO₂ out | Respiration & waste removal |
| Digestion / Absorption | Glucose, amino acids | Provide energy and building blocks |
| Excretion | Urea | Removes toxic substances |
| Photosynthesis (Plants) | CO₂ in, O₂ out | CO₂ needed; O₂ is waste |
| Water Balance (Osmosis) | H₂O | Maintains cell shape and function |
Final Statement:
Diffusion is essential for the survival of living organisms because it allows important substances like oxygen, carbon dioxide, glucose, amino acids, and urea to move into and out of cells naturally, supporting key life processes such as respiration, excretion, photosynthesis, and nutrient absorption.
Factors That Influence Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. This passive process plays a key role in transporting substances like oxygen, carbon dioxide, and glucose in living organisms. Several factors affect the rate of diffusion, making it faster or slower depending on the conditions.
1. Surface Area
A larger surface area allows more particles to pass through at once, increasing the rate of diffusion. Cells with structures like microvilli or alveoli are adapted to maximize surface area for faster exchange of gases or nutrients.
2. Temperature
Higher temperatures give particles more kinetic energy, causing them to move faster. This increases the speed at which particles spread out, so diffusion happens more quickly in warm conditions.
3. Concentration Gradient
The greater the difference in concentration between two regions, the faster the diffusion. A steep concentration gradient means particles move rapidly to balance the concentrations. Over time, as the gradient decreases, diffusion slows down.
Body temperature (~37°C) keeps diffusion efficient in humans especially in the lungs and intestines.
4. Distance
The shorter the distance particles have to travel, the faster they can diffuse. This is why cells are so small – a thin cell membrane reduces the diffusion distance, allowing for efficient exchange with the environment.
Example: Alveoli in lungs have very thin walls (one cell thick) to minimize diffusion distance for oxygen and carbon dioxide.
Summary Table
| Factor | Effect on Diffusion | Biological Example |
|---|---|---|
| Surface Area | ↑ Surface area = ↑ diffusion rate | Microvilli in intestines |
| Temperature | ↑ Temperature = ↑ particle speed | Warm blood helps gas exchange |
| Concentration Gradient | ↑ Gradient = faster diffusion | Oxygen moves into capillaries |
| Distance | ↓ Distance = faster diffusion | Thin alveolar membrane |