# Diffusion, Osmosis and Active Transport

> Biology · CIE IGCSE 0610
> Source: https://www.owlsprep.com/study/cie-0610-u2-diffusion-osmosis-and-active-transport/

This guide covers the three core cell transport mechanisms for CIE IGCSE Biology 0610 Unit 2, including definitions, biological examples, and tier-specific exam guidance for Core and Extended learners. You will learn to distinguish between the processes and answer structured exam questions correctly.

**Prerequisites:** [Basic cell structure and cell membrane function](https://www.owlsprep.com/study/cie-0610-u2-cell-structure/)

## Learning objectives

- Define diffusion, osmosis and active transport with biological examples
- Distinguish between passive (diffusion/osmosis) and active transport processes
- Explain osmosis effects on cells (Core: basic, Extended: water potential, turgidity, plasmolysis)
- Apply knowledge to answer structured exam questions for both tiers
- Avoid common exam pitfalls for this topic

## Diffusion (Core Content)

**Diffusion** — The net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.

*Example:* Oxygen diffusing from lung alveoli into red blood cells for transport to respiring tissues.

Diffusion is a passive process, meaning it does not require any energy input from the cell. It drives essential biological processes including gas exchange in lungs and leaves, and movement of small molecules across cell membranes. Factors that increase diffusion rate include higher temperature, larger surface area, steeper concentration gradient, and shorter diffusion distance.

**Worked example:** Carbon dioxide is produced as a waste product in respiring muscle cells. Explain how carbon dioxide leaves these cells using your knowledge of diffusion.

1. Step 1: Compare concentrations of carbon dioxide
2. The concentration of carbon dioxide is higher inside the respiring muscle cell than in the surrounding blood plasma.
3. Step 2: Describe diffusion movement
4. Carbon dioxide particles move down the concentration gradient, out of the muscle cell and into the blood plasma via diffusion.
5. Step 3: Link to biological function
6. This removes toxic waste carbon dioxide from the cell, preventing enzyme damage and cell death.

> **Exam tip:** Always include the phrase 'down a concentration gradient' in your diffusion definition to get full marks in exam answers.

## Osmosis (Core Content)

**Osmosis (Core definition)** — The net movement of water molecules from a region of higher water concentration to a region of lower water concentration, through a partially permeable membrane.

*Example:* Water uptake from soil into plant root hair cells via osmosis.

Osmosis is a special case of diffusion that only applies to water molecules moving across a partially permeable membrane. Like diffusion, it is a passive process with no energy requirement. Partially permeable membranes allow small molecules like water to pass through freely, but block larger solute molecules such as sugar, salt and protein.

**Worked example:** A student places a cube of potato into a beaker of pure water. After 30 minutes, the potato cube has increased in mass and feels firmer. Explain this observation (Core tier answer).

1. Step 1: Compare water concentrations inside and outside the potato cells
2. Pure water has a higher water concentration than the liquid inside potato cell vacuoles, which contains dissolved sugars and salts.
3. Step 2: Describe osmosis movement
4. Water moves into the potato cells through their partially permeable cell membranes via osmosis.
5. Step 3: Link to observations
6. The extra water inside the cells increases the mass of the potato cube and makes it firmer.

> **info**
>
> Extended tier learners will add details about water potential, turgidity and plasmolysis to osmosis answers, covered in the dedicated Extended section below.

> **Exam tip:** Never forget to mention the partially permeable membrane in your osmosis definition: this is a mandatory mark point for all exam answers.

## Active Transport (Core Content)

**Active Transport** — The movement of particles through a cell membrane from a region of their lower concentration to a region of their higher concentration, against a concentration gradient, using energy released from respiration.

*Example:* Uptake of mineral ions from low concentration soil into high concentration plant root hair cells.

Unlike diffusion and osmosis, active transport is an active process that requires energy input from the cell, produced via aerobic respiration. Cells that carry out large amounts of active transport (such as root hair cells and gut lining cells) have high numbers of mitochondria to produce the required energy.

**Worked example:** The cells lining the human small intestine absorb glucose from the gut cavity into the bloodstream, even when the concentration of glucose is higher in the cells than in the gut cavity. Name the process used and explain why it requires energy.

1. Step 1: Name the correct process
2. The process is active transport.
3. Step 2: Link to concentration gradient
4. Glucose is being absorbed against the concentration gradient (from lower concentration in the gut cavity to higher concentration inside the intestinal lining cells).
5. Step 3: Explain energy requirement
6. Passive processes cannot move particles against a concentration gradient, so energy released from respiration in the intestinal cells is needed to transport glucose across the membrane.

> **Exam tip:** If an exam question mentions movement against a concentration gradient or energy use from respiration, the process is always active transport for IGCSE Biology 0610.

## Extended Only: Water Potential, Turgidity and Plasmolysis

**Water Potential** — A measure of the tendency of water molecules to move from one location to another. Pure water has the highest possible water potential; adding dissolved solutes lowers water potential.

**Turgid** — Describes a plant cell that has absorbed maximum water via osmosis: the cytoplasm pushes against the rigid cell wall, making the cell firm. Turgidity provides structural support for plant stems and leaves.

**Plasmolysis** — Describes a plant cell that has lost large amounts of water via osmosis: the vacuole shrinks and the cytoplasm pulls away from the cell wall. Plasmolysis occurs when plant cells are placed in a solution with very low water potential (high solute concentration).

For Extended tier answers, replace references to water concentration with water potential, and include descriptions of turgidity or plasmolysis when describing osmosis effects on plant cells. Animal cells do not have cell walls, so they cannot undergo plasmolysis: they shrink in concentrated solutions and burst in pure water.

**Worked example:** A red onion cell is placed in a concentrated sucrose solution. Describe and explain what happens to the cell (Extended tier answer).

1. Step 1: Compare water potentials
2. The concentrated sucrose solution has a lower water potential than the cytoplasm and vacuole of the red onion cell.
3. Step 2: Describe osmosis movement
4. Water moves out of the cell down the water potential gradient through the partially permeable cell membrane via osmosis.
5. Step 3: Describe plasmolysis
6. The vacuole shrinks, and the cytoplasm pulls away from the rigid cell wall. This process is called plasmolysis, and the cell is described as plasmolysed.

> **Exam tip:** Never use the terms turgid or plasmolysed to describe animal cells: this is a common mistake that loses marks in Extended exams.

## Common pitfalls

- **Wrong:** Describing osmosis as movement of any substance across a membrane
  - Why it fails: Osmosis only refers to the movement of WATER molecules, not solute particles
  - Correct: Always specify osmosis involves net movement of water molecules in exam answers.
- **Wrong:** Stating that diffusion or osmosis requires energy
  - Why it fails: Diffusion and osmosis are passive processes that rely on random particle movement, with no energy input from the cell
  - Correct: Only active transport requires energy from respiration for IGCSE 0610 exams.
- **Wrong:** Confusing concentration gradient direction for transport processes
  - Why it fails: Diffusion and osmosis move DOWN concentration gradients, while active transport moves AGAINST concentration gradients
  - Correct: If movement is from low to high concentration, the process is active transport; otherwise it is diffusion or osmosis.
- **Wrong:** Forgetting to mention the partially permeable membrane in osmosis definitions
  - Why it fails: Osmosis only occurs across a partially permeable membrane, so this is a mandatory mark point
  - Correct: Always include 'through a partially permeable membrane' in your osmosis definition to get full marks.
- **Wrong:** Describing plasmolysis or turgidity in animal cells (Extended)
  - Why it fails: Animal cells lack a rigid cell wall, so they cannot undergo plasmolysis or become turgid
  - Correct: Only use turgid and plasmolysed to describe plant cells; use shrink or burst for animal cell osmosis effects.

## Cheatsheet

| Process | Concentration Gradient | Energy Required? | Core Example | Extended Key Terms |
| --- | --- | --- | --- | --- |
| Diffusion | Down (high → low) | No | Oxygen into red blood cells | N/A |
| Osmosis | Down (high → low water concentration/potential) | No | Water uptake by root hair cells | Water potential, turgid, plasmolysed |
| Active Transport | Against (low → high) | Yes (from respiration) | Mineral ion uptake by root hair cells | N/A |

## What's next

Now that you have mastered the three core cell transport mechanisms, you can apply this knowledge to related topics in the CIE IGCSE Biology 0610 syllabus. Next, you will learn about specialised cells (including root hair cells and red blood cells) that are adapted for efficient transport of substances. You will also cover the required practical investigation of osmosis in potato tissue, which is a common 5-6 mark structured question in both Core and Extended papers. Finally, you will build on this knowledge when you study gas exchange in humans and plants later in the course, where diffusion plays a central role in gas exchange systems.

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