# Movement of substances into and out of cells

> Biology · Edexcel IGCSE 4BI1 (2017)
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s2-movement-of-substances-into-and/

This guide covers the three core cell transport processes, factors affecting transport rate, and required practical assessments for Edexcel IGCSE Biology 4BI1 specification sections 2.15–2.17.

**Prerequisites:** [Cell structure and function](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-cell-structure/); [Basic particle theory](https://www.owlsprep.com/study/edexcel-igcse-combined-science-p1-kinetic-theory/)

## Learning objectives

- Define diffusion, osmosis and active transport correctly per Edexcel marking schemes
- Explain how 4 key factors affect the rate of substance movement into/out of cells
- Interpret results from diffusion and osmosis practicals (living and non-living systems)
- Distinguish between passive (diffusion, osmosis) and active transport processes
- Avoid common marking scheme pitfalls for transport-related exam questions

## Core Cell Transport Processes (2.15)

**Diffusion** — Net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), no energy required (passive process)

**Osmosis** — Net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) across a partially permeable membrane, passive process

**Active Transport** — Movement of substances against a concentration gradient (from lower to higher concentration), requires energy released from respiration, uses carrier proteins in the cell membrane

> **warning**
>
> Marking scheme alert: Never refer to osmosis as just 'diffusion of water' — you must always mention the partially permeable membrane to get full marks.

**Worked example:** A student defines osmosis as 'the diffusion of water from a dilute solution to a concentrated solution'. State one missing element of this definition required for full marks.

1. Step 1: Recall Edexcel's mandatory requirements for osmosis definitions.
2. Step 2: Identify that the definition mentions water movement and concentration direction, but omits the critical partially permeable membrane requirement.
3. Answer: The definition does not state that osmosis occurs across a partially permeable membrane.

*Calculator:* allowed

## Factors Affecting Transport Rate (2.16)

Four key factors affect how quickly substances move into or out of cells, all assessed regularly in exam questions:

- **Concentration gradient steepness:** The steeper the difference in concentration between two regions, the faster the rate of diffusion and osmosis.
- **Temperature:** Higher temperatures give particles more kinetic energy, so they move faster, increasing transport rate.
- **Diffusion pathway distance:** The shorter the distance particles have to travel, the faster the rate of transport.
- **Surface area to volume ratio (SA:V):** Smaller cells/organisms have a larger surface area relative to their volume, so exchange of substances happens faster. Larger organisms require specialized exchange surfaces to compensate for their small SA:V.

**Worked example:** Explain why a mouse has a faster rate of gas exchange per gram of body mass than an elephant.

1. Step 1: Link organism size to surface area to volume ratio.
2. Step 2: A mouse is much smaller than an elephant, so it has a far higher surface area to volume ratio.
3. Step 3: A higher SA:V means more surface area is available for gas exchange per gram of body mass, leading to faster exchange rates.

> **tip**
>
> When applying SA:V to examples like root hair cells or alveoli, always explicitly state whether the ratio is higher or lower when explaining your answer for full marks.

*Calculator:* allowed

## Required Practical: Diffusion and Osmosis (2.17)

You will be assessed on your understanding of both living (e.g. potato tissue) and non-living (e.g. agar, Visking tubing) practical investigations of diffusion and osmosis.

For the common potato osmosis practical: potato cores are cut to equal size, blotted dry, weighed, then placed in solutions of varying sugar/salt concentration for a fixed time, then re-blotting and re-weighing to calculate percentage change in mass.

- **Mass/length increase:** Net movement of water into potato cells, so the external solution is more dilute than potato cell sap. Cells become turgid.
- **Mass/length decrease:** Net movement of water out of potato cells, so the external solution is more concentrated than potato cell sap. Cells become flaccid or plasmolysed.
- **No change in mass/length:** The external solution has the same concentration as the potato cell sap, so there is no net movement of water.

**Worked example:** A potato core placed in a 0.5M sucrose solution loses 8% of its mass. Explain this result.

1. Step 1: Link mass loss to water movement via osmosis.
2. Step 2: The 0.5M sucrose solution is more concentrated than the potato cell sap, so water moves out of the potato cells across their partially permeable cell membranes.
3. Step 3: The net loss of water from the potato cells causes the core to decrease in mass.

> **tip**
>
> Always calculate percentage change in mass rather than absolute change when comparing samples, as starting masses may vary slightly, making absolute changes incomparable.

*Calculator:* allowed

## Comparing Passive and Active Transport

| Feature | Diffusion | Osmosis | Active Transport |
| --- | --- | --- | --- |
| Energy required? | No (passive) | No (passive) | Yes (from respiration) |
| Gradient direction | Down (high→low) | Down (high→low water) | Against (low→high) |
| Partially permeable membrane required? | No | Yes | No (uses carrier proteins) |
| Substances transported | Small soluble particles (O₂, glucose) | Water only | Ions, large molecules (minerals) |

**Check your understanding**

1. Which process requires energy from respiration to move substances against a concentration gradient?

   - Diffusion
   - Osmosis
   - Active transport

   *Answer:* Active transport

   *Why:* Active transport is the only active process listed, using energy from respiration to move substances up a concentration gradient.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Defining osmosis as 'diffusion of water' without mentioning a partially permeable membrane
  - Why it fails: Edexcel marking schemes explicitly require reference to the membrane for full marks, this is a common discriminator question
  - Correct: Always include 'across a partially permeable membrane' in any osmosis definition
- **Wrong:** Stating active transport uses energy from the concentration gradient
  - Why it fails: Active transport moves against the gradient, so it cannot use gradient energy
  - Correct: Explicitly state active transport uses energy released from aerobic respiration
- **Wrong:** Mixing up water movement direction in osmosis practicals
  - Why it fails: Many students mix up dilute/concentrated solutions and mass change, losing marks in practical interpretation questions
  - Correct: Remember water moves to the more concentrated solution: if external solution is more concentrated than cells, water leaves and mass decreases
- **Wrong:** Stating larger organisms have a larger surface area to volume ratio
  - Why it fails: SA:V decreases as organism size increases, a common multiple choice misconception
  - Correct: Smaller cells/organisms have higher SA:V, larger ones have lower SA:V
- **Wrong:** Using absolute mass change instead of percentage change for practical comparisons
  - Why it fails: Starting masses of potato cores are never identical, so absolute change does not allow fair comparison
  - Correct: Calculate percentage change in mass using % change = ((final mass - initial mass)/initial mass) × 100

## Cheatsheet

| Term | Exam-Ready Key Definition |
| --- | --- |
| Diffusion | Net movement of particles down concentration gradient, passive, no energy |
| Osmosis | Net movement of water down water concentration gradient, across partially permeable membrane, passive |
| Active Transport | Movement against concentration gradient, uses energy from respiration |
| Rate Factors | SA:V, temperature, concentration gradient, diffusion distance |
| Practical Rule | Mass ↑ = water in (external dilute), mass ↓ = water out (external concentrated) |

## What's next

Now that you have mastered core cell transport processes, you can apply this foundational knowledge to specialized exchange surfaces and transport systems across living organisms, a frequent topic in Edexcel IGCSE Biology exam papers. You will encounter diffusion in gas exchange systems in animals (alveoli) and plants (leaf stomata), osmosis in root hair cell water uptake, and active transport in mineral ion absorption in plant roots and nutrient uptake in the small intestine villi. Be sure to practice past paper questions on this topic to familiarize yourself with strict marking scheme requirements, especially for definition and practical interpretation questions, which are common high-mark questions across both Paper 1 and Paper 2.

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