Study Guide

Passive transport

CIE A-Level Biology· Unit 4: Cell Membranes and Transport· 15 min read

1. Simple Diffusion★★☆☆☆⏱ 3 min

📘 Definition

Simple diffusion

Net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, without the use of metabolic ATP energy.

Example:

Diffusion of oxygen and carbon dioxide across cell surface membranes

Diffusion relies on the random kinetic movement of molecules, so no external energy is required. The rate of diffusion is described by Fick's Law:

Rate of diffusion∝Surface Area×Concentration GradientMembrane Thickness\text{Rate of diffusion} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Membrane Thickness}}

Only small, non-polar, lipid-soluble molecules can diffuse directly through the hydrophobic phospholipid bilayer. Large, polar or charged molecules cannot pass through easily.

📐 Worked Example

Predict which of the following can cross a cell membrane by simple diffusion: (a) testosterone (steroid hormone), (b) sodium ion (Na⁺), (c) carbon dioxide (CO₂)

  1. 1
    1. Recall the properties of molecules that cross by simple diffusion: small, non-polar, lipid-soluble.
  2. 2
    1. (a) Testosterone is a small non-polar steroid lipid, so it can cross by simple diffusion.
  3. 3
    1. (b) Sodium ions are charged and polar, so they cannot cross by simple diffusion.
  4. 4
    1. (c) Carbon dioxide is small and non-polar, so it can cross by simple diffusion.

Exam tip:

You will often need to apply Fick's Law to explain adaptations for exchange e.g. thin alveolar walls increase diffusion rate.

2. Facilitated Diffusion★★☆☆☆⏱ 4 min

📘 Definition

Facilitated diffusion

Passive movement of large, polar or charged molecules down their concentration gradient, via specific channel or carrier proteins embedded in the cell membrane.

There are two main types of protein involved:

  • Channel proteins: Fixed-shaped pores that allow specific ions to pass through, many are gated (open/close in response to signals)

  • Carrier proteins: Change shape to move specific polar molecules (e.g. glucose) across the membrane; no energy is used for movement down a gradient

Unlike simple diffusion, the rate of facilitated diffusion is limited by the number of available transport proteins, because all proteins can become saturated at high substrate concentrations.

📐 Worked Example

A student measures the rate of glucose uptake into red blood cells at increasing external glucose concentrations. They observe the rate of uptake plateaus at high concentrations. Explain why this happens.

  1. 1
    1. Glucose is a large polar molecule that enters red blood cells via facilitated diffusion through specific glucose carrier proteins.
  2. 2
    1. At low glucose concentrations, the rate of uptake increases with increasing concentration gradient, as not all carriers are saturated.
  3. 3
    1. At high glucose concentrations, all available carrier proteins are fully occupied (saturated). The rate cannot increase further even if the concentration gradient is larger, leading to a plateau.

3. Osmosis and Water Potential★★★☆☆⏱ 5 min

📘 Definition

Osmosis

Net movement of water molecules from a region of higher water potential to a region of lower water potential, across a selectively permeable membrane.

Water potential (symbol (\Psi), measured in kPa) is the tendency of water to move out of a solution. By definition, pure water has a water potential of 0 kPa, the highest possible value. Adding solutes lowers water potential, so all solutions have negative water potential.

Solutions are described relative to a cell's cytoplasm: isotonic (same water potential), hypertonic (lower/more negative water potential), hypotonic (higher/less negative water potential).

📐 Worked Example

A plant cell with an internal water potential of -500 kPa is placed in a sucrose solution with water potential of -800 kPa. Predict the direction of net water movement and the final state of the cell.

  1. 1
    1. Recall water always moves from higher (less negative) water potential to lower (more negative) water potential.
  2. 2
    1. Compare the values: Cell (\Psi = -500) kPa, solution (\Psi = -800) kPa. -500 kPa is higher (less negative) than -800 kPa.
  3. 3
    1. Net water movement is out of the plant cell and into the surrounding solution.
  4. 4
    1. As water leaves, the vacuole shrinks and the cytoplasm pulls away from the rigid cell wall: the cell becomes plasmolysed.

Exam tip:

Always reference water potential explicitly when describing osmosis. Examiners will not accept answers that only mention solute concentration.

4. Factors Affecting Rate of Passive Transport★★★☆☆⏱ 3 min

Factor

Effect on rate of passive transport

Concentration gradient

Steeper gradient = faster rate (until saturation for facilitated diffusion)

Temperature

Higher temperature = faster rate (increases kinetic energy of molecules)

Surface area of membrane

Larger surface area = faster rate

Number of transport proteins

More proteins = faster facilitated diffusion, until saturation

Molecule size

Smaller molecules diffuse faster than larger molecules

Lipid solubility

More lipid-soluble molecules diffuse faster across the bilayer

✓ Quick check

Test your understanding of core concepts:

  1. Solution A has (\Psi = -300) kPa, Solution B has (\Psi = -100) kPa. What is the net direction of water movement across a selectively permeable membrane?

    • Net movement A → B

    • Net movement B → A

    • No net movement

    • Only movement of solutes

    Reveal answer
    Net movement B → A —

    Correct: B has higher (less negative) water potential than A, so water moves from B to A.

  2. Which process requires transport proteins but no ATP?

    • Simple diffusion

    • Active transport

    • Facilitated diffusion

    • Osmosis through the bilayer

    Reveal answer
    Facilitated diffusion —

    Correct: Facilitated diffusion uses proteins to move molecules down a gradient, so no ATP is required.

5. Common Pitfalls

Wrong move:

Stating water moves from low to high solute concentration without referencing water potential

Why:

While this is technically true, CIE examiners require explicit reference to water potential for full marks

Correct move:

Always state net water movement is from higher (less negative) water potential to lower (more negative) water potential across a selectively permeable membrane

Wrong move:

Classifying facilitated diffusion as active transport because it uses proteins

Why:

The key distinction between active and passive transport is whether ATP energy is required, not whether proteins are used

Correct move:

Classify transport as passive if movement is down a gradient and requires no ATP, regardless of the use of proteins

Wrong move:

Claiming pure water has a negative or positive non-zero water potential

Why:

By definition, pure water has the maximum possible water potential set to 0 kPa

Correct move:

Remember pure water = 0 kPa, all solutions have negative water potential

Wrong move:

Defining osmosis as diffusion of any solvent across a membrane

Why:

CIE definitions specifically restrict osmosis to the movement of water only

Correct move:

Define osmosis as net movement of water across a selectively permeable membrane down a water potential gradient

Wrong move:

Claiming rate of facilitated diffusion increases infinitely with increasing concentration gradient

Why:

There are a fixed number of transport proteins, which become fully saturated at high substrate concentrations

Correct move:

Recognize that the rate of facilitated diffusion plateaus once all transport proteins are saturated

6. Quick Reference Cheatsheet

Process

Down gradient?

Requires ATP?

Requires proteins?

Example substrate

Simple diffusion

Yes

No

No

O₂, CO₂, steroid hormones

Facilitated diffusion (channel)

Yes

No

Yes

Ions (Na⁺, K⁺)

Facilitated diffusion (carrier)

Yes

No

Yes

Glucose, amino acids

Osmosis

Yes (water potential)

No

No (or aquaporins)

Water only

7. Frequently Asked

Is facilitated diffusion active or passive?

Facilitated diffusion is always passive. It does not require ATP because molecules move down their concentration gradient, only requiring proteins for transport.

What is the difference between diffusion and osmosis?

Diffusion refers to net movement of any substance down its concentration gradient, while osmosis specifically refers to net movement of water across a selectively permeable membrane down a water potential gradient.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 · 1

    Multiple choice on osmosis direction

  • 2021 · 2

    Factors affecting diffusion rate

  • 2023 · 4

    Compare passive transport types

Going deeper

What's Next

Passive transport is a core foundation for understanding all transport across cell membranes, a theme that recurs across all units of CIE A-Level Biology. This sub-topic sets the stage for understanding energy-dependent transport processes, as well as how cells maintain water balance and exchange materials with their environment. Mastering passive transport concepts, especially osmosis and water potential, is essential for answering both multiple choice and extended response questions, and is often tested in combination with other topics like gas exchange, kidney function and plant water relations. Understanding the difference between passive and active transport is a common early exam question requirement.