# Cell Membranes and Transport

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u4-overview/
> Weight: n/a

This unit explores the structure of cell membranes and the mechanisms cells use to move substances across them, a core topic essential for understanding cellular physiology and whole-organism homeostasis.

**Prerequisites:** [Basic cell structure and organelle function](https://www.owlsprep.com/study/cie-9700-u2-cell-structure-overview/)

## Learning objectives

- Describe the structure of cell membranes according to the fluid mosaic model
- Explain how different factors affect membrane permeability and function
- Distinguish between passive, active and bulk transport processes across membranes
- Relate membrane transport mechanisms to core cellular functions and homeostasis

## Unit at a Glance

This unit follows a logical learning arc: we begin with the fundamental structure of cell membranes, then explore how that structure dictates what can cross it. After that, we break down all major classes of transport across the membrane, from small molecule movement to bulk transport of large particles.

Each concept builds on the previous one: understanding the fluid mosaic structure is required to explain membrane permeability, which in turn explains why different transport mechanisms evolved to move different types of molecules.

This unit is split into 5 core sub-topics:
- [Fluid mosaic membrane structure](https://www.owlsprep.com/study/cie-9700-u4-fluid-mosaic-membrane-structure/) — Learn the components and organization of the widely accepted fluid mosaic model of cell membranes.
- [Membrane permeability](https://www.owlsprep.com/study/cie-9700-u4-membrane-permeability/) — Explore how temperature, solvents and other chemicals affect membrane permeability and structure.
- [Passive transport](https://www.owlsprep.com/study/cie-9700-u4-passive-transport/) — Covers diffusion, osmosis and facilitated diffusion: all passive processes that do not require energy input.
- [Active transport](https://www.owlsprep.com/study/cie-9700-u4-active-transport/) — Understand energy-dependent movement of molecules against their concentration gradient via carrier proteins.
- [Bulk transport](https://www.owlsprep.com/study/cie-9700-u4-bulk-transport/) — Learn how large molecules and particles are moved into and out of cells via endocytosis and exocytosis.

## Common pitfalls

- **Wrong:** Stating water moves from high to low solute concentration during osmosis
  - Why it fails: This mixes up the gradient direction for solutes vs water molecules
  - Correct: Water moves from higher water potential (low solute concentration) to lower water potential (high solute concentration)
- **Wrong:** Assuming all passive transport occurs directly through the phospholipid bilayer
  - Why it fails: This forgets that large or charged molecules require protein channels for passive transport
  - Correct: Passive transport includes both simple diffusion (through bilayer) and facilitated diffusion (via channel proteins)
- **Wrong:** Thinking only active transport requires ATP
  - Why it fails: Many students forget bulk transport also requires energy for vesicle movement and membrane fusion
  - Correct: Both active transport and bulk transport are energy-dependent processes that use ATP

## Cheatsheet

| Concept | Key Summary |
| --- | --- |
| Fluid Mosaic Model | Phospholipid bilayer with embedded mobile proteins, cholesterol and glycoproteins |
| Water Potential | Pure water = 0 kPa; added solute lowers water potential to negative values |
| Osmosis | Net movement of water from higher to lower water potential across a partially permeable membrane |
| Passive Transport | Moves substances down a concentration gradient; does not require ATP input |
| Active Transport | Moves substances against a concentration gradient; requires ATP and carrier proteins |
| Endocytosis | Bulk transport of substances into the cell via vesicle formation from the cell membrane |
| Exocytosis | Bulk transport of substances out of the cell via vesicle fusion with the cell membrane |

## What's next

Start your study of this unit with the first sub-topic: the fluid mosaic model of membrane structure, which forms the foundation for all other concepts in this unit. Once you complete all sub-topics in this unit, you can move on to the next unit covering the cell cycle and cell division.

- [Fluid mosaic membrane structure](https://www.owlsprep.com/study/cie-9700-u4-fluid-mosaic-membrane-structure/)
- [Membrane permeability](https://www.owlsprep.com/study/cie-9700-u4-membrane-permeability/)
- [Passive transport](https://www.owlsprep.com/study/cie-9700-u4-passive-transport/)

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