Unit Overview
Cell Membranes and Transport
CIE A-Level BiologyΒ· 5 min read π n/a
1. 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
Learn the components and organization of the widely accepted fluid mosaic model of cell membranes.
β β β± 3 min
Membrane permeability
Explore how temperature, solvents and other chemicals affect membrane permeability and structure.
β β β± 3 min
Passive transport
Covers diffusion, osmosis and facilitated diffusion: all passive processes that do not require energy input.
β β β β± 4 min
Active transport
Understand energy-dependent movement of molecules against their concentration gradient via carrier proteins.
β β β β± 3 min
Bulk transport
Learn how large molecules and particles are moved into and out of cells via endocytosis and exocytosis.
β β β± 3 min
2. Common Pitfalls
Wrong move:
Stating water moves from high to low solute concentration during osmosis
Why:
This mixes up the gradient direction for solutes vs water molecules
Correct move:
Water moves from higher water potential (low solute concentration) to lower water potential (high solute concentration)
Wrong move:
Assuming all passive transport occurs directly through the phospholipid bilayer
Why:
This forgets that large or charged molecules require protein channels for passive transport
Correct move:
Passive transport includes both simple diffusion (through bilayer) and facilitated diffusion (via channel proteins)
Wrong move:
Thinking only active transport requires ATP
Why:
Many students forget bulk transport also requires energy for vesicle movement and membrane fusion
Correct move:
Both active transport and bulk transport are energy-dependent processes that use ATP
3. Quick Reference 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.