International A Level Biology / Membranes and transport
Diffusion and facilitated diffusion revision notes
Edexcel International A Level Biology (YBI11), chapter 4: Membranes and transport, specification point 2.5. Free lesson.
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Diffusion is the net movement of particles down a concentration gradient, using their own kinetic energy and no ATP. Small non-polar molecules cross the membrane directly, while ions and polar molecules need channel or carrier proteins for facilitated diffusion.
How diffusion moves particles across a membrane
Particles move randomly in both directions, but when their concentration is higher on one side, more move towards the lower concentration than return. This creates a net movement down the concentration gradient. The particles use their own kinetic energy, so the cell does not use ATP and the process is passive. In a respiring cell, oxygen is used up, so its concentration inside stays lower than outside. Oxygen therefore moves into the cell overall. Small, non-polar molecules such as O₂ and CO₂ dissolve in the membrane’s hydrophobic core and cross the phospholipid bilayer…
Why some molecules need a protein
The middle of the phospholipid bilayer is made of hydrophobic fatty acid tails. Small non-polar molecules can dissolve in this core, but charged ions and polar molecules are repelled by it. They need a protein to provide a route across. This is facilitated diffusion: the protein helps the particles cross, but does not provide energy. Simple diffusion: small, non-polar molecules such as O₂ and CO₂ cross straight through the bilayer. Facilitated diffusion: ions such as Na⁺ and polar molecules such as glucose cross through a channel or carrier protein. Both processes are passive and…
How a carrier protein moves glucose
A carrier protein moves molecules one at a time. Its binding site is specific, so only a particular molecule, or one very like it, can bind. Binding causes the carrier to change shape and open towards the other side of the membrane.
How a channel protein lets ions through
A channel protein spans the membrane and forms a water-filled pore. Ions pass through this pore, so they do not have to enter the hydrophobic core. Channels are specific: for example, a sodium channel lets sodium ions through. A channel may have a gate that opens, for example when the membrane voltage changes. When sodium ion concentration is higher outside the cell than inside, there is a net movement of sodium ions into the cell, down the concentration gradient.
How to tell channels and carriers apart
Both channel and carrier proteins are intrinsic proteins, which means they span the membrane. Their structures give particles different routes across, and their roles in transport are not the same. Channels form pores, often with gates, and carry specific ions such as Na⁺, K⁺ or Cl⁻. They are used in facilitated diffusion only. Carriers have specific binding sites and change shape when a molecule binds. They carry larger polar molecules such as glucose. Carriers can be used in facilitated diffusion or active transport. In active transport, ATP drives the shape change.
Use the membrane structure to explain the route
When explaining why a molecule uses a particular route, link its properties to the bilayer’s hydrophobic core. Non-polar oxygen can dissolve in the fatty acid tails, but polar glucose is repelled and needs a carrier protein. Facilitated diffusion is still passive. The particles move down their concentration gradient using their own kinetic energy. The protein provides a route, not energy.