A cell has to keep its contents in and keep most things out, while still letting oxygen wander in and carbon dioxide wander out. It manages that with a sheet two molecules thick that nothing builds and nothing holds together — it assembles itself, purely because water refuses to mix with oil.
📘 What you need to know
Cell membranes are built from phospholipid bilayers: a hydrophilic phosphate head plus two hydrophobic fatty acid tails.
Having both a water-loving and a water-hating region makes a phospholipid amphipathic.
In water, phospholipids form a monolayer at the surface, and a bilayer when there are enough of them.
The bilayer has two regions: a hydrophobic core (the tails) and a hydrophilic outer layer (the heads).
The heads face the water in the cytoplasm and the extracellular fluid; the tails are attracted to each other.
Large molecules cannot pass, because the core is tightly packed and has low permeability to them.
Polar molecules and ions cannot pass, because they will not interact with the hydrophobic tails.
Two arrangements in water
Drop a small number of phospholipids onto water and they collect at the surface. Heads dip into the water, tails stick up into the air, and you get a single sheet — a monolayer.
Now add more, and mix them in. There is no air for the tails to point into any more, so they do the next best thing: they point at each other. Two sheets form back to back, heads facing the water on both sides and tails buried in the middle. That is a bilayer.
The bilayer also repairs itself. Poke a small hole and the exposed tails are pushed back together, because leaving them in contact with water is the one thing the arrangement will not allow.
You met the phospholipid itself back in the Phospholipids page. What is new here is the consequence: once you have a bilayer, you automatically have a barrier, and the rest of this topic is about what that barrier will and will not let through.
The bilayer as a barrier
Look at the bilayer as three strips. The outer strips are heads — polar, charged, comfortable in water. The middle strip is tails — non-polar, tightly packed, and completely unwelcoming to anything charged.
Whether a substance crosses depends on whether it can get through that middle strip.
Large molecules are blocked. The tails are packed closely together, so the core has low permeability to anything bulky.
Polar molecules and ions are blocked. They are hydrophilic, so they will not interact with the hydrophobic tails at all.
Small non-polar molecules stroll through, because they dissolve in the tails.
Everything in the red section still gets into cells — but only through proteins. That is the whole reason the next page exists.
🤔 Why “cannot cross” is a feature, not a fault
If glucose, amino acids and ions could drift across freely, a cell could not hold onto anything it had made, and it could not build up a different concentration inside itself from outside. Because the bilayer blocks them, the only way in or out for those substances is through a protein — and the cell decides which proteins it puts in its membrane. Blocking things by default is what turns a barrier into a control system.
The phrase to use
the membrane is partially permeable — permeable to some substances, not to others
Worked examples
WORKED EXAMPLE
Explain why phospholipids form a bilayer rather than a single layer when mixed into water. [3]
Point 1
Phospholipids are amphipathic: hydrophilic phosphate heads and hydrophobic fatty acid tails.
Point 2
Surrounded by water on both sides, the heads face outwards into the water on each surface.
Point 3
The tails are repelled by water, so they point inwards towards each other, forming a hydrophobic core.
Heads out on both sides, tails in the middleA monolayer only works where there is a water surface and air above it.
WORKED EXAMPLE
Sodium ions are much smaller than steroid hormones, yet steroids cross the bilayer and sodium ions do not. Explain why. [3]
Point 1
The core of the bilayer is made of non-polar hydrocarbon tails.
Point 2
Steroid hormones are lipid-based and non-polar, so they dissolve in that core and diffuse through.
Point 3
A sodium ion is charged, so it is hydrophilic and will not interact with the tails; it needs a transport protein.
Polarity decides, not sizeIf your answer is only about size, you have answered a different question.
WORKED EXAMPLE
Suggest why a bilayer of phospholipids is described as a partially permeable barrier rather than an impermeable one. [2]
Point 1
Some substances do cross freely — small non-polar molecules such as oxygen and carbon dioxide.
Point 2
Others are blocked, including large molecules, polar molecules and ions.
Permeable to some substances but not othersGive an example on each side. Two named examples usually secures both marks.
💡 Exam tip
Use hydrophilic head and hydrophobic tails every time. Vague words like “one end likes water” lose marks.
Name the two regions: hydrophobic core and hydrophilic outer layers.
When explaining why something is blocked, say whether it is too large or charged/polar — they are different reasons.
Say partially permeable, not “semi-permeable” and not “impermeable”.
Remember the heads face water on both sides: cytoplasm inside and extracellular fluid outside.
A membrane is about 7 nm thick. Handy if a question gives you a scale bar or a magnification.
⚠ Common mix-up
Drawing the bilayer inside out. Tails never touch water. Heads always face outwards.
Saying the cell builds the bilayer and holds it together. It assembles itself; water does the work.
Thinking small always means it can cross. Ions are tiny and blocked.
Confusing monolayer and bilayer. Monolayer is one sheet at a water surface; bilayer is two sheets back to back.
Saying the membrane is “impermeable to water”. Water is polar but small, and it does cross slowly.
Forgetting that a triglyceride is not a phospholipid. Three tails cannot form a bilayer; you need the polar head.
Up next: Membrane Proteins — the parts stuck into that bilayer that do everything the phospholipids cannot.
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