Every cell you own is wrapped in a sheet that is only two molecules thick. Nobody builds it and nothing holds it together — it assembles itself, purely because of how one molecule behaves in water. Understand that molecule and the rest of this sub-topic follows on its own.
📚 What you need to know
Cell membranes are built from phospholipids arranged as a bilayer — two layers, back to back.
A phospholipid is a glycerol molecule carrying a phosphate head and two fatty acid (hydrocarbon) tails.
The phosphate head is polar, so it is soluble in water: hydrophilic.
The fatty acid tails are non-polar, so they are insoluble in water: hydrophobic.
Having both parts in one molecule makes a phospholipid amphipathic.
In water, phospholipids arrange themselves automatically: heads towards the water, tails away from it.
The bilayer therefore has a hydrophobic core and a hydrophilic surface on both sides.
That core acts as a barrier to large molecules, polar molecules and ions, which is how the cell controls what gets in and out.
One molecule, two personalities
A phospholipid is a small molecule with a very deliberate design. In the middle sits a glycerol molecule. Attached to it are three things:
a phosphate group, which carries a negative charge and forms the head
two fatty acid chains, long strings of carbon and hydrogen, which form the tails
The bonds joining the fatty acids to the glycerol are ester bonds. You do not need to draw them, but the word turns up in mark schemes, so it is worth recognising.
Draw it as a circle with two squiggles. That simple cartoon is all any exam question needs from you.
Students lose marks by saying the head “likes water” and stopping there. Say why: the phosphate group is polar (it has charged regions), and polar things dissolve in water because water is polar too. The tails are chains of carbon and hydrogen with no charged regions, so water simply will not interact with them.
What happens when you drop them in water
Now put a lot of phospholipids into water and think about what each half of the molecule wants.
The heads are perfectly happy surrounded by water. The tails are not — water molecules cling to each other and effectively squeeze the non-polar tails out of the way. The only arrangement that keeps everybody satisfied is one where the heads face outwards into the water and the tails are tucked away from it.
Depending on how many phospholipids are present, three arrangements can form:
The bilayer is the one that matters for cells, but all three are the same rule applied to different amounts of phospholipid.
Why this is so useful to a cell. A membrane that builds itself also repairs itself. Puncture a bilayer and the exposed tails are immediately squeezed back together by the surrounding water. No enzyme, no energy, no repair kit required.
The bilayer as a barrier
Look at a bilayer side-on and it has three distinct regions: a hydrophilic surface facing the cytoplasm, a hydrophilic surface facing the outside, and a hydrophobic core sandwiched between them where all the tails meet.
That core is the barrier. To cross the membrane unaided, a substance has to pass through a thick layer of tightly packed, non-polar hydrocarbon — and most substances cannot.
Substance
Can it cross unaided?
Why
Small, non-polar (oxygen, carbon dioxide)
Yes, easily
Non-polar, so it dissolves straight through the non-polar core
Small, polar (water, urea)
Slowly
Polar, but small enough to slip between the phospholipids at a low rate
Large, polar (glucose, amino acids)
No
Too big and too polar to pass through the hydrophobic core
Ions (sodium, potassium, chloride)
No
Charged, so the non-polar core repels them completely
This is the point of the whole membrane. Because the bilayer stops some things and allows others, the cell is not at the mercy of its surroundings — it can hold on to what it needs and keep out what it does not. Everything else in this sub-topic is about the workarounds cells use for the substances in the bottom two rows.
🧠
Remembering hydrophilic and hydrophobic
-philic as in “Anglophile” means loving; -phobic as in “arachnophobia” means fearing. Water-loving heads on the outside where the water is, water-fearing tails hiding in the middle.
Worked examples
WE 1
Explaining the arrangement
Explain why phospholipids form a bilayer when they are placed in water. (3 marks)
Point 1: name the property
Phospholipids are amphipathic — each one has a hydrophilic phosphate head and two hydrophobic fatty acid tails.
Point 2: what the heads do
The heads are polar, so they are attracted to water and face outwards into it on both sides.
Point 3: what the tails do
The tails are non-polar, so they are repelled by water and point inwards, away from it.
two layers, heads out and tails in, forms automaticallythe mark is for saying hydrophilic AND hydrophobic, not just “one end likes water”
WE 2
Predicting what can cross
Oxygen crosses the plasma membrane far more quickly than sodium ions, even though a sodium ion is smaller than an oxygen molecule. Suggest why. (2 marks)
Step 1: think about charge, not size
A sodium ion is charged; the core of the membrane is non-polar, so the two repel each other.
Step 2: compare with oxygen
Oxygen is a small non-polar molecule, so it dissolves in the hydrophobic core and passes straight through.
size is not the only factor — charge and polarity matter more herethe word “suggest” means apply the idea, not recall it; say why the core blocks a charge
WE 3
Reading an unfamiliar situation
Some drugs are designed to be more non-polar so that they can be taken as a tablet and absorbed easily. Explain how this design helps the drug reach the inside of a cell. (3 marks)
Point 1: the obstacle
To enter a cell the drug must cross the hydrophobic core of the phospholipid bilayer.
Point 2: why non-polar helps
A non-polar drug is soluble in that core, so it can pass through by simple diffusion without needing a protein.
Point 3: the comparison
A polar or charged drug would be blocked and would need a transport protein or another route into the cell.
non-polar means soluble in the core, so it diffuses straight throughexam data questions love applying membrane rules to drugs, toxins and poisons
💡 Exam tips
Use hydrophilic and hydrophobic by name. “Likes water” is worth nothing on its own.
Link hydrophilic to polar and hydrophobic to non-polar — that is the actual explanation.
Amphipathic is the single word that answers “why does a bilayer form?” Learn to spell it.
When drawing a phospholipid, always give it two tails. One tail is a mistake examiners look for.
Say hydrophobic core when explaining why something cannot cross, not just “the middle”.
Charge beats size: an ion is blocked even when it is very small.
⚠ Common mistakes
Saying the tails are “attracted to each other”. They are pushed together by water, which is a different explanation. Either wording usually scores, but the water-driven one is the correct mechanism.
Drawing the heads facing inwards. The heads face the water on both surfaces, always.
Calling the phosphate head a “protein”. It is part of the lipid; membrane proteins are separate molecules.
Saying nothing can cross the bilayer. Small non-polar molecules cross freely, and that is essential for gas exchange.
Confusing a bilayer with a monolayer. A monolayer is one layer at a water surface; a membrane is two.
Forgetting the glycerol. A full description of a phospholipid has three parts, not two.
Up next: Membrane Proteins — the bilayer on its own is a very good wall. Now we add the doors, the locks and the doorbells.
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