IB Biology SLTopic 2 — Carbohydrates & LipidsPaper 1 & 2Structure and function~10 min read
Phospholipids
Take a triglyceride, remove one fatty acid, and put a phosphate group in its place. That single swap creates a molecule with a water-loving end and a water-hating end — and drop enough of them into water and they arrange themselves, with no help from anything, into the double layer that every cell membrane on Earth is made of.
📘 What you need to know
A phospholipid is glycerol + two fatty acids + one phosphate group, joined by ester bonds.
The phosphate head is polar, so it is hydrophilic (soluble in water).
The fatty acid tails are non-polar, so they are hydrophobic (insoluble in water).
Having both regions makes a phospholipid amphipathic.
In water they form a monolayer at the surface, or a bilayer with the tails tucked inwards.
The bilayer is the basic structure of the cell membrane and a barrier to water-soluble substances.
Small non-polar molecules such as O2 and CO2, and steroid hormones, cross the bilayer easily.
Two tails, not three
The build is almost identical to a triglyceride. Glycerol still has three hydroxyl groups, but only two of them react with fatty acids. The third bonds to a phosphate group instead.
The swap that changes everything
glycerol + 2 fatty acids + phosphate → phospholipid
Phosphate carries a negative charge, so it interacts strongly with polar water molecules. The tails are plain hydrocarbon, so they do not. One molecule, two opposite personalities — that is what amphipathic means.
Compare this with the triglyceride diagram: same glycerol, same ester bonds, but the third position holds a phosphate instead of a fatty acid.
What happens when you drop them in water
Nobody arranges these molecules. They arrange themselves, because water pushes the tails out of the way and pulls the heads in. Two stable arrangements come out of that.
At the surface of water, the heads sit in the water and the tails stick up into the air. That is a monolayer.
Mixed into water, they form two sheets back to back — heads facing the water on both sides, tails hidden in the middle. That is a bilayer, and it is the basic structure of a cell membrane.
Two layers, tails facing inwards. It costs the cell nothing to build and nothing to hold together — the arrangement is simply the one water allows.
🤔 Why the bilayer forms on its own
Water molecules attract each other strongly. When a hydrophobic tail is in the way, water cannot form those attractions properly, so the tails get squeezed together into a group where they are out of the water’s way. The heads, meanwhile, are pulled towards the water and stay facing outwards. The bilayer is not built by the cell — it is the shape that leaves water most undisturbed, so it forms by itself and repairs itself if damaged.
Getting across the membrane
The middle of the bilayer is a hydrophobic strip. That is the whole barrier, and it explains what does and does not cross.
Small non-polar molecules such as oxygen and carbon dioxide dissolve in the tails and diffuse straight across. No protein needed.
Larger non-polar molecules such as steroid hormones also cross, because they are lipid-based.
Water-soluble substances — sugars, amino acids, proteins and ions — are blocked. They cannot leak out and unwanted ones cannot get in.
Cholesterol is the odd one out. It is a lipid with a hydrophilic hydroxyl end and a large hydrophobic hydrocarbon region, so it slots neatly into the bilayer. Steroid hormones such as oestradiol and testosterone are made from cholesterol, which is why they can cross membranes and even enter the nucleus, where they affect transcription.
A neat way to check your understanding: oxygen crosses in milliseconds, but a sodium ion — far smaller — cannot get through at all. Size is not the barrier. Charge and polarity are.
Worked examples
WORKED EXAMPLE
Explain why phospholipids form a bilayer when mixed with water. [3]
Point 1
Phospholipids are amphipathic: the phosphate head is hydrophilic and the fatty acid tails are hydrophobic.
Point 2
The heads are attracted to water, so they face outwards on both surfaces.
Point 3
The tails are repelled by water, so they point inwards, away from it, forming the centre of the bilayer.
Heads out, tails in — two layers back to backName both regions and say what each one does. That is three clean marks.
WORKED EXAMPLE
Oxygen crosses the phospholipid bilayer easily but sodium ions cannot, even though a sodium ion is smaller. Explain why. [3]
Point 1
The centre of the bilayer is made of non-polar hydrocarbon tails.
Point 2
Oxygen is a small non-polar molecule, so it dissolves in that region and diffuses through.
Point 3
A sodium ion is charged, so it is repelled by the hydrophobic centre and needs a transport protein.
Polarity decides, not sizeIf your answer only talks about size, you have missed the question.
WORKED EXAMPLE
A triglyceride and a phospholipid both contain glycerol and fatty acids. State two differences between them. [2]
Difference 1
A triglyceride has 3 fatty acids; a phospholipid has 2 plus a phosphate group.
Difference 2
A triglyceride is entirely hydrophobic; a phospholipid is amphipathic, with a hydrophilic head.
Different number of fatty acids, and different behaviour in waterA third difference if you need it: one stores energy, one builds membranes.
💡 Exam tip
Use the right pair of words: polar/hydrophilic for the head, non-polar/hydrophobic for the tails.
Amphipathic is worth a mark on its own. Use it, then explain it in the next clause.
For bilayer questions, describe the orientation: heads facing the water on both sides, tails inwards.
When asked what crosses a membrane, sort molecules by polarity first, then size.
Say the bilayer is a barrier to water-soluble substances rather than “to everything”.
Two fatty acids, not three. Simple, and easy to throw away under exam pressure.
⚠ Common mix-up
Drawing three tails on a phospholipid. That is a triglyceride.
Saying the tails are “attracted to each other”. More accurately, water pushes them together by attracting itself.
Getting the bilayer inside out. Heads always face the water; tails always face each other.
Thinking small molecules always cross. Ions are tiny and still blocked, because they are charged.
Mixing up hydrophobic and hydrophilic. Hydro-philic means water-loving. Phil, as in a friend.
Calling a monolayer a bilayer. Monolayer is one sheet on the surface of water; bilayer is two sheets back to back.
That completes Carbohydrates & Lipids. Up next: How Proteins Form — amino acids, peptide bonds, and how a chain folds into a shape that does a job.
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