IB Biology SLTopic 2 — Cell Membranes & TransportPaper 1 & 2Structure and function~8 min read
Glycolipids & Glycoproteins
A cell surrounded by other cells has a problem: how does anything tell it apart from its neighbours, or from a bacterium? The answer is a short branching sugar chain hanging off the outside of the membrane. Change one sugar in that chain and you change the cell’s identity — which is exactly what separates blood group A from blood group B.
📘 What you need to know
Glycoproteins are membrane proteins with a carbohydrate chain attached, on the extracellular (outside) side.
Glycolipids are lipids with a carbohydrate chain attached, also on the outer surface of the membrane.
The carbohydrate chain lets them act as receptor molecules, binding substances at the cell surface.
Receptor types include signalling receptors (hormones, neurotransmitters), receptors for endocytosis, and receptors for cell adhesion and stabilisation.
Cell adhesion lets cells attach to each other to form tissues.
Some act as cell markers, or antigens, for cell identification — how the immune system tells your own cells from a pathogen.
The ABO blood group antigens are glycolipids and glycoproteins that differ only slightly in their carbohydrate chains.
Same idea, two anchors
Both molecules are a carbohydrate chain attached to something already sitting in the membrane. The only difference is what it is attached to.
GLYCOPROTEIN
Carbohydrate chain + membrane protein.
Chain sits on the extracellular side
Classed as a protein
Often larger, branching chains
GLYCOLIPID
Carbohydrate chain + membrane lipid.
Also on the outer surface only
Attached to a phospholipid
Usually shorter chains
Read the word. “Glyco-” means sugar. Whatever follows tells you what the sugar is stuck to. Glyco + protein, glyco + lipid. Nothing more complicated than that.
The position is not decoration. Everything these molecules do — binding a hormone, sticking to a neighbour, being recognised by an immune cell — happens outside the cell.
What the carbohydrate chain does
A sugar chain is a good marker for two reasons. It sticks out where things can reach it, and its exact shape can be varied enormously by changing which sugars are used and how they branch. That makes it a molecule the cell can be recognised by.
Signalling receptors — hormones and neurotransmitters bind to a chain with a complementary shape, and the binding triggers a response inside the cell.
Endocytosis receptors — they help the membrane recognise a substance before taking it into the cell.
Cell adhesion and stabilisation — they let cells attach to their neighbours, which is how a group of cells becomes a tissue rather than a pile.
Cell markers, or antigens — they identify the cell. The immune system reads them to decide whether a cell belongs in the body or is a pathogen.
You have met this idea before on the Role of Glycoproteins page. The reason it appears twice is that it matters in two contexts: once as a property of carbohydrates, and once as a component of the membrane. The biology is the same; the question wording will differ.
How small a difference can matter
The ABO blood group antigens are glycolipids and glycoproteins on the surface of red blood cells. What separates group A from group B is not a different protein or a different lipid. The chain underneath is the same. Only the last sugar on the end differs.
An antibody binds a shape, and one different sugar is enough to change that shape. It is why a transfusion of the wrong group is dangerous and why the right group passes unnoticed.
🤔 Why sugars, rather than proteins, for markers
Carbohydrates branch. A protein chain runs in a line, but a sugar chain can split, fork and rejoin, so a short chain of a few sugars can be arranged in a huge number of different shapes. Combine that with the fact that they sit right on the outside surface, and they make an ideal identity badge — enormous variety, easy to reach, and cheap to build.
Worked examples
WORKED EXAMPLE
State one similarity and one difference between a glycoprotein and a glycolipid. [2]
Similarity
Both have a carbohydrate chain attached, and in both the chain faces the outside of the cell.
Difference
In a glycoprotein the chain is attached to a membrane protein; in a glycolipid it is attached to a lipid.
Same chain, different anchor“One similarity and one difference” means exactly one of each. Do not list four things.
WORKED EXAMPLE
Explain why the carbohydrate chains of glycoproteins are found only on the extracellular surface of the membrane. [2]
Point 1
Their functions are cell recognition, adhesion and binding signalling molecules such as hormones.
Point 2
All of these involve substances or cells outside the cell, so the chain must be exposed on that side to be reached.
Position matches functionA chain facing inwards could not be bound by anything outside, so it would do nothing.
WORKED EXAMPLE
The ABO blood group antigens differ only slightly in their carbohydrate chains. Suggest why such a small difference has such a large effect. [3]
Point 1
Antibodies bind to a region with a shape complementary to the antigen.
Point 2
Changing even one sugar changes the shape of the chain, so it is no longer complementary to the same antibody.
Point 3
The immune system therefore reads it as non-self and responds, which is why blood must be matched before a transfusion.
Small change in shape, complete change in recognition“Complementary shape” is doing the work in this answer. Use the phrase.
💡 Exam tip
Say the chain is on the extracellular side. That single word often carries a mark.
A glycoprotein is classed as a protein; a glycolipid is classed as a lipid. The sugar is the addition.
When listing functions, use the four the syllabus names: signalling, endocytosis, adhesion, identification.
Antigens are on cells; antibodies are in plasma. Keep the two words apart.
For recognition questions, always mention a complementary shape.
The ABO markers are both glycolipids and glycoproteins — not only proteins.
⚠ Common mix-up
Drawing sugar chains on both faces of the membrane. They only ever face outwards.
Saying a glycoprotein is a carbohydrate. It is a protein with a carbohydrate attached.
Confusing glycolipid with phospholipid. A glycolipid is a membrane lipid that has gained a sugar chain.
Thinking cell adhesion is the same as cell recognition. Adhesion sticks cells together; recognition identifies them.
Saying blood group O has an “O antigen”. It has the base chain and no extra sugar, so no A or B marker.
Assuming receptors are always proteins. Glycolipids act as receptors too.
Up next: The Fluid Mosaic Model — every component assembled into one diagram, and exactly what you have to label when an exam asks you to draw it.
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