Glycoproteins and cell–cell recognition

Molecules (Form and function) · Carbohydrates and lipids · note 5 of 8

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Glycoproteins and cell–cell recognitionSpec B1.1.7

In short

Glycoproteins are proteins with carbohydrate chains attached, found on the outer surface of the plasma membrane. The carbohydrate chains act as markers that other cells recognise, which is cell–cell recognition. ABO blood group antigens are an example: the carbohydrate on red blood cells differs between groups A, B, AB and O, so mismatched transfusions trigger an immune response.

A glycoprotein is a protein with one or more carbohydrate chains (short chains of monosaccharides) attached. In the plasma membrane the carbohydrate always faces outwards, into the extracellular fluid.

The sequence and branching of the carbohydrate chain is specific to a type of cell, tissue or individual. Other cells, including cells of the immune system, can bind to it. This is cell–cell recognition: it lets cells recognise self from non-self and helps cells of the same tissue stick together.

Example: ABO antigens

Red blood cells carry carbohydrate chains on their membrane glycoproteins (and glycolipids) that act as antigens. All groups share the same basic chain; groups A and B add one extra sugar to it, a different sugar in each case. The sugar names in the table are for interest only: you need to know that the antigens differ in their carbohydrate chain, not what the sugars are.

ABO antigens on red blood cells
Blood group	Antigen on red blood cells	Carbohydrate chain
O	Neither A nor B	Basic chain only
A	A antigen	Basic chain + N-acetylgalactosamine
B	B antigen	Basic chain + galactose
AB	A and B antigens	Both forms present

If a person receives red blood cells with an antigen their own cells do not carry, antibodies in their plasma bind to that antigen. The donated cells clump together (agglutination). This is why blood for transfusion must be matched.

Three sections of red blood cell plasma membrane, each with a glycoprotein crossing the phospholipid bilayer and a branched carbohydrate chain on the extracellular side. Group A has an extra N-acetylgalactosamine at the end of the chain (A antigen), group B has an extra galactose (B antigen) and group O has the basic chain only; the cytoplasm is below the membrane. — ABO antigens are carbohydrate chains on glycoproteins: groups A and B each add a different extra sugar to the same basic chain.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between alpha and beta glucose?

Alpha and beta glucose differ only at carbon 1: in alpha-glucose the –OH group is below the ring and in beta-glucose it is above. This small difference matters. Alpha-glucose forms coiled, branched starch and glycogen for energy storage, while beta-glucose forms straight cellulose chains that bundle into strong fibres.

Why are starch and glycogen good energy stores?

Starch and glycogen are good energy stores because coiling and branching make them compact, and their large size makes them relatively insoluble, so they have little osmotic effect. Glucose can easily be added by condensation or removed by hydrolysis, and glycogen's many branch ends let it release glucose quickly when needed.

Why are lipids better than carbohydrates for long-term energy storage?

Triglycerides release about twice as much energy per gram as carbohydrates when oxidised. They are insoluble, so they have no osmotic effect, and they are stored without water, whereas glycogen is stored bound to water. A fat store therefore adds much less mass for the same energy, which suits long-term storage.

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