IB Biology HLCarbohydrates & LipidsPaper 1 & 2~10 min read
The Role of Glycoproteins
Attach a carbohydrate to a protein, put it on the outside of a cell, and you have given that cell an identity badge. Your blood type is nothing more complicated than which badge your red blood cells are wearing.
📚 What you need to know
Carbohydrates and polypeptides combine, via covalent bonds, to make glycoproteins. These are classed as proteins.
Glycoproteins, along with glycolipids, form part of the structure of cell surface membranes.
They act as receptor molecules in cell recognition and identification, cell signalling, endocytosis, and cell adhesion and stabilisation.
Glycoproteins act as antigens, identifying cells as self or non-self. Non-self cells trigger an immune response.
A person’s blood type is determined by the glycoprotein antigens on their red blood cells: A, B, both (AB) or neither (O).
Antibodies in the plasma react with the wrong antigens, causing donated cells to clump together and block blood vessels — which can be fatal.
What a glycoprotein is
Definition
A glycoprotein is a carbohydrate covalently bonded to a polypeptide
The prefix “glyco” means sugar, so the name tells you the recipe. Glycoproteins are classed as proteins, and along with another group called glycolipids — a carbohydrate attached to a lipid — they form part of the structure of cell surface membranes.
The carbohydrate part faces outwards, into the space around the cell. That position is the whole reason glycoproteins can do the jobs they do.
Purple circles are phospholipid heads and the red block is a membrane protein. The green and orange chains are the carbohydrate part.
What glycoproteins do
They act as receptor molecules in a number of processes.
Role
What it means
Cell recognition and identification
Other cells, including immune cells, read the carbohydrate chain to work out what the cell is
Receptors for cell signalling
They bind signalling molecules such as hormones and neurotransmitters
Endocytosis
They help the membrane recognise and take in specific substances
Cell adhesion and stabilisation
They help cells stick to each other, holding tissues together
There is a reason a carbohydrate, rather than a protein alone, is used as the label. Sugars can be joined in a huge number of branching arrangements, so a short carbohydrate chain can carry an enormous amount of distinguishing detail in very little space.
Glycoproteins and ABO blood types
Glycoproteins can act as antigens, identifying cells as either self or non-self. Cells recognised as non-self will trigger an immune response within the organism.
A person’s blood type is determined by the glycoprotein antigens on the surface of their red blood cells.
Blue triangles are A antigens and green squares are B antigens. Type O has neither, which is why it carries both antibodies.
Blood type A individuals have type A glycoprotein antigens.
Blood type B individuals have type B glycoprotein antigens.
Blood type AB individuals have both types.
Blood type O individuals have neither.
The presence of antibodies in a person’s plasma creates an interaction with these glycoproteins if blood of the wrong type enters the body. Someone with type A antigens on their red blood cells will have antibodies against type B antigens.
Why the wrong transfusion is dangerous. The antibodies cause the incorrect antigens, on the transfused cells, to clump together. Clumps of red blood cells block blood vessels, which can be fatal.
Feature
Type A
Type B
Type AB
Type O
Antigens on red blood cells
A
B
A and B
None
Antibodies in plasma
Anti-B
Anti-A
None
Anti-A and anti-B
Can safely receive
A and O
B and O
A, B, AB and O
O only
🧠
Work the table out, do not memorise it
You carry antibodies against the antigens you do not have. Type A has A antigens, so it carries anti-B. Type O has none, so it carries both antibodies — which is why type O can only receive type O.
Worked examples
WE 1
Describing a glycoprotein
Describe what a glycoprotein is and state two of its roles in the cell surface membrane. (3 marks)
Point 1: the structure
A glycoprotein is a carbohydrate covalently bonded to a polypeptide, and is classed as a protein.
Point 2: role one
It acts as a receptor for cell signalling molecules such as hormones and neurotransmitters.
Point 3: role two
It is involved in cell recognition and identification, acting as an antigen that marks the cell as self or non-self.
Sugar plus protein, facing outwards, acting as a receptorcell adhesion and endocytosis are equally acceptable roles
WE 2
A transfusion goes wrong
A patient with blood type A is given blood of type B by mistake. Explain what happens. (4 marks)
Point 1: the antibodies present
A type A patient has A antigens on their red blood cells and carries anti-B antibodies in their plasma.
Point 2: what arrives
The donated cells carry B glycoprotein antigens, which are recognised as non-self.
Point 3: the reaction
The anti-B antibodies bind to those antigens, causing the donated red blood cells to clump together.
Point 4: the consequence
The clumps block blood vessels, which can be fatal.
Non-self antigen → antibody binds → clumping → blocked vesselssay “non-self” — it links the answer to the immune response, which is often the first mark
💡 Exam tips
Say the bond between the carbohydrate and the polypeptide is covalent.
State that glycoproteins are classed as proteins, and that glycolipids are the lipid version.
Learn all four roles: recognition, signalling, endocytosis, adhesion.
Use the terms antigen, antibody, self and non-self precisely.
Work out the ABO table from the rule rather than memorising all twelve boxes.
The danger in a transfusion is clumping blocking vessels, not the blood “reacting” vaguely.
⚠ Common mistakes
Swapping antigen and antibody. Antigens are on the cell; antibodies are in the plasma.
Saying type O has “O antigens”. It has none at all.
Saying AB can donate to anyone. AB can receive from anyone; O is the universal donor.
Calling a glycoprotein a carbohydrate. It is classed as a protein.
Putting the carbohydrate chain on the inside of the membrane. It faces outwards.
Up next: Lipids — why fat is insoluble, how a triglyceride is put together, and why it stores twice the energy of a carbohydrate.
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