IB Biology SLTopic 2 — Carbohydrates & LipidsPaper 1 & 2Structure and function~8 min read
The Role of Glycoproteins
Stick a short carbohydrate chain onto a protein and you get a glycoprotein. It sounds like a small detail. It is actually how your cells tell each other apart, how hormones find the right target, and why giving someone the wrong blood can kill them. Sugar on the outside of a cell is a name badge.
📘 What you need to know
A glycoprotein is a carbohydrate joined to a polypeptide by covalent bonds. It is classed as a protein.
Glycolipids are the same idea with a lipid instead of a protein. Both sit in the cell surface membrane.
The carbohydrate part sticks out on the outside of the cell, where other cells and molecules can reach it.
They work as receptors: cell recognition and identification, cell signalling (hormones, neurotransmitters), endocytosis, and cell adhesion.
Glycoproteins act as antigens, marking a cell as “self” or “non-self”. Non-self triggers an immune response.
Your ABO blood group is set by which glycoprotein antigens sit on your red blood cells.
Wrong-group blood makes antibodies clump the cells together (agglutination), blocking blood vessels.
What a glycoprotein actually is
Carbohydrates and polypeptides can bond together covalently. When they do, you get a glycoprotein — a protein with a short branched sugar chain attached. Because the protein part is embedded in the membrane and the sugar part faces outwards, the sugar is the bit the outside world touches first.
That position is the whole point. Anything arriving at the cell — a hormone, a virus, another cell — meets the carbohydrate chain before it meets anything else.
The protein is anchored in the membrane and the sugar hangs outside like a label. That is why a cell can be identified without anything having to go inside it.
The four jobs they do
Cell recognition and identification — your immune system reads these sugars to decide whether a cell belongs to you.
Receptors for cell signalling — hormones and neurotransmitters bind to a specific glycoprotein, and only cells with the matching receptor respond.
Endocytosis — they help the membrane recognise and take in particular substances.
Cell adhesion and stabilisation — they help cells stick to their neighbours so tissues hold together.
Notice how many of these are about specificity. A particular shape of carbohydrate is recognised by a particular molecule and nothing else. That idea — complementary shapes — runs through enzymes, antibodies and receptors, so it is worth getting comfortable with here.
Antigens: self and non-self
An antigen is any molecule that the immune system can recognise. Glycoproteins on your own cells are read as “self” and left alone. A cell carrying an antigen your body does not recognise is “non-self”, and an immune response follows.
This is helpful when the non-self cell is a bacterium. It is a serious problem when the non-self cell is a red blood cell from a transfusion or an organ from a donor.
Blood groups: glycoproteins with consequences
Your ABO blood group depends on which glycoprotein antigens sit on the surface of your red blood cells.
Read the pattern rather than memorising four separate facts: you make antibodies against whatever antigen you are missing.
Feature
Group A
Group B
Group AB
Group 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 from
A, O
B, O
A, B, AB, O
O only
Can donate to
A, AB
B, AB
AB only
A, B, AB, O
🤔 Why the wrong blood is dangerous
Say group B blood is given to a group A patient. The patient’s plasma is full of anti-B antibodies. Each antibody can bind more than one antigen, so it links the donated cells together and they clump — this is agglutination. The clumps are far bigger than a single cell and get stuck in narrow blood vessels, blocking blood flow to tissues. This can be fatal, which is why blood is always cross-matched before a transfusion.
🧠 A way to remember donors and recipients
O for “Owes nothing” — no antigens on its cells, so nobody’s antibodies can attack it. Universal donor. AB for “Accepts Both” — no antibodies in its plasma, so nothing gets attacked. Universal recipient.
Worked examples
WORKED EXAMPLE
Explain why blood group O can be given to a patient of any blood group. [2]
Point 1
Group O red blood cells carry neither A nor B antigens on their surface.
Point 2
So the recipient’s anti-A or anti-B antibodies have nothing to bind to, and no agglutination happens.
No antigens → no antibody binding → no clumpingAnswer about the donated cells, not about the donor’s plasma.
WORKED EXAMPLE
A patient with blood group A is accidentally given blood from a group B donor. Describe what happens and why it is dangerous. [3]
Step 1: what the patient already has
A group A patient has anti-B antibodies in their plasma.
Step 2: what arrives
The donated cells carry B antigens, which are recognised as non-self.
Step 3: the consequence
Antibodies bind the B antigens and link the cells into clumps (agglutination).
Clumps block blood vessels, so tissues are starved of oxygen. Can be fatal.Three stages: antibodies present, antigens arrive, clumping blocks vessels.
WORKED EXAMPLE
Suggest why a hormone affects only certain cells in the body, even though it travels in the blood to all of them. [2]
Point 1
The hormone binds to a glycoprotein receptor in the cell surface membrane.
Point 2
The receptor has a shape complementary to the hormone, and only target cells have that receptor.
No matching receptor, no response“Complementary shape” is the phrase mark schemes look for.
💡 Exam tip
Say covalently bonded when you define a glycoprotein. It is not a loose mixture of sugar and protein.
Antigens are on cells; antibodies are in plasma. Keep those two separate in every sentence you write.
You have antibodies against the antigen you lack. Work the group out from that rule instead of memorising a table.
Use the word agglutination and then explain it — the explanation is usually the mark.
For receptor questions, always mention a complementary shape between the signalling molecule and the receptor.
Remember glycolipids exist too. If a question says “carbohydrate on the membrane”, it may not be a protein.
⚠ Common mix-up
Swapping antigen and antibody. Antigen = the marker being recognised. Antibody = the protein doing the recognising.
Saying group O has “O antigens”. It has none at all. That is the entire reason it is the universal donor.
Thinking agglutination means the cells burst. They clump together and block vessels.
Putting the carbohydrate chain on the inside of the membrane. It always faces outwards.
Calling a glycoprotein a carbohydrate. It is classed as a protein that has a carbohydrate attached.
Assuming universal donor also means universal recipient. Group O can give to anyone but can only receive group O.
Up next: Lipids — why oil refuses to mix with water, how a triglyceride is put together, and why fat stores so much more energy than sugar.
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