2.1.4Additional lessons
Role of Glycoproteins — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 14 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Structure of Glycoproteins
Definition
- Definition A glycoprotein is a protein with one or more carbohydrate chains covalently attached to its polypeptide.
Attachment
- Attachment Short branched sugar chains are added to the polypeptide after it is made, a process called glycosylation.
Location
- Location Many glycoproteins sit in the plasma membrane with the carbohydrate chain pointing outwards into the extracellular fluid.
Secreted forms
- Secreted forms Some glycoproteins are released from cells, for example the mucins that make mucus slippery and protective.
Variety
- Variety Different sugar sequences give a huge number of possible shapes, which allows highly specific recognition.
Protection
- Protection Glycoproteins in mucus coat surfaces, trap microbes and stop the underlying cells from drying out or being damaged.
Section 2 of 4Functions on the Cell Surface
Glycoproteins appear on many cell surfaces and work through their sugar chains.
Functions on the Cell Surface
Cell recognition
- Cell recognition Carbohydrate chains act as markers, or antigens, that tell the immune system whether a cell belongs to the body.
Receptors
- Receptors Glycoproteins bind hormones, neurotransmitters or other signal molecules, which triggers a response inside the cell.
Cell adhesion
- Cell adhesion Glycoproteins link neighbouring cells together, helping to hold cells in tissues and stabilise their structure.
Uptake
- Uptake Some receptor glycoproteins bind molecules that are then taken into the cell by endocytosis.
Antibody binding
- Antibody binding Antibodies and receptors recognise the shape of a particular sugar chain, which explains why binding is so specific.
Section 3 of 4Examples and Roles
1
The ABO blood group antigens are carbohydrate chains on red blood cell surfaces, and different chains define type A, B or O.
2
The immune system recognises foreign antigens on pathogens and responds, but does not attack the body's own markers.
3
Hormone receptors on a target cell are glycoproteins that bind a specific hormone and start a signalling pathway.
4
Mucus glycoproteins coat surfaces such as the lining of the airways, trapping microbes and lubricating the tissue.
5
Because the sugar chains project outwards, they form a layer around the cell often called the glycocalyx.
Section 4 of 4Exam Checklist & Tips
2.1.4 – Key Points for the Exam
- Glycoprotein A protein with carbohydrate chains covalently attached.
- Antigen A molecule, often a glycoprotein or glycolipid, recognised by the immune system.
- Receptor A molecule that binds a specific signal molecule and triggers a cell response.
- Glycocalyx The carbohydrate-rich layer on the outer surface of the plasma membrane.
Common Mistakes
- Saying glycoproteins are made of protein only; they contain a carbohydrate chain.
- Placing the sugar chains on the inner (cytoplasmic) surface; they face the extracellular side.
- Describing every glycoprotein as a receptor, when some are antigens, adhesion molecules or secreted products.
- Confusing glycoproteins with glycolipids, which have a carbohydrate attached to a lipid.
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