2.1.4Inquiry Process
Role of Glycoproteins — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 40 of 160
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.
Part 1 of 5What Glycoproteins Are
Glycoproteins combine the properties of proteins with the recognition ability of carbohydrate chains.
Definition
- Definition A glycoprotein is a protein with one or more short carbohydrate chains covalently attached to it.
Attachment
- Attachment Sugar chains are added after translation, a process called glycosylation, mainly in the rough endoplasmic reticulum and Golgi apparatus.
Position
- Position Membrane glycoproteins have their carbohydrate chains on the outer surface, projecting into the extracellular fluid.
Variety
- Variety Different combinations of sugars give each cell type a distinct surface pattern.
Part 2 of 5Roles at the Cell Surface
Roles at the Cell Surface
Cell recognition
- Cell recognition The surface carbohydrate pattern lets cells identify one another and distinguish self from non-self.
Cell adhesion
- Cell adhesion Glycoproteins help cells bind to neighbouring cells and to the extracellular matrix, holding tissues together.
Receptors
- Receptors Many hormone and neurotransmitter receptors are glycoproteins that bind a specific signal molecule.
Antigens
- Antigens Blood group antigens and many pathogen markers are carbohydrate-containing molecules recognised by the immune system.
| Role | Example |
|---|---|
| Recognition | Blood group markers on red cells |
| Hormone receptor | Insulin receptor |
| Pathogen attachment | Viral surface glycoproteins binding host cells |
Part 3 of 5Glycoproteins Outside the Cell
Not all glycoproteins stay in the membrane; many are secreted and work in fluids or on body surfaces.
Mucus
- Mucus Mucins are heavily glycosylated proteins that hold water, forming a slippery protective layer on the gut and airways.
Antibodies
- Antibodies Immunoglobulins carry carbohydrate chains that help keep them soluble and stable in blood.
Hormones
- Hormones Some hormones, including FSH and LH, are glycoproteins that influence target cells.
Protection
- Protection The sugar coat of cell surfaces shields proteins from enzymes and physical damage.
| Glycoprotein | Where it works | Function |
|---|---|---|
| Mucin | Gut and airway lining | Lubricates and traps microbes |
| Antibody | Blood plasma | Binds specific antigens |
| FSH / LH | Blood, acting on gonads | Control gamete production |
| Membrane receptor | Plasma membrane | Binds signal molecules |
Part 4 of 5From Synthesis to Surface
1
A protein is made on ribosomes attached to the rough endoplasmic reticulum.
2
Carbohydrate chains are added to the polypeptide inside the endoplasmic reticulum lumen.
3
The glycoprotein is moved in a vesicle to the Golgi apparatus, where sugars are trimmed and modified.
4
Finished glycoproteins are packaged into vesicles and carried to the plasma membrane.
5
Vesicle fusion with the membrane places the carbohydrate chain on the external surface or releases the protein by exocytosis.
Part 5 of 5Exam Checklist & Tips
2.1.4 – Key Points for the Exam
- Glycoprotein A protein with carbohydrate chains attached to it.
- Glycosylation The addition of carbohydrate groups to a protein.
- Antigen A molecule recognised by the immune system as foreign or as self.
- Cell recognition The ability of cells to identify other cells by molecules on their surface.
Common Mistakes
- Placing the sugar chains on the cytoplasmic side of the membrane; they face outward.
- Treating glycoproteins and glycolipids as identical; one has a protein and the other a lipid core.
- Believing glycosylation happens at the ribosome alone; Golgi processing also occurs.
- Stating that mucus is a lipid; mucins are glycoproteins.
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