Conjugated Molecules: MDCAT Biology notes
Conjugated molecules MDCAT notes: glycoproteins, glycolipids, lipoproteins and nucleoproteins, their components, where they occur and their roles.
What a conjugated molecule is
A conjugated molecule forms when molecules from two different classes of biomolecules combine. Examples include a carbohydrate with a protein, or a lipid with a protein. The combined molecule can do jobs that neither part could do alone. Simple building blocks such as amino acids, monosaccharides and vitamins are not conjugated molecules.
The four main types
| Conjugated molecule | Made of | Where found | Role |
|---|---|---|---|
| Glycoprotein | Protein + carbohydrate | Outer surface of the plasma membrane; also secreted | Cell recognition, receptors, antigens, some hormones |
| Glycolipid | Lipid + carbohydrate | Outer surface of the plasma membrane | Cell recognition and markers |
| Lipoprotein | Lipid + protein | Cell membranes; blood plasma | Membrane structure; carry cholesterol and fats in blood |
| Nucleoprotein | Nucleic acid + protein | Chromosomes (DNA with histones), ribosomes (rRNA with protein) | Packaging and use of genetic material |
Glycoproteins in detail
Glycoproteins are the most frequently tested conjugated molecules. A glycoprotein is a protein carrying one or more carbohydrate (oligosaccharide) chains.
- Location: they project from the outer face of the plasma membrane, with the sugar chains facing outward. Mitochondria, chloroplasts and ribosomes are not where these are typically described.
- Cell recognition: the sugar chains act as identity tags. They let cells recognize one another, stick together in tissues and tell "self" from "non-self".
- Receptors: many hormone and neurotransmitter receptors are glycoproteins.
- Antigens: many surface antigens, including blood group markers, are glycoproteins or glycolipids.
- Secreted glycoproteins include antibodies (immunoglobulins), the mucins in mucus, and some hormones.
Glycoproteins get their sugar chains in the endoplasmic reticulum and Golgi apparatus, where carbohydrates are added to proteins before the proteins are sent to the membrane or secreted.
How to identify the parts
The name tells you the components. The prefix names the non-protein or non-lipid part, and the root names the main molecule:
- glyco- means sugar (carbohydrate).
- lipo- means lipid.
- nucleo- means nucleic acid.
- -protein or -lipid names the partner.
Worked example: What is a lipoprotein? Lipo (lipid) + protein, so it is a lipid combined with a protein. Another: Is a glycolipid made of fatty acid and carbohydrate? Yes, a lipid (containing fatty acids) with a carbohydrate attached. But "fatty acids and carbohydrates" is not the definition of a glycoprotein. Glycoproteins need a protein.
Common MDCAT traps
- Glycoprotein = carbohydrate + protein. Lipid + protein is a lipoprotein.
- Nucleic acid + protein is a nucleoprotein (chromatin), not a glycoprotein.
- Glycoproteins are typically placed on the plasma membrane surface.
- Vitamins, amino acids and monosaccharides are single-class molecules, not conjugated.
Quick revision
- Conjugated molecules combine two classes of biomolecule.
- Glycoproteins and glycolipids work in cell recognition.
- Chromatin is a nucleoprotein: DNA plus histones.
- Lipoproteins transport cholesterol in blood.
- Sugar chains are added in the ER and Golgi apparatus.