glycolipid
Summary
A glycolipid is a membrane lipid with a carbohydrate attached by a glycosidic bond, most often on a ceramide (sphingolipid) backbone. In animal cells it sits in the outer leaflet of the plasma membrane with its sugar facing the extracellular space, where it helps in cell recognition.
Detail
Meaning: A glycolipid is an amphipathic lipid whose polar head group is a sugar or oligosaccharide rather than a phosphate-containing group. In animals most are glycosphingolipids: ceramide (sphingosine plus a fatty acid) with sugar attached. Cerebrosides carry a single sugar. Gangliosides carry a branched oligosaccharide that includes sialic acid. The hydrophobic tails sit in the bilayer and the sugars project outward. Sugars are added inside the Golgi lumen, so when vesicles fuse with the plasma membrane the carbohydrate ends up on the extracellular face.
On the MCAT: Expect passages on membrane structure, asymmetry and cell recognition. Glycolipids and glycoproteins together form the glycocalyx, which functions in cell-cell recognition, adhesion and signaling. ABO blood group antigens are carbohydrate chains on red cell glycolipids and glycoproteins, so different sugars give different blood types. Questions may ask which leaflet carries the sugar, or how a glycolipid compares with a phospholipid or with cholesterol.
Common confusion: Glycolipids are not glycoproteins. In a glycolipid the carbohydrate is attached to a lipid, while in a glycoprotein it is attached to a protein. Both have sugars facing outward. A glycolipid is also not a phospholipid, because its head group is a sugar and has no phosphate. Spontaneous flip-flop across the bilayer is slow, which helps keep glycolipids on the outer leaflet.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.