Question

TRUE/FALSE

1.) Asymmetric distribution of phospholipids in the bilayers of cell membranes arises because phospatidylcholine and sphingomyelin spontaneously translocate from the cytoplasmic to the extracytoplasmic face of the membrane.

2.) The carbohydrate headgroups in glucosyl ceramide, lactosylceramide, and ganglioside are all attached to ceramide via a phosphoryl group.

Answer 1

1. The plasma membrane of normal eukaryotic cells have phosphatidylcholine (PC) and sphingomyelin (SM) in the extra cytosolic face of the membrane. Phosphatidylethanolamine (PE) and phosphatidylinositol (PI) phosphatidylserine (PS) is located in the cytoplasmic face of the plasma membrane. Asymmetry of these charged lipids contributes to membrane potential.

The biosynthesis of PC and SM occurs in the cytosolic side of Golgi membrane. They are processed in the lumen and translocated to extracytoplasmic face of plasma membrane. This transport is mediated by flopases. The ABC transporters transport the lipids from the cytoplasmic to extracytoplasmic face.

Right option is True.

2. Sphingolipids are lipids that can be simple sphingoid bases to complex ganglioside. They are components of cell membranes involved in apoptosis, cell growth, cell differentiation, and toxicity to metals, response to heat stress and bacteria /viral infections. Sphingoid bases are aliphatic amino alcohols with a long hydrocarbon tail, with hydroxyl groups at C1 and C3 and an amine group at C2. N-acetylation of the amine group of a sphingoid base with a fatty acid moiety, results in a ceramide molecule. Glucosylceramides are monoglyosylceramide that are precursors of lactocerebrosides and gangliosides. They have glucose bound with ceramide via a β-1,1 linkage. When galactose combines with glucose in a glucosylceramide via a β-1,4 glycosidic linkage, a lactosylceramide is formed. When galactose, mannose, N–acetylglucosamine and/or N–acetylgalactosamine (GalNAc) link to the glucose bonded to ceramide by β-1,4, β-1,3, β-1,2, α-1,4, α-1,3 linkages, oligosphingolipids are formed. Gangliosides are oligoglycosphingolipids containing a number of sialic acid moieties. Sialic acid is an N–acetylneuraminic acid (NeuAc, Neu5Ac), linked via an α-2,3 glycosidic bond to galactose in the carbohydrate structure. It can also be linked by an α-2,8 glycosidic bond to another N–acetylneuraminic acid unit.

There is no attachment to ceramide of any of these sphingolipid molecules by a phosphoryl group. There is linkage of glucose to ceramide by a β-1,1 linkage.

Right option is False.