Preparing the Future of Medicine

• Explanation

The cell membrane is a phospholipid bilayer that separates the interior of the cell (ie, intracellular space) from the external environment (ie, extracellular space). The two phospholipid layers are oriented such that the hydrophobic tails of the phospholipids in the inner and outer layers are in contact with each other, leaving the hydrophilic head groups in contact with either the intracellular or the extracellular fluid environment. The cell membrane is described by the fluid mosaic model as a nonrigid (fluid) phospholipid bilayer through which diverse proteins are scattered (the mosaic) and are able to migrate laterally.

In addition to phospholipids, animal cell plasma membranes also contain cholesterol, glycolipids (ie, lipids with attached sugar groups), glycoproteins (ie, proteins with attached sugar groups), and other proteins. Cholesterol, which is found in eukaryotic but not prokaryotic cell membranes, functions to regulate the fluidity of the bilayer. Due to their unique structure, cholesterol molecules decrease fluidity at higher temperatures and increase fluidity at lower temperatures. As eukaryotic cells, intestinal epithelial cell membranes contain cholesterol.

(Choice A) All cells, eukaryotic and prokaryotic, have a plasma membrane but some cell types are surrounded by an additional enveloping layer known as the cell wall. Unlike the plasma membrane, the cell wall is a rigid structure that defines the shape of the cell and provides added protection. Animal cells, including intestinal epithelial cells, do not have cell walls; however, nearly all prokaryotic cells have a cell wall. The major structural component of the bacterial cell wall is peptidoglycan, a polymer of sugars and short polypeptides.

(Choice C) Cytoskeletal filaments are found in the cytoplasm of cells and can interact directly with various components of the cell membrane (eg, by anchoring membrane proteins in place). However, they are not themselves part of the plasma membrane.

(Choice D) Cellulose, a polysaccharide, is the primary component of the cell wall in plant cells. Cellulose is not found in the membranes of animal cells.

Educational objective:
The plasma membrane is a fluid lipid bilayer that contains a diverse array of proteins scattered throughout. Certain cell types have a cell wall surrounding their plasma membrane; however, animal cells do not have a cell wall. The primary component of bacterial cell walls is peptidoglycan.

Study Guide References:

• 5.2 Plasma Membrane Components and Functions (see 5.2.01)