The plasma membrane has the same phospholipid bilayer construction as all intracellular membranes. All membranes are a fluid mosaic of proteins attached to or embedded in the phospholipid bilayer. The different proteins and to some extent, different phospholipids, structurally and functionally differentiate one kind of cellular membrane from another. Integral (trans-membrane) proteins span the phospholipid lipid bilayer, with one hydrophobic domain and two hydrophilic domains. In the case of the plasma membrane, the hydrophilic domains interact with the aqueous extracellular fluid on one side and the cytoplasm on the other, while the hydrophobic domain keeps the proteins anchored in the membrane. Once embedded in the fatty acid interior of a membrane, integral membrane proteins cannot escape! In contrast, peripheral membrane proteins bind to membrane surfaces, typically held in place by hydrophilic interactions with charged features of the membrane surface (phospholipid heads, hydrophilic surface domains of integral proteins). Integral membrane proteins are often glycoproteins whose sugars face the outside of the cell. Cells thus present a sugar coating, or glycocalyx, to the outside world. As cells form tissues and organs, they become bound to extracellular proteins and glycoproteins that they, or other cells, secrete to form an extracellular matrix. We will spend much of this chapter looking at characteristic structures and biological activities of plasma membrane proteins and their functions.
When you have mastered the information in this chapter, you should be able to:
- distinguish components of the membrane that can move (diffuse) laterally in the membrane from those that can flip (switch) from the outer to the inner surface of the phospholipid bilayer
- compare the fluid mosaic membrane to earlier membrane models and cite the evidence for and against each (as appropriate).
- describe how cells might make their plasma membranes and suggest an experiment that would demonstrate your hypothesis.
- distinguish between transmembrane and peripheral membrane proteins, and provide specific examples of each.
- decide whether a newly discovered protein might be a membrane protein.
- predict the effect of molecular and physical influences on membrane fluidity
- suggest how organisms living in warm tropical waters have adapted to the higher temperatures. Likewise, fish living under the arctic ice.
- explain how salmon are able to spend part of their lives in the ocean and another part swimming upstream in freshwater, without their cells shriveling or exploding
- list the diverse functions of membrane proteins.
- speculate on why only eukaryotic cells have evolved sugar coated cell surfaces.
- compare and contrast the glycocalyx and extracellular matrix of cells.