Consider the cross section of a typical cell membrane composed of lipid bilayer
ID: 1050291 • Letter: C
Question
Consider the cross section of a typical cell membrane composed of lipid bilayer with associated proteins. Propose a reason why cells form lipid bilayers instead of monolayers (or micelles). Is the inside of the membrane hydrophillic or hydrophobic? Explain. A cell membrane can vary in terms of how "fluid" or flexible it is. Describe if you would expect each of the following changes to make a particular membrane more fluid or more rigid. Explain your choices, an increase in the cholesterol content; an increase in the fraction of unsaturated fatty-acyl tails in the membrane glycerophospholipids. Would you expect a reindeer living in a cold climate in northern Canada to have a cell membrane composition similar to a red-tailed deer in a warm climate in the southern United States? If so, explain why. If not, how would you expect the membranes to differ?Explanation / Answer
Ans:
a. The interior of the phospholipids bi-layer is occupied by hydrophobic fatty acid chains, the membrane is impermeable to water-soluble molecules, including ions and most biological molecules. Second, bilayers of the naturally occurring phospholipids are viscous fluids, not solids.
The hydrophilic heads of phospholipids in a membrane bilayer face outward and the portions of an integral membrane protein found inside the membrane are hydrophobic.
b. The hydrophilic portion of a phospholipids is its head, which contains a negatively charged phosphate group and an additional small group which may be charged or polar. The hydrophilic heads of phospholipids in a membrane bi-layer face outward, contacting the aqueous fluid both inside and outside the cell. Since water is a polar molecule, it readily forms electrostatic interactions with the phospholipids heads.
The hydrophobic part of a phospholipids consists of its long, non polar fatty acid tails. The fatty acid tails can easily interact with other nonpolar molecules, but they interact poorly with water. Because of this, it is more energetically favorable for the phospholipids to tuck their fatty acid tails away in the interior of the membrane, where they are shielded from the surrounding water.
The phospholipids bi-layer formed by these interactions makes a good barrier between the interior and exterior of the cell, because water and other polar or charged substances cannot easily cross the hydrophobic core of the membrane.