102
Cell Biology:
Membranes
Answers
30.The answer is a.(Ross and Pawlina, pp 28, 30. Alberts, pp 583–585.)The
hydrophobic layer of the cell (plasma) membrane is labeled with the arrow. It
is responsible for the fundamental structure of the membrane and provides the
barrier to water-soluble molecules in the external milieu. It also provides a two-
dimensional solvent for membrane proteins. Other membrane functions are
performed primarily by proteins that function as receptors, enzymes (catalysis
of membrane-associated activities), and transporters (answers b, c, and d).
Connection to the cytoskeleton (answer e)is performed by members of the
spectrin family of proteins reinforcing the membrane on the cytosolic side.
The membrane consists of a bilayer of phospholipids with the nonpo-
lar, hydrophobic layer in the central portion of the membrane and the
hydrophilic polar regions of the phospholipids in contact with the aqueous
components at the intra- or extracellular surfaces of the membrane. Pro-
teins are generally dispersed within the lipid bilayer. The polar head groups
of the lipid bilayer react with osmium to create the trilaminar appearance
observed in electron micrographs of the plasma membrane. Cell mem-
branes range in thickness from 7 to 10 nm [1nm = 10–9m, 1 μm = 10–6m;
the diameter of a red blood cell (erythrocyte) is 7 μm].
31.The answer is b.(Alberts, pp 584, 612, 613. Junqueira, pp 23–25. Ross and
Pawlina, pp 23–25. Kasper, pp 1943–1944.)The child in the scenario suffers
from IgA deficiency, the most common immunoglobulin deficiency. IgA func-
tions in several ways, one of which is to coat pathogens with a negative charge
that repels the polyanionic charge on the cell surface. In IgA deficiency,
pathogens can more easily attach to the cell surface leading to persistent infec-
tions. The carbohydrate of biological membranes is found in the form of gly-
coproteins and glycolipids rather than as free saccharide groups (answer a).
The polyanionic charge of the membrane is produced by the sugar side chains
on the glycoproteins and glycolipids. Glycoproteins often terminate in sialic
acid side chains, which impart a negative (polyanionic) charge to the mem-
brane. Similarly, the glycolipids (also called glycosphingolipids), particularly