104 Anatomy, Histology, and Cell Biology
and in general contains a paucity of intramembranous particles (see upper
part of figure) compared with the P face (labeled with asterisks).
33.The answer is e.(Alberts, pp 604–605. Rubin, p 1039. Kumar, pp 625–626.
Kasper, pp 608–609.)In its anion exchanger role, band 3 protein exchanges
bicarbonate ion for chloride ion. Bicarbonate is transported by band 3 out of
the RBC in exchange for chloride, permitting the highly efficient transport of
CO 2 to the lungs as bicarbonate. In the absence of band 3 protein, the bicar-
bonate buffering of the blood is reduced, leading to acidosis or lowering of
blood pH. The result is reduced capacity to carry CO 2. In addition to its func-
tional, bidirectional anion exchanger role, band 3 plays a key membrane struc-
tural role, since the cytoplasmic domain of the protein interacts with spectrin
through an ankyrin bridge. Spectrin exists as dimers and trimers; the trimers
are bound together by actin, thus providing a connection to the cytoskeleton
maintaining the shape and stability of the RBC. The result of a null mutation in
band 3 is the formation of erythrocytes that are small and round instead of
biconcave (spherocytosis). Spherocytes are osmotically fragile because of their
decreased surface area per unit volume (answer a).The defective RBCs do not
readily pass through the small sinusoids of the spleen, resulting in destruction
and further membrane conditioning, which leads to accelerated destruction
(answer b)and, eventually, enlargement of the spleen (splenomegaly). The
accelerated hemolysis leads to increased bile production (answer c)and jaun-
dice. Hemoglobin production is also increased, as exemplified by an increase
in mean corpuscular hemoglobin concentration (MCHC) by about 35 to 40%.
The bone marrow compensates for the increased destruction of RBCs with
hyperplasia of erythroid precursors in the bone marrow (answer d)and
increase in the number of reticulocytes (polychromasia).
34.The answer is e.(Alberts, pp 584–592, 608–612.)The patient in the sce-
nario is suffering from cirrhosis in which there are alterations in plasma
lipoproteins. Binding of an antibody to a cell surface receptor results in lateral
diffusion of protein in the lipid bilayer, resulting in increased membrane flu-
idity—patching and capping. Rotational and lateral movements of both pro-
teins and lipids contribute to membrane fluidity. Restriction reduces
membrane fluidity (answer a).Phospholipids are capable of lateral diffusion,
rapid rotation around their long axis, and flexion of their hydrocarbon (fatty
acyl) tails. They undergo transbilayer movement (answer b),known as “flip-
flop,” between bilayers in the endoplasmic reticulum; however, in general this