Cell Biology: Membranes Answers 105does not occur in the plasma membrane. Other factors reduce membrane flu-
idity. An increase in the amount of cholesterol relative to phospholipid
(answer c)has been shown by a variety of physicochemical techniques to
decrease fluidity in both biological and artificial membranes by interacting
with the hydrophobic regions near the polar head groups and stiffening this
region of the membrane. Association or binding of integral membrane pro-
teins with cytoskeletal elements (answer d)on the interior of the cell and
peripheral membrane proteins on the extracellular surface limit membrane
mobility and fluidity.
35.The answer is a. (Alberts, pp 588–589.)Asymmetry of the lipid
bilayer is established during membrane synthesis in the endoplasmic retic-
ulum(answer a)before reaching the Golgi apparatus (answer b).Carbo-
hydrates are associated with the N terminals of transmembrane proteins
that extend from the extracellular surface, not the cytoplasmic surface
(answer c).Cholesterol is different from proteins and phospholipids that
are asymmetrically distributed within the bilayer (answer d).Cholesterol
is found on both sides of the bilayer. The small polar head group structure
of cholesterol allows it to flip-flop from leaflet to leaflet and respond to
changes in shape. In contrast to cholesterol, most proteins and phospho-
lipids are capable of only rare flip-flop (answer e).For example, transbi-
layer movement of phospholipid is limited mostly to the endoplasmic
reticulum.
36.The answer is b.(Alberts, pp 595–596, 868–870. Junqueira, pp 28–30.)
Albuterol binds to β-receptors, which are multipass G-protein-linked
receptors. Binding to G-protein-linked receptors activates or inactivates
enzymes bound to the plasma membrane (adenylyl cyclase or phospholi-
pase C) or opens or closes ion channels using G proteins. A table of G pro-
teins and their functions appears below. The β-receptors, as well as
muscarinic cholinergic receptors and rhodopsin, are multipass transmem-
brane(answer a)proteins consisting specifically of seven hydrophobic
spanning segments of the single polypeptide chain. The peptide bonds of
the spanning segments are polar. In the hydrophobic environment of the
lipid bilayer, in the absence of water, they form hydrogen bonds with each
other. There is a remarkable homology between the cell-surface receptors
linked to the G proteins. Ligand binding occurs on the extracellular surface
(answer c).Receptors with intrinsic enzyme activity belong to a separate