Chapter 1 - Plasma Membrane

Question 1

A herpetologist is bitten by a poisonous snake and is taken to the emergency department with progressive muscle paralysis. The venom is probably incapacitating his:
(A) Na+ channels
(B) Ca2+ channels
(C) phospholipids
(D) acetylcholine receptors
(E) spectrin

Answer 1

D. Snake venom usually blocks acetylcholine receptors, preventing depolarization of the muscle cell. The Na+ and Ca2+ channels are not incapacitated by snake venoms (see Chapter 1 IV B)

Question 2

Cholesterol functions in the plasmalemma to:
(A) increase fluidity of the lipid bilayer
(B) decrease fluidity of the lipid bilayer
(C) facilitate the diffusion of ions through the
lipid bilayer
(D) assist in the transport of hormones across the lipid bilayer
(E) bind extracellular matrix molecules

Answer 2

B. The fluidity of the lipid bilayer is decreased in three ways: (1) by lowering the temperature, (2) by increasing the saturation of the fatty acyl tails of the phospholipid molecules, and (3) by increasing the membrane’s cholesterol content (see Chapter 1 II A 2).

Question 3

The cell membrane consists of various components, including integral proteins. These integral proteins:
(A) are not attached to the outer leaflet
(B) are not attached to the inner leaflet
(C) include transmembrane proteins.
(D) are preferentially attached to the E-face
(E) function in the transport of cholesterol-based hormones

Answer 3

C. Integral proteins are not only closely associated with the lipid bilayer but also tightly bound to the cell membrane. These proteins frequently span the entire thickness of the plasmalemma and are thus termed transmembrane proteins (see Chapter 1 II B 1).

Question 4

Which one of the following transport processes requires energy?
(A) Facilitated diffusion
(B) Passive transport
(C) Active transport
(D) Simple diffusion

Answer 4

C. Active transport requires energy. Facilitated diffusion, which is mediated by membrane proteins, and simple diffusion, which involves passage of material directly across the lipid bilayer, are types of passive transport (see Chapter 1 III B).

Question 5

Which one of the following substances is unable to traverse the plasma membrane by simple diffusion?
(A) 02
(B) N2
(C) Na+
(D) Glycerol
(E) C02

Answer 5

C. Na+ and other ions require channel (carrier) proteins for their transport across the plasma membrane. The other substances are small nonpolar molecules and small uncharged polar molecules. The molecules can traverse the plasma membrane by simple diffusion (see Chapter 1 III A 2).

Question 6

Symport refers to the process of transporting:
(A) a molecule into the cell
(B) a molecule out of the cell
(C) two different molecules in opposite
directions
(D) two different molecules in the same direction
(E) a molecule between the cytoplasm and the nucleus

Answer 6

D. The coupled transport of two different molecules in the same direction is termed “symport” (see Chapter 1 III B).

Question 7

One of the ways that cells communicate with each other is by secretion of various molecules. The secreted molecule is known as:
(A) a receptor molecule
(B) a signaling molecule
(C) a spectrin tetramer
(D) an integrin
(E) an anticodon

Answer 7

B. Cells can communicate with each other by releasing signaling molecules, which attach to receptor molecules on target cells (see Chapter 1 IV A).

Question 8

Adrenocorticotropic hormone (ACTH) trav­els through the bloodstream, enters connective tissue spaces, and attaches to specific sites on target-cell membranes. These sites are:
(A) peripheral proteins
(B) signaling molecules
(C) G proteins
(D) G protein-linked receptors
(E) ribophorins

Answer 8

D. G protein-linked receptors are sites where ACTH and some other signaling molecules attach. Binding of ACTH to its receptor causes G5 protein to activate adenylate cyclase, setting in motion the specific response elicited by the hormone (see Chapter 1 IV B 2 c).

Question 9

Examination of the blood smear of a young patient reveals misshapen red blood cells, and the pathology report indicates hereditary sphe­rocytosis. Defects in which one of the following proteins cause this condition?
(A) Signaling molecules
(B) G proteins
(C) Spectrin
(D) Hemoglobin
(E) Ankyrin

Answer 9

C. Hereditary spherocytosis is caused by a defect in spectrin that renders the protein incapable of binding to band 4.1 protein, thus destabilizing the spectrin-actin complex of the cytoskel­eton. Although defects in hemoglobin (the respiratory protein of erythrocytes) also cause red blood cell anomalies, hereditary spherocytosis is not one of them (see Chapter 1 V A).

Question 10

Which of the following statements concern­ing plasma membrane components is TRUE?
(A) All G proteins are composed of three subunits
(B) The glycocalyx is usually composed of phospholipids
(C) Ion channel proteins are energy dependent (require adenosine triphosphate)
(D) Gated channels are always open
(E) Ankyrin binds to band 3 of the red blood
cell plasma membrane

Answer 10

E. Ankyrin is linked both to band 3 proteins and to spectrin tetramer, thus attaching the spectrin-actin complex to transmembrane proteins of the erythrocyte. There are two types of G proteins: trimeric and monomeric; glycocalyx (the sugar coat o n the membrane surface) is composed mostly of polar carbohydrate residues; only carrier proteins can be energy requiring; gated channels are open only transiently (see Chapter 1 V A) .