Chapter 10

Question 1

Amphiphilic

Answer 1

Having both hydrophobic and hydrophilic regions, as in a phospholipid or a detergent molecule.

Question 2

Cholesterol

Answer 2

Lipid molecule with a characteristic four-ring steroid structure that is an
important component of the plasma membranes of animal cells.

Question 3

Ganglioside

Answer 3

Any glycolipid having one or more sialic acid residues in its structure;especially abundant in the plasma membranes of nerve cells.

Question 4

Lipid Raft

Answer 4

Small region of the plasma membrane enriched in sphingolipids and
cholesterol.

Question 5

Liposome

Answer 5

Artificial phospholipid bilayer vesicle formed from an aqueous suspension of phospholipid molecules.

Question 6

Phosphoglyceride

Answer 6

The main type of phospholipid in animal cell membranes, with two fatty
acids and a polar head group attached to a three-carbon glycerol backbone.

Question 7

T or F: Although lipid molecules are free to diffuse in the plane of the bilayer,
they cannot flip-flop across the bilayer unless enzyme catalysts called phospholipid translocators are present in the membrane.

Answer 7

True; The hydrophobic interior of the lipid bilayer acts as a barrier to the
passage of the hydrophilic lipid head groups that must occur during flipflop. The energetic cost of this movement effectively prevents spontaneous
flip-flop of lipids, so that it occurs extremely rarely in the absence of specific catalysts known as phospholipid translocators.

Question 8

T or F: All of the common phospholipids—phosphatidylcholine, phosphatidylethanolamine,
phosphatidylserine, and sphingomyelin—carry a positively
charged moiety on their head group, but none carry a net positive charge.

Answer 8

True; The positively charged moieties in all cases are balanced by the negative charge on the phosphate group; thus, none of the common
phospholipids carries a net positive charge.

Question 9

T or F: Glycolipids are never found on the cytoplasmic face of membranes in
living cells.

Answer 9

True; Glycolipids are synthesized in the lumen of the Golgi apparatus,
which is topologically equivalent to the outside of the cell, and cannot flip-flop across the bilayer.

Question 10

Bacteriorhodopsin

Answer 10

Pigmented protein found in the plasma membrane of Halobacterium
halobium, where it pumps protons out of the cell in response to light.

Question 11

Lectin

Answer 11

Protein that binds tightly to a specific sugar.

Question 12

Carbohydrate Layer

Answer 12

The outer coat of a eukaryotic cell, composed of oligosaccharides linked
to intrinsic plasma membrane glycoproteins and glycolipids, as well as
proteins that have been secreted and reabsorbed onto the cell surface.

Question 13

Spectrin

Answer 13

Abundant protein associated with the cytosolic side of the plasma membrane
in red blood cells, forming a rigid network that supports the membrane.

Question 14

Multipass Transmembrane Protein

Answer 14

Protein whose polypeptide chain crosses the lipid bilayer more than
once.

Question 15

Cortex

Answer 15

The complicated cytoskeletal network in the cytosol just beneath the plasma membrane.

Question 16

T or F: Whereas all the carbohydrate in the plasma membrane faces outward on
the external surface of the cell, all the carbohydrate on internal membranes
faces toward the cytosol.

Answer 16

False; The carbohydrate on internal membranes is directed away from
the cytosol toward the lumen of an internal membrane-enclosed compartment.
Remember that the lumen of an internal compartment is topologically equivalent to the outside of the cell.

Question 17

T or F: Human red blood cells contain no internal membranes other than the
nuclear membrane.

Answer 17

False; Human red blood cells contain no internal membranes at all; at
an early stage in their development, they extrude their nuclei. The lack of
any internal membranes is the principal reason they have been used so
extensively to investigate the structure of the plasma membrane.

Question 18

T or F: Although membrane domains with different protein compositions are well known, there are at present no examples of membrane domains that differ in lipid composition.

Answer 18

False; n addition to lipid rafts, which are microdomains with distinct lipid compositions, the apical and basolateral surfaces of epithelial cells,
which are separated by intercellular tight junctions, also have different
lipid compositions.

Question 19

Glycosylphosphatidylinositol (GPI) anchor

Answer 19

Type of lipid linkage, formed as proteins pass through the endoplasmic
reticulum, by which some proteins are attached to the noncytosolic surface
of the membrane.

Question 20

What is the most likely reason that LDL particles are used to transport
cholesterol in the bloodstream?

Answer 20

Cholesterol is largely insoluble in aqueous solutions; Cholesterol is a hydrophobic lipid molecule with low solubility in aqueous solutions. Cholesterol must therefore be transported in the blood in a form that masks its hydrophobic nature. All the other choices are incorrect.

Question 21

What best describes the class of protein that apolipoprotein belongs to?

Answer 21

Amphiphilic proteins; Apolipoprotein must have hydrophilic surfaces that interact with the aqueous environment, as well as hydrophobic surfaces that interact with the lipid core of the LDL particle. It is therefore an amphiphilic protein. The other classes of protein listed in the question are found on cells, rather
than on LDL particles. In addition, LDL particles are not surrounded by a
lipid bilayer, so apolipoprotein cannot be a transmembrane protein.

Question 22

What is the most likely location of phospholipids in LDL particles?

Answer 22

On the surface interacting with aqueous environment; Phospholipids contain a polar head group that must interact with a polar
environment and a hydrophobic tail that must interact with a hydrophobic
environment. Thus, the most reasonable position for the phospholipid
is at the surface of the LDL particle, with the polar head group facing
the polar aqueous environment and the hydrophobic tail buried in
the hydrophobic core of the particle.

Question 23

What kind of molecule on the surface of a cell would be most likely to
serve as a receptor for LDL particles?

Answer 23

A transmembrane protein; The LDL receptor on the cell surface must bind to LDL particles with high
specificity and affinity, and then initiate a series of events that brings the
LDL particle into the cell. Integral membrane proteins are the most likely
candidates to serve this function and the LDL receptor is known to be
a single-pass transmembrane protein. Lipids such as cholesterol, phospholipids,
and glycolipids most commonly serve as structural—rather
than specificity—components of the membrane. A cholesterol-binding
protein is an unlikely choice because cholesterol is buried in the core of
the LDL particle and, therefore, unavailable for binding by a cell-surface
protein.

Question 24

What conditions would be likely to trigger increased uptake of LDL particles
by cells?

Answer 24

Increased plasma membrane growth; Cholesterol is an important component of the plasma membrane. Thus,
membrane growth, as occurs during cell proliferation, requires cholesterol,
which is acquired largely by uptake of LDL particles. The other
choices—glycolipid synthesis, protein secretion, and protein synthesis—
are unrelated to LDL particles.