Chapter 7

Question 1

what cell ability is fundamental to life? what makes this posible

Answer 1

. The ability of the cell to discriminate in its chemical exchanges is fundamental to life, and it is the plasma membrane and its component molecules that
make this selectivity possible.

Question 2

how are proteins distributed in the membrane and what have researchers proposed calling something related to them

Answer 2

The proteins are not randomly distributed in the membrane,
however. Groups of proteins are often associated in long-lasting,
specialized patches, where they carry out common functions.
Researchers have found specific lipids in these patches as well
and have proposed naming them lipid rafts, but there is ongoing controversy about whether such structures exist in living
cells or are an artifact of biochemical techniques.

Question 3

Like all models, the fluid mosaic model is

Answer 3

continually being refined as new

research reveals more about membrane structure.

Question 4

how do some memb proetins seem to move and why maybe?

Answer 4

Some membrane
proteins seem to move in a highly directed manner, perhaps
driven along cytoskeletal fibers in the cell by motor proteins connected to the membrane proteins’ cytoplasmic regions.
However, many other membrane proteins seem to be held
immobile by their attachment to the cytoskeleton or to the
extracellular matrix

Question 5

A membrane remains fluid as temperature decreases until

Answer 5

the phospholipids settle into a closely packed arrangement
and the membrane solidifies, much as bacon grease forms lard
when it cools

Question 6

Therefore, extreme environments pose

a challenge for

Answer 6

life, resulting in evolutionary adaptations

that include differences in membrane lipid composition.

Question 7

integral proteins

Answer 7

Integral proteins penetrate the hydrophobic interior of the
lipid bilayer. The majority are transmembrane proteins, which
span the membrane; other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions
of an integral protein consist of one or more stretches of nonpolar amino acids (see Figure 5.14), typically 20–30 amino
acids in length, usually coiled into α helices (Figure 7.6). The
hydrophilic parts of the molecule are exposed to the aqueous
solutions on either side of the membrane. Some proteins also
have one or more hydrophilic channels that allow passage
through the membrane of hydrophilic substances (even of
water itself; see Figure 7.1

Question 8

peripheral

Answer 8

Peripheral proteins are not
embedded in the lipid bilayer at all; they are loosely bound
to the surface of the membrane, often to exposed parts of
integral proteins

Question 9

Furthermore, a single membrane protein may itself

carry out

Answer 9

multiple functions. Thus, the membrane is not only
a structural mosaic, with many proteins embedded in the
membrane, but also a functional mosaic, carrying out a range of fxns

Question 10

Cell-cell recognition d and fxn

Answer 10

, a cell’s ability to distinguish one type of
neighboring cell from another, is crucial to the functioning
of an organism. It is important, for example, in the sorting
of cells into tissues and organs in an animal embryo

Question 11

how do cels recognize other cells

Answer 11

). Cells recognize other cells by binding to

molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane

Question 12

membrane carbs- how long, what are they bonded to and called consequentyl

Answer 12

ular surface of the plasma membrane (see Figure 7.7d).
Membrane carbohydrates are usually short, branched chains
of fewer than 15 sugar units. Some are covalently bonded to
lipids, forming molecules called glycolipids. (Recall that glyco
refers to carbohydrate.) However, most are covalently bonded
to proteins, which are thereby glycoproteins (see Figure 7.3).
The carbohydrates on the extracellular side of the plasma
membrane vary from species to species, among individuals of
the same species, and even from one cell type to another in
a single individual. T

Question 13

A transport protein is specific for

Answer 13

the substance it translocates (moves), allowing only a certain substance (or a small
group of related substances) to cross the membrane. For
example, a specific carrier protein in the plasma membrane
of red blood cells transports glucose across the membrane
50,000 times faster than glucose can pass through on its own.
This “glucose transporter” is so selective that it even rejects
fructose, a structural isomer of glucose.

Question 14

Thus, the selective

permeability of a membrane depends on

Answer 14

both the discriminating barrier of the lipid bilayer and the specific transport
proteins built into the membrane.

Question 15

osmosis (sugar water fake barrier example, j explan it)

Answer 15

Two sugar solutions of different
concentrations are separated by a membrane that the solvent (water)
can pass through but the solute (sugar) cannot. Water molecules
move randomly and may cross in either direction, but overall, water
diffuses from the solution with less concentrated solute to that
with more concentrated solute. This passive transport of water,
or osmosis, makes the sugar concentrations on both sides more
nearly equal. (The concentrations are prevented from being exactly
equal due to the effect of water pressure on the higher side, which
is not discussed here, for simplicity.)

Question 16

y. Note that each substance diffuses

down its own concentration gradient, unaffected by

Answer 16

the concentration gradients of other substances

Question 17

Much of the traffic across cell membranes occurs by diffusion. One important example is

Answer 17

the uptake of
oxygen by a cell performing cellular respiration. Dissolved
oxygen diffuses into the cell across the plasma membrane.
As long as cellular respiration consumes the O2 as it enters,
diffusion into the cell will continue because the concentration gradient favors movement in that direction.

Question 18

osmosis across an artificial barrier

Answer 18

Two sugar solutions of different
concentrations are separated by a membrane that the solvent (water)
can pass through but the solute (sugar) cannot. Water molecules
move randomly and may cross in either direction, but overall, water
diffuses from the solution with less concentrated solute to that
with more concentrated solute. This passive transport of water,
or osmosis, makes the sugar concentrations on both sides more
nearly equal. (The concentrations are prevented from being exactly
equal due to the effect of water pressure on the higher side, which
is not discussed here, for simplicity.)

Question 19

. However, tight clustering of water molecules
around the hydrophilic solute molecules makes some of the
water unavailable to cross the membrane. As a result

Answer 19

e solution with a higher solute concentration has a lower free water
concentration. Water diffuses across the membrane from the
region of higher free water concentration (lower solute concentration) to that of lower free water concentration (higher
solute concentration) until the solute concentrations on both
sides of the membrane are more nearly equal. The diffusion of
free water across a selectively permeable membrane, whether
artificial or cellular, is called osmosis

Question 20

why doesnt a paramecium cell burst when it has contractile vacuole

Answer 20

. The Paramecium cell doesn’t burst because it is also
equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it
enters by osmosis

Question 21

t, the bacteria and
archaea that live in hypersaline (excessively salty) environments
(see Figure 27.1) have cellular mechanisms that

Answer 21

balance the
internal and external solute concentrations to ensure that water
does not move out of the cell

Question 22

Plants that are not woody, such as most houseplants, depend

for mechanical support on

Answer 22

cells kept turgid by a surrounding
hypotonic solution. If a plant’s cells and surroundings are isotonic, there is no net tendency for water to enter and the cells
become flaccid (limp); the plant wilts.

Question 23

However, a cell wall is of no advantage if the cell is

immersed in a hypertonic environment

Answer 23

In this case, a plant
cell, like an animal cell, will lose water to its surroundings
and shrink

Question 24

This phenomenon is called facilitated diffusion

Answer 24

, many polar molecules and ions impeded by the lipid bilayer of the membrane
diffuse passively with the help of transport proteins that span the
membrane. T

Question 25

example of gated channels that are electrical

Answer 25

or some
gated channels, the stimulus is electrical. In a nerve cell, for
example, an ion channel opens in response to an electrical
stimulus, allowing a stream of potassium ions to leave the
cell. (See the potassium ion channel at the beginning of this
chapter.) This restores the cell’s ability to fire again

Question 26

Other
gated channels open or close when a specific substance other
than the one to be transported binds to the channel. These
gated channels are also important in

Answer 26

t in the functioning of the

nervous system

Question 27

Carrier proteins, such as the glucose transporter mentioned

earlier,

Answer 27

seem to undergo a subtle change in shape that somehow

translocates the solute-binding site across the membrane

Question 28

Such a change in shape may be triggered by

Answer 28

y the
binding and release of the transported molecule. Like ion channels, carrier proteins involved in facilitated diffusion result in the
net movement of a substance down its concentration gradient

Question 29

active transport examples and why it makes sense

Answer 29

. The
transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel
proteins. This makes sense because when channel proteins
are open, they merely allow solutes to diffuse down their
concentration gradients rather than picking them up and
transporting them against their gradients

Question 30

t. One way ATP
can power active transport is when its terminal phosphate
group is transferred directly to the transport protein

Answer 30

. This
can induce the protein to change its shape in a manner
that translocates a solute bound to the protein across the
membrane. One transport system that works this way is the
sodium-potassium pump, which exchanges Na+
for K+
across the plasma membrane of animal cells (Figure 7.15).
The distinction between passive transport and active transport is reviewed i

Question 31

The cytoplasmic side of the
membrane is negative in charge relative to the extracellular
side because

Answer 31

of an unequal distribution of anions and

cations on the two sides.

Question 32

The voltage across a membrane,

Answer 32

called a membrane potential, ranges from about -50 to
-200 millivolts (mV). (The minus sign indicates that the
inside of the cell is negative relative to the outside.)

Question 33

The membrane potential acts like

Answer 33

a battery, an energy
source that affects the traffic of all charged substances across
the membrane. Because the inside of the cell is negative compared with the outside, the membrane potential favors the
passive transport of cations into the cell and anions out of the
cell. T

Question 34

This combination of forces acting

on an ion is called the electrochemical gradient.

Answer 34

Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient,
which has been our sole consideration thus far in the chapter)
and an electrical force (the effect of the membrane potential
on the ion’s movement)

Question 35

One important use of proton gradients

in

Answer 35

the cell is for ATP synthesis during cellular respiration, as you
will see in Concept 9.4. Another is a type of membrane traffic
called cotransport.

Question 36

cotransport example

Answer 36

For instance,
a plant cell uses the gradient of H+
generated by its ATP-powered
proton pumps to drive the active transport of amino acids,
sugars, and several other nutrients into the c

Question 37

explain what results from plant sucrose h gradient

Answer 37

l. The resulting
H+
gradient represents potential energy that can be used for active
transport—of sucrose, in this case. Thus, ATP hydrolysis indirectly
provides the energy necessary for cotransport

Question 38

what do plants do with the scursoe made from photosynth

Answer 38

The vascular tissue of the plant can then distribute the sugar to
roots and other nonphotosynthetic organs that do not make
their own food.

Question 39

why treat diarrhea with a drink of nacl and glucose

Answer 39

The solutes are taken up by sodiumglucose cotransporters on the surface of intestinal cells and
passed through the cells into the blood. This simple treatment has lowered infant mortality worldwide.

Question 40

what do exo and endo cytosis nd

Answer 40

energy

Question 41

When the vesicle membrane

and plasma membrane come into contact (during travel of a vesicle to the membrane)

Answer 41

specific proteins
rearrange the lipid molecules of the two bilayers so that the
two membranes fuse. The contents of the vesicle spill out of
the cell, and the vesicle membrane becomes part of the plasma
membrane

Question 42

rme for cholesterol: how does it travel? what happens> what is hypercholesteremia

Answer 42

Human cells use receptor-mediated endocytosis to take
in cholesterol for membrane synthesis and the synthesis of
other steroids. Cholesterol travels in the blood in particles
called low-density lipoproteins (LDLs), each a complex of
lipids and a protein. LDLs bind to LDL receptors on plasma
membranes and then enter the cells by endocytosis. In the
inherited disease familial hypercholesterolemia, characterized by a very high level of cholesterol in the blood, LDLs
cannot enter cells because the LDL receptor proteins are
defective or missing:

Question 43

what happens in hpercholesterima

Answer 43

Consequently, cholesterol accumulates in the blood,
where it contributes to early atherosclerosis, the buildup of
lipid deposits within the walls of blood vessels. This buildup
narrows the space in the vessels and impedes blood flow,
potentially resulting in heart damage and stroke.

Question 44

In phagocytosis,

Answer 44

s, a cell engulfs a particle
by extending pseudopodia (singular,
pseudopodium) around it and packaging
it within a membranous sac called a food
vacuole. The particle will be digested after the
food vacuole fuses with a lysosome containing
hydrolytic enzymes

Question 45

In pinocytosis

Answer 45

a cell continually “gulps”
droplets of extracellular fluid into tiny
vesicles, formed by infoldings of the plasma
membrane. In this way, the cell obtains
molecules dissolved in the droplets. Because
any and all solutes are taken into the cell,
pinocytosis as shown here is nonspecific for
the substances it transports. In many cases, as
above, the parts of the plasma membrane that
form vesicles are lined on their cytoplasmic
side by a fuzzy layer of coat protein; the “pits”
and resulting vesicles are said to be “coated.

Question 46

Receptor-mediated endocytosis is

Answer 46

a
specialized type of pinocytosis that enables
the cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the plasma
membrane are proteins with receptor sites
exposed to the extracellular fluid. Specific
solutes bind to the receptors. The receptor
proteins then cluster in coated pits, and
each coated pit forms a vesicle containing
the bound molecules. The diagram shows
only bound molecules (purple triangles) inside
the vesicle, but other molecules from the
extracellular fluid are also present.
After the ingested material is liberated
from the vesicle, the emptied receptors are
recycled to the plasma membrane by the
same vesicle (not shown)

Question 47

Endocytosis and exocytosis also provide mechanisms for
rejuvenating or remodeling the plasma membrane. These
processes occur continually in most eukaryotic cells, yet

Answer 47

the
amount of plasma membrane in a nongrowing cell remains
fairly constant. The addition of membrane by one process
appears to offset the loss of membrane by the other.

Question 48

cells acquire

Answer 48

chemical energy to do the

work of life.