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Membrane
Structure and
Function
Chapter Concepts
4
...
68
• The membrane contains lipids and proteins
...
70

4
...
Ions and other types of molecules
need assistance to cross the membrane
...
3 Diffusion and Osmosis
• Molecules spontaneously diffuse (move from an
area of higher concentration to an area of lower
concentration), and some can diffuse across a
plasma membrane
...
74

4
...
76

4
...
78

A single cell about to be pierced by a fine probe so that DNA can
be removed by the suction tube on the bottom
...


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68

Part 1

4-2

Cell Biology

B

anners flying on a castle wall mark off the community
within from the surrounding countryside
...
Commands passed along from royalty to
knights to workers are obeyed by all
...
Only certain small creatures can
pass through the open slitlike windows, and the drawbridge
must be lowered for most needed supplies
...
Under the command of the nucleus, the organelles
carry out their specific functions and contribute to the working of the cell as a whole
...
The cell uses these nutrients as a
source of building blocks and energy to maintain the cell
...


4
...
4
...
The phospholipid bilayer has a fluid consistency,
comparable to that of light oil
...
This description of the plasma membrane is called the
fluid-mosaic model of membrane structure
...
The hydrophilic (water loving) polar heads of
the phospholipid molecules face the outside and inside of
the cell where water is found, and the hydrophobic (water
fearing) nonpolar tails face each other (Fig
...
1)
...
Glycolipids have a structure similar to phospholipids except that the hydrophilic
head is a variety of sugars joined to form a straight or

Outside cell

glycolipid

hydrophilic
heads

carbohydrate
chain

glycoprotein

hydrophobic
tails

hydrophilic
heads

phospholipid
bilayer

integral
protein

cholesterol

peripheral
protein

plasma
membrane

Inside cell
filaments of
the cytoskeleton

Figure 4
...


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The membrane is composed of a phospholipid bilayer in which proteins are embedded
...
The hydrophobic tails make up the interior of the membrane
...


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4-3

Chapter 4

branching carbohydrate chain
...
Cholesterol reduces the permeability of the membrane to most biological
molecules
...
Peripheral proteins occur either on the
outside or the inside surface of the membrane
...
Still
others are held in place by noncovalent interactions that can
be disrupted by gentle shaking or by change in the pH
...
)
The fluidity of a phospholipid bilayer means that cells are
pliable
...
This has been demonstrated
by fusing mouse and human cells, and watching the movement of tagged proteins (Fig
...
2)
...
The fluidity of the
membrane is needed for the functioning of some proteins
such as enzymes which become inactive when the membrane solidifies
...


hydrophilic
regions

mouse cell

Many integral proteins are glycoproteins, which have
an attached carbohydrate chain
...
Therefore it can be said that the plasma membrane is “sugarcoated
...
The carbohydrate chains of the glycolipids and proteins occur only on the outside surface and
the cytoskeletal filaments attach to proteins only on the inside
surface
...
Peripheral proteins are found on the
outside and inside surface of the membrane
...


The Fluidity of the Plasma Membrane

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At body temperature, the phospholipid bilayer of the
plasma membrane has the consistency of olive oil
...
In each monolayer,
the hydrocarbon tails wiggle, and the entire phospholipid
molecule can move sideways at a rate averaging about
2 µm—the length of a prokaryotic cell—per second
...
2

Experiment to demonstrate lateral drifting of
plasma membrane proteins
...


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70

Part 1

4-4

Cell Biology

The Mosaic Quality of the Membrane
The plasma membranes of various cells and the membranes
of various organelles each have their own unique collections
of proteins
...
When you consider that the plasma
membrane of a red blood cell contains over 50 different
types of proteins, you can see why the membrane is said to
be a mosaic
...
As we will discuss in more detail, certain
plasma membrane proteins are involved in the passage of
molecules through the membrane
...
Still others are receptors; each type of receptor
protein has a shape that allows a specific molecule to bind to
it
...
Some plasma membrane proteins are enzymatic proteins that carry out metabolic reactions directly
...

Figure 4
...


Channel Protein
Allows a particular molecule or ion to cross the
plasma membrane freely
...


Carrier Protein
Selectively interacts with a specific molecule
or ion so that it can cross the plasma
membrane
...


Cell Recognition Protein
The MHC (major histocompatibility complex)
glycoproteins are different for each person, so
organ transplants are difficult to achieve
...


The mosaic pattern of a membrane is dependent
on the proteins, which vary in structure and function
...
The possible diversity
of the chain is enormous; it can vary by the number of sugars (15 is usual, but there can be several hundred), by
whether the chain is branched, and by the sequence of the
particular sugars
...
Therefore,
they make cell–cell recognition possible
...

As you probably know, transplanted tissues are often rejected by the body
...
We also now know that a person’s particular
blood type is due to the presence of particular glycoproteins
in the membrane of red blood cells
...
Pygmies are short, not because
they do not produce enough growth hormone,
but because their plasma membrane growth
hormone receptors are faulty and cannot
interact with growth hormone
...
The membrane
protein, adenylate cyclase, is involved in ATP
metabolism
...


Figure 4
...


These are some of the functions performed by proteins found in the
plasma membrane
...

Certain organs produce chemicals that are needed by other
ware, has been done for quite some time, but researchers alcells
...
Now
producing insulin, a molecule that causes all cells to take up
researchers have learned how to grow all sorts of human cells
glucose and the liver to store glucose as glycogen
...
Insulin-producing panneed for organ transplantation obsolete
...
The cells
encounters two hurdles that are hard to overcome: (1) there is
are encased in plastic capsules called microreactors, because rean overwhelming need, but few human organs are available to
actors are typically large vats where chemicals are produced
be transplanted; and (2) immunosuppressive drugs must be ad(Fig
...
These capsules are so small they can be placed into the
ministered even if the organs are carefully matched to the
abdomen where they will float freely and produce insulin as
recipient, because the body tends to reject foreign organs
...
The membrane of the capsule contains pores that are
address these problems, some researchers have turned to pigs
large enough to allow oxygen and nutrients to flow in and
as a source of organs for humans
...
But the membrane
they have crippled the enzymes that produce plasma memof a microreactor will prevent immune cells from coming into
brane carbohydrate chains on pig cells; therefore, the human
contact with the enclosed pancreatic cells
...
Pigs
actually come in contact with transplanted cells, they cannot
carry viruses such as the one that causes swine flu, but pig
recognize them as foreign and destroy them
...
Thereeven busily growing implantable liver tissue
...

be safely done
...
Tissue engineering is an endeavor that prosupply Alzheimer patients with nerve cells, and cardiac paduces manufactured bioproducts that can replace normal structients with heart cells, and so forth
...
Integra is an artificial skin that consists of a
porous matrix made of the protein collagen and a derivative of
shark cartilage
...
Integra is
used to cover extensive burns
...

Researchers have also had success growing human
cartilage for knee operations
...
All patients reported that
they were doing much better following the procedure
...
It is possible to
grow tissues to bolster weak ureters that take urine back
to the kidneys instead of to the bladder where it belongs
...
If research continues to be successful,
nearly every human tissue is expected to undergo tissue
engineering
...

implants
...


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72

Part 1

4-6

Cell Biology

Table 4
...
2

The Permeability of the Plasma
Membrane
The plasma membrane is differentially (selectively) permeable
...
4
...
Macromolecules cannot diffuse across
the membrane because they are too large
...

Noncharged molecules such as alcohols and oxygen are
lipid-soluble and therefore can cross the membrane with
ease
...
4

+

How molecules cross the plasma membrane
...


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the membrane
...
These molecules follow their concentration
gradient which is a gradual decrease in concentration over
distance
...
Therefore oxygen follows its concentration gradient as it enters a cell
...

Special means are sometimes used to get ions and
charged molecules into and out of cells
...
1)
...
Carrier proteins combine with an ion or molecule before transporting it across the membrane
...
Our discussion in this chapter is
largely restricted to carrier proteins
...

Ways of crossing a plasma membrane are classified as
passive or active (Table 4
...
Passive ways, which do not use
chemical energy, involve diffusion or facilitated transport
...
Active ways, which do require chemical energy, include active transport, endocytosis, and exocytosis
...

Certain substances can freely pass through the
membrane and others must be transported across
either by carrier proteins or by vacuole formation
...
Crystal of
dye is placed
in water

Figure 4
...
Diffusion of
water and dye
molecules

c
...


Diffusion is spontaneous, and no chemical energy is required to bring it about
...
When dye crystals are placed in water, they are concentrated in
one area
...
The dye dissolves in the water, and there is a net movement of dye molecules from higher to lower concentration
...
c
...


4
...
Diffusion is a physical process that can be
observed with any type of molecule
...
4
...
Therefore, the dye is eventually dissolved in the water, resulting in a colored solution
...
In this case, the solute is the dye
and the solvent is the water molecules
...

As discussed, the chemical and physical properties of
the plasma membrane allow only a few types of molecules
to enter and exit a cell simply by diffusion
...
Also, consider the movement of oxygen from the alveoli (air sacs) of
the lungs to blood in the lung capillaries (Fig
...
6)
...
The principle of diffusion can be employed in the treatment of certain human disorders, as is
discussed in the Science Focus on page 71
...
6

Gas exchange in lungs
...


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Molecules diffuse down their concentration
gradients
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74

Part 1

4-8

Cell Biology

less water
(higher
percentage
of solute)

solution rises
due to movement
of water toward
lower percentage
of solute

net movement of water
to inside of thistle tube
solute

water

10%

<10%

5%

more water
(lower
percentage
of solute)

> 5%

a
...


differentially permeable membrane

Figure 4
...


Osmosis demonstration
...
The beaker contains a
5% sugar solution
...
There is a net movement of water toward the inside of the thistle tube, where there is a lower percentage of water molecules
...


Osmosis
Osmosis is the diffusion of water into and out of cells
...
4
...
The beaker contains more water
molecules (lower percentage of solute) per volume, and the
thistle tube contains fewer water molecules (higher percentage of solute) per volume
...
The solute is unable to
pass through the membrane; therefore, the level of the solution within the thistle tube rises (Fig
...
7c)
...
A differentially permeable membrane separates two
solutions
...

2
...

3
...

4
...
Why? Because there is now less
solute per volume
...
Why? Because there is
now more solute per volume
...
Osmotic pressure is
the pressure that develops in a system due to osmosis2
...
Due to
osmotic pressure, water is absorbed from the human large
intestine, is retained by the kidneys, and is taken up by
capillaries from tissue fluid
...
In the laboratory, cells are normally placed in isotonic solutions; that is, the solute concentration is the same
on both sides of the membrane, and therefore there is no net
gain or loss of water (Fig
...
8)
...

A 0
...
Therefore, intravenous solutions medically administered usually have this tonicity
...
The
prefix hypo means less than, and refers to a solution with a
lower percentage of solute (more water) than the cell
...

Any concentration of a salt solution lower than 0
...
Animal cells placed in such a
solution expand and sometimes burst due to the buildup of
pressure
...

1

Percent solutions are grams of solute per 100 ml of solvent
...


2

Osmotic pressure is measured by placing a solution in an osmometer and then
immersing the osmometer in pure water
...


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4-9

Chapter 4

Membrane Structure and Function

75

plasma
membrane

Animal
Cells

Under isotonic
conditions, there is no net
movement of water
...


cell
wall

nucleus

Plant
Cells

In a hypertonic environment,
water leaves the cell, which
shrivels (crenation)
...


Figure 4
...


In a hypertonic environment,
vacuoles lose water, the
cytoplasm shrinks (plasmolysis),
and chloroplasts are seen in the
center of the cell
...


The arrows indicate the net movement of water
...


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The swelling of a plant cell in a hypotonic solution creates turgor pressure
...
The plant cell
does not burst because the cell wall does not give way
...
If you forget to water your plants they wilt due to decreased turgor pressure
...
The prefix
hyper means more than, and refers to a solution with a
higher percentage of solute (less water) than the cell
...

Any solution with a concentration higher than 0
...
If animal

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cells are placed in this solution, they shrink
...
Meats are
sometimes preserved by salting them
...

When a plant cell is placed in a hypertonic solution, the
plasma membrane pulls away from the cell wall as the
large central vacuole loses water
...

Dead plants you see along a salted roadside after the
winter died because they were exposed to a hypertonic
solution
...
In a hypotonic solution, a cell gains
water
...


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76

Part 1

4-10

Cell Biology

Outside

Inside

carrier

1

Outside

carrier

1

protein

Inside

protein

2
2
ene

3

3
Membrane

Figure 4
...
10 Active transport
...


A carrier protein speeds the rate at which a solute crosses a membrane
from higher solute concentration to lower solute concentration
...
(2) Molecule is transported across the
membrane and exits on inside
...


Active transport allows a solute to cross the membrane from lower
solute concentration to higher solute concentration
...
(2) Chemical energy of ATP is needed to transport the molecule
which exits inside of cell
...


4
...
For example, various sugar molecules
of identical size might be present inside or outside the cell,
but glucose can cross the membrane hundreds of times
faster than the other sugars
...

The carrier for glucose has been isolated and a model
has been developed to explain how it works (Fig
...
9)
...
After glucose binds to the open end of a carrier, it closes behind the
glucose molecule
...
After glucose is released into the cytoplasm of the cell, the carrier
changes its conformation so that the binding site for glucose
is again open
...
Apparently, the cell has a pool of extra glucose
carriers
...
Some forms of diabetes are caused
by insulin insensitivity; that is, the binding of insulin does
not result in extra glucose carriers in the membrane
...
Neither diffusion, explained previously, nor facilitated transport requires an expenditure of chemical energy because the
molecules are moving down their concentration gradient in
the same direction they tend to move anyway
...
Yet, biologically useful molecules do enter and
exit the cell at a rapid rate because there are carrier proteins
in the membrane
...
It is not completely understood how carrier proteins function; but after a carrier
combines with a molecule, the carrier is believed to undergo
a change in shape that moves the molecule across the membrane
...
1)
...


Facilitated Transport

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Facilitated transport explains the passage of such molecules
as glucose and amino acids across the plasma membrane,
even though they are not lipid soluble
...
These carrier

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4-11

Chapter 4

Membrane Structure and Function

77

Active Transport
During active transport, ions or molecules move through
the plasma membrane, accumulating either inside or outside the cell
...
In these instances,
substances have moved to the region of higher concentration, exactly opposite to the process of diffusion
...

Both carrier proteins and an expenditure of energy are
needed to transport molecules against their concentration
gradient (Fig
...
10)
...
Therefore, it is not surprising that cells involved primarily in active transport, such as kidney cells,
have a large number of mitochondria near the membrane
through which active transport is occurring
...
One
type of pump that is active in all cells, but is especially associated with nerve and muscle cells, moves sodium ions
(Naϩ) to the outside of the cell and potassium ions (Kϩ) to
the inside of the cell
...
A change in carrier shape after the attachment, and again after the detachment, of a phosphate
group allows the carrier to combine alternately with
sodium ions and potassium ions (Fig
...
11)
...

The passage of salt (NaCl) across a plasma membrane
is of primary importance in cells
...
First,
sodium ions are pumped across a membrane, and then
chloride ions simply diffuse through channels that allow
their passage
...
3, the chloride ion
channels malfunction in persons with cystic fibrosis, and
this leads to the symptoms of this inherited (genetic)
disorder
...


3 Na+
carrier

ATP
ATP is split, and
phosphate group is
transferred to carrier
...
New shape
allows carrier to take up
potassium ions (K+)
...


K+

Change in shape results
that causes carrier to
release potassium ions (K+)
inside the cell
...


Figure 4
...


A carrier protein actively moves three sodium ions (Naϩ) to the
outside of the cell for every potassium ion (Kϩ) pumped to the inside
of the cell
...


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During facilitated transport, substances follow their
concentration gradient
...


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K+

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78

4
...


plasma
membrane

red blood
cell

Exocytosis
During exocytosis, vesicles often formed by the Golgi apparatus and carrying a specific molecule, fuse with the plasma
membrane as secretion occurs
...


vacuole
a
...
Pinocytosis

Inside

receptor
protein

solute

Notice that the membrane of the vesicle becomes a part of
the plasma membrane
...


Endocytosis

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During endocytosis, cells take in substances by vesicle formation (Fig
...
12)
...

When the material taken in by endocytosis is large, such
as a food particle or another cell, the process is called phagocytosis
...
When the endocytic vesicle fuses
with a lysosome, digestion occurs
...
Blood cells, cells that line the kidney
tubules or intestinal wall, and plant root cells all use this
method of ingesting substances
...


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c
...
12 Three methods of endocytosis
...
Phagocytosis occurs when the substance to be transported into
the cell is large; certain specialized cells in the body can engulf wornout red blood cells by phagocytosis
...
b
...
The result is a small vacuole or vesicle
...
Receptor-mediated
endocytosis is a form of pinocytosis
...
The vesicle that forms contains the ligand
and its receptor
...
13
...


b
...
13 Receptor-mediated endocytosis
...
(1) The receptors in the coated pits combine only with a solute
...
(3) Solutes leave the vesicle
...
b
...


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Receptor-mediated endocytosis is a form of pinocytosis
that is quite specific because it involves the use of a receptor
protein shaped in such a way that a specific molecule such
as vitamins, peptide hormones, and lipoproteins can bind to
it
...
This location is called a coated
pit because there is a layer of fibrous protein on the cytoplasmic side (see step 1, Fig
...
13)
...
The fate of the vesicle and its contents
depends on the kind of solute it contains
...
A spent hormone, on the other hand, may be digested when the vesicle
fuses with a lysosome
...

Aside from simply allowing substances to enter cells selectively from an extracellular fluid, coated pits are also involved
in the transfer and exchange of substances between cells
...

The importance of receptor-mediated endocytosis is
demonstrated by a genetic disorder called familial hypercholesterolemia
...
These individuals have inherited a gene that
causes them to have a reduced number and/or defective receptors for LDL in their plasma membranes
...


Substances are secreted from a cell by exocytosis
...
Receptormediated endocytosis allows cells to take up
specific kinds of molecules and then sort them
within the cell
...
Fox, a
favorite movie actor, have Parkinson disease
...
Due to the death of brain cells that
produce a substance called dopamine, motor control is not as smooth as it should be
...
As the disease worsens, patients
become unable to carry out even the simplest activities
...
The blood-brain barrier

is simply due to the impermeability of the
capillaries serving the brain
...
Most drugs can’t get through at all
...
Along the way, physicians and
patients are faced with a wide assortment
of adjunct remedies
...
Michael J
...
An experimental
surgical procedure, however, involves the
transplantation of dopamine-producing
fetal tissue into the brains of people with

Summarizing the Concepts
4
...
In the lipid bilayer, phospholipids are arranged with their hydrophilic heads at the surfaces and their hydrophobic tails in the
interior
...
Glycolipids and glycoproteins are involved in marking the cell as belonging to a particular
individual and tissue
...
Proteins act as receptors, carry on enzymatic reactions, join
cells together, form channels, or act as carriers to move substances
across the membrane
...
2 The Permeability of the Plasma Membrane
Some substances like gases and water are free to cross a plasma membrane, and others, particularly ions, charged molecules, and macromolecules, have to be assisted across
...
Active ways of crossing a plasma
membrane (active transport and vesicle formation) do require an expenditure of chemical energy
...
3 Diffusion and Osmosis

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Lipid-soluble compounds, water, and gases simply diffuse across the
membrane from the area of higher concentration to the area of lower
concentration
...
People who have received such transplants report a lessening of symptoms
...
Is it ethical to use fetal tissue to prevent
older people from having a debilitating
disorder? Why or why not?
2
...
Do you favor banning all research using
fetal tissue, or doing such research under
certain circumstances? Explain
...
Water moves across the membrane into the
area of lower water (higher solute) content
...


4
...

During facilitated transport, a carrier protein assists the movement of a molecule down its concentration gradient
...

During active transport, a carrier protein acts as a pump that
causes a substance to move against its concentration gradient
...
Energy in the form of ATP molecules is required
for active transport to occur
...
5 Exocytosis and Endocytosis
Larger substances can enter and exit a membrane by endocytosis and
exocytosis
...
Endocytosis includes
phagocytosis and pinocytosis which includes receptor-mediated endocytosis
...
Once specific solutes bind to
their receptors, the coated pit becomes a coated vesicle
...


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4-15

Chapter 4

Studying the Concepts
1
...
68–70
2
...
What are the mechanisms by which substances enter and exit
cells? Which are passive ways, and which are active ways? 72
4
...
73
5
...
Define isotonic, hypertonic, and hypotonic
solutions, and give examples of how these concentrations
affect red blood cells
...
Draw a simplified diagram of a red blood cell before and
after being placed in these solutions
...
Draw a simplified diagram of a plant cell before and after
being placed in these solutions
...
75
8
...
How does active transport differ from facilitated transport?
Give an example
...
Diagram and define endocytosis and exocytosis
...
78–79

Testing Yourself
Choose the best answer for each question
...
Label this diagram of the plasma membrane
...

a
...


b
...


f
...

i
...


h
...
The fluid-mosaic model of membrane structure refers to
a
...

b
...

c
...

d
...

e
...


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Membrane Structure and Function

81

3
...
The tails
are found
a
...

b
...

c
...

d
...

e
...

4
...
all molecules move only from the area of higher to lower
concentration
...
solvents move from the area of higher to lower concentration
...
there is a net movement of molecules from the area of
higher to lower concentration
...
a cell must be present for any movement of molecules to
occur
...
molecules move against their concentration gradient if
they are small and charged
...
When a cell is placed in a hypotonic solution,
a
...

b
...

c
...

d
...

e
...

6
...
solute exits the cell to equalize the concentration on both
sides of the membrane
...
water exits the cell toward the area of lower solute concentration
...
water exits the cell toward the area of higher solute
concentration
...
solute exits and water enters the cell
...
Both a and c are correct
...
Active transport
a
...

b
...

c
...

d
...

e
...

8
...
helps establish an electrochemical gradient across the
membrane
...
concentrates sodium on the outside of the membrane
...
utilizes a carrier protein and energy
...
is present in the plasma membrane
...
All of these are correct
...
A scientist observing a protozoan notices a vacuole discharging its contents at the plasma membrane
...
phagocytosis and vacuole formation
...
endocytosis and active transport
...
exocytosis and secretion
...
active transport and vacuole release
...
Both c and d are correct
...
Receptor-mediated endocytosis
a
...

b
...

c
...

d
...

11
...
Justify your conclusions
...


cell
wall

b
...
Considering the movement of molecules across the plasma
membrane:
a
...

b
...
4
...

c
...

d
...
Exocytotic vesicles add plasma membrane to the cell, and
endocytotic vesicles remove plasma membrane
...
In a cell in which the amount of plasma membrane stays
constant, how many exocytotic vesicles per endocytotic
vesicles would you expect?
b
...
If vesicle
formation is facilitating movement, where would you
expect exocytosis to be occurring? Where would you expect endocytosis to be occurring?
c
...
Imagine a virus that
enters a cell in this manner
...

Movement of molecules from a region of higher
concentration to a region of lower concentration
...

Internal pressure that adds to the strength of a cell
and builds up when water moves by osmosis into a plant cell
...

Solution that contains the same concentration of
solute and water as the cell
...

Passive transfer of a substance into and out of a
cell along a concentration gradient by a process that requires
a carrier
...

Process in which an intracellular vesicle fuses
with the plasma membrane so that the vesicle’s contents are
released outside the cell
...


Exploring the Internet
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...
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