Chapter 4: Transport of substances through cell membranes Flashcards
not miscible with either the extracellular fluid or the intracellular fluid.
lipid bilayer
it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments.
lipid bilayer
contains alarge amount of sodium but only a small amount of potassium
extracellular fluid
watery spaces all the way through the molecules and allow free movement of water, as well as selected ions or molecules
channel proteins or carrier proteins
bind with molecules or ions that are to be transported;conformational changes in the proteinmolecules thenmove the substances through theninterstices of the proteinto the other side of the
membrane
carrier proteins
Transport through the cell membrane, either directly through thelipid bilayer or through the proteins, occurs by one of twobasic processes
diffusion or active transport
means random molecular movement of substances molecule by molecule either through intermolecular spaces in the membrane or in combination with a carrier protein
diffusion
The energy that causes diffusion is the energy
of the?
normal kinetic motion of matter
movement of ions or other substances across the membrane in combination with a carrier protein in such a
way that the carrier protein
active transport
Motion of these particles is what
physicists call?
“heat”
—the greater the motion the higher the temperature—and the motionnever ceases
under any condition except at absolute zero temperature
continual movement of molecules among one another in liquids or in gases
diffusion
Diffusion through the cell membrane is divided into two subtypes called?
simple diffusion and facilitated diffusion
means that kinetic movement of molecules or ions occurs through a membrane openingor through intermolecular spaces without any interaction with
carrier proteins in the membrane
simple diffusion
requires interaction of a
carrier protein
facilitated diffusion
Simple diffusion can occur through the cell
membrane by two pathways:
(1) through the interstices of the lipid bilayer if the diffusing substance is lipid soluble
(2) through watery
channels that penetrate all the waythrough some
of the large transport proteins
one of the most important
factors that determines how rapidly a substance
diffuses through the lipid bilayer is the?
lipid solubility
can dissolve directly in
the lipid bilayer and diffuse through the cell membrane inthe same manner that diffusion of water solutes occurs in awatery solution
oxygen
nitrogen
carbon dioxide
alcohols
highly insoluble in the membrane lipids
water
composed of integral cell membrane proteins that form open tubes through the
membrane and are always open
pores
permit rapid passage of water through cell membranes but
exclude other molecules
aquaporins or water channels
The protein channels are distinguished by two important characteristics:
(1) They are often selectively permeable to certain substances
(2)
many of the channels can be opened or closed by
gates that are regulated by electrical signals
(voltage-gated channels) or chemicals that bind tothe channel proteins (ligand-gated channels).
permit passage of potassium ions across the cell membrane about 1000
times more readilythan they permit passage of sodium ions
potassium channels
What is the mechanism for this remarkable ion selectivity?
when the structure of a bacterial potassium channel was determined by x-ray crystallography
Potassium channels were found to have a
tetrameric
Even though water is highly insoluble in the
membrane lipids, it readily passes
through channels in protein molecules
the total
amount ofwater that diffuses in each direction
through the red cell membrane during each second
is about _________ as great as the volume of the red
cell itself
100 times
the diameter of the urea molecule is only ________ greater than that of water, yet its penetration
through the cell membrane pores is about 1000
times lessthan that of water
20 percent
structure consisting of four identical protein subunits surrounding a central pore
At
the top of the channel pore are
pore loops
Lining the selectivity filter are?
carbonyl oxygens
Different selectivity filters for the various ion
channels are believed to determine, in large part, the specificity of the channel for cations or anions or for particular ions, such as _______, _______, ________ that gain access to the channels
Na, K, Ca
one of the most important of the protein channels, only 0.3 by 0.5 nan-
ometer in diameter, but more important, the
inner surfaces of this channel are lined with amino
acids that are strongly negatively charged
sodium channel
strong negative charges can pull small dehydrated sodium
ions into these channels, actually pulling the sodium ions away from their hydrating water molecules
provides a means of controlling ion
permeability of the channels.
gating of protein channels
The opening and closing of gates are controlled
in two principal ways:
° voltage gating
° Chemical (ligand) gating
responds to the electrical potential across the cell membrane
voltage gating
Some protein channel gatesare opened by the binding of a chemical substance (aligand) with the protein; this
causes a conformationalorchemical bonding
change in the protein molecule thatopens or
closes the gate
chemical gating or ligand gating
one of the most important instances of chemical gating is the effect of acetylcholine on the so called?
acetylcholine channel
This gate is exceedingly important for
the transmission of nerve signals from one nerve cell to another
chemical gating
Facilitated diffusion is also called
carrier mediated diffusion
using a specific carrier protein to help.
That is, the carrier facilitates diffusion ofthe substance to the other side
facilitated diffusion
Facilitated diffusion differs from simple diffusion in the following important way:
✓ increases proportionately with the concentration of the diffusing substance
✓ facilitated diffusion approaches a maximumrate called the Vmax
in facilitated diffusion the rate of diffusion approaches a maximum
Vmax
most important substances that
cross cell membranes by facilitated diffusion are?
glucose and most of the amino acids
in the case of
glucose, at least five glucosetransporter molecules
have been discovered in various tissues
monosaccharides that have structures similar to that of glucose,
including;
galactose and fructose
one of these,
glucose transporter 4 (GLUT4), is activated by?
insulin
if an electrical potential is applied across the
membrane the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement
nernst potential
The positive
charge attracts the negative ions, whereas the
negative charge repels them
net diffusion
The pressure
is about ________ greater inside the capillary than
outside
20mm Hg
means the sum of all the forces
of the different molecules striking a unit surface
area at a given instant
pressure
most abundant substance that diffuses
through the cell membrane
water
Enough water
ordinarily diffuses in each direction through the
red cell membrane per second to equal about __________ the volume of the cellitself
100 times
process of
net movement of water caused by a concentration
difference of water
osmosis
exact amount of pressure required to
stop osmosis
osmotic pressure
The osmotic pressure exerted by
particles in a solution,whether they are molecules or ions, is determined by the
number of
particles per unit volume of fluid, not bythe mass
of the particles
terms of numbers of
particles, the unit called?
osmole
normally body temperature
37°C
can
develop across a membrane, just as concentration differences for other substances can
occur.
concentration difference to water
process of
net movement of water caused by a concentration
difference of water is called
osmosis
is the osmolar
concentration expressed as osmoles per liter of
solution rather than osmoles per kilogram of water
osmolarity
When a cell membrane moves molecules or ions
“uphill
” against a concentration gradient (or
“uphill
” against an electrical or pressure gradient),
the process is called
active transport
the energy is derived
directly from breakdown of adenosine triphosphate (ATP) or of some other high-energy
phosphate compound
primary active transport
the energy is derived secondarily from energy that has been stored inthe form of ionic
concentration differences of secondary molecular
or ionic substances between the two sidesof a cell
membrane, created originally by primary active
transport
secondary active transport
Among the substances that are transported
by primary active transport are;
° sodium
° potassium
° calcium
° hydrogen
° chloride
The active transport mechanism that has been
studied in greatest detail
sodium- potassium pump
a transport process
that pumps sodium ions outward through the
cell membrane of all cells and at thesame time
pumps potassium ions from the outside tothe
inside
sodium-potassium pump
This pump is responsible for maintaining
the sodium and potassium concentration differences acrossthe cell membrane, as well as for
establishing a negative electrical voltage inside the
cells
sodium-potassium pump
is a complex of two separate globular proteins
carrier proteins
large, with a molecular
weight of about 100,000
α subunit
large, with a molecular
weight of about 100,000
α subunit
smaller,with a molecular weight of about 55,
000
B subunit
the larger protein has three specific features that are important
for the functioning of the pump:
- It has three receptor sites for binding sodium
ions onthe portion of the protein that protrudes to the inside of the cell. - It has two receptor sites for potassium ions
on the outside. - The inside portion of this protein near the
sodiumbinding sites has ATPase activity
enzymes that catalyze the decomposition of ATP into ADP
ATPase
Another important primary active transport
mechanism
calcium pump
are normally maintained at extremely low concentration in the intracellularcytosol of virtually all
cells in the body, at a concentrationabout 10,000
times less than that in the extracellular fluid
calcium ions
At two places in the body, primary active transport hydrogen ions is important;
1) gastric glands of the stomach (2) late distal
tubules and cortical collecting ducts of the kidneys
have the most potent primary active mechanism for transporting hydrogen ions of any
part of the body.
parietal cells
In the renal tubules are special __________in thelate distal tubules and cortical collecting
ducts that alsotransport hydrogen ions by primary active transport
intercalated cells
this diffusion energy
of sodium can pull other substances along with the
sodium through the cell membrane. This phenomenon is called?
co-transport
co-transport is one form of?
secondary active transport
sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient
counter transport
the
conformational change takes place automatically, and the sodium and glucose are transported to
the inside ofthe cell at the same time.
sodium-glucose co-transport
especially
important mechanisms in transporting glucose
across renal and intestinal epithelial cells
sodium-glucose co-transport
other important co-transport mechanisms in
at least some cells include;
co-transport of chloride ions, iodine ions, iron ions, and urate ions
Two especially important counter-transport mechanisms;
sodium-calcium counter-transport sodium-hydrogen counter-transport
occurs through allor almost all cell membranes, with sodium
ions moving tothe interior and calcium ions to the
exterior, both bound to the same transport protein in a counter-transportmode
sodium-calcium counter-transport
occurs in
several tissues.
sodium-hydrogen counter transport
Active Transport Through Cellular Sheets occurs through the?
✓ intestinal epithelium
✓ epithelium of the renal tubules
✓ epithelium of all exocrine glands
✓ epithelium of
the gallbladder
✓ membrane of the choroid
plexus of the brain
The basic mechanism for transport of a
substance through a cellular sheet
active transport
the epithelial cells
are connected together tightly at the luminal pole
by means of junctions called?
kisses
