Download Transport through the cell membrane

Transport through the cell membrane either by
diffusion or active transport
Diffusion
divided into two subtypes called simple diffusion
and facilitated diffusion
Simple diffusion means that movement of
molecules or ions through the cell membrane
opening according to concentration gradient
(1) through the lipid bilayer if the diffusing substance is lipid soluble like
oxygen, nitrogen, carbon dioxide, and alcohols.
(2) through protein channels that penetrate all the way through the cell
membrane
The protein channels are distinguished by two
important characteristics:
1-they are often selectively permeable to certain
molecules or ions This depend on the diameter,
shape, and the nature of the electrical charges
2- can opened or closed by gates
a-Voltage gating. In this case opening of the gate responds to the
electrical potential across the cell membrane.
example is sodium channel when the inside of the cell membrane is
negative, this cause the sodium gates to remain closed; conversely,
when the inside of the membrane loses its negative charge, these
gates would open suddenly and allows sodium to pass inward
through the sodium pores.
b-Chemical (ligand) gating. They are opened by the
binding of a chemical substance with them. Example for
this is acetylcholine channel.
Facilitated Diffusion
carrier-mediated diffusion
substance diffuses through the membrane using a specific
carrier protein.
glucose and most of the amino acids
Factors That Affect Net Rate of
Diffusion
1-Concentration Difference
2-The electrical charges of the ions cause them to
move through the membrane even though no
concentration difference exists to cause
movement.
3-Effect of a Pressure Difference Across the
Membrane. The net movement of molecules from
the high-pressure side toward the low-pressure
side.
Osmosis
Diffusion of Water Across Selectively Permeable
Membranes from a region of low solute
concentration (high water concentration) to one
with a high solute concentration (low water
concentration)
Water is the most abundant substance that
diffuses through the cell membrane
ACTIVE TRANSPORT
movement of a substance from area of low
concentration of this substance to the area of
high concentration for that substance usually
need energy
Active transport is divided into two types
primary active transport and secondary active
transport
primary active transport
the energy is derived directly from ATP
Among the substances that are transported by
primary active transport are sodium, potassium,
calcium ,hydrogen, chloride, and a few other
ions
Na+-K+ pump
The liberated energy cause a
conformational change in the
carrier protein ,extruding the
three Na to the outside and the
two K to the inside.
net of apositive charge is moved
to the exterior. This creates
positivity outside the cell and
negativity on the inside.
Therefore, the Na+-K+ pump is
called electrogenic.
Calcium pump
the carrier protein has a binding site for Ca
And there is ATPase enzyme to liberate the energy
Calcium ions are maintained at low concentration
inside the cell. This is achieved mainly by calcium
pumps. One is in the cell membrane and pumps
calcium to the outside of the cell. The other pumps
calcium ions into the sarcoplasmic reticulum of
muscle cells and the mitochondria in all cells.
Hydrogen Ions
primary active transport of hydrogen ions is very
important at two places in the body, :
(1) in the gastric glands of the stomach
(2) in the late distal tubules and cortical
collecting ducts of the kidneys
In secondary active transport
the energy is derived secondarily from energy
that release in simple diffusion
Secondary Active Transport…. Co-Transport and
Secondary Active Transport ….Counter-Transport
Co-transport
excess sodium outside the cell cause sodium to diffuse to the inside
the cell. this diffusion of sodium can pull other substances with the
sodium through the cell membrane. This is called co-transport.
The carrier in this instance has as an attachment site for both the
sodium ion and the substance. Once they both are attached, the
energy gradient of the sodium ion causes both the sodium ion and the
other substance to be transported together .Co-Transport of Glucose
and Amino Acids
C0-transport
counter-transport
sodium attempt to diffuse to the interior of the
cell in this time, the substance to be
transported is the outside. Once both have
bound, a conformational change occurs, and
energy released by the sodium ion moving to
the interior causes the other substance to move
to the exterior.
sodium-calcium counter-transport and
sodium-hydrogen counter-transport.
Counter transport
Active Transport Through Cellular
Sheets
Transport of this type occurs through the
(1) intestinal epithelium,
(2)epithelium of the renal tubules
(3) epithelium of all exocrine glands
(4) epithelium of the gallbladder, and
The basic mechanism for transport of a
substance through a cellular sheet is
(1) active transport through the cell membrane
on one side of the transporting cells in the
sheet, and then (2) either simple diffusion or
facilitated diffusion through the membrane on
the opposite side of the cell.
sodium and water diffuse readily from the lumen into the interior of the
cell. Then, at the basal and lateral membranes of the cells, sodium ions
are actively transported into the extracellular fluid of the surrounding
connective tissue and blood vessels.
water diffuses from a region of low solute concentration (high water
concentration) to one with a high solute concentration (low water
concentration)
Transport through the cell membrane
Active transport
Diffusion
Facilitated
diffusion
Simple diffusion
Protein
channels
selective
Cell
memb.
pores
Open and
close by gate
Voltage
Primary active
transport
Co-transport
ligand
Secondary
transport
Counter
transport