Membrane transport: The set of transport proteins in the plasma
membrane, or in the membrane of an intracellular organelle, determines
exactly what solutes can pass into and out of that cell or organelle. Each
type of membrane therefore has its own characteristic set of transport
proteins.
Each type of
transport protein
transports a
particular type of
molecules –
selective set of
solute are
transported in or
out
Two major classes of membrane transmembrane proteins
Carrier proteins bind a solute on
one side and deliver it to the other
side through a change in shape.
Cells can also transport
macromolecules across the
membrane
Channel proteins form tiny
hydrophilic pores in the
membrane and the specific
molecules pass through by
diffusion from high to low
concentration. Most are ion
channels
The ion concentrations inside a cell are very different from those
outside
Because ions are
electrically charged,
their movements can
create powerful electric
forces across the
membrane. Important in
nerve cells, muscle cells,
and in the mitochondria,
for ATP synthesis using
the electron transport
chain.
Ion transport across cell
membranes is of central
importance in biology. Cells
maintain an internal ion
composition very different
from that in the fluid around
them and these differences are
crucial for the cell’s survival
and function.
Differential ion concentrations crucial.
Animal cells pump Na+ out.
If the pumping fails, water
flows in by osmosis and
causes the cell to swell and
burst.
The positive and negative
charges must be balanced by
an almost exactly equal
quantity both inside and
outside of the cell
nucleic acids, proteins, etc.
Carrier proteins are required for the transport of almost all small organic
molecules across the cell membranes. Each carrier is highly selective,
often transporting just one type of molecule.
Each cell, each
organelle has
particular
transport
proteins.
ADP
The membrane transport proteins studied have polypeptide chains that
traverse the lipid bilayer multiple times, forming a continuous proteinlined pathway allowing selected small hydrophilic molecules to cross
without coming into contact with the hydrophobic lipid bilayer.
A basic difference
between carrier
proteins and channel
proteins is the way they
discriminate between
solutes. Channel
proteins mainly go on
size and electric charge.
Carrier proteins
actually bind their
molecules it transfers,
and then changes
conformation
Passive transport – no energy needed, with the concentration gradient
Active transport - energy needed, against the concentration gradient
Passive transport with a carrier
protein = facilitated transport
Passive transport of glucose: carrier exists in at least to states
(shapes). Example: liver cell after a large meal – lots of glucose
outside in the extracellular fluid.
Opposite in liver when blood glucose becomes low. Liver cells
breakdown glycogen. Glucose levels are higher inside the cell now.
Passive transport moves glucose outside the cell.
Electrically charged molecules diffuse according to their concentration
gradient and according to their charge – electrochemical gradient.
Membranes
typically have
a voltage
difference
across them.
Negative
inside.
Na+ is at a higher
concentration
outside of the cell so
it tends to enter the
cell if given the
chance.
inside
outside
steep gradient
K+ is present at a
higher concentration
inside the cell –
therefore there is little
movement
Active transport moves solutes against their electrochemical gradients.
against
with
Animal cells use the Energy of ATP hydrolysis to pump Na+ out of the
cell to maintain the electrochemical gradient. This gradient is then used
to pump other molecules into or out of the cell against their
elecrochemical gradient.
Na+-K+ ATPase
or Na+-K+ pump
other ATP driven
pumps create
electrochemical
gradients of H+
ions. Next chapter
operates ceaselessly
Na+-K+ ATPase
or Na+-K+ pump
Animal cells use the Na+ gradient to take up nutrients actively.
The Na+-K+ pump helps maintain the osmotic balance of animal cells.
• Cytosolic Ca+2 concentrations are kept low
by Ca+2 pumps.
• Influx of Ca+2 is tightly regulated, since
Ca+2 binds molecules (enzymes) and alters
their activities (activation or inhibition).
• Influx of Ca+2 through Ca+2 channels is
often used as a signal to trigger other
intracellular events (muscle contraction).
• The cell maintains a low concentration, so
that signaling via increases is kept sensitive.
Ion channels are ion selective and gated. Tthey show ion selectivity
depending on the diameter and shape of the ion channel and on the
distribution of charged amino acids in its lining.. Most ion channels are
gated: they can switch between an open and a closed state by a change
in conformation, which is regulated by conditions inside and outside the
cell.
Ion channels are important in signaling in neurons
If the plasma membrane of animal cells was made permeable to Na+ and K+, the Na+K+ pump would:
A. be completely inhibited.
B. begin to pump Na+ in both directions.
C. begin synthesizing ATP instead of hydrolyzing it.
D. continue to pump ions and to hydrolyze ATP but the energy of hydrolysis
would be wasted, as it would generate heat rather than ion gradients.
E. continue to pump ions but would not hydrolyze ATP.
Ca2+ pumps in the plasma membrane and endoplasmic reticulum are important for:
A. maintaining osmotic balance.
B. preventing Ca2+ from altering the behavior of molecules in the cytosol.
C. providing enzymes in the endoplasmic reticulum with Ca2+ ions that are necessary
for their catalytic activity.
D. maintaining a negative membrane potential.
E. helping cells import K+.