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Summary Essential Cell Biology CH12
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Essential Cell Biology 5th Edition by Alberts
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Essential Cell Biology 5th Edition by Alberts
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COMPLETE TEST BANK: ESSENTIAL CELL BIOLOGY 5TH EDITION ALBERTS [ CONTAIN ANSWER KEY Questions with 100% correct Answer.
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7
Transport across cell membranes
Transporters = membrane protein that shifts small Cells contain two classed of membrane transport
organic molecules or inorganic ions from one side of proteins: transporters and channels.
the lipid bilayer to the other by changing shape. Each membrane transport protein allows the
Channels = membrane proteins that form tiny passage of only select members of a particular
hydrophilic pores across the membrane through which type. Channels discriminate mainly on the basis of
substances can pass by diffusion. size and electric charge: when the channel is open,
2 only ions of an appropriate size and charge can
These are membrane transport proteins that pass through. A transporter transfers only those
provide passageways across membrane for only
one particular small, water-soluble substance to molecules or ions that fit into specific binding sites
enter the cell. on the protein. Great specificity Selectivity
PRINCIPLES OF TRANSMEMBRANE TRANSPORT
Water-soluble molecules and ions have difficulty
crossing a lipid bilayer, because they have to enter
the fatty environment. This environment consists
of the hydrophobic tails of the lipid bilayers, which
are reluctant to interact with water.
Hydrophilic molecules can cross the cell membrane by
simple diffusion (this is very slow) and by facilitated
transport (passage is accelerated by specialised
membrane transport proteins). Solutes cross membranes either by passive or active
transport.
Any molecule can diffuse through Passive transport = transport of substances
the lipid layer but it takes time. through the cell membrane without using energy by
This time is dependent of on the the membrane transport protein. Channels and
:
transporters
size of the molecule and its spontaneous flow from high to low concentration
solubility properties. In general, Active transport = the movement of a solute
the smaller the molecule and the against its concentration gradient which need an
more hydrophobic, or nonpolar, it energy source to power the transport. Transporters pumps
called
is, the more rapidly it will diffuse
across the lipid bilayer.
- Small, nonpolar molecules: can
cross easily
- Uncharged polar molecules: small
ones can cross with a redacted
rate and large ones have
difficulty.
- Charged substances (ions): can't
cross
The ion concentrations inside a cell are very Both the concentration
different from those outside, because lipid bilayers gradient and membrane
are impermeable to inorganic ions. The differences potential influence the
in concentratieon is crucial for a cell's survival and
passive transport of
function. Inside the cell, K is the most abundant charged solutes. The
positively charged ion (balanced by inorganic and direction of uncharged
organic ions), whereas Na is the most outside molecules of passive
(balanced by Cl ). transport is determined
solely by its concentration
Differences in the concentration of inorganic ions gradient. A charged solute
across a cell membrane create a membrane will also tend to move
potential. down its concentration
Membrane potential = a voltage difference across gradient and the direction
the membrane created by electrical imbalances is also determined by the
Resting membrane potential = the voltage across membrane potential
the cell membrane of an unstimulated cell with its (voltage, positive in
movement of cation and anions across the negative out). This is
membrane is precisely balanced, steady-state called electrochemical
conditions. But isn't zero, between -20 and -200 mV. gradient.
ij
, The electrochemical gradient for sodium and For fuel the body: glucagon stimulates liver cells to
potassium ions are different. The membrane produce glucose by breaking down glycogen, since
potential and the concentration gradient for each glucose concentration is higher inside the liver cells
ion combine influence the movement across the
+
that outside, glucose is transported out and used by
plasma membrane. Na is abundant outside of the other cells.
cell and will flow into the cell with a net force being
great since voltage and concentration, gradient
+
works in the same direction. K is abundant inside
the cell, but the outside cell is pos. So voltage and
concentration gradient work in different direction
so net force is weak. (Read page 394 again)
Water moves across the cell membranes down its
concentration gradient - a process called osmosis.
Water can diffuse across the lipid bilayer, but this
happens very slowly. The flow of water is
facilitated by aquaporins. Water will enter due to Pumps actively transport a solute against its
osmolarity. The resulting osmotic gradient tends to electrochemical gradient. Cells depend on
pull water into the cell, low solute concentration to transmembrane pumps, active transporters, carrying
an area of high solute concentration (=osmosis). out active transport in three ways:
How is the osmotic equilibrium maintained in 1. Gradient-driven pumps: link uphill transport of one
different cells? solute across a membrane to downhill transport of
ANIMAL: using transmembrane pumps to expel another.
solutes 2. ATP-driven pumps: use energy released by the
PLANTS: are prevented form swelling by their tough hydrolysis of ATP drive uphill transport.
cell walls and so can tolerate a large osmotic 3. Light driven pumps: use energy derived from
difference across the plasma membrane; use sunlight to drive uphill transport.
osmotic swelling to keep their cell walls tense so
the stems are rigid and the leaves extended.
PROTOZOA: eliminate excess water using
contractile vacuoles that periodically discharge their
contents to the exterior.
+
The Na pump in animal cells uses energy supplied by
+
ATP to expel Na and bring in K . The sodium pumps in
+
animal cells uses the energy supplied by ATP hydrolysis
to maintain the concentration gradients of sodium
and potassium ions. The energy form ATP hydrolysis
fuels a stepwise series of conformational changes
t
that drives the exchange of Na and K ions. The
+
phosphate group removed of ATP gets transferred
TRANSPORTERS AND THEIR FUNCTION to the pump itself. The toxin ouabain inhibits the Na
1
pump by preventing the binding of extracellular K ,
arresting the cycle.
Passive transporters move a solute along its
electrochemical gradient. A glucose transporter
mediates passive transport in the plasma +
membrane, which crosses the membrane at least The Na pump generates a steep concentration
12 times, and can adopt several conformations. gradient of Na across the plasma membrane.
Glucose is uncharged so the electrochemical 2-1 2-1
gradient is zero and transport direction is Ca pumps keep the cytosolic Ca concentration
2-1
determined by its concentration gradient. low. Ca can bind tightly to a variety of proteins in2-1
Glucose, plentiful in outside cells, binds to the cell, altering their activities. An influx of Ca
2-1
external binding site on transporter, switches into the cytosol through Ca channels is used by
conformation, released into cytosol (low different cells as an intracellular signal.
concentration)(after meal).
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