Transcript Transport across cell membranes

Located in almost all
animals
 Carries electrical
impulses on the
specialized cell
membrane of the nerve
cells
 Nerve cells coordinate
the opening and closing
of ion channels in the
cell membrane to
create an action
potential.
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 Made
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of two layers of phospholipids
a.k.a. a lipid bilayer
 Proteins
and cholesterols are embedded in
the membrane
1. Passive transport
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Simple diffusion
Facilitated diffusion
2. Active transport
3. Endocytosis and exocytosis
a) Simple diffusion
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Materials move from areas of high concentration
to areas of low concentration
Energy from the cell is not needed
When concentrations are equal, molecules still
move but no net change results
 Things
like water, oxygen, and carbon
dioxide move by simple diffusion
 The
diffusion of water across a selectively
permeable membrane is called osmosis
Osmotic pressure = a force driven by differences in solute concentration; can
cause cells to shrink or swell
Hypertonic = “above strength,” more solute present
in the solution
Hypotonic = “below strength,” fewer solutes present
in the solution
Isotonic = “same strength,” same amount of solute
in solution as in cell
b) Facilitated diffusion
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Channel proteins and carrier proteins help larger
molecules cross the membrane
Many channels are gated for regulation
Carrier proteins change shape to move molecules
through
More specific than simple diffusion but still does
not require energy
 Things
like proteins and sugars move by
facilitated diffusion
 Cells
sometimes need to move substances
against the concentration gradient
 Often carrier proteins act as pumps
 Energy is required

Supplied by ATP
 Amino
acids, sugars, and ions are often
moved by active transport
a) Sodium ions normally are in high
concentration in extracellular fluid
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Sodium ions diffuse into cells
This reduces the amount of water inside the cell
As a result water diffuses into the cell
This is bad
Too much water means cells can swell and burst
High extracellular sodium concentration is also
necessary for glucose transport
The solution…..
 Sodium
potassium pump in action
1. Sodium ions flow into neuron, causing
depolarization; starts at the dendrites
2. If enough Na+ enters neuron (and K+ moves
out), the membrane reaches threshold
potential
3. An action potential is initiated at the axon
hillock (has higher density of Na+/K+ pumps)
4. The signal passes and the membrane
repolarizes
5. The signal is sent along the axon causing a
nerve impulse
 Myelin
sheath = electrical insulator that is
discontinuous along the axon of the neuron;
increases capacitance
 Axon
hillock = base of the axon; has higher
density of Na+/K+ pumps so action potential
is more easily initiated here
 Neural
synapse = gap between two neurons
along signal pathway
1. Electrical synapse – uses ion channels
connected axon to dendrite without a gap to
communicate between neurons.
1. Chemical synapse – uses vesicles to move
chemicals (neurotransmitters) across a small
gap between neurons
 Vesicles
move substances into and out of
cells
 Substances that are big molecules
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•
Proteins and polysaccharides are moved
in = ENDO
Proteins and waste are moved out = EXO
Exocytosis moves neurotransmitters into the
synaptic cleft
Summary Animation
a) Endocytosis
b) Exocytosis