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AKA Na+ K+ ATPase
ATP driven Na+ pump hydrolyzes ATP to ADP to transport Na+ out of the cell (doubling as a pump and enzyme)
Keeps [Na+] lower and [K+] higher inside the cell |
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| the movement of water from a region of high water concentration to a region of low concentration (high solute concentration) |
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| Specialized water channels, facilitate flow of water. |
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The driving force for water movement due to a difference in water pressure
Performed by the Na/K pump |
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| osmosis plus the active transport of ions into the cell – keeps plant cells rigid and leaved extended and regulated gas exchange through the stomata |
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| Three Ways to Maintain Osmotic Pressure |
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| Turgor Pressure (plants), Ion diffusion (animals), contractile vacuole (protozoan) |
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| Importance of Ca movement |
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Ca is kept in low concentrations in the cytosol compared with its high concentrations in the extracellular fluid
Ca pump is an ATPase that is phosphorylated and dephosphorylated during its cycle |
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| A gradient of any solute can be used to fuel the active transport of a second molecule. (Downhill gradient drives another uphill reaction) |
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| The transporter moves both solutes in the same direction across the membrane |
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| The transporter moves solutes in opposite directions across the membrane |
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| (not a coupled transporter) only moves one type of solute across the membrane (ie: passive glucose transporter) |
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Plants, fungi, and bacteria do not have Na/K pumps – instead they rely on an electrochemical gradient of H+
H+ higher outside than inside
Set up their H+ gradient by means of ATPases in plasma membranes that use energy of ATP hydrolysis to pump H+ out of the cell |
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| large pores between two adjacent cells |
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| form channels in the outer membrane of mitochondria |
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Show selective permeability
As ion concentrations are increased, the flow of ions through a channel at first increases proportionally but then levels off (saturates) at a maximum rate |
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| a specific stimulus triggers to switch between closed and open state |
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| Which has a higher rate of transport - ion channels or transporters? |
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| Ion channels can have a higher max rate of transport than transporters because the channel does not need to undergo conformational changes with each ion it passes |
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open/close controlled by membrane potential
Play a role in propagating electrical signals in nerve cells |
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| specialized charged protein domains that are sensitive to changes in membrane potential |
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| **A change in the membrane potential doesn’t affect how wide the channel opens, only the probability it will be found open** |
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| open/close controlled by binding of a ligand to the channel |
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| opening controlled by a mechanical force applied to the channel (ie: auditory hair cells in ears – sound vibrations pull channels open) |
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randomly flicker between open and closed states no matter what the conditions are in/out of the cell
i) Any transfer of positive charge to the exterior leaves behind unbalanced negative charge within the cell – creating membrane potential |
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| Resting Membrane Potential |
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Definition
| the membrane potential in steady state conditions. +/- flow of ions across the membrane is balanced (interior of cell (-) with respect to the outside) |
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| a nerve cell – receives, conducts, and transmits signals |
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| contains the nucleus – with long extensions radiating out from it |
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| conducts signals away from the cell body toward target cells |
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| providing an enlarged surface area to receive signals from the axons of other neurons |
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| allowing the message to be passed simultaneously to target cells |
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a) Action potential triggered by a local depolarization of the plasma membrane (shift towards 0)
i) Depolarization causes voltage gated Na channels to open, allowing a small amount of Na to enter the cell down the electrochemical gradient |
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| The signal is transmitted to the target cells at specialized junctions |
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| Presynaptic and postsynaptic cells separated by narrow synaptic cleft (electrical signal cannot cross) |
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| has moving parts and can shift small molecules from 1 side of the membrane to the other by changing its shape (inorganic ions or small O molecules) |
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| form tiny hydrophilic pores, solutes pass by diffusion (ion channels - inorganic ions only) |
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| Na most plentiful outside the cell |
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| K most plentiful inside the cell |
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| Depends on molecule size and polarity |
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| Have polypeptide chains that traverse the bilayer multiple times - forms a continuous protein-lined pathway that allows small hydrophilic molecules to cross without contacting the hydrophobic interior |
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| aka facilitated diffusion. requires no input of energy - solute moves "downhill" on the concentration gradient |
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| requires energy and specialized transporters to move solute against the concentration gradient (pumps) |
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| difference in electrical potential on each side of the membrane |
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| electrical potential + concentration gradient (determines direction of passive transport) |
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| couple the uphill transport of one solute across the membrane to the downhill transport of another |
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| couples uphill transport to the hydrolysis of ATP |
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| (bacterial cells) couple uphill transport to an input of energy from light |
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