Membrane Transport I, Passive Transport Flashcards
diffusion
spontaneous movement of solute from regions of high to low concentrations
what law does diffusion follow?
2nd law of thermodynamics
when could diffusion be working against the cell?
active transport
active transport
molecules & ions are moved against their concentration gradient, requires E (usually ATP)
what are PMs permeable to?
o Water
o A few small & uncharged molecules (O2 & CO2) gases
what are PMs not permeable to?
Ions such as
K+, Na+, Ca2+ (cations)
Cl-, HCO3 (anions)
Ions are small enough to get through but are charged, therefore, interact with hydrophobic interior & polar heads
Small hydrophilic molecules like glucose
Carried through transporters or move through channels
Macromolecules like PROs & RNA
How does the cell store E within membranes?
By accumulating molecules within membranes, E can be stored & later released by permitting some molecules to move down the gradient
osmosis
diffusion of a molecule through the cell membrane, net movement of water (or solvent) across a selectively permeable membrane
how does osmosis work?
Driven by a diff in solute concentrations on either side of the membrane
Low solute concentration to high solute concentration
when does osmosis occur?
when membranes are permeable to water but not to dissolved ions & smaller polar organic solutes
what 2 factors change when osmosis occurs?
Volume change (until solute concentration is equalized) water creates an expansion but the PM prevents this which leads to pressure in the cell
Pressure change
what determines the direction of water movement?
the envr the cell is situated
Hypertonic & what occurs to the cell?
high salt medium
By osmosis, water will flow into high salt region
Cell will shrink in size
Hypotonic & what occurs to the cell?
Lower salt medium, high water envr
o Water will flow by osmosis into the cell
o Cell will swell, increases to the point where it bursts or cell will work to prevent this
what occurs to a cell after a hypertonic or hypotonic state is reached?
Cells usually return to normal b/c salts re-enter or leave after a period of time (through channels)
Cells will pump ions in or out to compensate for the change in water flow
Undesirable
what organisms have a defense against a bursting cell? & what is it?
plants, fungi & bacteria
cell wall strong enough to fight pressure
if placed in a hypotonic solution, the cell won’t burst
cell wall
specialized & rigid extracellular matrix located on the outside of the PM
Porous & doesn’t provide a barrier to the diffusion of small molecules
isotonic & how is cell size affected?
salt concentration is the same in & out of the cell
Cells remain the same size
what happens if the cell is placed in a hypotonic envr?
cell will swell & burst
what happens if the cell is placed in a hypertonic envr?
cell will shrivel
turgor pressure
pressure of water that the membranes feel due to the pressure of water against them
what maintains movement through a plant?
plants tend to be hypertonic to their envr therefore, they keep salts in their cells & water is drawn inwards
transpiration keeps water moving through the plant
turgor pressure occurs
how do cells without a wall deal with osmosis?
pump water back into the contractile vacuole
ions are pumped into the vacuole through active transport causing water to move into the vacuole
exocytosis occurs
ions are pumped back into the cell via active transport
How are electric gradients established across membranes?
When a membrane is selectively permeable to ions
Net positive charge on one side of the membrane & net negative on the other
Occurs only if channels are present
Solutes move down their concentration gradient
what is an axon’s resting potential?
-70mV - slightly more negative on the inside than outside
cell acts as a capacitor
capacitor & what is it’s role in cells?
stores E in an electric field b/w a pair of conductors
Separates charge enabling the storage of E by building up potential E
what are 4 methods of transport across the membrane?
- Diffusion through lipids
Not ions
Water or gases - Diffusion through an aqueous channel or pore
Selective of ions - Facilitated diffusion: transmembrane PROs create a water-filled pore through which ions & some small hydrophilic molecules can pass by diffusion
o Channels can be open or closed depending on the needs of the cell
o Not an open channel, transporter of facilitated diffusion channel - Active transport: transmembrane PROs called transporters use ATP to force ions of small molecules through the membrane against their concentration gradient
what do aquaporins cause?
In some cell types the movement of water is much greater than it would be if it flowed through the lipid bilayer itself
role of aquaporins in the gut
several liters of water per day are moved into the lumen of the stomach, intestine & then are re-absorbed in the lower gut
Want to move water into the gut, food provides high amount of solutes in the intestine & then water wants to be released at the bowels
how is the movement of water through pores regulated?
Cell will be signaled to increase the amount of water absorption now, vesicles lined with aquaporins will move from the interior to the exterior PM & place those aquaporins
Aquaporin movement: internal membranes external membrane (PM)
when is the cell is signaled to do the opposite, aquaporins will be removed. Endocytotic vesicles will be formed
Flow is regulated by the location of aquaporins, not the aquaporins themselves
role of aquaporins in kidneys
Water is reabsorbed (less excretion) in response to the hormone vasopressin (antidiuretic hormone)
Presence of vasopressin promotes permeability & reabsorption
Aquaporins move from internal membranes to cell membrane
Vesicles lined with aquaporins, vesicles will move to the perimeter in response to antidurettic
Made in ER, to Golgi
Water flow was enhanced by placing aquaporins in the membrane –> water flows by diffusion
Stop the flow, remove aquaporins, vesicles will form endocytotic vesicles
transport of glucose within the cell via a carrier PRO
Diffusional transporter, designed only to capture glucose
Glucose fits into transporter, causing it to change shape
Opens up & dumps glucose into the cell
Once glucose is released, shape will revert
Can bind to another glucose molecule
Will take glucose away from the perimeter or phosphorylate it to maintain the low gradient in the cell
what are 2 types of channels for passive transport?
carriers –> ex: glucose transporter
channel –> membrane PROs form a highly selective pore
how is passive transport regulated?
presence or absence (expression) of PRO on membrane
“gated” PROs
Voltage is regulated
Ligand-regulated (hormone, neurotransmitter)
Mechanical or stretch (smooth muscle, sensory nerves)
what are the types of gated ion channels?
o Ligand gated
o Mechanically gated
o Voltage gated
o Light gated
types of facilitated diffusion (3) & exs of each
Ligand gated: ligand needs to bind to a particular place on the PRO for the channel to open
Ligands can be extracellular or intracellular
Ligand isn’t the substance being transported
Ex: Ach binds to neurotransmitters opening up the channel admitting Na+ to enter & initiate nerve impulse or muscle contraction
Mechanically gated ion channels: mechanical shift causes pores to open
Ex: sound waves bend cilia like projections causing the opening of ion channels
Voltage gated ion channels: response to changes in charge across the PM
Ex: neuron impulse, reduction in voltage opens Na+ channels allowing an influx of Na+ into the cell
Acts as a wave as Ach hits neurons
Pump ions back out to prepare for another signal
