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two problems to be considered with transport across cell membranes :

1. relative concentrations of molecules
2. lipid bilayers are impermeable to most essential molecules and ions


relative concentrations of molecules

molecules and ions move spontaneously down their concentration gradient (i.e from a regions of higher to a region of lower concentration) by diffusion



the spontaneous movement of salute from regions of high concentration to low concentration follows the 2nd law of thermodynamics
-molecules and ions can be moved against their concentration gradient, but this process, called active transport, requires the expenditure of energy (usually from ATP)


(Im)permeable lipid membranes

-the cell membrane is permeable to water molecules and a few other small, uncharges, molecules like Oxygen and carbon dioxide. these diffuse freely in and out of the cell
-lipid membranes are not permeable to:
-ions such as :
*K+, Na+, Ca2+ (cations)
*Cl-, HCO3- (anions)
-small hydrophilic molecules like glucose
-macromolecules like proteins and RNA


energetics of solute movement. the concentration gradient stores energy

-diffusion moves molecules down concentration or chemical gradients
-energy is proportion al to the difference in concentration on 2 sides of the membrane
-energy is dissipated when the molecules flow down the gradient


main point

by accumulating molecules within membranes, energy can be stored and later released by permitting some molecules to move down the gradient



a special case of diffusion of a molecule through the cell membrane. it is the net movement of water (or solvent) across a selectively permeable membrane driven by a difference in solute concentrations on the two sides of the membrane
-occurs when membranes are permeable to water but not to dissolved ions and small polar organic solutes
-the movement of solvent from regions of low solute concentration ot high solute concentration
-may manifest as volume change (unit solute conc. is equalized) and as pressure changes


osmosis in cells

cells are permeable to water but not to many salts
higher salt (hypertonic) medium-tendency to shrink
lower salt (hypotonic) medium-tendency to increase pressure
-cells usually return to normal because salts re-enter or leave after a period of time (through channels)


Dealing with osmosis :

-the water concentration gradient across the plasma membrane of most organisms leads to an influx of water into the cell.
-as water enters the cell, the plasma membrane can expand somewhat. if the influx of water continues, however, it will burst the membrane like an over-inflated balloon


organisms such as plants, fungi, and bacteria use _________ to deal with the influx of water

rigid cell wall
the cell wall is a specialized and relatively rigid extracellular matrix located outside of the plasma membrane. the cell wall is relatively porous and does not present a barrier to the diffusion of small molecules


plant cells are normally hypertonic to their environment

-water tends to flow in
-creating pressure - turgor pressure
- plants withstand this pressure b/c the cell wall is a rigid structure. it provides structural support to the cell
-if plants encounter a hypertonic environment, plasmolysis occurs and they wilt
lower salt outside and higher salt inside


dealing with osmosis without a cell wall

-animal cells do not have a rigid cell wall
-most free living protozoa live in dilute aqueous solutions, where osmotic effects are severe
-they deal with the constant in-flus of water by actively pumping the water that flows into the cell back out using an organelle known as the contractile vacuole
-water accumulates w/in the contractile vacuole, a membrane-bounded structure, which inflates. to expel the water the vacuole uses exocytosis to remove the water


how does the cell force water into the contractile vacuole

it expends a lot of energy pumping ions into the vacuole, and then expends even more energy recovering ions from the external environment


diffusion can create electrical gradients across cell membranes

-charged particles can also form an electrical gradient- a voltage potential-when membrane is selectively permeable
-this depends on selective permeability for a charged ion
-cells are highly selective, each ion can generate its own membrane potential
-solute movement down conc. gradient occurs only if channels present


electrical potential

-a voltage potential is generated when a membrane is selectively permeable to ions
trans membrane voltage develops
eg. high internal conc of K+ favours efflux from cell
-for potassium alone, the concentration diff leads to an electrical potential of 91 millivolts


cells resting potential

-typical axon cell membrane potential is -70mV
-negative charge on the inside of cell
-the membrane is only about 3.5 nm thick
-70mV/3.5X10^-7cm=200,000 V/cm
-compare this to high voltage transmission lines: ~100,000V/m


Cell membranes are great capacitors

a capacitor stores energy in an electric field b/w a pair of conductors


mechanisms of transport across membranes

1. diffusion through lipids
2. diffusion through an aqueous channel or pore
3. facilitated diffusion
4. active transport


facilitated diffustion

transmembrane proteins create a water-filled pore through which ions and some small hydrophilic molecules can pass by diffusion. the channels can be opened (or closed) according to the needs of the cell


active transport

transmembrane proteins, called transporters, used the energy of ATP to force ions of small molecules through the membrane against their concentration gradient


diffusion through pores - aquaporins

-in some cell types the movement of water is much greater than it would be if it flowed through the lipid bilayer itself
-due to the presence of trans-membrane aquaporin proteins (movement is still passive, no energy needed, down the conc. gradient)
eg.1 in the gut, several liters of water per day are moved into the lumen of stomach, intestine, then re-absorbed in the lower gut
eg.2 in the kidney, water permeability is regulated
aquaporin pore/channel allows water molecules to move through in single file



the presence of vasopressin promotes permeability and reabsorption. aquaporins move from internal membranes to cell membrane
-in the kidney, water is reabsorbed (i.e. less is excreted) in response to the hormone vasopressin (antidiuretic hormone)
even though the aquaporin channel itself does not regulate water flow, the flow is regulated by the location of aquaporin proteins


two types of channels for passive transport

1. carriers
2. channels
these may both be regulate by: presence/absence (expression) of the protein
-or they may be gated
*voltage regulated
*ligand regulated (hormone, neurotransmitter)
*mechanical or stretch (smooth muscle, sensory nerves)



eg. glucose transporter moves glucose through membrane, energy comes from conc. gradient
1. bind
2. transport
3. dissociate
4. recover
presence of insulin can regulate translocation of glucose transporter to cells membrane



membrane proteins form a highly selective pore through the membrane


facilitated diffusion of ions through channels

-facilitated diffusion of ions takes place through protein channels embedded in the plasma membrane. the ions can pass down their concentration gradient
-the transmembrane channels that permit facilitated diffusion can be opened of closed. they are said to be gated
some types of gated ion channels:
*ligand gated
*mechanically gated
*voltage gated
*light gated
potassium channel is selective for just K+; movement through channel is by diffusion; opening of channel is voltage gated


ligand gated channels

many ion channels open or close in response to binding a small signalling molecule or ligand. some ion channels are gated by extracellular ligands; some by intracellular ligands. in both cases, the ligands is not the substance that is transported when the channel opens
ex. acetylcholine (ACh) the binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction


mechanically gated ion channels

ex. sound waves bend cilia-like projections on the hair cells of the inner ear; the bending opens up ion channels, leading to the creation of nerve impulses to the brain


voltage gated ion channels

in neurons and muscle cells, some channels open or close in response to changes in the charge across the plasma membrane
ex. as an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane this allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse
-approx 7000 sodium ions pass through each channel during the millisecond that it remains open


importance of transport

cells acquire the molecules and ions they need from their surrounding extra-cellular fluid (ECF)-an unceasing traffic of molecules and ions