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Cell Membrane

Phospholipid Bilayer - Hydrophilic outside and hydrophobic inside; barrier for water and water-soluble molecules

Proteins - Allows for movement of substances through membrane


Proteins within Membrane

Ion channels

Carrier proteins (picks up a substance at one side of a protein and drops it at the other)

Pumps (uses energy because against concentration gradient)

Receptors (allow for communication between cells)



Types of transport through a membrane

Diffusion (no energy required) and active transport (energy required)



Random movement in all directions

2 types - simple diffusion and facilitated diffusion


Simple diffusion

Movement through membrane, channel protein, or pore

Lipid-soluble molecules move readily through the membrane (rate depends on lipid solubility) - oxygen, carbon dioxide, alcohol, and steroid hormones

Water can pass freely through any channel or pore

Water-soluble ions cross via SPECIFIC water-filled channels or pores


Selectivity of channels

Selective permeability due to different composition

Gated channels


Ungated vs Gated Channels

Ungated channels - always open (specific ion is always diffusing through pore); leak channels (important for electrical signals)

Gated channels - Open and close in response to a stimulus
1) Voltage-gated channels - sensitive and dependent; dependent on differences in electrical potentials across the membrane (gate is opened to restore potential difference)
2) Chemical (Ligand)-gated channel - open and close when stimulated by chemical binding (ex. - Acetylcholine)


Facilitated Diffusion

Required interaction of transported molecule with a carrier protein which aids passage


Rate of Diffusion

Simple diffusion = exponential rate

Facilitated Diffusion = logarithmic increase (dependent on the rate at which carrier protein can change conformation due to stimulus; Vmax cannot be greater than the rate of this change in conformation)


Net Diffusion

Average direction of diffusion


Net rate diffusion is affected by..

Concentration gradient (down concentration gradient)

Membrane electrical potential - electrical potential across membrane is equal in magnitude but opposite in direction to the concentration force (Nernst Potential - EMF (mV) = +/- 61 log (Co/Ci) )



Net diffusion of water


Osmostic Pressure

Amount of pressure required to counter osmosis (i.e. higher osmolarity = higher osmotic pressure)

Osmotic pressure caused by the number of particles present in a solution REGARDLESS of the molarity of the particle

Measured in HHmg



Number of osmotic particle / volume

Since osmotic pressure and therefore osmolarity is dependent on the number of particle NOT molarity, it is important to remember whether or not a compound dissociates in H20. If dissociation occurs, then there will be an increase in the number of particles and in result an increase in osmolarity and osmotic pressure