Membrane biology

Diffusion

• Random motion takes particles across the membrane
• The more particles on one side the greater the rate particles move across because each particle has an equal chance per unit time. The side with fewer particles crosses less often because there is a lesser chance.
• The rate at which particles diffuse is independent as long as there is no charge that complements it

Finks Equation

• To determine rates of diffusion we use Finks equation, that is the rate of diffusion for uncharged particles (dissolved solids or liquids-not gases)
• J=DA(C/X)
• J=rate of diffusion (mol/sec)
• D=permeability constant (temperature, nature of substance, nature of membrane)(cm^2/sec)
• A=area of diffusion (cm^2)
• c=concentration gradient (difference in concentration mol/cm^3)
• X=Distance of diffusion, how thick is the barrier in cm)

Electrochemical Gradient Example

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• Caption: In this example the membrane is permeable to Na but not Cl
• Once this system starts the first thing that happens is sodium diffuses from left to right
• As positive ions cross the membrane the left side becomes more negative so charge builds up across the membrane.
• Charge makes positive ions move slower to the right across the membrane so the electrical gradient pushes positive particles to the left while the concentration gradient moves particles to the right until a point is reached where there is a balance in equilibrium
• Charged particles that need to think about charge and concentration is referred to as electrochemical gradient
• Donnan equilibrium occurs once there is a balance and there are no longer any unbalanced forces or further charges, the molecule is in electrochemical equilibrium

Osmosis

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• Concentration of water is higher to the right than to the left so the water diffuses through the concentration gradient
• It does not make a difference what the solute is, just the total number of particles.
• Caption: Membrane is permeable to water but not to glucose

Osmolarity

• The total number of dissolved particles/liter regardless of its nature (milliosmoles)
• Anything that dissociates results in a greater number of particles
• Water is not effected by charge so it has no electrochemical gradient, it just moves from lower concentration of solution to a higher one

Osmolarity vs. Tonicity

• Isomotic: same concentration
• Hyposmotic: less concentrated
• Hyperosmotic: more concentrated
• Relative terms
• If the membrane is only permeable to water it moves from hyper to hype
• Isotonic, hypertonic, and hypotonic are different because they are used to refer to systems that are more dynamic and likely to change with time.

Membrane Passage Factors

• Size, little things pass through easier than larger, so the rate of diffusion is lower for bigger molecules than smaller
• How hydrophobic the molecule is. Lipophilic has an easier time passing through as well.
• CO2/O2 Ismall and nonpolar)>Alcohol>Water>Ions

Integral Proteins

• The lipid bilayer has integral proteins embedded in the bilayer who receive signals and are enzymatic. They are proteins that help move things through the bilayer

Simple Diffusion

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• Easiest way to get across is to just pass through

Channel Protein

• Permeation pore in the middle of the protein. Specific in what it lets through. (i.e., sodium)
• Can be open or closed
• Facilitated diffusion

Carrier Proteins

• Channel proteins can only be so big, so for bigger things carrier proteins are used (i.e., glucose)
• Changes conformation to move molecules
• Because the molecules are moving down the gradient, they are energetically favorable and don't require the input of energy
• Facilitated diffusion

Primary Active Transport

• Active transport: carrier proteins move one substance against concentration gradient.
• Primary is Ca2+ and ATP is paid at Ca2+
• Uniport
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Primary Active Transport

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• Sodium potassium pump
• Co-transport: transporting more than one substance at a time
• Anti transport-opposite
• Uses ATP

Secondary Active Transport

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• Symport: same direction. Gets energy for letting things move against gradient. Sodium gives energy to glucose.

Secondary Active Transport

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• Because we let sodium in we have to pay at some point to get it out. Coupled transporter-couples unfavorable and favorable reactions.