PSL300H1 Lecture Notes - Lecture 1: Semipermeable Membrane, Electromagnetism, Potassium Channel
PSL300
Lecture 1: Neurophysiology 1
Cell Membrane
• The cell membrane is not an inert bad holding the cell together
• It is composed of a phospholipid bilayer
• Lipid-soluble molecules and gases diffuse through readily
• Water-soluble molecules cannot cross without help
• Impermeable to organic anions (proteins)
o Proteins are large structures and organic ions cannot cross the membrane easily
• Permeability depends on molecular size, lipid solubility, and charge
Membrane Permeability
• If a substance can cross the membrane by any means, the membrane is permeable to that substance
• Gases can diffuse across the membrane
• Polar molecules and ions need the help of proteins (channels or carriers) to cross
• The membrane is selectively permeable
Simple Diffusion
• Small lipid-soluble molecules and gases (e.g. O2, CO2. Ethanol, urea etc..) pass either directly through the
phospholipid bilayer or through the pores
• Movement of substrate is down its concentration gradient
• The relative rate of diffusion is roughly proportional to the concentration gradient across the membrane
o Greater the gradient the greater the diffusion
• Passive: no energy input required from ATP
Facilitated Diffusion
• Facilitated diffusion is a process of diffusion, where molecules diffuse
across membrane, with the assistance of carrier proteins
• Carrier proteins aid the movement of polar molecules (e.g. sugars and
amino acids) across cell membrane
• Movement of substrate is down its concentration gradient – from high
to low
• The energy comes from the concentration gradient of the solute
• Passive: no energy input is required from ATP
• Carrier proteins are not continuous through the membrane
o One end is open and the other end is closed
o Not a hole like a pore or a channel
• Opening and closing happens through a conformational change
o Solute binds to a transporter on one side of the membrane
causing a conformational change resulting in the opening of
the protein on the other side of the membrane
▪ Translocation of the molecule across the membrane
• The number of transporters are finite
o The system can eventually saturate
▪ When the concentration of the solute molecule exceeds the number of the transport proteins the
system will be saturated – the rate of diffusion will be decreased
Active Diffusion
• Active transport is a mechanism to move selected molecules across cell membranes, against their concentration
gradient
• Substrate binds to a protein carrier that changes conformation to move substrate across the membrane
• Requires energy from ATP hydrolysis coupled with the movement of a substance across the membrane against its
concentration gradient
o ATPases (Na/K Pump)
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