Cell Membrane Permeability

• Artificial membranes are not equally permeable to all substances
• The smaller and more permeable a substance, the more likely it is to cross the membrane
• Synthetic lipid bilayers are impermeable to large uncharged polar molecules (e.g. some amino acids, glucose and nucleotides) and charged molecules (ions)
• Cell membranes are permeable to large uncharged polar molecules and charged molecules
• Transfer of water-soluble molecules depends on membrane transport proteins
• Each transport protein transfers a particular type of molecule, allowing an uneven concentration of that molecule to build up on either side of the membrane
• The set of transport proteins in a membrane determines exactly what water-soluble molecules can pass into and out of that cell or organelle
• Each type of membrane has its own characteristic set of transport proteins
• Unbalanced ion concentration gradients can cause a cell to be torn apart by electrical forces
• The positive external charge (Na+) is largely balanced by anions (Cl-)
• The positive internal charge (K+) is balanced mainly by nucleic acids and proteins
• Excesses of positive and negative charges accumulate at the plasma membrane, giving rise to the membrane potential
• The inside of the cell membrane is negative, and the outside is positive
• Ion gradients are the result of specific transporters that move specific ions

Types of transport:

• Passive transport does not require external energy
• Passive transport requires a concentration gradient, where the molecule travels down its concentration gradient (from high concentration to low)
• Active transport requires energy
• Active transport transports molecules against their concentration gradient

Passive transport (diffusion):

• Gases (O2, CO2), hydrophobic molecules (benzene), and small polar uncharged molecules (H₂O, ethanol) can dissolve in the lipid bilayer
• These molecules diffuse across the lipid bilayer, and then dissolve in the aqueous solution on the other side of the membrane
• For non-electrolytes, the rate of passive transport (diffusion) is dependent upon its partition coefficient
• Partition coefficient is a measure of a molecule's ability to partition between aqueous and hydrophobic environments
• Size also determines the rate of diffusion
• Of 2 molecules of equal partition coefficient, the smaller one diffuses faster than the larger
• Direction of transport is only determined by the concentration gradient
• A molecule can move in either direction until equilibrium is reached

Passive transport (diffusion) with molecules that cannot dissolve in the lipid bilayer:

• Large polar uncharged molecules (amino acids, nucleotides and sugars) and charged molecules (ions) are unable to dissolve in the lipid bilayer
• During facilitated diffusion, the passage of polar and charged molecules is mediated by proteins that enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior
• Facilitated diffusion requires membrane proteins and a concentration gradient, but no energy
• The molecule can either interact with the membrane protein or not

Active transport:

• Active transport moves proteins against their concentration gradient
• Active transport requires a membrane transport protein that is coupled to an energy-consuming reaction (hydrolysis of ATP)
• Examples include the proton pump of the lysosome and the sodium/potassium pump of the plasma membrane

Classes of transport proteins:

• ATP-powered pumps transport 1-1000 molecules/sec
• Channel proteins (ions) transport 107-108 molecules/sec
• Carrier proteins (transporters) transport 102-104 molecules/sec
• Hydrolysis of ATP is coupled to the transport of a molecule against its concentration gradient
• Ions are transported down their concentrations gradient through a hydrophilic pore in the membrane protein
• Channel proteins exist in an open or closed conformation and when open, many ions can pass simultaneously