Understanding:
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport
Cellular membranes possess two key qualities:
- They are semi-permeable [only certain materials may freely cross – large and charged substances are typically blocked]
- They are selective [membrane proteins may regulate the passage of material that cannot freely cross]
Movement of materials across a biological membrane may occur either actively or passively
Passive Transport
Passive transport involves the movement of material along a concentration gradient [high concentration ⇒ low concentration]
Because materials are moving down a concentration gradient, it does not require the expenditure of energy [ATP hydrolysis]
There are three main types of passive transport:
- Simple diffusion – movement of small or lipophilic molecules [e.g. O2, CO2, etc.]
- Osmosis – movement of water molecules [dependent on solute concentrations]
- Facilitated diffusion – movement of large or charged molecules via membrane proteins [e.g. ions, sucrose, etc.]
Active Transport
Active transport involves the movement of materials against a concentration gradient [low concentration ⇒ high concentration]
Because materials are moving against the gradient, it requires the expenditure of energy [e.g. ATP hydrolysis]
There are two main types of active transport:
- Primary [direct] active transport – Involves the direct use of metabolic energy [e.g. ATP hydrolysis] to mediate transport
- Secondary [indirect] active transport – Involves coupling the molecule with another moving along an electrochemical gradient
Types of Membrane Transport
Understanding:
• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport
Facilitated diffusion is the passive movement of molecules across the cell
membrane via the aid of a membrane protein
- It is utilised by molecules that are unable to freely cross the phospholipid bilayer [e.g. large, polar molecules and ions]
- This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins
Carrier Proteins
- Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
- Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
- Carrier proteins may move molecules against concentration gradients in the presence of ATP [i.e. are used in active transport]
- Carrier proteins have a much slower rate of transport than channel proteins [by an order of ~1,000 molecules per second]
Channel Proteins
- Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
- Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
- Channel proteins only move molecules along a concentration gradient [i.e. are not used in active transport]
- Channel proteins have a much faster rate of transport than carrier proteins
Channel Proteins versus Carrier Proteins
Application:
• Structure and function of sodium-potassium pumps for active transport and potassium channels for
facilitated diffusion in axons
The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane
- At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
- When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels
Potassium Channels
- Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
- The channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
- Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage
Voltage-Gated Ion Channels