Which process will be used to move molecules across the cell membrane to an area of higher concentration?

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

What process is used to move molecules across the cell membrane?

What process is used to move molecules across a cell membrane to space where the concentration of the molecules is higher?

What is it called when molecules move across the cell membrane from an area of high concentration to an area of low concentration with the help of a carrier protein?