Membrane Transport

• Cell membranes are permeable to large uncharged polar molecules (amino acids, glucose and nucleotides) and charged molecules (ions). Transfer of these water-soluble molecules depends on membrane transport proteins
•  If a cell is not to be torn apart by electrical forces, then the quantity of positive charge inside or outside the cell must be balanced by an almost exactly equal quantity of negative charge (outside or inside respectively)
• Nature has therefore employed mechanisms by which small molecules cross lipid bilayers
• passive diffusion → a molecule simply dissolves in the lipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane
• gases (O₂, CO₂), hydrophobic molecules (benzene) and small polar uncharged molecules (H₂O, ethanol) are able to dissolve in the lipid bilayer
• no membrane proteins are involved
• no external source of energy is required
• the net flow of molecules is always down their concentration gradient →  the direction of transport is determined only by the relative concentrations of the molecule inside and outside of the cell
• 
  
  facililated diffusion (passive transport) → 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
• large polar uncharged molecules (amino acids, nucleotides and sugars) and charged molecules (ions) are unable to dissolve in the lipid bilayer
• membrane transport proteins are involved
• no external source of energy is required
• the net flow of molecules is always down their concentration gradient → the direction of transport is determined only by the relative concentrations of the molecule inside and outside of the cell
• active transport
• membrane transport proteins are involved
• external source of energy is required
• the net flow of molecules is against their concentration gradient → energy provided by another coupled reaction (the hydrolysis of ATP) is used to drive the uphill transport of molecules

Transport Proteins

ATP-powered pumps

• Transport of molecules by the ATP-powered pumps is driven by the hydrolysis of ATP →  ADP + Pi

Typical ATP-powered pump

Example → the Na⁺/K⁺ ATPase is responsible for the coupled movement of K+ and Na+ into and out of the cell respectively

Hydrolysis of one molecule of ATP to ADP + Pi is coupled to the export of three Na⁺ ions and import of two K⁺ ions against their concentration gradients across the plasma membrane

Channel Proteins (ion channels)

• channel proteins transport specific types of ions down their concentration gradient and multiple ions move simultaneously through the ion channel



typical ion channel

• a channel protein forms a hydrophilic pore across the lipid bilayer through which specific inorganic ions can diffuse
• ion channels can exist in either an open or a closed conformation and transport only in the open conformation
• have two important properties:
• ion selectivity → are highly selective as narrow channels restrict ions by appropriate size and charge
• gating → are not permanently open but instead are gated, i.e. they can transition between an open and a closed stated through change in conformation in response to specific "stimuli"
• are found to be of three types:
• voltage-gated
• ligand-gated
• stress-activated

Carrier Proteins (transporters)

• carrier proteins bind one or few water-soluble molecules on one side of the membrane and deliver it to the other side through a change in the conformation 



typical transporter

• three groups of transporters are found:
• uniporters → transport a single type of molecule down its concentration gradient (for example: glucose or amino acids into mammalian cells)
• symporters → couple the movement of one type of ion/polar molecule against its concentration gradient (energetically unfavorable reaction) to the movement of a different ion/polar molecule down its concentration gradient (energetically favorable reaction) in the same direction
• antiporters → couple the movement of one type of ion/polar molecule against its concentration gradient (energetically unfavorable reaction) to the movement of a different ion/polar molecule down its concentration gradient (energetically favorable reaction) in the opposite direction