Transport Proteins
Lipid insoluble and large substances cannot cross the lipid bilayer.
Transport proteins provide passage for them to cross the membrane.
Are transmembrane proteins i.e. span the lipid bilayer.
Broad categories: Pumps, channels, carriers.
Pumps
Move substances uphill i.e. against an electrochemical gradient.
Use energy from ATP for uphill movement.
Type of transport is primary active.
Example: Na-K ATPase pump
Moves sodium from inside of the cell to outside and potassium from outside to inside.
Uses energy from ATP for transport.
Channels
Provide continuous passage through a membrane, like a tunnel.
Ions can freely diffuse through it.
Movement is passive, down the electrochemical gradient.
Often selective for a particular type of ion.
Types: Pores & gated channels
Pores
Are always open
Example:
Porin
Aquaporin
Nuclear pore complex
Gated Channels
Not always open.
Have gates that control the opening and closing of the channel.
When gate is closed: Ions cannot move through.
When gate is open: Multiple ions can move through it simultaneously.
Gates are controlled by ligands or voltage.
Ligand Gated Channels
Open or close upon binding of ligand
Example: sodium channels at the muscle end-plate open in response to acetylcholine.
Voltage-Gated Channels
Open or close depending on voltage across the cell membrane.
Example: Voltage-gated calcium channels in the heart open when the membrane depolarizes to a certain level.
Carriers
Do not provide continuous passage.
Have at least two gates and both are never open at a same time.
Open at one side
↓
Binds with a molecule that they carry
↓
closes from the open side
↓
Opens on the other side
↓
Release the molecule on that side
↓
Closes from that side
↓
This cycle is repeated to carry more molecules
Because of this multi-step process, transport through the carriers is slower than through channels.
Types: Uniporters, symporters & antiporters
Uniporters
Move a single type of substance down its electrochemical gradient.
E.g. glucose transporters, urea transporters.
Symporters and antiporters
Move at least two types of substances.
Move one substance downhill
↓
Harnesses the energy from this downhill movement
↓
Utilizes the energy to move the other substance uphill
Such uphill movement is called secondary active transport.
Symporters
Both the substances move in the same direction.
Example: Sodium-Glucose Cotransporter (SGLT) in apical membrane of proximal renal tubule.
Both sodium and glucose move from lumen into the cell.
Sodium concentration is higher in the lumen and glucose concentration is higher in the cell.
Energy from the downhill movement of sodium is used to move glucose uphill.
Antiporters
Move substances in opposite directions.
Example: Na-Ca Exchanger
Sodium moves from outside to inside of the cell and calcium moves from inside to outside.
Sodium and calcium both have higher concentrations outside as compared to inside.
Energy from downhill entry of sodium is used to extrude calcium uphill.