What components are involved in active transport?

Understanding:

•  Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport

    
Active transport uses energy to move molecules against a concentration gradient

This energy may either be generated by: 

• The direct hydrolysis of ATP (primary active transport)
• Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

• A specific solute will bind to the protein pump on one side of the membrane
• The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
• The solute molecule is consequently translocated across the membrane (against the gradient) and released

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

Sodium-Potassium Pump

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

The process of ion exchange against the gradient is energy-dependent and involves a number of key steps:

1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
2. A phosphate group is transferred to the pump via the hydrolysis of ATP
3. The pump undergoes a conformational change, translocating sodium across the membrane
4. The conformational change exposes two potassium binding sites on the extracellular surface of the pump

   Depending on the source of energy primary transport is differentiated from secondary transport. Primary transport uses energy directly: light or chemical energy is converted to electrochemical energy as electrochemical potential of the substances to be transported. This category comprises photosynthetic electron transport, light driven ion pumps, redoxenergy dependent respiratory chains, transport ATPases and sodium pumps utilizing decarboxylation energy.

   In secondary transport the electrochemical energy originates from the electrochemical potential of another substance that is used up in symport or antiport.

   Normally a molecule passes the membrane unchanged. However, in group translocation there is a chemical modification. This method is used by bacteria in the phosphoenolpyruvate:sugar phosphotransferase system (PTS).

   Transport systems for nutrients are directed into the cell; all prokaryotic as well as eukaryotic cells contain additionally systems with a reverse transport direction to get rid of toxic substances. These systems may severely impair therapeutic efforts (multidrug efflux systems). Special transport systems protect producers of antibiotics or toxins against their own poisons.

   The structures of few transport systems are known at atomic resolution. Among these are rhodopsins, some components of the PTS, and binding proteins used by bacteria in conjunction with transport ATPases. Binding proteins are found anchored to the cytoplasmic membrane of Gram-positive bacteria or free in the periplasm of Gram-negative bacteria (binding protein dependent transporters), they belong to the superfamily of ABC (ATP bindig cassette) transporters. Sugar specific binding proteins may change their conformation considerably upon binding a ligand, as the maltose binding protein of E. coli:

   
   
   
   MBP without substrate
   Glu45   Tyr 341
   domain I   domain IIMBP with maltoseMBP with maltotriose

   Examples shown here are the binding proteins involved with the transport of maltodextrins and mercury, for proton transport a bacteriorhodopsin and components of the glucose PTS from E. coli. As an exemption from the rule there is FhuA, which is an energy dependent transport system located in the outer membrane of E. coli specific for siderophor bound iron and misused for antibiotics, phage DNA and colicins. Click the structure icons for details.

   Which component is responsible for active transport?

   For the most part, carrier proteins mediate active transport while channel proteins mediate passive transport. Carrier proteins create an opening in the lipid bilayer by undergoing a conformational change upon the binding of the molecule.

   What is required for active transport?

   Active transport requires specialized carrier proteins and the expenditure of cellular energy. Carrier proteins allow chemicals to cross the membrane against a concentration gradient or when the phospholipid bilayer of the membrane is impermeable to a chemical (Fig. 1).

   Which processes are involved in active transport?

   The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane. Larger molecules such as starch can also be actively transported across the cell membrane by processes called endocytosis and exocytosis.

   What are 3 proteins associated with active transport?

   Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion. A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly.