Protein-mediated transport

• Membrane proteins form selective channels or gates that permit or enhance the passage of specific ions and molecules. There are two means by which this transport can occur:

• facilitated diffusion• active transport

• In each of these: – transport is more rapid than by simple diffusion – the channels are specific for particular

molecules, so transport is selective— some substances are transported and others are not

– the channels become saturated (fully occupied) as concentration of the transported substances increases

– transport of one substance is inhibited by the presence of another substance

• The principal difference between the two mechanisms is that active transport requires the expenditure of energy whereas facilitated diffusion does not.

• The principal difference between the two mechanisms is that active transport requires the expenditure of energy whereas facilitated diffusion does not.

TRANSPORT OF SMALL MOLECULES

Channel protein A channel protein allows a particular molecule or ion to cross the plasma membrane freely.

They allow water soluble molecules to pass thru by SIMPLE DIFFUSION (down a conc. gradient).

For eg. Neurotransmitter mol. Stimulate a nerve cell to open the channel that permit entry of Na+ into the cell. Functioning of nerves and muscles.

Ex Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl-) channel; a thick mucus collects in airways and in pancreatic and liver ducts.

Carrier protein

A carrier protein selectively interacts with a specific molecule or ion so that it may cross the plasma membrane (PASSIVE TRANSPORT).

After glucose binds to the open end of the carrier, the carrier closes behind the glucose molecule. As glucose moves along, the constricted end of the carrier opens in front of the glucose molecule. After glucose is released into the cytoplasm, the carrier returns to its former conformation. This process can occur as often as 100 times per second.

Eg. A faulty carrier for glucose may be the cause of diabetes mellitus in some persons. The cells starve in the midst of plenty, and glucose spills over into the urine.

Active transport

Active transport

MEMB TRANSORT OF MACROMOLECULES• Animal cells can also actively take in and turn out materials in masses of

large size.• Example: macromolecules, liquid droplets and solid particles• 3 processes• ENDOCYTOSIS• EXOCYTOSIS• PHAGOCYTOSISAll 3 processes involve• Movement• Folding• Fusion of cell membraneAll 3 are • Active• Energy requiring processes

Bulk transportEndocytosis

• Taking material in

Exocytosis

• Expelling material out

Pinocytosis

• Pinocytosis occurs when a macromolecule, such as a polypeptide, is to be transported into the cell. The resulting vesicle or vacuole is small.