MEMBRANE TRANSPORT PROTEINS © 2010 Paul Billiet ODWSODWS

MEMBRANE TRANSPORT PROTEINS

© 2010 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Passive Transport Driving forces

Diffusion along a concentration gradient

Electrochemical gradient: A membrane potential is set up due to a voltage (potential difference) across the membranePositive ions are encouraged to move in and Negative ions are encouraged to move out



Electrochemical gradientECF +ve

Cytoplasm -ve

Plasma membrane

K+

Cl-

Positive ions are encouraged to move in and negative ins are encouraged to move out



Diffusion and facilitated diffusion Diffusion may occur through any part

of the plasma membrane, e.g. N2 gas molecules.

Facilitated diffusion uses pores, e.g. glucose molecules



Diffusion and facilitated diffusion

Facilitate diffusion

Pores saturated

Rate of transport

Concentration

Simple diffusion



Channel Proteins

Permit the passive movement of molecules or ions of appropriate size (dialysis) through an aqueous pore



Carrier proteins

Bind to specific solutes to transport them across a membrane



Active Transport Uses energy Faster than diffusion Can move against a concentration or

electrochemical gradient Uses carrier proteins – very specific

selective transport



Evidence of active transport in marine algal cells

Concentration / m mol dm-3

Ion Sea water Cell sap

Sodium (Na+) 488 257

Potassium (K+) 12 337

Chloride (Cl-) 523 543



Uniport pore

One type of molecule transported

Change of configuration

Phosphorylation

ATP + H2O ADP + Pi

Dephosophorylation

P

PP



Coupled pores

Two molecules transported together

Symport: Both molecules move in the same direction

Phosphorylation

Change of configuration

ATP + H2O ADP + Pi

Dephosophorylation

PP P

Antiport pores

Molecules move in opposite directions (one in the other out)

e.g. Na+ (out) and K+ (in)

ATPase is an antiport pore protein

ATP is made on the mitochondria inner membranes by throwing an ATPase into reverse

Dephosophorylation

Phosphorylation Change of configuration

P PP

P

PP

Exocytosis and Endocytosis Transferring large molecules or particles or

large volumes in and out of the cell Mediated by special proteins Endocytosis may form small vesicles by

invaginating the plasma membrane = PinocytosisEndocytosis may also occur when a large cell flows round and engulfs a smaller cell = Phagocytosis.


Exocytosis Endocytosis

Two bilayers of phospholipid touchBilayer adherence

Two bilayers fuseBilayer joining

ECF

Cytoplasm

InvaginationECF

Cytoplasm

Secretion


Exocytosis and Endocytosis

Exocytosis may be continuous as a cell makes material for secretion

Exocytosis may be regulated, vesicles are stored in the cytoplasm waiting for a signal to be released

Endocytosis uses protein coated pits which form coated vesicles

The plasma membrane has receptor molecules on the outer surface

When the specific molecule attaches to the receptors the membrane invaginates


Phagocytosis Also works using

receptor molecules

Phagocytic white blood cells (neutrophils and macrophages) recognise and engulf microbes this way

Pseudopod


Phagocytosis Contact with prey Receptor

molecules on the plasma membrane recognise surface antigens


Phagocytosis Feeding cup forms

to engulf the prey The membrane

stays in contact with the prey


Phagocytosis

Bilayer adherance


Phagocytosis Bilayers fuse Food vacuole forms Lysosomes fuse with it The prey is digested

Food vacuole


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MEMBRANE TRANSPORT PROTEINS © 2010 Paul Billiet ODWSODWS