MEMBRANE TRANSPORT
PROTEINS
© 2010 Paul Billiet ODWS
Passive Transport

Driving forces


© 2010 Paul Billiet ODWS
Diffusion along a concentration gradient
Electrochemical gradient: A membrane
potential is set up due to a voltage (potential
difference) across the membrane
Positive ions are encouraged to move in and
Negative ions are encouraged to move out
Electrochemical gradient
ECF
K+
+ve
Plasma
membrane
Cytoplasm
Cl-
-ve
Positive ions are encouraged to move in and
negative ins are encouraged to move out
© 2010 Paul Billiet ODWS
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
© 2010 Paul Billiet ODWS
Diffusion and facilitated
diffusion
Simple diffusion
Pores saturated
Rate of
transport
Facilitate
diffusion
Concentration
© 2010 Paul Billiet ODWS
Channel Proteins
Permit the passive
movement of
molecules or ions
of appropriate size
(dialysis) through
an aqueous pore
© 2010 Paul Billiet ODWS
Carrier proteins
Bind to specific solutes to transport them
across a membrane
© 2010 Paul Billiet ODWS
Active Transport




Uses energy
Faster than diffusion
Can move against a concentration or
electrochemical gradient
Uses carrier proteins – very specific 
selective transport
© 2010 Paul Billiet ODWS
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
523
543
-
Chloride (Cl )
© 2010 Paul Billiet ODWS
Uniport pore
One type of molecule transported
Change of
configuration
P
P
Phosphorylation
© 2010 Paul Billiet ODWS
ATP + H2O  ADP + Pi
P
Dephosophorylation
Coupled pores
Two molecules transported together
Symport: Both molecules move in the
same direction
Change of
configuration
P
Phosphorylation
ATP + H2O ADP + Pi
P
P
Dephosophorylation
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
P
P
Phosphorylation
P
P
Change of
configuration
P
P
Dephosophorylation
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 =
Pinocytosis
Endocytosis may also occur when a large cell
flows round and engulfs a smaller cell =
Phagocytosis.
© 2010 Paul Billiet ODWS
Exocytosis
Two bilayers of
phospholipid touch
Bilayer adherence
ECF
© 2010 Paul Billiet ODWS
Invagination
ECF
Cytoplasm
Cytoplasm
Two bilayers fuse
Bilayer joining
Endocytosis
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
© 2010 Paul Billiet ODWS
Phagocytosis

Also works using
receptor molecules

Phagocytic white
blood cells
(neutrophils and
Pseudopod
macrophages)
recognise and engulf
microbes this way
© 2010 Paul Billiet ODWS
Phagocytosis


Contact with prey
Receptor
molecules on the
plasma membrane
recognise surface
antigens
© 2010 Paul Billiet ODWS
Phagocytosis


Feeding cup forms
to engulf the prey
The membrane
stays in contact
with the prey
© 2010 Paul Billiet ODWS
Phagocytosis
Bilayer adherance
© 2010 Paul Billiet ODWS
Phagocytosis




Bilayers fuse
Food vacuole forms
Lysosomes fuse with it
The prey is digested
Food
vacuole
© 2010 Paul Billiet ODWS