Active Transport
Active Transport
Cellular energy is used to to transport
substances across the membrane against a
concentration gradient
Energy is derived from splitting ATP
Pumps
A transporter protein that uses energy from
splitting ATP to change shape and carry a
substance across a cellular membrane
against its concentration gradient
Substances Transported
Na+
K+
H+
Ca++
ICl-
Sodium-Potassium Pump
Expels Na+ from the cell, brings K+ into
the cell
Acts as an enzyme to split ATP
Used to maintain a low [Na+] concentration
in the cytosol by pumping Na+ into the
extracellular fluid
Continuous Operation
Na+ and K+ slowly leak back across the
membrane down their gradients, so the
pumps must continuously move these ions
back across the membrane to maintain the
concentration gradient.
Concentration Gradient
The differing concentrations are crucial for
osmotic balance of the 2 fluids and also for
the ability of some cells to generate
electrical signals
Operation of the Sodium
Potassium Pump
1. Three Na+ in the cytosol bind to the
pump protein
2. Na+ binding triggers the splitting of ATP
into ADP plus a phosphate group which
attaches to the pump protein. This causes a
change in the shape of the pump protein
3. Three Na+ are expelled into the
extracellular fluid
Operation of the Sodium
Potassium Pump
4. The changed shape allows 2 K+ in the
extracellular fluid to bind to the protein
5. The binding of the K+ causes the
phosphate group to be released which
causes the pump protein to return to its
original shape
6. As the pump protein returns to its original
shape, it releases the 2 K+ into the cytosol.
The pump protein is then ready to bind Na+
and the cycle can repeat
Malfunctions of Transporters
Drugs – some turn off ATP production
which would stop active transport of
substances throughout the body
Cystic Fibrosis – a defective gene causes an
abnormal Cl- transporter. There is a failure
to secrete adequate amounts of Cl- resulting
in extremely thick mucus secretions that
result in obstruction and infection.
Transport in Vesicles
Vesicle – a small sac formed by the budding
off from an existing membrane
Movement of vesicles requires energy
supplied by ATP.
ENDOCYTOSIS
Materials move into a cell is a vesicle
formed from the plasma membrane
Phagocytosis
Bulk Phase Endocytosis
Phagocytosis
Large solid particles (like whole cells) are
taken in by the cell.
The particle binds to the plasma membrane
receptor, the cell extends projections called
pseudopods that surround the particle, then
the membranes fuse to create a vesicle in
the cytoplasm.
Any undigested material remain indefinitely
in a vesicle called a residual body
Phagocytosis occurs only in special cells
called phagocytes
They are specialized cells used to engulf and
destroy bacterial and other foreign substances
• These cells include certain white blood cells and
macrophages.
• Phagocytosis is one of the body’s defense
mechanisms against disease
Bulk Phase Endocytosis
Cells take up vesicles containing tiny
droplets of extracellular fluid and any
solutes dissolved in that fluid
The vesicle detaches from the membrane
and enters the cytosol
The vesicle fuses with a lysosome where
enzymes digest the solutes into smaller
molecules
Exocytosis
Results in secretion – materials exiting the
cell
Secretory cells give off digestive enzymes,
hormones, mucus or other secretions
Nerve cells give off neurotransmitters
Secretory Vesicle
Vesicle containing substance to be secreted
forms in the cytosol, fuses with the plasma
membrane then releases the contents into
the extracellular fluid
CHECKPOINT
What is the key difference between active
and passive transport?
Briefly compare/contrast facilitated
diffusion and active transport through a
membrane pump
Describe the process of phagocytosis.
Name one way phagocytosis helps to
maintain homeostasis.