Transcript 5 Active Transport,endoandexocytosis

Active Transport
• Quite often substances need to move
against their concentration gradient.
• Active Transport allows this to happen.
• I.e. Glucose and amino acids are pumped out
from urine back inside the blood.
• I.e. Gill cells in fish pump out sodium ions into
sea water
• I.e. Maintaining the pH level with in the cell by
pumping protons (H+) out.
• (40 % of your cell’s energy is used for active
transport.)
Active Transport Pumps
We will look at 3 types:
• Sodium – Potassium Pump
• Purpose:
• Transports
– Sodium ions (Na+) out of the cell
– Potassium ions (K+) into the cell
How does it work?
• With the energy from 1 ATP molecule,
– 3Na+ ions are able to move out and
– 2 K+ ions are able to move in.
• To get the energy stored from ATP, the pump must
‘hydrolize’ (or break a part)
– ATP ADP + Pi
– (Adenosine Triphosphate – Adenosine (Dipohsphate) +
1 phosphate)
The Process:
• 3 Na+ ions bind to the Na+ binding site on
cytosolic side of the pump
• 2 K+ ions bind to the K+ binding site on the
extracellular side of the pump
• This Triggers the Na+/K+ pump to hydrolyze 1
ATP molecule (ATP ADP + Pi)
• The pump then changes shape allowing 3 Na+
ions to move out and 2 K+ ions to move in
The end result from this main
pump
• helps other protein pumps in the membrane
transfer ions and molecules against their
concentration gradient by creating an
artificial concentration gradient.
• To achieve this,
– Na+ ions must continually be diffusing inside
the cell and
– K+ ions continually be diffusing out.
Two pumps that depend on this
are….
• 1. Na+/glucose pump
• I.e. Found in the
• lining of small
intestine
• Kidney tubules
• Here, glucose needs to move against a large
concentration gradient
• Na+ ions and glucose bind to their specific
binding sites of this carrier protein
• Shape of protein changes
• Na+ and Glucose move easily through
• 2. K+/H+ Pump
• I.e. Found in Stomach lining
• These protein pumps move H+ out of the
cell against its concentration gradient to
maintain a normal pH level in the cell.
Endocytosis
• The process where the cell membrane folds
around and traps substances from the
extracellular fluid to form a vesicle.
• There are 3 types:
1. Phagocytosis (Cell ‘eating’)
(p. 36)
• Seen in the
macrophages of our
immune system
• Eats up bacteria
• The cell envelops
bacteria and other
large particles and
then internalizes them.
2. Pinocytosis (‘drinking’)
• The cell takes up small
droplets of
extracellular fluid and
any material dissolved
in it.
3. Receptor-mediated
endocytosis (p. 37)
• Molecules in the extracellular fluid have proteins
on their surface that ‘fit’ with a receptor on the cell
membrane. (binding protein)
• The specific receptor on the cell surface
recognizes the molecule by its binding protein or
protein ‘tag’(I.e. ‘like a code’) and binds tightly to
it.
• This triggers endocytosis
• Results in a vesicle carrying the macromolecule
•
•
•
•
I.e. LDL
Cholesterol is not water soluble, thus,
Must be carried by a particle called LDL
LDL
– Droplets covered with a single layer of phospholipids
– Has a protein ‘tag’
• The protein ‘tag’ binds to the cell surface receptor
• Triggers endocytosis
• Once internalized, the vesicles empty its
contents
• The Membrane with receptors (from the
vesicle) returns to the surface
• Turns inside out so receptors face outside
once again.
• Process is repeated
HIV
• Is a virus with a
binding protein that
mimics a specific
binding protein
required for a receptor
on the cell surface
• It tricks the cell to
believe it is a
macromolecule needed
for the cell
• Once inside, the virus uses the cell’s
‘machinery’ to replicate
• Problem?
• The binding protein on the virus is
constantly changing to fit different receptors
on various cell surfaces therefore, the cell
does not recognize HIV as a virus.
Exocytosis
• Reverse of endocytosis
• A vesicle inside the cell moves toward the
surface
• Fuses with the membrane
• The contents of the vesicle are secreted out
into the extracellular fluid
• I.e. Specialized cells in the pancreas secrete
the hormone insulin through exocytosis