Transcript Transport Across a Cell Membrane

Transport Across a Cell
Membrane
• The makeup of a cell membrane effects
its permeability
• Three factors determine whether or not
a substance is easily able to cross a cell
membrane:
• 1. Polarity
• 2. Charge
• 3. Size
Passive Transport
• Passive transport involves the
movement of material across the cell
membrane without the input of energy
on the part of the cell
• There are three basic forms of passive
transport:
• 1. Simple diffusion
• 2. Facilitated diffusion
• Osmosis
Simple Diffusion
• Diffusion is the random movement of
particles from areas of high
concentration to areas of low
concentration
• Simple diffusion of material across a cell
membrane occurs only with substances
that can move easily across a cell
membrane
Facilitated Diffusion
• For molecules that, due to certain
characteristics, are not able to simply
diffuse across a cell membrane the
movement may be aided (facilitated) by
carrier proteins that provide an
environment acceptable to the
movement of these molecule along a
concentration gradient
Facilitated Diffusion
• Carrier proteins never provide an open
channel across the cell membrane
Passive Transport
Osmosis
• Osmosis is the diffusion of water across
a selectively permeable membrane in
response to differences in solute
concentration between the ECF and the
protoplasm
• Water will diffuse from the area of
higher water concentration to the area
of lower water concentration
Equilibrium
• Once the contraction of water become
equal on both sides of the membrane
the system has reached equilibrium
• Water molecules will continue to move
between the two sides but there will be
no net movement of water at equilibrium
Isotonic Solutions
• These are solutions in which the solutes
concentration outside the cell is equal
to that inside the cell
• The major function of blood is to keep
your internal environment in an isotonic
balance called homeostasis (the
maintenance of a constant environment
despite internal and external changes)
Hypotonic Solutions
• A hypotonic solution has a lower solute
concentration (higher water concentration)
than inside a cell
• This results in a net movement of water into
the cell
• If the hypotonic solution is maintained it will
result in excess water movement into a cell
causing it to explode (hemolysis/turgor)
Hypertonic Solutions
• A hypertonic solution has a higher solute
concentration (lower water concentration) and
inside a cell
• This results in a net movement of water out of
the cell
• If the hypertonic solution is maintained it will
result in excess movement of water out of the
cell causing it to shrink
(crenelation/plasmolysis)
• http://www.tvdsb.on.ca/westmin/science/sbi
3a1/Cells/Osmosis.htm
Active Transport
• Active transport is the movement of
materials across a cell membrane
against their concentration gradient with
the expenditure of energy in the form of
ATP
• There a three main types of active
transport:
• 1. Ion pumps
• 2. Co-transport
• 3. Endocytosis
Ion Pumps
• Primary active transport involves using
energy (through ATP hydrolysis) at the
membrane protein
• This causes a change in the shape of
the protein that results in the transport
of the molecule through the protein.
• AN example of this is the Na+-K+ pump.
• The Na+-K+ pump is an antiport, it
transports K+ into the cell and Na+ out
of the cell at the same time, with the
expenditure of ATP
Ion Pump
• Ion pump moves sulphur ions across
membrane
Sodium Potassium Pump
Co-Transport
• Other transporters use the energy
already stored in the gradient of a
directly-pumped ion
• First direct active transport of the ion
establishes a concentration gradient.
• Then this concentration gradient uses
facilitated diffusion to pass the ion back
into the cell
• The ion’ passage through a integral
protein pumps some other molecule or
ion along with it against its gradient
Co-Transport
• http://www.northland.cc.mn.us/biology/Biol
ogy1111/animations/transport1.html
Endocytosis
•
Endocytosis is the process by which
cells take in very large molecules
• There are two types of endocytosis:
1. Pinocytosis, where cells take up
dissolved molecules by engulfing
small amounts of the external solution
(Cells of the small intestine take up fat
droplets via pinocytosis.)
2. Phagocytosis, the process by which
cells engulf solid particles from the
external environment.
Exocytosis
• Exocytosis is the process by which
large molecules held within the cell are
transported to the external environment
(waste, proteins and other products of
cellular functions)
• Small vesicles break off from the Golgi
apparatus and move toward the cell
membrane.
• The vesicles fuse with the cell
membrane and the material is released
into the external environment
Exocytosis