Cell Transport Systems
Diffusion
• Mixing of materials by their own random motion
• Mixing occurs from an area of high to low
concentrations
Osmosis
•
•
•
Basically the diffusion
of H2O through a
membrane
Since H2O molecules
are small – they can
pass through
membranes (even nonpolar part)
Follows the same
principles of regular
diffusion (High to low
concentration)
Osmotic pressure
• When a cell is in fresh water, the concentration of water is
greater on the outside than the inside of a cell. This is called a
Hypotonic solution. When this happens, water can fill the cell
and put pressure on the side of it causing pressure – Osmotic
pressure. If osmotic pressure gets too much it can “burst” a
cell. This is called Plasmolysis.
• Many fresh water organisms have built in “sump pumps” to
remove excess H2O. Other Salt water organisms must pump in
water
• Conversely, in a salt-water environment, cells must pump
water into the cell. This is called a hypertonic solution. The
cell can lose water and "shrivel" this is called crenation.
• Cells in larger organisms are surrounded by solution with
roughly equal concentrations of H2O and solvents. This is
called an isotonic solution.
Osmosis illustrated
Isotonic
Hypertonic
Hypotonic
Selective Transports
• Used for molecules which cannot easily
pass through cell membrane because they
are either…
• Too large to pass between phospholipids
• Are polar and cannot pass through non-polar
region eg. ions
• Two basic types
Facilitated Diffusion
• Works by having channel proteins of different sizes (like
a sieve) or ion channels which allow larger, or charged
molecules to diffuse in/out of cell
Active Transports
•
•
•
•
Lets in/out molecules via membrane proteins which actively pump in/out
molecules.
This requires energy from the cell.
Moves molecules in 1 direction AGAINST THE CONCENTRATION
GRADIENT
Example is the Sodium-Potassium Pump.
Sodium / Potassium Pump (Adv. Bio)
Pumps Potassium into & Sodium
out-of cell
6 step process
1.
2.
3.
4.
5.
6.
3 Na+ (Sodium) Ions bind to a
special receptor protein
Energy (from ATP) is added
to the protein.
Protein changes shape,
releasing Na+ from the cell.
2 K+ (Potassium) ions now
bind to the protein
Phosphate is released, protein
returns to its original shape.
K+ ions are released into the
cell interior
Endocytosis
• Large particles enter cells via endocytosis
• Membrane actively surrounds & engulfs particle or fluid
and forms a vacuole.
• Phagocytosis – Cell eating – involves large particles.
• Pinocytosis – Cell Drinking – involves liquids.
Exocytosis
• Large particles exit the cell via Exocytosis
• Vessicle moves to and merges with the plasma membrane.
• Vessicle ruptures outwards, releasing its contents
Receptor mediated endocytosis
• Receptor mediated endocytosis: ligands bind to specific
receptors on cell surface (coated pits)
• Example: human cells take in cholesterol by this process