Cell Transport SystemsCell Transport Systems

DiffusionDiffusion

• Mixing of materials by their own random motionMixing of materials by their own random motion• Mixing occurs from an area of high to low Mixing occurs from an area of high to low

concentrationsconcentrations

OsmosisOsmosis

• Basically the diffusion Basically the diffusion of Hof H22O through a O through a membranemembrane

• Since HSince H22O molecules O molecules are small – they can are small – they can pass through pass through membranes (even non-membranes (even non-polar part)polar part)

• Follows the Follows the same principles of same principles of regular diffusion (High regular diffusion (High to low concentration)to low concentration)

Osmotic pressureOsmotic pressure

• When a cell is in fresh water, the concentration of water is 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 greater on the outside than the inside of a cell. This is called a HypotonicHypotonic solution. When this happens, water can fill the cell solution. When this happens, water can fill the cell and put pressure on the side of it causing pressure and put pressure on the side of it causing pressure – – Osmotic Osmotic pressurepressure. If osmotic pressure gets too much it can “burst” a . If osmotic pressure gets too much it can “burst” a cell. This is called cell. This is called PlasmolysisPlasmolysis..

• Many fresh water organisms have built in “sump pumps” to Many fresh water organisms have built in “sump pumps” to remove excess Hremove excess H22O. Other Salt water organisms must pump in O. Other Salt water organisms must pump in waterwater

• Conversely, in a salt-water environment, cells must pump Conversely, in a salt-water environment, cells must pump water into the cell. This is called a water into the cell. This is called a hypertonichypertonic solution. The solution. The cell can lose water and "shrivel" this is called cell can lose water and "shrivel" this is called crenationcrenation..

• Cells in larger organisms are surrounded by solution with Cells in larger organisms are surrounded by solution with roughly equal concentrations of Hroughly equal concentrations of H22O and solvents. This is O and solvents. This is called an called an isotonicisotonic solution. solution.

Osmosis illustrated

IsotonicIsotonicHypotonicHypotonic

HypertonicHypertonic

Selective TransportsSelective 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 DiffusionFacilitated Diffusion

• Works by having Works by having channel proteinschannel proteins of different sizes (like of different sizes (like a sieve) or a sieve) or ion channelsion channels which allow larger, or charged which allow larger, or charged molecules to diffuse in/out of cell molecules to diffuse in/out of cell

Active TransportsActive 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)Sodium / Potassium Pump (Adv. Bio)

Pumps Potassium into & Sodium Pumps Potassium into & Sodium out-of cellout-of cell

6 step process6 step process

1.1. 3 Na3 Na++ (Sodium) Ions bind to a (Sodium) Ions bind to a special receptor proteinspecial receptor protein

2.2. Energy (from ATP) is added Energy (from ATP) is added to the protein.to the protein.

3.3. Protein changes shape, Protein changes shape, releasing Nareleasing Na++ from the cell. from the cell.

4.4. 2 K2 K++ (Potassium) ions now (Potassium) ions now bind to the proteinbind to the protein

5.5. Phosphate is released, protein Phosphate is released, protein returns to its original shape. returns to its original shape.

6.6. K+ ions are released into the K+ ions are released into the cell interiorcell interior

EndocytosisEndocytosis• Large particles enter cells via Large particles enter cells via endocytosis endocytosis • Membrane actively surrounds & engulfs particle or fluid Membrane actively surrounds & engulfs particle or fluid

and forms a vacuole.and forms a vacuole.• PhagocytosisPhagocytosis – Cell eating – involves large particles. – Cell eating – involves large particles.• PinocytosisPinocytosis – Cell Drinking – involves liquids – Cell Drinking – involves liquids..

ExocytosisExocytosis

• Large particles exit the cell via Large particles exit the cell via ExocytosisExocytosis• Vessicle moves to and merges with the plasma membrane.Vessicle moves to and merges with the plasma membrane.

• Vessicle ruptures outwards, releasing its contentsVessicle ruptures outwards, releasing its contents

Receptor mediated endocytosisReceptor mediated endocytosis

• Receptor mediated endocytosisReceptor mediated endocytosis: ligands bind to specific : ligands bind to specific receptors on cell surface (coated pits) receptors on cell surface (coated pits) • Example: human cells take in cholesterol by this process Example: human cells take in cholesterol by this process