Cell Transport Systems

1. Cell Transport Systems

2. Diffusion • Mixing of materials by their own random motion • Mixing occurs from an area of high to low concentrations

3. Osmosis • Basically the diffusion of H2O through a membrane • Since H2O molecules are small – they can pass through membranes (even non-polar part) • Follows the same principles of regular diffusion (High to low concentration)

4. 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 Hypotonicsolution. 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.

5. Osmosis illustrated Hypotonic Isotonic Hypertonic

6. 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

7. 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

8. 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.

9. Sodium / Potassium Pump (Adv. Bio) Pumps Potassium into & Sodium out-of cell 6 step process • 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

10. 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.

11. Exocytosis • Large particles exit the cell via Exocytosis • Vessicle moves to and merges with the plasma membrane. • Vessicle ruptures outwards, releasing its contents

12. 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