Cellular Transports

Diffusion & Osmosis

• Diffusion: Defn': Net movement of molecules from regions of higher concentrations to regions of lower concentration. This is the result of random molecular motions, and results in a uniform distribution of molecules.

Diffusion

• Molecules move from High to low concentrations. The difference in concentrations is known as a Concentration Gradient.
• Energy can be obtained by the cell from utilizing the concentration gradient. An example is the function of ATP synthetase which utilizes a gradient of H⁺ ions to create ATP molecules from ADP and Pi.

ATP production utilizing a concentration gradient

• Osmosis is essentially describes the diffusion of H₂O across a semipermiable membrane
• A semipermiable membrane refers to a membrane which allows the passage of H2O, but not 1 or more solutes.
• H₂O like other molecules will diffuse from areas of high concentration to low concentrations.
• Regions of high solute concentrations Þ Low H₂O concentrations
• Regions of low solute concentrations Þ High H₂O concentrations

• In the presence of a solute, water will travel across a membrane from high to low concentrations.
• This creates an osmotic pressure against the motion of water.
• When the osmotic pressure = force of water across the membrane through osmosis, the system is in equilibrium.

Osmosis & Osmotic Pressure

 

• In the living Cell, this can cause major problems.
• In environments with low solute concentrations (Hypotonic Solutions), Since higher solute concentrations are within the cell, water will move into the cell, increasing the pressure within the cell. This can cause cells to burst or "lyse"
• In environments with high solute concentrations(Hypertonic solutions), higher solute concentrations are found outside cell, and net water movement will be out of the cell. This can result in shrinkage or "crenation" of the cell.
• Cells in larger organisms are surrounded by solution with roughly equal concentrations of H₂O and solvents. This is called an isotonic solution.

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Selective Transport

1. 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

1. 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 transport. 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

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Example is the Sodium-Potassium Pump.

• Pumps Potassium into & Sodium Out-of cell
• 5 step process

1. 3 Na+ (Sodium) Ions bind to a special receptor protein
2. Energy (from ATP) is added to the protein.
3. Protein changes shape, releasing Na+ from the cell.
4. 2 K+ (Potassium) ions now bind to the protein
5. Phosphate is released, protein returns to its original shape.
6. K+ ions are released to the inside of the membrane.

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Large Molecules/Particles/Organisms

1. Large particles get in via Endocytosis

• Membrane actively surrounds & engulfs particle or fluid and forms a vacuole.
• Phagocytosis – Cell eating – involves large particles
• Pinocytosis – Cell Drinking – involves liquids.

• Is an active transport system since this requires (much) energy from the cell.

2.  Large particles exit the cell via Exocytosis

• Vessicle moves to and merges with the plasma membrane
.
• Vessicle ruptures outwards, releasing its contents

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Receptor mediated endocytosis: ligands bind to specific receptors on cell surface (coated pits)

• Example: human cells take in cholesterol by this process