Transport Across the Membrane

selectively

• Endocytosis - large molecules
• Phagocytosis - another cell/big structure
• Pinocytosis - liquid

Exocytosis - eg waste products

• Passive diffusion
• Facilitated diffusion
• Active transport

When lipid soluble molecules move across the membrane from a high to a low concentration down a concentration gradient (using no energy)

When lipid insoluble molecules move across the membrane using transport proteins - either carrier proteins or channel proteins.  This is a specific and passive process (still no energy is used).

When molecules move across the membrane against a concentration gradient.  This requires energy and specific transport molecules

The sodium-potassium pump

3

2

• Inside a cell there is a low concentration of sodium ions.  Outside a cell there is a low concentration of potassium ions.
• Three sodium ions inside the cell cytosol binds to sites in the sodium-potassium pump
• This binding enables the proteins ATPase function to hydrolyse ATP to ADP + Pi phosphorylating the protein (the phosphate adheres to the protein)
• The binding and protein phosphorylation causes a conformational change (the ion pump opens up to the outside of the cell).  The phosphorylation of protein reduces the affinity for sodium)
• The reduced affinity of sodium sites for Na⁺ means the sodium is released to the outside of the cell
• When the sodium has been released, the potassium binding sites can then be occupied by two K⁺ ions from outside the cell
• The binding of potassium causes the phosphate to be released
• The dephophorylation restores the proteins original form.  The channel faces inside the cell and potassium is released into the cell.  Affinity for Na⁺ is high again.