Transport Across the Membrane
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The cell membrane is said to be _____ permeable?
How are large molecules/cells taken into the cell?
- Endocytosis - large molecules
- Phagocytosis - another cell/big structure
- Pinocytosis - liquid
How are large molecules/particles removed from the cell?
Exocytosis - eg waste products
How can small molecules pass over the membrane?
- Passive diffusion
- Facilitated diffusion
- Active transport
What is passive diffusion?
When lipid soluble molecules move across the membrane from a high to a low concentration down a concentration gradient (using no energy)
What is facilitated diffusion?
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).
What is active transport?
When molecules move across the membrane against a concentration gradient. This requires energy and specific transport molecules
What is an example of a specific case of active transport across the cell membrane?
The sodium-potassium pump
How many sodium ions does the sodium-potassium pump pump out of the cell?
How many potassium ions does the sodium-potassium pump pump into the cell?
Describe how the sodium-potassium pump works
- 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.
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