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The cell membrane is said to be _____ permeable?
selectively
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?
3
How many potassium ions does the sodium-potassium pump pump into the cell?
2
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.