Simple Diffusion is Limited to Small, Nonpolar Molecules Flashcards
What are liposomes?
liposomes are small vesicles, each consisting of a closed, spherical, lipid bilayer that is devoid of membrane proteins
What did Bangham show?
Bangham showed that it is possible to trap solutes such as potassium ions in the liposomes as they form, and then measure the rates at which solutes escape by diffusion across the liposome bilayer
What did Banghams results show?
they showed that ions such as potassium and sodium were trapped in the vesicles for days, whereas small, uncharged molecules such as oxygen exchange so rapidly that the rates could hardly be measured
What was the conclusion of Banghams experiment?
the lipid bilayer represents primary permeability barrier of membrane, ie small uncharged molecules could escape through the barrier by simple diffusion, whereas Na+ and K+ ions could barely pass at all
What are the three main factors affecting diffusion of solutes?
Size, Polarity, Charge
Solute Size: relationship between solute size and the lipid bilayer?
lipid bilayers are more permeable to smaller molecules than to larger molecules
Solute Size: Examples of small molecules? How do they cross the cell membrane?
small molecules include H2O, O2, CO2. Membranes are quite permeable to these molecules which require no specialized transport process for moving into and out of the cells
Solute Size: How do larger molecules get across the cell membrane? Example of lager molecule?
Larger molecules such as glucose cannot diffuse across membranes - cells therefore need specialized proteins in their plasma membranes to facilitate the entry of glucose and numerous other solutes
Solute Polarity: relationship between solute polarity and lipid bilayer? Why?
lipid bilayer is more permeable to nonpolar molecules and less permeable to polar molecules. This is b/c nonpolar molecules dissolve more readily in the hydrophobic phase of the lipid bilayer and can therefore cross the membranes much more rapidly than polar molecules of similar size
Solute Polarity: In general…
the more nonpolar (or hydrophobic) a substance is, the more readily and rapidly it can move across a membrane
Solute Charge: relationship between water and polar substances?
the molecules of water form a shell of hydration around polar substances
Solute charge: How do polar solutes cross the membrane? What affect does their association with water molecules have on their transport across membranes?
the water molecules must be stripped off, and an input of energy is required to eliminate the bonding between the ions and water molecules. Therefore the association of ions with water molecules to form shells of hydration drastically restricts ion transport across the membranes.
Why is the impermeability of membranes to ions important for cell activity?
b/c in order to function, every cell must maintain an electrochemical potential across its plasma membrane.
What is the electrochemical potential in most cases?
In most cases this potential is a gradient of either Na+ ions (animal cells) or protons (most other cells)
