Hello everyone!
I would just like to ask if anybody could explain the modes of transport of soluble substances across the plasma membrane. And explain each of the following: Simple diffusion, facilitated diffusion, osmosis and active transport.
Thanks.
Okay to start us off, diffusion is the movement of molecules down the concentration gradient. What this means is that the molecules are moving from a region of high concentration to a region of lower concentration.
If this diffusion refers to the movement of water, then we call this osmosis.
If the diffusion is of anything else (i.e. only refers to solute molecules) then it's just diffusion (which can have added bits tacked onto the name (e.g. simple)
Because the molecules in diffusion are moving down the concentration gradient (a.k.a. from a region of high concentration of the molecule to a region of lower concentration), they will do it with out needing any energy input.
Simple diffusion occurs when a molecule (which has to be non-polar as the fatty tails of the phospholipid bilayer will not all ow polar molecules to pass through - the exception to this is water, which although polar, can occasionally just slip through because it's soooo tiny) basically just passes in between the phospholipids that constitute the cell membrane.
Facilitated diffusion is the same as simple diffusion, except the molecule passing through (such as glucose or water) has to pass through a protein channel which 'facilitates' (helps) it. This also requires no energy input as again, it is simply diffusion. Water, for example, most commonly passes through a protein channel known as an aquaporin. I'm not sure about this next bit I'm about to say, so could someone please verify? But I also believe that this aquaporin can be shut if the cells wishes (well cells don't 'wish'
), meaning that the cell won't absolutely fill up with water and lyse (burst/explode) if it is placed in pure water as otherwise the water would move in to balance the concentration of all the stuff inside the cell compared to the outside. Right?
As I've said earlier, osmosis is simply the diffusion of water across the semi-permeable membrane. Again, it is diffusion, so it doesn't require any expenditure of energy. This can be seen for example when you have the classic 'U-shaped' container thing (guys, what's it called LOL
with a semi-permeable membrane in the centre-bottom bit, in which the SP membrane is permeable for water, but not salt. If there are different concentrations of salt on either side, then the water will move (diffuse) from the area of lower concentration of the salt (as this is the area with the higher concentration of the water) to the area of lower concentration of the salt. This is osmosis in action!
(google osmosis - the image I've described should come up)
Active transport also refers to the movement of molecules across semi-permeable membranes, but the molecules are moved 'up' their concentration gradient (imagine there were two pools of equal size; one pool had 5 ppl in it and the other had 10. People would be moving up the concentration gradient if even more people from the less populated pool moved to the more populated one). For this reason, active transport requires the expenditure of energy (which is sourced from the breakdown of ATP into ADP + Pi [Pi just means a free phosphate]).
Basically all this movement across the cell membrane ideally intends to allow the cells to take in and get rid of what it needs (although am I forgetting things like endo and exocytosis?)
Endocytosis is the movement of larger, non-soluble molecules into the cell via a process in which a part of the phospholipid bilayer forms a vesicle configuration around the thing being moved based on the same principles of hydophilia/phobia that cause them to generally form the cell membrane. Exocytosis is the same, but isthe movement of the molecules out of the cell. Both of these DO require energy expenditure.
An example of exocytosis is when neurotransmitters move out of the axon of one neuron via exocytosis, diffuse across the synapse (so they go down the concentration gradient) and then reach the dendrite of the receiving neuron, and then activate a postsynaptic receptor to continue the series of things but that's nerves so eh imma stop soon
Also interesting to know that the organelles inside a (eukaryotic) cell are also membrane bound (as that's pretty much the definition of a eukaryotic cell) so they too are involved in some of these forms of transport.
I hope this is helpful (As mr andersen says [watch his video on movement across cell membranes!!!!!!!!!! {Bozeman Bio}])
EDIT: Haha I lose *raspberry @ cosine & sunshine*