gas flow - (Apr/05/2009 )

I'm not sure where this question should go, as there is some chemistry involved. But I saw this concept in my biology book, so I placed my super hard question here. Any help is greatly appreciated.

NOTE: This is NOT a homework question. This is just a question I stumbled upon as I was trying to understand the text.

We know that gas flows from a region of higher pressure to a region of lower pressure (a very important concept in the mammarian breathing mechanism). WHY does this happen???????

I guess my confusion stems from the fact that (according to my book) the property above and diffusion are totally different things, as evident in transpiration. Diffusion would take forever, but thanks to transpirational pull ( which utilizes the property above), transpiration does not a decade to bring a water molecule up the stem. But I can't think of anything else either than diffusion's mechanism that would explain how the property that gas flows from a region of higher pressure to a region of lower pressure works. Please help!!!!

I thank you in advance.

Don't confuse diffusion and bulk flow, which you have done in your answer. Diffusion is the process by which random movements in a molecule (which are related to temperature and molecular weight) move them though a space or substance with no other driving force (e.g salt moving into a cell). Bulk flow is the movement of large volumes by input of direct force (e.g. water flowing downhill, breath being pushed out of our lungs)

The reason gas (or any molecule) diffuses from high concentration to low concentration is due to it being the lowest energy state for this system under the second law of thermodynamics. One way to think about it is - if you take a classroom full of people and pack them all into a toilet cubicle (high pressure), they will all be bumping against each-other and getting kind of hot, now you open the door and let them move out into a hall (low pressure), there will still be the same number of people but they are taking up a much larger area and will be less hot (and annoyed) and as such have a lower energy.

Transpiration is a special case - it relies on diffusion to work, but there is a key difference which is surface area. You will have seen capillary action in class before and you will know that plant cell walls are made up of lots of interwoven fibers (sort of like a tissue - look at one under a microscope some time, and note that it is partly space with fibers crossing it). So the fibres in the cell wall act just like a tissue, there are lots and lots of gaps between them which all soaking up water from below, which is then evaporated off the outside of the cell, drawing more water up behind it to fill those gaps. The fibers have a very large surface area if you added it all up, much much larger (can't remember the figure, but it will be thousands of times) than just the area of a leaf. This system is so efficient that plants actually have to cut down the amount of water that they lose though this process, so they have stomata on the leaves.

The lungs work on a similar principle, except it is the muscles of the chest and the diaphragm that draw the air in by bulk flow, then the bronchi and bronchioles that provide the surface area for the gases to diffuse across.