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For an organism (think single-celled) living in just the right kind of environment, it might just be possible to survive using only facilitated diffusion, at least as far as small molecules are concerned. This kind of organism would have to maintain exactly the right concentration of the molecules it wants to keep or get rid of, based on the external ...

AndroidPenguin's answer has really muddied the water, but I can't fit my response into a comment. When a glucose molecule moves from the blood into, for example, a muscle cell, it is indeed diffusing down a concentration gradient, and the GLUT transporter protein acts to facilitate this process by providing a route for the polar glucose molecule to get ...

Diffusion relies on the fact particles move randomly in whatever they're in. If they are concentrated (I.e. there are lots of them in one place) they will fill out the whole place. With respect to cell membranes, anything that can diffuse freely will be the same concentration inside and outside the cell. It doesn't require metabolic energy as the cell ...

Diffusion is always a passive process that doesn't require energy. Therefore it would seem that A is correct, B doesn't really make a whole lot of sense. In the case of the cell membrane diffusion will often be in the form of 'facilitated diffusion' through carrier proteins, however the transport is still not active.

Bilayer components will 'flip-flop' at measurable rates, but these are very different for different lipid classes. Here are the results of an experiment using fluorescently-labelled analogues. Bai, JN and Pagano, RE (1997) Measurement of spontaneous transfer and transbilayer movement of BODIPY-labeled lipids in lipid vesicles. Biochemistry 36:8840-8848 ...