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I got this question and don't really understand the difference between the answers.

Diffusion is (in cell membrane):

a) passive by nature, no metabolic energy is needed

b) driven process by pressure or voltage

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3 Answers

up vote 1 down vote accepted

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.

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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 doesn't need to do a thing. Thus the answer is A.

Facilitated diffusion is where the cell puts protein channels on its membrane to allow diffusion of things that can't pass through the cell membrane normally or do so really slowly. This can be driven by a voltage or by pressure, which "facilitate" or help the diffusion. Glucose for example moves like this into the cell. It uses the fact that there's a high concentration of sodium outside the cell that "wants" to move in because of the cell inside being negative and low concentration inside. So glucose hops onto a sodium ion whilst it is moving in.

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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 across the membrane. This is facilitated diffusion. The movement of the glucose molecules requires no input of metabolic energy. That part of AP's answer is ok, and really that's all that the OP was about.

In direct contrast to this, enterocytes can transport glucose from the gut lumen into their cytosol even though the gradient of glucose concentration is in the other direction. They do this by linking the inwards movement of the glucose molecule to the outwards movement of Na+ ions, down the electrochemical gradient for these ions. This Na+ gradient is, in turn, generated by a pump which uses ATP to move Na+ out and K+ in. So in this example the energy derived from ATP hydrolysis is 'stored' in a gradient of Na+ concentration, and this gradient is tapped as a source of energy to power movement of glucose. This is NOT an example of facilitated diffusion. The ATP-powered movement of ions is an example of primary active transport, and the action of the Na+/glucose transporter is an example of secondary active transport. Note also that the glucose and the Na+ move in opposite directions (antiport) so there is no question of a direct interaction between the ion and the glucose.

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