(d)+Passive+Transport

Giang Le and Masatoki Teranishi  PASSIVE TRANSPORT (1) Carefully define the terms **diffusion** & **osmosis**.


 * Diffusion** is when moluecules spread out into available space.Substances diffuse from a more concentrated environment to a less concentrated environment. The diffusion of substances across membrances is called **passive transport.**

The rate of diffusion is affected by the permeability of the membrane. For example, water diffuses across cell membranes freely and easily while other substances do not. They must be helped across by facilitated diffusion.
 * Osmosis** is the process when molecules pass through a semipermeable membrane, moving from an area where the solute is is present in low concentrations to an area where there is high concentrations. Osmosis results in equal amount of of fluid on either side of the barrier, creating a state which is known as "isotonic".

(2) As a general rule, which substances can diffuse across a cell membrane and which substances cannot?

Generally, this is the order from hard to diffuse to easy to diffuse: Solid -> Liquid -> gas

As a general rule, smaller and non-polar molecules diffuse the easiest

(3) What is meant by **facilitated diffusion**? Does it require an energy input? De scribe the process.

When a molecule is attached to the carrier protein, the carrier protein starts to change shape in order to let the molecule go through a hollow tube in which leads to inside or outside of a cell. When there is a higher concentration of molecules outside of a cell, once a molecule is put into it, a molecule tries to get out from the other end, which would be the inside of a cell. On the other hand, if there is a higher concentration of molecules inside a cell, then those molecules will be taken out of a cell via facilitated diffusion. Therefore, just like normal diffusion, a cell wouldn’t have to use any input of energy.
 * Facilitated diffusion** is when a carrier protein is used to transport specific molecules right for its own (particular sugar or amino acid). Facilitated diffusion is used instead of normal diffusion because some molecules that are being transported is too big to be diffused through membrane.

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