Difusion And Osmosis

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PASSIVE TRANSPORT – no energy required
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Diffusion
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The molecules of a gas such as oxygen are moving about all the time. So are themolecules of a liquid, of a substance such as sugar dissolved in water. As a result of this movement, the molecules spread themselves out evenly to fill all the available space. The process by which molecules spread from a higher concentration to a lower concentration areas is called Diffusion. Diffusion is the net movement of molecules down their concentration gradient (a difference betweenconcentrations in a space) Diffusion can occur in gases, in liquids, or through solids even in solutions. The factors that affect the rate of diffusion are the temperature of the solution and the steepens of the concentration gradient.
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This process iscalled diffusion. One effect of diffusion is that the molecules of a gas, a liquid or a dissolved substance will move from a region of high concentration to regions of low concentration until the concentration everywhere is the same.
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A) Greater concentration outside
(B) Concentrations equal on both sides of the cell membrane
F1

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The effect of this difference in concentration is to make the molecules diffuse into the cell until the concentration insideand outside is the same. (F1)
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Whether this will happen or not depends on whether the cell membrane will let the molecules through. Small molecules such as water (H²O), carbon dioxide (CO²) and oxygen (O²) can pass through the cell membrane fairly easily. So diffusion tends to equalize the concentration of these molecules inside and outside thecell all the time.
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Rates of diffusion
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The speed with which a substance diffuses through a cell wall of cell membrane will depend on temperature, pressure and many other conditions including the distance it has to diffuse, its concentration inside and outside the cell and the size of its moleculesor ions.
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Cell membranes are all about the same thickness but plant cell walls vary in their thickness and permeability. Generally speaking, the thicker the wall, the slower is the rate of diffusion.
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The bigger the difference in concentration of a substance on either side of amembrane, the faster it will tend to diffuse. The difference is called a concentration gradient of diffusion gradient. If a substance on one side of a membrane is steadily removed, the diffusion gradient is maintained. When oxygen molecules enter a red cell they combine with a chemical which takes them out of solution. Thus the concentration of free oxygen molecules inside the cell is kept very low and...