FACTORS AFFECTING THE RATE OF DIFFUSION

I) Surface Area vs. Volume - Cells are limited in their size because they must maintain a high “Surface Area : Volume” ratio.

- Nutrients (oxygen and glucose) must enter a cell while wastes (carbon dioxide and urea) must exit a cell through the cell membrane.

- The amount of cell membrane = Surface Area of the cell. - Inside the cell, organelles use up nutrients and produce wastes. The larger the cell, the more nutrients needed and the more wastes produced.

- The size of the cell (based on the number of organelles) = Volume of the cell.
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Cell S.A Volume S.A. : Volume LxWx6 LxWxH Ratio A 6 units2 1 unit3 6 : 1 B 24 units2 8 units3 3 : 1 C 96 units2 64 units3 1.5 : 1 D 384 units2 512 units3 0.75 : 1
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- Small cells have a high “SA:Volume” ratio, they can supply the organelles with plenty of nutrients and rapidly remove wastes.

- Large cells have a low “SA:Volume” ratio because volume rises faster than surface area. If a cell gets too big, wastes will build up and nutrients will run out; cell cannot survive. - Therefore, cells are limited in their size. The more active a cell the smaller it must be. Active cells need more nutrients and they produce more wastes. - How do cells occupy the large volumes associated with large organisms? The cells reproduce frequently through mitosis to maintain their small size (high “SA:Volume” ratios) while creating millions of cells to fill these larger volumes.
(As opposed to using fewer larger cells) - Some cells can achieve a large size while still maintaining a high SA:Volume ratio. This is achieved by producing folds (of cell membrane) to increase SA.

[pic][pic]
- These folds allow the cell to gain more surface area without a large increase in volume. Checkout : http://www.hillstrath.on.ca/moffatt/bio3a/cellbio/sa-vsoln.htm II) FACTORS THAT WILL AFFECT THE RATE OF DIFFUSION 1. Concentration gradient:
– the difference in concentration between two areas.
-- the greater the [ ] gradient, the faster the diffusion. [pic]
2. The size and shape of the molecules:
- Smaller molecules that fit through pores will pass at a faster rate. 3. Temperature:
– Warmer = faster (more kinetic energy in the particles) http://mutuslab.cs.uwindsor.ca/schurko/animations/waterphases/status_water.htm Click on the first button 4. Type of medium: – Diffusion is faster in gases than in liquids (more empty space between the molecules of a gas as well as greater Kinetic E) [pic][pic]
5. Movement of the medium
–Water or air currents speed up diffusion.
Example: Stirring or heating can create currents (promotes mixing).
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Higher Concentration

Lower Concentration