Factors Affecting the Rate of Diffusion.

factors Affecting the Rate of Diffusion Size Small molecules can slip by the polar heads of the phospholipids and through the membrane to the other side. Oxygen gas, carbon dioxide and water can move in this manner. Very large molecules like proteins cannot diffuse across the membrane at all. Shape Glucose is able to get into cells much faster than other sugars. This is accomplished by facilitated diffusion. A carrier protein specific for glucose (not other sugars) combines with it on the outer surface, closes around it, and then opens to the inside of the cell where the glucose is released. The carrier then returns to its original shape and is ready to transport another glucose molecule. These carriers can move up to 100 glucose molecules per second across the cell membrane. Concentration The greater the concentration gradient between the outside and inside of the membrane the greater the rate of diffusion. If the concentration of oxygen outside the cell increases then it will diffuse more quickly into the cell. The opposite is also true. If a muscle cell for example is working hard and using up large quantities of oxygen in cellular respiration producing ATP, then the low levels inside the cell will increase the concentration gradient compared to outside and the rate of diffusion of oxygen into the cell will increase. The same conditions in a muscle cell would create high concentrations of carbon dioxide inside the cell and increase the rate of diffusion from inside to outside. Charge (+/-) Ions or molecules with a charge cannot pass through the lipid bilayer by diffusion. Other mechanisms involving protein carriers and ATP energy are required. The sodium/potassium ion pump is an example of this type of transport. Lipid Solubility Lipid soluble molecules can move through the lipid bilayer. Generally these molecules are other lipids. Steroid hormones like testosterone and estrogen are examples of such molecules. This easy access to cells explains the powerful and wide ranging effects of such hormones. Temperature In general, increases in temperature cause all molecules to move faster. Diffusion is a passive movement of molecules so quicker molecule movement translates into quicker diffusion.

Rate of diffusion increases as: * Diffusion distance decreases * Concentration gradient increases * Surface area increases

Investigation to Discover the Factors Affecting Rates of Diffusion between Agar blocks and Hydrochloric Acid. In this investigation I hope to discover the factors affecting the rate of diffusion between Agar blocks and Hydrochloric acid. I will hope to measure how varying either the concentration of the acid or the temperature affects the rate of diffusion. I will try the experiment with a consistent size of agar blocks, but varying concentration and note down the times it takes for each block to diffuse completely. I will be given a 1cm deep agar plate containing 1% agar. The agar has been made alkaline by the addition of NaOH (Sodium Hydroxide) and stained pink using phenolphthalein. As the HCl diffuses into the agar, the alkali is neutralised and the phenolphthalein becomes colourless. I will time how long it takes for the acid to diffuse into the different size blocks of agar. The equipment I will be able to use in the experiment is listed below: * 1mol/dm3 of Hydrochloric acid * Heating equipment. E.g. Bunsen burner, Water bath. * Thermometers * Straws * Test Tubes * Pipettes * Usual Glassware (Beakers, Boiling tubes, etc.) * Agar Borer There are 3 main factors affecting the rate of diffusion. These are: 1. Temperature 2. Surface Area 3. Concentration Temperature affects the rate of diffusion in the way that heat gives energy to the particles. This causes the particles to move around faster over a larger distance. This results in the particles spreading out over a wider area of space faster than if they were at normal temperature, hence diffusion would be faster. However if you lower the temperature it will have the opposite effect on the particles. The particles will lose energy and will slow down resulting in the cold, slow particles not covering as much space as the hotter, faster particles. This means that the diffusion will take a much longer time. Surface area affects the rate of diffusion in the way that the larger the surface area, the more space there is for the particles to diffuse through, which therefore means a faster rate of diffusion. This means that if you want I wanted the experiment to have a fast rate of diffusion I would somehow have to give the agar block a small mass but a large surface area. If the agar block had a large mass and a large surface area it would still take longer than the small mass and large surface area