Investigate Osmosis using Potato Strips
Diffusion and Osmosis
There are many laboratory experiments to demonstrate diffusion and osmosis. Once you understand the concept of osmosis you will be able to apply your knowledge to understanding the experiments and answering the discussion questions asked by your teacher. Follow the links on this page to access some of the common experimental set-ups for demonstrating osmosis, discussion questions and answers. Find below explanations of some of the jargon/terms used when discussing diffusion and osmosis.
Definition of Diffusion
The net movement of particles of a substance along its concentration gradient (from an area of high concentration to an area of low concentration) until equilibrium is established. Diffusion is a passive process, that is, it does not require the input of energy.
Definition of Osmosis
Osmosis is a form of transport in living organisms. Osmosis is defined a net movement of particles of solvent along its concentration gradient, across a selectively permeable membrane, until equilibrium is established. Osmosis is a passive process – it does not require the input of energy in the form of adenosine triphosphate (ATP). It is worth noting that osmosis is a special type of diffusion.
Concentration gradient refers to the difference in the concentration of solute in a solution. If the difference is great i.e. concentration of Solution A is high and concentration of solution B is low, the concentration gradient is said to be large or steep. And, if the difference is small, then the concentration gradient is said to be small.
Although it is the amount solute particles - in a given volume of solvent - which determine the concentration of solutions, it is the concentration of solvent which is considered in osmosis. Solvent particles are small enough to penetrate the selectively permeable membrane, whereas the solute particles are not. It is important that you understand this concept, in order to predict which direction the solvent will flow. Water is the ‘universal’ solvent and certainly the solvent of choice of living organisms and in your lab work.
Isotonic, Hypotonic & Hypertonic Solutions
Isotonic, hypotonic and hypertonic are terms used to compare the concentration of a solution to the concentration of another (reference) solution. Sometimes the reference solution is the cytoplasm of cells. For the purpose of this discussion, I will call the reference solution ‘Solution A’ and the solution to be compared solution ‘B’. Solutes commonly used in biology osmosis experiments include sodium chloride and glucose.
Iso = same. An isotonic solution has the same concentration of solute as the reference solution, ie. [A]=[B]. Using arbitrary numbers: [A] = 35 g/L and [B] = 35 g/L. In osmosis, there will be no net movement of solvent between the two solutions.
Hypo = low. A hypotonic solution will have a lower concentration of solute than the reference solution i.e. [A] > [B]. Using arbitrary numbers [A] = 35 g/L and [B] = 20 g/L. However, it must be noted that although the hypotonic solution has a lower concentration of solute, its solvent concentration is higher. Therefore solvent particles will move down the concentration gradient i.e. from the area where they are higher (hypotonic solution B) to the area where their concentration is lower (reference solution A). In osmosis, the net movement of water will be from the hypotonic to the reference solution.
Hyper = high. A hypertonic solution will have a higher concentration of solute than the reference solution i.e. [A] >[B]. Again, using our arbitrary numbers, [A] = 35g/L and [B] = 45 g/L. It must be noted, however, that although the hypertonic solution has a higher concentration of solute, its solvent concentration is lower. Therefore solvent particles will move down the concentration gradient from the area where they are higher (reference solution A) to the area where they are lower (hypertonic solution B). In osmosis, the net movement of water will be from the reference solution to the hypotonic solution.
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Diffusion & Osmosis