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"Principles of Diffusion and Osmosis" is an .interactive tutorial written and designed by Ian Emerson, Associate Professor, Department of Biology; graphics and text programmed by Dean William Barnes
Each molecules moves independently in a different direction. The probability of molecules moving in one direction would be the same as the probability of moving in the opposite direction. That is, there is no net movement in any one direction.
Perfume evaporating on front desk in classroom.
Since the perfume molecules are in constant random motion they bump into each other causing some molecules to be occasionally sent hurtling out of the mass. Gradually the molecules spread out from the drop on the front desk and are sensed by students in the front of the classroom. See figure 2.2 below.
Perfume Molecules Spreading Out From Drop on the Front Desk
Perfume Molecules Spreading Out in Classroom
Eventually the perfume molecules are evenly intermixed with the nitrogen, oxygen and carbon dioxide molecules of the air such that one litre of air from the front of the room would contain the same number of perfume molecules as a litre of air from the back of the room.
Pile of Salt Crystals in a Tank of Water
In water, sodium chloride molecules quickly dissociate into charged atoms called ions and become dissolved in the water.
Above the pile of crystals, a dense concentration of ions (Na+ and Cl-) begins to form. The further away from the pile the fewer the number of ions of Na+ and Cl- that exist thus producing a decreasing concentration gradient.
Na+ and Cl- Dissociating and Producing a High Concentration Above Pile of Salt Crystals
Diffusion now acts to further spread out the Na+ and Cl- ions from the centre of concentration and to intermix the Na+ and Cl- ions with the H2O molecules. Note also that the water molecules are also diffusing from an area of higher concentration of water (away from the pile of salt crystals) to an area of lower concentration of water (near the pile of salt crystals). Hence the ions (Na+ and Cl-) and the water molecules are behaving as separate entities: both are moving down their individual concentration gradients but in this case in opposite directions (salt away from the crystals, water toward the crystals).
Diffusion of Sodium and Choride Ions Away From Pile of Crystals and Osmosis of Water Toward Pile of Salt Crystals
Eventually when all the salt has dissolved in the water and diffusion has evened out the concentration gradients a homogeneous solution of NaCl in water will exist. The liquid part of the solution (in this case water) is referred to as the solvent and the NaCl is the solute.
Completion of Osmosis and Diffusion to Produce a Homogeneous Solution of NaCl in Water
Click on Fish for an interactive question
Aquarium Divided By an Artificaial Membrane Separating Pure Water (Side A) From a 5% Solution of Sodium Chloride (Side B)
Since the artificial membrane, in our example, is permeable to both ions (Na+ and C1-) and to the water molecule, diffusion occurs in both directions across the membrane: the Na+ and Cl- ions diffuse from their area of higher concentration (side B) to their area of lower concentration (side A); whereas, the water diffuses from its area of higher concentration (side A) to its area of lower water concentration (side B). To convince yourself that each particle type is moving down its own gradient calculate the concentration of each of the solute and solvent components on each side of the membrane.
In this last example note that diffusion stopped when the concentration on either side of the membrane became equal (in other words, the concentration gradient no longer existed). The continuous random movement of molecules continued (i.e., solute and solvent particles moved back and forth) but no net movement of molecules in either direction occurred.
Diffusion proceeds until the concentration gradient no longer exists. At this point the two sides will be equal in concentration and are said to be isotonic. (Isotonic refers to two solutions having equal concentration [of a particular solute]). In this example for every 100 g of slution, 2.5 g of NaCl diffuse from side B to side A and 2.5 g of water diffuse from side A to side B.
Cellophane is an Artificial Membrane Which is Permiable to Water Molecules But Impermiable to Protein Molecules
Diffusion of H2O Molecules From Pure Water into a Hypertonic Solution
The Effect of Gravity on the Occurrence of Diffusion
Percentage by weight is the a method of expressing concentration of a solution. It is calculated as the number of grams of solute per 100 grams of solvent multiplied by 100.
Diffusion is the movement of particles (ions or molecules) from an area of higher concentration to an area of lower concentration.
Osmosis is the diffusion of water from a hypotonic solution into a hypertonic solution across and selectively permeable membrane.
Passive Transport is the movement of ions or molecules across a membrane without the expenditure of energy. Diffusion and osmosis are examples of passive transport.
Ions are charged atoms or groups of atoms, i.e. they have a positive or negative charge. When salts dissolve in water they dissociate in ions: e.g.
SALT Dissociation IONS
Molecules consist of two or more atoms joined together by chemical bonds. Overall, molecules are neutral, i.e. do not have a net positive or negative charge ( as do ions ), e.g. H2O is the formula for a water molecule.
Thermal Motion is the constant random movement of particles in a liquid or gas resulting from the absorption of heat from the particles' surroundings. The more heat absorbed the faster the speed of the particle.
Concentration Gradients A concentration gradient exists when two or more solutions of differing concentrations are in close proximity ( for example: two solutions of differing concentrations separated by a membrane).
Biological Membranes are membranes produced by living organisms: for example, the plasma membrane (= cell membrane) and the membranes which surround organelles.
Living Systems are found within living organisms and consist of organic molecules and carry out energy converting chemical reactions.
Homogeneous refers to an even distribution (or uniform mixture). Solutions are homogenous because their components ( solute and solvent ) are evenly intermixed. Two samples of equal volumes from a solution would contain exactly the same numbers of particles of solute and solvent.
Hypotonic refers to the solution in a concentration gradient having the lesser concentration ( of solute ). [ A hypotonic solution would have a greater concentration of solvent but a lesser concentration of solute than a hypertonic one. ]
Hypertonic refers to the solution in a concentration gradient having the greater concentration ( of solute ). [ A hypertonic solution would have a lesser concentration of solvent but a greater concentration of solute than a hypotonic one. ]
Isotonic means having equal concentration.
Plasma Membraneis the outer membrane of the cell. Plasma membrane and cell membrane are synonymous.
Random Movement describes the movement of molecules in a liquid or gas meaning that each molecule moves independently of the other molecules and in its own individual direction. Continuous random movement is sometimes called Brownian Movement or Thermal Motion.
Selectively Permeable describes membranes. A membrane which is selectively permeable allows certain molecules to pass through but not others. (It is equivalent to the less appropriate term, semipermeable which means half permeable.)
Solute is one component of a solution (the other is the solvent).The solute is the smaller of the two in quantity. We generally think of the solute as being dissolved in the solvent, e.g., in a salt water solution the salt is the solute and the solvent is water.
Solute Concentration refers to the amount of solute per unit volume of solvent (usually Litres - solute concentration is the same as the concentration of the solution.).
The expression "5% solution" refers to a solution in which the solute represents 5% of the solution by weight and water represents the other 95% by weight, i.e., 100 g of solution of sodium chloride contains 95 g of water and 5 g of NaCl. Note that solutions are named after the solute and that water, the "universal solvent" is assumed to be the solvent unless stated otherwise. e.g. a 5% solution (by weight ) would contain 5 g of solute for every 95 g of solvent.
Solution(s) is a one phase homogeneous mixture of two (or more) forms of matter. The components of solutions are called solute and solvent. In living systems the most common types of solutions are a solid dissolved in water (such as salt water) or a gas dissolved in water (for example oxygen dissolved in water).
Solvent is the component in which something is dissolved eg. the water in a salt water solution. The solute is what is dissolved in the solvent eg. the salt in a salt water solution. Most solvents in biological systems are water.
Solvent Concentration refers to the concentration of solvent (usually water) in a solution. The most simplest way to understand this is view it as the number of moles of solvent per unit volume the whole solution actually occupies.