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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


Diffusion and osmosis are examples of passive transport whereby ions or molecules driven by thermal motion move down concentration gradients set up between solutions separated by biological membranes in living systems.

Diffusion occurs in solutions consisting of particles

Air and drinking water are both examples of solutions consisting of mixtures of different types of particles. In air these particles are mainly molecules of nitrogen, oxygen, and carbon dioxide; whereas, drinking water consists of molecules of water and some dissolved ions

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Diffusion results from the movement of particles in liquids and gases.

Regardless of their type, particles in liquids and gases are in constant motion driven by energy which they absorb from the heat of their surroundings. This movement, called thermal motion, speeds up as the temperature of their surroundings warms up.
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Thermal motion is random

The molecules or ions of liquids and gases move in straight lines until disturbed, changing direction upon impact with other moving particles or with hard surfaces. Overall the movement of these particles is random, each moving in a different direction.

Figure 1

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.

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Random motion drives diffusion

Two consequences of continuous random molecule movement are important to an understanding of diffusion and osmosis: (a) molecules tend to move away (disperse) from centres of concentration (b) molecules of different types tend to intermix.

Example One:

Evaporation of perfume in a classroom demonstrates diffusion

Consider the following example as a demonstration of dispersion and intermixing of molecules. A drop of perfume is placed on a desk top at the front of a classroom. Being volatile the perfume quickly begins to evaporate creating a high concentration of perfume molecules above the drop.

Figure 2.1

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.

Figure 2.2

Perfume Molecules Spreading Out From Drop on the Front Desk

Diffusion causes molecules to disperse from a centre of concentration

Further spreading of molecules carries the perfume molecules to the middle of the classroom and finally to the back of the room. See Figure 2.3 below.


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.

Solutions are homogeneous one-phase mixtures

Mixtures such as this mixture of air and perfume molecules which have an even distribution of molecules are called homogeneous. Any homogeneous one-phase mixture is called a solution; therefore, the mixture of perfume and air is actually a solution even though most of us think of solutions as just being homogeneous mixtures of a liquid and a solid (such as salt dissolved in water). All solutions, because they are homogeneous, look like they are one-phase; that is, the individual components (e.g., salt and water) are no longer distinguishable as separate entities.

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Example Two:

Salt dissolving in water demonstrates that the components of solutions can behave as separate entities

Consider the following example as an illustration of this principle. Imagine an aquarium full of pure water with a tablespoon of salt crystals piled on the bottom. (N.B. non- iodized table salt consists of molecules of sodium chloride [NaCl] organized into a crystal form).

Figure 3.1

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.

Figure 3.2

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.

Figure 3.5

Completion of Osmosis and Diffusion to Produce a Homogeneous Solution of NaCl in Water

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Example Three:

Diffusion will occur through an artificial membrane if the membrane is permeable to the solute or the solvent and seperates two solutions of different concentration.

Since cells are surrounded by a plasma membrane, most biological systems involve the diffusion of ions and/or water molecules across cell membranes (in other words into and out of cells). Before we consider diffusion involving cell membranes, let's consider an example involving an artificial membrane. An ordinary cellophane food wrap can serve as an artificial membrane: it is a thin barrier perforated with tiny holes which allow ions and small molecules to pass through. A membrane which allows molecules to pass through it is said to be permeable (to those molecules)

Suppose an aquarium is divided into two chambers by an artificial membrane. One chamber (side A) is filled with pure water; the other chamber (side B) is filled with a 5% solution of sodium chloride.

Figure 4.1

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.

Calculation of solute concentration

For the solute (salt): On side A, the solute concentration is 0%, on side B the solute concentration is 5% percentage by weight; therefore the solute diffuses from the higher concentration (side B) to the lower concentration (side A).

Figure 4.2

Solute Diffuses From Side B to Side A

For the solvent (water): On side A, the solution consists solely of solvent (100%), on side B the solvent 95% percentage by weight; therefore the solvent moves from the higher concentration (side A) to the lower concentration (side B).

Figure 4.3

Water Diffuses From Side A to Side B

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Osmosis is a special case of diffusion

Diffusion of water across a membrane (as in the last example) is called osmosis. Note that in the process of osmosis water diffuses into a solution having a greater solute concentration; but, in fact osmosis is just a special case of diffusion because water also diffuses from an area of greater water concentration into an area of lesser water concentration.

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.

Example Four:

Diffusion through an artificial permeable membrane will occur when the membrane separates two same-solute solutions of different concentration

It should be obvious by now that diffusion (and osmosis) is controlled by membrane permeability and the presence of concentration gradients. Let's consider another example just from the point of view of concentration gradients. Again imagine the divided aquarium setup, but this time there is a 5% NaCl solution on side A and a 10% NaCl solution on side B. The artificial membrane is permeable to both ions (Na+ and Cl-) and water molecules. When two solutions of differing concentrations (as in this example) are separated by a membrane they are referred to as a concentration gradient. The terms hypotonic and hypertonic are used to describe these two solutions: the solution having the greater total (solute) concentration is referred to as being hypertonic (side B in our example); and the solution having the lesser total (solute) concentration is referred to as being hypotonic.

Figure 5.1

As diffusion proceeds, the solute (Na+ and Cl- ions) will diffuse from site B to side A, i.e., the direction of solute diffusion is from hypertonic to hypotonic; similarity, the solvent (water) will diffuse from side A to side B. i.e., the direction of solvent (water) diffusion (i.e., osmosis) is from hypotonic to hypertonic.


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.

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Example Five:

Selectively permeable membranes allow some particles to pass through but not others

This example considers what happens when the membrane is permeable to the solvent (water) but not to the solute (in this case large protein molecules). Membranes of this type are called selectively permeable membranes. Cellophane food wrap serves this purpose because it is permeable to the smaller water molecules but impermeable to the larger protein molecules. Again consider the divided aquarium set up (shown below) but this time with a 5% protein solution on side A and a 10% protein solution on side B.

Figure 6.0

Cellophane is an Artificial Membrane Which is Permiable to Water Molecules But Impermiable to Protein Molecules

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Osmosis again occurs because a concentration gradient exists and the membrane is permeable to water molecules; however, diffusion of solute does not occur even though a concentration exists because the membrane is impermeable to the large protein molecule.

Figure 6.1

Diffusion of H2O Molecules From Pure Water into a Hypertonic Solution

Other factors may influence the rate or occurrence of diffusion

If osmosis was the only factor at play here, eventually sufficient water from side A would diffuse into side B to equalize the concentrations on each side. The result however, where side B has a much greater volume than side A would not happen. As the volume of side B rises above side A gravity would oppose the diffusion of water from side A; hence, diffusion would stop and the concentration of sides A and B would never become equal.

Figure 6.2

The Effect of Gravity on the Occurrence of Diffusion

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Concentration of a solution is the amount of solute per unit volume of solvent. Since water is the solvent for most biological solutions, the solution is specified by the concentration of solute

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 Membrane is 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.

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