Introduction to Diffusion, Osmosis, Transport and Active Transport

See also Surface exchange of substances in animal organisms

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

What is diffusion? Why does diffusion happen?

What is osmosis? How does osmosis work?

Why is osmosis so important in plants and animals?

What is active transport? How does active transport work?

Why is active transport needed in plants and animals?

You should appreciate that it is important that dissolved substances must be able to get in and out of a cell through the cell membranes, otherwise the cell could not live or reproduce!



1a. DIFFUSION

Experiments to show diffusion (adapted from my states of matter page)

Particles are always moving at random and this causes them to spread throughout in a container if a gas or spread out throughout a solution if dissolved in a solvent.

This is the process of diffusion and naturally occurs in gases and liquids.

It is this continuous random movement of particles that allows diffusion to take place.

Diffusion is the natural net movement of particles from a higher concentration to a lower concentration.

and

In both experiment you start with a container of a colourless medium (air or water), add a coloured material (gas or soluble solid), make sure the container is sealed to prevent any air disturbance (or gas escaping).

The container is left to stand, preferably at a constant temperature to prevent mixing due to convention. Immediately the coloured particles spread (gases mix, solid dissolves and spreads) due to random natural particle movement, from an area of high concentration to one of low concentration.

The spreading is self-evident and direct experimental evidence for the natural constant random movement of particles (molecules or ions).

After many hours, due to diffusion, the colour is evenly distributed due to the random movement of ALL the particles in the gas or liquid mixture.

As you can see, diffusion readily occurs in liquids or gases and it is faster in gases because of the greater distance between the particles.

Diffusion is almost impossible in solids because of the stronger interparticle bonding forces holding the particles in fixed positions.

 

Another demonstration of diffusion - with a bit of added chemistry!

1. Agar gel cubes are prepared with a little sodium hydroxide solution and a few drops of phenolphthalein indicator added. Phenolphthalein turns pink-red in alkaline solution.

The agar jelly cubes are placed in a beaker of dilute hydrochloric acid.

2. When the beaker is left to stand, the acid will slowly diffuse into the agar jelly cubes and neutralise the alkali ...

hydrochloric acid + sodium hydroxide ===> sodium chloride + water

HCl(aq)  +  NaOH(aq)  ===> NaCl(aq)  +  H2O(l)

... and the agar jelly cubes begin to turn colourless.

3. Gradually, all of the alkali is neutralised, and the whole of the agar gel turns colourless as the acid diffuses right to the centre of the cubes.

Notes:

It is also true to say that the alkali can also diffuse out of the cubes and be neutralised in the acid solution. Either way, it doesn't matter, all the particles are on the move constantly at random in all directions.

At the start, the diffusion gradient for the acid is into the agar gel cubes.

You can do an investigation with different concentrations of acid to change the diffusion gradient and time the results. You need to keep the recipe of the agar jelly cubes constant and conduct the experiments at the same temperature. You should be able to predict the pattern of results!


A particle model and factors affecting the rate of diffusion

A particle model of diffusion in gases and liquids: This picture could represent diffusion of molecules or ions in cell fluids or blood stream or gases in the lungs. Imagine the diffusion gradient from left to right for the green particles added to the blue particles on the left. The blue particles could we water and the green particles could be a sugar, protein or carbon dioxide molecule. So, for the green particles, net migration is from left to right and will continue, in a sealed container, until all the particles are evenly distributed (as pictured). BUT, as in living organism, if the green particles are removed or used in some process on the right, then net migration (net diffusion) would continue until there was not enough green particles to create a diffusion gradient from left to right i.e. become evenly very dilute.

===> ===>
Be able to define diffusion as the movement of particles from an area of high concentration to an area of lower concentration.

You experience the gas diffusion experiment (or the diffusion particle picture above!) if somebody sprays perfume or deodorant into a room (green particles in the diagram above!).

Even without draughts or convection, the odour will eventually enter your nose and be detected by your sense of smell in any area of the room.

Similarly you can smell petrol or diesel fumes throughout garage due to the diffusion of fuel vapour molecules,

You should know that all liquid or dissolved particles have kinetic energy and so in constant random motion in all directions and tend to spread in all directions, BUT, on average, they will tend to migrate from a region of higher concentration to a region of lower concentration.

The two experiments described above illustrate this random spreading, but by the nature of the experiment design you will see initially the spreading on average is upwards because the coloured substance starts off at the bottom of the container where the concentration will be very high.

Note:

(i) The bigger the concentration difference between two adjacent regions, the steeper the diffusion gradient and the faster the rate of diffusion takes in terms of the net transfer of a particular molecule or ions (eg sugar or sodium ions etc.).

(ii) If the system is warmer, at a higher temperature, the particles gain kinetic energy and can on average move faster and so diffusion is faster.


Factors affecting the rate of diffusion and Fick's Law of diffusion

The diffusion situation might be exchange of gases in the lungs or movement of molecules and ions through a cell membrane. Three rate of diffusion factors are described and explained in the context of transferring substances through a membrane.

Factors affecting the rate of diffusion of particles through a membrane - you may be talking about diffusion or osmosis and active transport.

Expressed as how diffusion rate is increased - since that's usually what you want in organisms!

(i) The larger the surface area of the membrane, the greater the rate of diffusion,

(ii) The steeper the concentration gradient, meaning the greater the difference between the highest concentration and the lowest concentration area on either side of membrane, the greater the rate of diffusion.

So, in a given time, more particles will diffuse from the area of highest concentration to the area of lowest concentration

If the concentration was uniform i.e. equal on both sides of the membrane, there would be no net diffusion - no net transfer (ignoring active transport which can operate against a diffusion gradient).

(iii) The shorter the distance the particles have to diffuse - the thinner the membrane.

The shorter the time needed to transfer particles, the greater the rate of diffusion - think of how thin membranes are!

(iv) Factor (iii) can also be expressed as increase in surface/volume ratio also increases the rate of substance transfer.

(v) Rates of transfer of substance will increase with increase in temperature - the particles (molecules, ions etc.) have more kinetic energy and their average speed increases - so particles can move in and out of cells more rapidly.

Fick's Law on the rate of diffusion of particles relating to a membrane

Fick's Law expresses the three diffusion factors (i) to (iii) described above in a ,proportional' mathematical formula:

Rate of diffusion (surface area x concentration difference) (thickness of membrane)

Rate factor (i) x factor (ii) factor (iii)  !!!

(i) A bigger surface area of membrane - bigger rate of diffusion.

If you can double or triple the surface area in an organ, you can double or triple the rate of diffusion = rate of transfer of substances.

This assumes a constant diffusion gradient and thickness of membrane.

(ii) A bigger concentration difference - bigger rate of diffusion

For a given membrane of fixed surface area and thickness, the bigger the difference in concentration between the two sides of the membrane, the steeper the diffusion gradient.

Suppose in terms of concentrations on either side of a membrane

(a) the concentrations were 0.05 mol/dm3 and 0.10 mol/dm3

(b) the concentrations were 0.025 mol/dm3 and 0.15 mol/dm3

concentration differences:

(a) 0.10 - 0.05 = 0.05;  (b) 0.15 - 0.025 = 0.125

For a given membrane (constant surface area and thickness) the ratio of the rates of diffusion will be 0.125/0.05 = 2.5

In other words the rate of diffusion in situation (b) is 2.5 times faster than situation (a).

(iii) A thinner membrane - bigger rate of diffusion - less time needed for transfer.

If you can halve the thickness of a membrane you can double the rate of diffusion through it.

This assumes a constant surface area and diffusion gradient.

 

Examples of diffusion in living organisms are described in detail on

See also Surface exchange of substances in animal organisms

 


1b. The action of cell membranes - selective diffusion

Although a cell membrane holds the cell together it lets substances in and out, but these substances must be dissolved in water in order pass to and fro through the cell membrane by diffusion.

You can think of partially permeable membrane (semi-permeable) as having tiny molecular sized holes in it, that only allow certain small, but NOT large, particles through.

However, only small molecules and ions can diffuse through the cell membrane e.g. glucose and oxygen for respiration, waste carbon dioxide from respiration, amino acids for protein synthesis and of course water itself, as well as being the solvent.

BUT big molecules cannot get through the cell membrane e.g. starch and proteins.

In the particle model of a cell membrane on the right, the thick black dotted line represents the membrane.

Think of the grey circles as the larger molecules like proteins or starch which cannot pass from left to right through the cell membrane.

Imagine the blue circles are water - they can pass through the membrane in any direction.

Imagine the green circles are small molecules or ions - they can also pass through the cell membrane in either direction, but the concentration is greater on the left than the right.

Therefore the diffusion gradient is from left to right and there is a net movement of the green particles (smaller molecules) from the left higher concentration to the right lower concentration passing through the cell membrane in the process.

Also bare in mind that the larger the surface area of a membrane, the faster the net rate of diffusion of a particular molecule or ion.


Examples of diffusion in living organisms

The process of respiration.

The thin cell membranes allow the diffusion of small molecules in and out of cells.

Since the capillaries are thin and numerous, the diffusion distance from cells is short, so transfer of nutrients in, and waste products out, is as efficient as possible.

As the cells respire they use up oxygen/glucose, so their concentration falls in the cell. Therefore the external concentrations (e.g. in capillaries) is higher, so more oxygen/glucose will diffuse into the cell.

At the same time, the concentration of the waste product carbon dioxide builds in the cell, and so carbon dioxide will then naturally diffuse out of the cell to the lower concentration region in the capillaries.

For more details on gas exchange see Surface exchange of substances in animal organisms

 


2a. OSMOSIS

Know and understand that water often moves across boundaries by osmosis - a special case of particle (water) diffusion down a concentration gradient.

Know that osmosis is the diffusion or bulk movement of water from a dilute to a more concentrated solution through a partially permeable membrane (semi-permeable membrane) that allows the passage of very small molecules like water (diagram on right).

A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through e.g. even relatively small molecules like sucrose will not pass through a partially permeable membrane (see experiment in section 2b. below).

So, in living organisms, only water gets through and depending on the relative concentration of dissolved substances either side of the membrane, osmosis can happen in either direction - meaning water can diffuse through the membrane in either direction.

Although the water molecules (and any other particles) are moving around at random, there will be a net transfer of water in one direction at a time through a partially permeable membrane.

The net direction of diffusion of water is from a less concentrated solute solution (more water molecules) to a more concentrated solute solution (less water molecules) i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane.

Solutes include sugars, oxygen and carbon dioxide gases, amino acids, proteins etc.

Therefore a more concentrated solution becomes more dilute in the process.

This osmosis diffusion can occur in either direction depending on the relative concentration of the solutes in the cell fluids or tissue fluids and concentrated solutions e.g. of sugars, will tend become diluted by water passing through the partially permeable membrane.

The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

(a) If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established.

The term water potential is used to describe these situations.

You can talk about the water potential gradient across a partially permeable membrane.

Water will diffuse from a high water potential to a low water potential.

In situation (a), the fluid outside the cells has a high water potential than the solute solution in the cells, so water moves into the cells.

(b) Conversely, if the cell solution is too dilute, then water will diffuse out by osmotic action across the semi-permeable membrane of the cell wall.

In situation (b), the solution inside the cells has a high water potential than the fluid outside the cells, so water moves out of the cells.

 

Note on plant cells and water potential

(i) When you water a plant it increases the water potential of the soil around it.

Therefore the plant cells will draw water in by osmosis until they become turgid - fatter and swollen.

The cell fluids (contents of the cell) will push against the cell wall, known as turgor pressure, and this helps support the plant tissues (therefore the plant as a whole).

(ii) If the soil is very dry, lacking in water, the plant starts to wilt and the water potential of the plant is greater than the surrounding soil.

The result is the plant cells become flaccid and begin to lose water.

The plant doesn't droop (flop) completely and retains much of its shape because the strong cellulose cell wall is relatively inelastic and helps the plant retain its shape.

Note on animal cells and water potential

In the case of animal cells, they do not have strong walls and can respond adversely to change in the ambient water pressure.

If animal cells are surrounded by a solution of greater water potential (less concentrated in solutes), they can absorb so much water by osmosis that they burst - which kills the cells. In extreme cases you can die of over-hydration, but its a complicated effect that reduces the level of salt (= sodium ions) in the blood to dangerous levels.

Osmosis is important for the function of many animal organs

e.g. water is absorbed into the bloodstream from the large intestine to form faeces in the appropriate physical state!

See also Homeostasis - osmoregulation - ADH - water control  gcse biology revision notes

and Transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure

 


2b. A simple demonstration of osmosis

Set-up and method

You can do a simple experiment to demonstrate osmosis by placing blocks or cylinders of potato into pure water and then a series of sugar solutions (e.g. glucose/sucrose) of increasing in concentration (increasingly higher molarity e.g. from 0.0 to 1.0 mol/dm3) i.e. from pure water, and a dilute to a very concentrated sugar solution.

The dependent variable is the potato mass and the independent variable is the concentration of the sugar solution.

The different concentrations of sugar represent different water potentials - the more concentrated, the lower the water potential.

All the other variables should be kept constant - so make sure the original potato blocks are identical in size and mass, same temperature, same time left to change, same sugar and same volume of liquid - all about a 'fair test'.

You measure and record the initial mass of the potato blocks and place them individually in pure water and the range of sugar solutions in a series of beakers.

Leave the beakers for as longer times as possible - best for class if it can be done in a lesson.

Carefully remove the potato blocks from the liquid, dry them with a paper towel and re-weigh them.

Different pupil groups can use one particular concentration and use 3 lots of potato and submit an average to the whole class results.

Results

From the weighings work out the mass gain/loss from each potato block.

You can convert the weighing into % mass gain/loss.

% change in mass = 100 x (final mass - initial mass) / initial mass

You can plot a graph of mass gain/loss (g or %) versus the sugar concentration (mol/dm3), and the graph might not be quite what you expect!

Osmosis is taking place with water and most of the sugar solutions - but not always in the same direction!

Typical results from an osmotic experiment using potato tubes or blocks.

Initially the potato blocks gain mass, then there is little change in mass and then the blocks lose mass.

Explanation

(i) Initially the concentration of the sugar in the water is less than the solute concentration in the potato cells of starch.

You can say the water concentration in the external sugar solution is greater than that in the potato cells - pure water has a greater water potential than the cell fluids.

Therefore when osmosis takes place with pure water, or very dilute solutions of sugar, the potato cells absorb water by osmosis giving a percentage mass increase.

The water will diffuse through the partially permeable membranes of the potato cells to try and dilute the internal solute solution of the potato cells - osmosis is happening.

(ii) At a 'medium' concentration of sugar, its concentration matches the concentration of the solutes in the potato cells and there is no net osmosis.

You can say the water concentration in the external sugar solution is the same as in the potato cell - no osmosis - both the potato cells and sugar solution have the same water potential.

When there is no change in mass, the two solutions have the same water concentration and the solutions are said to be isotonic - identical water potentials.

(iii) At higher external sugar concentrations, the osmotic effect reverses direction and water will diffuse through the partially permeable membranes out of the potato cells to try and dilute the external more concentrated sugar solution.

You can say the water concentration in the external sugar solution is less than that in the potato cell, so the water diffusion gradient is out of the potato cells giving a net mass loss. In other words, the water potential of the potato cell fluids is greater than the external sugar solution water potential.

Ultimately the greater the concentration, the greater the osmotic effect - a higher concentration of sugar will draw out more water - a greater rate of diffusion and osmosis - the more concentrated the sugar solution, the greater the mass loss of the potato block.

Sources of error

Obviously, all experiment can be repeated e.g. groups of students in the same class, this reduces errors - some groups of pupils might be more careful than others. You can then use mean values in your numerical analysis.

If the potato blocks are not completely dried, your will record a smaller mass of water loss than actually happened.

Especially if the room is warm, evaporation would increase the concentration of the sugar solution, this would increase the mass loss. This error can be eliminated by putting 'lids' over the beaker - paper covers held with an elastic bands will do.

Other similar experiment

You can repeat the experiments using common salt (sodium chloride, NaCl) and you should get a similar pattern of results.

 

Drinks and hydration

Most soft drinks contain water, sugar and ions.

Sports drinks contain sugars to replace the sugar used in energy release during the activity.

They also contain water and ions to replace the water and ions lost during sweating.

The sports drinks are supposed to be isotonic.

Know and understand that if water and ions are not replaced, the ion / water balance of the body is disturbed and the cells do not work as efficiently.

 


3. ACTIVE TRANSPORT

See also Examples of surfaces for the exchange of substances in animal organisms

What is 'active transport'

Active transport is the movement of particles across a membrane against a concentration gradient.

So, sometimes, substances are absorbed by cells against a concentration gradient - a net transfer against the normal diffusion gradient action described above in section 1.

This means transfer occurs in the opposite direction to the natural direction of the diffusion gradient.

e.g. active transport enables cells to absorb ions from very dilute solutions.

BUT, this movement of chemicals across a cell membrane against a natural diffusion gradient, requires the use of energy from respiration and the overall process is called active transport.

Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration

 

Examples of active transport

The gut and digestion

The diagram illustrates the movement of molecules (green spheres) being moved through the membrane of the gut from the gut into the bloodstream, in the opposite direction to the natural diffusion gradient.

(The blue circles represent water molecules - solvent medium.)

The red circles represent the relatively large red blood cells, which are too large to get through the membrane, so staying in the bloodstream, to be joined by nutrient molecules (green circles) and ions via active transport.

Active transport is required to absorb nutrients (green circles) like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

If the concentrations of nutrients (e.g. sugars, amino acids) in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

If the concentration gradient flow is in the direction of the blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

So active transport enables the gut to move nutrients like into the blood even though the natural concentration gradient (diffusion gradient) is the wrong way round.

Glucose can be transferred into the blood stream, even if its concentration is higher in the blood stream, and so conveyed to cells for respiration.

For more on the gut see Surfaces for the exchange of substances in animal organisms

 

Plants

Active transport is used in the absorption of nitrates and other ions by plant roots.

For details see Transport and gas exchange in plants, transpiration, absorption of nutrients etc.

 

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Typical learning objectives  Diffusion, Osmosis, Transport and Active Transport

  • Appreciate that we need to understand how biological and environmental systems operate when they are working well in order to be able to intervene when things go wrong.

  • Appreciate that modern developments in biomedical and technological research allow us to do so.

  • Know and understand that the cells, tissues and organs in plants and animals are adapted to take up and get rid of dissolved substances.

  • Know that different conditions can affect the rate of transfer.

  • Sometimes energy is needed for transfer to take place - active transport.

  • You should be able to use your skills, knowledge and understanding to:

    • evaluate the development and use of artificial aids to breathing, including the use of artificial ventilators,

    • evaluate the claims of manufacturers about sports drinks,

    • analyse and evaluate the conditions that affect water loss in plants.

  • Know and understand that differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

    • The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

    • The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

    • If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established. Conversely, if the cell solution is too dilute, then water will diffuse out from osmotic action across the semi-permeable membrane of the cell wall.

  • Know and understand that substances are sometimes absorbed against a concentration gradient.

    • This means transfer occurs in the opposite direction to the natural direction of diffusion and osmosis.

    • Know that this requires the use of energy from respiration and this process is called active transport.

    • Know that active transport enables cells to absorb ions from very dilute solutions.

    • Active transport is required to absorb nutrients like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

    • If the concentrations of nutrients in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

    • If the concentration gradient flow is in the direction of blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

    • Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration


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