Introduction to Diffusion, Osmosis, Transport and Active Transport

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

UNDER DEVELOPMENT

For plants see Transport in plants notes

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!

 

 

understand that osmosis (a specific case of diffusion) is the overall movement of water from a dilute to a more concentrated solution through a partially permeable membrane

know that the movement of water into plant roots occurs by osmosis

HT only: understand that active transport is the overall movement of chemicals across a cell membrane requiring energy from respiration

HT only: know that active transport is used in the absorption of nitrates by plant roots



1. DIFFUSION

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

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) 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 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.

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!). 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.

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.

The bigger the concentration difference between two adjacent regions, the steeper the diffusion gradient and the faster the diffusion takes place in terms of the net transfer of the molecules or ions involved (eg sugar or sodium ions etc.).

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

The action of cell membranes

Although cell membranes hold the cell together they let substances in and out and these dissolved substances pass to and fro through the cell membrane by diffusion.

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

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

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.

??

 


2. OSMOSIS

a) Know and understand that dissolved substances move by diffusion and by active transport.

b) Know and understand that water often moves across boundaries by osmosis - a specific case of particle diffusion.

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 water molecules.

A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through.

Other molecules eg even relatively small molecules like sugars or amino acids and any larger molecules like proteins or glycogen, will not pass through a semi-permeable membrane.

So, in living organisms, only water gets through and depending on the concentration of dissolved substance, osmosis can happen in either direction across a partially permeable membrane.

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 water will diffuse from a less concentrated solution to a more concentrated solution i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane

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.

You can do simple experiments to demonstrate this by placing blocks or cylinders of potato into pure water and a series of sugar solutions (e.g. glucose) increasing in concentration (increasingly higher molarity mol/dm3).

The potato 'chips' will shrink and lose mass the more concentrated the sugar solution because the water will undergo osmosis and diffuse through the partially permeable membranes of the potato cells to try and dilute the sugar solution.

c) 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.

d) Know that most soft drinks contain water, sugar and ions.

e) Know and understand that 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.

f) 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.

 Be able to define osmosis as the movement of water molecules from an area of higher concentration of water to an area of lower concentration of water through a partially permeable membrane.

Appreciate that dissolved substances move by diffusion and by active transport.

Appreciate that water often moves across boundaries by osmosis - a specific case of particle diffusion.

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 water molecules.

A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through.

Other molecules eg even relatively small molecules like sugars or amino acids and any larger molecules like proteins or glycogen, will not pass through a semi-permeable membrane.

So, in living organisms, only water gets through and depending on the concentration of dissolved substance, osmosis can happen in either direction across a partially permeable membrane.

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 water will diffuse from a less concentrated solution to a more concentrated solution i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane

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.

You can do simple experiments to demonstrate this by placing blocks or cylinders of potato into pure water and a series of sugar solutions (e.g. glucose) increasing in concentration (increasingly higher molarity mol/dm3).

The potato 'chips' will shrink and lose mass the more concentrated the sugar solution because the water will undergo osmosis and diffuse through the partially permeable membranes of the cell walls.

 


3. ACTIVE TRANSPORT

g) 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

h) Know and understand that many organ systems are specialised for exchanging materials.

Know that the effectiveness of an exchange surface is increased by:

having a large surface area,

being thin, to provide a short diffusion path,

(in animals) having an efficient blood supply,

(in animals, for gaseous exchange) being ventilated.

i) Know that gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness.

It is essential that the transfer processes of moving sugars, amino acids, oxygen etc. into cells and the removal of waste products, can happen as efficiently as possible.

Therefore exchange surfaces have evolved to maximise the rate of transfer of wanted and unwanted chemicals.

To increase the probability of exchange the exchange surface needs to be ...

a large surface area to increase diffusion rate eg alveoli in lungs, villi in intestine

a thin layer so diffusion times are short - cell membranes are usually quite thin,

lots of thin blood vessels to bring in essential molecules for life and carry waste molecules away eg the thin bronchiole tubes in the lungs,

j) Know that the size and complexity of an organism increases the difficulty of exchanging materials.

One reason for this increased difficulty in exchanging materials is that the distance from the exchange surface is getting further away from where the nutrients and oxygen are needed and the waste to be removed.

k) Know and understand that in humans:

the surface area of the lungs is increased by the alveoli - tiny sacs of the end of the tiny bronchiole tubes in the lungs,

the surface area of the small intestine is increased by villi.

l) Know and understand that the villi in the small intestine provide a large surface area with an extensive network of thin blood capillaries to absorb the products of digestion by diffusion and active transport.

The tissue lining in the small intestine is covered with millions of protuberances called villi, which poke up from the intestine surface into the partially or wholly digested food /mush'.

This considerably speeds up the absorption process because the villi massively increase the effective digested food absorbing surface area of the small intestine.

Each villus (of the millions of villi) has single layer of surface cells and each villus contains a multitude of fine blood capillaries into which the small digested food molecules can rapidly diffuse into, a good blood supply is needed to efficiently carry the digested food away to where they are needed.

 

 

 

 

 

 

 

TOP OF PAGE


IGCSE revision notes diffusion osmosis active transport KS4 biology Science notes on diffusion osmosis active transport GCSE biology guide notes on diffusion osmosis active transport for schools colleges academies science course tutors images pictures diagrams for diffusion osmosis active transport science revision notes on diffusion osmosis active transport for revising biology modules biology topics notes to help on understanding of diffusion osmosis active transport university courses in biological science careers in science biology jobs in the pharmaceutical industry biological laboratory assistant apprenticeships technical internships in biology USA US grade 8 grade 9 grade10 AQA biology science GCSE notes on diffusion osmosis active transport Edexcel biology science notes on diffusion osmosis active transport for OCR 21st century biology science OCR GCSE Gateway  biology science notes WJEC gcse science CCEA/CEA gcse science

KS3 SCIENCE QUIZZES ALPHABETICAL INDEX
GCSE grade 9-1 & IGCSE CHEMISTRY Doc Brown's Travel Pictures & Notes
ADVANCED LEVEL CHEMISTRY [SEARCH BOX] - see below
GCSE 9-1 Physics Revision Notes GCSE 9-1 Biology Revision Notes
All website content Dr Phil Brown 2000 onwards. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial. Email doc b: chem55555@hotmail.com

 Doc Brown's Biology

*

 For latest updates see https://twitter.com/docbrownchem

 Have your say about doc b's website

TOP OF PAGE