www.BioTopics.co.uk
Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

HOW SUBSTANCES GET INTO
(AND OUT OF) CELLS

Knowledge about diffusion is the key to understanding how the lungs and blood system work, as well as many forms of transport, especially in lower animals and plants.

Background information about the physics and chemistry of diffusion in fluids


The term fluids comprises gases (including vapours), and liquids (including solutions) which consist of particles in constant motion, but gases' (and vapours') particles move faster.
Liquids have a fixed volume, but the others can expand into the maximum space available. In biological applications, these particles are usually either molecules or ions.

The random 3-dimensional movement of these moving particles in a fluid explains diffusion - a gradual spreading or migration of particles, intermingling as far as possible with their surroundings. Diffusion is not involved in mixing due to currents caused by pressure differences, or convection, e.g. draughts, winds, pumped liquids, etc.

Illustration: Solute and water movement in a (previously) divided container
In diffusion, we always think that particles will move from high to low concentration, i.e. down along a concentration gradient. If there is no such difference, no net movement occurs, but we say a dynamic equilibrium exists.

SPEED OF DIFFUSION

Diffusion is faster in gases than in liquids, and at higher temperatures, because heat speeds up the process by making particles move faster.

Look at this picture of a cell alongside a blood vessel.

food and oxygen supply to & carbon dioxide leaving cell

In what form will "food" and oxygen be supplied to the cell?
> in solution/dissolved

Will "food" and oxygen really be coming from different directions?
>no

Will oxygen and carbon dioxide really be coming from different directions?
>yes - in relation to blood flow

What makes these substances move in the directions they do?

> concentration gradient

PERMEABILITY OF MEMBRANES

Most natural (and some Man-made) membranes can be thought of as having extremely tiny pores or holes of various sizes; such partially permeable membranes allow some substances to pass but not others, depending on the relative particle sizes.

This effect is responsible for 2 phenomena which Biologists (including examiners) think are important: OSMOSIS and DIALYSIS.

THE PROCESS OF OSMOSIS

Osmosis is a special form of diffusion:
the MOVEMENT OF WATER
from a dilute solution to a more concentrated one
through a PARTIALLY PERMEABLE MEMBRANE.

This type of membrane (also called semi-permeable) allows only water, but not other (dissolved) substances to pass through.

OSMOSIS animation OSMOSIS diagram

All cells are surrounded by a cell membrane, which is partially permeable, and the cytoplasm contains dissolved substances (solutes), e.g. sugars, salts, proteins. Whether or not water enters or leaves the cell depends on the relative concentrations of the cytoplasm and the fluid surrounding them.

It is important to remember that a concentrated (strong) solution has a lower concentration of water than a dilute (weak) one, or water itself.

cell in water- water enterscell in concentrated solution - water leaves
Move the mouse pointer over the boxes above to show the movement of water in the case of these two cells placed into two different liquids.
The movement of water molecules can also set up a pressure, called osmotic pressure, which depends on the difference between the concentrations on either side of the membrane.

DEMONSTRATION OF OSMOSIS USING VISKING TUBING


The Visking Osmometer "Visking" tubing is a form of processed cellulose or cellophane which has pores in it through which water (and other small molecules) can pass, so it can be considered as a PARTIALLY PERMEABLE MEMBRANE. If it is sealed at one end, attached to a glass tube, and filled with a liquid such as sugar solution, and immersed in another liquid such as water, then water should pass through the visking tubing and cause the level of liquid to rise inside the glass tube.
Note the changes which occur during the lesson and the next day.
Then try to answer the questions beneath.

2 Visking Osmometers osmometer animation
What has caused the change in levels within the visking osmometers?
(i.e. what movement? - answer is not just "osmosis"!)

> movement of water NOT SOLUTION

What is the difference between the results from the levels inside the two setups?
Container 1) >liquid level rose Container 2) >liquid level fell

What do you think has happened to the volume of liquids in the surrounding containers?
Container 1) >liquid level fellContainer 2) >liquid level rose
Apart from liquid movement, what else does the change in liquid levels indicate?
> buildup of pressure

click to see photos of osmometers

DEMONSTRATION OF OSMOSIS IN POTATO TISSUE

Return to see and record the results of this demonstration at the end of the lesson, or next day. Note only the presence or absence of liquid.
Do not worry about apparent decay or odd colours, and do not bother referring to starch.
Click here to see a diagram showing results to this experiment, or click the camera icon below to see actual photos. actual photos of the experiment

In answering the following questions, you must be clear about the difference between a result, i.e. an observation, and a conclusion, i.e. what you think explains it.

(a) What results do you notice and (b) what can you conclude by comparing potato number 1 with potato number 2?
>(a) Both fill up with liquid
>(b) Sugar and salt have the same effect - causing accumulation of liquid

(a) What results do you notice and (b) what can you conclude by comparing potato number 1 with potato number 3?
>(a)Cavity in 1 fills whereas 3 stays dry
>(b) Presence of salt causes liquid - i.e. liquid flows uphill, but only if a solution is there

(a) What results do you notice and (b) what can you conclude by comparing potato number 1 with potato number 4?
>(a)Cavity 1 fills, whereas no 4 stays more or less dry

>(b)Boiling of potato prevents osmosis process - INTACT MEMBRANE required

Where has the liquid in the cavity of some of the potatoes come from? (2 possible answers)
> cells in potato
> water at base

What is happening to the liquid in the dish?
(2 answers, apart from evaporation)
> taken up into potato
> overflowing down the outside

What will happen when the liquid in the cavity overflows?
(answer has 2 stages)
>solute concentration increases below
>flow stops - so it is NOT a PERPETUAL process

Please explain these findings. You may find the following diagram useful.
potato outline - try clicking!



This topic has connections with other units on:-

CELL STRUCTURE
HOW SUBSTANCES GET INTO AND OUT OF CELLS - 2 (PLASMOLYSIS + DIALYSIS)
PUPILS' EXPTS: OSMOSIS IN CARROT DISCS
HOMEOSTASIS, EXCRETION AND THE KIDNEYS
TRANSPORT & SUPPORT IN PLANTS
Experiment to show some methods of preserving food
Food spoilage and additives

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