Diffusion
Animations Osmosis Osmosis Animations Osmosis and Cells
Osmoregulation Osmoregulation in Animal Cells
Osmoregulation in
Plant Cells Osmosis and Food
Preservation
DIFFUSION
Diffusion is the movement of particles (atoms, ions or molecules) from a
region in which they are in higher concentration to regions of lower
concentration. A good example of
diffusion is food colouring. If you place a drop of red food colouring in a
beaker of water eventually the entire beaker of water will have a red tint. The
food colouring moved through the water until it was equally distributed
throughout the beaker. Diffusion takes place along a concentration gradient. A concentration gradient exists until the
diffused substance is evenly distributed.
Other everyday examples of
diffusion are:
1.
Sugar
will diffuse through tea until the entire cup of tea is sweet. (We stir the tea
to speed up the diffusion.)
2.
The
odour of food cooking diffuses throughout the kitchen. If you open the kitchen
door it will spread into the next room.
The
movement of these molecules is said
to be passive. No energy is needed
to be provided. The natural kinetic energy of the particles supplies the
energy.
Examples
of diffusion in science are:
1.
Carbon
dioxide entering the stomata of leaves.
2.
Oxygen
diffusing out of the stomata and lenticels of leaves.
This animation shows the purple molecules diffusing throughout
the box.
This
animation shows a graduated cylinder. Note how the bromine diffuses throughout
the cylinder when the glass plate it removed.
(click
the reload or F5 button to view this
again)
Click here for an excellent interactive animation of osmosis and diffusion
Here the molecules move
out of the liquid and into the air by diffusion.
Click
here to view an animation of diffusion
OSMOSMOSIS
Osmosis is a special example
of diffusion. It is the diffusion of a substance through a semipermeable membrane from a more dilute solution to a more
concentrated solution. This process is also passive since no external energy is needed.
A semipermeable membrane is a barrier that permits the passage of
some substances but not others. Cell membranes are described as selectively
permeable because not only do they allow the passage of water but also allow
the passage of certain solutes (dissolved substances).
Some major examples of osmosis:
- Absorption of water by plant roots.
- Reabsorption of water by the proximal and
distal convoluted tubules of the nephron.
- Reabsorption of tissue fluid into the venule
ends of the blood capillaries.
- Absorption of water by the alimentary canal —
stomach, small intestine and the colon.
Here is an example of
particles moving through a plasma membrane.
This animation shows osmosis of
particles. Note that when the particles are distributed equally on both sides
of the membrane the osmosis stops.
(Click
the reload or F5 button to view this again)
In
another example, the solute can’t fit through the membrane but the water can.
The water moves through until both sides’ concentrations are equal.
Here, the
concentration is the same on both side of the membrane. The water moves through
the membrane from both sides at equal rates.
The movement of liquids in and out cells is dependant on the
concentration of the solution surrounding it.
There are 3
types of situations in which this could vary:
1.
Isotonic: Here the external solution concentration and
the internal concentration of the organism are the same.
2.
Hypotonic: Here the
external solution concentration is less than the concentration of the organism.
In this case water will rush into the organism.
3.
Hypertonic: Here the
external solution concentration is greater than the concentration of the
organism. In this case the water will rush out of the organism.
Osmoregulation is the means by which
cells keep the concentration of cell cytoplasm or blood at a suitable
concentration.
OSMOREGULATION
IN ANIMAL CELLS
(A)
Amoeba,
living in freshwater, uses a contractile vacuole to expel the excess water from
its cytoplasm. This was discussed in the amoeba
web page which can be found on the homepage.
Paramecium, another protest, also gets rid of
excess water using a vacuole:
(B)
The
kidneys maintain the blood at the correct concentration.
Click
here to see an interactive animation of osmoregulation with the kidneys
Plant cells have a cell wall
that allows all materials to enter. It then has a semi-permeable membrane
surrounding the cell’s cytoplasm. Plant cells are able to take in water and
swell because the cell wall is strong enough to withstand the pressure within
the cell. This pressure is called turgor
pressure. As a result of turgor pressure the cell becomes firmer and
stronger. Plant cells with the correct internal water pressure are said to be turgid. Plants with too little internal
water are said to be flaccid.
A good example of this is a
plant that is watered compared to a plant that is not watered. The non-watered
plant will shrivel and will because it has lost its Turgor pressure.
The loss of water from a plant is called plasmolysis. The cell then becomes plasmolysed. The cell on the right
(below) is plasmolysed. Deplasmolysis is
the process by which the cell become turgid by gaining water.
Food can be preserved by causing any
microorganism that comes in contact with it to become plasmolysed and,
therefore, shrivel and die. To do this food is placed in a high salt or sugar
medium. The salt or sugar concentration is higher
than the cytoplasm of bacteria or fungi. Bacteria or fungi, that contaminate
the food, will lose water by osmosis and their metabolism will decline. Many
will die but some bacteria may survive by forming dormant resistant endospores.
Meat and fish are often preserved in salt. Fruit is commonly preserved in sugar
as in jam or syrup.