DIFFUSION, OSMOSIS AND ACTIVE TRANSPORT

In Biology, movement of particles in an out of cells can be described in three ways: DIFFUSION, OSMOSIS and ACTIVE TRANSPORT.

WHAT IS DIFFUSION?
Diffusion is the movement of particles (ions or molecules) from a region where they are in higher concentration to a region where they are in lower concentration down a concentration gradient.

The rate of diffusion depends on the following factors:

• The concentration gradient - the steeper the gradient the faster the rate.
• The size of the particles - the smaller the size the faster the rate and the larger the size the slower the rate.
• The distance it has to diffuse - cell membranes are all about the same thickness but plant cell walls vary in their thickness and permeability. Generally speaking, the thicker the wall the slower the rate of diffusion.

Importance of diffusion (examples):

• Gaseous exchange between the leaf and the atmosphere. Oxygen diffuses out of the leaf through the stomata if the oxygen concentration is higher in the inter-cellular air spaces in the leaf than in the atmosphere. Carbon dioxide diffuses into the leaf through the stomata if the carbon dioxide concentration is higher in the atmosphere than in the inter-cellular air spaces in the leaf.
• Gaseous exchange in the alveoli in the lungs. Oxygen diffuses from the alveoli into the blood in the blood capillaries from a region of its higher concentration (alveoli) to a region of its lower concentration (blood in the blood capillaries). Likewise carbon dioxide diffuses out of the blood into the alveoli from a region of its higher concentration (blood) to a region of its lower concentration (alveoli).

WHAT IS OSMOSIS?
This is actually a special case of diffusion for water molecules only. It is defined as the movement of water molecules from a solution of higher water potential to a solution of lower water potential across a partially permeable membrane (also called selectively or semi permeable membrane and in cells this partially permeable membrane is the cell membrane).

Note: Osmosis is solely used for describing movement of WATER MOLECULES only and not for others. And students often confused the usage of water molecules and water potential. Sometimes students described osmosis as the movement of water potential rather than the movement of water molecules. Always remember that water potential is a measure of the tendency of water molecules to move from one place to another. A dilute solution contains more water molecules per unit volume than a concentrated solution so it has a higher water potential than a concentrated solution.

Concentration of solutions can be described in the following ways:
(Note: the following terms are to be used only when comparing two solutions)

• Hypotonic solution: this is a term used to describe a weak (or dilute) solution in comparison to another solution which is a strong (a concentrated) solution.
• Hypertonic solution: this is a term used to describe a strong (or concentrated) relative to another solution which is a weak (or dilute) solution.
• Isotonic solution: this is a term used to described two solution of the same strength. For example solution A and solution B have the same concentration so we say that solution A and solution B are isotonic to each other.

Effects of osmosis on plant cells and animal cells in hypotonic and hypertonic solutions:

In hypotonic solution (having higher water potential), water molecules diffuses in into the cells. In plant cell, the cell will increase in size but the cell will not burst due to the presence of cell wall. This condition is called TURGIDITY and is very important in plant by providing support to the whole plant and it is also important in leaves because it helps to increase the surface area of the leaves and this in turn helps the leaves to absorb as much light energy as possible to be used for photo synthesis.

In case of animal cell the entry of water by osmosis will increase the cell and because of the absence of cell wall, the cell will burst and we say that the cell LYSED. In case of red blood cells, we say that the red blood cells HAEMOLYSED.

In hypertonic solution (having lower water potential), water molecules diffuses out of the cells. In plant cell, the cell will decrease in size and volume. This condition is called PLASMOLYSIS and is very important in closing the stomata in leaves, thus this helps to reduce loss of water due to transpiration. Plasmolysis is also important in reducing the surface area of the leaves of certain plants when the environmental temperature is very high. This is actually an adaptation for certain plants in reducing the rate of transpiration.

In case of animal cell the passage of water out of the cell by osmosis will cause the cell to shrivel. In case of red blood cells, we say that the red blood cells undergo CRENATION.


WHAT IS ACTIVE TRANSPORT?
This process described the transport of a substance from a region of lower concentration to a region of higher concentration across a partially permeable membrane against a concentration gradient in the presence of energy.

Importance of active transport (examples):

• Absorption of mineral salts from the soil by the root hair cells. Movement is from low salt concentration (in the soil) to high salt concentration (in the cell sap of the root hair cells) across the cell membrane of the root hair cells. Energy for this absorption is provided by the mitochondria in the cytoplasm of the root hair cells.
• Absorption of glucose and amino acids from the villi in the ileum into the blood in the blood capillaries inside the villi.