Diffusion is the net movement of particles from an area of high concentration to an area of low concentration until they are evenly distributed.
In diffusion, the particles move down the concentration gradient. The concentration gradient is the difference in concentration between two areas. So, the larger the difference in concentration, the steeper the concentration gradient.
In diffusion, the particles always move down the concentration gradient, towards the area of lower concentration.
The reason that particles behave this way is due to a phenomenon known as Brownian Motion. Particles are constantly in motion, randomly moving and colliding with one another. So, when a high concentration of a certain particle is found within a closed space, those particles will collide with one another frequently. Eventually, the random movements and collisions of the particles will result in a net movement towards the area of lower concentration.
For example, if a woman who has sprayed a copious amount of perfume on her clothing enters a poorly ventilated room, whatever tasteless fragrance she is wearing will eventually spread around the whole room, ruining everyone's day. The particles which carry the fragrance of the perfume move from the area of high concentration around her clothing towards the area of low concentration in the surrounding room.
Diffusion is very important in several biological processes. Think about the structure of a cell, for example: the cell membrane is made up of a differentially/selectively permeable membrane. That is, it only allows certain substances (e.g. ions, compounds or molecules) to pass through it.
As a result, a cell is able to take in certain substances from it environment based on differences in concentration. In the human body, cells are brought into contact with oxygenated blood, and through diffusion, oxygen enters the cell. Carbon dioxide (a waste product of respiration) diffuses out of the cell and into the blood to be excreted.
Diffusion is also essential in other biological processes:
Gaseous exchange in the alveoli of the lungs- carbon dioxide diffuses out of deoxygenated blood and into the air while oxygen diffuses out of the air and into the red blood cells
Gaseous exchange in plants- Carbon dioxide diffuses into the air spaces of the spongy mesophyll through the stomata and into the photosynthesising palisade mesophyll cells. Oxygen diffuses out of the mesophyll cells as a waste product of photosynthesis and out into the atmosphere through the stomata.
Transfer of transmitter substances- Acetylcholine (a transmitter substance) diffuses out of the presynaptic membrane and into the postsynaptic membrane at a synapse between neurons.
Osmosis is the movement of water molecules from an area of high water potential to an area of low water potential through a semi-permeable membrane until the water potential is equalized. Water potential is basically the difference between a given sample and pure water. A substance with a higher water potential is closer to pure water, while a lower water potential has more dissolved solute.
Essentially, water moves through a semi-permeable membrane from a dilute solution to a more concentrated solution.
Thus, in osmosis, water moves down the water potential gradient.
Osmosis can be observed through an experiment using visking tube (artificial semi-permeable membrane used in separation techniques).
We can set up the visking tube to be filled slightly with concentrated salt water, tied with thread at both ends and suspended in a beaker containing distilled (pure) water:
As you can see, there was a movement of water molecules into the visking tubing from the outside due to the difference in water potential.
NB: Several terms are used to refer to membranes based on what passes through them, selectively permeable, differentially permeable and semi-permeable, though you may not be familiar with each of their meanings:
Selectively permeable- allows certain molecules or ions to pass through it via active transport or passive transport (osmosis, diffusion)
Differentially permeable- allows the passage of smaller molecules, but not larger ones.
Semi-permeable- allows the passage of certain ions or molecules.
Both selectively and differentially permeable membranes are types of semi-permeable membranes.
There are different types of solutions based on tonicity. Tonicity is the measure of the water potential gradient of two solutions separated by a semi-permeable membrane. The tonicity decides the direction fo the movement of water molecules. There are hypertonic, hypotonic and isotonic solutions.
The effects are different for plant cells and animal cells in each type of solution.
In a hypotonic solution, the surrounding solution is more dilute than the cell (it has a higher water potential than the cell). So, water enters the cell.
In the plant cell, the cytoplasm and the vacuole increase in volume. The cell membrane is pressed up against the cell wall, causing the cell to stretch slightly and become turgid (Note that only plant cells can be turgid).
In the animal cell, the cytoplasm increases in volume. However, the cell membrane cannot contain the cell contents, and splits open. This is called cell lysis, or cytolysis.
It is the cell wall of the plant cell that prevents it from becoming lysed.
In hypertonic solutions, the surrounding solution has a lower water potential than the cell. Thus, the net movement of water is out of the cell. Both the plant and animal cells become flaccid. However, over time, the plant cell may become plasmolysed as the cell membrane shrinks away from the cell wall.
In an isotonic solution, the water potential inside the cell is the same as the water potential in the surrounding solution. So, there is no net movement of water molecules. The cell remains flaccid.
Osmosis is used in several biological processes:
Absorption of water by plant roots
Re-absorption of water by the proximal and distal convoluted tubules of the nephron.
Re-absorption of tissue fluid (lymph) into the venule ends of the blood capillaries
Absorption of water by the alimentary canal — stomach, small intestine and the colon
Diffusion and osmosis are both types of passive transport, that is, no energy (ATP) is used to transport the particles. These process occur naturally.