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Diffusion, facilitated diffusion and osmosis

The main properties of molecules that determine how they may be transported across a membrane are solubilitysize and charge.

Large molecules can't cross the membrane, charged molecules also can't, and naturally, lipid-repelling (or water-attracting) molecules can't. Conversely, small molecules can cross the membrane barrier, alongside molecules with no charge (nonionised) as well as lipophilic (hydrophobic) molecules.

It's important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption.


Diffusion = the spread of particles from a region of higher concentration to a region of lower concentration, until the particles are evenly spread out.

Diffusion takes place when you use a spray in a room, for example. The particles in the spray move randomly, knocking each other, which results in them spreading throughout the room gradually, from high concentration to low concentration. Therefore, diffusion acts down (or along) a concentration gradient.

It is important to know what affects the rate of diffusion. These are:

1. Surface area - the greater the surface area, the faster diffusion will occur
2. Difference in concentration - the higher the difference (the steeper the gradient), the faster diffusion will take place
3. The thickness of the exchange surface - the thicker the exchange surface, the slower the rate of diffusion.

Of course there are other factors such as temperature (increased kinetic energy results in faster diffusion) and the diffusion pathway (distance). The latter is a side effect of (3.) The thickness of the exchange surface, in some respects.

Facilitated diffusion

In some cases, diffusion is aided by certain proteins. This is called facilitated diffusion. The responsible proteins speed up diffusion of substances which would otherwise take longer to pass through the plasma membrane. The key points about facilitated diffusion which differentiate it from active transport (which also uses proteins):

-it occurs down a concentration gradient
-it uses no metabolic energy

Two kinds of protein achieve facilitated diffusion: carrier proteins and ion channels. Carrier proteins transport substances from one side of the membrane to the other, usually by co-transport. For example, glucose is transported along with an Na+ ion.

Ion channels are proteins with gates that can be open or closed to allow or stop certain ions from entering, e.g. Na+ (sodium) and K+ (potassium) ions.


Osmosis is the diffusion of water across a semi-permeable membrane. The "concentration" of water is referred to as water potential (ψ). So osmosis is the movement of water from a higher water potential to a lower water potential across a membrane.

For osmosis to occur it is essential that there is a semi-permeable membrane separating two environments with a different solute concentration. The solute must be unable to cross the membrane (molecules too big), but the water molecules are free to pass through and lead to an equilibrium. In the above image, the right side of the beaker has a higher water potential than the left side, so water moves in from right to left.

You must also learn the term isotonic. A solution is isotonic when it has the same water potential as another solution. An example is Ringer's solution which has the same water potential as blood plasma, so can be used to keep tissues alive.

The terms that describe the overall water potential (ψ) are solute potential (ψπ) and pressure potential (ψp), amongst other factors. Solute potential refers to the property determined by the concentration of solute dissolved in water, while pressure potential refers to the mechanical pressure that the water system is under. The sum of these gives the overall water potential.

ψ = ψπ + ψp

Knowing the values for these terms enables easy calculation of overall water potential.

Extreme shifts is cell water potential in animal and plant cells result in the plasma membrane being pulled away from the cell wall, or by the cell lysing.

In animal cells which do not have cell walls, this is called lysis and is caused by too high a water potential (drawing in excess water and leading to bursting). If the water potential is very low and solutes are drawn in while water leaves the cell, it causes it to shrink. This effect is called crenation.

Plant cells have cell walls, so these can offer some resistance to water pushing against the cell wall if the water potential runs high. Cell bursting is resisted by default due to the turgor conferred by the cell wall. In the opposite scenario of water leaving the cell, the plasma membrane starts pulling away from the cell wall. This is called incipient plasmolysis. When the plasma membrane has completely detached from the cell wall, plasmolysis is complete.

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