OBJECTIVES: upon the completion of this exercise you should be able to:

1. Understand the concept of a concentration gradient in simple diffusion
2. Understand the effect of temperature, molecule weight (size) and solute concentrations on diffusion rates
3. Determine the hypotonic, hypertonic and isotonic solutions for a cell through
exposure to differing molar solutions

The ability of molecules and ions to be transported into or out of a cell or even within a cell is important to the functioning of a cell.

Many molecular processes are dependent on the frequency in which molecules come into contact with one another. The movement of molecules (or any material, really) in the environment can be due to a variety of factors. One of the most common driving forces underlying the passive movement of molecules in the environment is concentration. The movement of materials is typically from areas of high concentration to areas of lower concentration. This directional movement will continue to occur until the concentration gradient has broken down. This is called reaching equilibrium.

We refer to the movement of any substance based on such a concentration gradient as diffusion. When referring to this process, it is said that a substance travels down its concentration or diffusional gradient. Environmental factors such as temperature or molecular size can impact the rate of diffusion.

We are interested not only in the diffusion of molecules through empty space, but also their diffusion across biological membranes. Not all molecules will be able to cross biological membranes by themselves. In fact few molecules can cross the membrane without assistance. Oxygen and Carbon Dioxide can diffuse across the cell membranes without assistance. Molecules that follow their concentration gradient across the membrane unaided are said to exhibit simple diffusion

The plasma membrane is referred to as a selectively permeable membrane, and this feature is conferred by the membrane’s amphiphilic (having both hydrophobic and hydrophilic regions) nature.  Just because a molecule cannot cross a membrane by itself does not necessarily mean that the molecule will never be able to get across the membrane. If a cell needs to transport such molecules across the membrane, transmembrane transport proteins (channel and carrier) are used. This process is known as facilitated diffusion. Both simple diffusion and facilitated diffusion are referred to as passive transport, as movement of materials always follows their concentration gradient, and therefore occurs without an input of energy.

Since water is a polar molecule, the bulk of its transmembrane movement cannot occur by way of simple diffusion. One special case of facilitated diffusion is the diffusion of water across a membrane, which is referred to as osmosisAquaporin proteins are the channel proteins which are responsible for facilitating water’s transport across membranes.

In osmosis, the water will move from an area of lower solute concentration into an area of higher solute concentration. So in osmosis, solute concentrations are important. Solutes are molecules or atoms which can dissolve into a solvent. In osmosis, water is the solvent and sugars and salts can be the solute. In the figure to the right, the water will move from the inside of the cell to the outside of the cell because the solute concentration is higher outside of the cell. The cell will dehydrate becoming smaller and losing weight.

When discussing the concentration of solutes in solution, there are two solutions of interest from a cellular perspective: the concentration of solutes inside the cell and the concentration of solutes outside the cell. We usually talk about the solute concentration of the environment as it compares to that inside of the cell. In general, this concept is known as tonicity. A solution which has a greater solute concentration than the cell is said to be hypertonic. A solution which has a lower solute concentration than the cell is said to be hypotonic. When solute concentration is equal on both sides of the membrane we say the solutions are isotonic.

A cell in a hypertonic solution will dehydrate becoming smaller and losing weight. Cells in a hypotonic solution will gain water, gain weight and become larger. Cells in an isotonic solution will exhibit no change in hydration, weight or size. See the figure to the right.

However, materials do not always move from areas of high concentration to areas of lower concentration. In some instances, cells need to accumulate materials on one side of the membrane or the other. In this case, proteins again facilitate the movement of materials across the membrane. This process is referred to as active transport, and requires an investment of metabolic energy (often in the form of ATP).