Osmotic Pressure

Osmotic Pressure Definition

Osmotic pressure can be thought of as the pressure that would be required to stop water from diffusing through a barrier by osmosis. In other words, it refers to how hard the water would “push” to get through the barrier in order to diffuse to the other side.

Osmotic pressure is determined by solute concentration – water will “try harder” to diffuse into an area with a high concentration of a solute, such as a salt, than into an area with a low concentration.

In reality of course, osmotic pressure is not a “desire” of water to move, but rather an extension of the natural law that all matter will become randomly distributed over time. When the concentrations of substances are different in two areas and the areas have contact with each other, the random motion of particles will cause the substances to diffuse until the solution is uniform throughout the whole area.

Osmosis is the particular diffusion of water through a semi-permeable membrane. So in the case of osmosis, the solutes cannot move because they cannot pass through the membrane. However, the water can move, and it does – passing through the membrane to an area with higher solute concentration.

This can cause the total volume of water on each side of the membrane to change: the side of the membrane with more solutes may end up with much more water. This can lead to problems for cells, such as bursting (if too much water moves into the cell), or becoming dehydrate (if too much water moves out).

This is a very important factor in biology because the intracellular environment is different from the extracellular environment. If the extracellular environment changes, it may cause water to flow into or our of cells.

Some organisms, such as plants that use osmotic pressure to move water, have taken advantage of this principle. But it can also threaten the health of cells and organisms when there is too much or too little water in the extracellular environment compared to the inside of the cell.

Osmotic Pressure Equation

Osmotic pressure can be calculated using the following equation:

π = MRT

In this equation:

π

Importantly, π does not equal 3.14… in this equation!

Instead, here “π” is the symbol used to denote osmotic pressure. You can think of this equation as solving for“π” just like solving for X.

There is nothing special about the symbol “π” except that it is the symbol which chemists have internationally agreed upon to mean osmotic pressure.

M

M is the molar concentration of the solute. Molar concentration refers to the actual number of atoms, ions, or molecules of the solute.

This is important because it is the number of particles that determine how the particles interact in osmosis – not the volume or weight.

How much a mole – a unit of measurement of particles – weighs, or how much volume it takes up, will vary depending on the molecular weight and density of the solute.

R

R is the ideal gas constant.

Although the ideal gas constant was created to refer to gasses and how they diffuse and behave, it also applies to liquids.

In chemistry, both liquids and gases are considered “fluids” – particles that are able to diffuse freely, as opposed to solids, whose particles are held in place by strong bonds.

T

T is the temperature in degrees Kelvin.

One degree Kelvin is the same as one degree Celsius – but there is an important difference between the two measuring systems.

Temperature is a measure of the energy in molecules. At higher temperatures, molecules move faster; at lower temperatures, they move slower. This is why temperature is so important to this equation: the faster particles are undergoing random molecular motion, the faster they will diffuse.

In Celsius “zero degrees” is considered to be the freezing point of water. This is an arbitrary number that was picked by scientists because freezing water is a common phenomenon. Celsius temperatures can be positive (above zero) or negative (below zero).

But in Kelvin, “zero degrees” is the temperature at which no molecular motion occurs. This is actually absolute zero – it is impossible to get colder than “no molecular motion.”

As a result, Kelvin is used in many chemistry equations, because it is an absolute measure of heat. If a substance is at 300 degrees Kelvin, you know exactly how much heat is in the substance: 300 Kelvins total.

This is much better for scientists than calculating based on Celsius, and having to figure out how much heat is in water at “-30 Celsius,” for example.

Fun fact: absolute zero – zero Kelvin – is -273.15 degrees Celsius. Another way to say that is that zero degrees Celsius – the freezing point of water – occurs at 273.15 Kelvins.

Examples of Osmotic Pressure

Wilting Plants

Many plants actually use osmotic pressure to maintain the shape of their stems and leaves.

If you have kept potted plants, you probably know that your plants can become very wilted very quickly if they are not watered. But within just minutes of watering, they can perk right back up!

This is because the stems and leaves of many plants are essentially “inflated” by osmotic pressure – the salts in the cells cause water to be drawn in through osmosis, making the cell plump and firm.

If not enough water is available, the plant will wilt because its cells are becoming “deflated.” In scientific terms, they are “hypertonic” – which means “the concentration of solute is too high.”

Plants can also demonstrate the power of osmotic pressure as they grow.

You may have seen plants springing up through asphalt, or tree roots growing through bricks or concrete.

This, too, is made possible by osmotic pressure: as plants grow, their cells draw in more water. The slow but inexorable pressure of water moving through the plant cell’s membranes can actually push through asphalt!

Effects of Dehydration – And Overhydration

We all know the dangers of dehydration, where lack of water can cause dangerous effects in our body. What we might not realize is that these effects are directly related to osmotic pressure.

When our bodies don’t have enough water, water can actually move out of our cells into our blood. This can cause the concentrations of salts and other solutes in our cells to become too high, interfering with cellular function.

When we drink water, the water enters the body through our bloodstreams, and is able to diffuse back into our cells through osmosis, restoring their proper function.

The opposite is also possible: it is actually possible to die from drinking too much water.

It is hard to accidentally “overdose” on water, but in extreme cases such as water-drinking contests, it is possible to drink so much water that too much of it diffuses into your cells. In extreme cases this can cause swelling of the brain.

Rapid rehydration after severe dehydration can be dangerous for the same reason. It is advised to undertake rehydration slowly, because filling dehydrated cells suddenly with large volumes of water can cause them to burst!

The graphic below shows how dehydration – or overhydration – can affect our blood cells by causing them to shrivel or burst:
Osmotic pressure on blood cells

Related Biology Terms

  • Diffusion – A process by which fluids and solutes become evenly distributed through random molecular motion.
  • Temperature – A measure of the average kinetic energy of particles in a substance. “Warmer” substances have more particle motion, while “cooler” substances have less.
  • Water – The “universal solvent” upon which life on Earth is based. Water’s unique properties make it an ideal carrier for the molecules of life such as sugars, amino acids, and nucleic acids.

Quiz

1. Which of the following is NOT true of osmosis?
A. It refers specifically to the movement of water through a semi-permeable membrane.
B. It can only occur where there is a membrane that is permeable to water.
C. It can apply to any diffusion process involving water.
D. None of the above.

Answer to Question #1

2. Which of the following is NOT a variable in the equation for osmotic pressure?
A. π – The value 3.14…
B. M – The molar concentration of the solute.
C. R – The ideal gas law.
D. T – The temperature of the solution in degrees Kelvin.

Answer to Question #2
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