Concentration Gradient

Concentration Gradient Definition

A concentration gradient occurs when a solute is more concentrated in one area than another.

“Concentration” refers to how much of a solute there is compared to solvent. A corner of a water tank that has just had salt dumped into it, for example would have a much higher concentration of salt than the opposite end of the tank, where no salt has reached.

Over time, solutes always move down their concentration gradient to “try” to produce an equal concentration throughout the whole solution.

The solute does not really “want” anything, of course. But the laws of thermodynamics state that due to the constant movements of atoms and molecules, substances will move from areas of higher concentration to lower concentration, in order to produce a random solution. This animation illustrates how and why this process occurs:

Chemical surface diffusion slow

This can be easily demonstrated at home by adding a drop of food coloring to a glass of water. At first, the food coloring will only occupy the small spot in the water glass where it was added. But over time, the colored particles will spread, creating an equal distribution of colored particles throughout the bottom of the glass.

Some life forms use this tendency of solutes to move from an area of high concentration to low concentration in order to power life processes.

Neurons, for example, are able to send signals so rapidly because they use a concentration gradient of charge particles to create an electrochemical impulse when they need to fire. 20-25% of all calories consumed by the human body are used to maintain this vital concentration gradient!

Function of Concentration Gradients

Concentration gradients are a natural consequence of the laws of physics. However, living things have found many ways to use their properties to accomplish important life functions.

Organisms that need to move a substance in or out of their cells, for example, may use the movement of one substance down its concentration gradient to transport another substance in tandem.

Organisms may also use concentration gradients to accomplish sudden changes or movements by releasing high concentrations of solute to move to low-concentration areas. Neurons are an example of cells that use high concentrations of solutes to accomplish rapid changes.

Examples of Concentration Gradients

Neurons and the Sodium/Potassium Pump

Neurons spend a huge amount of energy – about 20-25% of all the body’s calories, in humans – pumping potassium into their cells, and sodium out. The result is an extremely high concentration of potassium inside of nerve cells, and a very high concentration of sodium outside.

When cells communicate, they open ion gates that allow sodium and potassium to pass through. The sodium/potassium concentration differences are so strong that the ions “want” to instantly rush out of the cell. Because ions are electrically charged, this actually changes the electrical charge of the cell.

This “electrochemical” signal travels much faster than a merely chemical signal would, allowing us to perceive, think, and respond rapidly.

Problems which interfere with the neurons’ sodium/potassium pump can cause death very quickly, because the heart muscle itself relies on these electrochemical impulses to pump blood to keep us alive.

This makes the sodium/potassium concentration gradient in neurons arguably the most important concentration gradient to human life!

Glucose/Sodium Symport Pump

The glucose-sodium symport pump also takes advantage of the sodium/potassium gradient.

One challenge faced by cells is moving glucose – which are large and difficult to move, compared to tiny sodium ions – and which often need to be moved against their concentration gradient.

To solve this problem, some cells have “coupled” the movement of glucose with the movement of potassium, using proteins that will permit sodium to move down its concentration gradient – if it takes a glucose molecule with it.

This is just one more example of the ways in which cells use the basic laws of physics in innovative ways to accomplish the functions of life.

Lungs and Gills

The most common examples of concentration gradients involve solid particles dissolved in water. But gases can have concentration gradients, too.

Human lungs and fishes’ gills both use concentration gradients to keep us alive. Because oxygen follows the rules of concentration gradients just like any other substance, it tends to diffuse from areas of high concentration into areas of low concentration. That means that it diffuses from the air into our oxygen-depleted blood.

Lungs and gills make this process more efficient by rapidly running our most oxygen-depleted blood across the surfaces of our lungs and gills. This way, oxygen is constantly diffusing into the blood cells that need it most.

Related Biology Terms

  • Concentration – The density of a solvent within a solute. Often measured in a unit of parts per million, or “ppm,” describing how much of the solution consists of a particular substance.
  • Solute – A particle that is dissolved in a solvent. Solutes may be solids, liquids, or gases.
  • Solvent – A medium in which a solute is dissolved, usually a liquid or gas.

Quiz

1. Which of the following laws describes how concentration gradients work?
A. An object in motion tends to stay in motion, unless acted upon by an outside force.
B. Systems always progress toward a state of higher randomness.
C. Substances diffuse from areas of high concentration to areas of low concentration.
D. Both B and C.

Answer to Question #1

2. Which of the following is NOT true of the sodium/potassium concentration gradient?
A. You can move a substance against its concentration gradient without expending energy, if you have the right transport protein.
B. Transport proteins that move substances against their concentration gradients need to be supplied with energy in order to function.
C. Because cells must break down molecules and expend energy, to move substances against their concentration gradient, this movement does not break the laws of thermodynamics.
D. None of the above.

Answer to Question #2

3. Which of the following would we not be able to do if substances didn’t tend to move down their concentration gradients?
A. Think
B. Move
C. Breathe
D. All of the above

Answer to Question #3
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