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What Is a Concentration Gradient?
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A difference in concentration of molecules in physical space is called a concentration gradient. An easy to understand example from everyday life is the application and subsequent fading of perfume or aftershave scent.
Everyday Examples of Concentration Gradient
The concentration of scent molecules is highest on areas of the skin that have had perfume or aftershave directly applied. Others can smell the scent because some of those molecules are always traveling away from the perfumed person, the source, out into the air—moving down the concentration gradient, from a high concentration to a lower concentration. Eventually the scent molecules are so widely dispersed that they can no longer be perceived.
Think about the unpleasant scent of skunk stink, when a skunk has sprayed or been struck by a vehicle. As a person gets closer to the source of the skunk stink, the scent gets stronger, because the “stink molecules” are more highly concentrated closer to the source.
Molecules not only travel through air, but through other media as well. When a person puts creamer in his or her coffee, the cream molecules would eventually bounce around in the cup, moving down the concentration gradient until evenly distributed. However, most coffee drinkers do not wait for this to happen. They introduce additional energy by stirring the coffee and speeding up the process.
Concentration Gradients and the Plasma Membrane of Biological Cells
The plasma membrane controls traffic of materials moving into and out of biological cells. It is composed primarily of phospholipids and proteins, and is semi-permeable, allowing some materials to easily move across and barring the transport of others.
Phospolipids are polar molecules, each with a water-loving (hydrophilic) head and a water-hating (hydrophobic) tail. Their polar nature causes phospholipids to spontaneously arrange into a lipid bilayer; two layers of phospholopids with the hydrophilic heads pointing outward and the hydrophobic tails pointing towards each other.
Even in the presence of a concentration gradient, many molecules can't move across the cellular membrane. Large molecules cannot fit through the double layer of phospholipids, charged molecules are repelled, and hydrophilic molecules have trouble crossing the hydrophobic interior of the membrane. This often results in higher concentrations of chemicals on one side of the membrane than on the other.
Electrochemical Gradients
When charged molecules are present in higher concentrations on one side of the membrane than on the other, the result is an electrical gradient. Electrical gradients can provide the energy for passive transport of molecules across the plasma membrane—processes such as diffusion, facilitated diffusion and osmosis. Passive transport means that the cell does not need to contribute any of its own ATP energy in order to move the molecules. The energy is provided by the electrochemical gradient.
For more biochemistry information, see the educational websites Chem 4 Kids or Science Prof Online Chemistry Main Page.
Sources
- Bauman, R. (2007) Microbiology with Diseases & Taxonomy, Pearson Benjamin Cummings.
- Nester, E. et al (2001) Microbiology: A Human Perspective. McGraw Hill.
This article originally appeared on Suite101 online magazine.
Article Summary: What is a concentration gradient and how does it provide energy for the movement of molecules? Here's a simple explanation.
What Is a Concentration Gradient?
Second hand smoke that you can breath in when near a smoker has traveled down a concentration gradient.
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