Nadine has taught nursing for 12 years and has a PhD in Nursing research
In this lesson, you'll learn about the concentration gradient and its processes, as well as about the two types of diffusion, passive and facilitative, that happen along the concentration gradient.
Concentration Gradient Defined
The formal definition of a concentration gradient is the process of particles, which are sometimes called solutes, moving through a solution or gas from an area with a higher number of particles to an area with a lower number of particles. The areas are typically separated by a membrane. This membrane can be permeable, semi-permeable, or non-permeable. Permeable is defined as a membrane that can be crossed by particles, ions, or water. Semi-permeable means that some particles, ions, or water can cross the membrane. Finally, non-permeable membrane means that no particles, ions, or water can cross the membrane.
An example that might help you understand the different types of membranes would be different types of fences. A wooden log fence would allow many things to pass through - this would be an example of a permeable membrane. A chain link fence would allow some small items to pass through it - this would be like a semi-permeable membrane. A solid plastic fence would not allow items to pass through it at all - this would represent a non-permeable membrane.
Concentration Gradient Process and Diffusion Types
Solutes moving through solution (liquid or gas) happen by random motion until there are equal numbers of particles in the two areas. Random motion is defined as movement that occurs by chance because there is no order or regular system by which the movements of the particles shift in the solution or gas.
The actual movement is called diffusion. There are three types of diffusion: passive diffusion, facilitative diffusion, and active transport. Because active transport does not happen along the concentration gradient, it will not be covered in this lesson. Instead, let's first discuss passive diffusion.
Passive diffusion does not require energy; it happens by random motion. In a solution or gas that has an area of high numbers of particles and an area of low numbers of particles, the particles will diffuse or move from the area of higher to the area of lower concentration. A common example of this is a cup of water that you drop food coloring into. The food coloring is concentrated when dropped into the cup of water; however, after a few seconds pass, the particles become lighter in color as they move to the lower concentration level. Again, this happens without energy so it is known as passive diffusion.
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Another type of diffusion that is passive because it does not require energy is facilitative diffusion. This is how particles move across a membrane. This usually refers to a cell membrane, so that is the description we will use. A cell membrane is a barrier that usually cannot be crossed by all particles or ions. Ions are just charged particles, which means they have either a positive charge or a negative change. The particles cannot move freely across the membrane into and out of the cell; they must be assisted by a carrier. This is called facilitation.
This carrier is usually a protein that is able to cross the cell membrane. It has a special shape and usually can carry a small particle or ion across the cell membrane through a specific channel. This process can either happen at any time, or the channel may be opened and closed at different times depending on the cell's needs.
To assist you with this explanation, think of the process of a person having surgery. The person having surgery cannot go into the operating room; they must first be taken into the surgery suite. Also, the surgery door would not always be open to the person - the doors would only be open when they are scheduled for surgery. In addition, patients cannot go into surgery by themselves - the nurse takes the person through the operating room doors. Do you see how this relates to the process of facilitated diffusion? Remember, the protein carries the particle or ion across the membrane through a channel in the cell membrane wall.
The concentration gradient is an important process for understanding how particles and ions move in random motion in a solution or gas. It is the process used for particles moving from an area of higher concentration in a solution to an area of lower concentration. This occurs until there are equal parts of particles in the solution or gas.
Processes that use the concentration gradient include passive diffusion and facilitated diffusion, which occur without the assistance of energy. Passive diffusion happens by random motion, while facilitated diffusion requires the assistance of proteins that travel through channels in a membrane.
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