Simple Diffusion

Reviewed by: BD Editors

Simple Diffusion Definition

Simple diffusion is the process by which solutes are moved along a concentration gradient in a solution or across a semipermeable membrane. Simple diffusion is carried out by the actions of hydrogen bonds forming between water molecules and solutes. Water molecules move in to surround individual solute molecules, which maximizes hydrogen bonding. Hydrogen bonds are extremely temporary, however, and the solution is constantly stirred as a result. This helps distribute the solute evenly throughout the solution. If the molecules are small enough, this simple diffusion can happen across cell membranes, between the individual phospholipids that make up the membrane. Water can move along its concentration gradient through a cell membrane in this manner, a form of simple diffusion known as osmosis.

Unlike simple diffusion, cell membranes often incorporate specialized membrane proteins which help transport substances across the membrane. This is known as facilitated diffusion. Facilitated diffusion includes both the active and passive transport of solutes across the membrane. Active transport uses ATP to provide energy to the proteins providing the transport. Unlike in simple diffusion, molecules can be moved against their gradient using active transport systems. Simple diffusion is present in a number of biological systems, including the delivery of oxygen, water, and other small molecules to the cells of the body. While many solutes have the ability to traverse the membrane via simple diffusion, cells will often have protein channels to help speed the process.

Examples of Simple Diffusion

Carbon Dioxide

Carbon dioxide is a small molecule that can be dissolved into water. If you’ve ever enjoyed a bubbly soda, you know this. However, you might not know that the same mechanism is transporting the carbon dioxide that your cells create into your bloodstream and out of your body via your lungs. Carbon dioxide is small enough to move through simple diffusion through your tissues and out of your body. If you hold your breath for a short time, you will begin to feel a burning “desire to breathe”. This is caused by the accumulation of carbon dioxide in sensitive nerve tissues in your bloodstream, lungs and brain. When you begin to breathe again, the carbon dioxide diffuses out of your system. Many gases are able to do this through your lungs including oxygen, nitrogen, and many others in the atmosphere.


Being the simple organisms they are, bacteria have no way to intake nutrients other than diffusing them across the cell membrane. While they do use facilitated diffusion to transport most nutrients, they rely on simple diffusion to deliver oxygen, water and small nutrients to the cytoplasm. Within their cells, there are no specialized organelles to hold or transport substances, so bacteria rely on the simple diffusion of material within their cells to ensure materials are present for the reaction that control their life processes.

  • Facilitated Diffusion – The distribution of a substance across a cell membrane using specialized embedded transport proteins.
  • Passive Transport – The use of proteins to facilitate the diffusion of a solute along its gradient across a cell membrane.
  • Active Transport – The use of proteins and energy to transport substance, even against their gradient across a membrane.
  • Osmosis – The simple diffusion of water along its gradient in a solution or across a membrane.


1. A cube of salt is dropped into a glass of water. After an hour, the cube has disappeared. Which of the following happened?
A. Simple diffusion
B. Facilitated diffusion
C. Osmosis

Answer to Question #1
A is correct. This is a case of simple diffusion. Table salt is made of a matrix of two ions: sodium (Na+) and chloride (Cl). The water in the glass immediately started working molecules off of the block as soon as it was dropped into the glass. The hydrogen bonds formed between water molecules and the salt ions pull the salt ions apart, and distribute them evenly between the water molecules. As water works its way into to the block, the salt molecules eventually become completely surrounded by water molecules. The hydrogen bonds continue to move the water molecules around and completely surround each salt ion, until the salt is no longer visible in the solution.

2. Some organisms have proteins in their cell membranes called aquaporins, which provide a route for water to pass easily through the membrane. However, water will travel by osmosis through the membrane without these proteins. If the cell is exposed to a highly hypotonic environment, water rushes through both the aquaporins and the cell membrane into the cell. Which of the following is happening?
A. Simple Diffusion
B. Osmosis
C. Osmosis and Facilitated Diffusion

Answer to Question #2
C is correct. The simple diffusion of water is known as osmosis. It has a separate name to distinguish when water travels without its solutes, as in the case of water traveling through the cell membrane. The solutes originally distributed in the water are left behind, as the water rushes to balance the concentration of solutes within the cell. Aquaporin are special membrane proteins that facilitate this diffusion of water into the cell.

3. Oxygen used for respiration is a very small molecule. In some small organisms, no respiratory organs are needed, because the oxygen moves directly through the cell membrane and into each cell. Which process is responsible for this?
A. Facilitated Diffusion
B. Osmosis
C. Simple Diffusion

Answer to Question #3
C is correct. Oxygen travels throughout marine environments thanks to simple diffusion. Oxygen usually forms a bond with a second oxygen molecule and travels as O2. These small molecules can form hydrogen bonds and are also small enough to diffuse directly through cell membranes. Even in large organisms, no special proteins are needed to move oxygen into cells, it diffuses directly from the lungs into blood cells and throughout the body.

Cite This Article

MLAAPAChicago Editors. "Simple Diffusion." Biology Dictionary,, 10 Feb. 2017, Editors. (2017, February 10). Simple Diffusion. Retrieved from Editors. "Simple Diffusion." Biology Dictionary., February 10, 2017.

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