SIMPLE DIFFUSION: PROCESS AND EXAMPLES - BIOLOGY - 2025

The simple diffusion is the physical process by which the substances are moved from a region where you are highly concentrated to areas where the concentration of the substance is lower. Simple diffusion is a spontaneous process that does not require energy, but is promoted by a concentration gradient.

The substances that are dissolved are called solutes, they move through a solution (or a gas) by random movements. Random motion is defined as motion that occurs by chance, where there is no regular order or system by which the motions of particles change in a solution or gas.

Simple diffusion

If the molecules are small enough, this simple diffusion can occur across cell membranes, between the individual phospholipids that make it up. The membranes can be permeable, impermeable, or semi-permeable.

A permeable membrane is one that allows any substance to pass through, an impermeable membrane does not allow any substance to pass through and a semipermeable membrane only allows certain substances to pass through.

Simple diffusion process

Simple diffusion is carried out through actions of the hydrogen bonds that form between water molecules and the solutes dissolved in it. The water molecules move randomly and eventually surround individual solute molecules, maximizing the opportunity to form hydrogen bonds.

Thus, if a mixture of a substance were made in water, initially the substance would be concentrated in a single place, the initial one. At that time the concentration gradient is very high.

As time passes the molecules move and collide with each other; this movement promotes the formation of bonds between the solute particles and the solvent. However, these bonds break quickly because they are very weak and this is the property that allows them to be distributed throughout space until equilibrium is reached.

At that point, where the solute is uniformly distributed throughout the solution, the force of the concentration gradient ends.

Simple diffusion examples

Gas exchange in the respiratory system

In the respiratory system of lung animals, oxygen (O2) and carbon dioxide (CO2) are constantly exchanged. Animals absorb oxygen and expel carbon dioxide by simple diffusion.

In the lungs, the oxygen concentration is higher than in the blood, so there is a concentration gradient between these two regions, and this causes the diffusion of oxygen from the lungs to the blood to be promoted.

Similarly, as there are more carbon dioxide molecules in the blood than in the lungs, they will tend to move from the blood to the lungs.

Spreading the tea in hot water

When a cup of tea is brewed initially there is only hot water without any solute. By placing the tea bag in the hot water, it begins to release the tea particles from the bag - where there is a higher concentration of tea - into the water, where the concentration of tea is lower.

The movement of the molecules is favored by the initial concentration gradient of the tea. Eventually, the diffusion of the tea causes the concentrations to equalize throughout the cup and the solution to become homogeneous (it all looks the same color). At this point there is no longer a concentration gradient.

Differences between simple diffusion and facilitated diffusion

- In simple diffusion, the particles cross the cell membrane freely, without the "help" of other structures. In contrast, in facilitated diffusion, the molecules are transported through small protein channels that cross the entire membrane.

- Simple diffusion is slower than facilitated diffusion.

- Simple diffusion is not specific to the solute, only that the particles to be transported have to be hydrophobic so that they can cross the cell membrane without problem. In contrast, in facilitated diffusion specific solutes are transported.

- Simple diffusion cannot be inhibited, while facilitated diffusion can be inhibited by specific molecules that bind to transporter channels.

- Simple diffusion is always a passive process, that is, it does not require the energy from ATP. In contrast, facilitated diffusion can be an active or passive mechanism, depending on whether or not it requires the energy of ATP.

- In simple diffusion the substances always move in favor of their concentration gradient. On the contrary, in facilitated diffusion the substances pass through the membrane against or in favor of its concentration gradient.

- Simple diffusion allows the passage of small and nonpolar molecules (hydrophobic), while facilitated diffusion allows the passage of large and polar molecules (hydrophilic).

Differences between simple diffusion and osmosis

- Simple diffusion refers to the movement of any substance as a function of its concentration gradient; on the other hand, osmosis refers exclusively to the movement of water as a function of its water potential.

- Simple diffusion occurs in solid liquids and gases. On the contrary, osmosis can only occur in a liquid medium.

- Simple diffusion does not require a semi-permeable membrane, while osmosis is a process that occurs through a semi-permeable membrane.

- Simple diffusion helps to equalize the concentrations of dissolved solutes in the available space. Osmosis does not favor the equality of water concentration on both sides of the semipermeable membrane.

- Simple diffusion can occur between different types of solvents. In contrast, osmosis can only occur between similar solvents and in biological systems. This solvent is generally water.

- In simple diffusion the particles are distributed in all directions, in osmosis the flow of molecules is basically in the same direction.

- The simple diffusion process cannot be stopped or reversed. Instead, osmosis can be stopped or reversed if additional pressure is applied on the side of the membrane where there is less water available.

References

1. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K. & Walter, P. (2014). Molecular Biology of the Cell (6th ed.). Garland Science.
2. Campbell, N. & Reece, J. (2005). Biology (2nd ed.) Pearson Education.
3. Lodish, H., Berk, A., Kaiser, C., Krieger, M., Bretscher, A., Ploegh, H., Amon, A. & Martin, K. (2016). Molecular Cell Biology (8th ed.). WH Freeman and Company.
4. Purves, W., Sadava, D., Orians, G. & Heller, H. (2004). Life: the science of biology (7th ed.). Sinauer Associates and WH Freeman.
5. Solomon, E., Berg, L. & Martin, D. (2004). Biology (7th ed.) Cengage Learning.
6. Thibodeau, P. (2013). Anatomy and Phisiology (8 ^th). Mosby, Inc.
7. Tortora, G. & Derrickson, B. (2012). Principles of Anatomy and Physiology (13th ed.). John Wiley & Sons Inc.