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Substances must move in and out of the cells to ensure the proper functioning of an organism. This movement of substances in and out of the cells is accomplished by three processes: diffusion, osmosis, and active transport. In this article, we will only discuss the process of diffusion.
Diffusion Defined
Diffusion is defined as follows:
The net movement of substances from an area of their higher concentration to an area of their lower concentration down a concentration gradient, as a result of their random movement is referred to as a diffusion
Diffusion is a natural process that occurs when particles spread from the region of a higher concentration, i.e. the area where there are so many of them to the region of lower concentration, i.e. the area where there are quite fewer of them. The example of diffusion in our daily lives includes the spreading of smell in the air and the movement of concentrated liquids when placed in water.
What is Concentration Gradient?
Substances can move in and out of the cell through cell membranes. The rate at which the materials move in and out of the cell depends on the concentration gradient. If more particles are present in a certain volume, then those particles are deemed as concentrated. These particles will tend to move at a place of lower concentration. A concentration gradient is when there is an area of high concentration and an area of low concentration.
Examples of Concentration Gradient in Our Bodies
Products of digestion that are dissolved in water can move across the wall of the small intestine through this process. The concentration of these products is higher in the small intestine than their concentration in the blood. Hence, there is a concentration gradient from the intestine to the blood.
Water dissolved substances such as oxygen and carbon dioxide are transported through diffusion in the lungs. It is only possible because of the concentration gradient.
- Oxygen is transported down a concentration gradient from the air in the alveoli to the blood.
- Carbon dioxide is transported down a concentration gradient from the blood to the air in the alveoli.
It means that the dissolved materials will only diffuse when a concentration gradient is present.
Diffusion in Our Cells Occurs Through Cell Membrane
Diffusion also occurs in our cells. Certain substances such as carbon dioxide, oxygen, dissolved nutrients, water, and urea move in and out of the cell through this process. The process occurs across the cell membrane which is a selectively permeable membrane. For cells in living organisms, the principle of the transportation of materials down the concentration gradient is the same, however, the cell membrane surrounds the cell that restricts the free movement of substances. Dissolved or gaseous particles must move through the cell membrane to enter or leave our cell.
- The cell membrane is actually a partially permeable membrane which means that it enables few substances to move easily. On the other hand, the cell membrane does not allow few substances to move freely in and out of the cells.
- The size of the molecules also influence the movement of the materials in and out of the cell
Apart from cells, diffusion is also essential for other functions of our bodies.
Why Substance Transportation is Important in Cells?
It is critical for the substances such as oxygen, carbon dioxide, water, dissolved food molecules, mineral ions, and urea to be moved in and out of the cells in the organisms. Waste substances must be eliminated from the cell and useful substances must enter the cell for their proper functioning. Diffusion is one of the three processes that make this movement of substances possible.
Rate of Diffusion
Molecules are moving constantly in and out of our cells for the proper functioning of our bodies. Certain factors can affect the diffusion rates of the substances. These factors are discussed below:
- Concentration gradient: A larger difference in the high and low concentration enhances the rate of diffusion. In other words, the greater there is a difference in the concentration levels, the greater is the rate of diffusion.
- Temperature: A higher temperature also increases the diffusion rate significantly because it provides more kinetic energy to the particles. Lower temperatures decrease the diffusion rates because of the low energy of the particles.
- Surface area: A larger surface area also results in an increased diffusion rate.
Surface Area to Volume Ratio
As the volume of the cell increases, it needs more substances, for instance, oxygen for various cell reactions. As the surface area of the cell expands, the number of substances that diffuse in the cell increases. When the radius of the cell increases, then its surface area increases as the square of its radius and volume as the cube of its radius. It means that the volume increases much more quickly than surface area.
Hence, as the size of the cell increases, its surface area to volume ratio decreases. But the question arises that why cells are so small? Well, the answer is that when the cell is larger, there is more surface area available for diffusion. When the size of the cell increases, the volume also increases and hence the demand for nutrients. The increase in the size of the cell has a greater impact on the cell's volume than its surface area. If the cell is large enough, then it means that substances cannot diffuse through the cell quickly.
Diffusion in Multi-cellular Organisms
Multicellular organisms, i.e. the organisms having multiple cells have developed organs such as lungs which have increased surface area for internal gas exchange. As there is a distance between lungs and other body cells, so a transport system like the circulatory system is also present in multi-cellular organisms to provide all cells with essential substances such as oxygen. In other words, we can say that multi-cellular organisms have specialized surfaces and organ systems to enable the exchange of materials. Besides lungs, small intestine, gills in fish, roots, and leaves in plants are all specialized for transporting materials.
In short, diffusion helps the living organisms to:
- meet much of their daily needs
- eliminate waste materials
- carry out gas exchange through lungs
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