B iological membranes are known to serve as the physical barrier between the intracellular environment
In this article, we will explore the two types of transport mechanisms in biological membranes that, at first glance, may seem almost interchangeable, namely diffusion and osmosis . Also, we’ll see the main difference between diffusion and osmosis in detail.
What is Diffusion? Diffusion in action (Source: Wikimedia) Also called as passive transport, diffusion is a type of transport process that allows small lipid-soluble (
lipophilic ) molecules to enter and leave the cell easily. This becomes possible because usually, the
cellular membranes of living organisms are held by weak forces that allow these molecules to travel from the inside to the outside, and vice versa.
One of the main drivers that make possible diffusion in cells is the difference in the concentration on each side of a biological membrane. This is called the concentration gradient [1] or the chemical gradient. The movement of substances is from a region of high concentration (of that molecule or substance) to a region of relatively lower concentration. Diffusion does not require a semipermeable membrane. Apparently, there are two types of diffusion[2] : passive and facilitated .
Passive diffusion is a type of diffusion that requires no protein channels during transports across the semipermeable membrane. On the other hand, facilitated diffusion
Why is it Important? Diffusion affects almost every aspect of biology, making it one of organisms’ most important life processes. Living organisms use diffusion to facilitate the transport of substances within short distances. Aside from that, this process is very sustainable since no energy is required for it to happen.
Examples In living organisms, the process of diffusion is quite widespread and happens in almost all cells. The following are some of these examples:
1. Plants Many diffusion processes occur in plants, one of which is during gas exchange. Plants utilize diffusion to facilitate the entry of
carbon dioxide [3] into the leaves for
photosynthesis .
First, the carbon dioxide molecules diffuse inward through the various pores in the leaves called stomata . After that, air spaces inside the leaves become filled with water vapor. This process highly depends on various environmental factors such as light and humidity.
2. Animals Diffusion, too, occurs in animal cells in several ways–either to facilitate the entry of molecules into the cells or the exit of wastes out of them.
For instance, during digestion [4] , food molecules like amino acids Another example is the diffusion of waste products like urea from the body cells to the blood.
What is Osmosis? The definition of osmosis can vary depending on two perspectives. For one, osmosis is the movement of water from a lower solute concentration to one with a higher one. In other words, it can also mean the movement of water from a region with a higher water concentration to a lower concentration.
Osmosis process over a semi-permeable membrane (blue dots represent particles driving the osmotic gradient) – Source: Wikimedia It is important to note that osmosis only occurs across semipermeable membranes[5] and is highly dependent upon the reduction of free energy of the solvents involved. Furthermore, it is also dependent on other factors like the solute potential, and the number of particles of solutes dissolved. The main force regulating such process is called the osmotic pressure [6] . Osmotic pressure is needed to stop the water flow across a membrane to separate the solutions with varying concentrations.
Check out the Osmosis simulation here .
Why is it Important? Like diffusion, osmosis is highly important in living organisms as it facilitates the distribution of needed compounds and the exit of waste products out of cells. Without this process to regulate everything, substances essential for life enter or leave cells aimlessly, leading to potential biological failure.
Examples There are many examples of osmotic processes in biological systems. The following are some of the examples of osmotic processes occurring in plant and animal cells
1. Plants The osmosis process is essential for vascular plants and is perhaps one of the main drivers for their evolution to land.
The roots of these plants first absorb water and minerals from the soil, and the osmosis process allows them to move up the plant through the transport tissue xylem. Another osmotic process in plants is the transpiration of water molecules through their evaporation from the leaves.
2. Animals For instance, one way is to concentrate the urine in humans. This will be initiated by the movement of water from the blood filtrate resulting in the formation of urine across the
epithelial cell layer in the tubules (in the kidney) and ultimately into the blood.
If this process does not occur by any chance, an average person will have to urinate several liters daily.
Main Difference Between Diffusion and Osmosis Features Diffusion Osmosis Direction of Movement Solutes move from a region of higher solute concentration to a lower concentration Water molecules move from a region of higher water concentration to a lower one Type of Molecules Involved Solutes like ions, or any solid, liquid, or gas molecules Water molecules only Type of Membrane Requirement Any type Semipermeable membrane Major Energy Requirement Free energy of diffusing substance Free energy chemical potential of water Main Drivers to Occur Difference in concentration gradient Osmotic pressure Duration Occurs faster than osmosis Relatively lower than diffusion
Other Differences As mentioned earlier, both diffusion and osmosis[7] occur through passive transport. No additional energy is needed to drive them; they involve the movement of particles from a higher concentration to a lower one. Aside from those said earlier, one of the major differences is the type of medium where these processes happen. On the one hand, diffusion happens in any medium, be it solid, liquid, or gas, whereas osmosis can only happen in a liquid environment. Moreover, diffusion aims to attain equilibrium in the energy concentration in the biological system. Osmosis, on the other hand, requires the equilibrium of the concentration of solute among the intra-cellular environment and its outside environment.
To have a detailed illustration, you may want to view this video:
Now, after learning about their unique properties, can you now easily differentiate the two?
Cite This Page
[1] – Diffusion and Transport Across Cell Membranes . Accessed August 28, 2017. PDF Link .[2] – “What Are the Two Main Types of Diffusion & Osmosis? | Education – Seattle PI” . Accessed August 28, 2017. Link .[3] – “1.1.3 – CO2 diffusion to chloroplasts | Plants in Action” . Accessed August 28, 2017. Link .[4] – “BBC – Standard Grade Bitesize Biology – Cells and diffusion: Revision, Page 3” . Accessed August 28, 2017. Link .[5] – “Difference between Diffusion and Osmosis | Major Differences” . Accessed August 28, 2017. Link .[6] – “Osmosis, Water Channels, and the Regulation of Cell Volume – Molecular Cell Biology – NCBI Bookshelf” . Accessed August 28, 2017. Link .[7] – “What Is the Difference Between Osmosis and Diffusion?” . Accessed August 28, 2017. Link .
