What is the movement of particles from an area of higher concentration to an area of lower concentration of particles?

A water solution that contains nutrients, wastes, gases, salts and other substances surrounds cells. This is the external environment of a cell. The cell’s outer surface of the plasma membrane is in contact with this external environment, while the inner surface is in contact with the cytoplasm. Thus, the plasma membrane controls what enters and leaves the cell.

The membrane permits the passage of some materials, but not all. The cell membrane is said to be selectively permeable. Small molecules, for example, may pass through the membrane. If no energy is required for substances to pass through the membrane, the process is called passive transport. We will discuss two examples of passive transport in this tutorial: diffusion and osmosis.

Diffusion
Although you may not know what diffusion is, you have experienced the process. Can you remember walking into the front door of your home and smelling a pleasant aroma coming from the kitchen? It was diffusion of molecules from the kitchen to the front door of the house that allowed you to detect the odors.

Diffusion is defined as the net movement of molecules from an area of greater concentration to an area of lesser concentration.

The molecules in a gas, a liquid or a solid are in constant motion due to their kinetic energy. Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas. Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the blood to the body tissues.

This spread of particles through random motion from an area of high concentration to an area of lower concentration is known as diffusion. This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of diffusion. In living systems, diffusion is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.

Figure \(\PageIndex{1}\). (CC BY-NC-SA)

Osmosis

Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. 

Semi-permeable membranes are very thin layers of material which allow some things to pass through them, but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.

Figure \(\PageIndex{2}\). (CC BY-NC-SA)

Example: If there was a semi-permeable membrane with more water molecules on one side as there were on the other, water molecules would flow from the side with a high concentration of water to the side with the lower concentration of water. This would continue until the concentration of water on both sides of the membrane were equal (dynamic equilibrium is established).

Figure \(\PageIndex{3}\). (CC BY-NC-SA)

Osmotic Pressure
Adding sugars to water will result in a decrease in the water concentration because the sugar molecules displace the water molecules. 

Figure \(\PageIndex{4}\). osmotic pressure (CC BY-NC-SA; LadyOfHats)

If the two containers are connected, but separated by a semi-permeable membrane, water molecules would flow from the area of high water concentration (the solution that does not contain any sugar) to the area of lower water concentration (the solution that contains sugar). 

Figure \(\PageIndex{5}\). osmotic pressure (CC BY-NC-SA; LadyOfHats)

This movement of water would continue until the water concentration on both sides of the membrane is equal, and will result in a change in volume of the two sides. The side that contains sugar will end up with a larger volume.

Figure \(\PageIndex{6}\). osmotic pressure (CC BY-NC-SA; LadyOfHats)

Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution. Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and sugar would be characterized as a sugar solution.

The classic example used to demonstrate osmosis and osmotic pressure is to immerse red blood cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution.

1. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation the cell is in an isotonic solution (iso = equal or the same as normal). A red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell.

2. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). In this situation, a red blood will appear to shrink as the water flows out of the cell and into the surrounding environment.

3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). A red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell.

Figure \(\PageIndex{4}\). (CC BY-NC-SA)

Figure \(\PageIndex{4}\). (CC BY-NC-SA)

Diffusion n., plural: diffusions [dɪˈfjuːʒən]

Definition: net passive movement of molecules from region of higher concentration to region of lower concentration

Diffusion is the net passive movement of molecules or particles from regions of higher to regions of lower concentration. For diffusion to occur there must be a concentration gradient. The dissimilarity in the amounts of solutes, particles, or molecules between the two regions will cause them to move between the two regions. To define diffusion in the biological context, imagine the ions, respiratory gases, glucose, or other particles that move randomly between the crowded and the less-crowded regions (fluids), often across a semi-permeable membrane. The unequal concentration of the particles between two fluids will generate a gradient that will incite them to move in order to equalize the disparity in concentrations. Nevertheless, the movement of solutes during the diffusion process is not a one-way route. There could be movements to and fro. However, the movement is inclined towards the area of lower concentration. Thus, diffusion is characterized by a net movement of the particle down their concentration gradient — or in other words, from the area of greater concentration to an area of lower concentration. The net movement of oxygen and carbon dioxide across the alveolar-capillary membrane of mammalian lungs and the net movement of glucose down the concentration gradient are diffusion examples in biological systems.

Diffusion is the passive movement of molecules or particles from an area of higher concentration to an area of lower concentration. The concentration gradient incites them to diffuse. Since the movement is downhill, there is no chemical energy expenditure (as opposed to active transport that uses chemical energy). The transport of molecules across a biological membrane without the aid of membrane proteins is called simple diffusion.

The word diffusion comes from the Latin diffusionem, diffusio, meaning “a pouring forth”.

Diffusion in Physics, Chemistry, and Biology

Diffusion in physics is the movement of particles from an area of higher concentration to an area of lower concentration as driven by thermal energy. (1) This definition is affirmed as well in chemistry. The particles suspended in liquids and gases, for instance, struck each other resulting in their random constant motion. This motion due to the collisions of particles is called pedesis or the Brownian movement. When these particles increase in number, they become compacted and the Brownian movement is seemingly lost as what can be observed in a concentrated solution. When provided with an augmented space they will start to move but in a rather organized manner from an area of high concentration to an area of low concentration, following Fick’s laws.(2)

The factors that affect diffusion rates are temperature, concentration, distance, and material.(3) In particular, a hot temperature causes the particles to acquire more kinetic energy (i.e. the energy possessed by the particles in motion), and therefore will move and push other particles more. Thus, the diffusion of particles becomes faster when the temperature is higher. Conversely, it is slower at a lower temperature. As for particle concentration, the greater the number of particles in a solution, the rate of diffusion will also be faster. As for the distance, the shorter the distance for the diffusing particles to travel, the faster the rate will be. The kind of material in the solution also affects the rate of diffusion. Smaller and lighter particles can spread more easily than larger and heavier ones. Thus, gaseous particles diffuse faster than the particles in liquids or solids. Liquids, in turn, diffuse quicker than solids. Diffusion in chemistry is s exemplified by food dye dropped in an aqueous solution. The dye particles can be seen moving as they spread out throughout the solution. Another example is when spraying perfume the scent will be picked up as it diffuses throughout the room.

In physics and chemistry, diffusion is basically defined as the “spreading out” of the objects from the initial area of higher concentration. Diffusion in biology applies the same principle but the process involves a biological system, such as a semipermeable membrane. Furthermore, it is defined as a net movement of particles or molecules. In biological systems, it is a type of passive transport. Passive transport refers to the type of cellular transport wherein the net movement of substances is down the concentration gradient. In contrast, cellular transport wherein substances have to be moved to an area that is already saturated or high in concentration is called active transport. Because the movement of substances in passive transport does not go against the concentration gradient it, therefore, does not require chemical energy (e.g. ATP) to proceed. Rather, it is driven by kinetic and natural energy. Other examples of passive transport are filtration and osmosis.

Types of Diffusion

Some molecules such as polar and large molecules cannot readily pass across a biological membrane. The plasma membrane, for instance, transports these molecules into and out of the cell by using membrane proteins. This type of transport is called facilitated diffusion.

Diffusion is of two major types. The first one does not require assistance when moving down the concentration gradient. This type is called simple diffusion. In contrast, facilitated diffusion, as the name implies, is one in which assistance is required. The assistance comes, for instance, from the proteins embedded in a biological membrane. Basically, these two types of diffusion differ in the mechanism by which substances move – one that occurs without assistance and the other one that occurs with the help of transport proteins. Thus, in facilitated diffusion, the transport only occurs when the molecule is able to bind to the membrane protein transporter. Both of them result in the net downhill movement of substances and do not require chemical energy to proceed.

Diffusion vs. Active Transport

While diffusion is the movement of particles down their concentration gradient, active transport is the movement of particles against the concentration gradient. Since the movement is characteristically uphill this type of transport requires energy often in the form of adenosine triphosphate (ATP).

Diffusion and Osmosis

Diffusion and osmosis are both types of passive transport. Thus, both of them occur in a downhill manner and without energy expenditure. The difference is the diffusing molecules or particles. In diffusion, the diffusing particles are the solutes of a solution. In osmosis, the diffusing particles are the solvent of the solution, i.e. water molecules. In osmosis, water molecules diffuse from an area of high water concentration to an area of low water concentration across a biological membrane. Water that is drawn towards a concentrated solution but does not involve a biological membrane is not osmosis.

Diffusion in Cells

The cell regulates the entry and exit of substances through its plasma membrane. Not all molecules can readily pass across this selective membrane due to its structure. The lipid bilayer feature of the plasma membrane prevents the passage of polar molecules. Nevertheless, small nonpolar molecules and ions can pass through the lipid bilayer. The way through which polar molecules can pass is via the proteins in the plasma membrane. The transmembrane integral proteins are membrane proteins involved in the transport of substances into and out of the cell. (4, 5)

In this table, the mechanism employed by certain types of molecules and ions in order to pass across the plasma membrane down their concentration gradient is shown.

Diffusion in Plants and Animals

In plants and animals, diffusion is exemplified by the diffusion of gasses. In plants, the openings formed by the guard cells called stomata are where carbon dioxide enters and oxygen leaves the plant. The plant takes in carbon dioxide since it is one of the major reactants in photosynthesis. Oxygen, in turn, is a byproduct of photosynthesis and is then released as it diffuses into the environment through the stomata.

Similarly, in animals, respiratory gases are transported generally by simple diffusion. In humans, the diffusion of respiratory gases takes place at the capillary beds that separate blood from the tissue fluid. In the lungs, carbon dioxide is released by diffusing from the blood into the alveoli and then breathed out. Oxygen, in turn, is breathed in, and then diffuses from the alveoli of the lungs into the blood. The oxygen then diffuses from the circulating blood to the various tissues of the body.

Try to answer the quiz below to check what you have learned so far about diffusion.

References

1. Lin, Y.C., Phua, S. C., Lin, B., & Inoue, T. (2013). Visualizing molecular diffusion through passive permeability barriers in cells: conventional and novel approaches. Current Opinion in Chemical Biology, 17(4), 663–671. https://doi.org/10.1016/j.cbpa.2013.04.027
2. Lecture 3: Diffusion: Fick’s first law. (n.d.). Retrieved from https://my.eng.utah.edu/~lzang/images/lecture-3.pdf
3. Vinzant, A. (2017, September 7). DIY: Diffusion Science Experiment – Astrocamp School. Retrieved from Astrocamp School website: https://astrocampschool.org/diy-diffusion/
4. Pratt, C. A., Voet, D., Voet, J. G. (2002). Fundamentals of biochemistry upgrade. New York: Wiley. pp. 264–266.
5. TRANSPORT IN AND OUT OF CELLS. (2019). Retrieved from Estrellamountain.edu website: https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBooktransp.html

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