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Factors affecting the rate of diffusion

Learn about and revise transport into and out of cells with this BBC Bitesize Combined Science AQA Synergy study guide.

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Transport into and out of cells

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Factors affecting the rate of diffusion

If a crystal of a coloured chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles.

The potassium manganate(VII) is the solute.

The water is the solvent.

The potassium manganate(VII) has dissolved.

The mixture that results is the solution.

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The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

The rate of diffusion

The rate of diffusion can be affected by a number of factors:

The concentration gradient

The greater the difference in concentration, the quicker the rate of diffusion.

The temperature

The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.

The surface area of the cell membrane separating the different regions

The greater the surface area, the faster the rate of diffusion.

Diffusion, surface area and volume

For a bacterium, substances diffuse into and out of the bacterial cell across its surface. Once inside, because of the bacterium's size, substances will need to diffuse 1 μm or less to where they are needed, for example, for respiration.

For simple multicellularorganisms, such as small plants like mosses, substances diffuse into the leaves and roots over their surface. Again, once inside the plant, they don't need to move far.

Substances move into and around the moss plants by diffusion and osmosis.

Simple organisms take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. For example, the surface area to volume ratio of a puppy is several times greater than that of an adult dog.

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Suggest why puppies are more at risk of losing body heat than adult dogs.

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What are the factors affecting the rate of diffusion?

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What are the factors affecting the rate of diffusion?

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Diffusion is the passive movement of substances from a region of higher concentration to a region of lower concentration. Diffusion of substances plays an important role in cellular transport in plants. The rate of diffusion is affected by the concentration gradient, membrane permeability, temperature, and pressure. Diffusion takes place as long as there is a difference between the concentrations of a substance across a barrier. However, diffusion stops, when the concentrations of the substance on either side of the barrier become equal. The permeability of a membrane affects the rate of diffusion. Diffusion rate increases as membrane permeability increases. Changes in temperature and pressure values also affect the diffusion of substances. Pressure plays an important role in the diffusion of gases as gases diffuse from a region of higher partial pressure to a region of lower partial pressure.

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5.2C: Diffusion

Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.

5.2C: Diffusion

Last updated Aug 15, 2020

5.2B: Selective Permeability

5.2D: Facilitated transport

Boundless Boundless

Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.

Learning Objectives

Describe diffusion and the factors that affect how materials move across the cell membrane.

Key Points

Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients.

After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium.

Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.

Key Terms

diffusion: The passive movement of a solute across a permeable membraneconcentration gradient: A concentration gradient is present when a membrane separates two different concentrations of molecules.

Diffusion

Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, or spread away, from the bottle; gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell ‘s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated.

Figure 5.2C.1 5.2C.1

: Diffusion: Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm).

Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium.

Factors That Affect Diffusion

Molecules move constantly in a random manner at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion:

Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.

Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules.

Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion.

Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials. An example of this is a person experiencing dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells.

Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.

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