Diffusion plays an important role in many chemical and biological processes. Organisms undergo a constant and ongoing diffusion process in every part, component and area of their system.

Even as you read this, oxygen is diffusing out of your vascular system's capillaries and infusing your muscles. This is a passive diffusion; no other molecules help the oxygen saturate the tissue they diffuse into. Other diffusion functions are helped along by various molecules.

Like practically every other natural phenomenon, diffusion follows the laws of nature and physics. What those laws are and why they exist; how they're followed and the factors that impact diffusion is what your Superprof looks at now.

Diffusion Highlights
Diffusion is the net movement of molecules from a high-concentration area to one of low concentration.
Diffused material can be solid, liquid or gas
Materials may diffuse into a solid, liquid or gaseous environment.
Diffusion denotes molecule movement along a concentration gradient.
Diffusion rates depend on the interaction of the medium and the material it diffuses into.
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Diffusion Particulars

Diffusion describes a fairly broad scope of activity across all states of matter - gas, solid and liquid. Even plasma, sometimes considered matter's fourth state. Still, for all the variety of matter that may take part in diffusion, it boils down to a simple process.

Diffusion is the act of molecule movement from an area of highly concentrated particles to where particles are less concentrated.

Milk in hot coffee acts very different than milk in cold coffee
As you pour cold milk into hot coffee, you can see the milk particles rushing to areas of low particle concentration. Photo credit: Croydon Clicker on VisualHunt

Do you put milk in your tea or coffee? If you're not a coffee/tea consumer, have you ever watched a coffee or tea drinker doctor their beverage? If so, you may have noticed that the milk doesn't instantly turn the entire drink a lighter colour. At first, only the point where the milk was poured was white; it took some stirring to balance the mix properly.

Adding milk to coffee is a good example of diffusion, even if you had to help it along a bit by stirring.

Not all diffusions are instantaneous; that's why we have to consider the rate of diffusion. Two simple liquids, such as food colouring and water would diffuse into each other fairly quickly. Conversely, testing liquids that don't mix - petrol and water, for instance, we'd find that only the smallest contact layer of the two liquids diffuse into one another, and exceedingly slowly, at that.

One critical factor distinguishes diffusion from osmosis: the presence of a membrane to diffuse through. Osmosis is defined as diffusing molecules through a semi-permeable membrane; diffusion writ large means molecules moving along a concentration gradient with or without a barrier in place.

Nevertheless, osmosis is a form of diffusion; one discussed at length in a different article.

In this article's introduction, we touched on oxygen diffusing through capillaries and into our bodies' tissues. Delving deeper into that concept, we find that aerobic respiration plays a role in the diffusion process.

The carbon dioxide generated as a by-product of cellular respiration raises the concentration of those molecules within the cells. As the number of molecules increases, they are then pushed outward, toward the capillaries, where the force exerted by the flow of blood sweeps it out of the tissues, through the vascular system and into the lungs, to be expelled as we breathe.

In this example, we can see how broad the act of diffusion is in biological systems. Even as oxygen molecules are constantly entering the tissues, carbon dioxide molecules are perpetually moving out. It proves that one substance or process of diffusion may happen independently of all other diffusion processes that may be going on at the same time.

Over all, diffusion is vital to the proper functioning of cell structures...

Types of Diffusion

As diffusion is universal - a process that every type of matter takes in every state, and crucial for maintaining organisms, it should come as no surprise that there would be two methods of diffusion.

Simple diffusion represents molecules moving along a concentration gradient without input, influence or interference from other molecules or forces.

Earlier, we mentioned carbon dioxide diffusing into the vascular system for its journey to the lungs and ultimate expulsion. The carbon dioxide molecule is small enough to diffuse easily, without any help - what is called facilitated diffusion; the type we'll talk about in just a moment.

Osmosis falls into the category of simple diffusion. the water molecules passing through our cell walls require no additional help; they are small enough to fit through the pores in those barriers without any specialised proteins.

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Solutes travel through solvents to equalise the concentration of particles
In this image, we see solute dispersing through a solvent. Photo credit: Ian Hayhurst on VisualHunt.com

Facilitated Diffusion

While simple diffusion is essentially the laws of nature and physics at work, facilitated diffusion takes a little help to accomplish desired results. Another molecule lends its energy to move a larger or more polar molecule across the hydrophobic/lipid bilayer.

High-energy molecules like ATP aren't necessarily involved in every facilitated diffusion process because the diffusion process, whether facilitated or simple, is molecules moving along their concentration gradients. Still, ATP and its related molecule, GTP (guanosine triphosphate) may play an indirect role.

Because of the uncomplicated structure, bacteria have no other recourse than simple diffusion to take in oxygen and some nutrients. Facilitated diffusion is used to transport nutrients in bacteria because these organisms have no organelles; thus, no way to grab a hold of and transport food.

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Diffusion Functions

Within a liquid or gas, molecules and ions are constantly moving about, seeking to be evenly distributed within the solvent. Even though these particles ultimately attain a stable concentration throughout the medium, they will continue to move, and they will move in any direction.

What's all that movement for? Besides maintaining equilibrium,

  • in animals:
    • oxygen and carbon dioxide are diffused in and out of the blood
    • carbon dioxide is diffused out of the lungs and into the air
    • nutrients and oxygen are diffused into tissues
    • particles diffuse to provide cells with nutrients, water and oxygen, and to carry out waste products.
  • in plants:
    • carbon dioxide is diffused from the air into the plants' leaves
    • oxygen is diffused out of the plants' leave
    • nutrients are diffused throughout the plant transport system, facilitated by proteins

Photosynthesis, the process of creating glucose and oxygen from sunlight, relies on diffusion. The structure of plant cells - and animal cells, for that matter, are optimal for the type of gas exchange; one function of diffusion.

Furthermore, they are constructed in such a way as to allow the diffusion of larger molecules, particularly mineral and nutrient particles, although they are more likely to employ facilitated diffusion.

Factors that Impact Diffusion

Regardless of the solutes and solvents involved, diffusion is continuous. However, four factors that can change the rate and degree of diffusion.

Particle Size

Regardless of conditions, smaller particles will diffuse faster than larger ones. Additionally, smaller particles may not need 'helper' proteins to keep them moving. Larger molecules, particularly those with more complex shapes/surfaces and greater masses bend to the laws of physics. They will move more slowly along their concentration gradients than smaller, more streamlined particles.

For instance, an oxygen molecule, relatively simply constructed and light, will move faster than, say, a molecule of iodine gas.

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Particles will continue to search for low concentration areas
Depending on how saturated the solvent is, particles will continue to search for low concentration areas they can occupy. Photo credit: Ian Hayhurst on Visualhunt

Interaction Area

To make this point clear, let's recall our cup of coffee, to which we just added milk. The spot we poured the milk in is lighter and, depending on how much we poured in, waves of milk have diffused throughout the drink. If we do nothing else, the milk will eventually pool at the bottom of the cup because it is slightly denser than coffee.

Remember that particle size is another factor that affects diffusion; hence, the milk's limited distribution throughout the coffee. At this point, the area of interaction between particles is very small. We need to stir our drink to increase the interaction area.

Stirring drives the milk particles towards areas where there are fewer milk particles. After a couple of seconds of stirring, the milk particles have diffused throughout the darker beverage.

Increasing the interaction area by any means allows for a faster diffusion process.

Temperature

Higher temperatures cause the release of more kinetic energy. You can verify this physics law for yourself by trying the milk/coffee experiment. You'll need a hot cup of coffee or tea, and an identical one that is only tepid. Now, pour a pre-measured amount of milk into both cups and observe how the milk reacts.

In the hot cup, the milk will appear more lively; it will bounce from the bottom and create visible waves just below the liquid's surface. By contrast, the milk in the tepid cup of coffee will act sluggish, settling on the bottom and likely not creating any bouncing waves.

The greater the temperature difference, the more lively the lower-temp molecules become. They start zinging off of each other and away from the cold centre, launching themselves into the hotter environment.

This is the same principle that causes ice to melt faster on a hot day than a cooler one.

The Steepness of the Gradient

Diffusion is the phenomenon of particles moving from a high-concentration area to an area that has a lower concentration of particles. Thus, it stands to reason that, the lower the concentration of particles, the faster they will move to occupy that area. Conversely, the more saturated an area becomes, the slower those particles will move.

Remember that diffusion is a constant and ongoing process, and it is multidirectional. So, if an area becomes saturated, particles will continue to travel until they find an area where equilibrium has yet to be reached.

The net movement of molecules and the factors that affect diffusion have a great impact on cell biology - in all organisms. It's important to know how these interactions work and how they sustain life.

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