Diffusion is the physical process of the natural movement of ions or molecules. It occurs in both liquids and gasses and is important to all living organisms for many different reasons.
First of which is survival. Diffusion makes the nervous system function, promotes cellular respiration, and allows nutrients to be distributed among cells, among many others.
We may not be aware of it, but diffusion is ever-present in our lives. Get to know more about this molecular process and how we benefit from it every day.
Main Types of Diffusion
This type of diffusion occurs when molecules move across a semipermeable membrane, without the protein channels helping the movement take place. Cell membranes are made out of a bilayer made of phospholipid, which has a middle layer that is nonpolar, or water-fearing. The middle layer is protected on both sides by water-loving, or polar hydrophilic surfaces.
Hydrophobic molecules always pass easily through a cell membrane, and this includes certain gases such as carbon dioxide, oxygen, and nitrogen. Electrically charged, large polar molecules are unable to pass freely through a cell membrane.
When diffusion occurs across a cell membrane, this is considered a type of passive transport, and it requires no energy. Keep in mind that the cell membrane is a phospholipid bilayer and that both the inside and outside of the cell are water-based.
There is also a hydrophobic area in the middle, which is a vital barrier to anything that is charged and large, or hydrophilic. Simple diffusion allows the molecules that are hydrophobic to pass through the cell membrane, much like the hydrophobic region.
Because of this, simple diffusion provides unassisted passage of hydrophobic, nonpolar molecules that are quite small, and the passage starts at a higher concentration and goes to a lower concentration. Even if the cells are hydrophilic, tiny molecules can still slip through the membrane of the cell, simply because of their small size.
Facilitated diffusion allows for the flow of molecules down a concentration gradient and across the cell’s membrane, but the process requires help from a protein. Two categories of proteins exist that help this type of diffusion.
The first are carrier proteins, which can be thought of like a taxi cab in a cell membrane because they shuffle the molecules from one side of the membrane to the other side.
The second is channel proteins, which resemble tunnels and create a hole across the cell membrane. In these cases, channels open which allow the molecules to flow through them. Although facilitated diffusion involves proteins, the proteins involved require no use of the energy molecule known as ATP.
Molecules such as oxygen and carbon dioxide can diffuse across the plasma membrane directly, while other molecules need assistance to cross the hydrophobic core.
In facilitated diffusion, the molecules diffuse across the plasma membrane with help from the proteins of the membrane, including proteins such as carriers and channels. Because a concentration gradient exists for those molecules, they have the potential to diffuse either out of or into the cell itself by moving down through it.
Because they are polar or charged, however, they are unable to cross the phospholipid part of the membrane without a little assistance. In other words, the proteins that are part of the facilitated transport actually protect the molecules from the hydrophobic core of that membrane, which provides a route that they can cross.
In summary, the two major classifications of proteins for facilitated transport are carrier proteins and channels.
Some experts list three types of diffusion instead of two: simple, channel, and facilitated. In these descriptions, channel diffusion is considered a passive process that involves the ions and charged particles moving through a specific channel protein or pore in the wall of the cell.
In this case, no limits to the number of particles that travel through each of those channels exist. The proteins are actually embedded in the cell membrane, and they can open and close, which allows the compounds or molecules to go into or out of the cell. This type of diffusion is regulated very easily by the proteins of the membrane.
Details to Help You Understand Diffusion
The Difference Between Diffusion and Osmosis
Osmosis is a specific type of diffusion that consists of water crossing a semipermeable membrane and going into an area that has greater solute concentration. By contrast, standard diffusion often lets both solutes and solvents move freely towards equilibrium.
As a general rule, diffusion involves the net movement of molecules in a solution with a higher concentration to an area of a lower concentration. Osmotic pressure has the potential to be very powerful, and it is the only force that is required to move water from the very lowest roots to the tops of the tallest trees anywhere in the world.
If water dissolves any other substance, it is usually because the water’s molecules, which are polar, and the solute are actually attracted to one another. Each ion, atom, or molecule of solute has a charge in a minimum of one region which attracts one or another side of the water molecule.
Molecules are in random and constant motion around one another when they’re in the liquid water, but because of this attraction, the water molecules are a tiny bit more likely to move towards water molecules, rather than other solvent particles.
This is a net movement that gradually results in a more even distribution of the solute. In the process of osmosis, only the water molecules can move and equalize concentrations.
Osmosis and diffusion are, in fact, related but are not the exact same thing. With diffusion, molecules move from a high-concentration area to a low-concentration area, whereas osmosis is simply the diffusion of water across any type of semipermeable membrane.
Both processes have to do with molecules moving down a concentration gradient; however, the term “osmosis” refers to the movement of water molecules specifically, while diffusion can also involve any other type of molecules.
Diffusion and osmosis are both spontaneous in nature, which simply means that they occur without the assistance of any type of outside energy. “Osmosis” essentially refers only to the movement of water which is in a liquid state, but the term “diffusion” can also mean the movement of molecules either in a gaseous or a liquid state.
An example includes when carbon dioxide gas gets released into the center of a room, and it subsequently diffuses throughout that room until the carbon dioxide concentration is even and uniform across the entire room.
The term “osmosis” is frequently used in sciences such as physiology and cell biology. If the cell is placed in a hypertonic solution or a solution with a higher concentration of solute than that which is found inside of the cell, it results in water moving out of the cell spontaneously – or osmosis.
In both of these cases, it is easy to say that the diffusion of water actually occurred across the membrane of the cell.
Examples of Types of Diffusion
Some examples of different diffusion types include:
- When the tea is dispersed in hot water
- When smoke from a lit cigarette is spread around in the air
- When bubbles of carbon dioxide diffuse from an opened soda bottle and cause the soda itself to become flat as the carbonation is lost
- When food is digested, which is caused by oxygen being transferred into the blood from the lungs and into the muscles from the cells of the blood
- When a woman is pregnant and food and oxygen travel from the mother’s body into the fetus
- When a hot liquid such as coffee is being poured, and the cup is heated because the heat actually diffuses
- When you place a sugar cube into a liquid and it dissolves, because it is actually diffusing through the liquid and sweetening it evenly, even if you aren’t stirring it
- When you deflate a helium balloon because it deflates when it loses helium slowly from the balloon
- When you smell perfume or cologne on another human being because the perfume or cologne is diffusing into the air around it
- When you water your plants, the water is diffused into them and prevents the leaves from wilting; in other words, the carbon dioxide is diffused from the leaves from air pockets located between the various mesophyll cells and transfers it to the chloroplast
- When warm waters around the equator are diffused by ocean currents, and if you’re in an area where the air that is moist and warm rises within the air that is cold, that water vapor can turn into rainstorms
Examples of passive, or simple diffusion include:
- Carbon dioxide. Carbon dioxide is very small, and the molecule can be dissolved in water. One example of this is the burning sensation that you get when you hold your breath, which results in a strong desire to breathe. This happens because carbon dioxide accumulates in the nerve tissues of your bloodstream, which are very sensitive, as well as in your brain and lungs. Once you start to breathe again, carbon dioxide starts to diffuse out of your system. Moreover, many gases can do this because your lungs diffuse gases such as nitrogen, oxygen, and many others during processes such as this one.
- Bacteria. Bacteria are simple organisms that have no way to intake nutrients unless they diffuse them across the cell membrane. Bacteria do not use facilitated diffusion to transport most of these nutrients, but instead, they rely on simple diffusion to deliver water, oxygen, and small nutrients to the cytoplasm. There are no specialized organelles with their cells to transport or hold substances, and therefore the bacteria rely on the simple diffusion of the various materials contained in their cells to guarantee that those materials are present for the reactions that control all of their life processes.
- Oxygen. The eighth element of the periodic table, it is a good example of a molecule that is transported using passive diffusion. When combined with glucose to produce carbon dioxide, oxygen plays a very important role in the respiration process, which uses passive diffusion to complete.
- Steroid hormones. This is a great example of simple diffusion. Steroid hormones are similar to cholesterol and they can move freely across different membranes if it is down their concentration gradient. Steroids will bind to a hormone-binding site that prevents a certain binding complex from attaching to a specific DNA binding domain, which results in the DNA binding site being exposed, triggering the transcription-activating domain and altering gene transcription.
NOTE: To learn about active transport, check out our article “7 Different Types of Active Transport”
One example of facilitated diffusion is when K+ ions are passed through a membrane and they are aided by a potassium transport protein, as well as glucose and amino acids that are passed with the assistance of proteins known as permeases.
Retinol-binding protein can act as a water-soluble carrier for fatty acids and retinol. Carrier proteins are molecule specific and therefore, the diffusion rate is always limited by the number of carriers. When all carriers for a particular molecule are occupied, the so-called saturation point is reached.
Substances that utilize facilitated diffusion include:
- The skin of a cell. The plasma membrane is a very thin layer that maintains the cell’s integrity and encloses the cell itself, mostly by containing cellular fluid, or cytoplasm, and organelles, which are specialized structures. The plasma membrane regulates all of the substances that enter or leave the cell’s interior. Cells move molecules through the cell through a variety of methods, including two general categories – active transport and passive transport. Cells have to expend energy in order to accomplish active transport, whereas passive transport requires no cellular energy. Passive transport includes the activity of facilitated diffusion.
- The facilitation of glucose. Glucose is a sugar molecule and is the main energy source for many types of cells. Outside of a cell, a person’s bloodstream is constantly supplying glucose, while cellular metabolism continuously consumes glucose on the inside of the cell. What results is a concentration of glucose located outside of the cell, which stays at a higher concentration level inside of the cell. Nevertheless, the glucose molecule is too big to get through the plasma membrane without assistance. Because this is so, the cell provides glucose specific carrier proteins that actually stick to glucose molecules, allowing them to enter the cell.
- Molecules flow from high to low. Since some molecules, during the diffusion process, are unable to enter or exit a cell while being influenced by a concentration gradient because they are incompatible with the cell’s plasma membrane, facilitated diffusion can assist some of these molecules when they’re trying to pass through the plasma membrane. They do this by binding the molecules to special carrier proteins or by the opening of channels located between the cell and the surrounding environment.
- Ion channels. Carrier proteins being used for facilitated diffusion is quite common, and some examples include galactose and fructose, which are monosaccharides just like glucose is; amino acids, which make up a protein’s building blocks; and nucleosides, what is required for the synthesis of RNA and DNA to occur. There is a different type of facilitated diffusion that involves channel proteins. These proteins don’t bind to molecules, but instead, they open a channel which allows smaller molecules and ions – such as potassium, calcium, sodium, and chlorine – to transport quickly.
- General reasons for facilitated diffusion. There are a few polar, very large molecules that are lipid-insoluble or electrically charged which require help in order to diffuse across the membrane of the plasma. In these instances, facilitated diffusion which uses ion channels or carrier proteins allow these important molecules – for example, glucose – to actually cross the membrane.
In channel diffusion, the materials pass through one end, and the ion channels open and let them pass through if solutes are also present. It is a diffusion that is carried out by a protein channel, and therefore it is also called a channel-mediated diffusion. A lot of molecules have to have help moving across the cell membrane, thanks to their polarity and size.
The proteins that are embedded in the membrane act as a gateway for diffusion. Molecules will move down a concentration gradient via a protein pore that is open, which is a process called channel diffusion. There are many practical examples of channel diffusion available, and there are many websites that go into great detail on this topic.
Glossary of Terms Related to Diffusion
Active Transport: Active transport is a process that requires energy and in which there is a movement of ions or molecules that goes against a concentration gradient; it goes from an area of low concentration to one that has a high concentration.
Active Transport Occurrences: Active transport can only occur in a living tissue where there is energy from the ATP present. Active transport allows cells to take up ions or nutrients even with low concentrations of substances outside of the cell; allows the cells to get rid of substances that are unwanted when the concentration is greater outside of the cell; in plant cells, the phloem is actively loaded with sucrose; enables the Na/K pump to work with ions which move in opposite directions; enables a Ca pump to work at neuromuscular junctions in order for the muscles to contract; and in the kidney, useful substances such as ions, are reabsorbed against the concentration gradient.
Cell-Mediated Endocytosis: This occurs when molecules bind to special receptors in the membrane and are then taken into the cell via a vesicle that is created by the plasma membrane itself.
Crenation: When red blood cells dehydrate.
Dialysis: A process whereby the blood is filtered in order to remove toxins.
Diffusion: The movement of a molecule from its higher to its lower concentration, until it reaches equilibrium.
Endocytosis: This refers to the bulk importation of various substances into the cells.
Endocytosis Process: During this process, molecules are actually too big to fit through the membranes.
Endocytosis Types: The three main types of endocytosis are phagocytosis, pinocytosis, and cell-mediated endocytosis.
Exocytosis: This refers to the bulk secretion of large molecules from the cells.
Exocytosis Process: This process occurs when molecules are packed into Golgi vesicles or lysosomes by the Golgi and the vesicles then move into the cell surface membrane, with help from macro tubules and ATP. They then fuse with the plasma membrane and release the contents to the exterior.
Facilitated Diffusion: In facilitated diffusion, the molecules pass through either channel or carrier proteins located in the membrane, and the movement of the molecules is facilitated by a protein. In addition, the molecules are moving down their concentration gradient in facilitated diffusion, not up the gradient.
Facilitated Diffusion Differences: The two main differences are: with channeled proteins, there is no opening and closing, and there is no binding site; when it comes to carrier proteins, they do open and close, and there is always a binding site.
Facilitated Diffusion Types
There are two types of facilitated diffusion – carrier proteins and channeled proteins
Hemolysis: Refers to a situation whereby a red blood cell takes in the maximum amount of water and therefore, it bursts.
Hydrostatic Pressure: Liquid pressure.
Hypertonic: Low water potential.
Incipient Plasmolysis: When half, or 50% of cells, are considered slightly plasmolyzed.
Isotonic: Equal on both sides.
Molecules That Pass Through Facilitated Diffusion: Only large polar molecules pass through facilitated diffusion, including amino acids, glucose, and nucleotides.
Molecules That Pass Through Simple Diffusion: Only small molecules can pass through simple diffusions, such as H2O, O2, and CO2.
Osmosis: Osmosis is the passive movement of water molecules from an area with a low solute concentration to one that has a high solute concentration through a membrane that is semipermeable.
Phagocytosis: Also known as cell eating, phagocytosis refers to the process whereby large molecules are taken into a phagocytic vacuole where they are then digested. The vacuole is called a phagosome.
Pinocytosis: Pinocytosis refers to the process whereby molecules are taken into a vesicle that has been formed by the membrane of the plasma; also referred to as cell drinking.
Plasmolysis: Plasmolysis describes when plant cells are exposed to an outward osmotic flow of water, which results in the shrinking of the cytoplasm, shrinking it away from a living cell wall.
Simple Diffusion: In simple diffusion, very small molecules have the capability to move through gaps that are located between the various phospholipid molecules in the cell membranes.
Turgid: This is when a plant cell has taken in the maximum amount of water.
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