The Difference Between Passive Diffusion & Active Transport: With Examples

If you have ever opened an oven and noticed how the smell from your cooking fills the whole room, then you have already experienced the principle of diffusion first hand. 

Passive diffusion and active transport are two important processes that govern the movement of chemicals and substances through a given medium. 

Be it odor causing particles traveling through air, or ions traveling through water, either diffusion or active transport is likely to be taking place.

While both processes are very similar in many respects, there are crucial differences that set them apart. 

You can find examples of active transport and diffusion happening around us every day, both in the outside world and within our own bodies.

Understanding them is vital to understanding even the most basic biological processes, from digestion to the production of energy during respiration. 

In this article, we will list some basic examples that highlight the different kinds of diffusion and active transport. We will also provide a full breakdown of the similarities and differences between these two processes.

Passive Diffusion 

Passive diffusion is defined as the movement of particles along a concentration gradient. Particles involved in this process will naturally move from an area of high concentration to an area of low concentration. 

Since the substance in question is moving to equally distribute itself through the given medium, diffusion does not require any energy to take place. 

Think of a water tank with a dividing wall in the middle. One side is full to the brim, while the other side is only one third full.

 When the divider is removed, the water from the full side will move into the side that is only one third full until the water is evenly distributed across both halves. 

The following sections contain some popular examples used to explain and understand the process of passive diffusion. 

Osmosis

Osmosis is a special kind of passive diffusion that specifically describes the movement of water along a concentration gradient. 

Just like in our example above, the water moves from an area of high concentration to an area of low concentration without the need for energy. 

This movement takes place across a semipermeable membrane, such as the plasma membrane of a cell. 

As such, osmosis is one of the primary methods by which cells absorb and release water, which is essential for respiration and many other chemical processes. 

However, water is not the only factor to consider when determining the direction of osmosis. Pure water has very little osmotic potential and is unlikely to move without the presence of ions. 

Ions dissolved in water will reduce its osmotic potential. If these ions cannot travel across the semipermeable membrane, then they will force water to move from the area of high osmotic potential to the area of low potential. 

As such, the water will move from areas where there are very few dissolved ions to areas that are more rich in solutes. 

Due to the above, osmosis governs not just the movement of water in and out of the cell, but also the transport of dissolved ions and other substances. 

These substances may be waste products that the cell wishes to expel, or essential nutrients for carrying out biochemical reactions. 

Osmosis In Animals

A great example of the principle of osmosis in animals can be seen in the kidneys. Here, water can be seen moving from the tubules in the kidneys and the gastrointestinal tract into the bloodstream. 

This allows animals to retain water and prevent too much of it being lost in the production of urine. 

This process requires no energy, nor does it rely on the movement of ions or proteins to make it occur. 

However, in many cases, osmosis will be reliant on a strong difference in the concentration of dissolved ions to force it through certain membranes made of a phospholipid bi-layer. 

These membranes have a strong hydrophobic zone that would prevent water freely flowing through them without a strong osmotic gradient. 

Osmosis In Plants

For another good example of osmosis, look no further than the stomatal guard cells found in the epidermis of most plant species. 

The stomata are the pores of the plant, commonly found on the underside of the leaves, and control the movement of water vapor in and out of the plant (also known as transpiration). 

These pores are surrounded by a pair of stomatal guard cells, which are specialized cells with large vacuoles for storing water. 

When the plant is low on water, it creates an osmotic gradient that forces water out of the vacuoles. The guard cells become flaccid, closing the stomata and conserving water. 

When water is abundant, the opposite process occurs, and water rushes into the vacuoles along the osmotic gradient. 

This causes the guard cells to become turgid, opening the stomata so that transpiration can resume. 

Simple Diffusion

Osmosis is a special type of diffusion that refers exclusively to the movement of water and dissolved solutes. 

Simple diffusion on the other hand applies to the movement of any particle from an area of high concentration to an area of low concentration. 

A classic example of simple diffusion is the movement of lipophilic molecules across the cell membrane. 

Examples of lipophilic substances include steroid hormones, fats and other substances that don’t have a polarity. 

This method is used to allow drugs to enter our bodies, by binding them to lipophilic compounds. 

As long as there is a concentration gradient, these particles will flow across the cell membrane until they are evenly distributed. 

As well as the concentration gradient, other factors that determine the speed of diffusion include the ionization state of the particle in question. 

Ionized substances are less soluble, which means they will diffuse more slowly. Larger molecules will also take longer to diffuse across the membrane, or may not be able to diffuse at all if they are too big.  

Facilitated diffusion is another type of passive diffusion, since it doesn’t require energy to take place. However, it does require transporter proteins in the cell membrane to allow substances in and out of the cell. 

Simple Diffusion In Respiration

One of the clearest examples of simple diffusion is respiration. Oxygen molecules in the blood flow into the cell along the concentration gradient. 

Here they are used to generating energy, producing carbon dioxide in the process. The carbon dioxide is then excreted from the cell back into the blood, where it is carried to the lungs. 

Here, the CO₂ moves across the semipermeable membrane of the alveoli, so it can enter the lungs and be expelled when we exhale. 

This is just one of hundreds of examples of simple diffusion occurring in our bodies, as well as the bodies of all plants and animals. 

Facilitated Diffusion In Glucose Absorption

Glucose is another substance that is essential for respiration and the production of energy. However, molecules of glucose are far too large to pass through the cell membrane by themselves. 

As such, transporter proteins in the membrane are required to remove glucose from the blood and carry it into the cell via facilitated diffusion. 

These transported proteins will often have a specific shape that allows them to only bind with glucose or other specific substances.

This means they can carry larger molecules into the cell across a concentration gradient without also letting in harmful or unwanted particles. 

Facilitated Diffusion In Ion Transport

Ions are polar molecules by nature and as such cannot move across plasma membranes with a similar polarity. For this to happen, they need to move across polarized transmembrane proteins called ion channels. 

The channels are tunnels that move through the membrane, allowing ions that would normally not be able to pass through to enter and exit the cell. 

They are highly specific, with different channels for all the various types of ion that the cell needs. As such, they allow for rapid diffusion along a concentration gradient. 

Facilitate Diffusion Of Oxygen Molecules

Oxygen is carried in our blood by attaching to a special protein called Hemoglobin. However, in our muscles it needs to be transported by a different carrier protein called Myoglobin.  

As such, these two proteins work together to facilitate the diffusion of oxygen from our blood into our muscles, where it is used for respiration. 

A similar mechanism is used for the transport of carbon dioxide and carbon monoxide back out of our muscles and into the blood. 

Active Transport

Active transport is the opposite of diffusion, in that it is the movement of molecules against a concentration gradient. 

This means that the particles move from an area of low concentration to an area of higher concentration. Since this is working against diffusion, it requires energy to force the substance in question to move against the concentration gradient. 

There are two main types of active transport that are differentiated by the source of energy they use for moving the particles. 

Primary active transport uses energy generated from the breakdown of adenosine triphosphate (ATP) which is the main source of all the energy our bodies use. 

Secondary active transport on the other hand uses electrochemical energy to move ions against a concentration gradient. Much like facilitated diffusion, active transport makes use of specific transporter proteins to force particles into areas of higher concentration. 

The Active Transport Of Calcium Ions Out Of The Cell

Scientists have observed that the concentration of calcium ions is roughly 1000 times greater outside the cells than it is inside them. 

This indicates that calcium ions are being actively transported out of the cells against the natural concentration gradient. 

When calcium ions react with ATP, they will form calcium phosphate crystals that are lethal to the cell in high concentrations.

This is the primary reason that calcium ions need to be actively removed from the cell, to prevent the build up of these harmful crystals. 

When calcium ions enter the cell via their ion channels, they pass through two gates. The extracellular gate is on the outside of the cell membrane, while the cytosolic gate is located on the inside.

This is the same for when they are forced back out of the cell by calcium pump proteins. 

When enough calcium ions enter the cell, it causes changes within the cell that trigger the release of ATP. 

The excess ions bind to the cytosolic gate on a calcium pump protein, causing it to open. The pump protein has an affinity for calcium ions, which draws them into the channel protein. 

Adenosine triphosphate (ATP) is then converted into adenosine diphosphate (ADP) which releases energy. A single phosphate group produced by this reaction binds to the cytosolic gate, causing it to close. 

The extracellular gate then opens, and the energy produced by the breakdown of ATP is used to pump calcium ions out of the cell against the concentration gradient. 

Since the energy required for this process comes from the breakdown of ATP, this is an example of primary active transport.

The pump protein has no affinity for calcium ions coming from the extracellular fluid, meaning they cannot re-enter the cell by diffusion except through their ion channels. 

These channels will only open when the cell needs to take in calcium ions, meaning that the cell only takes in what it needs without any unwanted excess. 

Active Transport For Vesicular Transport

Large molecules are often carried around the body and cells inside packages called vesicles. These vesicles are often made from a similar phospholipid bi-layer to the plasma membrane of the cell. 

The large molecules contained in vesicles are too big to pass through the cell membrane on their own.

As such, vesicles merge with the cell membrane to carry these molecules into and out of the cell by active transport. 

Movement of macromolecules outside of the cell is called exocytosis, while movement into the cell is referred to as endocytosis.

By these two principles, cells can absorb and expel molecules that would normally be too big to pass through their outer membrane. 

Scientists have found that endocytosis and exocytosis can not occur as efficiently in the absence of ATP. This has led to the conclusion that both processes work by active transport rather than diffusion. 

Active Transport Of Glucose In The Small Intestine

Earlier, we covered the facilitated diffusion of glucose molecules from the bloodstream into the cells. However, active transport is required to absorb glucose into the blood in the first place. 

When carbohydrates enter our small intestine, they have already been broken down into their composite monosaccharides by our stomach acid.

As such, there is a higher concentration of glucose in the small intestine than there is in the blood contained in the capillaries of the villi. 

Therefore, glucose can diffuse into the bloodstream, until there is a high enough concentration of it in the epithelial cells of the small intestine to eliminate the concentration gradient. 

To prevent any glucose being wasted, the remaining molecules are then pumped into these cells by active transport. 

For this to happen, sodium ions are first pumped out of the epithelial cells by primary active transport. Sodium-potassium pumps use ATP to accomplish this.

This creates a higher concentration of sodium ions in the small intestine than there is in the blood. 

Which sets up a concentration gradient. The sodium ions then diffuse back into epithelial cells through sodium-glucose channels, which means that they can carry the glucose molecules with them. 

Since the energy required to carry glucose against the concentration gradient is generated by the diffusion of the sodium ions and not ATP, this is an example of secondary active transport. 

The glucose can then diffuse into the blood through glucose transporter proteins via facilitated diffusion. 

Active Transport In The Root Hair Cells Of Plants

Plants absorb water into their roots via osmosis, but in order to do this they need to use active transport as well. The root hair cells contain a far higher concentration of mineral ions than the water outside of the roots. 

This is maintained by active transport, which carries these ions against the concentration gradient into the root hair cells. 

The reason for this is that the higher concentration of solutes in the root hair cell creates a much lower osmotic potential. 

As such, the active transport of mineral ions allows the root hair cells to maintain an osmotic gradient, so that water can be constantly absorbed into the roots. 

This is a great example of how active transport and diffusion can occur at the same time, with one process often being dependent on the other. 

The Differences Between Active Transport And Diffusion

Both active transport and diffusion involve the movement of molecules across a semipermeable membrane. However, while they are similar in many respects, they are very different from each other. 

In this section, we will take you through some crucial distinctions between these two processes. 

The Concentration Gradient

Both passive and facilitated diffusion are completely dependent on the presence of a concentration gradient. 

This means there needs to be an area of high concentration and an area of lower concentration. Diffusion occurs down this gradient, while active transport works against it. 

Sometimes active transport will be needed to create this gradient in the first place, so that diffusion can naturally occur. 

However, while the gradient is important, there are other factors that may limit diffusion. Large molecules, or those that are polarized, will struggle to move across the cell membrane as quickly as smaller, non-polar molecules. 

Facilitate diffusion can allow some of these particles to move across the membrane through transporter proteins. However, there are other instances where these molecules will need to be actively transported regardless of the concentration gradient. 

The Need For Energy

Diffusion will occur naturally, provided there is a difference in concentration for the particles to follow. Since it is the concentration gradient itself that drives this process, it does not require any energy to take place. 

Active transport on the other hand requires energy to take place, since it is working against the concentration gradient. Without this energy, the particles would have no impetus to move from an area of low concentration to an area of higher concentration. 

The energy for this process often comes from the breakdown of ATP into ADP. The spare phosphate, produced by this reaction, is used alongside the energy to open protein pumps. 

These pumps earn their name, since they force molecules against the concentration gradient either into or out of the cell. 

However, sometimes the energy for this process comes from the electrochemical movement of other particles, like diffusion. We saw this in the movement of glucose molecules from the small intestine into the bloodstream. 

This is why secondary active transport is often called cotransport, since one molecule is carrying another molecule into or out of the cell. Hence, the diffusion of one substance facilitates the active transport of another. 

Often, primary active transport is required to set up the concentration gradient that allows secondary active transport to take place. 

As we saw in the small intestine, the sodium ions were first pumped out of the epithelial cells of the small intestine by active transport. 

This established a gradient that allowed the sodium ions to diffuse back into the cells, while actively transporting molecules of glucose with them. 

Direction of transport

Diffusion can happen in any direction and as the concentration of various molecules change, it will often fluctuate from moving one way to the other. Even facilitated diffusion that uses special transporter proteins can occur in both directions.

This is not the case for active transport, which will only occur in one direction regardless of concentration gradients. This is because active transport is carried out by specific transporter proteins embedded in the cell membrane. 

Unlike the proteins for facilitated diffusion, these transporters are a one way route into or out of the cell. The reason for this is that if active transport were allowed to occur both ways, it would not be able to push molecules against the concentration gradient. 

Any substance pumped out would simply be able to diffuse back to its original area, hence the need for active transport to be a one way process. 

Size Of The Molecules Being Transported 

Diffusion is limited by several factors, as well as the concentration gradient. For one, polarized molecules will not be able to cross the phospholipid bilayer by themselves and will need ion channels to be able to do so. 

Furthermore, very large molecules such as glucose won’t fit between the gaps in the phospholipid bi-layer, which means they won’t be able to diffuse into or out of a cell without the help of a transporter protein. 

Active transport is generally used for larger molecules or those that are strongly polarized. This is done with transporter proteins that help the large molecules to pass through the cell membrane. 

It should be noted that this is not always the case, and there are transporter proteins that will allow a substance into or out of the cell down the concentration gradient by facilitated diffusion.

An Experiment To Demonstrate Diffusion

If you want to witness one type of passive diffusion, specifically osmosis, for yourself, then you can try out this simple experiment. 

First fill two glasses with water, preferably distilled water if you can. Add up to three tablespoons of salt to one of the glasses and stir until it has completely dissolved, leave the other glass pure. 

Now take a potato and cut it into small strips similar in size to french fries. Observe the color and flexibility of the potato pieces before placing them in the water. Put one or two pieces of potato in each glass and leave them overnight. 

You will notice that the potato left in salt water will have turned brown and is much more bendy than the potato left in pure water.

This is because the salt water creates an osmotic gradient that draws water out of the potato, causing it to turn brown and lose its structural integrity. 

The potato left in pure water, on the other hand, will be white and much stiffer, snapping when bent too far. This is because the potato will contain more ions than the pure water, thus causing the water to flow into it by osmosis. 

This is a simple yet effective experiment that will allow you to witness the effects of osmosis for yourself. 

Conclusion

Diffusion and active transport are two essential processes that govern many of the fundamental biological processes in our body. 

They are responsible for digestion of food, respiration, water retention and many, many more systems that allow our bodies to function the way they do. 

Hopefully, after reading this article, you now understand the difference between these two processes and how vital they are to all living things.

https://www.youtube.com/watch?v=U61e-dFpkuE