Simple and Facilitated Diffusion - Online Tutor, Practice Problems & Exam Prep

Simple and facilitated diffusion are forms of passive transport, requiring no energy as molecules move down their concentration gradients. Simple diffusion allows small uncharged molecules to pass directly through the phospholipid bilayer, while facilitated diffusion involves transport proteins, such as porins and carriers, to assist charged molecules. Porins create channels for ions and water, enhancing osmosis, whereas carriers undergo conformational changes to transport substances across the membrane. Both processes are essential for maintaining cellular homeostasis without energy expenditure.

Simple and Facilitated Diffusion

In this video, we're going to talk about simple and facilitated diffusion. Both simple diffusion and facilitated diffusion are types of passive transport, which means that the transport across the membrane occurs passively without any energy input. There's absolutely no energy involved with passive transport, and that means there's no energy involved with either simple or facilitated diffusion. Recall from our previous lesson videos that the reason passive transport requires no energy is because molecules are transported down their concentration gradients from areas of high concentration to areas of low concentration, and that's why no energy is required.

Simple diffusion, as its name implies, is quite simple. It involves the direct diffusion of small uncharged molecules directly through the cell membrane, squeezing their way between the phospholipids from one side of the membrane to the other. On the other hand, facilitated diffusion, which also involves no energy due to it being a form of passive transport, typically transports charged molecules. However, as its name implies, facilitated diffusion is facilitated by a transport protein. So, facilitated diffusion requires facilitation by a transport protein, whereas simple diffusion does not require any facilitation whatsoever.

Let's take a look at our image below to get a better understanding of this. Notice that on the left-hand side, we're showing you a snippet of our map here where you can see that passive transport, which requires absolutely no energy input, includes both simple diffusion and facilitated diffusion. Simple diffusion is straightforward and requires no facilitation from a protein, so no protein is involved with simple diffusion. Whereas facilitated diffusion does involve a transport protein, and the transport protein facilitates this diffusion.

Over here on this half of the image, we're showing you simple diffusion, which allows small uncharged molecules to simply diffuse directly through the membrane. These molecules are uncharged and small enough to squeeze their way between the phospholipids and get to the other side of the membrane from an area of high concentration to an area of low concentration. Because it's going from a high concentration to a low concentration down the concentration gradient, no energy is required for this type of transport, simple diffusion.

On the right-hand side of this image, notice that we're showing you facilitated diffusion. Facilitated diffusion also transports molecules down their concentration gradients from an area of high concentration, and no energy is needed. The main difference is that facilitated diffusion primarily transports ions or charged molecules, whereas simple diffusion occurs only with small uncharged molecules, not with ions. If ions want to get to the other side of the membrane, they need some facilitation by a transport protein. This protein, embedded in the membrane, creates a channel, basically a tunnel through the membrane that allows these ions, these charged molecules, to diffuse down their concentration gradients across the membrane. No energy is needed because the molecules move from a high concentration to a low concentration.

This concludes our introduction to simple and facilitated diffusion, and we'll learn more about these processes as we move forward in our course. I'll see you in our next video.

Which of the following processes includes all of the others?

• A. Osmosis
• B. Facilitated diffusion
• C. Passive transport
• D. Transport of an ion down its electrochemical gradient

The difference between simple and facilitated diffusion is that facilitated diffusion:

• A. Requires a protein transporter.
• B. Moves molecules against their concentration gradient.
• C. Requires energy.
• D. Freely diffuses molecules against their concentration gradient.

Transport Proteins of Facilitated Diffusion

So now that we know from our last lesson video that facilitated diffusion requires transport proteins, in this video, we're going to talk about some of the transport proteins of facilitated diffusion. And really there are 2 main types of transport proteins involved in facilitated diffusion. And we have those 2 numbered down below, number 1 and number 2. And so the first type of transport protein involved in facilitated diffusion are the porins, otherwise known as channels. And so as their name implies, porins channels, they're going to form an obvious membrane-spanning tunnel that will allow molecules to move across the membrane by traveling through the tunnel that these porins or channels create. And so porins implies the word pour and pour is like a hole or like a tunnel. And channels, once again, are basically like tunnels as well. Now aquaporins are specifically used to transport water molecules across or through the cell membrane, essentially facilitating osmosis so that osmosis, the diffusion of water across a semi-permeable membrane, can occur at a faster rate, thanks to the aquaporins.

Now, the second type of transport protein involved in facilitated diffusion are the transporters, otherwise known as carriers. Now the transporters or carriers, unlike the porins and channels, they do not form an obvious membrane-spanning tunnel. Instead, the transporters or carriers, they're only open on one side of the membrane at a time, and they must undergo conformational changes in order to move molecules across the membrane. So let's take a look at our image down below to get a better understanding of these ideas. So notice on the left-hand side over here what we're showing you are porins and channels. And so notice that porins and channels, they're going to create a membrane-spanning tunnel that is basically going to allow molecules to move through the membrane, through the tunnel, and cross to the other side. Now aquaporins are specifically porins that allow water molecules to diffuse across the membrane at faster rates, essentially allowing osmosis to occur faster. And you can see that channels, like ion channels like this, can allow ions that, have charges like a positive charge, for example, to move through the tunnel here and get to the other side of the membrane.

And so over here, what we see is that there are obvious membrane-spanning tunnels, and so there's an opening on both sides of the membrane. Now when we take a look at the right-hand side of the image over here, notice that we're showing you transporters or carrier proteins. And so, when you look at these, notice that it does not form an obvious membrane-spanning tunnel like these do over here. Instead, the transporters are only open to one side of the membrane at a time. And so in order to transport these molecules from one side of the membrane to the other side of the membrane, this transporter or carrier protein must undergo a conformational change. And so notice that over here, the transporter or carrier has undergone a conformational change that can allow the molecule on the inside to be released to the other side of the membrane. And so, what you'll notice is that, here, the transporter or carrier protein is only open to one side of the membrane, which is over here, and it's closed on this side of the membrane. And so, transporters or carriers are only going to be open to one side of the membrane at a time, and it's only after the conformational change occurs where they can change their shape and be open on the other side of the membrane. And so both of these are types of transport proteins involved in facilitated diffusion allowing molecules to be transported down their concentration gradients passively without any energy input. And so this here concludes our lesson on this, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Which of the following does not accurately describe a channel or a carrier?

• A. Channel – open to both sides of the membrane simultaneously.
• B. Carrier – open to one side of the membrane at a time.
• C. Carrier – requires a conformation change to complete function.
• D. Channel – not selective for molecules that move through it.

Which type(s) of molecules cannot enter/exit the cell via simple diffusion and require facilitated diffusion?

• A. Nonpolar oxygen gas molecules.
• B. Charged Ca²⁺ ions
• C. Nonpolar carbon dioxide gas molecules.
• D. Nonpolar water molecules
• E. Charged Na⁺ ions
• F. b and e only
• G. a, c, and d only

Here’s what students ask on this topic:

What is the difference between simple diffusion and facilitated diffusion?

Simple diffusion and facilitated diffusion are both forms of passive transport, meaning they do not require energy. Simple diffusion allows small, uncharged molecules to pass directly through the phospholipid bilayer of the cell membrane. In contrast, facilitated diffusion involves the use of transport proteins to help move charged molecules or larger substances across the membrane. These transport proteins can be porins, which create channels, or carriers, which undergo conformational changes to transport molecules. Both processes move substances down their concentration gradients, from areas of high concentration to low concentration.

How do transport proteins facilitate diffusion?

Transport proteins facilitate diffusion by providing a pathway for molecules that cannot easily pass through the cell membrane. There are two main types of transport proteins: porins and carriers. Porins, also known as channels, create membrane-spanning tunnels that allow ions and water molecules to move through the membrane. Carriers, on the other hand, do not form tunnels but undergo conformational changes to transport molecules. They are only open to one side of the membrane at a time and change shape to move the molecule to the other side. Both types of proteins help move substances down their concentration gradients without the need for energy.

Why does facilitated diffusion not require energy?

Facilitated diffusion does not require energy because it is a form of passive transport. In passive transport, molecules move down their concentration gradients, from areas of high concentration to areas of low concentration. This movement is driven by the natural kinetic energy of the molecules and does not require additional energy input from the cell. Transport proteins, such as porins and carriers, assist in this process by providing pathways for molecules that cannot easily pass through the cell membrane, but they do not use energy to do so.

What types of molecules are transported by simple diffusion?

Simple diffusion primarily transports small, uncharged molecules directly through the phospholipid bilayer of the cell membrane. Examples of such molecules include oxygen (O₂), carbon dioxide (CO₂), and small lipophilic molecules. These molecules are able to move freely across the membrane without the need for transport proteins because they are small enough to pass between the phospholipids and are not repelled by the hydrophobic core of the membrane.

What role do aquaporins play in facilitated diffusion?

Aquaporins are a specific type of porin that facilitate the transport of water molecules across the cell membrane. They create channels that allow water to move through the membrane more efficiently, enhancing the process of osmosis. Osmosis is the diffusion of water across a semi-permeable membrane, and aquaporins help this process occur at a faster rate by providing a dedicated pathway for water molecules. This is particularly important in cells that require rapid water transport to maintain homeostasis.