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

Simple diffusion and facilitated diffusion are both forms of passive transport, requiring no energy as molecules move from high to low concentration. Simple diffusion allows small uncharged molecules to pass directly through the membrane, while facilitated diffusion uses transport proteins to assist charged molecules. Two main types of transport proteins are porins (channels) that create tunnels for molecules, and transporters (carriers) that change shape to move substances across the membrane. Both processes are essential for maintaining cellular homeostasis and nutrient absorption.

Simple and Facilitated Diffusion

In this video, we're going to differentiate between simple and facilitated diffusion. Simple diffusion and facilitated diffusion are both types of passive transport, which you might recall from our previous lesson videos just means that the transport of molecules across the membrane occurs passively with no energy input, and so no energy is needed. Recall from our previous lesson videos that the reason no energy is needed for passive transport is that molecules are being transported across the membrane down or with their concentration gradients from areas of high concentration down to areas of low concentration, which is the natural tendency for molecules to diffuse. This will be true for both simple and facilitated diffusion.

Now, simple diffusion, as its name implies, involves the simple and direct diffusion of small uncharged molecules directly through the cell membrane without any facilitation from a membrane protein. On the other hand, facilitated diffusion is defined as non-energetic diffusion, and "non-energetic" just means that no energy is involved because it's a type of passive transport. Unlike simple diffusion, which transports uncharged molecules, facilitated diffusion typically transports charged molecules that either have a positive or a negative net charge. These charged molecules, which you might recall are unable to directly diffuse across the membrane, require facilitation by a transport protein. The biggest difference between facilitated diffusion and simple diffusion is that facilitated diffusion utilizes a transport protein to facilitate the transport, whereas simple diffusion does not utilize a transport protein.

If we take a look at our image down below, we can clear up some of these ideas and differentiate further between simple and facilitated diffusion. Notice on the left-hand side over here, we're showing you a little snippet of our map of the lesson on membrane transport, showing you how passive transport absolutely requires no input of energy. There are two types of passive transport discussed here, which includes simple diffusion and facilitated diffusion. Again, the biggest difference between simple and facilitated diffusion is that simple diffusion requires absolutely no facilitation from a membrane protein. In the example of simple diffusion shown in the image, small uncharged molecules are allowed to directly diffuse across the membrane without any facilitation from a membrane protein, always diffusing from an area of high concentration towards an area of low concentration across the membrane, making this a passive process that requires no energy input.

Now, on the right-hand side of the image, we're showing you an example of facilitated diffusion. With facilitated diffusion, what you'll notice is that it does indeed utilize the membrane protein embedded in the membrane. It still allows molecules to diffuse from high concentration towards an area of low concentration across the membrane. Typically, these molecules are going to be ions, or in other words, charged molecules. These charged molecules do not have a natural tendency to diffuse across the membrane without facilitation. They cannot get across the membrane unless there is facilitation by a membrane protein. Hence, facilitated diffusion allows for charged ions and molecules to diffuse across the membrane again from an area of high concentration to an area of low concentration. Still, no energy is needed, and it is still a passive process.

This concludes our brief lesson on simple and facilitated diffusion. We'll be able to get some practice applying these concepts and learn more about membrane transport as we move forward. I'll see you all 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. There are two main types of transport proteins involved in facilitated diffusion, which we have numbered down below: number 1 and number 2. The first type of transport protein involved in facilitated diffusion are the porins, otherwise known as channels. As their name implies, porins and 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. Porins imply the word "por," which is like a hole or a tunnel. And channels, once again, are basically like tunnels as well. Aquaporins are specifically used to transport water molecules across or through the cell membrane, essentially facilitating osmosis. Thus, osmosis, the diffusion of water across a semipermeable membrane, can occur at a faster rate, thanks to the aquaporins.

The second type of transport protein involved in facilitated diffusion are the transporters, otherwise known as carriers. Unlike the porins and channels, transporters or carriers do not form an obvious membrane-spanning tunnel. Instead, transporters or carriers are 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. Let's take a look at our image down below to get a better understanding of these ideas. On the left-hand side, we are showing porins and channels. Notice that porins and channels create a membrane-spanning tunnel that allows molecules to move through the membrane, through the tunnel, and cross to the other side. Aquaporins, specifically, allow water molecules to diffuse across the membrane at faster rates, essentially allowing osmosis to occur faster. You can see that channels, like ion channels, can allow ions with charges, like a positive charge, for example, to move through the tunnel here and get to the other side of the membrane. Over here, we see obvious membrane-spanning tunnels, and there is an opening on both sides of the membrane.

Now, looking at the right-hand side of the image, we see transporters or carrier proteins. These do not form an obvious membrane-spanning tunnel, like those on the left. Instead, transporters are only open to one side of the membrane at a time. In order to transport these molecules from one side of the membrane to the other side, this transporter or carrier protein must undergo a conformational change. Notice that over here, the transporter or carrier has undergone a conformational change that allows the molecule on the inside to be released to the other side of the membrane. Here, the transporter or carrier protein is only open to one side of the membrane, and it is closed on the other side of the membrane. Transporters or carriers are only open to one side of the membrane at a time, and it is only after a conformational change occurs that they can change their shape and be open on the other side of the membrane. 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. This concludes our lesson on this, and we will be able to get some practice applying these concepts as we move forward in our course. 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 types of passive transport, meaning they do not require energy. Simple diffusion allows small, uncharged molecules to pass directly through the cell membrane from an area of high concentration to an area of low concentration. In contrast, facilitated diffusion involves the use of transport proteins to help move charged molecules or larger molecules across the membrane. These transport proteins can be either porins (channels) or transporters (carriers). While simple diffusion does not involve any proteins, facilitated diffusion relies on these proteins to assist in the movement of molecules.

How do transport proteins function in facilitated diffusion?

Transport proteins in facilitated diffusion assist in moving molecules across the cell membrane without the use of energy. There are two main types of transport proteins: porins (channels) and transporters (carriers). Porins create a tunnel-like structure that spans the membrane, allowing molecules to pass through. Aquaporins, a type of porin, specifically facilitate the movement of water molecules. Transporters, on the other hand, do not form a continuous tunnel. Instead, they undergo conformational changes to move molecules from one side of the membrane to the other. These proteins are essential for transporting charged or larger molecules that cannot diffuse directly through the membrane.

Why is no energy required for simple and facilitated diffusion?

No energy is required for simple and facilitated diffusion because both processes involve the movement of molecules down their concentration gradient, from an area of high concentration to an area of low concentration. This movement is driven by the natural tendency of molecules to spread out and achieve equilibrium. In simple diffusion, small uncharged molecules pass directly through the membrane. In facilitated diffusion, transport proteins assist in moving charged or larger molecules across the membrane. Since the movement is along the concentration gradient, it does not require additional energy input.

What types of molecules are transported by simple diffusion?

Simple diffusion primarily transports small, uncharged molecules directly through the cell membrane. Examples of such molecules include oxygen (O₂), carbon dioxide (CO₂), and small lipophilic molecules. These molecules can easily pass through the lipid bilayer of the cell membrane without the need for transport proteins. The process occurs naturally as these molecules move from an area of high concentration to an area of low concentration, following their concentration gradient.

What role do aquaporins play in facilitated diffusion?

Aquaporins are a specific type of porin (channel) that facilitate the movement of water molecules across the cell membrane. They create a tunnel-like structure that allows water to pass through more efficiently than it would by simple diffusion alone. This process is essential for osmosis, the diffusion of water across a semipermeable membrane. By providing a dedicated pathway for water, aquaporins help maintain cellular homeostasis and regulate water balance within the cell, allowing osmosis to occur at a faster rate.