Introduction to Membrane Transport - Online Tutor, Practice Problems & Exam Prep

Biological membranes are selectively permeable, allowing only specific molecules to cross. Molecules that can freely diffuse across membranes are typically small, uncharged, and nonpolar, such as oxygen and carbon dioxide. In contrast, larger, charged, or polar molecules, like sugars and proteins, require protein facilitation to cross. Membrane transport can be categorized into molecular transport, including passive and active transport, and bulk transport, which encompasses processes like endocytosis and exocytosis. Understanding these mechanisms is crucial for grasping cellular function and homeostasis.

Introduction to Membrane Transport

In this video, we're going to begin our introduction to membrane transport. It's important to know that biological membranes are actually semi-permeable, which really just means that biological membranes can act as barriers to essentially prevent diffusion of some molecules across the membrane. It's also important to note that the term semi-permeable is synonymous with the term selectively permeable. Selectively permeable and semi-permeable really just mean the same thing. Again, all it's saying is that membranes are really, really picky about what types of molecules they allow to cross the membrane. The term permeable means how penetrable something is, and the term selectively, of course, just means picky. It is going to select the specific types of molecules that are able to cross the membrane freely. The root semi means partial, and so semi-permeable just means partially penetrable. Really, all of this is just saying that biological membranes are selectively permeable or semi-permeable meaning that they're super picky about what types of molecules can cross and what types cannot cross.

If we take a look at our image down below, we'll be able to see an example of a selectively permeable biological membrane. Notice in this cartoon image here in the middle we're showing you our biological membrane, which we know is a semi-permeable membrane or a selectively permeable membrane. Again, all this means is that it's really picky about what it allows to cross the membrane. So, notice on the left-hand side we're showing you two molecules, a sodium ion and an oxygen molecule or an O₂ molecule. Notice that only the O₂ molecule in this cartoon is able to cross the membrane freely, and the sodium ion is not able to cross the membrane. This is because of the properties of these molecules, and we'll be able to talk more about what exact properties these molecules need to have in order to be able to cross the membrane freely in our next lesson video. But here in this cartoon, notice that the membrane here is saying, "Excuse me, do you have an appointment?" And when it's talking, it's talking about the sodium ion. The sodium ion does not have an appointment in this cartoon. So, he's saying this isn't fair, stupid barrier, I'm not able to get through. Whereas the oxygen molecule here is able to get through, and he's saying we have an appointment, let's go, and it's able to cross. This is just a silly cartoon, but hopefully, it drives home the point of how biological membranes are semi-permeable.

This concludes this video, and I'll see you all in our next one.

Which Molecules Freely Cross Membranes?

So from our last lesson video, we already know that biological membranes are semi-permeable or selectively permeable, which really just means that they're really picky about what types of molecules they allow across the membrane freely. In other words, being semi-permeable just means that it is capable of preventing the diffusion of some types of molecules across the membrane depending on the properties of those molecules. But here's a question for you: Which types of molecules are able to freely cross membranes and which types of molecules are not able to freely cross membranes? Well, that's exactly what we're going to focus on here in this video.

Some molecules can freely diffuse across a membrane without any facilitation from a protein whatsoever. The molecules that can freely diffuse across a membrane without any facilitation from a protein whatsoever are going to have very specific properties that include having a very small size, being uncharged with a neutral net charge of 0, and being nonpolar or hydrophobic. Again, these three features—being small in size, uncharged, and being nonpolar or hydrophobic—are all features that correspond with molecules that can freely diffuse across a membrane without any facilitation from a protein.

On the other hand, molecules that cannot freely diffuse across a membrane without any facilitation from a protein are going to have the opposite features. Instead of being really small, these molecules are going to be really large. Instead of being uncharged, these molecules are going to be charged with an overall net charge that is positive or negative. And instead of being nonpolar or hydrophobic, these molecules will be polar or hydrophilic.

If we take a look at our image down below, we'll be able to get a better understanding of which molecules are able to freely diffuse across the membrane and which ones cannot freely diffuse across the membrane without facilitation from a protein. In this image, note that in the middle, we're showing you our biological membrane. On this side of the membrane, this is representing the inside of our cell or the cytosol, the cytoplasm. And on the outside over here, of course, it's representing the outside of our cell.

Notice that we have these different categories of molecules, and we're looking at their ability to freely cross the membrane without facilitation from a protein. The first block of molecules that we have here are all molecules that are relatively small, uncharged with a neutral net charge of 0, and nonpolar or hydrophobic molecules. These molecules include oxygen gas, carbon dioxide gas, and nitrogen gas. Being small, uncharged, and nonpolar, you might recall are all tendencies that correspond with being able to freely diffuse across the membrane without facilitation from a protein. That means that these molecules will easily be able to cross this membrane, from the outside of the cell to the inside of the cell. Notice that we have this really big green thick arrow going across the membrane to represent that these molecules have no problem whatsoever freely crossing the membrane since they have all of the correct tendencies and correct properties that allow them to do so.

If we look at the next category of molecules, notice that we have mixed features that correspond with being able to freely diffuse and that correspond with not being able to freely diffuse across the membrane. Notice that we have molecules that are going to be small and uncharged, which correspond with features of molecules that can freely diffuse, but these molecules are also polar. Being polar or hydrophilic is a feature of not being able to freely diffuse across the membrane. These molecules include water, the steroid that you see right here, and glycerol. Notice over here this other category of molecules are molecules that are either charged or polar molecules. Being charged or polar are features of not being able to freely diffuse across the membrane without facilitation from a protein. These molecules here include molecules that are sugars, molecules that are charged ions, and amino acids like this one which again are going to have these charges.

Last but not least, this category of molecules here are large macromolecules. Recall that the root 'macro' means large, and again being large is going to be a feature that corresponds with not being able to freely diffuse across the membrane. These large macromolecules include substances like polypeptides or proteins, polysaccharides or long carbohydrates, as well as nucleic acids such as DNA or RNA. Notice here that we have this red arrow that is bouncing off the membrane, indicating that these large molecules cannot freely diffuse across the membrane without any facilitation.

This here concludes our brief lesson on which types of molecules can freely cross membranes and which ones cannot. Again, the molecules that can freely diffuse across a membrane without protein facilitation are going to be small, uncharged, and nonpolar hydrophobic, whereas the ones that cannot freely diffuse across the membrane without facilitation are going to be large, charged, and polar hydrophilic. We'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Which molecule most easily diffuses across a biological membrane's lipid bilayer, without help of a protein?

• A. H₂O
• B. O₂
• C. H₂PO₄^-
• D. Glucose
• E. Na⁺

Map of the Lesson on Membrane Transport

In this video, we're going to introduce our map of the lesson on membrane transport, which is down below right here. Notice that at the very top of this map, it is titled membrane transport, and membrane transport can really be broken up into 2 major categories, which is why we have these two branches coming off of the membrane transport. Membrane transport can be broken up into molecular transport, which is really just going to be the transport of relatively small molecules across the membrane, and then it can also be broken up into bulk transport, which is going to be the transport of relatively large molecules across the membrane.

Now, because this is our map of the lesson, you can use this image as a map or a table of contents to figure out what topics we're going to cover next and to figure out where you are within the lesson as you move forward in the course. We're going to be exploring the leftmost branches first. So we'll start off by exploring molecular transport, distinguishing passive transport, which is no energy, from active transport, which requires energy. And then we'll explore the leftmost branches first. So we'll talk about osmosis, simple diffusion, facilitated diffusion including carriers and transporters, and proteins and channels, and then after we explore the leftmost branches, we'll zoom back out and explore the right branches.

So we'll talk more about active transport, distinguishing between primary active transport and secondary active transport, and then after we explore these branches, of course, we'll zoom back out and explore the right branches, so we'll talk more about bulk transport of larger molecules, including endocytosis, such as phagocytosis - cellular eating, and pinocytosis - cellular drinking, and of course, another type of pinocytosis, which is receptor-mediated endocytosis. And then after we explore these, of course, we'll zoom back out and explore exocytosis.

That is really the plan that we have moving forward in our course. Again, you should be referring back to this image as we move forward to figure out what topics we're going to be covering next and to figure out where you are within the lesson. That being said, this here concludes our brief introduction to our map of the lesson on membrane transport, and I'll see you all in our next video to talk more about molecular transport and distinguish between passive transport and active transport. I'll see you all there.

According to the map above, what kind of large molecule transport is also called the process of cell 'eating'?

• A. Phagocytosis
• B. Receptor-mediated Endocytosis
• C. Exocytosis
• D. Pinocytosis
• E. Facilitated Diffusion

Here’s what students ask on this topic:

What does it mean for a biological membrane to be selectively permeable?

A biological membrane being selectively permeable means it allows certain molecules to pass through while blocking others. This selective permeability is crucial for maintaining cellular homeostasis. Small, uncharged, nonpolar molecules like oxygen (O₂) can diffuse freely across the membrane. In contrast, large, charged, or polar molecules require specific transport proteins to facilitate their movement. This selective nature ensures that essential molecules like nutrients enter the cell, while waste products and harmful substances are kept out.

Which types of molecules can freely cross a biological membrane?

Molecules that can freely cross a biological membrane are typically small, uncharged, and nonpolar. Examples include gases like oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). These molecules can diffuse across the membrane without the need for protein facilitation due to their size and lack of charge, which allows them to pass through the hydrophobic core of the lipid bilayer easily.

What is the difference between passive and active transport in membrane transport?

Passive transport involves the movement of molecules across a membrane without the use of cellular energy (ATP). It relies on the concentration gradient, moving substances from areas of high concentration to low concentration. Examples include simple diffusion, facilitated diffusion, and osmosis. Active transport, on the other hand, requires energy to move molecules against their concentration gradient, from low to high concentration. This process involves transport proteins like pumps and is essential for maintaining cellular functions such as ion balance.

What are the main types of bulk transport in cells?

Bulk transport in cells involves the movement of large molecules or particles across the membrane and includes two main types: endocytosis and exocytosis. Endocytosis is the process of taking substances into the cell and can be further divided into phagocytosis (cellular eating), pinocytosis (cellular drinking), and receptor-mediated endocytosis. Exocytosis is the process of expelling substances from the cell, often involving vesicles that fuse with the plasma membrane to release their contents outside the cell. Both processes are crucial for nutrient uptake, waste removal, and cell signaling.

How do charged or polar molecules cross the biological membrane?

Charged or polar molecules cannot freely diffuse across the hydrophobic core of the biological membrane. Instead, they require the assistance of specific transport proteins. These proteins can be channels or carriers that facilitate the movement of these molecules. Channels provide a passageway for ions and small polar molecules, while carriers undergo conformational changes to transport substances across the membrane. This facilitated diffusion allows essential molecules like glucose and ions to enter and exit the cell efficiently.