Introduction to Membrane Transport: Videos & Practice Problems

Biological membranes are selectively permeable, allowing only certain molecules to cross. Small, uncharged, nonpolar molecules can diffuse freely, while large, charged, or polar molecules require protein facilitation. Membrane transport is categorized into molecular transport, including passive processes like diffusion and osmosis, and bulk transport, which involves 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. Biological membranes are semi-permeable, and what this means is that they can act as barriers to essentially prevent diffusion of molecules. Now semi-permeable, or in other words, selectively permeable are terms that are synonyms to one another. Selectively permeable and semipermeable are synonyms. And, really, what this means, selectively permeable or semipermeable, it just means that biological membranes, they're really picky about what they allow to cross the membrane.

Permeable is really a word that just refers to how penetrable something is, and selectively just means that it's picky. It's going to select what is allowed to cross and what isn't allowed to cross. And semi is basically a word that means partial, and so semi-permeable means partially penetrable. Again, just meaning that it's really picky about what it allows to cross the membrane. So let's take a look at our example image down below at the selectively permeable biological membranes.

Notice here in the middle what we have is our biological membrane, which is a semi-permeable biological membrane. Notice that some molecules like this yellow molecule here are able to get through the membrane really easily, but not all molecules can get through the membrane. Some molecules are not able to cross, and so notice these molecules are like a stupid barrier. They cannot get through, and the membrane is, again, acting as a barrier. It's essentially saying, excuse me.

Do you, red guys over here, have an appointment to get across the membrane? And so essentially this whole cartoon right here is just supposed to represent that membranes act as biological barriers to prevent the diffusion of some molecules. And so these red molecules are not able to diffuse across from an area of high concentration to an area of low concentration. But other molecules like this yellow one over here who's laughing at the red ones that can't get through, they can get through the membrane, and so that's something important to keep in mind. And so in terms of what molecules are allowed to cross the membrane and what molecules are not allowed to cross the membrane, we'll get to talk about that in our next lesson video.

So I'll see you all there.

Which Molecules Freely Cross Membranes?

So from our last lesson video, we know that biological membranes are selectively permeable or semi-permeable, meaning that they act as barriers to prevent the diffusion of some molecules. But which molecules freely cross membranes and which molecules do not freely cross membranes? Well, that's exactly what we're going to talk about in this video. Now some molecules can freely diffuse across a membrane without any facilitation from proteins whatsoever. And so the molecules that can freely diffuse across the membrane without any facilitation from a protein whatsoever are molecules that are really really small in size, molecules that are uncharged or have a neutral charge, and molecules that are nonpolar or hydrophobic.

Whereas, molecules that cannot freely cross or freely diffuse across a membrane without facilitation from a protein are molecules that are really really large, molecules that are charged with either a positive or a negative charge, and molecules that are polar or hydrophilic. And so once again, molecules with these features here cannot freely diffuse across the membrane without facilitation from a protein. But molecules that have these features at the top, they can freely diffuse across a membrane without facilitation. So let's take a look at this image down below to get a better understanding of this idea of the diffusion of molecules across the membrane. Notice over here in the top left what we are showing you are small uncharged nonpolar molecules and notice that they have all three of these features.

They're small, they're uncharged, and they're nonpolar or hydrophobic molecules. And so when the molecules have all of these features, these molecules are able to cross the membrane freely without any facilitation whatsoever, and that's why we have a big thick green arrow here to show that these molecules have no problems getting across the membrane. And so this includes molecules such as gases like oxygen gas, carbon dioxide gas, and nitrogen gas as well. All of these molecules are capable of diffusing across the membrane with no problem whatsoever. Now notice over here we have some molecules that have a little bit of some mixed features here.

They are small and uncharged, but notice that they are polar or at least they have some polar components. Polar is a component of features that cannot freely diffuse across, and so because these molecules have a little bit of polar notice that they can still get across the membrane, but they don't get across the membrane as easily as these other molecules over here, and that's why we have a thin skinny green arrow to show that they still get across but not as easily as the ones that have all of the features that can freely diffuse. And so this includes molecules like water, a type of steroid, and a molecule called glycerol. These molecules are small and uncharged, but yet they are polar. And so they can get through the membrane, but not as easily as these other ones over here.

Now notice over here in this box, we're showing you molecules that are charged or polar, and these are features of molecules that cannot freely diffuse across the membrane. So notice that these molecules cannot diffuse across the membrane. They cannot get through the membrane without any facilitation. Notice that this last category of molecules down below are large macromolecules. Macro is a prefix that means large.

So these are really, really large molecules, like large polypeptides or large proteins, as well as large polysaccharides and nucleic acids. These molecules, because they are large, features of, molecules that cannot freely diffuse across the membrane. So once again, notice that these molecules are not able to diffuse across the membrane, and so they cannot get through. And so, really, this here concludes our introduction to which molecules can freely cross the membranes and which molecules cannot freely cross the membrane. And once again, this little table here summarizes what you should know.

The molecules that can freely diffuse are going to be very very small, uncharged, nonpolar, and hydrophobic; whereas the molecules that cannot freely diffuse across the membrane will be large, charged, either positively or negatively charged, or polar or hydrophilic. So we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you guys 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⁺

Answer: (B)

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 at the very top of this map, we have the words "membrane transport." Transport and membrane transport can really be grouped into two large categories, which is why we have two branches coming off of "membrane transport": one for molecular transport and one for bulk transport. Molecular transport is for the transportation of very small molecules, whereas bulk transport is for the transport of very large molecules.

Now, because this image here is a map of the lesson, that means that we can use this image like a map to make predictions about what topics we're going to cover next. We're going to be exploring this map by following the leftmost branches first. We're going to start off our lesson by talking about molecular transport, focusing on passive transport, then talking about osmosis, simple diffusion, and facilitated diffusion, including carriers and transporters, and pores and channels. Once we've explored the leftmost branches to their end, we'll zoom back out and focus more on the other branches. We'll talk about active transport, distinguishing primary from secondary active transport.

Then, once we've explored these leftmost branches, we'll zoom back out and start to explore bulk transport or the transport of very large molecules, which includes endocytosis like phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis. Once we've explored these, we'll end the lesson by talking about exocytosis. This is how the map can be utilized. Moving forward throughout our course, you should be referring back to this map so that you can make predictions about what topics we're covering next and also where we are within the map. This here concludes our introduction to the map of a lesson on membrane transport, and we'll be able to get started with our lesson by talking about molecular transport.

Essentially talking about passive transport and distinguishing it from active transport. So, I'll see you guys in our next video.

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

Answer: (A)

Here’s what students ask on this topic:

What is the difference between selectively permeable and semi-permeable membranes?

Selectively permeable and semi-permeable membranes are terms that are often used interchangeably. Both refer to biological membranes that allow certain molecules to pass through while blocking others. The term 'selectively permeable' emphasizes the membrane's ability to 'select' which molecules can cross, based on factors like size, charge, and polarity. 'Semi-permeable' highlights that the membrane is only partially penetrable, meaning it allows some substances to pass while preventing others. In essence, both terms describe the same property of biological membranes, which is crucial for maintaining cellular homeostasis by regulating the internal environment.

Which molecules can freely diffuse across a biological membrane?

Molecules that can freely diffuse across a biological membrane are typically small, uncharged, and nonpolar. Examples include gases like oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). These molecules do not require any facilitation from proteins to cross the membrane. Their small size and lack of charge allow them to move easily through the lipid bilayer, which is hydrophobic in nature. This type of diffusion is a passive process, meaning it does not require energy input from the cell.

What are the main types of membrane transport?

Membrane transport can be categorized into two main types: molecular transport and bulk transport. Molecular transport involves the movement of small molecules and can be further divided into passive transport (e.g., simple diffusion, osmosis, and facilitated diffusion) and active transport (e.g., primary and secondary active transport). Bulk transport involves the movement of large molecules and includes processes like endocytosis (phagocytosis, pinocytosis, and receptor-mediated endocytosis) and exocytosis. Understanding these mechanisms is essential for grasping how cells maintain homeostasis and perform various functions.

What is the role of proteins in membrane transport?

Proteins play a crucial role in membrane transport, especially for molecules that cannot freely diffuse across the membrane. These proteins can be classified into two main types: carrier proteins and channel proteins. Carrier proteins bind to specific molecules and undergo conformational changes to transport them across the membrane. Channel proteins form pores that allow specific ions or molecules to pass through. Both types of proteins facilitate the movement of large, charged, or polar molecules, enabling the cell to regulate its internal environment effectively.

How does osmosis differ from simple diffusion?

Osmosis is a specific type of passive transport that involves the movement of water molecules across a selectively permeable membrane. It occurs from an area of low solute concentration to an area of high solute concentration, aiming to equalize solute concentrations on both sides of the membrane. Simple diffusion, on the other hand, involves the movement of solute molecules from an area of high concentration to an area of low concentration, without the need for a membrane. Both processes are passive and do not require energy, but osmosis specifically pertains to water movement.