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

Biological membranes are selectively permeable, allowing only certain molecules to cross. Small, uncharged, nonpolar molecules can freely diffuse, while large, charged, or polar molecules require protein facilitation. Membrane transport is categorized into molecular transport, involving small molecules, and bulk transport for larger molecules. Key processes include passive transport, which requires no energy, and active transport, which does. Understanding these mechanisms is crucial for grasping cellular function and interactions within biological systems.

Introduction to Membrane Transport

In this video, we're going to begin our introduction to membrane transport. Biological membranes are semi-permeable. This means that they can act as barriers to essentially prevent the diffusion of molecules. Now semi-permeable, or in other words, selectively permeable, are terms that are synonymous with one another. Selectively permeable and semi-permeable are synonyms. And really what this means, selectively permeable or semi-permeable, it just means that biological membranes are really, 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. So semi-permeable means partially penetrable. Again, just meaning that it's really picky about what it allows to cross the membrane.

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 stubborn barrier; they cannot get through, and the membrane is, again, acting as a barrier. It's asking, "Excuse me, do you red guys over here have an appointment to get across the membrane?" Essentially, this whole cartoon right here is just supposed to represent that membranes act as biological barriers to prevent the diffusion of some molecules. 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.

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. Some molecules can freely diffuse across a membrane without any facilitation from proteins whatsoever. The molecules that can freely diffuse across the membrane without any facilitation from a protein whatsoever are molecules that are 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. Once again, molecules with these features right 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.

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, non-polar molecules. They have all three of the features: They're small, they're uncharged, and they're nonpolar or hydrophobic molecules. When the molecules have all of the 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. This includes molecules such as gases like oxygen gas and carbon dioxide gas as well as nitrogen gas, 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 bit of mixed features. 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. Because these molecules have a 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. This includes molecules like water and a type of steroid, and a molecule called glycerol. These molecules are small and uncharged but are polar. Hence, they can get through the membrane but not as easily as the other ones over here.

Now notice over here in this box, we're showing you molecules that are charged or polar, and so those are features of molecules that cannot freely diffuse across the membrane. Notice that these molecules cannot diffuse across the membrane. They cannot get through the membrane without any facilitation. And notice that this last category of molecules down below are large macromolecules. 'Macro' is a prefix that means large. These are really large molecules like large polypeptides or large proteins as well as large polysaccharides and nucleic acids. Because they are large, they are features of molecules that cannot freely diffuse across the membrane. Once again, notice that these molecules are not able to diffuse across the membrane, and so they cannot get through.

This concludes our introduction to which molecules can freely cross the membranes and which molecules cannot freely cross the membrane. This little table here summarizes it, what you should know. The molecules that can freely diffuse are going to be very small, uncharged, nonpolar, and hydrophobic, charged either positive or negative, or polar or hydrophilic. We'll be able to get some practice applying these concepts as we move forward in our course, so 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⁺

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.

We'll talk about osmosis, simple diffusion, facilitated diffusion, including carriers, transporters, proteins, and channels. After we explore the leftmost branches, we'll zoom back out and explore the right branches. We'll talk more about active transport, distinguishing between primary active transport and secondary active transport. 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. 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 the internal environment of a cell. Small, uncharged, nonpolar molecules can freely diffuse across the membrane, while large, charged, or polar molecules require assistance from proteins to cross. This selective nature ensures that essential molecules like oxygen and carbon dioxide can enter and exit the cell easily, while harmful substances or unnecessary molecules are kept out. This property is vital for cellular function and homeostasis.

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 free diffusion is essential for processes like respiration and cellular metabolism.

What is the difference between passive and active transport across a membrane?

Passive transport is the movement of molecules across a membrane without the use of energy. It relies on the concentration gradient, moving molecules from an area of high concentration to low concentration. Examples include simple diffusion, osmosis, and facilitated diffusion. Active transport, on the other hand, requires energy, usually in the form of ATP, to move molecules against their concentration gradient, from low to high concentration. Examples include primary active transport, like the sodium-potassium pump, and secondary active transport, which uses the energy from the movement of one molecule to drive the movement of another.

What are the main types of bulk transport in cells?

Bulk transport in cells involves the movement of large molecules or particles and can be categorized into endocytosis and exocytosis. Endocytosis is the process of taking in large molecules by engulfing them in a vesicle. It includes phagocytosis (cellular eating), pinocytosis (cellular drinking), and receptor-mediated endocytosis. Exocytosis is the process of expelling large molecules from the cell by vesicles fusing with the plasma membrane. These processes are essential 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 biological membrane due to their hydrophilic nature. They require the assistance of membrane proteins to cross. This can occur through facilitated diffusion, where molecules move down their concentration gradient via channel proteins or carrier proteins, or through active transport, where molecules are moved against their concentration gradient using energy. Examples include ion channels for ions like Na⁺ and K⁺, and transporters like the glucose transporter.