Microbiology, part 16: Prokaryotes & Eukaryotes - Membrane Transport

Hi, I'm Cathy with Level Up RN. In this video, I'm going to continue my review of key topics that you need to know for your microbiology class. Specifically, I'll be talking about membrane transport, including a discussion of passive versus active transport, osmosis, exocytosis, and endocytosis. At the end of the video, I'm going to give you guys a quiz to test your understanding of some of the key information I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards on membrane transport so you can follow along with me, and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to see show up on a micro exam.

There are two basic ways that substances can cross the plasma membrane of a cell. One is passive transport, and the other is active transport. With passive transport, the movement of molecules across the membrane does not require energy input. So ATP is not needed. Examples of passive transport mechanisms include simple diffusion, facilitated diffusion, and osmosis. Simple diffusion is where we have the movement of molecules down a concentration gradient. So we are going from an area of high concentration to an area of low concentration. So the molecules are like, "It's too crowded here on this side of the membrane, so I'm going to cross the membrane where there's more room." With facilitated diffusion, certain molecules, such as polar molecules and charged molecules, can't just cross the membrane on their own. They require assistance from a carrier protein or a channel protein to move across the membrane. However, it's important to note that facilitated diffusion is still a form of passive transport.

Osmosis is another form of passive transport. But before we go into details about osmosis, let's first review active transport. With active transport, it takes energy, or ATP, to move molecules across the membrane because we are moving them against the concentration gradient. So we're moving them from an area of low concentration to an area of high concentration. And this is often done using a pump. Group translocation is a specific form of active transport that is used by prokaryotic cells only. With group translocation, the molecule is chemically altered as it is transported across the membrane. An example of this is the phosphotransferase system, which adds a phosphate ion to a sugar molecule as it enters the cell.

Let's now get into the details of osmosis, which is the movement of water across a semi-permeable membrane from an area with low solute concentration, meaning more water to an area of high solute concentration, so less water. So water is like, "Wow. It's really concentrated over there across the membrane. Let me cross the membrane and dilute things." So as a reminder, osmosis is a form of passive transport. There are three types of osmotic solutions that have different effects on a cell. This includes a hypertonic solution, an isotonic solution, and a hypotonic solution. Let's take a look at the effect of these solutions on cells without a cell wall and cells with a cell wall.

For a cell without a cell wall, such as a red blood cell as shown in this image, if the cell is in a hypertonic solution, this means that the solute concentration is greater outside the cell than inside the cell. So water is going to move via osmosis outside the cell, causing crenation of the cell or shriveling due to dehydration. If the cell is in an isotonic solution, this means the solute concentration is the same inside and outside the cell. So there's going to be no net movement of water. And then if the cell is in a hypotonic solution, this means that the solute concentration is greater inside the cell than outside the cell. So water is going to move via osmosis into the cell, causing the cell to expand and swell. And eventually, this will cause the cell to lyse or burst.

Now let's look at the effect of these same three osmotic solutions on a cell with a cell wall, such as a bacterial cell or a plant cell. If the cell is in a hypertonic solution, this will cause water to leave the cell, resulting in plasmolysis, which is where the plasma membrane shrinks and detaches from the cell wall. If the cell is in an isotonic solution, there will be no net movement of water across the membrane. And then if the cell is in a hypotonic solution, then we will have the movement of water into the cell, which will cause the cell to swell. However, in cells with a cell wall, the cell wall provides increased protection against lysis.

Next, let's talk about some membrane transport mechanisms that are specific to eukaryotic cells. The first is endocytosis, which is the uptake of matter through the cell's plasma membrane. Phagocytosis is the uptake of large particles. So during phagocytosis, the cell membrane surrounds the particle and then pinches off to form an intracellular vacuole. Pinocytosis is the uptake of small dissolved materials or liquids. And our cool chicken hint to help you remember this is pinocytosis is like drinking a Pinot, which, of course, is liquid as well. Just like with phagocytosis, with pinocytosis, the cell membrane surrounds the liquid or dissolved substance and then pinches off to form an intracellular vesicle. And then with receptor-mediated endocytosis, endocytosis is triggered when specific molecules bind with receptors on the cell's surface. Finally, let's talk about exocytosis. So with endocytosis, we were bringing matter into the cell. With exocytosis, matter is being ejected from the cell. So during exocytosis, an intracellular vesicle containing the matter fuses with the plasma membrane, and the contents are ejected out of the cell.

All right. It's quiz time, and I have five questions for you. Question number one, which passive membrane transport mechanism requires the assistance of a carrier or channel protein to move a molecule across the membrane. The answer is facilitated diffusion. Number two, which type of endocytosis involves the uptake of a liquid into a cell? The answer is pinocytosis. Number three, is the movement of molecules across a membrane from an area of low concentration to an area of high concentration, a form of passive or active transport? The answer is active transport. Number four, during blank, an intracellular vesicle fuses with the cell's membrane, and the contents of the vesicle are ejected out of the cell. The answer is exocytosis. Number five, if a cell with a cell wall is in a hypertonic environment, this will cause A, crenation, B, plasmolysis, C, lysis, or D, no change in the cell? The answer is B, plasmolysis.

All right. Hope you did great with that quiz, and I hope you found this video to be helpful. Take care and good luck with studying.

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And then if the cell requires the assistance of a carrier or channel.