Introduction to Membrane Transport - Video Tutorials & Practice Problems
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Introduction to Membrane Transport
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2m
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In this video we're going to begin our introduction to membrane transport. And so 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. Now it's also important to note that the term semi permeable is synonymous with the term selectively permeable. And so selectively permeable and semi permeable really just mean the same thing. And again, all it's saying is that membranes are really, really picky about what types of molecules they allow to cross the membrane. And so the term permeable is a term that actually means how penetratable 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. And again the root semi is a root that means partial and so semi permeable just means partially penetratable. And so 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 of molecules cannot cross. And so if we take a look at our image down below we'll be able to see an example of a selectively permeable biological membrane. And so 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. And 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 2 molecules, a sodium ion and an oxygen molecule or an O2 molecule. And notice that only the o two molecule in this cartoon is able to cross the membrane freely and the sodium ion is not able to cross the membrane. And again this is because of the properties of these molecules and we'll be able to talk more about what exact properties do 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 to you, it's talking about the sodium ion. And 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. And so this is just a silly cartoon, but hopefully it drives home the point of how biological membranes are semi permeable. And so this concludes this video and I'll see you all in our next one.
2
concept
Which Molecules Freely Cross Membranes?
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5m
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Video transcript
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 allowed across the membrane freely or 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. And so, some molecules can freely diffuse across a membrane without any facilitation from a protein whatsoever. And so 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 non polar or hydrophobic. And so again these three features being small in size, uncharged, and being non polar or hydrophobic are all features that correspond with molecules that can freely diffuse across a membrane without any facilitation from a protein. Now 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. So instead of being really, really small, these molecules are going to be really, 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 hydrophobic, these molecules will be polar or hydrophilic. And so 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. And so in this image notice that in the middle here we're showing you our biological membrane. And 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 going to represent the outside of our cell. And so 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. And so 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 non polar or hydrophobic molecules. And so these molecules include oxygen gas, carbon dioxide gas and nitrogen gas. And so being small, uncharged, and non polar you might recall are all tendencies that correspond with being able to freely diffuse across the membrane without facilitation from a protein. And so 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. And so, 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. Now 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. And so notice that we have molecules that are going to be small and uncharged which correspond, with features of molecules that can freely diffuse, but we also have, these molecules are also polar and being polar or hydrophilic is a feature of not being able to freely diffuse across the membrane. So these molecules here that we're showing have mixed features of being able to cross and not being able to cross. And so it turns out that these molecules here are able to still diffuse across the membrane. However, notice that the green arrow here is not nearly as thick as this other green arrow over here. And what this is representing is that these molecules can still cross the membrane but they do not cross as quickly and they do not cross as efficiently as these other molecules on the left that we showed with the bigger, thicker green arrow. Such as such as water, this steroid that you see right here, and this molecule over here which is Glycerol. Now notice over here this other category of molecules are molecules that are either charged or polar molecules. And again, being charged or polar are features of not being able to freely diffuse across the membrane without facilitation from a protein. And so notice that we have this red arrow that is going backwards bouncing off of the membrane which is indicating that these molecules cannot freely diffuse across this membrane if they don't have facilitation from a protein. And so, these molecules here include molecules that are sugars, molecules that are charged, these ions here, and again amino acids like this one which again are going to have these charges. Now, last but not least, this category of molecules here are large macromolecules and 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. And so these large macromolecules include substances like polypeptides or proteins, polysaccharides or long carbohydrates as well as nucleic acids such as DNA or RNA. And again notice here that, we have this red arrow that is bouncing off of the membrane indicating that these large molecules cannot freely diffuse across the membrane without any facilitation. And so this here concludes our brief lesson on which types of molecules can freely cross membranes and which ones cannot. And again, the molecules that can freely diffuse across a membrane without protein facilitation are going to be small, uncharged and non polar hydrophobic whereas the ones that cannot freely diffuse across the membrane without facilitation are going to be large, charged and polar hydrophilic. And so 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.
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Problem
Problem
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+
4
concept
Map of the Lesson on Membrane Transport
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2m
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Video transcript
In this video we're going to introduce our map of the lesson on membrane transport which is down below right here. And so 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. And so 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. And so we're going to be exploring the left most 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 portends 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. And so that is really the plan that we have moving forward in our course. And so 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. And so that being said, this here concludes our brief introduction to our map of the lesson of memory 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. So I'll see you all there.
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Problem
Problem
According to the map above, what kind of large molecule transport is also called the process of cell 'eating'?