Cell Membranes: The Phospholipid Bilayer | A-level Biology | OCR, AQA, Edexcel

hi guys in this video we'll be looking at the cell membrane the phospholipid bilayer and then we'll finish with a summary so all cells of any organism has to contain what we call a plasma membrane and it's kind of like a sheet structure which helps divide areas from each other the plasma membrane that surrounds the whole cell as an entirety is known as the cell surface membrane so for example if we were to take an animal cell like this so this could be a cell from a human for example it's got to have a barrier which separates the cell itself from the outside world and if it didn't have such a barrier then all of the contents would simply spread out and there would be no entity to the cell whatsoever so this means that this barrier around the edge of the cell is known as the cell surface membrane and it really helps if you start imagining cells as 3d objects rather than just how they're drawn usually which is a disc with a nucleus in the center imagining the self-service membrane as a coating around the whole cell like a sphere we don't just see this in animal cells we see this in prokaryotes as well so for example bacteria and again just like animal cells they have to have a cell surface membrane encapsulating all of their contents and some cells including prokaryotes and some plant cells also will have cell walls around them but regardless of whether they have cell walls or not the membrane is still an important part of the cell in eukaryotic cells like animal cells and plant cells we actually have compartments within the cell as well and these compartments are also enclosed in another cell membrane and we call these compartments organelles so again if we look at a general cell so if we call this an animal cell we can recognize the original cell surface membrane we were talking about and when you look inside the cell it's not just one big diffuse mess of content it's divided into very strict compartments which need to interact with each other so we have for example the nucleus we have endoplasmic reticulum and we have the golgi apparatus don't worry so much about what these do for now but just be aware that actually there are membranes surrounding these organelles as well which give them their own independent structure and their own sort of entity as a compartment within a cell so this is also cell membrane and these are all termed as organelles so we have organs which take part in lots of functions in the body and organelle is almost like the organ of a cell try and think of it in that way so the cell membranes act as barriers they help to compartmentalize the cell from other cells and obviously the organelles from other organelles in the cell so they're acting as kind of the barrier or boundary controlling what can enter and what can exit these cells or their organelles so if we think about a cell in general but because the cell membrane is there it regulates what can enter and what can exit in its own accord and as well as this we have organelles that lie within the cell like the golgi apparatus or mitochondria they can also regulate what enters and exits them based on the properties of their own membrane and this is really really important because the environments of these organelles and of the cell itself needs to be very tightly controlled so that it can carry out the right functions for example the levels of ions the level of water and how many enzymes are around if this wasn't regulated and there was no barrier to control this then everything would fall apart very quickly so cell membranes are able to act as a barrier because of the structure which makes up the cell membrane and the structure which makes them up is made of a molecule called a phospholipid so a cell membrane whether that's the membrane around the cell or around an organelle is made of two layers of a type of molecule the type of molecule is known as a phospholipid and it's basically lots of phospholipids arranged into one layer on top of another layer so we call this the phospholipid bilayer so the bilayer refers to the fact that we've got two layers and the phospholipids are the individual molecules making this layer up if we were to say this was a cell and we zoomed in on part of the membrane here what you would see is you would see layer number one and layer number two and they're kind of mirroring each other but each one is made up of lots of phospholipids and because there's two layers we call this a bilayer so this is found in the cell membrane and also the membrane of organelles inside the cell too so why does the molecule form a bilayer well it's more to do with the properties of the individual molecules or the phospholipids which causes this they have two parts to them so this is one phospholipid molecule this is one single phospholipid molecule and it has two parts to it it has a hydrophilic phosphate head so that's this part here so hydrophilic means water attracting or water loving and it's called a phosphate head so the head has a phosphate group which contains phosphorus and oxygen atoms don't worry about the ion structure for now but that's where it gets its name from and it attracts water so if there was ever water around then water tends to cluster and get attracted to the head of the molecule the other part of the molecule is these two tails and these are hydrophobic fatty acid tails so here we have two of them hydrophobic meaning water repelling and the fatty acid refers to the idea that there are two very long lipids attached as the tails so these ones repel water so if water was ever brought near them the water would be sent away and they want to try and avoid water so this is one phospholipid molecule a hydrophilic phosphate head which attracts water and two hydrophobic fatty acid tails which repel water so in aqueous conditions i.e conditions where there is water present for example in our cells and in the body the phospholipids arrange themselves in this bilayer form because the hydrophilic heads which like water face the outside world and they form the two outer rows where they can face the water but the hydrophobic tails protect themselves in a core all together as a cluster trying to avoid the water so imagine we've just dunked a load of these phospholipids which are represented again the phosphate head and the two fatty acid tails imagine if we then immerse lots of these into a watery environment well the hydrophilic heads like the water so they're going to try and face the water and attract it so you can see that all of the hydrophilic heads face outwards towards wherever the water is and all the tails start clustering together in the center of the two layers and they exclude any water that's found there so the water will leave this area because the water and the tails don't like each other so then what happens is you get this happy bi-layer structure or this very stable bi-layer where all of the heads are facing the water but all of the tails are facing away from the water and this is the only arrangement that they want to be in so because it forms this phospholipid bilayer the membrane gets a certain number of properties which help to perform its function as a barrier so first of all it creates a partially permeable barrier so when you've got this bilayer structure and if you imagine this is all around the cell it kind of creates a boundary as we said in the beginning so what it does is it allows certain substances to pass through for example oxygen can pass through and carbon dioxide can pass through and this is important for diffusion for respiration but it doesn't allow other things to pass through for example charged ions like sodium or chloride can't get through because of certain properties of the bilayer and that means they wouldn't be able to get through with the other side either so this is really important they're partially permeable because they only allow certain things in not others and this is important for controlling the internal and the external environment of the cell and why do we want to control the environment of a cell or an organelle because the right number of solutes the right fluidity and the right composition is very important for an organelle or a cell to carry out the right processes if we have too many ions in a cell it can become electrically too active if we have too much carbon dioxide this can be toxic etc etc and the bilayer also allows the membranes to be very very fluid but also very stable so if we're looking down at a phospholipid bilayer from the head-on so if we had a by-layer looking like this and we were to look head-on then we would see all of these phosphate heads and the membrane itself is not a very rigid structure but it's actually very fluid and what we mean by fluid is not completely liquid but it means that they move around each other and they're quite sort of mobile it's a very soft moving structure and because of this it allows things to pass through and allows things to change shape as well because not all cells stick to the same shape they're not often round cells cells need to be able to move for example neutrophils and white blood cells have to be able to engulf bacteria cells move around the body as well so the membrane is very fluid but it's stable so it doesn't fall apart or break down and this is a very important balance for the membrane to do what it needs to do and in a fluid environment like the cell and the tissue it's very important that membranes are flexible and able to bend with the environment to allow this hey guys i hope you enjoyed the video if you're looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park

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Keywords: snaprevise, a-level, alevel, biology, alevel biology, ocr, aqa, snab, edexcel b, wjec, eduqas, ial, cie, Cell Membranes: The Phospholipid Bilayer, The Cell Membrane, The Phospholipid Bilayer

Id: Ak1NTUzJxAM

Length: 9min 39sec (579 seconds)

Published: Mon Apr 29 2019