Random as it may seem, I'm about to revisit the plasma membrane in biology (am resitting A levels xD), and it would be good if I revised it! Up to the age of 16, we were tought the cell had this thing called a membrane, kinda like our skin but it was a line on the diagram and that's pretty much it. Then you do A level and you're like......WOW. Token AWESOME video ("Inner life" on the top row; there's 3 speeds): multimedia.mcb.harvard.edu/media.html
Ok, so the first time, you're only vaguely aware of what the hell is going on, but it has nerd value, and if you're doing science after compulsory education, watch it a second time and it makes way more sense.
As a Catholic, this detail in science really strengthens my faith and makes me so grateful for how amazing life is; regardess of your personal religious choices, and whether you have one or not, you've still got to admit that life is incredible.
SO.....what's on the membrane?
A membrane is really just like the skin of a cell; it protects it, and let's some stuff in and some stuff out. Our skin lets moisture in, and it lets sweat out. And that really depends on what's going on in the body. Equally, what goes in and out of the membrane depends on what's going on in the cell. Here's a mint picture from Ltaing's site:
This is what the cell membrane looks like. It has proteins sticking out of the membrane, which have a huge array of functions. Some of them have carbohydrate chains attached to a protein molecule, to make glycoproteins. Glycoproteins are like badges to say who the cell is, and which department it works in. Just like your key card at work, it lets the cell in personnel-only areas. So when bacteria have different key cards, and are caught in personnel-only areas, the big burly security cells (white blood cells) in your body kick the bacteria out. These glycoproteins also allow the cell to communicate to other cells. The message depends on which type of glycoprotein has been stuck onto the cell membrane; these glycoproteins are often changed by the cell, so they can send different messages.
Other proteins look like North American donuts - they have a hole in the middle to let particular substances through. These donut-like proteins are called channel proteins. Another protein which lets stuff in and out of the cell is a carrier protein. Carrier proteins grab a molecule, and pull it to the other side of the membrane.
Then, there's them phospholipids. In the diagram, they're the orange balls with the black lines. Here's a pretty picture from The Wikk:
Other proteins look like North American donuts - they have a hole in the middle to let particular substances through. These donut-like proteins are called channel proteins. Another protein which lets stuff in and out of the cell is a carrier protein. Carrier proteins grab a molecule, and pull it to the other side of the membrane.
Then, there's them phospholipids. In the diagram, they're the orange balls with the black lines. Here's a pretty picture from The Wikk:
If you want to ignore the bit on the left, the red ball and the blue wavy lines - that's the phospholipid. The diagram on the left-hand side shows the chemical composition of the lipid bit of the phospholipid; it's a diglyceride, made of 2 unsaturated fatty acids (because of TEH KINKEH TAILZ), which means you could put more H's on it (more hydrogen atoms). The 'P' in the red box is the rest of the molecule, which is a bit more complicated, and so cruelly ignored in this blog. Maybe when I'm drunk I'll explain (yes, I do that and I REALLY wish I didn't - it's ok though, my partner and I both do it). The 2 fatty acids (aka blue wavy lines) are joined to the rest of the molecule with an ester bond (or ester linkage). Definition of an ester bond?: (thefreedictionary.com) "the bonding between fatty acids and glycerol that characterizes true fats". Really. That's all it means. :3
RIGHT: components down - now structure and functions!
Righty.
- Phospholipids have their tails touching because their tails love water (are hydrophillic) and the rest of the phospholipid hates water (is hydrophobic). This means stuff which dissolves in water cannot pass through the phospholipids. The exceptions are REALLY REALLY small molecules. Like water, ironically enough.
- Channel proteins accept water-soluble ions, like Cl- (chloride) ions. These ions help keep the water where it should be - in the cell. So when cholera bacteria come along and rough up the protein channels in a small intestine, the chloride ions come out with loads of water molecules. When the water is lost in a cell, dehydration occurs. This is why the diahorrea in a cholera victim is called 'ricewater', and it's how cholera can kill someone so fast.
- Glycoproteins have their sticky out bits on the outside so they can:
- communicate with other cells (the body can distinguish its own cells by the type of glycoprotein)
- adhere to other cells (white blood cells do this in the "Inner Life" video - they 'velcro' to the blood vessel wall)
- Cholesterol exsists in the membrane. Lots of cholesterol bulks up the membrane. Less cholesterol makes the membrane more fluid.
- Fluidity of the cell membrane allows vesicles to carry crap to the cell membrane, and 'spit it out' by fusing with the membrane, thus releasing the contents outside the cell. Vesicles are membrane-bound sacs of digestive enzymes or hydrogen peroxide, which break down old organelles in the cell and bring them to the cell membrane to be 'spat out'. This process is called exocytosis.
Love and science!
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calm - Music:Telly.
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