Staring at a Map of the World

When it comes down to it we are all fat people. You, me, the world. Hell, even your cat, his dog, and her chinchilla are fat. I’ll go so far as to say that the bacteria crawling over the week old pizza in your fridge is also fat.

Yes, when we talk about our biological structure, proteins usually make the top of the list. And true, they make up the majority of our dry mass, but then again, they contain much in the way of oxygen, nitrogen and sulphur – relatively heavy elements.

The essential elements of life

But we simply would not exist without fat, which comparatively contains less oxygen and nitrogen per molecule. A type of fat – phospholipid – makes up the cell membranes which surround every cell on our planet. It’s why we need fat in our diet. We take in dietary fats and then convert them to structural fats. It’s fundamental to our very existence. They make the ‘skin’ or the ‘bag’ that contains all of our cellular… stuff. The proteins, the cytoplasm, the organelles, the enzymes – everything. And the fact that it is fat, and not protein, that does this job of making the cell membrane is very important. The cell membrane is more than just a static skin around a cell (despite me having just used the analogy a couple of sentences back). Think about it. Your skin is more than just a bag that holds your bones in. Your skin allows you to feel, contact and is an interface with the world. It keeps harmful stuff out. Well the cell membrane has a similar role. It is the interface between the delicate components of the inner cell (like your genes and your DNA and stuff) and the outside world. It acts as a selective barrier to molecules, blocking some, but allowing others to enter. I could go on and on. Of course, proteins are required to mediate many of these functions, but they must be embedded in the membrane itself. And that in itself requires the properties of the lipid membrane to be just right.

Structure of a phospholipid. The tadpole like structure is a standard diagram for all phospholipids.

So then. Our fatty membrane. The lipid membrane. The cell membrane. The plasma membrane. Call it what you may, it is all the same thing. It is made of lots of phospholipids, and a fair amount of protein. There’s also some carbohydrates and cholesterol in there also. The membrane obtains a bilayer structure due to very structure of the individual phospholipid molecules which build the membrane. They have a water hating ‘hydrophobic’ end, and a water tolerant ‘hydrophillic’ end. This is all determined by entropy, where water doesn’t like to be in contact with long chains of carbon and hydrogen as it can’t interact with them, so it just pushes them out of the way. We can say that the phospholipids are upsetting the entropy of the water. Water is always present in the cell and usually present outside of the cell. Eventually, hydrophobic molecules are forced to cluster together because they get pushed in that position. Water likes to interact with the hydrophillic ends as they contain oxygen and sometimes nitrogen, and water being a polar molecule, can form dipole attractions with the oxygen and the nitrogen. This causes the bilayer conformation to be taken, with hydrophobic ‘tails’ facing into the bilayer and hydrophilic ‘heads’ facing towards the water.

The membrane of the cell is more dynamic than just being 2 layers of fat, though. Proteins which are specially designed to sit in the membrane get embedded in there and they act as channels and pores to allow specific substances in and out. Water needs to be allowed in, and water needs to be allowed out, so the cell doesn’t get too full of water, or too dehydrated. These proteins can also acts as transducers, so when a signal from outside the cell contacts them, they change shape and initiate a cascade of reactions within the cell which can ultimately lead to changes in the chemical composition within the cell, or even changes of which genes are being expressed. The membrane allows free diffusion of uncharged gasses though, so oxygen can cross the membranes freely, without the aid of proteins. This is crucial to allow the cell to respire. We wouldn’t want to have trouble respiring simply because we hadn’t had a lot of protein recently. But there’s more.

Despite being the bag that holds each and every single cell in existence together – the cell membrane isn’t actually solid. It’s more fluid. So all of the fluidy, watery, soupy, innards of our cells are being held in place by another fluid-like substance. To put that into perspective, that’s like your shopping carrier bag being made of jelly. Try taking a 6-pack home in that. The lipid membrane is constantly mixing around and moving around. None of the lipids stay in one place. They all diffuse around laterally. In fact, even the individual lipids that build up the membrane aren’t the same. There’s loads of different types of them, and its’ not random either. Different ones have different functions, and different cells have different amounts of different lipids in their membranes. And in a given cell, this composition of different lipids can change, depending on the conditions.

When a cell feels under stress and thinks it may cause a horrid gooey death which will then be harmful to neighbouring cells, (which also known as necrosis), the cell decides to commit suicide. This is a programmed cell death, which is controlled. The cell has specific mechanisms in place should it need to commit suicide. When it is doing this, it signals to Macrophages (a type of immune-system cell) by presenting a type of lipid called phosphatidylserine in the outer layer of the membrane. Ordinarily, this is only present on the inner membrane. This tells the macrophage that the cell is about to kill itself and that it needs something nearby to clean up the mess. Once the cell has killed itself, the macrophage comes along and eats up the dead cell. This goes on all the time in multicellular organisms, including us humans, and this process is actually crucial to our survival. Apoptosis, or programmed cell death (aka cell suicide), is crucial to our correct development. We make MANY more neurons than we’ll ever need in our lifetimes, during development in the womb, and then the excess ones just kill themselves when they realise they’re useless. They’re killing themselves for the common good of all the cells in the body. I suppose our cells have a kind of communist system going.

When a cell (in higher multicellular animals) has been infected by a virus, a similar strategy is implemented. The virus is within the cell, and the cell detects foreign proteins inside if itself. Trouble is, the virus will have already integrated its own DNA with the host cells’ DNA so even if the virus in the cell is killed, the cells own DNA will just produce more viruses (See my post on viruses here ). The cell chops up the virus proteins, and then sticks them to a special protein called MHC (Major Histocompatibility Complex), and this protein then inserts itself into the membrane (insertion is easy as the membrane is fluid like, remember). This protein then presents this foreign virus protein on the outside of the cell (like waving a flag saying ‘Over here! I’m infected!’) which is recognised by B-Cells (a type of immune system cell). They recruit cytotoxic T-Cells (aka Killer Cells, yet another type of cell from the immune system), and the Killer cells come along and kill the infected cell. This leaves a characteristic hole in the cell, kind of like being shot in the head I suppose. Brutal. But for the cells, it’s vital. If the cell is left to continue living, the virus DNA will just produce more virus, these will burst out of the cell and infect all neighbouring cells, continuing the process. This is also used against cancer cells. Imagine that, if the government had a system to shoot infected people in the head.