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Keeping Things In and Keeping Things Out
Keeping Things In and Keeping Things Out
Cells are truly amazing things. They represent what we refer to as the 'smallest unit of life'. Thus, they are the smallest thing that we can call alive, much like a single atom of hydrogen is the small thing we can still accurately refer to as hydrogen. But do not let their size fool you. You have already seen some of the amazing things going on inside of a cell, but have you thought about how impressive it is that there is an inside and an outside? Cells are able to keep all of those organelles inside despite the fact that all of these things are in constant motion and within an aqueous solution (the cytosol).
This may seem trivial at first, lots of things have barriers - think about the balloon we examined when discussing cell size. The latex of the balloon represents a barrier. However, cells can do one better. Not only do they have a barrier, but it is a barrier that allows a cell to keep what it wants inside and what it doesn't want on the outside. That is incredibly useful and, in fact, necessary, for a cell. In order to maintain life, you create a lot of waste, and that waste needs to be able to leave a cell, just as the supplies to power the cell need to be brought in.
For the record, the area inside of a cell is called the intracellular space and the area outside of a cell is referred to as the extracellular space. This will become absolutely crucial to know when we start talking about how things enter or leave a cell.
The Phospholipid
The Phospholipid
So how does the barrier work? Well, firstly let's get our terminology down. The barrier of a cell is referred to as the cell membrane. Hopefully, that term is familiar to you, especially after we looked into the macromolecules a little bit. If you'll recall, there was one kind of lipid, the phospolipid, that I mentioned would be very prevalent shortly. The phospholipid structure can be seen here, and understanding its structure is absolutely essential to understanding its function, as with all things in biology.
The absolute most important thing to notice about the phospholipid is that there are two major components: the head and the tails. The hydrophilic head is exactly that - hydrophilic. Remember that this means that the molecule (or in this case, the portion of the molecule) is polar. You can tell this also by looking at the atoms within the head. The phosphate group has a charged oxygen ion and there are numerous oxygen-carbon bonds.
The hydrophobic tails are, you guessed it, hydrophic. This means that they are nonpolar. You can see from the structure that the tails are almost entirely carbon-hydrogen bonds. Recall from earlier chapters that these are nonpolar because carbon and hydrogen have very similar electronegativities.
This phospolipid structure is particularly important because this one molecule is ambivalent - it is both polar and nonpolar all at once. This may not be too amazing by itself, but remember that like dissolves like. So if a phospholipid is placed in water, the hydrophilic head will be comfortable interacting with the water, whereas the hydrophobic tail will be trying to get away from all those pesky polar water molecules. Imagine if you walked into a room and immediately your legs were attempting to leave and your torso was trying to stay - that is the experience of a phospholipid when it tries to live in an aqueous solution on its own.
Phospholipid Bilayer
Phospholipid Bilayer
Again, I cannot overstate the significant of the phospolipid being partly polar and partly nonpolar. As you know, if you put oil in water, the oil will rest on top of the water (assuming the oil is less dense than the water). So if we toss some phospholipids in the water, they will organize themselves accordingly. The hydrophilic heads will hang out in the water and the hydrophobic rails will hang out in the oil (see top of beaker shown here).
If you have a single droplet of oil in the water, the phospholipids will still orient themselves so that the tails are near the oil and the heads near the water, effectively keeping the oil in a bubble via a single layer of phospholipids (see oil droplet in image). Because this is a single layer of phospholipids, it can be referred to as a monolayer, but you do not need to worry about that term - I mention it only to contrast it to the phospholipid bilayer you will become familiar with (and that is shown in the bottom-right of the diagram).
Phospholipid Bilayer
Phospholipid Bilayer
If you put enough of these phospholipids together in an aqueous solution, the tails will be seeking out anything nearby that is just like them - nonpolar. And they will inevitably find tails of nearby phospholipids. These tails will all group up and the heads will all group up - they're very cliquey like that. This structure, shown here, is the phospolipid bilayer. It is called this for a few reasons that are relatively obvious when you look at it closely. It is made of phospholipids, obviously, but it is also a barrier made up of a layer with a width of two (that's where the 'bi' comes from) phospholipids. Hence, phospholipid bilayer.
When you get a lot of these together, they will form one giant bubble with an inside and an outside - THAT is your cell. But membranes will only get more complicated for us, so it is important that we understand these fundamentals before getting into something like what is shown below.
Cell Membrane
Cell Membrane
Note that there are some proteins and even some carbohydrates. So now all of our macromolecules (well nearly all - we will see nucleic acids more in a bit) are coming together and interacting to form this amazing thing known as a cell.
Each of these macromolecules present on the bilayer are important, but only the phospholipid is capable of forming that incredible barrier that lets some things in and keeps some things out.
Because the membrane can let some things in, it is referred to as semipermeable. To permeate something means to get through it. Semi means 'somewhat' or 'not fully'. So it is semipermeable because some things can get though, but not all. We will be exploring what things might be getting through shortly. Sometimes this is referred to as selectively permeable.
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