Our gates were strong, our walls were thick, So smooth and high, no man could win A foothold there, no clever trick Could take us, have us dead or quick. Only a bird could have got in.
The Castle, Edwin Muir
It was wonderful taking a tour of our castle of a cell, so before we move on, I want to point out a few more points of interest. The first is all of the carts and baskets that you see around – so interesting right? Right?! What, you don’t think those are interesting? Well, let me tell you, these are important because they are the place where all of the things made in the castle are put before they can be taken to the village nearby. In the cell, this job is done by the delightfully named Golgi body ([goal-gee] also known as the Golgi apparatus), which packages the proteins that are made by the ribosomes into structures called vesicles [vess-ick-uhls] so that they can be transported around or out of the cell. Those are kind of like the pages and other workers that scurry about the castle moving the goods and crops where the lord wants them to go. Vesicles also look like spheres, but slightly larger than a ribosome; a Golgi body looks like a series of different sized rubber bands. (You might have noticed an abundance of spheres in cells. Thinking back to fat cells, can you figure out why that might be? Spheres are excellent for storage, which is often needed in cells of all types.)
All people everywhere have garbage and waste to deal with, and that was no different in a castle. The most troublesome type of waste is that created by humans, and in castles this was generally taken care of through use of a privy (you know, ye olde water closet…without the water) which often sent the waste out of the castle into the moat. Farmers would then come to collect the waste to fertilize their fields. (By the way, this practice is still done today with the use of “biosolids,” which is just a fancy name for human waste that can be used to grow food. Enjoy those carrots!) In the cell, the lysosome [lie-zoe-zomes] deals with old parts of the cell, recycling them like the farmer’s fertilizer for reuse in the cell. Lysosomes (more spheres – a bit bigger still than the vesicles) also protect the cell from viruses and other invaders, as well as helping to recycle the entire cell at the end of its life.
So far, we have been talking about eukaryotic cells (those with a nucleus, including plants, animals, fungi, and protists), but what about prokaryotic cells (those without a nucleus, bacteria and archaea)? Do they have all the same organelles as plant and animal cells? To illustrate how these cells are similar and different, let’s leave a defensive war machine for an offensive one – a tank.
Tanks, like castles, have a strong outer defense with controlled entry points (the armor and the hatch). Similarly, bacteria have a cell membrane similar to those found in animal cells. Before we go in the tank, you might notice that tanks have the ability to move (usually on treads), a feature somewhat lacking in a castle. Many bacteria have cilia (like little hairs) or flagella (like tails) that allow them to move around. Some animal cells can also move, but most stay in one place.
Going into the tank, you will notice that it is significantly smaller than a castle (just as most bacteria are smaller than most animal cells), but that they both have open areas inside: prokaryotic cells have cytoplasm in them just like eukaryotic cells. Inside the tank, you will, of course, find soldiers who work together as a group to run the tank. In a bacteria cell, since it lacks a nucleus, there is not a central place where the DNA is kept. Instead there is a nucleoid [new-klee-oid], DNA distributed around the cell. Like a team of soldiers working together, rather than a single lord, the bacteria’s DNA still gives instructions to the cell but in a more diffuse way.
Bacteria have one other important organelle – the ribosomes. These protein-producing powerhouses take the instructions from the DNA, just like in a eukaryotic cell, as a computer in a tank takes the instructions from the soldiers and then actually makes the tank move this way or that.
You might be asking yourself, why are bacteria cells missing such random things – a packing center for proteins, storage spaces, a container for its DNA? Why these things – they don’t seem to have anything in common. The thing that connects all the missing structures is that they all have membranes – wrappers around them that separate them from the cytoplasm. While bacteria do have a cell membrane to separate themselves from the rest of the world, they do not have any membrane-bound organelles (as we call this category of cell parts). Although it may seem like this is a weakness, the Creator must have known what He was doing, after all bacteria and archaea are everywhere, doing amazing things in our world.