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Up until now, we have spent some time (OK, maybe lots of time…stop looking at us like that) describing the junk, er, different components you might expect to find in different kinds of cells. We have also spent a lit—lots of time talking about what each of these unique components do for the cell. That is, we have talked about their functions. Hopefully, by now, you have begun to notice that, in almost every case, the structure of a given cellular component has a lot to do with its function. In fact, one mantra of biology encapsulates this idea perfectly: "Structure dictates function" (you should probably memorize this phrase now). The name for these relationships are, uh, structure-function relationships. To really appreciate how true this idea is, let’s look at a few examples in detail.
Let's zip back to mitochondria and chloroplasts. These organelles are really nothing more than membranes within membranes, with a little space between said membranes. The main function of mitochondria is to convert the energy in glucose to ATP, a usable form of energy for the cell, through the process of cellular respiration. This exceedingly important function is only possible because of the unique structure of the mitochondrial membranes, which allow for an intermembrane space to form where protons can accumulate, and for a matrix to which the protons can flow.
Without the inner mitochondrial membrane, or IMM, there would be no "Hoover Dam" to hold back protons and force them to flow through the ATP synthase rotor. Moreover, the IMM is folded into structures called cristae, which pave the way for millions of ATP synthase complexes to jam into a single mitochondrion. Sounds a little crowded. Without the unique folded structure of cristae, cells would need millions of mitochondria in order to produce the same amount of energy produced by just a few with cristae. Structure dictates function.
As for chloroplasts, without the thylakoid membranes separating the stroma from the lumen, there would be no space for protons to accumulate and flow back into. Without the products produced by the thylakoid membrane proteins, including ATP (we know; he's everywhere), and without a space for glucose to be made, or the stroma, photosynthesis would not occur, and life on Earth as we know it would cease to exist. Are you ready to acknowledge the vital relationship between structure and function yet, or what? Do you want the world to end? DO you?!
In the end, only the structures of the mitochondria and chloroplasts allow the processes of cellular respiration and photosynthesis to take place. In both cases, the presence of a membrane allows for compartments to form. Those compartments can have different concentrations of hydrogen ions, and it is those differences in concentration that drive formation of important substances.
Ribosomes provide another good example of structure determining function. These small cellular components are made of protein and ribosomal RNA (rRNA). Their main function is to translate messenger RNA, or mRNA, into strings of amino acids called proteins.
Ribosomes are composed of two main parts:
Let's go back to our picture of a complete ribosome:
The small subunit has a special groove that allows for mRNA to bind to it. Once the mRNA is bound, the large subunit attaches on top, and a complete ribosome is formed. mRNA is pulled through the space between the two subunits as another molecule, transfer RNA (tRNA), binds to a second groove in the ribosome and to the mRNA, leaving behind an amino acid in yet a third groove.
For every three base pairs of mRNA, tRNA leaves behind one specific amino acid. When the end of the mRNA strand is reached, the ribosome subunits detach and let both the mRNA and the newly formed string of amino acids, aka the protein, run free into the big wide world. The grooves of the ribosome allow for mRNA to be held in place while tRNA reads the "code" that determines which amino acid is next in the sequence. It is the very structure of ribosomes that completes the Central Dogma of Biology, or DNA to RNA to Protein.
Without proteins, a big, fat nothing would get done in the cell. N.O.T.H.I.N.G.
By this point, you may be tired of hearing about mitochondria and chloroplasts. Enough already, right? Well, we could lie and say that there is no more, but, well… we'd be lying. Sorry. Don’t give up now, though, because the most interesting part (we swear) about these seemingly benign organelles is yet to come. We promise, you won’t be disappointed. (This promise is not associated with any monetary refund.)
Our story begins many, many eons ago..."yeeeeeeah, back when the world was new, the planet Earth was down on its luck." Oh, sorry. We had a bit of a Hercules flashback there. Carry on.
We begin our story many eons ago, probably not long after the first eukaryotic cells evolved. At some point, during the earliest stages of life on Earth, a eukaryotic cell named Eukie engulfed, through the process of phagocytosis, a prokaryotic cell named Prokie capable of converting chemical energy into ATP. Eukie did not destroy the engulfed Prokie, but instead, the two got along swimmingly. Eukie provided a plethora of sugars for Prokie, and Prokie provided a substantial amount of energy for Eukie. A match made in heaven. Why isn't there a Disney movie about this yet?
Through time, Prokie divided again and again until there were many daughter prokaryotes living inside Eukie. OK, maybe this is why this story is not a Disney movie… When Eukie replicated, she passed some of Prokie's daughter cells onto her descendents. The energy from the prokaryotes gave these new eukaryotes a huge advantage, and more of them survived than almost any other eukaryotes on the planet. This story is a little weird, but so far so good…
Through time, nearly every other eukaryotic cell on Earth went extinct (poor guys!) while the eukaryote carrying the prokaryote thrived. Way to go, Eukie and Prokie. Along the way, one of the thriving eukaryotes ingested yet another prokaryote (getting weird again…), this one capable of converting sunlight into sugar. Therefore, through time, some eukaryotic cells had both types of prokaryotes, and some had only the first type of prokaryote. Those that had both types no longer had to spend time looking for food, but instead could focus on producing food through the process of photosynthesis.
Although it is impossible to know for sure how all the details played out—our story above is a bit contrived, for sure—biologists are fairly certain that the ancestors of both mitochondria and chloroplasts were originally bacteria that had been engulfed by primitive eukaryotes. Some of the evidence for this idea, called the Endosymbiotic Theory, is that both mitochondria and chloroplasts have their own DNA that codes for some of the proteins necessary for cellular respiration and photosynthesis. This DNA is circular, just like that seen in prokaryotic cells.
In addition, multiple membranes surround both mitochondria and chloroplasts, a structure only seen in prokaryotes. Both mitochondria and chloroplasts divide by binary fission, or the splitting of a single cell into two nearly equal daughter cells, just like bacteria, and both contain ribosomes that are structurally similar to prokaryotic ribosomes. Last but not least, the thylakoid structure in chloroplasts is a structure only seen in cyanobacteria. These, and quite a few other lines of evidence, lead the vast majority of biologists to accept the Endosymbiotic Theory as an accurate explanation the presence of mitochondria and chloroplasts in eukaryotic cells. It is truly a fascinating case study in the evolutionary relationship between prokaryotic and eukaryotic cells.