The Theme of Structure and Function in Cells
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:
- A large subunit
- A small subunit.
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.