My notes and thoughts from Biology 111, for Wednesday, September 24, 2008. The entire series can be found here.
Forgive the delay in this, the next installment of the series. At about this point in the semester, the workload and involvement in school just took off, and I needed to devote as much time as possible to homework and studying.
When last we left off, we had just begun a quick tour of the eukaryotic cell and its structures. We’d gone over the Nucleus and the Ribosomes, and some of the membrane-bound organelles like the Endoplasmic Reticulum (or ER), the Golgi Apparatus (or GA), and the Lysosomes.
We’ll pick it up here with number 7, the Mitochondria (another membrane-bound organelle), and we’ll go into more depth when we get to Chapter 9.
The mitochondria are sites of aerobic respiration. Recall that C:H bonds have a high potential energy because of the maximum distance of electrons from the nuclei of the Carbon and Hydrogen atoms. In other words, the electrons they share equally are midway between the C and the H.
(The lecture notes continue below the fold.)
So to get energy, a eukaryote (like a human, for instance) eats sugar and breathes air. In the air, there is Oxygen. What aerobic respiration does is take the energy out of the C:H bonds in the sugar by combining it with oxygen. The chemical reaction goes like this:
C6H12O6 + 6O2 —> 6CO2 + 6H2O
What does that mean? Well, to each molecule of hexose (a sugar with 6 Carbons), six Oxygen atoms are added. Now, all those atoms have to go somewhere, they don’t just disappear. By rearranging the atoms into six Carbon Dioxide molecules and six water molecules (both polar molecules, meaning the electrons are closer to one atom than the other), the mitochondria lower the potential energy of the molecules. But that energy also has to go somewhere, it also doesn’t just disappear. (First Law of Thermodynamics, remember?) Where does it go? Well, the body uses it to do stuff, like make your muscles contract!
In fact, most of the energy our bodies use comes from this very reaction. The mitochondria are the body’s power plant. This is pretty cool, especially since the mitochondria is just an infection, really.
Well, not exactly, but yeah. It turns out that way back in the day, millions of years ago, probably before Ronald Reagan was born even, what is now your body’s power plant was a bacterium. Remember when we discussed some of the differences between prokaryotic cells and eukaryotic cells? Let’s put that table back up, since it’s been a while.
|in cytoplasm||in nucleus|
|not present||Membrane Bound Organelles||present|
|1 – 10 µm||Size||10 – 100 µm|
|*circular or linear: meaning the ends of the chromosome either connect or don’t connect – as they are tangled up in a little ball, it is important to remember that the overall shape is not a circle or straight line|
|**70s and 80s are just a reference to size, with 80s being somewhat bigger than 70s|
It turns out that the mitochondria has one circular chromosome, no nucleus so the chromosome is in the matrix (cytoplasm), has its own 70s ribosomes, and is the size of a prokaryote. The mitochondria probably evolved from what is known as an alpha proteobacteria by a process known as endocytosis (‘endo = inside; cytosis = more than the usual number of cells’). Endocytosis occurs when a cell literally wraps itself around another cell (in this case, the proteobacterium) and sort of swallows it up. Here, the proteobacterium survived the process, and multiplied. The eukaryotic cell benefited from the power being generated by the proteobacterium, and the proteobacterium benefited from the physical protection of the eukaryotic cell. In anthropomorphic terms, the cell said, “Dude! Make me some juice and I won’t let the other cells eat you, lol!”
Over the course of time, the proteobacteria gave up some of its independence, and the eukaryotic cell stopped generating its own power. They became interdependent. This relationship is termed endosymbiotic, meaning a relationship where one partner is inside the other and both partners benefit from the relationship.
This is a very cool illustration of how evolution can do the neatest things with what’s around it.
Also, the mitochondria divide (by binary fission, a prokaryote technique) separately from the rest of the cell. This is yet further support for the evolution of the mitochondria from a proteobacterium. There can be little doubt that the mitochondria was at one time a prokaryote.
This leads us to number 8, Chloroplasts.
Chloroplasts are very similar to mitochondria, but are found only in plants and algae. Since plants and algae also have mitochondria, it would seem that the proteobacterial infection that led to the evolution of the mitochondria must have occurred so long ago that it was before animals and plants went their separate ways on the evolutionary bush. Not so with chloroplasts. Since we see them in plants and algae, but not other eukaryotes, we can infer that chloroplasts evolved after plants and algae split from other eukaryotes, but before they split from each other.
Now here’s the really cool thing: chloroplasts work exactly the opposite from mitochondria. Instead of turning sugar and oxygen into carbon dioxide and water, chloroplasts take carbon dioxide and water, add a little energy from sunlight, and turn them into oxygen and sugar. This is how plants and algae take raw materials from the environment and store that sunlight as energy in the form of C:H bonds until they need it. When they need energy, the mitochondria reverse the chemical reaction, and voila!
So we eat plants. The plants have sunlight stored in the sugar. We eat the sugar and do exactly what the plants do with it with the exact same process! That is just totally cool.
Now, the chloroplast also has 70s ribosomes and divide independently of the cell, etc etc. They have a little bit different structure than the mitochondria though, including a thylakoid membrane (which is functionally equivalent to the matrix of the mitochondria) and stroma (which is functionally equivalent to the inner membrane space). It seems the chloroplast descended from a different bacterium, called a cyanobacterium, and also became part of the cells through endosymbiosis.
Wow. Nifty, nifty, nifty.
Ok, moving on to number 8.
8. The Cytoskeleton
The cytoskeleton is not bound by a membrane. It is a network of protein fibers that extend from the nuclear envelope to the plasma membrane. It keeps the organelles in place inside the cell, and organizes internal structures. It’s very important for the ER and the GA, for instance. It also helps to support the cell and maintain the shape of the cell, as the name implies.
It also serves as a sort of train track for the transport vesicles, and the Harvard/XVIVO animation I mentioned earlier has some very good shots of transport vesicles traveling along the fibers. You should go watch Inner Life of the Cell again. (Actually, you should do that again anyway, just because it’s so freaking cool.)
The cytoskeleton is made up of three types of fibers:
a) microtubules, which are basically little hollow straws
b) microfilaments, which are braided cables
c) intermediate filaments, which are big, fat cables
And this brings us to
9. Cilia and Flagella
Microtubules can sort of stick out of the cell (but not break the cell membrane which still surrounds them). They can form a long tail (a flagellum), or a bunch of them can stick out a shorter distance (cilia). A cross section of a flagellum or a cilium is exactly the same. Eight pairs of microtubules encircles one pair of microtubules. Because microtubules can contract or extend, they can create motion for the cell. When a cell uses a flagellum for locomotion, the flagellum moves like a swimming snake to produce directional motion. When a cell uses cilia, it’s more like the oars of a boat.
Now, most eukaryotic cells have neither flagella nor cilia. Out of about 200 kinds of cells in the human body, only the sperm have a flagellum. The cilia appear in the reproductive tract of females. Instead of sticking out of an egg cell and swimming the egg along, however, the cilia are actually in the tract itself, and sort of “pass” the egg along. They are also found in the upper respiratory tract.
The final stop on our quick tour of the cell before we go into more depth is
10. The Cell Wall
Ok, the main thing here is not to confuse this with the plasma membrane. The cell wall is a distinct cell feature not found in all eukaryotes. It resides outside the plasma membrane in plants, algae, and fungi, but not in animals. It is somewhat rigid, unlike the plasma membrane. Mostly it helps to support and protect the cell. It is composed primarily of polysaccharides. In plants and algae, it’s made of cellulose, and in fungi it’s made of chitin.
In the next lecture, we pick up with the structure and function of the cell membrane.
From whence came the art:
The first image is of our textbook, Biology, Eighth Edition, by Campbell & Reese et al.
Other images by me and are licensed under the Creative Commons Attribution- NonCommercial- Share Alike 3.0 License.