On most Sundays, I won’t be around to post, except in the evening, half-brain dead from ISL class. Anyhow, I’m a day off to recuperate from last week, so I have time to post my very first “Monday Organism”, and a day early, at that!
Since this is the first weekly organism, I think it’s appropriate to explain why there is, in fact, a weekly organism. Since this blog is about biology, it’d be mighty improper unless it had periodical items about animals, don’t you think? I mean, come on, it’s no use running a blog about biology without fluffy animals in it (or angry wobbly ones or, well, extremely tiny ones).
Also, the Monday Organism is sometimes going to be about higher taxa as well (usually very high taxa, mainly to illustrate an interesting point about evolutionary biology)
The first Monday Organism is actually not an Organism, but a Phylum: Cyanobacteria.
Cyanobacteria literally means “blue bacteria”, but they’re actually called “blue algae” in Hebrew. The wiki on Cyanobacteria states that the taxonomy of Cyanobacteria is under revision, which is no surprise. In class, this group was even (I think most appropriately) called “Cyanophyta”, meaning “blue algae”.
Cyanobacteria are a fascinating group, and their existence is sound evidence for various evolutionary theories, the most important one is probably the evolution of the chloroplast organelle, the organelle in plant cells in which photosynthesis occurs.
The truly amazing thing about Cyanobacteria is the fact that they’re actually prokaryotes (having no distinct cell nuclei), and yet, they have photosynthetic pigments in their cells which are used to produce organic material by absorbing light energy from the sun. This means, in effect, that Cyanobacteria are the evolutionary precursor for the eukaryotic plants.
While it is obvious that all algae are commonly related, the truly interesting characteristics of Cyanobacteria are the ones that point out to the evolution of plant organelles. When I first learnt about Endosymbiont theory, I was plainly told that “endosymbiont bacteria eventually became permanent organelles”. Now these endosymbiont bacteria have a name: Cyanobacteria. In fact, the evidence shows that the Cyanobacteria themselves evolved into the chloroplast, and it is quite possible that every plant cell is, in a way, a symbiotic colony of eukaryotes and prokaryotic photosynthetic bacteria!
Obviously, the radiation of photosynthetic taxa is prolific enough to rule out such a simplistic story, but the evidence shows similar genetic and biochemical traits in modern day chloroplasts and in the makeup of Cyanobacteria. Since this isn’t an encyclopedic article and I rather focus only on one interesting concept at the time, I’ll give just one example for “evidence” of the common descent of CB and chloroplasts : the genetic makeup of chloroplast DNA (yes, they have their own DNA and they replicate on their own!) is similar to Cyanobacteria DNA. This alone is solid evidence for common descent for the two.
There’s lots of special cases of endosymbiosis that show not-so-common descent, but rather “common descents”, but I’ll leave that to the avid reader.
The main point of this post is not so much to tell about CB anatomy (warning: other posts might deal with interesting anatomy and physiology!), rather it is to illustrate classic tools in evolutionary research: genetic, anatomical, biochemical and physiological comparison as instruments for detecting common descent. It’s a crucial way of thinking in all of biology, and it highlights the sometimes elusive practical value in evolutionary theory: knowing the genetic relationship between different taxa can be critical in any biological endeavor. If one seeks to find antibiotic weaponry against infection and disease, knowing the culprit’s phylogeny can be of tremendous use, and phylogeny is best derived from the comparative tools I’ve briefly illustrated here.
Tags: Algae, Bacteria, Biology, Cyanobacteria, Evolution, Monday Organism, Plants, Science
Obsessed With Reality 2.0 
December 1, 2008 at 3:24 am |
I knew the chloroplast is equivalent of the mitochondria, the mitochondria has it’s own DNA so I wasn’t surprised chloroplasts did, but, I had no idea they could replicate on themselves!
Mitochondria originated in the same way too, endosymbiosis, only from different bacteria.
I only knew more about the mitochondria because the biology I studied was about humans, not plants. And seeing this, I kind of wonder why both structures are so similar.
Well, this gives something to look forward to for mondays. 🙂
December 1, 2008 at 5:51 pm |
Actually, since I didn’t learn about Chloroplasts yet (I will in a few weeks :-> ) I wasn’t aware of it either, and I had to double and triple check it on the internet before being sure. Yes, Chloroplasts do in fact have their own DNA and it replicates autonomously. The wiki article states what you wrote, as well, that chloroplasts and mitochondria have different endosymbiont origins, and that either origins have been detected (cyanobacteria is not the origin of mitochondria, but in some ancient version of a group of bacteria called Rickettsia, which I admit I know nothing about other than them being aerobic bacteria, unsurprisingly so).
I think you’re wrong about one thing, chloroplasts and mitochondria are not similar in structure, in fact, if they were, it’d be good evidence that there wasn’t separate endosymbiosis for the two, since it’d mean they both originated from the same ancient bacteria, Cyanobacteria. I checked the article and it says that CB are oxygen-tolerate, so I’m assuming the relationship between Rickettsia and mitochondria was detected through other types of comparison.
The only thing that’s definitely the same in the two is the fact that they reproduce on their own (not a big surprise since they were both independent organisms in the ancient past) and they’re both enclosed by a membrane (ditto).
Other than that, I don’t know of any genetic or physiological similarity between the two.
December 3, 2008 at 1:08 am |
Well I’m just learning about choloplasts now, so I could be wrong, yes. But I do think they’re pretty similar!
– They have their own DNA (agreed)
– They are both endosymbionts. (agreed)
– Both chloroplasts and mitochondria are the energy transducers of their respective cells. Of course their reactions are kind of opposite: the mitochondria takes care of the production of ATP (energy unit) from the conversion of small molecules into CO2 and water… and the chloroplast does the reverse taking water + CO2 and converting it into small molecules with the help of light energy.
Anyway you could say they power their respective cells.
– They are not only surrounded by a membrane, but a double membrane (although this happens because of their endosymbiotic origin). The inner membrane is ruffled in both. So they even look alike…
That’s what leaves me confused. In my opinion they have a very similar structure, but evidence says they have different origins… so what caused different cells to do the same kind of processes with different microorganisms? It’s like animal cells and plant cells had gone to a meeting and agreed to go home and digest the organisms that would help them in energy production. It makes no sense.
December 3, 2008 at 6:55 am |
Wow, wow, wow. I haven’t studied about chloroplasts yet (only yesterday we’ve started studying about ferns, but we haven’t gotten deep into the plant cell yet), but I do know what happens with mitochondria. Mitochondria are like cellular lungs. they use oxygen (they were aerobic bacteria, once) to enhance glycolysis by allowing the newer, oxygen-present ATP production cycles. This process is quite different from photosynthesis, or at least, I find it almost incredible to believe that the processes are similar in any way and still are considered to be of different origins. Glycolysis is plain and simple compared to the Krebs-cycle, in which all the fun happens for animal cells. Mitochondria are present in plant cells, too, remember?
I had to study the Krebs cycle once, it wasn’t pretty. The metabolic pathways are very intricate and I find it almost impossibly unlikely that it’s anything like what happens in the metabolic process in photosynthesis.
The double-layer issue is indeed conundrum, I thought it might have to do with the fact that gram-positive bacteria have an inner and outer membrane, which is a pretty well-known phenomenon in bacteria. This could explain the double-layer in both organelles, and since there’s an enormous amount of bacteria, it’s highly unlikely that the double layer is a reason to consider them of common descent.