This is bad form on the part of Mercedes-Benz. Somebody forgot to tell them that it's only men who are supposed to be stupid in TV commercials.
[Hat Tip: Pretty shaved ape at Canadian Cynic
[Hat Tip: Pretty shaved ape at Canadian Cynic
The nitrogen needed for amino acids (and for the heterocyclic bases of nucleotides) comes from two major sources: nitrogen gas in the atmosphere and nitrate (NO3⊖) in soil and water. Atmospheric N2 which constitutes about 80% of the atmosphere, is the ultimate source of biological nitrogen. This molecule can be metabolized, or fixed, by only a few species of bacteria. N2 and NO3⊖ must be reduced to ammonia in order to be used in metabolism. The ammonia produced is incorporated into amino acids via glutamate, glutamine, and carbamoyl phosphate.
N2 is chemically unreactive because of the great strength of the triple bond (N≡N). Some bacteria have a very specific, sophisticated enzyme—nitrogenase1—that can catalyze the reduction of N2 to ammonia in a process called nitrogen fixation. In addition to biological nitrogen fixation there are two additional nitrogen-converting processes. During lightning storms, high-voltage discharges cause the oxidation of N2 to nitrate and nitrite (NO2⊖). Industrially, nitrogen is converted to ammonia for use in plant fertilizers by an energetically expensive process that requires high temperature and pressure as well as special catalysts to drive the reduction of N2 by H2. The availability of biologically useful nitrogen is often a limiting factor for plant growth, and the application of nitrogenous fertilizers is important for obtaining high crop yields. Although only a small percentage of the nitrogen undergoing metabolism comes directly from nitrogen fixation, this process is the only way that organisms can use the huge pool of atmospheric N2.
The overall scheme for the interconversion of the major nitrogen-containing compounds is shown in Figure 17.1. The flow of nitrogen from N2 to nitrogen oxides, ammonia, and nitrogenous biomolecules and then back to N2 is called the nitrogen cycle. Most of the nitrogen shuttles between ammonia and nitrate. Ammonia from decayed organisms is oxidized by soil bacteria to nitrate. This formation of nitrate is called nitrification. Some anaerobic bacteria can reduce nitrate or nitrite to N2 (denitrification). Most green plants and some microorganisms contain nitrate reductase and nitrite reductase, enzymes that together catalyze the reduction of nitrogen oxides to ammonia.
This ammonia is used by plants, which supply amino acids to animals. Reduced ferredoxin (formed in the light reactions of photosynthesis) is the source of the reducing power in plants and photosynthetic bacteria.
Let’s examine the enzymatic reduction of N2. Most nitrogen fixation in the biosphere is carried out by bacteria that synthesize the enzyme nitrogenase. This multisubunit protein catalyzes the conversion of each molecule of N2 to two molecules of NH3 (ammonia). Nitrogenase is present in various species of Rhizobium and Bradyrhizobium that live symbiotically in root nodules of many leguminous plants, including soybeans, peas, alfalfa, and clover (Figure 17.2). N2 is also fixed by freeliving soil bacteria such as Agrobacteria, Azotobacter, Klebsiella, and Clostridium and by cyanobacteria (mostly Trichodesmium spp.) found in the ocean. Most plants require a supply of fixed nitrogen from sources such as decayed animal and plant tissue, nitrogen compounds excreted by bacteria, and fertilizers. Vertebrates obtain fixed nitrogen by ingesting plant and animal matter.
Nitrogenase is a protein complex that consists of two different polypeptide subunits with a relatively complicated electron-transport system. One polypeptide (called iron protein) contains a [4 Fe–4 S] cluster, and the other (called iron–molybdenum protein) has two oxidation–reduction centers, one containing iron in an [8 Fe–7 S] cluster, and the other containing both iron and molybdenum. Nitrogenases must be protected from oxygen because the metalloproteins are highly susceptible to inactivation by O2. For example, strict anaerobes carry out nitrogen fixation only in the absence of O2. Within the root nodules of leguminous plants, the protein leghemoglobin (a homolog of vertebrate myoglobin) binds and thereby keeps its concentration sufficiently low in the immediate environment of the nitrogen-fixing enzymes of rhizobia. Nitrogen fixation in cyanobacteria is carried out in specialized cells (heterocysts) whose thick membranes inhibit entry of O2.
A strong reducing agent—either reduced ferredoxin or reduced flavodoxin (a flavoprotein electron carrier in microorganisms)—is required for the enzymatic reduction of N2 to NH3. An obligatory reduction of 2 H⊕ to H2 accompanies the reduction of N2. For each electron transferred by nitrogenase, at least two ATP molecules must be converted to ADP and Pi (inorganic phosphate) so the six-electron reduction of a single molecule of N2 (plus the two-electron reduction of 2 H⊕) consumes a minimum of 16 ATP.
In order to obtain the reducing power and ATP required for this process, symbiotic nitrogen-fixing microorganisms rely on nutrients obtained through photosynthesis carried out by the plants with which they are associated.
Is there a limit to how far back I can send email?Think carefully before using this new feature. Each person only gets ten messages.
Yes. You'll only be able to send email back until April 1, 2004, the day we launched Gmail. If we were to let you send an email from Gmail before Gmail existed, well, that would be like hanging out with your parents before you were born -- crazy talk.
When I teamed up with Matthew Nisbet a year ago to talk about the subject of framing science--which I still believe to be a very important one--it was not my goal to alienate or outrage a group that I consider one of my most important audiences, namely, ScienceBlogs bloggers and readers. And yet when you look at the latest blowup over what I have posted, Sheril has posted, and Nisbet has posted about Expelled, it's undeniable that there is now an audience that reacts very negatively even to any basic mention of the concept of framing.Good for you Chris. The irony has been apparent to many of us and it's really good to see you confess to having created the problem. My respect for you just went up several notches.
And there's just no other way to spin it--this is a painfully ironic communication failure on the part of those of us who wanted to introduce what I view as a very important communication tool to the science world. If we can't explain something so useful to an important segment of our own audience, how can we possibly hope to use it to counter the other side?
Now, to be sure, the concept of framing has been quite influential already for many people who care about science, but who are not seemingly well represented on ScienceBlogs. When I go around lecturing with Matt Nisbet, we constantly encounter enthusiastic, receptive scientist-laden audiences at universities. There is simply nothing like the response that we've seen here over the last week. Indeed, I believe the reactions at lectures may have skewed my perceptions, and made me neglect or dismiss, to a significant extent, the way our ideas were faring in the science blogosphere.It's very common for people on the lecture circuit to get an exaggerated—and false—impression of their message. This is because the only people who come to your talks are the true believers. When dissenters do show up it's often hard for them to debate the speaker just by posing questions from the audience.
But no success on the lecture circuit can change the fact that somehow--and I'll have ideas about how it happened in later posts--the concept of framing has been blackened on Scienceblogs, which I consider a truly tragic occurrence. And while I'm hardly the only guilty party here, I certainly played a role in that, whether actively or by omission.
1. Framing is deliberately altering what you want to say in order to make it more acceptable to your audience.
Welcome to Gene Genie #24: with a heavy emphasis on Personal Genetics.The beautiful logo was created by Ricardo at My Biotech Life.
1. Just to clarify for my American friends ... this was not a tournament where only teenagers in colleges like UNC could play and it was not a tournament confined to a single country. This is a world title. The USA didn't even make the playoffs.
[Photo Credit: Ford World Women's Curling Championship]
[Hat Tip: Jane at Beer with Chocolate.]
[Image Credit: Dixon and Kahn (2004) based on the structure PDB 1n2c by Schindelin et al. (1997)]
Dixon, R. and Kahn, D. (2004) Genetic regulation of biological nitrogen fixation. Nature Reviews Microbiology 2, 621-631. doi:10.1038/nrmicro954
Schindelin, H., Kisker, C., Schlessman, J.L., Howard, J.B. and Rees, D.C. (1997) Structure of ADP x AIF4(-)-stabilized nitrogenase complex and its implications for signal transduction. Nature 387: 370-376 [PubMed]
Darn. I guess I’ll have to take a rain check on that apology – because I don’t agree with this – and not just because Maurice et al. (2008) are cited incorrectly. Here’s why.Biological evolution never means just change over time, but that's not the real problem with Wells' post. You're going to have to scoot on over to Evolution News & Views and read the whole thing.
"Evolution" has many meanings. It can mean simply "change over time." The present is different from the past. The cosmos evolves. Technology evolves. No sane person denies evolution in this sense.
[HatTip: Friendly Atheist: How to Ruin a Trip to the Museuam.]
Hedges, in Toronto recently to promote his book, attacks both fundamentalists and the new breed of atheists as not only intolerant, but wrong about both the Jesus story and the nature of sin.Sin, as I understand it, is when you violate a moral rule of some kind. It usually means you are disobeying the wishes of supernatural beings. It's not a word used by atheists.
Sin, he says, is a personal thing that will always be with us. Humans will never outgrow it through evolution, as the atheist authors contend, nor can anyone (Jesus included) relieve us of it, as fundamentalist Christians believe, Hedges says.
The best we can do, he says, is try to mitigate evil by living a good life, and having democratic institutions in place to ensure that we can get rid of bad leaders from time to time.
For fundamentalists, evil and sin are an external force – often represented by Satan – to be vanquished by giving yourself over to Jesus. For the new atheists, evil and sin are the fault of religion, and can be defeated by getting rid of all religion and giving yourself over to scientific, reasoned thought.Just because religions can be evil does not mean that all evil is due to religion. No atheist believes this, as far as I know.
For Hedges, it's the same argument: People are basically good, but external forces make us do bad things. Get rid of those forces, and people will be good.
No more effort is needed to achieve utopia, he says.
Hedges draws a distinction between the new breed of atheists and such past non-believers as Albert Camus or scientists of the Enlightenment, whose skepticism he says helped drive human knowledge and understanding.The "new atheists" claim there are no supernatural beings because there's no evidence of such beings. This has nothing to do with evil and sin. When are these sophisticated Christians going to address the real question instead of going off on weird tangents? If they have a good argument for the existence of god then let's hear it. Otherwise, their "sophistication" looks more like "obfuscation" to me. (Or like The Emperor's New Clothes and the Courtier's Reply.)
But Hedges has few such hopes for the new atheists. Where atheists of the past used their disbelief as a stepping off point to find something else to believe in, the new atheists claim to have already found it in what he calls a "cult of science."
Anything that can't be proved scientifically is simply discounted, Hedges says, warning that such a narrow approach to study thwarts the pursuit of knowledge by denying a voice to those who disagree.
1. In spite of his confusion about religion, he seems to be a pretty good guy. He was right about Iraq, for example.