< PREV | NEXT > | INDEX | SITEMAP | GOOGLE | LINKS | UPDATES | BLOG | EMAIL | $Donate? | HOME

[7.0] Biochemistry

v1.3.0 / chapter 7 of 15 / 01 jun 16 / greg goebel

* During the 20th century, researchers acquired a broad understanding of the chemistry of life, opening the door to major advances in biomedicine and biotechnology in the 21st century. This chapter provides a short overview of modern biochemistry.

DNA molecule


[7.1] THE CELL
[7.2] PROTEINS
[7.3] LIPIDS & CARBOHYDRATES
[7.4] NUCLEIC ACIDS
[7.5] METABOLISM / PHOTOSYNTHESIS
[7.6] VITAMINS & OTHER MISCELLANEOUS BIOMOLECULES
[7.7] FOOTNOTE: THE ORIGINS OF LIFE

[7.1] THE CELL

* In the modern era, the cell is known to be made up of several classes of components:

The cell is not a static entity. Its machinery is always in operation, with metabolic processes keeping the cell alive and active. Plant cells also perform photosynthesis to support their life processes, and indirectly support the life processes of all other life-forms on Earth.

There are two general classes of cells, "prokaryotes" and "eukaryotes". Prokaryotes include bacteria and the superficially similar "archaea", once called "archaebacteria", both of which look like bags of biomolecules and lack distinct internal "organelles". Eukaryotes include the single-celled "protists", as well as all plants, fungi, and animals. Compared to prokaryotes, eukaryotes are much larger and have much more elaborate structures -- including a distinct central "nucleus" containing the nucleic acids, as well as a number of specialized organelles. Some of these organelles contain their own simple nucleic acid codes; it is believed that eukaryotes evolved by the collaborative combination of different prokaryotes. The sections below detail the components of the cell.

BACK_TO_TOP

[7.2] PROTEINS

* The body's proteins make up about half the dry weight of animals, with humans featuring about 30,000 different types of proteins. They are all polymer chains of "amino acids", which are organic molecules that can be thought of as a combination of amines and organic acids, with an amino (NH2) group on one side of a carbon atom and a carboxyl (COOH) group on the other.

amino acids

The central carbon atom has a side group that varies depending on the amino acid. The side chains vary in complexity, some of them being much larger and more elaborate than the amino acid core. Human proteins use 20 different amino acids; the exact formula for them is of no great importance here, but it is useful to know their names:

   _____________________________________________________

   alanine (ala)                  leucine (leu)    
   arginine (arg)                 lysine (lys)
   asparagine (asn)               methionine (met) 
   aspartic acid (asp)            phenylalanine (phe) 
   cysteine (cys)                 proline (pro)    

   glutamic acid (glu)            serine (ser)
   glutamine (gln)                threonine (thr)  
   glycine (gly)                  tryptophan (trp)    
   histidine (his)                tyrosine (tyr)   
   isoleucine (ile)               valine (val)
   _____________________________________________________

Each chain of amino acids is linked together by "peptide bonds", which are connections between the amino group on one amino acid to the carboxyl group on another.

formation of peptide bond

The polypeptide chain of amino acids makes up the "primary structure" of a protein, but the interactions between the units of the chain cause it to fold up in complicated ways. The folding will involve certain general "secondary structures", such as the "alpha helix" or "beta pleated sheet", and in some cases will involve a higher-level "tertiary structure".

primary structure of beef insulin

These tertiary structures are due to the fact that some of the side chains of the amino acids in the protein are negatively charged and some are positively charged, resulting in the weak linkages that coil the protein chain up on itself. In addition, the amino acid cysteine, which has a CH2SH side group, can link up with another cysteine on the chain to form a strong "disulfide bond", often found in stiff structural proteins. Some proteins actually consist of multiple chains working together, with such configurations said to have a "quaternary structure".

While it is relatively straightforward (at least with modern technology) to determine the primary structure of proteins, determining their high-level structures can be very difficult, requiring massive amounts of computing power. Incidentally, it is these high-level structures that account for the somewhat unusual tendency of liquid proteins such as egg white to become solid on heating: the heating unwinds or "denatures" the structure of the proteins and the resulting solid tangle can't ever be put back into its original order.

* There are two classes of proteins: fibrous proteins and globular proteins. Fibrous proteins include:

The globular proteins include:

Globular proteins also can form up into tiny "microtubules" that provide connectivity and structural reinforcement in cells, and also help make up the threadlike "cilia" on the surface of some microorganisms. The microtubules are formed as a coil of two proteins, appropriately named "alpha & beta tubulin".

The following table gives the relative sizes of various proteins, listing the number of amino acids in the protein, the molecular weight of the protein, and the number of chains that make up the protein:

   protein                 amino_acids   molecular_weight    chains
   __________________________________________________________________

   insulin (hormone)                51              5,700         2
   cobra venom toxin                62              7,000         1
   myoglobin                       153             16,900         1
   keratin                         204             21,000         1
   hemoglobin                      574             64,500         4
   gamma-globulin antibody       1,250            150,000         4
   collagen                      3,000            300,000         3
   __________________________________________________________________

Some proteins are commercially important for medical use -- insulin being one of the most prominent examples. In some cases, insulin also being one of them, it is possible to extract the valuable proteins from animals. However, in modern times bioengineering techniques can be used to modify organisms to produce desired proteins. The "classic" approach is to insert the genetic information to produce a particular protein into the human colon bacterium, Escherichia coli, with the resulting bacterial cells cultivated, collected, and purified into the desired protein. Other work has been done to genetically modify goats to produce valuable proteins through their milk, chickens to produce them through their eggs, and tobacco plants to produce them through their leaves.

BACK_TO_TOP

[7.3] LIPIDS & CARBOHYDRATES

* The lipids are distinct from other types of organic compounds in that they won't dissolve in water, but will dissolve in alcohol or other organic solvents. There are several categories of lipids.

The triglycerides -- fats and oils -- were mentioned in the discussion of organic chemistry. They are used for the body's energy storage, providing twice as much energy per mass as carbohydrates or proteins. The most common fats are the "fatty acids", which are combinations of glycerol and long-chain carboxylic acids. Fatty acids include:

   arachidic acid (from peanut oil):               CH3(CH2)18-COOH
   butyric acid (from coconut oil):                CH3(CH2)2-COOH
   stearic acid (from animal and vegetable fats):  CH3(CH2)16-COOH
   oleic acid (from corn oil):                     CH3(CH2)7=CH(CH2)7-COOH

Notice that the first three fatty acids are unsaturated, while oleic acid, with its double bond, is saturated.

The "phospholipids" are a bit like fatty acids, also being esters of glycerol, but they only have two fatty acid chains, the third chain being in the form of a phosphate-based group. This results in a molecule with a nonpolar end -- the dual fatty acid chains -- and a polar head -- the phosphate-based group.

lecithin (a phospholipid)

Phospholipids are used to form cell membranes, forming a "bilayer" -- a dual layer with the polarized heads on the surfaces. The cell membrane may feature embedded proteins -- sometimes running through the membrane, operating as a selective pore -- or proteins may loosely adhere to the polar surface.

phosphlipid cell membrane

The "waxes" are also esters, but unlike fatty acids and phospholipids they are based on simple alcohols, not glycerol. They are secreted as protective waterproof coatings on plant leaves and the skin of animals.

beeswax

The "steroids" are built around a characteristic structure of four merged carbon rings. They include a number of key hormones, for example estrogen and testosterone, the female and male sex hormones respectively, as well as cholesterol, which contributes to heart disease, and the skin poisons of some species of toads. Some steroids found in human skin turn into vitamin D after exposure to sunlight.

general steroid structure, testosterone

* The carbohydrates are organic molecules with a general formula of Cm(H2O)n, hence the name, though the early idea that they were "hydrates"-- actually incorporating water molecules -- didn't pan out. As mentioned, they serve as both cellular energy sources and structural elements.

There is a hierarchy of carbohydrates. At the bottom are the simple sugars, or "monosaccharides", which are simple ring compounds including "fructose (C6H12O6)" and in particular "glucose (C6H12O6)", which is one of the primary energy fuels in the human body. (Although fructose and glucose have the same general formula, they have different structures -- fructose is based on a five-carbon ring, glucose on a six-carbon ring.)

sugars

Two simple sugars linked together to form "dissacharides", the most familiar being "sucrose (C12H22O11)" -- ordinary table sugar. Two other common dissacharides" are "maltose", derived from malt, and "lactose", derived from milk.

amylose polysaccharide

Long chains of simple sugars form "polysaccharides", which are either used as fuel storage -- in the form of "starches" such as "amylose" for plants or the similar "glycogen" for animals, both a chain of about 25 units -- or for structures -- in the form of "cellulose", a chain of 100 to 200 units of six-carbon sugars. Cellulose makes up plant cell walls in combination with "hemicellulose" -- similar to cellulose but is based on five-carbon sugars -- along with a reinforcing material known as "lignin", a messy crosslinked structure of aromatic hydrocarbons. Lignin is particularly common in woody plants. The external "exoskeletons" of insects, crabs, and the like is made of "chitin", a polysaccharide along the lines of cellulose.

BACK_TO_TOP

[7.4] NUCLEIC ACIDS

* The nucleic acids provide central control over the cells in our body. All the cell's components are assembled and ultimately controlled by a molecular "program" in the form of "deoxyribonucleic acid (DNA)", assisted by "ribonucleic acid (RNA)". DNA and RNA are in the form of double-chain molecules, build up of three classes of molecules.

DNA / RNA bases

The core of a chain is built around a set of four "nitrogenous bases" -- there's actually five different bases, with one differing between DNA and RNA:

   adenine (A)
   guanine (G)
   thymine (T) in DNA and uracil (U) in RNA
   cytosine (C)

It is the arrangement of these bases that provides the information in DNA. Each base is linked to a sugar molecule that helps form the chain structure. The sugar is "deoxyribose" in DNA and "ribose" in RNA.

2-deoxyribose, ribose

The sugars form the links in the chain, being connected by phosphate (PO4) groups:

DNA nucleotide chaining

A base, sugar, and phosphate form up a "nucleotide group". Each of the two chains in a DNA / RNA strand are made up of a sequence of such groups. The two chains are linked to each other by weak hydrogen bonds between the bases, with the bases matching up in a "complementary" fashion as follows:

The primary function of DNA is to "code" the production of proteins. The DNA chain is organized in "triplets" or "codons" of bases, coding a particular amino acid in a protein chain as follows:

   _____________________________________________________________________

   ala:   GCU GCC GCA GCG              leu:   UUA UUG CUU CUC CUA CUG
   arg:   CGU CGC CGA CGG AGA AAG      lys:   AAA AAG
   asn:   AAU AAC                      met:   AUG
   asp:   GAU GAC                      phe:   UUU UUC
   cys:   UGU UGC                      pro:   CCU CCC CCA CCG

   glu:   GAA GAG                      ser:   UCU UCC UCA UCG AGU AGC
   gln:   CAA CAG                      thr:   ACU ACC ACA ACG
   gly:   GGU GGC GGA GGG              trp:   UGG
   his:   CAU CAC                      tyr:   UAU UAC
   ile:   AAU AUC AUA                  val:   GUU GUC GUA GUG

   START  AUG
   STOP   UAG UGA UAA
   _____________________________________________________________________

A particular DNA strand may encode a number of proteins, with a section encoding a single protein called a "gene" and marked out by the START and STOP sequences listed above.

Protein synthesis involves DNA, RNA, enzymes, and a cellular organelle called the "ribosome". Under the control of enzymes, a DNA strand is split in half down the middle, and then forms a matching half-strand of RNA. This "messenger RNA (mRNA)" then leaves the cell nucleus and links to a ribosome. The ribosome scans along the mRNA half-strand, obtaining amino acids tagged by short strands of RNA called "transfer RNA (tRNA)", "translating" the original DNA sequence into a chain of amino acids to form a protein. Once complete, the protein then spontaneously coils into its final structure.

protein synthesis from DNA

The half-strand of mRNA is called a "sense" half-strand because its nucleotide sequences -- CAA CCC AAU CUG -- define the amino-acid sequence in the final protein. The matching half-strand in the original DNA -- CAA CCC AAT CTG -- is similarly called the sense strand, while the complementary or "mirror" half-strand in the DNA -- GTT GGG TTA GAC -- is called the "antisense" half-strand.

In creation of new cells, the DNA in the cell nucleus also must "replicate", being split down the middle and reformed into two complete chains using an enzyme known as a "DNA polymerase". Errors can occur during the replication process, or sometimes due to damage to the DNA itself by radiation or other factors. These "mutations" can be disastrous, for example when mutations cause cells to replicate wildly as cancers. However, by the odds mutations are generally harmless, and in some cases mutations can be beneficial to an organism.

Incidentally, in many viruses -- the influenza virus being a good example -- the viral genome is RNA, not DNA. The two work about the same as far as coding goes, but RNA is less stable than DNA, the result being that RNA viruses tend to undergo mutations more readily than DNA viruses. The mutability of RNA viruses tends to quickly produce new strains that can evade the body's immune defenses.

* Applications of genetic technology fall into two categories: analysis and modification. With the development of advanced genomic sequencing technology, ever-increasing numbers of genome sequences -- of significant plants and animals, as well as humans -- have become available. The sequences of plants and animals have permitted a far more accurate understanding of their evolutionary ancestry, and also are useful for breeding improved crop plants and domestic animals. Databases of human genomes are becoming a significant tool in public health.

Whole-genome sequencing is powerful, but laborious, and overkill for many applications. Useful analysis can be provided by examination of the presence or absence of various sets of "marker" sequences in the genome -- the markers allowing identification of the genome of a particular individual, or the ethnic derivation of that individual, and possibly providing hints to potential health problems.

Another useful innovation in genetic analysis is the "DNA microarray" or "DNA chip" -- a slide, usually glass, marked using micro-fabrication techniques with a grid of samples of different DNA fragments that can match to target DNA sequences. A sample of DNA to be tested is broken down into fragments, the fragments being tagged with a fluorescent molecule, and then applied to the DNA chip. The chip is then washed, removing all the fragments except those that stick to cells, with the chip scanned by a laser to see which cells fluoresce, identifying a match. The DNA chips have proven useful for identifying toxins, pathogens, and cancer genes; new uses are continuously being found for them.

Early work on DNA modification was laborious and somewhat hit-or-miss, but it has now become much easier and more accurate. Some years back, biologists discovered an odd feature in the genomes of some bacteria that they described as "clustered, regularly interspaced short palindromic repeats (CRISPR)". Bacteria use them to make little bits of RNA, with the CRISPR RNA binding to a piece of DNA with a complementary sequence. An enzyme named "Cas9" recognizes the structure made when a CRISPR RNA binds to a piece of DNA, and reacts by cutting through the DNA at precisely that point.

Bacteria make CRISPR RNAs that recognize the DNA of viruses which infect on them, marking that DNA for destruction by Cas9, and so protecting the bacteria from infection. It is not difficult to synthesize RNAs that target any DNA sequence researchers want; and because of the way that cells repair broken DNA, if they put a new gene into a cell along with the CRISPR-Cas9 system, they can replace the old gene with a new one.

The effect of CRISPR-Cas9 is revolutionary, giving genome researchers something that works like the find-&-replace function on a word processor. However, genetic modification of crop plants and domestic animals has proven highly controversial, forcing agritech companies to rely on forced mutation of organisms by chemicals or radiation, with the modified organisms then bred into production product by selective breeding. Such "mutagenesis breeding" is really nothing more than a very crude form of genetic modification, but for various reasons, nobody makes much fuss about it -- one reason being that it would all but shut down agriculture if it were banned. As far as genetic modification of humans goes, at present nobody dares to try it.

BACK_TO_TOP

[7.5] METABOLISM / PHOTOSYNTHESIS

* The metabolic processes that keep the cell in operation fall into two categories: "anabolism" or "synthesis", which involves the construction and maintenance of cellular components; and "catabolism" or "destruction", which involves the use of cellular energy sources just to keep the cell working -- for example, providing energy to muscle cells to provide motion to an animal. Catabolism also supports body temperature and the degradation of waste products for excretion. When anabolism exceeds catabolism, a life-form grows; when catabolism exceeds anabolism, the life-form feeds off its own reserves and tissues, to gradually run down and die.

The anabolic "pathways" begin with simple components called "intermediates" and use enzymes to synthesize a set of "end products", particularly complicated molecules such as carbohydrates, proteins, and fats. The catabolic pathways use a different mix of enzymes to break down complicated molecules into simple ones, producing energy. The fundamental unit of "fuel" for biological processes is "adenosine triphosphate (ATP)", a simple organic molecule with three high-energy phosphate bonds. One phosphate group can be expended, resulting in "adenosine diphosphate (ADP)"; expending another yields "adenosine monophosphate (AMP)".

adenosine triphosphate (ATP)

The metabolic system is extremely complicated. It is not only controlled at the cellular level, it is also controlled at an overall level by the nervous system (in animals), as well as hormones -- "chemical messengers" -- released by the pituitary and adrenal glands. Inputs are amino acids from the digestion of proteins; carbohydrates, mostly obtained from digestion in the form of glucose; plus glycerin and fatty acids. Amino acids are synthesized into proteins; glucose is used for energy, with any surplus first converted to starches and the surplus above that to fats; and fats are incorporated into organism structures, with the surplus also stored for lean times.

Metabolic processes produce carbon dioxide and water as waste, with the carbon dioxide exhaled and the water excreted, carrying with it other waste products, indigestibles, and toxins rejected by the digestive system. Trace vitamins are needed to keep the system working.

* Some metabolic processes are consumers of energy, while others are producers of energy -- with the important fine print that the energy has to be obtained from food in the first place. It is the photosynthesis of plants that drives the entire food chain, obtaining energy from the Sun and converting it into fuel and plant structure. At a high level, it is a seemingly simple process, in which carbon dioxide and water are converted into oxygen and the sugar glucose, along with some water:

   6CO2 + 12H2O  --> C6H12O6 + 6O2 + 6H2O

The glucose can then be converted into cellulose, the structural material that makes up the cell walls. Excess glucose is converted into starch and other carbohydrates, once again to be stored for lean times.

The chemical formula given above is just a "black box" description of photosynthesis, showing the inputs and outputs but not detailing the highly complicated parts in between. Photosynthesis in plants is performed by a cellular organelle named the "plastid" or "chloroplast", with a few dozen in each plant leaf cell. Like the mitochondrion, the chloroplast has its own genome and clearly had a free-living prokaryotic ancestor. The chloroplast is oval-shaped and stores what looks like a stack of disks, each of which is filled with chlorophyll, a green pigment that is central to the photosynthetic process, as well as other "accessory" pigments, plus enzymes and other molecules needed to support photosynthesis.

photosynthesis

The actual chemical reaction is very complicated, but in broad sweep it consists of two stages:

* While this discussion describes photosynthesis in plants, it also occurs in some bacteria, most notably the "blue-green algae". Since prokaryotes don't have distinct organelles, photosynthetic bacteria don't have chloroplasts, instead containing chlorophyll in folds of their surface membranes called "chromatophores". Some of the archaea also perform a variation on photosynthesis, though instead of using water they use hydrogen sulfide (H2S) and produce sulfur instead of oxygen. There are those who suspect that life got started around sulfurous volcanic vents and that these archaea are the distant descendants of the first true living organisms.

BACK_TO_TOP

[7.6] VITAMINS & OTHER MISCELLANEOUS BIOMOLECULES

* Along with a discussion of the primary biomolecules -- proteins, lipids, carbohydrates, nucleic acids -- it is worthwhile to consider the class of biomolecules referred to as "vitamins", since the term is in such wide usage.

Vitamins are used by the body to synthesize enzymes, hormones and other chemical necessities. Humans need 13 vitamins to survive; they are all relatively simple organic molecules, with a few dozen carbon atoms at most, featuring ring and chain structures. Vitamins, also called "micronutrients" since they are only required in minute quantities, can be grouped in two categories:

Nutritionists say that it's better to get vitamins through food, the main reason being that the vitamins in fruit, vegetables, and so on also include large numbers of other "phytochemicals" -- plant nutrients that aren't essential for life, but seem to boost health. These phytochemicals include carotenoids in carrots and tomatoes, isothiocyanates in broccoli and cabbage, and flavonoids in soy, cocoa and red wine. Taken together, vitamins and phytochemicals seem to have much more effect than they do when taken individually.

Nutritionists do recommend that healthy adults take a multivitamin pill and extra vitamin D if they don't get much sun. There is debate over the usefulness of large doses of other vitamins -- overdoses of vitamin A are known to be hazardous -- but folic acid (AKA vitamin B9) is recommended for pregnant women to prevent birth defects, vitamin B12 supplements are useful for people over 50 because it gets harder to absorb the vitamin in the digestive tract, and AIDS patients can benefit from multivitamins, since they boost immunity. Ironically, most of the people who take multivitamins are the affluent and health-conscious, and are generally the ones who need them the least.

* A second class of simple biomolecules are the "opsins", which are photosensitive dyes used for vision. They consist of a small protein chain attached to a variant of the vitamin A molecule. Minor variations in the amino acid structure make the opsins sensitive to different colors of light -- humans have three opsins, for red, green and blue. Most other mammals only have two and see in two colors, but birds and some other animals have four and can see into the ultraviolet in some cases. There is a related molecule named "rhodopsin" that is not very color-sensitive but is very light-sensitive. It is used for night vision; nocturnal (night living) and deep sea creatures often only have rhodopsins, no opsins.

* A third interesting minor class of biomolecules are the "semiochemicals", which are "messenger molecules" that send a message from one organism to another. The semiochemicals consist of the "allomones", "kairomones", and the well-known "pheromones".

The allomones provide an advantage of some sort to the organism that produces them. Antibiotics -- chemicals that attack bacterial infections -- were famously discovered in molds that killed off bacterial pathogens. Nicotine is produced by tobacco plants, not to gratify smokers, but to discourage caterpillars from eating the plants, in other words acting as a natural insecticide. The "perfumes" of sweet-smelling flowers act as attractants to pollinators such as bees.

The kairomones provide an advantage to the species that detects them, not the species that produces them. In some cases, allomones and kairomones are one and the same thing -- the perfumes of flowers are allomones to flowers and kairomones to bees. In other cases, kairomones are smells produced by an organism that its adversaries have learned to recognize -- for example, some marine molluscs can smell a starfish nearby and make an escape.

Pheromones are messages sent between members of the same species:

Humans have long leveraged off attractants and sex pheromones to produce perfumes -- though in modern times, scent manufacturers have generally turned to direct synthesis of odors, instead of laboriously extracting them from flowers or the musk glands of animals. The old-fashioned reliance on nature is cost-effective for production of expensive, low-volume perfumes, but the bulk of the industry's production is focused on scents for air fresheners and laundry detergents.

Scent manufacturers still rely on nature for inspiration. "Headspace" sampling was invented in the 1970s and is now being widely used. The technique involves obtaining samples in sealed jars, then later releasing the odor and passing it through a charcoal or silica filter to catch all the organic molecules. The molecules are then sorted out with a gas chromatograph, and tested by a professional sniffer to see if they are valuable. Once screened, the scents are filed in a database for future reference.

BACK_TO_TOP

[7.7] FOOTNOTE: THE ORIGINS OF LIFE

* One of the most intriguing "leading edge" research topics in biochemistry is the question of "abiogenesis", the origins of life on Earth. While we have a good understanding of the evolution of life after its beginnings, exactly how life began in the first place is a matter of intensive investigation.

Critics of abiogenesis studies like to insist that it is impossible for life to have arisen from nonlife, pointing to the complexities of even simple bacterial cells and performing calculations based on those complexities that show the probabilities of life spontaneously arising from nonlife are vanishingly small -- comparing it to the idea that a jumbo jetliner could be thrown together by a tornado running through a junkyard.

However, the fact of the matter is that life did arise from nonlife, otherwise we wouldn't be here, and so obviously it was possible. As far as the probability calculations go, they're silly. Texts on probability often use, say, playing cards as examples, deriving the probability of obtaining certain hands of cards. That's fine in itself, but with playing cards, we have an effectively complete knowledge of the scenario, knowing the numbers and types of cards, and the rankings of the various hands.

In the case of the origins of life, we have nothing close to a complete understanding of the scenario -- that's why the research is being done. A simple calculation of the probabilities of the synthesis of a molecule on the basis of its molecular arrangement is meaningless, since we don't really know how that molecule was formed. After all, on the basis of its arrangement, the probability of the formation of a salt crystal ten atoms on a side is no less than one in (2^1000)/24 = 4.17E300. Of course, such a tiny salt crystal is perfectly ordinary, its formation not being dependent on random processes.

The "tornado in a junkyard" comparison is based on a misunderstanding of abiogenesis studies. Nobody working in the field believes that even a bacterial cell could have spontaneously assembled. All research from the earliest days of the field has assumed that life began with some simpler system that gradually became more elaborate. Critics insist the odds of the emergence of even a simple self-replicating molecular system are vanishingly small -- but given high rates of chemical reactions on a young and geologically-active Earth, working as a planetary-scale "bioreactor", over hundreds of millions of years, the emergence of life might well have been a certainty.

Until we learn more, all we can say about the probability of the spontaneous emergence of life on Earth is that it's somewhere between ZERO to ONE. Life may indeed be a freak accident of the cosmos; the Earth could be the only planet in the entire Universe where it has arisen. Few would find that idea any less than staggering: there are 100,000,000,000 stars in our own Galaxy, and even if life is a one-in-a-million thing, that means 100,000 star systems where life has arisen, in forms that could resemble life on Earth or could be radically different from it. We are simply ignorant of how widespread life actually is, and ignorance gives us no sensible basis for argument about anything.

* The first serious work on abiogenesis was conducted in the 1920s, when the British biologist J.B.S. Haldane (1892:1964) and the Soviet scientist Alexander Oparin (1894:1980) proposed relatively detailed models for how life might have arisen. Haldane postulated the rise of biomolecules in a warm body of water saturated with simple biomolecules, which Haldane called the "primordial soup". Oparin started out with a similar environment, but suggested that oily blobs could have led to the creation of simple cellular structures.

The "primordial soup" notion is now seen as misleading, since nothing much is likely to happen in an unenergetic broth of chemicals. In modern times the focus has shifted to ocean-floor volcanic vents, which are not only chemically diverse and energetic environments, but also support the growth of "chimneys" above the vents -- tall porous mineral structures that provide excellent substrates for chemical synthesis.

In any case, nobody paid too much attention to the ideas of Haldane and Oparin, mostly because the era's knowledge of biochemistry was so primitive. However, an American chemist named Harold C. Urey (1893:1981) was impressed and eventually decided to perform a simple test of the matter. In 1953, he set a graduate researcher named Stanley Miller (1930:2007) to the task, with Miller recirculating what Urey believed to be a sample of the primordial atmosphere -- consisting of hydrogen, ammonia, and methane, but no oxygen, or what became known as a "reducing" atmosphere -- in a flask that was subjected to electric sparks. After a few weeks of this treatment, organic materials, including the amino acids used to build up proteins.

The Miller-Urey experiment was trumpeted up in the textbooks as evidence that the spontaneous origin of life was at least plausible, but in hindsight it wasn't that dramatic a revelation. Hydrogen is a light gas and tends to dissipate into space from the atmospheres of relatively small planets with low gravity like the Earth, and most geoscientists believe today that the primordial atmosphere was "nonreducing", composed of carbon dioxide and some nitrogen, and at least initially experiments that tried to duplicate the Miller-Urey experiment with such an atmosphere were duds.

Even the production of amino acids was not all that impressive. They're easily generated, actually being found in interstellar gas clouds. The chemical reactions that generate them are exothermic -- they release energy -- and so it's thermodynamically straightforward. Assembling amino acids into proteins is an endothermic reaction -- it requires energy. That doesn't rule out the self-assembly of proteins, but it makes it much more troublesome to consider.

In addition, life can be divided into two primary components: replication based on DNA and RNA, and metabolism based on proteins, with the two sets of processes strongly interlinked, neither being particularly useful without the help of the other. The Miller-Urey experiment focused on the synthesis of protein components and said nothing about the synthesis of DNA and RNA. Early experiments to simulate the spontaneous origin of RNA proved frustrating.

* Although the Miller-Urey experiment has been heavily criticized, it does have its place of honor in the history of science as the first serious experiment to investigate abiogenesis. It has been followed by improved variants operating on a range of scenarios, and recent experiments have been encouraging. It turns out that a reducing atmosphere does a perfectly good job of producing amino acids, as long as reactive nitrogen compounds formed by the sparks are soaked up by trace "buffering" chemicals. Similarly, difficulties in the synthesis of nucleic acids increasingly seem to have been rooted in our past failure to understand that there are many different ways in which nucleic acids can be synthesized; experiments based on revised assumptions are now showing it's not as troublesome as it was once thought to be.

New theoretical models have been developed in parallel with improved experiments. One scheme, with its roots in the 1960s with the British-American chemist Leslie Orgel (1927:2007) of the Salk Institute in the US and promoted by German biophysicist Manfred Eigen (born 1927) of the Max Planck Institute in Germany, postulates that in the beginning, life was based on RNA. One of the interesting things about RNA is that, unlike DNA, it does not need proteins for replication. RNA is capable of acting as an (admittedly inefficient) catalyst to support its own replication, with such catalytic RNAs known as "ribozymes".

Even more intriguingly, test-tube experiments show that if the components of RNA are mixed in solution with a metal to act as a catalyst, they will slowly form up full RNA molecules. Biologist Jack Szostak of Harvard (born 1952), a Nobel-prize-winning biochemist and a well-known figure in the field of abiogenesis research, has conducted experiments that show that if RNA is mixed with fatty acids, a fatty envelope will grow around the RNA, forming what might well be a protocell. These experiments have been able to produce envelopes that can grow by accumulating more fatty acid molecules, and allow nucleic acids to leak through.

Biochemists tinkering with RNA have also found that it easily associates with short protein segments or "peptides", forming "ribonucleoprotein complexes" that could represent the roots of the mutualistic system of replication and metabolism seen in modern organisms. It is becoming apparent the emergence of the first protocells really was in one step -- not RNA or peptides or lipids, but all three of them together, achieving a synergy in collaboration, a kind of molecular "mutualism" or "symbiosis".

* Critics ask why we see no evidence such protocell systems in the world today, but the answer's obvious: as each more efficient level of cell systems arose, it shoved the previous one into extinction by soaking up all resources available and devouring its predecessor. Modern organisms are found all over the planet, in surprisingly hostile environments, even deep into the Earth, and it is very difficult to set up a lab environment free of biological contamination. Any resources that could support a second abiogenesis would be quickly gobbled up by the life that exists. The only way we will likely observe any early protocell system is in a lab experiment, and since researchers don't have a planetary-scale bioreactor like the primordial Earth and hundreds of millions of years to experiment, that remains a challenge.

Abiogenesis researchers clearly have challenges, but the overblown attacks by their critics amount to a con job -- in which the origin of life is declared impossible, with the inconvenient fact of its existence then attributed to some arbitrary, unexplained, permanently unexplainable magic process. Whatever the limitations of current abiogenesis research, invoking magic based on "arguments of ignorance" is lame. Researchers point out to critics that it wasn't so long ago, in the era of vitalism, that life processes were regarded as entirely mysterious and unexplainable. Nobody seriously believes in the "non-answer" of vitalism any longer, and future generations may see the idea that life origins had to be some sort of unexplainable magic as just as silly a "non-answer".

We may never be able to do more than construct a number of plausible models of the origins of life, and not be able to choose conclusively among them -- but if we are able to imagine plausible schemes by which it could have happened, nobody will be able to credibly claim that the origins of life are "unimaginable", and declared magical. It will be exciting enough just to show that there are schemes that are workable.

BACK_TO_TOP
< PREV | NEXT > | INDEX | SITEMAP | GOOGLE | LINKS | UPDATES | BLOG | EMAIL | $Donate? | HOME