Essentials for Life...

Having argued that water is the ideal transport medium for life, let us next
look at the further building materials of organisms which guarantee rigidity
and mobility, a reproducible structure, and fast supply during construction and
maintenance. Lehninger has classified 6 chemical elements as of class I (H,
C, N, O, P, S), another 5 as of class II (Na, K, Mg, Ca, Cl), and additional
16 trace elements as of class III (Mn, Fe, Co, Cu, Zn, B, Al, V , Mo, I,
Si, Sn, Ni, Cr, F, Se), all of which are essential in the nutrition of at least
one species. Man may even require more than ten additional elements for rare
but essential purposes (in enzymes). Apparently, life makes use of a significant
fraction of all the chemical elements, all of which happen to be available at the
surface of the Earth thanks to the stirring action of deep-rooted volcanism
(including plate tectonics) whose feeding plumes (tubes) drain on its fluid
core. And a close look at all the various molecules used in plants and animals
leaves me with the impression that organic chemistry is essentially the science
of the biological building blocks, which function like tinker toys, or rather like
LEGO blocks.
A leading role in organic chemistry is played by the element carbon, for
which no substitute is readily in sight: Its central position in the periodic
table of elements allows it to bind strongly to itself, forming chains, rings,
and tubes, as well as to various other atoms. Its alloys outnumber those of all
other elements. Without carbon, life would hardly find enough building blocks
to choose from. Its nearest rival is silicon whose corresponding hydrogen alloys
are so weakly bound that their chemistry would require temperatures too low
for reasonable reaction rates. Living organisms consist to 71% by weight of
water, and to 20% of carbon, another 7% being almost equally shared by Ca,
P, and (excess) H. (The constituent elements in hydrocarbons occur in the
approximate number ratios H : C : O : N ≈ 5 : 3 : 2 : 0.1 ). CO2, the burning
product of carbon, is a gas of which man exhales a kg per day. It has an
anomalously high solvability in water, in violation of Dalton’s law, and is an
efficient buffer of the acidity (pH value) of a solution.
The basic building blocks of living organisms are autonomous cells which
commit suicide (apoptosis) when malfunctioning. They are separated from
their surroundings by at least one insulating, hydrophilic, phospholipid membrane,
_ 10−2.3μm thick, consisting of two adjacent layers of (hydrophobic)
fatty acids topped on both sides by phosphate heads. These membranes contain
many protein sluices for exchanges with the outside world, specific for
each type of cell. For pulling big molecules (such as glucose) through a watertight
sluice, an electric voltage of 10−1.15±0.15 V is provided by permanently
active Na-K pumps which suck Na+ ions to the outside in order to charge
it positive. Na+ions can thus serve as electric engines pulling cargoes electrostatically
to the interior through narrow channels. The pumps are thought
to act as heat pumps, arresting the Na+ ions on the outside once they have
managed to thermally bounce across the opposing voltage [Kundt, 1998b]. For
this to be possible without violating the second law, the gate keeper has to be
powered each time by an ATP molecule (= adenosin triphosphate) – the biological
energy unit (= 0.32 eV) – which is hydrolysed in this process to ADP
(plus H3PO4). Of course, steady-state operation requires a power station inside
the cell – the mitochondrion – which recycles ADP to ATP. No work is
performed in compartments lacking a mitochondrion, or some similar organ,
like a chloroplast. Note that a high thermodynamic efficiency (of order 0.66)
is required for all the cellular engines in order to avoid overheating during
action. Multicellular life has probably used them right from the beginning.
Ion pumps similar to the Na-K pumps, common to all plant and animal
cells, have diffusive charging times of order ms, which sets a lower limit to
biological reaction times. They are also used by some fish for orientation in
turbid waters: such fish emit electric pulses of several V, obtained by stacking
several batteries in series, and map with their lateral-line organ (so to speak)
the mirror charges in their surrounding conductors, being sensitive to field
strengths of _ 5 nV/cm. Much higher voltages, of up to 0.8 kV, are generated
by the torpedo ray, the South-American electric eel, and the African catfish
in order to paralyse their prey, by stacking some 104 batteries in series; their
discharge power peaks at 10 kW, for a small fraction of a second.
Voltage-gated proteinic ion channels through membranes in nerve cells,
muscle cells, and other biological cell types, from bacteria to humans, function
as (sensitive) transistors , with sensitivities dJ/dU of their currents J (of
potassium, sodium, or calcium ions) twice as large as their electronic counter
parts .
Returning to cells as the building blocks of higher organisms, it should be
mentioned that their static solidity is achieved by means of rods and ropes
crossing their interiors (as with tents), the rods consisting of tubulin tubes,
the ropes of actin filaments. Ordered transport inside cells takes place along
these 1-d structures, via molecular engines, viz. via dynein and myosin motors
which slide along them, reminiscent of conveyor belts at airports. Even inside
individual cells, traffic is not left to diffusion! Moreover, the higher rigidity requirements
of plants (compared with animals) are met by additional (primary
and secondary) cell walls, formed primarily from cellulose fibers, of thickness
_ 0.3 μm. These walls have to take pressures of _ bar, exerted by stepwise
jumps in the osmotic pressure, a phenomenon known as turgor. During cell
growth (through factors of _ 106 in volume), these strong walls are distended
by transiently loosening the cohesive network of polysaccharides (cellulose microfibrils)
via the action of expansin proteins [Nature, 2000: 407, 321]. The
number of cells of an organism grows with its size: {fungi, plants, mammals}
consist of _ {1012, 1015, 1016.4} cells, of some {4, 20, 102} different types,
respectively.
There is no present-day organism without a building plan: in 1953,Watson
and Crick discovered the structure of this carrier of information: a double helix
called DNA (= deoxyribose nucleic acid ) whose rails (strands) are formed
alternatingly from sugar and phosphate, and whose ties (rungs), stretching
between successive sugar links, are formed from the two base pairs AT and GC
of nucleic acids called adenine, thymine, guanine, and cytosine, respectively,
which are linked by (weak) {double, triple} bindings of equal total length
(20 ˚A), comparable to a zipper [Hoyle, 1975]. Whilst the strands consist of
carbohydrates and phosphorus, the rungs require nitrogen. DNAs of {viruses,
bacteria, fungi, plants, insects, amphibians, mammals (including man)} have
lengths of {_ 104, 106.5±0.5, 107.5±0.5, 109.5±1.5, 109±1, 1010±1, 109.6±0.4}
rungs. Rungs have separations of 3.4 ˚A, resulting in a total length of a human
DNA of 109.8+0.5˚A = 2 m. This giant DNA molecule, with the topology of
a ladder or railroad track endowed with an intrinsic direction, can be orderly
packed, or hierarchically folded into 46 rod-shaped chromosomes (for man),
each _ 10 μm long, via coiling and winding onto (positively charged) proteins
made of histones to form nucleosomes, then coiling again to form a solenoid,
and finally via chromatine loops. Imagine the full genetic code, of macroscopic
length, packed into a few dozen mesoscopic rods which fit into the nucleus of
every cell!
Because of the unique pairing of the four bases, a DNA can be unzipped
into two strands, each storing the full information. When a DNA is transcribed
in vivo, each sugar S := deoxyribose is replaced by a ribose S’, in order to
be distinguishable from the original, and each thymine replaced by a uracil.
The genetic code stored in a thus-obtained, single-stranded RNA is such that
each 3 successive rungs form a letter, composed of arbitrary combinations of
A,C,G, and U so that there are 43 = 64 letters. Of these, AUG stands for
a beginning, and UAA, UAG, and UGA stand for an end. The remaining
60 letters code for 20 different known amino acids. Every DNA is composed
of a long sequence of subunits, a small percentage of them called genes, each
_ 103 basepairs long, which code for proteins. Proteins are chains of _ 102.5
amino acids; they are the worker bees of a cell. In this way, a DNA stores the
information of a long sequence of different amino acids, some 106 for man, and
is (even) able to synthesize them. It is not clear at this time what percentage
of the remaining DNA segments – among them transposons and retroviral
sequences – serves exclusively their host, or the host’s future evolution, or
perhaps nothing useful at all(junk DNA) .
Remarkably, the amino acids (and proteins) of all living creatures have
left-handed chirality whilst their nucleic acids (sugars, DNAs) are all righthanded.
Should one conclude that life sprang forth from just one common
ancestor, whose choice was made by accident, or did the 1/3 dominance of
left-handed amino acids in the carbonaceous chondrites of the solar system
influence this ambiguity of molecular symmetry for life? Or can the amino-acid
catalyst proline create the asymmetry?
Among the basic organic molecules are also the chlorophylls which use
photon power during the photosynthesis of plants to drag electrons to one side
of the thylakoid membrane, ready to attract an equal number of protons from
the other side whose electrostatic energy (and free-fall momentum through
a channel) is used for ATP synthesis. Chlorophyll is related to haemoglobin:
replace the central magnesium atom in its porphyrin head by an iron atom,
and you get one of the four oxygen- and CO2-binding haem groups in the
haemoglobin of the red blood corpuscles of animals. The chemistry of life
shows remarkable uniformity and order!
Essential for life are also the pumps which pressurize the blood of animals
to make it circulate, or the water in plant roots to make it rise, their
hearts. Remarkably, the human heart has the longest lifetime measured by
the number of its beats, 109.6, some four-times more than the typical 109
beats of animal hearts. The analogous situation for plants is ill-treated in the
literature [Kundt, 1998c]: Large trees can lift a ton of water per day to their
crowns, part of it overnight, and occasionally under conditions of saturated
water-vapour pressure. They can hold their water columns by means of capillarity
and osmotic suction, which replace evaporation losses whenever ground
water is available. The motor driving the rise of water is often said to be transpiration,
but transpiration removes water, it does not supply it; the hearts
of the plants sit in their root tips. The roots take in water from the ground
via osmotic suction, and a reverse osmosis is required in order to allow for
a second osmotic pull of their crowns. This faculty has been known for over
275 years through the phenomenon of exudation, or root pressure, which can
reach 6 bar in tomato plants, 10 bar in grass stalks, or even up to 60 bar
in certain desert plants. Root pressure is provided by billions of subcellular
mechanical pumps, the desmotubuli, in the pores of the endodermis and pericycle
of the root-hair zone, in all young root tips, _ weeks old. Here, some 103
plasmodesmata per cell wall – each encompassing a (central) desmotubule –
achieve the indispensable reverse osmosis, forced by myosin VIII that strains
an actin spiral.
Another essential for life is metabolism. Among the best examples of metabolism
is the South-American bull frog, 20 cm long, with a splendid appetite,
which eats everything up to the size of 1.5-m long snakes which in turn do not
mind eating frogs. It is a matter of speed and strength whose head enters the
other’s throat for good. Once its head has been swallowed, the victim’s further
fate is left exclusively to subconscious processes which squeeze it down
the gullet and apply all sorts of organic and inorganic chemistry, involving
rough-surface wall catalysis in the stomach, to dissolve and metamorphose it
into the other’s replenishment. It works both ways, and takes a day or longer.
Most impressive is the chemical perfection at which complex creatures are
reprocessed, with seemingly quite modest growth of the overall entropy.
Even plants can digest animals, the meat-eating ones. Moreover, plants
use the atmospheric carbon available in the form of carbon dioxide, via photosynthesis,
employing chlorophyll to convert daylight into electric voltage
which makes endergonic carbon reactions feasible. These photovoltaic engines
are carefully constructed in maximising integrated sunlight during sparse supply,
via suitable orientation and chemistry, and by avoiding burnout during
excess supply, through non-photochemical quenching (dissipation) via zeaxanthin
and possibly lutein. Animals owe their existence to carbon-providing
plants.

pt ca nu stiu engleza si uneltele lingvistice de la google nu traduc de nici o culoare mi-a rezultat un amalgam de cuvinte si informatii pe care nu le pot lega sa obtin cheia ...