Figure 7.1. Leonardo da Vinci’s illustration of a fetus in the womb.
7. THE MATRIX
Water is the most important liquid for our existence and plays an essential role in physics, chemistry, biology and geoscience. What makes water unique is not only its importance but also the anomalous behaviour of many of its macroscopic properties.… If water would not behave in this unusual way it is most questionable if life could have developed on planet Earth.
—ANDERS NILSSON AND LARS G. M. PETTERSSON1
THE TINY LEUKOCYTE CELL CHASING A BACTERIUM IN THE VIDEO mentioned in Chapter 1 makes compelling viewing.2 The leukocyte senses the chemical “aroma” of the bacterium. It crawls in a specific direction along a chemical gradient, following the smell of the bacterium. And it does this while continuously making transient selective adhesions to the substratum. These skills are remarkable any time, but we find by far their most spectacular application in the miracle of embryology.
In the embryo it is not just one cell moving towards a specific target but millions of cells, each moving towards specific targets in an ever-changing kaleidoscope of different embryonic cells and chemical signals, with each cell obeying a strictly choreographed program, a program directing the timing of gene expression and a unique succession of changes in cell shape and cell surface proteins and adhesive properties in different cells in different regions of the embryo.
While crawling through the embryo and sticking to their assigned partners at precise moments and in precise places in the developing embryo, the cells continually pick up chemical and physical signals from the mass of surrounding cells, signals that direct all manner of molecular and genetic activities within the individual cells. These involve precisely timed expression of particular sets of genes and exquisitely ordered movements of actin filaments, molecular motors, and microtubules, which act together to generate continual changes in the cells’ architecture. In one place and time an embryonic cell may turn into a neuron, in another into a red blood cell, in another into a leukocyte, in another into a photoreceptor.
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Figure 7.1. Leonardo da Vinci’s illustration of a fetus in the womb. |
It is hard to envisage any material phenomenon more complex than the development of the embryo, involving as it does the integration of such a myriad of precisely coordinated events. The totality is beyond grasp. It involves a seeming infinity of subtle and exquisitely choreographed changes in the architecture and shape of cells in different parts of the embryo; precise spatial and temporal ordering of myriads of changes in cell surface properties in different parts of the embryo; numerous precisely ordered cell divisions, particularly in the earlier stages of embryogenesis; a vast diversity of emergent cell movements and cellular morphologies; the setting up of all manner of diffusion gradients; and the programming of numerous cell types to read their exact position in these gradients at precisely predetermined times.
The development of the embryo with its staggering panoply of continuously morphing cells—each finding its unique way through the seething and dynamic yet highly ordered embryonic web of cellular matter, touching and feeling its neighbors in search of spatial and temporal clues and obediently changing its own chemical, genetic, and physical state in response—is by far the most complex phenomenon on Earth, far more complex by many orders of magnitude than the assembly of the most complex human artifact ever built.
Indeed, the developing embryo is a phenomenon far beyond anything in the realm of our ordinary, or extraordinary, experience. The unimaginable immensity of spatial and temporal molecular clues and molecular and genetic responses exploited by this innumerable host of nanobots navigating the embryonic ocean is far greater than all the maps, charts, and devices used by all the mariners who ever navigated the oceans of Earth. No human machine built to date nor any on the drawing board of even the most ambitious and farsighted gurus of nano-technology is remotely as complex as a developing embryo.
One key to the miracle is another ensemble of fitness in nature, this one arising from the unique properties of that most familiar yet most remarkable of liquids—water. As Albert Szent-Györgyi famously commented, “Water is in any case the central substance of living nature. It is the cradle of life, the mother of life and its medium. It is our mater and matrix.”3
Early chapters touched on some of the unique properties of water essential to cellular function. Several of these and others are described in more detail below.4
ONE PROPERTY of water not touched on in this book thus far, which has a critical bearing on the function of all carbon-based cells but particularly on the function of the large complex cells in advanced multicellular organisms like ourselves (and embryos) is its viscosity. Water’s viscosity determines two important parameters. One is the diffusion rate of solutes such as oxygen and nutrients in aqueous solutions. The second, viscous drag, is the resistance experienced in moving an object in a liquid (think of moving a spoon through honey). Diffusion rates are inversely proportional to the viscosity of a liquid (as indicated by the Stokes-Einstein equation, D = k/m, where D is rate of diffusion, k is a constant and m is viscosity5) while viscous drag is directly proportional to viscosity.6 As we shall see, if the viscosity of water was not very close to what it is, there would be no embryos, no multicellular organisms, and no cells capable of the crawling and morphing talents of the leukocyte and its embryonic cousins.
NOT ALL cells can crawl or change shape like a leukocyte or the cells of an embryo. Many cells, including plant and fungal cells, are encased in a rigid cell wall and are unable to crawl or change shape. Bacterial cells can move using a flagellum, but they cannot crawl or adopt an endless series of different morphologies as the leukocyte can. Only cells bounded by the pliable and remarkably deformable bilayer lipid cell membrane can crawl, change shape, and use these two skills to build the embryo.
The absence of a rigid cell wall is not the only factor that precludes the escaping bacterium from being able to crawl and morph like the leukocyte. Cell motility and morphing also depend on a suite of specialized cytoplasmic molecular motors and other macromolecular components. Their extraordinary complexity is described in many major texts.7 Most bacterial cells are far too small to contain this suite of cytoskeletal components. An E. coli bacterial cell, for instance, may contain a mere two million proteins,8 compared to many human cells containing several billion protein molecules,9 several million actin monomers (which polymerize to make actin filaments), and about a million myosin motor molecules (providing the contractile force involved in crawling).10
The maximum possible diameter of cells is constrained by a fundamental physical parameter: the rate of diffusion of molecules in water. Diffusion is very rapid and effective over short distances but increasingly slow and inefficient over long distances. More formally, diffusion time increases with the square of the diffusion distance. Thus, as Knut Schmidt-Nielsen explains, if one were to impose a stepwise increase in oxygen at a given point, it would diffuse one micron in one ten-thousandth of a second. But for the oxygen to travel ten times as far would take one hundred times as long. Thus an average diffusion distance of ten microns (the distance across some cells) takes one hundredth of a second; one millimeter takes one hundred seconds; ten millimeters, about three hours; and one meter, about three years.11
Since diffusion time increases with the square of diffusion distance, and volumes increase with the cube of a sphere’s diameter, it follows that if diffusion rates in water had been slower than they are—that is, as slow as they are in many other fluids (see below)—utilizing oxygen and nutrients at the same rate as our own body cells or the cells of the embryo use them would require decreasing the sizes of the cells substantially. For example, if diffusion rates were ten times less than they are, then maintaining the same oxygen consumption would require the volume of a roughly spherical cell to shrink by a thousand fold. If diffusion rates were one hundred times less than they are, then the same cell would need to shrink by a million fold to maintain the same oxygen consumption—in both cases, almost certainly too small to contain the complex cytoskeletal components to enable crawling and the other abilities cells need to create an embryo.
Also, because the surface area of such hypothetical mini-cells would be one hundred to ten thousand times less, the total number and diversity of cell-surface adhesion molecules also would be drastically reduced. Such tiny cells would hardly be able to put out the complex arrays of micro-protrusions or filipods (mentioned in Chapter 4) that detect chemical signals in the environment and initiate cell-cell and cell-matrix adhesion.12 These abilities are crucial for the pathfinder or pioneer neurons in the developing nervous system.13
In sum, the existence of cells large enough to crawl and morph depends on the diffusion rates of oxygen and other solutes in water being not significantly less than what they are. If these diffusion rates were appreciably lower, active aerobic cells large enough to contain the molecular machinery for crawling and morphing could never have emerged.
BECAUSE OF diffusional constraints, the aerobic cells in complex multicellular organisms, including embryos, require a set of narrow tubes—capillaries about five microns across—that permeate all the tissues and organs of the organism. These carry blood enriched in oxygen, which supplies via diffusion sufficient oxygen to satisfy the energetic demands of these organisms. Mammalian tissues, for example, are permeated by 1,000 capillaries per square micron14 so that most capillaries are about forty microns apart and most tissue cells are between one to three cell widths from a capillary.15
The design of the capillary bed is constrained by another physical parameter arising from viscosity—viscous drag. The pressure (P) required to pump a fluid through a pipe rises with the viscosity of the fluid (m),16 because the greater the viscosity the greater the viscous drag. Think of the difficulty of drawing honey up a straw compared with a far less viscous fluid such as water.
If blood’s viscosity (largely determined by water’s viscosity) were increased two or three times, the pressure needed to pump blood through the capillary bed would be too great. As things are, the head of pressure at the arterial end of a human capillary is thirty-five mm Hg, which is considerable, about one-third that of the systolic pressure in the aorta. This relatively high pressure is necessary to force blood through the capillaries. This would have to be increased proportionately if water’s vicosity were higher. But then the thin endothelial lining of the capillaries (necessary if nutrients and oxygen are to diffuse unimpeded into the tissues) would be subject to greater pressures and the threat of rupture.
From these considerations it is clear that the functioning of the circulatory system and particularly the capillary bed depends on two different physical parameters both being very close to what they are: the diffusion rates of oxygen and nutrients in water, and the viscous drag of water. The high diffusion rates enable the transfer of oxygen and nutrients from the capillaries in sufficient quantities to supply the energy needs of the cells, but only because water’s low viscous drag enables the perfusion of the capillary bed at a rate sufficient to satisfy those needs.
If viscosity were significantly higher, trying to compensate for that by increasing the number and size of the capillaries would take up too much volume. The organism or embryo would be reduced to a bag of fluid, leaving no room, as Stephen Vogel put it, for “guts or gonads.”17
On the other hand, if the viscosity were much lower, then diffusion rates would be greater and viscous drag less, but the delicate cytoarchitecture inside the cell would be subject to more intense Brownian bombardment, since particle mobility in a fluid is inversely related to viscosity.18 Thus, things would be far less stable. The half-lives of the cell’s key macromolecules would also be decreased and the energetic burden of maintaining cellular homeostasis increased. It is highly doubtful if anything remotely resembling a living cell would be feasible if the viscosity of water approached, for instance, that of a gas. The low viscosity of gases is the reason most authors reject them as suitable media to instantiate a chemical living system like the carbon-based cell.19 Gases are also far too volatile and labile to be considered seriously as candidates for the chemical matrix of life, as Arthur Needham comments:
Only systems based on a fluid medium could display the properties which we should accept as Life. Gaseous systems are too volatile and lack the powers of spontaneously segregating sub-systems… In gases… the tendency towards a uniform distribution of energy is very rapid but in liquids is slow enough for local differences to be maintained… and for steady-states to be set up. All gases mix freely with any other, but not all liquids. Some form discontinuities or interfaces, with solid-state properties, where they meet another liquid, and complex polyphasic systems are readily formed which further… increase the potentialities for steady-state perpetuation.20
Needham notes that clouds are a rare exception to the rule that segregating sub-systems are unusual in a gas. However, the very transience of cloud patterns graphically illustrates the unsuitability of gas as a medium for the support of stable segregating sub-systems.
It is not possible to determine precisely how narrow is the range of viscosities and diffusion rates compatible with a functioning circulatory system and aerobic cells large enough to possess the crawling and shape-morphing abilities of a leukocyte (or a motile embryonic cell). But all the evidence suggests that it must be very close to what it is, within a range of about 0.5 mP-s to 3 mP-s.
That the fitness of the viscosity of water must fall within such a narrow range highlights just how fine-tuned is the natural order for life. The viscosity of common substances varies greatly.21 Measured in millipascals-seconds, the viscosity of air is 0.017, water 1.0, olive oil 84, glycerin 1420 and honey 10,000.22 The total range of viscosities of substances on our planet is more than twenty-seven orders of magnitude, from the viscosity of air to the viscosity of crustal rocks.23 Thus, the range of life-friendly viscosities is a tiny, vital band within the inconceivably vast range of viscosities in nature.
In sum, it is ultimately water that permits cells in a multicellular organism or developing embryo to be adequately supplied with oxygen and nutrients via a circulatory system. It is also water that permits cells in multicellular organisms to grow large enough so that they can crawl and morph. Only then can cells in the embryo perform that greatest of miracles—the directed assembly during development of the trillions of embryonic cells into the mature form of the organism.
ANY FLUID serving as the cell matrix must also be a good solvent, able to carry in solution a vast range of ions and biochemicals, including oxygen and various nutrients necessary for cells to function. Water wonderfully fits the bill.24
As a solvent, water is without peer. “Liquid water is such a good solvent, in fact, that it is almost impossible to find naturally occurring pure samples and even producing it in the rarefied environment of the laboratory is difficult,” writes Alok Jha. “Almost every known chemical compound will dissolve in water to a small (but detectable) extent. Related to this, because it will interact with everything, over long periods of time water is also one of the most reactive and corrosive chemicals we know.”25 The sentiment is echoed by the late Felix Franks, one of the leading authorities on the properties of water: “The almost universal solvent action of liquid water” makes “its rigorous purification extremely difficult. Nearly all known chemicals dissolve in water to a slight, but detectable extent.”26
Water is indeed the alkahest, the universal solvent the alchemists sought. No other liquid comes close. As Lawrence Henderson commented, “As a solvent there is literally nothing to compare with water… In the first place the solubility in water of acids, bases and salts, the most familiar classes of inorganic substances, is almost universal.”27
Virtually all organic substances that carry either an ionic charge or contain polar regions—which include the great majority of all organic compounds in the cell and biological fluids—dissolve readily in water. This includes proteins and other big molecules, provided they have polar or ionic regions on their surfaces.28 (See Chapter 4.)
AS WE also saw in Chapter 4, there is one exception to the solvation powers of water, which is worth briefly recalling. It is common knowledge that oil and water don’t mix. If you pour oil on water, it doesn’t dissolve. Instead it forms a separate layer on the surface. The reason is that oils consist of long hydrocarbon chains in which C-H bonds are nonpolar (the electrons are equally distributed over the carbon and hydrogen atoms in the chain), so there are no positively or negatively charged regions along the chain. This lack of charged regions precludes the formation of hydration shells, which are involved in rendering a substance soluble in water.
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Figure 7.2. A water drop on a leaf shows the power of the hydrophobic force. |
The hydrophobic effect can be observed in the beading of water on nonpolar surfaces, such as waxy leaves after a rain shower. The water molecules are unable to form hydrogen bonds with the waxy hydrophobic surface and are forced to clump together into beads away from the surface.
As we saw in Chapter 4, both the assembly of cell membranes and the folding of proteins depend on hydrophobic forces. Water’s inability to dissolve hydrophobic compounds is no defect but a vital element of water’s fitness for the carbon-based cell.
AS SCIENCE has progressed and the properties of bio-matter have become better understood, new elements of the fitness of water for life have been revealed. Recent research has shown, for example, that water is a far more active player in cellular physiology than was previously believed, thoroughly supporting Albert Szent-Györgyi’s celebrated claim that “life is water dancing to the tune of solids.”29
Philip Ball concurs: “It has become increasingly clear over the past two decades or so that water is not simply ‘life’s solvent’ but is indeed a matrix more akin to the one Paracelsus envisaged: a substance that actively engages and interacts with biomolecules in complex, subtle, and essential ways.”30 Ball gives as an example DNA, noting that its double helical structure “relies on a subtle balance of energy contributions present in aqueous solution.” If water weren’t there to “screen the electrostatic repulsions between phosphate groups,” the double helix would cease to be viable, as evidenced by the fact that “DNA undergoes conformational transitions, and even loses its double helix, in some apolar solvents.”31
In concluding his article, Ball sums up the developing view:
Water plays a wide variety of roles in biochemical processes. It maintains macromolecular structure and mediates molecular recognition, it activates and modulates protein dynamics, it provides a switchable communication channel across membranes and between the inside and outside of proteins. Many of these properties do seem to depend, to a greater or lesser degree, on the “special” attributes of the H2O molecule, in particular its ability to engage in directional, weak bonding in a way that allows for reorientation and reconfiguration of discrete and identifiable three-dimensional structures. Thus, although it seems entirely likely that some of water’s functions in biology are those of a generic polar solvent rather than being unique to water itself, it is very hard to imagine any other solvent that could fulfill all of its roles—or even all of those that help to distinguish a generic polypeptide chain from a fully functioning protein.32
The increasing evidence and awareness that water may be an active player in cellular physiology is described in the text Water and the Cell, which defends the notion in the preface: “Water within cells is to a major extent ordered differently than bulk water, and functions not as an inert solvent, but as an active player… Understanding water order in biological systems is key to an understanding of life processes.”33 Some of the chapter titles emphasize just how important water’s active role may turn out to be for cellular physiology: “Information Exchange within Cellular Water,” “Biology’s Unique Phase Transition Drives Cell Function,” “Some Properties of Interfacial Water,” and “Biological Significance of Active Oxygen Dependent Processes in Aqueous System.”
Considering the fantastic number of ways water is fit for life on Earth and how more examples keep coming to light as scientific knowledge advances, it is likely there are many more to be discovered. Despite its importance and the massive research effort34 devoted to understanding it, the structure of intracellular water is still more mysterious than the geophysics of Mars or the chemistry of hydrothermal vents.
ONE INTRIGUING element of fitness for bioenergetics and proton pumping arises directly out of water’s hydrogen-bonded network,35 which provides so-called “proton wires” consisting of long chains of linked water molecules for moving protons (H ions) around in the cell and across the inner mitochondrial membrane.
While, as Alok Jha points out, other charged particles involved in cellular functions have to move themselves physically from one place to another, “protons can pass their energy along a hydrogen-bonded water wire without moving themselves at all, thanks to the so called Grotthuss mechanism.” A proton attaches to one end of the wire, he explains, and in a split second, “each of the hydrogen bonds further along the length of the wire spin around in sequence so that a proton drops off the water molecule at the other end of the wire. The initial proton has not moved any further than the starting end of the wire but its charge and energy have been ‘conducted’ along the wire’s length.”36
Biophysicist Harold Morowitz discusses the unique fitness of these water wires for bioenergetics. “The past few years have witnessed the developing study of a newly understood property of water [proton conductance] that appears to be almost unique to that substance, is a key element in biological-energy transfer, and was almost certainly of importance in the origin of life,” he writes. “The more we learn the more impressed some of us become with nature’s fitness in a very precise sense.”37
Nick Lane, author of The Vital Question, also sees proton conductance as playing an essential role in the origin of life, driving the formation of organic compounds.38 If Lane is correct and proton flows provided essential energy for the synthesis of organic compounds in the earliest protocells,39 then water would indeed be the cradle and mother of life, as Szent-Györgyi claimed more than fifty years ago.
THERE IS one further very critical element of water’s fitness for cellular life to consider, touched on in a previous chapter: the temperature range in which living things thrive on Earth, and which is fit for the chemistry of life, is almost exactly the same temperature range in which water is a liquid.
The currently established upper temperature limit for metazoan organisms is about 50°C.40 This limit applies even to animals adapted to the hot waters of hydrothermal vents, which can only survive short periods at temperatures above 45°C.41 One exception is a species of desert ant, which can survive for short periods at temperatures of 55°C.42 Current record-holders for the most thermo-tolerant microorganisms are hyper-thermophilic species of bacteria which can survive in temperatures of as much as 120°C and are found in hot springs throughout the world, such as those in Yellowstone Park in the United States and in water close to oceanic hydrothermal vents. (In the deep ocean, water temperature can climb over 100°C because the pressure is much higher than atmospheric pressure at sea level.) The current record-holder is a methanogen discovered in the Indian Ocean, in black smoker fluid of the Kairei hydrothermal field. It survives and reproduces at 122°C.43
As mentioned previously, the lower limit for life is hard to determine exactly because water freezes at 0°C. However, many organisms protect themselves against freezing by cryoprotectants, and microbial metabolism has been reported down to –20°C.44 A midge in the Himalayas survives in temperatures of –18°C.45 However, at temperatures much below –20°C, no matter what cryoprotectants are used, freezing inevitably occurs. Even if ice crystal formation is avoided, intracellular water eventually undergoes a glass transition—vitrification. This effectively stops all metabolic processes.46
Consequently, a temperature of about –20° marks the lower recorded limit of life, or at least of active metabolism on Earth. This limit is imposed by the properties of the medium in which all life processes occur—water47—and thus might seem a defect in water’s fitness for life. However, if metabolism could be maintained at colder temperatures in something other than water, in a matrix that was a liquid at colder temperatures (ammonia comes to mind, which is a liquid between –78°C and –33°C), it would be glacially slow.48 In fact, life at –40°C would be sixty-four times slower than life at –20°C. Or, more properly, the rates of chemical reaction would be sixty-four times slower.
This means that at whatever constraints the rate of biochemical processes impose on biological evolution in our actual world, that rate would arguably be as much as sixty-four times slower in such a hypothetical cold world. It took more than 300-million years to get from multicellular jellyfish (Haootia) to the first mammals. Multiply that by sixty-four and we’re looking at around 20 billion years for evolution to follow that path. By then the sun would long have burned out, rendering Earth uninhabitable.
A world capable of evolving creatures like ourselves, then, would appear to require a matrix with a liquid range similar to water’s, or at least not one where life must exist at substantially colder temperatures.
What about the possibility of carbon-based life at temperatures much above 100°C? It is likely that the currently highest known temperature in which bacteria can survive, 122°C, is close to the maximum possible for carbon-based life on Earth. And it must be presumed that this upper limit is imposed because, as discussed previously, the organic compounds used in the cell become increasingly unstable as temperatures rise above 100°C. In fact, whatever theoretical reasons one might adduce for the upper temperature range of life (e.g., the instability of covalent and particularly weak bonds), nature has in effect already indicated the upper temperature limit empirically by the absence of any organisms on Earth able to withstand temperatures much above 100°C. After four billion years of experimentation, nature has provided clear empirical evidence that the fecund chemistry of carbon is of little use in constructing biological systems at temperatures beyond this.49
The weak bonds, being ten to twenty times weaker than covalent bonds, are even more susceptible to thermal disruption as temperatures rise above the ambient range. As I noted in Nature’s Destiny, the relative weakness of the weak bonds compared to the strong covalent bonds is apparent in cooking. “The disruption of weak bonds occurs in two very familiar processes in the kitchen—in the heating and beating of egg white, both of which causes the egg white to whiten and coagulate”50 and also in the slow cooking of meat between 85° and 90°C . The gentle heating causes the familiar softening of the meat. Neither the beating of egg whites nor the slow cooking of meat under 90°C breaks the covalent bonds in the egg white or the meat. The softening of the meat and the coagulation of the egg white involve the breaking of the weak bonds which hold proteins in their native 3-D shapes, causing their unraveling and denaturation, which occurs at temperatures considerably below those required to break covalent bonds (beyond 100°C for most covalent bonds). Because covalent bonds are far more robust, neither beating nor gentle heating has any significant effect on them and most remain intact. The extreme sensitivity of the weak bonds to high temperatures is another reason why few organisms can survive at temperatures above 100°C.51
Although a temperature range of –20°C to 122°C (a range of 142°C) appears from our mundane perspective to be considerable, as pointed out in Chapter 2 such a range is an unimaginably tiny fraction of the total range of all temperatures in the cosmos. Temperatures in the cosmos range from 1032°C (10 followed by thirty-one zeros), which was the temperature of the universe shortly after the Big Bang, to very close to absolute zero, or –273.15°C. The temperature inside some of the hottest stars is several thousand million degrees.52 Even inside our own Sun, which is not a particularly hot star, the temperature is on the order of fifteen million degrees,53 and its surface temperature is just below 6,000°C.54 So, out of the enormous range of temperatures in the cosmos, there is only one tiny temperature band, about one 10–29 of the total range, where water is a liquid. Within this tiny temperature band, the energy levels of the covalent bonds of the organic domain can be manipulated by living systems; the weak bonds can be used for stabilizing the 3-D forms of complex molecules; and water, the only compound known to possess the many other properties essential to serve as the matrix of life, exists in the liquid state.
This is little short of a miracle. If this coincidence did not hold, water would not be fit to form the matrix of the cell. All the myriad other elements of fitness of this unique fluid would be to no avail. Almost certainly there would be no carbon-based life in the cosmos. Or to put that matter more positively, it is surely an awesome coincidence, indicative of the profound fitness of water and, by extension, of nature for carbon-based life, that the optimum temperature range for the complex atomic and molecular manipulations essential for life is precisely the temperature range in which water, the ideal matrix in so many other ways, exists as a liquid at ambient conditions on Earth.
Unrivaled
THE EVIDENCE is overwhelming: water possesses an ensemble of unique properties which uniquely equip it to function as the matrix of the carbon-based cell. There is hardly an author conversant with the facts who would contest Henderson’s verdict that no substance can rival the fitness of water as the milieu intérieur of carbon-based life.55 And discoveries since Henderson’s time have merely deepened and broadened the evidence for that conviction. Kevin Plaxco and Michael Gross recently reviewed the fitness of water in the introductory chapter of Astrobiology and concluded that water’s “ability to form the basis of biochemistry may well be unique... no other liquid has even a fraction of the favorable attributes of water.”56
Finally, water also has many unique properties essential for creatures of our physiological design. The high diffusion rate of solutes in water makes possible a circulatory system to supply our body cells with oxygen and nutrients. It also makes possible cells sufficiently large to contain the necessary cytoplasmic machinery for cellular motility and shape changing, essential for so many cellular functions, including embryo assembly. Again, our warm bloodedness depends critically on the thermal properties of water—its high heat capacity and high latent heat of evaporation. Water also plays a vital role in the transport of carbon dioxide from the tissues for excretion in the lungs. In short, in the properties of water, nature appears to have had not only life in mind, but creatures like ourselves.
