14

Crystals formed through biological activity

Various inorganic and organic crystals are formed in living bodies through the biological activity in animals and plants. Some are indispensable, such as: hydroxyapatite, which constitutes teeth and bones; aragonite and calcite, which are the main constituents of shells and exo-skeletons; and those formed through the excretion of components that are either unnecessary or due to disease. The examples given respectively show the characteristic morphology of crystals and the textures of polycrystalline aggregates. In this chapter, we will summarize and discuss which features crystal growth in inorganic systems and in living bodies have in common and which are different. To do this we will consider the morphology of the crystals.

14.1Crystal growth in living bodies

Many inorganic and organic crystals grow in living bodies and act as indispensable major components in the function of cells and organs. Teeth and bones consist of hydroxyapatite crystals, and shells, pearls, and the exo-skeletons of coral or coccolithophores are mainly composed of carbonate crystals, such as aragonite and calcite. There are magnetite crystals of single magnetic domain in size aligned in a rosary form in cells of magnetotactic bacterium, which acts as a direction sensor. Similar magnetite rosaries are found in the brain cells of pigeons, dolphins, and salmon, and it is suggested that they act as sensors for homing, wandering, or recurring instincts.

Crystals (or amorphous substances) are also formed in living bodies as a result of biological activities to act as a reservoir for necessary components (for example, amorphous silica in grass cells, and calcium oxalate or inuline crystals in dahlias and begonias), or crystals are formed by excretion processes or due to illness (for

262Crystals formed through biological activity

Table 14.1 Crystals formed by biomineralization

[I]Bone, exo-skeleton, shell, carapace, spicule, etc.

Calcite: coccolith, foraminifera, mollusks Mg-containing calcite: octocorals, spicules of sea urchin

Aragonite: corals, mollusks, gastropods, cephalopods, fish Vaterite: gastropods, sea squirt

Amorphous CaCO3: crustaceans, birds’ eggs, plants Hydroxyapatite: vertebrates, fish

Octacalcium phosphate: vertebrates Celestite: acantharia

Amorphous SiO2: diatoms, radiolarians

[II] Tooth

Hydroxyapatite: vertebrates, mammals, fish

Octacalcium phosphate: vertebrates

Amorphous SiO2: limpets, chitons

Magnetite: chitons

Goethite: limpets

Phosphoferrite: chitons

[III] Sensors or receptors for orientation or gravity

Magnetite: magnetotactic bacteria, tuna, salmon, pigeons

Calcite: mammals, fish

Gypsum: jellyfish

Barite: charr

[IV] Storage

Whewellite CaC2O4· H2O: various plants

Weddellite CaC2O4· 2H2O: various plants

Amorphous silica: plants, grasses

[V]Excretion

Sodium uric acid: gout Cholesterol: gallstones

[VI] Function not clarifieda

Sulfides (pyrite, sphalerite, wurtzite, galena, greigite, mackinawite)

Oxides (magnetite, todorokite)

Carbonates (calcite, hydroxycalcite, siderite, aragonite)

Sulfates (barite, gypsum)

Phosphates (jarosite)

a All minerals in [VI] are found as selectively precipitated minerals on the surface of bacteria cells

example, calculus in various organs, or sodium uric acid in the case of gout). There remain some situations in which the reasons for crystal formation or function are still not well understood (for example, bacteria around which inorganic crystals precipitate selectively). Examples of these are summarized in Table 14.1.

There is no essential difference between crystal growth in living bodies and that

14.1 Crystal growth in living bodies 263

of inorganic crystals in aqueous solution. In both cases, conditions in driving force higher than a critical value, such as a supersaturated state, should be achieved first, and then growth proceeds in the processes of nucleation and growth, through which the morphology of the crystals and their textures are determined. Since the textures formed in this manner play an essential role in organ function, the morphology and textures of the crystals are assumed to have been controlled purposely through biological activity. We may summarize the characteristic points of crystal growth in living bodies as follows.

(1)The environmental phase is limited, for example in cells or organs. In other words, this situation is comparable to crystal growth in a partly closed vessel.

(2)There are cases in which proteins or saccharoids, or organic sheets (protein foil) made up of these substances, play a cooperative role in crystal growth, and cases in which there is no relation. We may evaluate the degree of cooperation by evaluation of a coherency or misfit ratio between the crystal and the protein, or by the degree of epitaxial relation. The role of the protein may be variable. In crystal growth that is indispensable for biological activity, the role of the protein will be definitive, but for crystals formed in excretion processes, this cooperation will either not occur or will be very weak.

(3)All crystal growth takes place in low-temperature, low-pressure aqueous solution (at 1 atmospheric pressure and room temperature). This suggests a higher probability of formation of an amorphous state, phases of low crystallinity, and metastable phases as precursors, and therefore subsequent transformation to stable or metastable phases.

(4)Some crystals may remain in the cells, which act as vessels, and form textures consisting of crystals and protein films; in other cases, crystals protrude from the cell, where they may connect with others formed in neighboring cells or discharge from the cell and connect with other crystals to form higher-order textures, such as an exo-skeleton.

It is important to understand how these characteristics affect the morphology of the crystals and the textures of their aggregations. These specific characteristics allow crystals formed due to biological activity to exist as amorphous states or in metastable phases, and they bring about different characteristics in the morphology and textures of crystals from those present in uncontrolled inorganic systems.

The same crystal species may be formed either as an indispensable component for biological activity, or as an unnecessary (subsequently excreted) product; but the two may be different in their morphological features. It is the purpose of this chapter to investigate systematically crystals formed through biological activity,