Nacre known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer. It is strong and iridescent. Nacre is found in some of the most ancient lineages of bivalves and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, this results in a non-iridescent shine, or more in non-nacreous iridescence such as flame structure as is found in conch pearls; the outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae. Nacre is composed of hexagonal platelets of aragonite 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules.
These layers are separated by sheets of organic matrix composed of elastic biopolymers. This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa. Strength and resilience are likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation; this structure, at multiple length sizes increases its toughness, making it as strong as silicon. The statistical variation of the platelets has a negative effect on the mechanical performance because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours; the crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have different c-axis orientation randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis, this way inclined; the interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers. Nacre formation is not understood; the initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles within an organic matrix that arrange in fibre-like polycrystalline configurations.
The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset and form of crystal growth. Individual aragonite "bricks" are believed to grow to the full height of the nacreous layer, expand until they abut adjacent bricks; this produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis is slow in nacre, fast in fibrous aragonite. Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs; the nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer known as mother of pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues.
The process is called encystation and it continues as long as the mollusc lives. The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, in cephalopods, they are pseudohexagonal monocrystals, which are twinned; the main commercial sources of mother of pearl have been the pearl oyster, freshwater pearl mussels, to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century. Used for pearl buttons during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus; the international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries. Nacre has been used for centuries for a variety o
Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
The mollusc shell is a calcareous exoskeleton which encloses and protects the soft parts of an animal in the phylum Mollusca, which includes snails, tusk shells, several other classes. Not all shelled; the ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid and some smaller groups such as the caudofoveata and solenogastres, the derived Xenoturbella. Today, over 100,000 living species bear a shell. Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, this is called conchology—although these terms used to be, to a minor extent still are, used interchangeably by scientists. Within some species of molluscs, there is a wide degree of variation in the exact shape, pattern and color of the shell. A mollusc shell is formed and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are reflected in the shape and form and color of the shell.
When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced; the mantle edge secretes a shell. The organic constituent is made up of polysaccharides and glycoproteins; this organic framework controls the formation of calcium carbonate crystals, dictates when and where crystals start and stop growing, how fast they expand. The shell formation requires certain biological machinery; the shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle; the periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur.
The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills and epithelium, transported by the haemolymph to the calcifying epithelium, stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required; the organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition. Nucleation is endoepithelial in Neopilina and Nautilus, but exoepithelial in the bivalves and gastropods; the formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are conserved, but the proteins in the secretome are derived and evolving. Engrailed serves to demark the edge of the shell field.
In gastropod embryos, Hox1 is expressed. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – though diatoms use silica, not calcite, to form their tests! The shell-secreting area is differentiated early in embryonic development. An area of the ectoderm thickens invaginates to become a "shell gland"; the shape of this gland is tied to the form of the adult shell. The gland subsequently evaginates in molluscs. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and protrudes in the form of flaps well beyond the visceral mass itself. In many species of molluscs the epidermis of the mantle secretes calcium carbonate and conchiolin, creates a shell. In sea slugs there is a progressive loss of the shell and the mantle becomes the dorsal surface of the animal; the words mantle and pallium both meant cloak or cape, see mantle. This anatomical structure in molluscs resembles a cloak because in many groups the edges of the mantle referred to as the mantle margin, extend far beyond the main part of the body, forming flaps, double-layered structures which have been adapted for many different uses, including for example, the siphon; the mantle cavity is a central feature of molluscan biology. This cavity is formed by a double fold of mantle which encloses a water space; this space contains the mollusc's gills, osphradium and gonopores. The mantle cavity functions as a respiratory chamber in most molluscs.
In bivalves it is part of the feeding structure. In some molluscs the mantle cavity is a brood chamber, in cephalopods and some bivalves such as scallops, it is a locomotory organ; the mantle is muscular. In cephalopods the contraction of the mantle is used to force water through a tubular siphon, the hyponome, this propels the animal rapidly through the water. In gastropods it is used as a kind of "foot" for locomotion over the surface. In Patella the foot includes the entire ventral surface of the animal; the foot of the Bivalvia is a fleshy process adapted by its form to digging rather than to locomotion. In shelled molluscs, the mantle is the organ that forms the shell, adds to the shell to increase its size and strength as the animal grows. Shell material is secreted by the ectodermic cells of the mantle tissue; the mantle of many gastropods is fully or hidden inside the gastropod shell. In species where the shell is small compared to the size of the body, more of the mantle shows. Shell-less slugs have the mantle visible.
The dorsal surface of the mantle is called the notum, while the ventral surface of the mantle is called the hyponotum. In the family Philomycidae, the mantle covers the whole back side of the body. Mollusc shell, formed by the mantle Siphon, a part of the mantle in some groups of molluscs
The periostracum is a thin organic coating or "skin", the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs it is seen in snails and clams, i.e. in gastropods and bivalves, but it is found in cephalopods such as Allonautilus scrobiculatus. Periostracum is an integral part of the shell, it forms as the shell forms, along with the other shell layers. Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial and marine habitats, may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs; the word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is the more calcareous part of the shell. Technically the calcareous part of the shell can be referred to as the "ostracum", but that term is only rarely used; this shell layer is composed of a type of protein known as conchiolin. Conchiolin is composed of quinone-tanned proteins, which are similar to those found in the epidermal cuticle.
The formation of a shell requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs; this caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur; the accumulation of ions is driven by ion pumps packed within the calcifying epithelium. The organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc; the periostracum was essential in allowing early molluscs to obtain large size with a single valve. The periostracum is secreted from a groove in the mantle, termed the periostracal groove.
When secreted, it consists of the soluble protein periostracin. Periostracum is yellowish or brownish in color. In some species it is black; the periostracum is often a different color than the underlying layer of the shell. In the shells of species which have periostracum, this shell layer is quite physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more formed areas of the shell. Periostracum can in some cases be quite thin, smooth and transparent, such that it looks like a thin yellow varnish, or it can be thicker and more or less opaque; when it is thick it is relatively rough in texture and dull. In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel, see. In some species the periostracum adheres tightly to the underlying shell surface, in others the periostracum is less attached. In certain marine species, such as for example certain species of cone snails, a heavy periostracum obscures the color patterns that exist on the calcareous layer of the shell.
In many aquatic species, once a shell has been removed from the water and has had time to dry out the periostracum may become brittle and start to flake or peel off of the surface of the shell. It is not uncommon for shell collectors to deliberately remove a periostracum layer if they feel that a shell is more attractive without it; however the periostracum is an important part of the shell, is of interest to malacologists. Details of the periostracum can sometimes be helpful in identifying a species. Haired shells occur in gastropods in several species of the Stylommatophoran families Polygyridae and Hygromiidae; these families are only distantly related, suggesting that this features has evolved several times independently. Haired shells are exclusively observed in species living in moist microhabitats, like layers of fallen leaves, broad-leaved vegetation, damp meadows or wet scree; such a correlation suggests an adaptive significance of the trait in such a habitat. These hairs can reach varying lengths.
In some cases hardly visible, they confer an furry impression to the shell in others. These semi-rigid structures are part of the periostracum, a thin protein layer secreted by the snail to cover the calcareous shell. Building hairs requires the snail to have specialised glandular tissue and complex strategies to form them; this trait can be assumed to be costly and should thus present a selective advantage to its bearers in order to be conserved. Experiments by Pfenninger et al. on genus Trochulus showed an increased adherence of haired shells to wet surfaces. Haired shells appeared to be the ancestral character state, a feature most lost three times independently; the possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high. The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts; the periostracum of brachiopods is made of chitin.
New cells on the edges of the brachiopod mantle secrete material that extends the periostracum, but are displace
Sporopollenin is one of the most chemically inert biological polymers. It is a major component of the tough outer walls of plant spores and pollen grains, it is chemically stable and is well preserved in soils and sediments. The exine layer is intricately sculptured in species-specific patterns, allowing material recovered from lake sediments to provide useful information to palynologists about plant and fungal populations in the past. Sporopollenin has found uses in the field of paleoclimatology as well. Sporopollenin is found in the cell walls of several taxa of green alga, including Phycopeltis and Chlorella. Spores are dispersed by many different environmental factors, such as water or animals. If the conditions are suitable the sporopollenin-impregnated walls of pollen grains and spores can persist in the fossil record for hundreds of millions of years, since sporopollenin is resistant to chemical degradation by organic and inorganic chemicals; the chemical composition of sporopollenin has long been elusive due to its unusual chemical stability and resistance to degradation by enzymes and strong chemical reagents.
Analyses have revealed a mixture of biopolymers, containing long chain fatty acids, phenylpropanoids and traces of carotenoids. Tracer experiments have shown that phenylalanine is a major precursor, but other carbon sources contribute, it is that sporopollenin derives from several precursors that are chemically cross-linked to form a rigid structure. In 2019, researchers at MIT determined via thioacidolysis degradation and solid-state NMR the molecular structure of pine sporopollenin, finding it composed of polyvinyl alcohol units alongside other aliphatic monomers, all crosslinked through a series of acetal linkages. Electron microscopy shows that the tapetal cells that surround the developing pollen grain in the anther have a active secretory system containing lipophilic globules; these globules are believed to contain sporopollenin precursors. Chemical inhibitors of pollen development and many male sterile mutants have effects on the secretion of these globules by the tapetal cells. Chitin Conchiolin Tectin Boavida, L. C..
A.. "The making of gametes in higher plants". The International Journal of Developmental Biology. 49: 595–614. Doi:10.1387/ijdb.052019lb. PMID 16096968. Guilford, W. J.. "High Resolution Solid State 13C NMR Spectroscopy of Sporopollenins from Different Plant Taxa". Plant Physiology. 86: 134–136. Doi:10.1104/pp.86.1.134. JSTOR 4271095. PMC 1054442. PMID 16665854
Chitin n, a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, the scales of fish and lissamphibians; the structure of chitin is comparable to another polysaccharide - cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal and biotechnological purposes; the English word "chitin" comes from the French word chitine, derived in 1821 from the Greek word χιτών, meaning covering. A similar word, "chiton", refers to a marine animal with a protective shell; the structure of chitin was determined by Albert Hofmann in 1929. Chitin is a modified polysaccharide; these units form covalent β--linkages. Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an acetyl amine group.
This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength. In its pure, unmodified form, chitin is translucent, pliable and quite tough. In most arthropods, however, it is modified, occurring as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and molluscs, chitin produces a much stronger composite; this composite material is much harder and stiffer than pure chitin, is tougher and less brittle than pure calcium carbonate. Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar to the stiff, light elytron of a beetle. In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various iridescent colors serving phenotypic signaling and communication for mating and foraging.
The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in biomimicry. Scarab beetles in the genus Cyphochilus utilize chitin to form thin scales that diffusely reflect white light; these scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of nanometres, which serve to scatter light. The multiple scattering of light is thought to play a role in the unusual whiteness of the scales. In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper. Chitosan is produced commercially by deacetylation of chitin. Nanofibrils have been made using chitosan. Chitin-producing organisms like protozoa, fungi and nematodes are pathogens in other species. Humans and other mammals have chitinase-like proteins that can degrade chitin. Chitin is sensed in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells.
Keratinocytes in skin can react to chitin or chitin fragments. According to in vitro studies, chitin is sensed by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, mannose receptors; the immune response can sometimes clear the chitin and its associated organism, but sometimes the immune response is pathological and becomes an allergy. Plants have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein; the first chitin receptor was cloned in 2006. When the receptors are activated by chitin, genes related to plant defense are expressed, jasmonate hormones are activated, which in turn activate systematic defenses. Commensal fungi have ways to interact with the host immune response that as of 2016 were not well understood; some pathogens produce chitin-binding proteins. Zymoseptoria tritici is an example of a fungal pathogen. Chitin was present in the exoskeletons of Cambrian arthropods such as trilobites; the oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a scorpion encased in amber.
Chitin is a good inducer of plant defense mechanisms for controlling diseases. It has been assessed as a fertilizer that can improve overall crop yields. Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods. Processes to size and strengthen paper employ chitosan. How chitin interacts with the immune system of plants and animals has been an active area of research, including the identity of key receptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, mechanisms by which immune systems respond. Chitin and chitosan have bee