1.
Actin
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Actin is a family of globular multi-functional proteins that form microfilaments. It is found in all eukaryotic cells, where it may be present at a concentration of over 100 μM. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes, in vertebrates, three main groups of actin isoforms, alpha, beta, and gamma have been identified. The alpha actins, found in tissues, are a major constituent of the contractile apparatus. The beta and gamma actins coexist in most cell types as components of the cytoskeleton and it is believed that the diverse range of structures formed by actin enabling it to fulfill such a large range of functions is regulated through the binding of tropomyosin along the filaments. Other enzymes or organelles such as cilia can be anchored to this scaffolding in order to control the deformation of the cell membrane. It can also produce movement either by itself or with the help of molecular motors, Actin therefore contributes to processes such as the intracellular transport of vesicles and organelles as well as muscular contraction and cellular migration. It therefore plays an important role in embryogenesis, the healing of wounds, the evolutionary origin of actin can be traced to prokaryotic cells, which have equivalent proteins. Actin homologs from prokaryotes and archaea polymerize into different helical or linear filaments consisting of one or multiple strands, however the in-strand contacts and nucleotide binding sites are preserved in prokaryotes and in archaea. Lastly, actin plays an important role in the control of gene expression, a large number of illnesses and diseases are caused by mutations in alleles of the genes that regulate the production of actin or of its associated proteins. The production of actin is also key to the process of infection by some pathogenic microorganisms, mutations in the different genes that regulate actin production in humans can cause muscular diseases, variations in the size and function of the heart as well as deafness. The make-up of the cytoskeleton is also related to the pathogenicity of bacteria and viruses. Actin was first observed experimentally in 1887 by W. D. Halliburton, in 1942, Straub developed a novel technique for extracting muscle protein that allowed him to isolate substantial amounts of relatively pure actin. Straubs method is essentially the same as used in laboratories today. Adding ATP to a mixture of both proteins causes a decrease in viscosity, the hostilities of World War II meant Szent-Gyorgyi and Straub were unable to publish the work in Western scientific journals. Actin therefore only became known in the West in 1945. Straub suggested that the transformation of ATP-bound actin to ADP-bound actin played a role in muscular contraction, in fact, this is true only in smooth muscle, and was not supported through experimentation until 2001. The amino acid sequencing of actin was completed by M. Elzinga, the crystal structure of G-actin was solved in 1990 by Kabsch and colleagues
2.
Algae
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Algae is an informal term for a large, diverse group of photosynthetic organisms which are not necessarily closely related, and is thus polyphyletic. Included organisms range from unicellular genera, such as Chlorella and the diatoms, to forms, such as the giant kelp. Most are aquatic and autotrophic and lack many of the cell and tissue types, such as stomata, xylem, and phloem. No definition of algae is generally accepted, one definition is that algae have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells. Some authors exclude all prokaryotes thus do not consider cyanobacteria as algae, Algae constitute a polyphyletic group since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria, diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Algae exhibit a range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction. Algae lack the various structures that characterize land plants, such as the phyllids of bryophytes, rhizoids in nonvascular plants, and the roots, leaves, and other organs found in tracheophytes. Most are phototrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some other heterotrophic organisms, such as the apicomplexans, are derived from cells whose ancestors possessed plastids. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago, the singular alga is the Latin word for seaweed and retains that meaning in English. Although some speculate that it is related to Latin algēre, be cold, a more likely source is alliga, binding, entwining. The Ancient Greek word for seaweed was φῦκος, which could mean either the seaweed or a red dye derived from it, the Latinization, fūcus, meant primarily the cosmetic rouge. It could be any color, black, red, green, accordingly, the modern study of marine and freshwater algae is called either phycology or algology, depending on whether the Greek or Latin root is used. The name Fucus appears in a number of taxa, most algae contain chloroplasts that are similar in structure to cyanobacteria. Chloroplasts contain circular DNA like that in cyanobacteria and presumably represent reduced endosymbiotic cyanobacteria, however, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. The table below describes the composition of the three groups of algae. Their lineage relationships are shown in the figure in the upper right, many of these groups contain some members that are no longer photosynthetic
3.
Embryophyte
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The Embryophyta are the most familiar group of green plants that form vegetation on earth. Living embryophytes include hornworts, liverworts, mosses, ferns, lycophytes, gymnosperms and flowering plants, the Embryophyta are informally called land plants because they live primarily in terrestrial habitats, while the related green algae are primarily aquatic. All are complex multicellular eukaryotes with specialized reproductive organs, the name derives from their innovative characteristic of nurturing the young embryo sporophyte during the early stages of its multicellular development within the tissues of the parent gametophyte. With very few exceptions, embryophytes obtain their energy by photosynthesis, the evolutionary origins of the embryophytes are discussed further below, but they are believed to have evolved from within a group of complex green algae during the Paleozoic era. Charales or the stoneworts may be the best living illustration of that developmental step, embryophytes are primarily adapted for life on land, although some are secondarily aquatic. Accordingly, they are called land plants or terrestrial plants. On a microscopic level, the cells of embryophytes are broadly similar to those of green algae and they are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. Embryophyte cells also generally have a central vacuole enclosed by a vacuolar membrane or tonoplast. In common with all groups of multicellular algae they have a cycle which involves alternation of generations. The mature sporophyte produces spores which grow into a gametophyte. Embryophytes have two related to their reproductive cycles which distinguish them from all other plant lineages. Firstly, their gametophytes produce sperm and eggs in multicellular structures and this second feature is the origin of the term embryophyte – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell. In the bryophytes the sporophyte dependent on the gametophyte, while in all other embryophytes the sporophyte generation is dominant. Embryophytes also differ from algae by having metamers, metamers are repeated units of development, in which each unit derives from a single cell, but the resulting product tissue or part is largely the same for each cell. The whole organism is thus constructed from similar, repeating parts or metamers, accordingly, these plants are sometimes termed metaphytes and classified as the group Metaphyta. All green algae and land plants are now known to form an evolutionary lineage or clade. According to several molecular clock estimates the Viridiplantae split 1,200 million years ago to 725 million years ago into two clades, chlorophytes and streptophytes, the chlorophytes are considerably more diverse and were originally marine, although some groups have since spread into fresh water. The streptophyte algae are less diverse and adapted to fresh very early in their evolutionary history
4.
Lipid bilayer
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The lipid bilayer is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a barrier around all cells. The cell membranes of almost all living organisms and many viruses are made of a bilayer, as are the membranes surrounding the cell nucleus. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed, Lipid bilayers are ideally suited to this role because, even though they are only a few nanometers in width, they are impermeable to most water-soluble molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations, biological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Phospholipids with certain groups can alter the surface chemistry of a bilayer and can, for example. Just like the heads, the tails of lipids can also affect membrane properties, the packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. Many of these properties have been studied with the use of model bilayers produced in a lab. Vesicles made by model bilayers have also used clinically to deliver drugs. Biological membranes typically include several types of other than phospholipids. A particularly important example in animal cells is cholesterol, which strengthen the bilayer. Cholesterol also helps regulate the activity of certain integral membrane proteins, integral membrane proteins function when incorporated into a lipid bilayer, and they are held tightly to lipid bilayer with the help of an annular lipid shell. Because bilayers define the boundaries of the cell and its compartments, certain kinds of membrane proteins are involved in the process of fusing two bilayers together. This fusion allows the joining of two structures as in the fertilization of an egg by sperm or the entry of a virus into a cell. Because lipid bilayers are quite fragile and invisible in a traditional microscope, experiments on bilayers often require advanced techniques like electron microscopy and atomic force microscopy. When phospholipids are exposed to water, they self-assemble into a sheet with the hydrophobic tails pointing toward the center of the sheet. This arrangement results in two “leaflets” that are each a single molecular layer, the center of this bilayer contains almost no water and excludes molecules like sugars or salts that dissolve in water. The assembly process is driven by interactions between hydrophobic molecules, an increase in interactions between hydrophobic molecules allows water molecules to bond more freely with each other, increasing the entropy of the system
5.
Sugar
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Sugar is the generic name for sweet, soluble carbohydrates, many of which are used in food. There are various types of derived from different sources. Simple sugars are called monosaccharides and include glucose, fructose, the table sugar or granulated sugar most customarily used as food is sucrose, a disaccharide of glucose and fructose. Sugar is used in prepared foods and it is added to some foods, in the body, sucrose is hydrolysed into the simple sugars fructose and glucose. Other disaccharides include maltose from malted grain, and lactose from milk, longer chains of sugars are called oligosaccharides or polysaccharides. Some other chemical substances, such as glycerol may also have a sweet taste, low-calorie food substitutes for sugar, described as artificial sweeteners, include aspartame and sucralose, a chlorinated derivative of sucrose. Sugars are found in the tissues of most plants and are present in sufficient concentrations for efficient commercial extraction in sugarcane, the world production of sugar in 2011 was about 168 million tonnes. The average person consumes about 24 kilograms of sugar each year, equivalent to over 260 food calories per person, since the latter part of the twentieth century, it has been questioned whether a diet high in sugars, especially refined sugars, is good for human health. Sugar has been linked to obesity, and suspected of, or fully implicated as a cause in the occurrence of diabetes, cardiovascular disease, dementia, macular degeneration, the etymology reflects the spread of the commodity. The English word sugar ultimately originates from the Sanskrit शर्करा, via Arabic سكر as granular or candied sugar, the contemporary Italian word is zucchero, whereas the Spanish and Portuguese words, azúcar and açúcar, respectively, have kept a trace of the Arabic definite article. The Old French word is zuchre and the contemporary French, sucre, the earliest Greek word attested is σάκχαρις. The English word jaggery, a brown sugar made from date palm sap or sugarcane juice, has a similar etymological origin – Portuguese jagara from the Sanskrit शर्करा. Sugar has been produced in the Indian subcontinent since ancient times and it was not plentiful or cheap in early times and honey was more often used for sweetening in most parts of the world. Originally, people chewed raw sugarcane to extract its sweetness, sugarcane was a native of tropical South Asia and Southeast Asia. Different species seem to have originated from different locations with Saccharum barberi originating in India and S. edule, one of the earliest historical references to sugarcane is in Chinese manuscripts dating back to 8th century BC that state that the use of sugarcane originated in India. Sugar was found in Europe by the 1st century AD, but only as an imported medicine and it is a kind of honey found in cane, white as gum, and it crunches between the teeth. It comes in lumps the size of a hazelnut, sugar is used only for medical purposes. Sugar remained relatively unimportant until the Indians discovered methods of turning sugarcane juice into granulated crystals that were easier to store, crystallized sugar was discovered by the time of the Imperial Guptas, around the 5th century AD
6.
Green fluorescent protein
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The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range. Although many other organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the jellyfish Aequorea victoria. The GFP from A. victoria has an excitation peak at a wavelength of 395 nm. Its emission peak is at 509 nm, which is in the green portion of the visible spectrum. The fluorescence quantum yield of GFP is 0.79, the GFP from the sea pansy has a single major excitation peak at 498 nm. In cell and molecular biology, the GFP gene is used as a reporter of expression. GFP can be introduced into animals or other species through transgenic techniques, and maintained in their genome, to date, GFP has been expressed in many species, including bacteria, yeasts, fungi, fish and mammals, including in human cells. Tsien, Osamu Shimomura, and Martin Chalfie were awarded the 2008 Nobel Prize in Chemistry on 10 October 2008 for their discovery and development of the green fluorescent protein. In the 1960s and 1970s, GFP, along with the separate luminescent protein aequorin, was first purified from Aequorea victoria, in A. victoria, GFP fluorescence occurs when aequorin interacts with Ca2+ ions, inducing a blue glow. Some of this luminescent energy is transferred to the GFP, shifting the overall color towards green, however, its utility as a tool for molecular biologists did not begin to be realized until 1992 when Douglas Prasher reported the cloning and nucleotide sequence of wtGFP in Gene. The funding for this project had run out, so Prasher sent cDNA samples to several labs. The lab of Martin Chalfie expressed the coding sequence of wtGFP, with the first few amino acids deleted, in cells of E. coli and C. elegans. Frederick Tsujis lab independently reported the expression of the recombinant protein one month later, remarkably, the GFP molecule folded and was fluorescent at room temperature, without the need for exogenous cofactors specific to the jellyfish. The first reported crystal structure of a GFP was that of the S65T mutant by the Remington group in Science in 1996, one month later, the Phillips group independently reported the wild-type GFP structure in Nature Biotechnology. These crystal structures provided vital background on chromophore formation and neighboring residue interactions, researchers have modified these residues by directed and random mutagenesis to produce the wide variety of GFP derivatives in use today. Due to the potential for widespread usage and the needs of researchers. The first major improvement was a point mutation reported in 1995 in Nature by Roger Tsien. This matched the spectral characteristics of commonly available FITC filter sets, a 37 °C folding efficiency point mutant to this scaffold, yielding enhanced GFP, was discovered in 1995 by the laboratories of Thastrup and Falkow
7.
Flower
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A flower, sometimes known as a bloom or blossom, is the reproductive structure found in plants that are floral. The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs, Flowers may facilitate outcrossing or allow selfing. Some flowers produce diaspores without fertilization, Flowers contain sporangia and are the site where gametophytes develop. Many flowers have evolved to be attractive to animals, so as to them to be vectors for the transfer of pollen. After fertilization, the ovary of the flower develops into fruit containing seeds, the essential parts of a flower can be considered in two parts, the vegetative part, consisting of petals and associated structures in the perianth, and the reproductive or sexual parts. A stereotypical flower consists of four kinds of structures attached to the tip of a short stalk, each of these kinds of parts is arranged in a whorl on the receptacle. The four main whorls are as follows, Collectively the calyx, corolla, the next whorl toward the apex, composed of units called petals, which are typically thin, soft and colored to attract animals that help the process of pollination. Androecium, the whorl, consisting of units called stamens. Stamens consist of two parts, a called a filament, topped by an anther where pollen is produced by meiosis. Gynoecium, the innermost whorl of a flower, consisting of one or more units called carpels, the carpel or multiple fused carpels form a hollow structure called an ovary, which produces ovules internally. Ovules are megasporangia and they in turn produce megaspores by meiosis which develop into female gametophytes and these give rise to egg cells. The gynoecium of a flower is described using an alternative terminology wherein the structure one sees in the innermost whorl is called a pistil. A pistil may consist of a carpel or a number of carpels fused together. The sticky tip of the pistil, the stigma, is the receptor of pollen, the supportive stalk, the style, becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma. The relationship to the gynoecium on the receptacle is described as hypogynous, perigynous, although the arrangement described above is considered typical, plant species show a wide variation in floral structure. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species, the four main parts of a flower are generally defined by their positions on the receptacle and not by their function. Many flowers lack some parts or parts may be modified into other functions and/or look like what is typically another part, in some families, like Ranunculaceae, the petals are greatly reduced and in many species the sepals are colorful and petal-like. Other flowers have modified stamens that are petal-like, the flowers of Peonies and Roses are mostly petaloid stamens
8.
Eukaryote
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A eukaryote is any organism whose cells contain a nucleus and other organelles enclosed within membranes. Eukaryotes belong to the taxon Eukarya or Eukaryota, the presence of a nucleus gives eukaryotes their name, which comes from the Greek εὖ and κάρυον. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, plants and algae contain chloroplasts. Eukaryotic organisms may be unicellular or multicellular, only eukaryotes form multicellular organisms consisting of many kinds of tissue made up of different cell types. Eukaryotes can reproduce asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two identical cells. In meiosis, DNA replication is followed by two rounds of division to produce four daughter cells each with half the number of chromosomes as the original parent cell. These act as sex cells resulting from genetic recombination during meiosis, the domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features, eukaryotes represent a tiny minority of all living things. However, due to their larger size, eukaryotes collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1. 6–2.1 billion years ago, in 1905 and 1910, the Russian biologist Konstantin Mereschkowsky argued three things about the origin of nucleated cells. Firstly, plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, secondly, the host had earlier in evolution formed by symbiosis between an amoeba-like host and a bacteria-like cell that formed the nucleus. Thirdly, plants inherited photosynthesis from cyanobacteria, the split between the prokaryotes and eukaryotes was introduced in the 1960s. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton, the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1938 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes. However he mentioned this in one paragraph, and the idea was effectively ignored until Chattons statement was rediscovered by Stanier. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in cells in her paper. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA and this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles, mitochondria and chloroplasts
9.
Molecule
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A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge, however, in quantum physics, organic chemistry, and biochemistry, the term molecule is often used less strictly, also being applied to polyatomic ions. In the kinetic theory of gases, the molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are in fact monoatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one element, as with oxygen, or it may be heteronuclear. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances and they also make up most of the oceans and atmosphere. Also, no typical molecule can be defined for ionic crystals and covalent crystals, the theme of repeated unit-cellular-structure also holds for most condensed phases with metallic bonding, which means that solid metals are also not made of molecules. In glasses, atoms may also be together by chemical bonds with no presence of any definable molecule. The science of molecules is called molecular chemistry or molecular physics, in practice, however, this distinction is vague. In molecular sciences, a molecule consists of a system composed of two or more atoms. Polyatomic ions may sometimes be thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i. e, according to Merriam-Webster and the Online Etymology Dictionary, the word molecule derives from the Latin moles or small unit of mass. Molecule – extremely minute particle, from French molécule, from New Latin molecula, diminutive of Latin moles mass, a vague meaning at first, the vogue for the word can be traced to the philosophy of Descartes. The definition of the molecule has evolved as knowledge of the structure of molecules has increased, earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties. Molecules are held together by covalent bonding or ionic bonding. Several types of non-metal elements exist only as molecules in the environment, for example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements, a covalent bond is a chemical bond that involves the sharing of electron pairs between atoms
10.
Protein
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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, 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 rarely considered to be proteins and are commonly called peptides, or sometimes oligopeptides. The individual amino acid residues are bonded together by peptide bonds, the sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the code specifies 20 standard amino acids, however. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors, proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes. Once formed, proteins only exist for a period of time and are then degraded and recycled by the cells machinery through the process of protein turnover. A proteins lifespan is measured in terms of its half-life and covers a wide range and they can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly 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, many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, other proteins are important in cell signaling, immune responses, cell adhesion, and 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. Methods commonly used to study structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance. 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, and a variable side chain are bonded. Only proline differs from this structure as it contains an unusual ring to the N-end amine group. The amino acids in a chain are linked by peptide bonds. Once linked in the chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen. The peptide bond has two forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar
11.
Brown algae
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The Phaeophyceae or brown algae, are a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form, for instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m in length and forms prominent underwater forests. Another example is Sargassum, which creates unique habitats in the waters of the Sargasso Sea. Many brown algae, such as members of the order Fucales, some members of the class, such as kelp, are used as food for humans. Worldwide, about 1500–2000 species of algae are known. Some species, such as Ascophyllum nodosum, are of sufficient commercial importance that they have become subjects of research in their own right. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name, genetic studies show their closest relatives to be the yellow-green algae. Brown algae exist in a range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike cells no more than a few centimeters long, some species have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of algae grow to much larger sizes. The rockweeds and leathery kelps are often the most conspicuous algae in their habitats, kelps can range in size from the two-foot-tall sea palm Postelsia to the giant kelp Macrocystis pyrifera, which grows to over 45 m long and is the largest of all the algae. In form, the brown algae range from small crusts or cushions to leafy free-floating mats formed by species of Sargassum and they may consist of delicate felt-like strands of cells, as in Ectocarpus, or of foot-long flattened branches resembling a fan, as in Padina. Regardless of size or form, two visible features set the Phaeophyceae apart from all other algae, first, members of the group possess a characteristic color that ranges from an olive green to various shades of brown. The particular shade depends upon the amount of fucoxanthin present in the alga, second, all brown algae are multicellular. There are no species that exist as single cells or as colonies of cells. Whatever their form, the body of all brown algae is termed a thallus and this does not mean that brown algae completely lack specialized structures. Although not all brown algae are structurally complex, those that are typically one or more characteristic parts. A holdfast is a structure present at the base of the alga
12.
Cytoskeleton
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A cytoskeleton is present in all cells of all domains of life. It is a network of interlinking filaments and tubules that extend throughout the cytoplasm. The cytoskeletal systems of different organisms are composed of similar proteins, the structure, function and dynamic behaviour of the cytoskeleton can be very different, depending on organism and cell type. Even within one cell the cytoskeleton can change through association with other proteins, there is a multitude of functions that the cytoskeleton can perform. Primarily, it gives the cell its shape and mechanical resistance to deformation, the cytoskeleton can also actively contract, thereby deforming the cell and the cells environment and allowing cells to migrate. Furthermore, it forms specialized structures, such as flagella, cilia, lamellipodia, a large-scale example of an action performed by the cytoskeleton is muscle contraction. During contraction of a muscle, within each cell, myosin molecular motors collectively exert forces on parallel actin filaments. This action contracts the muscle cell, and through the process in many muscle cells. In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton, eukaryotic cells contain three main kinds of cytoskeletal filaments, microfilaments, microtubules, and intermediate filaments. Each cytoskeletal filament has a shape and intracellular distribution, additionally, the filaments are formed by polymerization of different types of sub-units. The microfilament consist of the polymers of the protein actin which has a diameter of 7 nm, the microtubules are made up of the protein called tubulin which has a diameter of 25 nm. Intermediate filaments are made up of proteins which varies depending on the cell type. These type of filament normally have diameters ranging from 8-12 nm, the cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment. Cytoskeletal elements interact extensively and intimately with cellular membranes, a number of small molecule cytoskeletal drugs have been discovered that interact with actin and microtubules. These compounds have proven useful in studying the cytoskeleton and several have clinical applications, all filaments interact with accessory proteins that regulate and link the filaments to other cell compounds and each other. The accessory proteins are essential for controlled assembly of cytoskeletal filaments in particular locations, microfilaments are composed of linear polymers of G-actin proteins, and generate force when the growing end of the filament pushes against a barrier, such as the cell membrane. They also act as tracks for the movement of molecules that attach to the microfilament. Myosin motoring along F-actin filaments generates contractile forces in so-called actomyosin fibers, actin structures are controlled by the Rho family of small GTP-binding proteins such as Rho itself for contractile acto-myosin filaments, Rac for lamellipodia and Cdc42 for filopodia
