Gravitropism is a coordinated process of differential growth by a plant or fungus in response to gravity pulling on it. It is a general feature of all many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism; that is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be demonstrated with any potted plant; when laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Hebaverns stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. Root growth occurs by division of stem cells in the root meristem located in the tip of the root, the subsequent asymmetric expansion of cells in a shoot-ward region to the tip known as the elongation zone. Differential growth during tropisms involves changes in cell expansion versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out.
Gravity is sensed in the root tip and this information must be relayed to the elongation zone so as to maintain growth direction and mount effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity. Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport; this was described in the 1920s in the Cholodny-Went model. The model was independently proposed by the Russian scientist N. Cholodny of the University of Kiev in 1927 and by Frits Went of the California Institute of Technology in 1928, both based on work they had done in 1926. Auxin exists in nearly every organ and tissue of a plant, but it has been reoriented in the gravity field, can initiate differential growth resulting in root curvature. Experiments show that auxin distribution is characterized by a fast movement of auxin to the lower side of the root in response to a gravity stimulus at a 90° degree angle or more.
However, once the root tip reaches a 40° angle to the horizontal of the stimulus, auxin distribution shifts to a more symmetrical arrangement. This behavior is described as a "tipping point" mechanism for auxin transport in response to a gravitational stimulus. Gravitropism is an integral part of plant growth, orienting its position to maximize contact with sunlight, as well as ensuring that the roots are growing in the correct direction. Growth due to gravitropism is mediated by changes in concentration of the plant hormone auxin within plant cells; as plants mature, gravitropism continues to guide development along with phototropism. While amyloplasts continue to guide plants in the right direction, plant organs and function rely on phototropic responses to ensure that the leaves are receiving enough light to perform basic functions such as photosynthesis. In complete darkness, mature plants have little to no sense of gravity, unlike seedlings that can still orient themselves to have the shoots grow upward until light is reached when development can begin.
Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the forces of gravity. In both roots and stems, auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity. In stems, the auxin accumulates on the lower side, however in this tissue it increases cell expansion and results in the shoot curving up. A recent study showed that for gravitropism to occur in shoots, only a fraction of an inclination, instead of a strong gravitational force, is necessary; this finding sets aside gravity sensing mechanisms that would rely on detecting the pressure of the weight of statoliths. Plants possess the ability to sense gravity in several ways, one of, through statoliths. Statoliths are dense amyloplasts, organelles that synthesize and store starch involved in the perception of gravity by the plant, that collect in specialized cells called statocytes.
Statocytes are located in the starch parenchyma cells near vascular tissues in the shoots and in the columella in the caps of the roots. These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector; the statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels. The gravitropic signal leads to the reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in the root cap and root as a whole; the changed relations in concentration of auxin leads to differential growth of the root tissues. Taken together, the root is turning to follow the gravity stimuli. Statoliths are found in the endodermic layer of the hypocotyl and inflorescence stock; the redistribution of auxin causes increased growth on the lower side of the shoot so that it orients in a direction opposite that of the gravity stimuli. Phytochromes are red and far-red photoreceptors that help induce changes in certain aspects of plant development.
Apart being itself the tropic factor, light may suppress the gravitropic reaction. In seedlings and far-red light both inhibit negative gravitropism in seedling hypocotyls causing growth in random directions. However, the hypocotyls re
Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. A few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, are present in most of its habitats. Bacteria inhabit soil, acidic hot springs, radioactive waste, the deep portions of Earth's crust. Bacteria live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, only about half of the bacterial phyla have species that can be grown in the laboratory; the study of bacteria is known as a branch of microbiology. There are 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are 5×1030 bacteria on Earth, forming a biomass which exceeds that of all plants and animals. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere.
The nutrient cycle includes the decomposition of dead bodies. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Data reported by researchers in October 2012 and published in March 2013 suggested that bacteria thrive in the Mariana Trench, with a depth of up to 11 kilometres, is the deepest known part of the oceans. Other researchers reported related studies that microbes thrive inside rocks up to 580 metres below the sea floor under 2.6 kilometres of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere—they're adaptable to conditions, survive wherever they are."The famous notion that bacterial cells in the human body outnumber human cells by a factor of 10:1 has been debunked. There are 39 trillion bacterial cells in the human microbiota as personified by a "reference" 70 kg male 170 cm tall, whereas there are 30 trillion human cells in the body.
This means that although they do have the upper hand in actual numbers, it is only by 30%, not 900%. The largest number exist in the gut flora, a large number on the skin; the vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, though many are beneficial in the gut flora. However several species of bacteria are pathogenic and cause infectious diseases, including cholera, anthrax and bubonic plague; the most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and are used in farming, making antibiotic resistance a growing problem. In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium and other metals in the mining sector, as well as in biotechnology, the manufacture of antibiotics and other chemicals.
Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two different groups of organisms that evolved from an ancient common ancestor; these evolutionary domains are called Archaea. The word bacteria is the plural of the New Latin bacterium, the latinisation of the Greek βακτήριον, the diminutive of βακτηρία, meaning "staff, cane", because the first ones to be discovered were rod-shaped; the ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species.
However, gene sequences can be used to reconstruct the bacterial phylogeny, these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The most recent common ancestor of bacteria and archaea was a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. Bacteria were involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves related to the Archaea; this involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Some eukaryotes that contained mitochondria engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants; this is known as primary endosymbiosis. Bacteria display a wide diversity of sizes, called morphologies.
Bacterial cells are about one-tenth the size of eukaryotic cells
Cellular differentiation is the process where a cell changes from one cell type to another. The cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create differentiated daughter cells during tissue repair and during normal cell turnover; some differentiation occurs in response to antigen exposure. Differentiation changes a cell's size, membrane potential, metabolic activity, responsiveness to signals; these changes are due to controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation never involves a change in the DNA sequence itself. Thus, different cells can have different physical characteristics despite having the same genome. A specialized type of differentiation, known as'terminal differentiation', is of importance in some tissues, for example vertebrate nervous system, striated muscle and gut.
During terminal differentiation, a precursor cell capable of cell division, permanently leaves the cell cycle, dismantles the cell cycle machinery and expresses a range of genes characteristic of the cell's final function. Differentiation may continue to occur after terminal differentiation if the capacity and functions of the cell undergo further changes. Among dividing cells, there are multiple levels of cell potency, the cell's ability to differentiate into other cell types. A greater potency indicates a larger number of cell types. A cell that can differentiate into all cell types, including the placental tissue, is known as totipotent. In mammals, only the zygote and subsequent blastomeres are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques. A cell that can differentiate into all cell types of the adult organism is known as pluripotent; such cells are called meristematic cells in higher plants and embryonic stem cells in animals, though some groups report the presence of adult pluripotent cells.
Virally induced expression of four transcription factors Oct4, Sox2, c-Myc, KIF4 is sufficient to create pluripotent cells from adult fibroblasts. A multipotent cell is one that can differentiate into multiple different, but related cell types. Oligopotent cells are more restricted than multipotent, but can still differentiate into a few related cell types. Unipotent cells can differentiate into only one cell type, but are capable of self-renewal. In cytopathology, the level of cellular differentiation is used as a measure of cancer progression. "Grade" is a marker of. Three basic categories of cells make up the mammalian body: germ cells, somatic cells, stem cells; each of the 37.2 trillion cells in an adult human has its own copy or copies of the genome except certain cell types, such as red blood cells, that lack nuclei in their differentiated state. Most cells are diploid; such cells, called somatic cells, make up most such as skin and muscle cells. Cells differentiate to specialize for different functions.
Germ line cells are any line of cells that give rise to gametes—eggs and sperm—and thus are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells, they are best described in the context of normal human development. Development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. In humans four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst; the blastocyst has an outer layer of cells, inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass go on to form all of the tissues of the human body. Although the cells of the inner cell mass can form every type of cell found in the human body, they cannot form an organism.
These cells are referred to as pluripotent. Pluripotent stem cells undergo further specialization into multipotent progenitor cells that give rise to functional cells. Examples of stem and progenitor cells include: Radial glial cells that give rise to excitatory neurons in the fetal brain through the process of neurogenesis. Hematopoietic stem cells from the bone marrow that give rise to red blood cells, white blood cells, platelets Mesenchymal stem cells from the bone marrow that give rise to stromal cells, fat cells, types of bone cells Epithelial stem cells that give rise to the various types of skin cells Muscle satellite cells that contribute to differentiated muscle tissue. A pathway, guided by the cell adhesion molecules consisting of four amino acids, glycine and serine, is created as the cellular blastomere differentiates from the single-layered blastula to the three primary layers of germ cells in mammals, namely the ectoderm and endoderm; the ectoderm ends up forming the skin and the nervous system, the mesoderm forms the bones and muscular tissue, the endoderm forms the internal organ tissues.
Andreas Franz Wilhelm Schimper
Andreas Franz Wilhelm Schimper was a German botanist and phytogeographer who made major contributions in the fields of histology and plant geography. He travelled to the Caribbean as part of the 1899 deep-sea expedition, he coined the terms tropical rainforest and sclerophyll and is commemorated in numerous specific names. Schimper was born into a family of eminent scientists, his father Wilhelm Philippe Schimper was Director of the Natural History Museum in the same town, Professor of Geology, a leading bryologist. His father's cousin was Georg Wilhelm Schimper, prominent collector and explorer in Arabia and North Africa. Schimper studied at the University of Strassburg from 1874 to 1878, acquiring a Ph. D, he worked in Lyon, in 1880 travelled to the United States, becoming a Fellow at Johns Hopkins University. In 1882, he moved back to the University of Bonn working with Eduard Strasburger, becoming a private docent. In 1883, Schimper postulated the endosymbiotic origin of chloroplasts and paved the way to the symbiogenesis theory of Konstantin Mereschkowski und Lynn Margulis.
In 1886, he was appointed Extraordinary Professor at the University of Bonn, worked on cell histology and starch metabolism. He had become interested in phytogeography and plant ecology, undertaking expeditions to the West Indies and Venezuela in 1882-1883. In 1886, he stayed with Fritz Müller in Brazil, in 1889-1890 in Ceylon, the Malaya and Botanical Garden in Buitenzorg, concentrating on mangroves and littoral vegetation; this resulted in his account of the Rhizophoraceae in Engler & Prantl's Natürliche Pflanzenfamilien. In 1898, he became Professor of Botany at the University of Basel and the same year joined the German Valdivia-Expedition; this was a deep-sea expedition aboard the SS Valdivia led by Professor Carl Chun. The trip lasted 9 months, during which they visited the Canary Islands, Cape Town, New Amsterdam and Cocos Islands, the Maldives, the Seychelles and the Red Sea. In 1899, he became Professor of Botany at the University of Basel, his health had been affected by malaria contracted in Cameroon and Dar-es-Salaam, he died of complications of malaria at the age of 45 in 1901.
Schimper is best known for Pflanzengeographie auf physiologischer Grundlage, published at the University of Jena in 1898 where he aimed to explain the expansion and ecology of plants based on the ecological knowledge of the time. In this book he coined the terms tropical sclerophyll, he wrote in the preface: "Nur wenn sie in engster Fühlung mit der experimentellen Physiologie verbleibt, wird die Ökologie der Pflanzengeographie neue Bahnen eröffnen können, denn sie setzt eine genaue Kenntnis der Lebensbedingungen der Pflanze voraus, welche nur das Experiment verschaffen kann". His classification of plant formations was important for the development of the botanical sciences: „Nach dem Vorhergehenden sind zwei ökologische Formationsgruppen zu unterscheiden, die klimatischen oder Gebietsformationen, deren Vegetationscharakter durch die Hydrometeore beherrscht, und die edaphischen oder Standortsformationen, wo derselbe in erster Linie durch die Bodenbeschaffenheit bedingt ist“. At the same time as his Russian soil science colleagues, Schimper discussed the hypothesis of vegetation being limited to climate zones versus those that are azonal, elaborated by Frederic Edward Clements and geobotanist Heinrich Walter amongst others.
In 1894, Schimper was one of the 4 original authors of the textbook of botany Lehrbuch der Botanik and until the 5th edition 1902 editor of the chapter spermatophyta or seed-bearing plants. Rudolf Marloth wrote an account of the Cape floral region for Chun's proposed Wissenschaftliche Ergebnisse der deutschen Tiefsee-Expedition auf dem Dampfer Valdivia 1898-1899, and Schimper contributed two chapters on "Gebiet der Hartlaubgehölze" and "Der Knysnawald". Schimper is commemorated in specific names such as Acokanthera Harpachne schimperi. In 1892, he was voted a member of the Deutsche Akademie der Naturforscher Leopoldina gewählt. Works by Andreas Franz Wilhelm Schimper at Project Gutenberg Works by or about Andreas Franz Wilhelm Schimper at Internet Archive Books by and about A. F. G. Schimper on WorldCat Digital edition: "Anleitung zur mikroskopischen Untersuchung der vegetabilischen Nahrungs- und Genussmittel" 2nd ed. by the University and State Library Düsseldorf Plant-geography upon a physiological basis by A.
F. W. Schimper, 1903 Biodiversity Heritage Library Botanical Exploration of Southern Africa Mary Gunn & LE Codd
Chlorophyll is any of several related green pigments found in cyanobacteria and the chloroplasts of algae and plants. Its name is derived from the Greek words chloros and φύλλον, phyllon. Chlorophyll is essential in photosynthesis, allowing plants to absorb energy from light. Chlorophylls absorb light most in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum, which it reflects, producing the green color of chlorophyll-containing tissues. Two types of chlorophyll exist in the photosystems of green plants: chlorophyll a and b. Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817; the presence of magnesium in chlorophyll was discovered in 1906, was the first time that magnesium had been detected in living tissue. After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940.
By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming, in 1990 Woodward and co-authors published an updated synthesis. Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010. Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves three functions; the function of the vast majority of chlorophyll is to absorb light. Having done so, these same centers execute their second function: the transfer of that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems; this pair effects the final function of charge separation, leading to biosynthesis.
The two accepted photosystem units are photosystem II and photosystem I, which have their own distinct reaction centres, named P680 and P700, respectively. These centres are named after the wavelength of their red-peak absorption maximum; the identity and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent, these chlorophyll pigments can be separated into chlorophyll a and chlorophyll b; the function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation; the removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an electron transport chain; the charged reaction center of chlorophyll is reduced back to its ground state by accepting an electron stripped from water.
The electron that reduces P680+ comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as plants produce O2 gas, is the source for all the O2 in Earth's atmosphere. Photosystem I works in series with Photosystem II. Electron transfer reactions in the thylakoid membranes are complex and the source of electrons used to reduce P700+ can vary; the electron flow produced by the reaction center chlorophyll pigments is used to pump H+ ions across the thylakoid membrane, setting up a chemiosmotic potential used in the production of ATP or to reduce NADP+ to NADPH. NADPH is a universal agent used to reduce CO2 into sugars as well as other biosynthetic reactions. Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of that happening under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins all cooperatively absorb and funnel light energy to the reaction center.
Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment–protein antenna complexes. Chlorophylls are numerous in types, but all are defined by the presence of a fifth ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, they share a common biosynthetic pathway as porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which feature iron at the center of the tetrapyrrole ring, chlorophylls bind magnesium. For the structures depicted in this article, some of the ligands attached to the Mg2+ center are omitted for clarity; the chlorin ring can have various side chains including a long phytol chain. The most distributed form in terrestrial plants is chlorophyll a; the structures of chlorophylls are summarized below: When leaves degreen in the process of plant senescence, chlorophyll is converted to a group of colourless tetrapyrroles known as nonfluorescent chlorophyll cataboli
Algae is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not closely related, is thus polyphyletic. Including organisms ranging from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata and phloem, which are found in land plants; the largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example and the stoneworts. No definition of algae is accepted. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Although cyanobacteria are referred to as "blue-green algae", most authorities exclude all prokaryotes from the definition of algae.
Algae constitute a polyphyletic group since they do not include a common ancestor, 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 wide 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, the roots 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 unicellular species of green algae, many golden algae, euglenids and other algae have become heterotrophs, sometimes parasitic, relying on external energy sources and have limited or no photosynthetic apparatus.
Some other heterotrophic organisms, such as the apicomplexans, are derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago. The singular alga retains that meaning in English; the etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold", no reason is known to associate seaweed with temperature. A more 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 the cosmetic rouge; the etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך, "paint", a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean.
It could be any color: black, green, or blue. 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. The algae contain chloroplasts. Chloroplasts contain circular DNA like that in cyanobacteria and are interpreted as representing 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 major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members; some retain plastids, but not chloroplasts. Phylogeny based on plastid not nucleocytoplasmic genealogy: Linnaeus, in Species Plantarum, the starting point for modern botanical nomenclature, recognized 14 genera of algae, of which only four are considered among algae.
In Systema Naturae, Linnaeus described the genera Volvox and Corallina, a species of Acetabularia, among the animals. In 1768, Samuel Gottlieb Gmelin published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the new binomial nomenclature of Linnaeus, it included elaborate illustrations of seaweed and marine algae on folded leaves. W. H. Harvey and Lamouroux were the first to divide macroscopic algae into four divisions based on their pigmentation; this is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae, brown algae, green algae, Diatomaceae. At this time, microscopic algae were discovered and reported by a different group of workers studying the Infusoria. Unlike macroalgae, which were viewed as plants, microalgae were considered animals because they are motile; the nonmotile microalgae were sometimes seen as stages of the lifecycle of plants, macroalgae, or animals. Although used as a taxonomic category in some pre-D
Fat is one of the three main macronutrients, along with carbohydrate and protein. Fats molecules consist of carbon and hydrogen atoms, thus they are all hydrocarbon molecules. Examples include cholesterol and triglycerides; the terms "lipid", "oil" and "fat" are confused. "Lipid" is the general term, though a lipid is not a triglyceride. "Oil" refers to a lipid with short or unsaturated fatty acid chains, liquid at room temperature, while "fat" refers to lipids that are solids at room temperature – however, "fat" may be used in food science as a synonym for lipid. Fats, like other lipids, are hydrophobic, are soluble in organic solvents and insoluble in water. Fat is an important foodstuff for many forms of life, fats serve both structural and metabolic functions, they are a necessary part of the diet of most heterotrophs and are the most energy dense, thus the most efficient form of energy storage. Some fatty acids that are set free by the digestion of fats are called essential because they cannot be synthesized in the body from simpler constituents.
There are two essential fatty acids in human nutrition: linoleic acid. Other lipids needed by the body can be synthesized from other fats. Fats and other lipids are broken down in the body by enzymes called lipases produced in the pancreas. Fats and oils are categorized according to the number and bonding of the carbon atoms in the aliphatic chain. Fats that are saturated fats have no double bonds between the carbons in the chain. Unsaturated fats have one or more double bonded carbons in the chain; the nomenclature is based on the non-acid end of the chain. This end is called the n-end, thus alpha-linolenic acid is called an omega-3 fatty acid because the 3rd carbon from that end is the first double bonded carbon in the chain counting from that end. Some oils and fats are therefore called polyunsaturated fats. Unsaturated fats can be further divided into cis fats, which are the most common in nature, trans fats, which are rare in nature. Unsaturated fats can be altered by reaction with hydrogen effected by a catalyst.
This action, called hydrogenation, tends to break all the double bonds and makes a saturated fat. To make vegetable shortening liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce saturated fats, which have more desirable physical properties e.g. they melt at a desirable temperature, store well, whereas polyunsaturated oils go rancid when they react with oxygen in the air. However, trans fats are generated during hydrogenation as contaminants created by an unwanted side reaction on the catalyst during partial hydrogenation. Saturated fats can stack themselves in a packed arrangement, so they can solidify and are solid at room temperature. For example, animal fats tallow and lard are solids. Olive and linseed oils on the other hand are liquid. Fats serve both as energy sources for the body, as stores for energy in excess of what the body needs immediately; each gram of fat when burned or metabolized releases about 9 food calories. Fats are broken down in the healthy body to release their constituents and fatty acids.
Glycerol itself can be converted to glucose by the liver and so become a source of energy. There are many different kinds of fats. All fats are derivatives of fatty acids and glycerol. Most fats are glycerides triglycerides. One chain of fatty acid is bonded to each of the three -OH groups of the glycerol by the reaction of the carboxyl end of the fatty acid with the alcohol. Water is eliminated and the carbons are linked by an -O- bond through dehydration synthesis; this process is called esterification and fats are therefore esters. As a simple visual illustration, if the kinks and angles of these chains were straightened out, the molecule would have the shape of a capital letter E; the fatty acids would each be a horizontal line. Fats therefore have "ester" bonds; the properties of any specific fat molecule depend on the particular fatty acids. Fatty acids form a family of compounds that are composed of increasing numbers of carbon atoms linked into a zig-zag chain; the more carbon atoms there are in any fatty acid, the longer its chain will be.
Long chains are more susceptible to intermolecular forces of attraction, so the longer ones melt at a higher temperature. Fatty acid chains may differ by length categorized as short to long. Short-chain fatty acids are fatty acids with aliphatic tails of fewer than six carbons. Medium-chain fatty acids are fatty acids with aliphatic tails of 6–12 carbons, which can form medium-chain triglycerides. Long-chain fatty acids are fatty acids with aliphatic tails of 13 to 21 carbons. Long chain fatty acids are fatty acids with aliphatic tails of 22 or more carbons. Any of these aliphatic fatty acid chains may be glycerated and the resultant fats may have tails of different lengths from short triformin to long, e.g. cerotic acid, or hexacosanoic acid, a 26-carbon long-chain saturated fatty acid. Long chain fats are exemplified by tallow. Most fats found in foo