Wednesday, December 16, 2009

Biology

Biology
Biology deals with the study of the many varieties of living organisms(clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle
Biology (from Greek βιολογία - βίος, bios, "life"; -λογία, -logia, study of) is the natural scienceconcerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy.[1] The term biology in its modern sense appears to have been introduced independently by Karl Friedrich Burdach (1800), Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802), and Jean-Baptiste Lamarck (Hydrogéologie, 1802).[2][3]
Biology is a vast subject containing many subdivisions, topics, and theories. Five unifying principles can be said to form the fundamental axioms of modern biology: cell theory, evolution, gene theory,energy, and homeostasis.[4]
These fields are further divided based on the scale at which organisms are studied and the methods used to study them: biochemistry examines the rudimentary chemistry of life; molecular biologystudies the complex interactions of systems of biological molecules; cellular biology examines the basic building block of all life, the cell; physiology examines the physical and chemical functions of the tissues, organs, and organ systems of an organism; and ecology examines how various organisms interrelate with their environment.[5]
The classification, taxonomy, and nomenclature of biological organisms is administered by theInternational Code of Zoological Nomenclature, International Code of Botanical Nomenclature, andInternational Code of Nomenclature of Bacteria for animals, plants, and bacteria, respectively. Viruses, viroids, prions, and all other sub-viral agents that demonstrate biological characteristics are controlled by the International Code of Virus classification and nomenclature.[6][7][8][9] However, several other viral classification systems do exist.

History of biology

History
Ernst Haeckel's Tree of Life (1879)
Although biology in its modern form is a relatively recent development, sciences related to and included within biology have been studied since ancient times. Natural philosophy was studied as early as the ancient civilizations of Mesopotamia, Egypt, the Indian subcontinent, and China. However, the origins of modern biology and its approach to the study of nature are most often traced back to ancient Greece.[10]While the formal study of medicine dates back to Hippocrates, it was Aristotle who contributed most extensively to the development of biology. Especially important are his History of Animals and other works where he showed naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books onbotany that survived as the most important contribution of antiquity to botany, even into the Middle Ages. Significant advances in the study and development of biology were promoted through the efforts of suchMuslim physicians as the Afro-Arab scholar al-Jahiz (781–869) in zoology,[11] the Kurdish biologist Al-Dinawari (828–896) in botany,[12] and the Persian physician Rhazes (865–925) in anatomy and physiology. These philosophers elaborated on, expanded, and improved the Greek biological theories and systematics. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.
Biology began to quickly develop and grow with Antony van Leeuwenhoek's dramatic improvement of the microscope. It was then that scholars discovered spermatozoa, bacteria, infusoria and the sheer strangeness and diversity of microscopic life. Investigations by Jan Swammerdam led to new interest inentomology and built the basic techniques of microscopic dissection and staining.[13]
Advances in microscopy also had a profound impact on biological thinking itself. In the early 19th century, a number of biologists pointed to the central importance of the cell. In 1838 and 1839, Schleiden and Schwann began promoting the ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, though they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory.[14]
Meanwhile, taxonomy and classification began to present a focal point in the study of natural history. Carolus Linnaeus published a basictaxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species.[15] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Though he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought; his work would influence the evolutionary theories of both Lamarck and Darwin.[16]
Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck. However, it was the British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Thomas Malthus's writings on population growth, and his own morphological expertise, that created a more successful evolutionary theory based on natural selection; similar evidence led Alfred Russel Wallace to independently reach the same conclusions.[17]
The discovery of the physical representation of heredity came along with evoluttionary principles and population genetics. In the 1940s and early 1950s, experiments pointed to DNA as the portion of chromosomes (and perhaps other nucleoproteins) that held genes. A focus on new model organisms such as viruses and bacteria, along with the discovery of the double helical structure of DNA in 1953, marked the transition to the era of molecular genetics. Since the 1950s to present times, biology has been extensively defined in the molecular form. The DNA code was cracked by Har Gobind Khorana, Robert W. Holley and Marshall Warren Nirenberg after DNA was proven to be composed solely of codons. Finally, the Human Genome Project was launched in 1990 as an attempt to map out the general human genome. This project was essentially completed in 2003, with further analysis still being published. The Human Genome Project was the first step in the globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body.

volution

volution
Natural selection of a population for dark coloration.
A central organizing concept in biology is that life changes and develops throughevolution, and that all life-forms known have a common origin. Introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809,[19] Charles Darwin established evolution fifty years later as a viable theory by articulating its driving force: natural selection.[20][21] (Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution).[22]Evolution is now used to explain the great variations of life found on Earth.
Darwin theorized that species and breeds developed through the processes of natural selection and artificial selection or selective breeding.[23] Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory.[24]
The evolutionary history of the species— which describes the characteristics of the various species from which it descended— together with its genealogical relationship to every other species is known as its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms inpaleontology.[25]
Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics. For a summary of major events in the evolution of life as currently understood by biologists, see evolutionary timeline.
Historically, it wasn't evolution that was theorized to be the reason for biological speciation. Up into the 19th century, spontaneous generation, the belief that life forms could appear spontaneously under certain conditions, was widely accepted.[26] This misconception was challenged byWilliam Harvey, who even before the invention of the microscope was led by his studies to suggest that life came from invisible 'eggs.' In the frontispiece of his book Exercitationes de Generatione Animalium (Essays on the Generation of Animals), he expressed the basic principle ofbiogenesis: "Omnia ex ovo" (everything from eggs).[27]
A group of organisms have a common descent if they share a common ancestor. All organisms on the Earth, both living and extinct, have been or are descended from a common ancestor or an ancestral gene pool. This last universal common ancestor of all organisms is believed to have appeared about 3.5 billion years ago.[28] Biologists generally regard the collective universality of the genetic code as definitive evidence in favor of the theory of universal common descent for all bacteria, archaea, and eukaryotes

Genetics

Genetics
A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
Genes are the primary units of inheritance in all organisms. A gene is a unit of heredity and a region of DNAthat influences a particular characteristic in an organism. All organisms, from bacteria to animals, share the same basic machinery that copies and translates DNA into proteins. Cells transcribe a DNA gene into an RNA version of the gene, and a ribosome then translates the RNA into a protein. Additionally, DNA codes for identical or at least strongly similar proteins within all organisms. A sequence of DNA that codes for insulin in humans will also code for insulin when inserted into other organisms, such as plants.[30][31]
DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell and any other hereditary information found in the mitochondria, chloroplasts, or other locations is collectively known as its genome. A chromosome is an organized structure consisting of DNA and histones. Genomic DNA is located in the cell nucleus of eukaryotes, as well as small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid.[32] The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype

Homeostasis

Homeostasis
Homeostasis is the ability of an open system to regulate its internal environment to maintain a stable condition by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living organisms, whether unicellular or multicellular, exhibit homeostasis.[34]
In order to maintain dynamic equilibrium and effectively carry out certain functions, a system must detect and respond to perturbations. After the detection of a perturbation, a biological system will respond through at least one of the two forms of feedback: negative feedback andpositive feedback.[35] Negative feedback consists of reducing the output or activity of an organ or system back to its normal range of functioning. One example is the human body's release of insulin when blood sugar levels are too high. Another example is the release ofglucagon when sugar levels are too low. Positive feedback mechanisms are designed to accelerate or enhance an output, and not necessarily to maintain an equilibrium. One example of a positive feedback event in the human body is the activation of blood platelets, which, in turn, release chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth

Energy

Energy
The survival of a living organism depends on the continuous input of energy. Chemical reactions that are responsible for its structure and function are tuned to extract energy from substances that act as its food and transform them to help form new cells and sustain them. In this process, molecules of chemical substances that constitute food play two roles; first, they contain energy that can be transformed for biologicalchemical reactions; and also develop molecular structures made up of biomolecules.
Nearly all of the energy needed for life processes originates from the Sun.[37] Plants and other autotrophs use solar energy via a process known as photosynthesis to convert raw materials into organic molecules, such as ATP, whose bonds can be broken to release energy.[38] A fewecosystems, however, depend entirely on energy extracted by chemotrophs from methane, sulfides, or other non-luminal energy sources.[39]The molecules used by organisms to release energy are sometimes also, in biology, referred to as chemical energy.
Some of the captured energy is used to produce biomass to sustain life and provide energy for its growth and development. The majority of the rest of this energy is lost as heat and waste molecules. The most common processes for converting the energy trapped in chemical substances into energy useful to sustain life are metabolism[40] and cellular respiration.[36]

Structural

Structural
Schematic of typical animal cell depicting the variousorganelles and structures.
Main articles: Molecular biology, Cell biology, Genetics, and Developmental biology
Molecular biology is the study of biology at a molecular level.[41] This field overlaps with other areas of biology, particularly with genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interrelationship of DNA, RNA, and protein synthesis and learning how these interactions are regulated.
Cell biology studies the physiological properties of cells, as well as their behaviors, interactions, and environment. This is done both on a microscopic and molecular level. Cell biology researches both single-celled organisms like bacteria and specialized cells in multicellular organisms like humans.
Understanding cell composition and how they function is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important in the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types.
Genetics is the science of genes, heredity, and the variation of organisms.[42][43] Genes encode the information necessary for synthesizing proteins, which in turn play a large role in influencing (though, in many instances, not completely determining) the final phenotype of the organism. In modern research, genetics provides important tools in the investigation of the function of a particular gene, or the analysis ofgenetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.
Developmental biology studies the process by which organisms grow and develop. Originating in embryology, modern developmental biology studies the genetic control of cell growth, differentiation, and "morphogenesis," which is the process that progressively gives rise to tissues,organs, and anatomy. Model organisms for developmental biology include the round worm Caenorhabditis elegans,[44] the fruit fly Drosophila melanogaster,[45] the zebrafish Danio rerio[46], the mouse Mus musculus,[47], and the weed Arabidopsis thaliana.[48][49] A model organism is aspecies that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms.[50]