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Heredity And Variability
Evolutionary Development
Organismes and environment State of ecosystems, habitats and species
Human Impact On The Natural Environment
Applied integrated sciences Biochemistry and molecular biology (mcdb)
Cell Biology

Growth And Development
“Development” and “growth” are sometimes used interchangeably in conversation, but in a botanical sense, they describe separate events in the organization of the mature plant body.
Development is the progression from earlier to later stages in maturation, e.g. a fertilized egg develops into a mature tree. It is the process whereby tissues, organs, and whole plants are produced. It involves: growth, morphogenesis (the acquisition of form and structure), and differentiation. The interactions of the environment and the genetic instructions inherited by the cells determine how the plant develops.

Growth is the irreversible change in size of cells and plant organs due to both cell division and enlargement. Enlargement necessitates a change in the elasticity of the cell walls together with an increase in the size and water content of the vacuole. Growth can be determinate—when an organ or part or whole organism reaches a

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certain size and then stops growing—or indeterminate—when cells continue to divide indefinitely. Plants in general have indeterminate growth.
Differentiation is the process in which generalized cells specialize into the morphologically and physiologically different cells . Since all of the cells produced by division in the meristems have the same genetic make up, differentiation is a function of which particular genes are either expressed or repressed. The kind of cell that ultimately develops also is a result of its location: Root cells don't form in developing flowers, for example, nor do petals form on roots.
Mature plant cells can be stimulated under certain conditions to divide and differentiate again, i.e. to dedifferentiate. This happens when tissues are wounded, as when branches break or leaves are damaged by insects. The plant repairs itself bydedifferentiating parenchyma cells in the vicinity of the wound, making cells like those injured or else physiologically similar cells.

Plants differ from animals in their manner of growth. As young animals mature, all parts of their bodies grow until they reach a genetically determined size for each species. Plant growth, on the other hand, continues throughout the life span of the plant and is restricted to certain meristematic tissue regions only. This continuous growth results in:

Two general groups of tissues, primary and secondary.
Two body types, primary and secondary.

Apical and lateral meristems.

Apical meristems, or zones of cell division, occur in the tips of both roots, stems of all plants, and are responsible for increases in the length of the primary plant body as the primary tissues differentiate from the meristems. As the vacuoles of the primary tissue cells enlarge, the stems and roots increase in girth until a maximum size (determined by the elasticity of their cell walls) is reached. The plant may continue to grow in length, but no longer does it grow in girth. Herbaceous plants with only primary tissues are thus limited to a relatively small size.
Woody plants, on the other hand, can grow to enormous size because of the strengthening and protective secondary tissues produced by lateral meristems, which develop around the periphery of their roots and stems. These tissues constitute the secondary plant body.
Heredity And Variability
Heredity refers to the genetic transmission of traits from parents to offspring. Heredity helps explain why children tend to resemble their parents, as well as how a genetic disease runs in a family. Some genetic conditions are caused by mutations in a single gene. These conditions are usually inherited in one of several straightforward patterns, including autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, codominant, and mitochondrial inheritance patterns. Complex disorders and multifactorial disorders are caused by a combination of genetic and environmental factors. These disorders may cluster in families, but do not have a clear-cut pattern of inheritance.
Evolution : a process of development in which an organ or organism becomes

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more and more complex by the differentiation of its parts; a continuous and progressive change according to certain laws and by means of resident forces
bathmic or orthogenic evolution : evolution due to something in the organism itself independent of environment
convergent evolution : the appearance of similar forms and/or functions in two or more lines not sufficiently related phylogenetically to account for the similarity. The concept that chance reigns supreme may ring less true when it comes to complex behaviors. A study of the similarities between the webs of different Tetragnatha spider species on different Hawaiian Islands provides fresh evidence that behavioral tendencies can actually evolve rather predictably, even in widely separated places. The spiders' webs vary significantly, with tissue -like 'sheet webs', disorganized cobwebs and spiral-shaped 'orb webs' as three of the most common types. Each species had its own characteristic type of web. But the scientists found that in several cases, separate species of Tetragnatha spiders on different islands constructed extremely similar orb webs, right down to the number of spokes, and the lengths and densities of the sticky spiral that captures bugs. Was this an example of similar environments producing the same complex behavior, or did the spiders with corresponding webs share a common ancestor? The tree that linked spiders through their web-constructing behavior proved highly improbable as it was very complicated, and contradicted the relationships suggested by their DNA. It is likely that similar forest types support similar mixes of prey, which could elicit similar web structures. Previous research has found that physical traits, for example legs or wings, can arise independently in similar environmental conditions. And various groups have looked at the evolution of simple behaviors, such as where species locate themselves within a habitat, like a branch or lake. But the evolution of complex behaviors is less well understood : predictable evolutionary convergence of behavior applies far beyond spiders, and happens more often then some believe

emergent evolution : the assumption that each step in evolution produces something new and something that could not be predicted from its antecedents.

organic evolution : the origin and development of species; the theory that existing organisms are the result of descent with modification from those of past times.

parallel evolution : the independent evolution of similar structures in two or more rather closely related organisms

salutatory evolution : evolution showing sudden changes; mutation or saltation.

o halmatogenesis / salutatory variation : a sudden alteration of type from one generation to another

darwinism / darwinian theory : the theory of evolution by Charles Robert Darwin according to which higher organisms have developed from lower ones through the influence of natural selection

o adaptive plasticity in response to environmental pressures : snake populations that persistently encounter large prey may accumulate gene mutations that specify a large head size, or head growth may be increased in individual snakes to meet local demands (adaptive developmental plasticity).

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monogenesis : the theory of evolution according to which the course of evolution is fixed and predetermined by law, no place being left for chance

an adaptations programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. Some criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analyzed as integrated wholes, with Bauplane so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. Some fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. Some support Darwin's own pluralistic approach to identifying the agents of evolutionary change

the theory of intelligent design (ID)makes the claim that the existence of complex systems and phenomena, lacking any justification for their existence that is known to us, implies that such systems exist as the purposeful result of the activity of a powerful, conscious being that designed the visible complexity into them. This is not a scientific explanation, as it posits the existence of something that cannot be tested or demonstrated by experiment, but must be taken on faith. The contrast between the theory of intelligent design and the theory of special creation is that the latter names the designer "God" and declares the story in the biblical book of Exodus as the whole truth, whereas the former does not name the designer nor does it declare any particular story of the designer's works and actions to be historical truth. However, both of these theories are theology, not biology, and while not identical, are both out of place in a life science journal. Theologians, and even scientists, are entitled to logically debate questions of faith surrounding the problems of first causes, complexity, the existence of evil, and so forth, but not in scientific publications. Albert Einstein is quoted as having said, "Science without religion is lame; religion without science is blind." Let us be clear, however: science is about knowledge gained by hypothesis testing, and religion is about faith gained from reason, inspiration, and introspection. We must keep them properly separated to understand the difference between that which we can know and that which we must choose, or choose not, to believe.

first proposed by W.D. Hamilton in 1964, the theory of kin selection holds

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that altruistic cooperative behavior preferentially directed at helping a relative is favored because it helps that relative do better and reproduce, which indirectly helps the cooperator to pass on its genes. Generating siderophores is costly to producer Pseudomonas aeruginosa (cooperators), but others around it can use the siderophores to their own benefit without paying the price (cheaters). When relatedness is high, the cooperators spread to fixation and take over; and when relatedness is low, the cheaters spread to take over, meaning that higher relatedness had a tendency to favor selection for more altruism or cooperation. Another more subtle effect of kin selection is the scale of competition—whether competition is local (competition between close relatives) or global (competition between unrelated bacteria of the same species). Relatedness increases cooperation, so that over time, a localized group of highly related organisms emerges. But eventually, these would also become the closest competitors in the local area, so they were the ones you had to compete with for spots in the gene pool in the next generation. The experimental effects of relatedness on the scale of competition explained > 90% of the variation in the frequency of cooperators versus cheaters at the end of the experiment. The work has implications for social insects : if individual insects are close relatives but are going be dispersing to some other area, or maybe foraging in different areas or looking in different areas for mates, then the scale at which competition might take place is going to vary quite a bit depending on the ecology of that particular insect.
Selection
Selection generally refers to the pressures on crops and organisms to evolve. These pressures include natural selection, and, in eukaryotic cells that reproduce sexually, sexual selection. Certain phenotypic traits (characteristics of an organism)—or, on a genetic level, alleles of genes—segregate within a population, where individuals with aadaptive advantages or traits tend to succeeded more than their peers when they reproduce, and so contribute more ooffspring to the succeeding generation. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring inherit them from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved.
Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryos, and gamete eggs and sperm all undergo selection. Factors fostering selection include sexual selection, primarily caused by mate choice in the mating phase of sexual reproduction, limits on resources (nourishment, habitat space, mates) and the existence of threats (predators, disease, adverse weather). Biologists often refer to such factors as selective or evolutionary pressures.

Natural selection has, since the 1930s, included sexual selection because biologists at the time did not think it was of great importance though it has become to be seen as more important in the 21st Century.Other subcategories of natural selection include ecological selection, stabilizing selection, disruptive selection and selection. Selective can be seen in the breeding of dogs, and the domestication of

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farm animals and crops, now commonly known as selective breeding.

Selection is hierarchically classified into natural and artificial selection. Natural selection is further sub classified into ecological and sexual selection

Selection occurs only when the individuals of a population are diverse in their characteristics—or more specifically when the traits of individuals differ with respect to how well they equip them to survive or exploit a particular pressure. In the absence of individual variation, or when variations are selectively neutral, selection does not occur.

Meanwhile, selection does not guarantee that advantageous traits or alleles become prevalent within a population. Another process of gene frequency alteration in a population is called genetic drift, which acts over genes that are not under selection. But, this drift can't overcome natural selection itself, as it is a 'random sampling' process and Natural Selection is actually an evaluative force. In the face of selection, even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of "weak" selection (e.g., an infectious disease with only a low mortality rate) or small populations.

Though deleterious alleles may sometimes become established, selection may act "negatively" as well as positively. Negative selection or purifying selection decreases the prevalence of traits that diminish individuals' capacity to succeed reproductively (i.e., their fitness), while positive selection increases the prevalence of adaptive traits.
Evolutionary Development
Charles Darwin's theory of evolution builds on three principles: natural selection, heredity, and variation. At the time that Darwin wrote, the principles underlying heredity and variation were poorly understood. In the 1940s, however, biologists incorporated Gregor Mendel's principles of genetics to explain both, resulting in the modern synthesis. It was not until the 1980s and 1990s, however, when more comparative molecular sequence data between different kinds of organisms was amassed and detailed, that an understanding of the molecular basis of the developmental mechanisms began to form.
Currently, it is well understood how genetic mutation occurs. However, developmental mechanisms are not understood sufficiently to explain which kinds of phenotypic variation can arise in each generation from variation at the genetic level. Evolutionary developmental biology studies how the dynamics of development determine the phenotypic variation arising from genetic variation and how that affects

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phenotypic evolution (especially its direction). At the same time evolutionary developmental biology also studies how development itself evolves.
Thus the origins of evolutionary developmental biology come both from an improvement in molecular biology techniques as applied to development, and from the full appreciation of the limitations of classic neo-Darwinism as applied to phenotypic evolution. Some evo-devo researchers see themselves as extending and enhancing the modern synthesis by incorporating the findings of molecular genetics and developmental biology into an extended evolutionary synthesis.
Evolutionary developmental biology can be distinguished from earlier approaches to evolutionary theory by its focus on a few crucial ideas. One of these is modularity: as has been long recognized, plants and animal bodies are modular: they are organized into developmentally and anatomically distinct parts. Often these parts are repeated, such as fingers, ribs, and body segments. Evo-devo seeks the genetic and evolutionary basis for the division of the embryo into distinct modules, and for the partly independent development of such modules.
The statistician Ronald Fisher (1890 – 1962) helped to form the modern evolutionary synthesis of Mendelian genetics and natural selection.
J. B. S. Haldane (1892 – 1964) helped to create the field of population genetics. Microbiology has recently developed into an evolutionary discipline. It was originally ignored due to the paucity of morphological traits and the lack of a species concept in microbiology. Now, evolutionary researchers are taking advantage of a more extensive understanding of microbial physiology, the ease of microbial genomics, and the quick generation time of some microbes to answer evolutionary questions. Similar features have led to progress in viral evolution, particularly for
bacteriophages.

Many biologists have contributed to our current understanding of evolution. Although the term had been used sporadically starting at the turn of the century, evolutionary biology in a disciplinary sense gained currency during the period of "the evolutionary synthesis" (Smocovitis, 1996). Theodosius Dobzhansky and E. B. Ford were important in the establishment of an empirical research programmer for evolutionary biology as were theorists Ronald Fisher, Sewall Wright and J. S. Haldane. Ernst Mayr, George Gaylord Simpson and G. Ledyard Stebbins were also important discipline-builders during the modern synthesis, in the fields of systematics, palaeontology and botany, respectively. Through training many future evolutionary biologists, James Crow,^[1] Richard Lewontin, Dan Hartl, Marcus Feldman, and Brian Charlesworth^[6] have also made large contributions to building the discipline of evolutionary biology.

Organismes and environment
State of ecosystems, habitats and species
The expansion of humans activities into the natural environment, manifested by urbanization, recreation, industrialization, and agriculture, results in increasing uniformity in landscapes and consequential reduction, disappearance, fragmentation or isolation of habitats and landscapes.

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It is evident that the increasing exploitation of land for human use greatly reduces the area of each wildlife habitat as well as the total area surface throughout Europe. The consequences are:
A decreased species diversity, due to reduced habitable surface area which corresponds to a reduced "species carrying capacity".
The reduction of the size of habitats also reduces the genetic diversity of the species living there. Smaller habitats can only accommodate smaller populations, this results in an impoverished gene pool.
The reduction of genetic resources of a species diminishes its flexibility and evolutionary adaptability to changing situations. This has significant negative impacts on its survival.
The conditions under which the reduction of habitats often occur prevent living organisms making use of their normal ways to flee their threatened habitat. Those escape routes include migration to other habitats, adaption to the changing environment, or genetic interchange with populations in nearby habitats. Particular concern is:

The abrupt nature of human intervention; human projects are planned and implemented on a much shorter time scale than natural processes;

Furthermore human intervention, such as the construction of buildings, motorways or railways results in the fragmentation of habitats, which strongly limits the possibility for contact or migration among them;
In extreme cases, even the smallest, narrowest connections between habitats are broken off. Such isolation is catastrophic for life in the habitat fragments.
Loss of Species of Fauna and Flora
Although relatively few species of Europe's fauna and flora have actually become extinct during this century, the continent's biodiversity is affected by decreasing species numbers and the loss of habitats in many regions. Approximately 30 % of the vertebrates and 20 % of the higher plants are classified as "threatened". Threats are directly linked to the loss of habitats due to destruction, modification and fragmentation of ecosystems as well as from overuse of pesticides and herbicides, intensive farming methods, hunting and general human disturbance. The overall deterioration of Europe's air and water quality add to the detrimental influence.
Agriculture
Europe's natural environment is inextricably linked with agriculture and forestry. Since agriculture traditionally depends on sound environmental conditions, farmers have a special interest in the maintenance of natural resources and for centuries maintained a mosaic of landscapes which protected and enriched the natural environment.
As a result of needs for food production since the 1940s, policies have encouraged increased pro- diction through a variety of mechanisms, including price support, other subsidies and support for research and development. The success achieved in agricultural production has however entailed increased impact on the environment.
Modern agriculture is responsible for the loss of much wildlife and their habitats in Europe, through reduction and fragmentation of habitats and wildlife

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populations. The drainage of wetlands, the destruction of hedgerows and the intensive use of fertilizers and pesticides can all pose a threat to wildlife. Highly specialized monoculture are causing significant loss in species abundance and diversity. On the other hand, increased production per hectare in intensive areas, raising of livestock volume, and lower prices for agricultural products also caused marginalization of agricultural land, changing the diversity of European landscapes into the direction of two main types: Intensive Agriculture and Abandoned land.
Energy

Abandonment can be positive for nature, but this is not necessarily so. Land abandonment increases the risk of fire in the Mediterranean Region, causes a decline of small-scale landscape diversity and can cause decrease in species diversity.

All energy types have potential impacts on the natural environment to varying degrees at all stages of use, from extraction through processing to end use. Generating energy from any source involves making the choices between impacts and how far those impacts can be tolerated at the local and global scale. This is especially of importance for nuclear power, where there are significant risks of radioactive pollution such as at Chernobyl.
Shell Oil Company and IUCN have jointly drafted environmental regulations for oil-exploitation in Arctic areas of Siberia. Other oil companies are aware of this and use these environmental regulations voluntarily for developing oil fields.
Into the future, the sustainability of the natural environment will be improved as trends away from damaging energy uses, extractive methods reduce, and whilst real cost market forces and the polluter pays principle take effect.
Fisheries

The principle of the fisheries sector is towards sustainable catches of wild aquatic fauna. The principle environmental impact associated with fisheries activities is the unsustainable har- vesting of fish stocks and shellfish and has consequences for the ecological balance of the aquatic environment. The sector is in a state of "crisis", with over capacity of the fleet, overexploitation of stocks, debt, and marketing problems.

Growing aquaculture industry may increase water pollution in Western Europe, and is appearing to be a rising trend in the Mediterranean and Central/East Europe.
Fishing activities have an impact on cetaceans and there is concern that large numbers of dolphins, and even the globally endangered Monk seal, are being killed.
Forestry

Compared to other land uses, forest management has the longest tradition in following sustainable principles due to which over 30% of Europe is still covered with trees. Without such an organized approach, forests are likely to have already disappeared from Europe's lowlands. However, as an economic sector, forestry has also impacted severely on the naturalness of Europe's forests: soils have been drained, pesticides and fertilizers applied, and exotic species planted. In many areas monocultures have replaced the original diverse forest composition. Monocultures are extremely sensitive to insect infestations, fires or wind, and so can lead to financial losses as well as biological decline. The inadequate afforestation practices characterize new trends in impacting on the sustainability of the natural environment.

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Industry
Almost all forms of industry have an impact on the natural environment and its sustainability. The impact varies at different stages in the life cycle of a product, depending upon the raw materials used through to the final end use of the product for waste residue, re-use or recycling. Industrial accidents and war damage to industrial plants can also endanger the natural environment.
Transport and Infrastructure
Transport is perhaps the major contributor to pollution in the world today, particularly global envy- ronmental issues such as the greenhouse effect. The key impacts of transportation include frag- mentation of habitats and species and genetic populations, disruption of migration and traffic mortalities to wildlife. Since the 1970s transport has become a major consumer of non-renewable resources, 80% of oil consumption coming from road transport.
Human Impact On The Natural Environment

Agriculture

Main article: Environmental impact of agriculture
The environmental impact of agriculture varies based on the wide variety of agricultural practices employed around the world. Ultimately, the environmental impact depends on the production practices of the system used by farmers. The connection between emissions into the environment and the farming system is indirect, as it also depends on other climate variables such as rainfall and temperature.
There are two types of indicators of environmental impact: "means-based", which is based on the farmer's production methods, and "effect-based", which is the impact that farming methods have on the farming system or on emissions to the environment. An example of a means-based indicator would be the quality of groundwater, that is effected by the amount of nitrogen applied to the soil. An indicator reflecting the loss of nitrate to groundwater would be effect-based.^[11]
The environmental impact of agriculture involves a variety of factors from the soil, to water, the air, animal and soil diversity, plants, and the food itself. Some of the environmental issues that are related to agriculture are climate change, deforestation, genetic engineering, irrigation problems, pollutants, soil degradation, and waste.

Natural environment is of crucial importance for social and economic life. We use the living world as

a resource for food supply

an energy source

a source for recreation
a major source of medicines

natural resources for industrial products

In this respect the diversity of nature not only offers man a vast power of choice for his current needs and desires. It also enhances the role of nature as a source of solutions for the future needs and challenges of mankind.

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Applied integrated sciences
Biochemistry and molecular biology (mcdb)
What is the difference between biochemistry, molecular biology, and genetics?

Genetics is the most distinct of the three. It studies genes, genomics, and heredity. This can include molecular genetics, which deals directly with the DNA and it includes population genetics, which has more to do with how different alleles spread in a population.

I have yet to see a definition of molecular biology that does not overlap with biochemistry. The two are nearly identical sciences. The closest I have found to a meaningful distinction is that molecular biologists are biologists and biochemists are chemists. Molecular biologists concern themselves with the biological processes; the cells, the tissues, the organisms. Biochemists are more about the chemicals, which just happen to be in a living thing; reaction mechanisms, thermodynamics, bond angles and the like. Not that what I am saying here is universally agreed upon.
At the end of the day, the amount of overlap is massive and we are splitting hairs by saying somebody is absolutely one and not the other. One can have a degree in molecular biology, be a member of a genetics department, and look at the structural biochemistry of how a protein binds to DNA.
Biochemistry has to do with chemical properties and interactions of biological molecules. So for example we can take an isolated enzyme add substrate and measure the kinetics of a reaction in a test tube. The experiments try to isolate specific chemical properties, not necessarily mimicking cellular environment (which is most often the case).

Molecular biology has to do with biological effects of specific molecules - we add X to cell culture - do the cells die? Do they become cancerous?

Genetics looks at heritability of traits and tries to find what are the molecules that have to do with that trait. How much of susceptibility to X can be attributed to genetics? What is the gene that makes eyes blue?
In current research these disciplines closely intertwine, and it is almost impossible to publish a good paper in only one of them, without having some evidence from others. So genetics identifies the players, biochemistry says how they likely function, and molecular biology asks how this function influences biological properties of an organism.

Biochemistry focuses on the protein part of life functions. It studies the components independent of the organism.

Genetics focuses on the gene part. Usually mutants are used. So, it is organism without the component.

Molecular Biology integrates those two, as can be quite well ascertained from the "central dogma" i.e., genes-> proteins.

So, for e.g., if one is interested in studying what imparts red color to a fruit fly’s eyes, this is probably how the three would work:
A biochemist would make a puree of the fruit fly, isolate the component responsible for the eye color and characterize it.
A geneticist would look for flies that have different eye colors, and compare

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each of them, breed them in various combinations, observe how the traits are inherited. So essentially, one can be blissfully unaware of the chemical nature of the said component (gene/ protein) but still figure out how the trait is passed on/ affect a population.
A molecular biologist would isolate the gene, study it, and arrive at the protein therefrom.
Biochemistry is the study of chemical processes within and relating to living organisms. By controlling information flow through biochemical signaling and the flow of chemical energy through metabolism, biochemical processes give rise to the complexity of life. Molecular biology is a branch of science concerning biological activity at the molecular level. The field of molecular biology overlaps with biology and chemistry and in particular, genetics and biochemistry. and, Genetics is the study of genes, heredity, and genetic variation in living organisms. It is generally considered a field of biology, but it intersects frequently with many of the life sciences and is strongly linked with the study of information systems.
Cell Biology
Cell biology is the study of cell structure and function, and it revolves around the concept that the cell is the fundamental unit of life. Focusing on the cell permits a detailed understanding of the tissues and organisms that cells compose. Some organisms have only one cell, while others are organized into cooperative groups with huge numbers of cells. On the whole, cell biology focuses on the structure and function of a cell, from the most general properties shared by all cells, to the unique, highly intricate functions particular to specialized cells.
The starting point for this discipline might be considered the 1830s. Though scientists had been using microscopes for centuries, they were not always sure what they were looking at. Robert Hooke's initial observation in 1665 of plant-cell walls in slices of cork was followed shortly by Antoine van Leeuwenhoek's first descriptions of live cells with visibly moving parts. In the 1830s two scientists who were colleagues — Schleiden, looking at plant cells, and Schwann, looking first at animal cells — provided the first clearly stated definition of the cell. Their definition stated that that all living creatures, both simple and complex, are made out of one or more cells, and the cell is the structural and functional unit of life — a concept that became known as cell theory.
As microscopes and staining techniques improved over the nineteenth and twentieth centuries, scientists were able to see more and more internal detail within cells. The microscopes used by van Leeuwenhoek probably magnified specimens a few hundredfold. Today high-powered electron microscopes can magnify specimens more than a million times and can reveal the shapes of organelles at the scale of a micrometer and below. With confocal microscopy, a series of images can be combined, allowing researchers to generate detailed three-dimensional representations of cells. These improved imaging techniques have helped us better understand the wonderful complexity of cells and the structures they form.
There are several main subfields within cell biology. One is the study of cell

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energy and the biochemical mechanisms that support cell metabolism. As cells are machines unto themselves, the focus on cell energy overlaps with the pursuit of questions of how energy first arose in original primordial cells, billions of years ago. Another subfield of cell biology concerns the genetics of the cell and its tight interconnection with the proteins controlling the release of genetic information from the nucleus to the cell cytoplasm. Yet another subfield focuses on the structure of cell components, known as subcellular compartments. Cutting across many biological disciplines is the additional subfield of cell biology, concerned with cell communication and signaling, concentrating on the messages that cells give to and receive from other cells and themselves. And finally, there is the subfield primarily concerned with the cell cycle, the rotation of phases beginning and ending with cell division and focused on different periods of growth and DNA replication. Many cell biologists dwell at the intersection of two or more of these subfields as our ability to analyze cells in more complex ways expands.
In line with continually increasing interdisciplinary study, the recent emergence of systems biology has affected many biological disciplines; it is a methodology that encourages the analysis of living systems within the context of other systems. In the field of cell biology, systems biology has enabled the asking and answering of more complex questions, such as the interrelationships of gene regulatory networks, evolutionary relationships between genomes, and the interactions between intracellular signaling networks. Ultimately, the broader a lens we take on our discoveries in cell biology, the more likely we can decipher the complexities of all living systems, large and small.

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