Organelle

From Wikipedia, the free encyclopedia
Jump to: navigation, search


A typical animal cell. Within the cytoplasm, the major organelles and cellular structures include: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (6) Golgi apparatus (7) cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytosol (12) lysosome (13) centriole.
In cell biology, an organelle (pronounced /ɔrɡəˈnɛl/) is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane.
The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. Prokaryotes were once thought not to have organelles, but some examples have now been identified.[1]

History and Terminology

In biology, organs are defined as confined functional units within an organism. The analogy of bodily organs to microscopic cellular substructures is obvious, as from even early works, authors of respective textbooks rarely elaborate on the distinction between the two.
Credited as the first[2][3][4] to use a diminutive of organ (i.e. little organ) for cellular structures was German zoologist Karl August Möbius (1884), who used the term "organula" [5] (plural form of organulum, the diminutive of latin organum). From the context, it is clear that he referred to reproduction related structures of protists. In a footnote, which was published as a correction in the next issue of the journal, he justified his suggestion to call organs of unicellular organisms "organella" since they are only differently formed parts of one cell, in contrast to multicellular organs of multicellular organisms. Thus, the original definition was limited to structures of unicellular organisms.
It would take several years before organulum, or the later term organelle, became accepted and expanded in meaning to include subcellular structures in multicellular organisms. Books around 1900 from Valentin Häcker,[6] Edmund Wilson[7] and Oscar Hertwig[8] still referred to cellular organs. Later, both terms came to be used side by side: Bengt Lidforss wrote 1915 (in German) about "Organs or Organells".[9]
Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex", i.e., flagella and their anchoring)[10] and other protist structures, such as ciliates.[11] Alfred Kühn wrote about centrioles as division organelles, although he stated that, for Vahlkampfias, the alternative 'organelle' or 'product of structural build-up' had not yet been decided, without explaining the difference between the alternatives.[12]
In his 1953 textbook, Max Hartmann used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists.[13]
Later, the now-widely-used[14][15][16][17] definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles. However, the more original definition of subcellular functional unit in general still coexists.[18][19]
In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli.[20][21] This new definition, however, did not win wide recognition

DNA expressed as mRNA transcripts

Which of the following is a marker for regions of the DNA expressed as mRNA transcripts, and characterized by cDNA cloning? EST To characterize sequences from the human genome that are coding regions for mRNAs (i.e. "expressed" sequences), cDNA copies of mixtures of cellular mRNA templates are synthesized with reverse transcriptase. The cDNAs can be converted to double stranded cDNAs, inserted into plasmid vectors using recombinant DNA technology, purified by cloning in bacterial cells, and sequenced.
The term "EST" is now used to describe the DNA sequence of a short fragment of a cDNA. As of October 1996, there were about 450,000 EST sequences from human DNA in public DNA sequence databases. Many are redundant cDNAs from different regions of the same mRNA. Nevertheless, approximately 49,000 unique human ESTs are known, representing half of the predicted 100,000 expressed sequences in the human genome.

DNA expressed as mRNA transcripts

Which of the following is a marker for regions of the DNA expressed as mRNA transcripts, and characterized by cDNA cloning? EST To characterize sequences from the human genome that are coding regions for mRNAs (i.e. "expressed" sequences), cDNA copies of mixtures of cellular mRNA templates are synthesized with reverse transcriptase. The cDNAs can be converted to double stranded cDNAs, inserted into plasmid vectors using recombinant DNA technology, purified by cloning in bacterial cells, and sequenced.
The term "EST" is now used to describe the DNA sequence of a short fragment of a cDNA. As of October 1996, there were about 450,000 EST sequences from human DNA in public DNA sequence databases. Many are redundant cDNAs from different regions of the same mRNA. Nevertheless, approximately 49,000 unique human ESTs are known, representing half of the predicted 100,000 expressed sequences in the human genome.

Tutorial: Inheritance of an X-linked recessive trait

Red-green color blindness is X-linked in humans. If a male is red-green color blind, and both parents have normal color vision, which of the male's grandparents is most likely to be red-green color blind? Parents
If both parents have normal vision, the mother of the affected male must be heterozygous for the X-linked, recessive alleles for red-green color blindness. The father of the affected male could not have been the source of the red-green color blind allele since fathers can only pass X-linked traits to their daughters, and Y chromosomes to their sons. Grandparents
The two possible pedigrees for inheritance from a maternal grandparent are shown in the pedigree charts labeled A andWhich of the mother's parents, the maternal grandmother (pedigree chart B) or maternal grandfather (pedigree chart A), is more likely to both be
1. red-green color blind, 2. the source of the allele inherited by their grandson? Males with only a single X chromosome are more commonly affected by X-linked, recessive traits than are females with two X chromosomes. Why?
A male need only inherit the recessive allele from a heterozygous female carrier. A female would need to inherit the recessive allele from both parents, an affected father and a carrier (or affected) mother. B.

Cell Membranes Tutorial

This exercise introduces the dynamic complexes of proteins, carbohydrates, and lipids that comprise cell membranes. You should learn that membranes are fluid, with components that move, change, and perform vital physiological roles as they allow cells to communicate with each other and their environment. We also show that membranes also are important for regulating ion and molecular traffic flow between cells,and that defects in membrane components lead to many significant diseases.
Instructions: The following problems have multiple choice answers. Correct answers are reinforced with a brief explanation. Incorrect answers are linked to tutorials to help solve the problem.

The cell is a unit of organization

Cells are classified by fundamental units of structure and by the way they obtain energy. Cells are classified as prokaryotes or eukaryotes, which will be covered in more detail in the next two pages of this tutorial.
Living things are classified in six kingdoms based on structure. Within prokaryotes, which appeared 3.5 billion years ago, are the kingdoms Monera (Eubacteria) and Archaea. Within eukaryotes, which evolved 1.5 billion years ago, are the kingdoms Protista, Plantae, Fungae, Animalia.
Cells are also defined according the need for energy. Autotrophs are "self feeders" that use light or chemical energy to make food. Plants are an example of autotrophs. In contrast, heterotrophs ("other feeders") obtain energy from other autotrophs or heterotrophs. Many bacteria and animals are heterotrophs. Multicellular Organisms
Multicellular organisms are created from a complex organization of cooperating cells. There must be new mechanisms for cell to cell communication and regulation. There also must be unique mechanisms for a single fertilized egg to develop into all the different kinds of tissues of the body. In humans, there are 1014 cells comprising 200 kinds of tissues!

Condensed matter

Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. In particular, it is concerned with the "condensed" phases that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong.
The most familiar examples of condensed phases are solids and liquids, which arise from the bonding and electromagnetic force between atoms. More exotic condensed phases include the superfluid and the Bose-Einstein condensate found in certain atomic systems at very low temperature, the superconducting phase exhibited by conduction electrons in certain materials, and the ferromagnetic and antiferromagnetic phases of spins on atomic lattices.
Condensed matter physics is by far the largest field of contemporary physics. Historically, condensed matter physics grew out of solid-state physics, which is now considered one of its main subfields. The term condensed matter physics was apparently coined by Philip Anderson when he renamed his research group — previously solid-state theory — in 1967.
In 1978, the Division of Solid State Physics at the American Physical Society was renamed as the Division of Condensed Matter Physics.[21] Condensed matter physics has a large overlap with chemistry, materials science, nanotechnology and engineering.

Application and influence

Applied physics is a general term for physics research which is intended for a particular use. An applied physics curriculum usually contains a few classes in an applied discipline, like geology or electrical engineering. It usually differs from engineering in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem.
The approach is similar to that of applied mathematics. Applied physicists can also be interested in the use of physics for scientific research. For instance, people working on accelerator physics might seek to build better particle detectors for research in theoretical physics.
Physics is used heavily in engineering. For example, Statics, a subfield of mechanics, is used in the building of bridges and other structures. The understanding and use of acoustics results in better concert halls; similarly, the use of optics creates better optical devices. An understanding of physics makes for more realistic flight simulators, video games, and movies, and is often critical in forensic investigations.
With the standard consensus that the laws of physics are universal and do not change with time, physics can be used to study things that would ordinarily be mired in uncertainty. For example, in the study of the origin of the Earth, one can reasonably model Earth's mass, temperature, and rate of rotation, over time. It also allows for simulations in engineering which drastically speed up the development of a new technology.
But there is also considerable interdisciplinarity in the physicist's methods, and so many other important fields are influenced by physics: e.g. presently the fields of econophysics plays an important role, as well as sociophysics

History

Since antiquity, people have tried to understand the behavior of the natural world. One great mystery was the predictable behavior of celestial objects such as the Sun and the Moon. Several theories were proposed, the majority of which were disproved.
Early physical theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. Many of the commonly accepted works of Ptolemy and Aristotle are not always found to match everyday observations.
Even so, Chinese and Indian philosophers and astronomers gave many correct descriptions in atomism and astronomy, and the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics. A more experimental physics began taking shape among medieval Muslim physicists, while modern physics largely took shape among early modern

Theory and experiment

The culture of physics has a higher degree of separation between theory and experiment than many other sciences. Since the twentieth century, most individual physicists have specialized in either theoretical physics or experimental physics. In contrast, almost all the successful theorists in biology and chemistry (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, although this is changing as of late.
Theorists seek to develop mathematical models that both agree with existing experiments and successfully predict future results, while experimentalists devise and perform experiments to test theoretical predictions and explore new phenomena. Although theory and experiment are developed separately, they are strongly dependent upon each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot explain, or when new theories generate experimentally testable predictions, which inspire new experiments.
It is also worth noting there are some physicists who work at the interplay of theory and experiment who are called phenomenologists. Phenomenologists look at the complex phenomena observed in experiment and work to relate them to fundamental theory.
Theoretical physics has historically taken inspiration from philosophy and metaphysics; electromagnetism was unified this way.[9] Beyond the known universe, the field of theoretical physics also deals with hypothetical issues,[10] such as parallel universes, a multiverse, and higher dimensions. Theorists invoke these ideas in hopes of solving particular problems with existing theories. They then explore the consequences of these ideas and work toward making testable predictions.
Experimental physics informs, and is informed by, engineering and technology. Experimental physicists involved in basic research design and perform experiments with equipment such as particle accelerators and lasers, whereas those involved in applied research often work in industry, developing technologies such as magnetic resonance imaging (MRI) and transistors. Feynman has noted that experimentalists may seek areas which are not well explored by theorists

Scope and aims

Physics covers a wide range of phenomena, from the smallest sub-atomic particles (protons, neutrons and electrons), to the largest galaxies. Included in this are the very most basic objects from which all other things are composed, and therefore physics is sometimes said to be the "fundamental science".
Physics aims to describe the various phenomena that occur in nature in terms of simpler phenomena. Thus, physics aims to both connect the things we see around us to root causes, and then to try to connect these causes together in the hope of finding an ultimate reason for why nature is as it is.
For example, the ancient Chinese observed that certain rocks (lodestone) were attracted to one another by some invisible force. This effect was later called magnetism, and was first rigorously studied in the 17th century.
A little earlier than the Chinese, the ancient Greeks knew of other objects such as amber, that when rubbed with fur would cause a similar invisible attraction between the two. This was also first studied rigorously in the 17th century, and came to be called electricity.
Thus, physics had come to understand two observations of nature in terms of some root cause (electricity and magnetism). However, further work in the 19th century revealed that these two forces were just two different aspects of one force – electromagnetism. This process of "unifying" forces continues today (see section Current research for more information).