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Macromolecule
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"Macromolecules" redirects here. For the journal, see Macromolecules (journal).
"Macromolecular chemistry" redirects here. For the journal formerly known as Macromolecular Chemistry, see Macromolecular Chemistry and Physics.
Chemical structure of a polypeptide macromolecule
A macromolecule is a very large molecule, such as protein, commonly created by the polymerization of smaller subunits (monomers). They are typically composed of thousands of atoms or more. The most common macromolecules in biochemistry are biopolymers (nucleic acids, proteins, carbohydrates and lipids) and large non-polymeric molecules (such as lipids and macrocycles).[1] Synthetic macromolecules include common plastics and synthetic fibers as well as experimental materials such as carbon nanotubes.[2][3]
Contents
[hide]
1 Definition
2 Properties
3 Linear biopolymers
3.1 Structural features
3.1.1 DNA is optimised for encoding information
3.1.2 Proteins are optimised for catalysis
3.1.3 RNA is multifunctional
4 Branched biopolymers
5 Synthetic macromolecules
6 See also
7 References
8 External links
Definition[edit]
IUPAC definition
Macromolecule
Large molecule
A molecule of high relative molecular mass, the structure of which essentially
comprises the multiple repetition of units derived, actually or conceptually, from
molecules of low relative molecular mass.
Notes
1. In many cases, especially for synthetic polymers, a molecule can be regarded
as having a high relative molecular mass if the addition or removal of one or a
few of the units has a negligible effect on the molecular properties. This statement
fails in the case of certain macromolecules for which the properties may be
critically dependent on fine details of the molecular structure.
2. If a part or the whole of the molecule fits into this definition, it may be described
as either macromolecular or polymeric, or by polymer used adjectivally.[4]
The term macromolecule (macro- + molecule) was coined by Nobel laureate Hermann Staudinger in the 1920s, although his first relevant publication on this field only mentions high molecular compounds (in excess of 1,000 atoms).[5] At that time the phrase polymer, as introduced by Berzelius in 1833, had a different meaning from that of today: it simply was another form of isomerism for example with benzene and acetylene and had little to do with size.[6]
Usage of the term to describe large molecules varies among the disciplines. For example, while biology refers to macromolecules as the four large molecules comprising living things, in chemistry, the term may refer to aggregates of two or more molecules held together by intermolecular forces rather than covalent bonds but which do not readily dissociate.[7]
According to the standard IUPAC definition, the term macromolecule as used in polymer science refers only to a single molecule. For example, a single polymeric molecule is appropriately described as a "macromolecule" or "polymer molecule" rather than a "polymer," which suggests a substance composed of macromolecules.[8]
Because of their size, macromolecules are not conveniently described in terms of stoichiometry alone. The structure of simple macromolecules, such as homopolymers, may be described in terms of the individual monomer subunit and total molecular mass. Complicated biomacromolecules, on the other hand, require multi-faceted structural description such as the hierarchy of structures used to describe proteins. In British English, the word "macromolecule" tends to be called "high polymer".
Properties[edit]
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Macromolecules often have unusual physical properties that do not occur for smaller molecules.
For example, DNA in a solution can be broken simply by sucking the solution through an ordinary straw because the physical forces on the molecule can overcome the strength of its covalent bonds. The 1964 edition of Linus Pauling's College Chemistry asserted that DNA in nature is never longer than about 5,000 base pairs.[9] This error arose because biochemists were inadvertently breaking their samples into fragments. In fact, the DNA of chromosomes can be hundreds of millions of base pairs long, packaged into chromatin.
Another common macromolecular property that does not characterize smaller molecules is their relative insolubility in water and similar solvents, instead forming colloids. Many require salts or particular ions to dissolve in water. Similarly, many proteins will denature if the solute concentration of their solution is too high or too low.
High concentrations of macromolecules in a solution can alter the rates and equilibrium constants of the reactions of other macromolecules, through an effect known as macromolecular crowding.[10] This comes from macromolecules excluding other molecules from a large part of the volume of the solution, thereby increasing the effective concentrations of these molecules.
Linear biopolymers[edit]
All living organisms are dependent on three essential biopolymers for their biological functions: DNA, RNA and proteins.[11] Each of these molecules is required for life since each plays a distinct, indispensable role in the cell.[12] The simple summary is that DNA makes RNA, and then RNA makes proteins.
DNA, RNA, and proteins all consist of a repeating structure of related building blocks (nucleotides in the case of DNA and RNA, amino acids in the case of proteins). In general, they are all unbranched polymers, and so can be represented in the form of a string. Indeed, they can be viewed as a string of beads, with each bead representing a single nucleotide or amino acid monomer linked together through covalent chemical bonds into a very long chain.
In most cases, the monomers within the chain have a strong propensity to interact with other amino acids or nucleotides. In DNA and RNA, this can take the form of Watson-Crick base pairs (G-C and A-T or A-U), although many more complicated interactions can and do occur.
Structural features[edit]
DNA
RNA
Proteins
Encodes genetic information Yes Yes No
Catalyzes biological reactions No Yes Yes
Building blocks (type) Nucleotides Nucleotides Amino acids
Building blocks (number) 4 4 20
Strandedness Double Single Single
Structure Double helix Complex Complex
Stability to degradation High Variable Variable
Repair systems Yes No No
Because of the double-stranded nature of DNA, essentially all of the nucleotides take the form of Watson-Crick base pairs between nucleotides on the two complementary strands of the double-helix.
In contrast, both RNA and proteins are normally single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, and so fold into complex three-dimensional shapes dependent on their sequence. These different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific binding pockets, and the ability to catalyse biochemical reactions.
DNA is optimised for encoding information[edit]
DNA is an information storage macromolecule that encodes the complete set of instructions (the genome) that are required to assemble, maintain, and reproduce every living organism.[13]
DNA and RNA are both capable of encoding genetic information, because there are biochemical mechanisms which read the information coded within a DNA or RNA sequence and use it to generate a specified protein. On the other hand, the sequence information of a protein molecule is not used by cells to functionally encode genetic information.[1]:5
DNA has three primary attributes that allow it to be far better than RNA at encoding genetic information. First, it is normally double-stranded, so that there are a minimum of two copies of the information encoding each gene in every cell. Second, DNA has a much greater stability against breakdown than does RNA, an attribute primarily associated with the absence of the 2'-hydroxyl group within every nucleotide of DNA. Third, highly sophisticated DNA surveillance and repair systems are present which monitor damage to the DNA and repair the sequence when necessary. Analogous systems have not evolved for repairing damaged RNA molecules. Consequently, chromosomes can contain many billions of atoms, arranged in a specific chemical structure.
Proteins are optimised for catalysis[edit]
Proteins are functional macromolecules responsible for catalysing the biochemical reactions that sustain life.[1]:3 Proteins carry out all functions of an organism, for example photosynthesis, neural function, vision, and movement.[14]
The single-stranded nature of protein molecules, together with their composition of 20 or more different amino acid building blocks, allows them to fold in to a vast number of different three-dimensional shapes, while providing binding pockets through which they can specifically interact with all manner of molecules. In addition, the chemical diversity of the different amino acids, together with different chemical environments afforded by local 3D structure, enables many proteins to act as enzymes, catalyzing a wide range of specific biochemical transformations within cells. In addition, proteins have evolved the ability to bind a wide range of cofactors and coenzymes, smaller molecules that can endow the protein with specific activities beyond those associated with the polypeptide chain alone.
RNA is multifunctional[edit]
RNA is multifunctional, its primary function is to encode proteins, according to the instructions within a cell’s DNA.[1]:5 They control and regulate many aspects of protein synthesis in eukaryotes.
RNA encodes genetic information that can be translated into the amino acid sequence of proteins, as evidenced by the messenger RNA molecules present within every cell, and the RNA genomes of a large number of viruses. The single-stranded nature of RNA, together with tendency for rapid breakdown and a lack of repair systems means that RNA is not so well suited for the long-term storage of genetic information as is DNA.
In addition, RNA is a single-stranded polymer that can, like proteins, fold into a very large number of three-dimensional structures. Some of these structures provide binding sites for other molecules and chemically-active centers that can catalyze specific chemical reactions on those bound molecules. The limited number of different building blocks of RNA (4 nucleotides vs >20 amino acids in proteins), together with their lack of chemical diversity, results in catalytic RNA (ribozymes) being generally less-effective catalysts than proteins for most biological reactions.
Branched biopolymers[edit]
Raspberry ellagitannin, a tannin composed of core of glucose units surrounded by gallic acid esters and ellagic acid units
Carbohydrate macromolecules (polysaccharides) are formed from polymers of monosaccharides.[1]:11 Because monosaccharides have multiple functional groups, polysaccharides can form linear polymers (e.g. cellulose) or complex branched structures (e.g. glycogen). Polysaccharides perform numerous roles in living organisms, acting as energy stores (e.g. starch) and as structural components (e.g. chitin in arthropods and fungi). Many carbohydrates contain modified monosaccharide units that have had functional groups replaced or removed.
Polyphenols consist of a branched structure of multiple phenolic subunits. They can perform structural roles (e.g. lignin) as well as roles as secondary metabolites involved in signalling, pigmentation and defense.
Synthetic macromolecules[edit]
Structure of a polyphenylene dendrimer macromolecule reported by Müllen, et al.[15]
Some examples of macromolecules are synthetic polymers (plastics, synthetic fibers, and synthetic rubber), graphene, and carbon nanotubes. Polymers may be prepared from inorganic matter as well as for instance in inorganic polymers and geopolymers. The incorporation of inorganic elements enables the tunability of properties and/or responsive behavior as for instance in smart inorganic polymers.
See also[edit]
List of biophysically important macromolecular crystal structures
References[edit]
^ Jump up to: a b c d e Stryer L, Berg JM, Tymoczko JL (2002). Biochemistry (5th ed.). San Francisco: W.H. Freeman. ISBN 0-7167-4955-6.
Jump up ^ Life cycle of a plastic product Archived 2010-03-17 at the Wayback Machine.. Americanchemistry.com. Retrieved on 2011-07-01.
Jump up ^ Gullapalli, S.; Wong, M.S. (2011). "Nanotechnology: A Guide to Nano-Objects" (PDF). Chemical Engineering Progress. 107 (5): 28–32.
Jump up ^ "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)" (PDF). Pure and Applied Chemistry. 68 (12): 2287–2311. 1996. doi:10.1351/pac199668122287.
Jump up ^ Staudinger, H.; Fritschi, J. (1922). "Über Isopren und Kautschuk. 5. Mitteilung. Über die Hydrierung des Kautschuks und über seine Konstitution". Helvetica Chimica Acta. 5 (5): 785. doi:10.1002/hlca.19220050517.
Jump up ^ Jensen, William B. (2008). "The Origin of the Polymer Concept". Journal of Chemical Education. 85 (5): 624. Bibcode:2008JChEd..85..624J. doi:10.1021/ed085p624.
Jump up ^ van Holde, K.E. (1998) Principles of Physical Biochemistry Prentice Hall: New Jersey, ISBN 0-13-720459-0
Jump up ^ Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). "Glossary of Basic Terms in Polymer Science" (PDF). Pure and Applied Chemistry. 68 (12): 2287. doi:10.1351/pac199668122287. Archived from the original (PDF) on 2007-02-23.
Jump up ^ Pauling, Linus (1964). College Chemistry. W.H. Feeman and Company.
Jump up ^ Minton AP (2006). "How can biochemical reactions within cells differ from those in test tubes?". J. Cell Sci. 119 (Pt 14): 2863–9. doi:10.1242/jcs.03063. PMID 16825427.
Jump up ^ Berg, Jeremy Mark; Tymoczko, John L.; Stryer, Lubert (2010). Biochemistry, 7th ed. (Biochemistry (Berg)). W.H. Freeman & Company. ISBN 1-4292-2936-5. Fifth edition available online through the NCBI Bookshelf: link
Jump up ^ Walter, Peter; Alberts, Bruce; Johnson, Alexander S.; Lewis, Julian; Raff, Martin C.; Roberts, Keith (2008). Molecular Biology of the Cell (5th edition, Extended version). New York: Garland Science. ISBN 0-8153-4111-3. . Fourth edition is available online through the NCBI Bookshelf: link
Jump up ^ Golnick, Larry; Wheelis, Mark. The Cartoon Guide to Genetics. Collins Reference. ISBN 978-0-06-273099-2.
Jump up ^ Takemura, Masaharu (2009). The Manga Guide to Molecular Biology. No Starch Press. ISBN 978-1-59327-202-9.
Jump up ^ Roland E. Bauer; Volker Enkelmann; Uwe M. Wiesler; Alexander J. Berresheim; Klaus Müllen (2002). "Single-Crystal Structures of Polyphenylene Dendrimers". Chemistry: A European Journal. 8 (17): 3858. doi:10.1002/1521-3765(20020902)8:17<3858::AID-CHEM3858>3.0.CO;2-5.
Tanford, Charles (1961). Physical Chemistry of Macromolecules. New York, NY: John Wiley & Sons.
External links[edit]
Synopsis of Chapter 5, Campbell & Reece, 2002
Lecture notes on the structure and function of macromolecules
Several (free) introductory macromolecule related internet-based courses
Giant Molecules! by Ulysses Magee, ISSA Review Winter 2002–2003, ISSN 1540-9864. Cached HTML version of a missing PDF file. Retrieved March 10, 2010. The article is based on the book, Inventing Polymer Science: Staudinger, Carothers, and the Emergence of Macromolecular Chemistry by Yasu Furukawa.
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Blood type diet
From Wikipedia, the free encyclopedia
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The blood type diets are fad diets[1] advocated by several authors, the most prominent of whom is Peter J. D'Adamo.[2] These diets are based on the notion that blood type, according to the ABO blood group system, is the most important factor in determining a healthy diet, and each author recommends a distinct diet for each blood type.
The consensus among dietitians, physicians, and scientists is that these diets are unsupported by scientific evidence.[2][3][4][5][6]
Contents
[hide]
1 Diet
2 Evidence
3 See also
4 References
5 External links
Diet[edit]
The underlying hypothesis of blood type diets is that people with different blood types digest lectins differently, and that if people eat food that is not compatible with their blood type, they will experience many health problems. On the other hand, if a person eats food that is compatible, they will be healthier.[4]
That hypothesis is, in turn, based on an assumption that each blood type represents a different evolutionary heritage. "Based on the ‘Blood-Type’ diet theory, group O is considered the ancestral blood group in humans so their optimal diet should resemble the high animal protein diets typical of the hunter-gatherer era. In contrast, those with group A should thrive on a vegetarian diet as this blood group was believed to have evolved when humans settled down into agrarian societies. Following the same rationale, individuals with blood group B are considered to benefit from consumption of dairy products because this blood group was believed to originate in nomadic tribes. Finally, individuals with an AB blood group are believed to benefit from a diet that is intermediate to those proposed for group A and group B."[6]
Evidence[edit]
As of 2017 there is no scientific evidence to support the blood type diet hypothesis and no clinical evidence that it improves health.[2] Peter J. D'Adamo is the most prominent proponent of blood type diets.[2]
Luiz C. de Mattos and Haroldo W. Moreira point out that assertions made by proponents of blood type diets that the O blood type was the first human blood type requires that the O gene have evolved before the A and B genes in the ABO locus;[7] phylogenetic networks of human and non-human ABO alleles show that the A gene was the first to evolve.[8] They argue that it would be extraordinary, from the perspective of evolution, for normal genes (those for types A and B) to have evolved from abnormal genes (for type O).
Yamamoto et al. further note: "Although the O blood type is common in all populations around the world, there is no evidence that the O gene represents the ancestral gene at the ABO locus. Nor is it reasonable to suppose that a defective gene would arise spontaneously and then evolve into normal genes.[7]
See also[edit]
Blood type
Blood types in Japanese culture
List of diets
References[edit]
Jump up ^ Baldwin, EJ (2004). "Fad diets in diabetes" (PDF). Br J Diabetes Vasc Dis. 4: 333–7. doi:10.1177/14746514040040050901.
^ Jump up to: a b c d Leila Cusack; Emmy De Buck; Veerle Compernolle; Philippe Vandekerckhove (2013-07-01). "Blood type diets lack supporting evidence: a systematic review.". The American Journal of Clinical Nutrition. 98 (1): 99–104. PMID 23697707. doi:10.3945/ajcn.113.058693.
Jump up ^ May-Jean King (2000-07-04). "2: ABO Polymorphisms and their putative biological relationships with disease". Human Blood Cells (Consequences of Genetic Polymorphisms and Variations). World Scientific Pub Co Inc. p. 44. ISBN 978-1860941962. doi:10.1142/9781848160309_0002. As it is not possible to comparatively re-interpret all of the published data, we have tried to present this data with a reasonably "open mind", so that you may "find your own truth". However, it must be stated that an "open mind" should not extend to some of the non-scientific literature where there are books on the ABO system of pure fantasy. 206 The most recent and incredulous of these claims that individuals of each ABO blood type must subscribe to a specific diet in order to stay healthy, live longer and achieve an ideal weight!
^ Jump up to: a b Katherine Zeratsky (2010-08-12). "Blood type diet: What is it? Does it work?". Mayo Clinic. Archived from the original on 2011-06-12. Retrieved 2013-08-21.
Jump up ^ David C K Roberts (200). "Quick weight loss: sorting fad from fact". The Medical Journal of Australia. 175: 637–40. PMID 11837873. Archived from the original on 2011-08-24.
^ Jump up to: a b Wang, Jingzhou; Bibiana García-Bailo; Daiva E. Nielsen; Ahmed El-Sohemy (15 January 2014). "ABO Genotype, ‘Blood-Type’ Diet and Cardiometabolic Risk Factors". PLoS ONE. 9 (1): e84749. PMC 3893150  . PMID 24454746. doi:10.1371/journal.pone.0084749.
^ Jump up to: a b Mattos, Luiz C. de; Moreira, Haroldo W. (2004). "Genetic of the ABO blood system and its link with the immune system". Revista Brasileira de Hematologia e Hemoterapia. 26. doi:10.1590/S1516-84842004000100012.
Jump up ^ Saitou, Naruya; Yamamoto, Fumi-ichiro (1997). "Evolution of Primate ABO Blood Group Genes and Their Homologous Genes". Molecular Biology and Evolution. 14 (4): 399–411. PMID 9100370. doi:10.1093/oxfordjournals.molbev.a025776.
External links[edit]
Edward Blonz (2003-03-18). "Book Review: Eat Right 4 Your Type (1996)". Quackwatch. Retrieved 2013-08-22.
"blood type diet". The Skeptic's Dictionary. 2012-08-22. Retrieved 2013-08-22.
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Cell theory
From Wikipedia, the free encyclopedia
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Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed.
In biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells. Cells are the basic unit of structure in all organisms and also the basic unit of reproduction. With continual improvements made to microscopes over time, magnification technology advanced enough to discover cells in the 17th century. This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientists. Most of these debates involved the nature of cellular regeneration, and the idea of cells as a fundamental unit of life. Cell theory was eventually formulated in 1839. This is usually credited to Matthias Schleiden and Theodor Schwann. However, many other scientists like Rudolf Virchow contributed to the theory.
The three tenets to the cell theory are as described below:
All living organisms are composed of one or more cells.
The cell is the basic unit of structure and organization in organisms.
Cells arise from pre-existing cells.
The first of these tenets is disputed, as non-cellular entities such as viruses are sometimes considered life-forms.[1]
Contents
[hide]
1 Microscopes
2 Discovery of cells
3 Cell theory
4 Modern interpretation
5 The modern version of the cell theory
6 Opposing concepts in cell theory: history and background
6.1 Evolution of the membrane and bulk phase theories
6.2 The emergence of the steady-state membrane pump concept
6.3 The reemergence of the bulk phase theories
7 Types of cells
8 See also
9 References
10 Bibliography
11 Further reading
12 External links
Microscopes
Anton van Leeuwenhoek's microscope from the 17th century with a magnification of 270x.
Robert Hooke's microscope
The discovery of the cell was made possible through the invention of the microscope. In the first century BC, Romans were able to make glass, discovering that objects appeared to be larger under the glass. In Italy during the 12th century, Salvino D’Armate made a piece of glass fit over one eye, allowing for a magnification effect to that eye. The expanded use of lenses in eyeglasses in the 13th century probably led to wider spread use of simple microscopes (magnifying glasses) with limited magnification.[2] Compound microscope, which combine an objective lens with an eyepiece to view a real image achieving much higher magnification, first appeared in Europe around 1620[3][4] In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia. Hooke also used a simpler microscope with a single lens for examining specimens with directly transmitted light, because this allowed for a clearer image.[5]
Extensive microscopic study was done by Anton van Leeuwenhoek, a draper who took the interest in microscopes after seeing one while on an apprenticeship in Amsterdam in 1648. At some point in his life before 1668, he was able to learn how to grind lenses. This eventually led to Leeuwenhoek making his own unique microscope. His were a single lens simple microscope, rather than a compound microscope. This was because he was able to use a single lens that was a small glass sphere but allowed for a magnification of 270x. This was a large progression since the magnification before was only a maximum of 50x. After Leeuwenhoek, there was not much progress for the microscopes until the 1850s, two hundred years later. Carl Zeiss, a German engineer who manufactured microscopes, began to make changes to the lenses used. But the optical quality did not improve until the 1880s when he hired Otto Schott and eventually Ernst Abbe.[6]
Optical microscopes can focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than the wavelengths of visible light. Later in the 1920s, the electron microscope was developed, making it possible to view objects that are smaller than optical wavelengths, once again, changing the possibilities in science.[6]
Discovery of cells
Drawing of the structure of cork by Robert Hooke that appeared in Micrographia.
The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia. In this book, he gave 60 ‘observations’ in detail of various objects under a coarse, compound microscope.[5] One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning ‘a small room’ like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb. However, Hooke did not know their real structure or function.[7] What Hooke had thought were cells, were actually empty cell walls of plant tissues. With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing. Therefore, he did not think the "cellulae" were alive.[8] His cell observations gave no indication of the nucleus and other organelles found in most living cells. In Micrographia, Hooke also observed mould, bluish in color, found on leather. After studying it under his microscope, he was unable to observe “seeds” that would have indicated how the mould was multiplying in quantity. This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause. Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovers generation is achieved otherwise.[5]
Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did. He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x.[8] Under these microscopes, Leeuwenhoek found motile objects. In a letter to The Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms. Over time, he wrote many more papers in which described many specific forms of microorganisms. Leeuwenhoek named these “animalcules,” which included protozoa and other unicellular organisms, like bacteria.[6] Though he did not have much formal education, he was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676. He also found for the first time the sperm cells of animals and humans. Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter the egg cell. This put an end to the previous theory of spontaneous generation. After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries.[5]
The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying. Biologists believed that there was a fundamental unit to life, but were unsure what this was. It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants.[9] This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization”,[10] Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit.
In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells", meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen.[11] Before, it had been thought that cells shared walls and the fluid passed between them this way.
Cell theory
Matthias Jakob Schleiden (1804–1881)
Theodor Schwann (1810–1882)
Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden.[12] While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it. In 1839, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells. He also suggested that cells were made by a crystallization process either within other cells or from the outside.[13] However, this was not an original idea of Schleiden. He claimed this theory as his own, though Barthelemy Dumortier had stated it years before him. This crystallization process is no longer accepted with modern cell theory. In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures.[14] This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants. From these conclusions about plants and animals, two of the three tenets of cell theory were postulated.[9]
1. All living organisms are composed of one or more cells
2. The cell is the most basic unit of life
Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker.[6] In 1855, Rudolf Virchow added the third tenet to cell theory. In Latin, this tenet states Omnis cellula e cellula.[9] This translated to:
3. All cells arise only from pre-existing cells
However, the idea that all cells come from pre-existing cells had in fact already been proposed by Robert Remak; it has been suggested that Virchow plagiarized Remak and did not give him credit.[15] Remak published observations in 1852 on cell division, claiming Schleiden and Schawnn were incorrect about generation schemes. He instead said that binary fission, which was first introduced by Dumortier, was how reproduction of new animal cells were made. Once this tenet was added, the classical cell theory was complete.
Modern interpretation
The generally accepted parts of modern cell theory include:
All known living things are made up of one or more cells[16]
All living cells arise from pre-existing cells by division.
The cell is the fundamental unit of structure and function in all living organisms.[17]
The activity of an organism depends on the total activity of independent cells.[citation needed]
Energy flow (metabolism and biochemistry) occurs within cells.[citation needed]
Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm.[18]
All cells are basically the same in chemical composition in organisms of similar species .[citation needed]
The modern version of the cell theory
The modern version of the cell theory includes the ideas that:
Energy flow occurs within cells.[19]
Heredity information (DNA) is passed on from cell to cell.[19]
All cells have the same basic chemical composition.[19]
Opposing concepts in cell theory: history and background
The cell was first discovered by Robert Hooke in 1665 using a microscope. The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s. In this theory the internal contents of cells were called protoplasm and described as a jelly-like substance, sometimes called living jelly. At about the same time, colloidal chemistry began its development, and the concepts of bound water emerged. A colloid being something between a solution and a suspension, where Brownian motion is sufficient to prevent sedimentation. The idea of a semipermeable membrane, a barrier that is permeable to solvent but impermeable to solute molecules was developed at about the same time. The term osmosis originated in 1827 and its importance to physiological phenomena realized, but it wasn’t until 1877, when the botanist Pfeffer proposed the membrane theory of cell physiology. In this view, the cell was seen to be enclosed by a thin surface, the plasma membrane, and cell water and solutes such as a potassium ion existed in a physical state like that of a dilute solution. In 1889 Hamburger used hemolysis of erythrocytes to determine the permeability of various solutes. By measuring the time required for the cells to swell past their elastic limit, the rate at which solutes entered the cells could be estimated by the accompanying change in cell volume. He also found that there was an apparent nonsolvent volume of about 50% in red blood cells and later showed that this includes water of hydration in addition to the protein and other nonsolvent components of the cells.
Evolution of the membrane and bulk phase theories
Two opposing concepts developed within the context of studies on osmosis, permeability, and electrical properties of cells.[20] The first held that these properties all belonged to the plasma membrane whereas the other predominant view was that the protoplasm was responsible for these properties. The membrane theory developed as a succession of ad-hoc additions and changes to the theory to overcome experimental hurdles. Overton (a distant cousin of Charles Darwin) first proposed the concept of a lipid (oil) plasma membrane in 1899. The major weakness of the lipid membrane was the lack of an explanation of the high permeability to water, so Nathansohn (1904) proposed the mosaic theory. In this view, the membrane is not a pure lipid layer, but a mosaic of areas with lipid and areas with semipermeable gel. Ruhland refined the mosaic theory to include pores to allow additional passage of small molecules. Since membranes are generally less permeable to anions, Leonor Michaelis concluded that ions are adsorbed to the walls of the pores, changing the permeability of the pores to ions by electrostatic repulsion. Michaelis demonstrated the membrane potential (1926) and proposed that it was related to the distribution of ions across the membrane.[21] Harvey and Danielli (1939) proposed a lipid bilayer membrane covered on each side with a layer of protein to account for measurements of surface tension. In 1941 Boyle & Conway showed that the membrane of frog muscle was permeable to both K+ and Cl-, but apparently not to Na+, so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to K+, H+, and Cl- as well as the impermeability to Na+, Ca+, and Mg++. Over the same time period, it was shown (Procter & Wilson, 1916) that gels, which do not have a semipermeable membrane, would swell in dilute solutions. Loeb (1920) also studied gelatin extensively, with and without a membrane, showing that more of the properties attributed to the plasma membrane could be duplicated in gels without a membrane. In particular, he found that an electrical potential difference between the gelatin and the outside medium could be developed, based on the H+ concentration. Some criticisms of the membrane theory developed in the 1930s, based on observations such as the ability of some cells to swell and increase their surface area by a factor of 1000. A lipid layer cannot stretch to that extent without becoming a patchwork (thereby losing its barrier properties. Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties. In 1938, Fischer and Suer proposed that water in the protoplasm is not free but in a chemically combined form—the protoplasm represents a combination of protein, salt and water—and demonstrated the basic similarity between swelling in living tissues and the swelling of gelatin and fibrin gels. Dimitri Nasonov (1944) viewed proteins as the central components responsible for many properties of the cell, including electrical properties. By the 1940s, the bulk phase theories were not as well developed as the membrane theories. In 1941, Brooks & Brooks published a monograph The Permeability of Living Cells, which rejects the bulk phase theories.
The emergence of the steady-state membrane pump concept
With the development of radioactive tracers, it was shown that cells are not impermeable to Na+. This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove Na+ as it permeates cells. This drove the concept that cells are in a state of dynamic equilibrium, constantly using energy to maintain ion gradients. In 1935, Karl Lohmann discovered ATP and its role as a source of energy for cells, so the concept of a metabolically-driven sodium pump was proposed. The tremendous success of Hodgkin, Huxley, and Katz in the development of the membrane theory of cellular membrane potentials, with differential equations that modeled the phenomena correctly, provided even more support for the membrane pump hypothesis.
The modern view of the plasma membrane is of a fluid lipid bilayer that has protein components embedded within it. The structure of the membrane is now known in great detail, including 3D models of many of the hundreds of different proteins that are bound to the membrane. These major developments in cell physiology placed the membrane theory in a position of dominance and stimulated the imagination of most physiologists, who now apparently accept the theory as fact—there are, however, a few dissenters.[citation needed]
The reemergence of the bulk phase theories
In 1956, Afanasy S. Troshin published a book, The Problems of Cell Permeability, in Russian (1958 in German, 1961 in Chinese, 1966 in English) in which he found that permeability was of secondary importance in determination of the patterns of equilibrium between the cell and its environment. Troshin showed that cell water decreased in solutions of galactose or urea although these compounds did slowly permeate cells. Since the membrane theory requires an impermanent solute to sustain cell shrinkage, these experiments cast doubt on the theory. Others questioned whether the cell has enough energy to sustain the sodium/potassium pump. Such questions became even more urgent as dozens of new metabolic pumps were added as new chemical gradients were discovered.
In 1962, Gilbert Ling became the champion of the bulk phase theories and proposed his association-induction hypothesis of living cells.
Types of cells
Main article: Cell types
Prokaryote cell.
Eukaryote cell.
Cells can be subdivided into the following subcategories:
Prokaryotes: Prokaryotes are relatively small cells surrounded by the plasma membrane, with a characteristic cell wall that may differ in composition depending on the particular organism.[22] Prokaryotes lack a nucleus (although they do have circular or linear DNA) and other membrane-bound organelles (though they do contain ribosomes). The protoplasm of a prokaryote contains the chromosomal region that appears as fibrous deposits under the microscope, and the cytoplasm.[22] Bacteria and Archaea are the two domains of prokaryotes.
Eukaryotes: Eukaryotic cells are also surrounded by the plasma membrane, but on the other hand, they have distinct nuclei bound by a nuclear membrane or envelope. Eukaryotic cells also contain membrane-bound organelles, such as (mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, vacuoles).[23] In addition, they possess organized chromosomes which store genetic material.[citation needed]
Animals have evolved a greater diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants, fungi, and protoctista.[24]
See also
Biology portal
Cell adhesion
Cell biology
Cellular differentiation
Germ theory of disease
Membrane models
References
Jump up ^ Villarreal, Luis P. (August 8, 2008) Are Viruses Alive? Scientific American
Jump up ^ Atti Della Fondazione Giorgio Ronchi E Contributi Dell'Istituto Nazionale Di Ottica, Volume 30, La Fondazione-1975, page 554
Jump up ^ Albert Van Helden; Sven Dupré; Rob van Gent (2010). The Origins of the Telescope. Amsterdam University Press. p. 24. ISBN 978-90-6984-615-6.
Jump up ^ William Rosenthal, Spectacles and Other Vision Aids: A History and Guide to Collecting, Norman Publishing, 1996, page 391 - 392
^ Jump up to: a b c d Gest, H (2004). "The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the Royal Society". Notes and Records of the Royal Society of London. 58 (2): 187–201. doi:10.1098/rsnr.2004.0055. PMID 15209075.
^ Jump up to: a b c d Mazzarello, P. (1999). "A unifying concept: the history of cell theory". Nature Cell Biology. 1 (1): E13–5. doi:10.1038/8964. PMID 10559875. Archived from the original on 2015-06-03.
Jump up ^ Inwood, Stephen (2003). The man who knew too much: the strange and inventive life of Robert Hooke, 1635–1703. London: Pan. p. 72. ISBN 0-330-48829-5.
^ Jump up to: a b Becker, Wayne M.; Kleinsmith, Lewis J.; Hardin, Jeff (2003). The World of the Cell. Benjamin/Cummings Publishing Company. p. 1. ISBN 978-0-8053-4854-5.
^ Jump up to: a b c Robinson, Richard. "History of Biology: Cell Theory and Cell Structure". Advameg, Inc. Retrieved 17 March 2014.
Jump up ^ Dutrochet, Henri (1824) "Recherches anatomiques et physiologiques sur la structure intime des animaux et des vegetaux, et sur leur motilite, par M.H. Dutrochet, avec deux planches"
Jump up ^ Kalenderblatt Dezember 2013 – Mathematisch-Naturwissenschaftliche Fakultät – Universität Rostock. Mathnat.uni-rostock.de (2013-11-28). Retrieved on 2015-10-15.
Jump up ^ Sharp, L. W. (1921). Introduction To Cytology. New York: McGraw Hill Book Company Inc.
Jump up ^ Schleiden, M. J. (1839). "Beiträge zur Phytogenesis". Archiv für Anatomie, Physiologie und wissenschaftliche Medicin: 137–176.
Jump up ^ Schwann, T. (1839). Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Berlin: Sander.
Jump up ^ Silver, GA (1987). "Virchow, the heroic model in medicine: health policy by accolade". American Journal of Public Health. 77 (1): 82–8. doi:10.2105/AJPH.77.1.82. PMC 1646803  . PMID 3538915.
Jump up ^ Wolfe
Jump up ^ Wolfe, p. 5
Jump up ^ Wolfe, p. 8
^ Jump up to: a b c "The modern version of the Cell Theory". Retrieved 12 February 2015.
Jump up ^ Ling, Gilbert N. (1984). In search of the physical basis of life. New York: Plenum Press. ISBN 0306414090.
Jump up ^ Michaelis, L. (1925). "Contribution to the Theory of Permeability of Membranes for Electrolytes". The Journal of General Physiology. 8 (2): 33–59. doi:10.1085/jgp.8.2.33. PMC 2140746  . PMID 19872189.
^ Jump up to: a b Wolfe, p. 11
Jump up ^ Wolfe, p. 13
Jump up ^ Margulis, L. & Chapman, M.J. (2009). Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth ([4th ed.]. ed.). Amsterdam: Academic Press/Elsevier. p. 116.
Bibliography
Wolfe, Stephen L. (1972). Biology of the cell. Wadsworth Pub. Co. ISBN 978-0-534-00106-3.
Further reading
Turner W (January 1890). "The Cell Theory Past and Present". J Anat Physiol. 24 (Pt 2): 253–87. PMC 1328050  . PMID 17231856.
Tavassoli M (1980). "The cell theory: a foundation to the edifice of biology". Am. J. Pathol. 98 (1): 44. PMC 1903404  . PMID 6985772.
External links
Mallery C (2008-02-11). "Cell Theory". Retrieved 2008-11-25.
"Studying Cells Tutorial". 2004. Retrieved 2008-11-25.
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