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Small intestine
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Small Intestine
Diagram showing the small intestine and surrounding structures
Details
Artery
Superior mesenteric artery
Vein
Hepatic portal vein
Nerve
Celiac ganglia, vagus[1]
Lymph
Intestinal lymph trunk
Identifiers
Latin
Intestinum tenue
MeSH
A03.556.124.684
Code
4453467
Dorlands
/Elsevier
Small intestine r
TA
A05.6.01.001
FMA
7200
Anatomical terminology
[edit on Wikidata]
The small intestine or small bowel is the part of the gastrointestinal tract between the stomach and the large intestine, and is where most of the end absorption of food takes place. The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum is the shortest part of the small intestine and is where preparation for absorption begins. It also receives bile and pancreatic juice through the pancreatic duct, controlled by the sphincter of Oddi. The primary function of the small intestine is the absorption of nutrients and minerals from food, using small finger-like protrusions called villi.[2]
Contents
[hide]
1 Structure
1.1 Size
1.2 Parts
1.3 Blood supply
1.4 Histology
1.5 Development
2 Function
2.1 Digestion
2.2 Absorption
2.3 Immunological
3 Gene and protein expression
4 Clinical significance
5 Other animals
6 Society and culture
7 Additional images
8 See also
9 References
10 Bibliography
11 External links
Structure[edit]
Size[edit]
The length of the small intestine can vary greatly, from as short as 2.75 m (9.0 ft) to as long as 10.49 m (34.4 ft).[3] The average length in a living person is 3m-5m.[4][5] The length depends both on how tall the person is and how the length is measured.[3] Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty.[3]
It is approximately 1.5 cm in diameter in newborns after 35 weeks of gestational age,[6] and 2.5–3 cm (1 inch) in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm.[7][8] On CT scans, a diameter of over 2.5 cm is considered abnormally dilated.[7][9] The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square meters.[10]
Parts[edit]
The small intestine is divided into three structural parts.
The duodenum is a short structure ranging from 20 cm (7.9 inches) to 25 cm (9.8 inches) in length, and shaped like a "C".[11] It surrounds the head of the pancreas. It receives gastric chyme from the stomach, together with digestive juices from the pancreas (digestive enzymes) and the liver (bile). The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme.
The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m long, and contains the plicae circulares, and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum.
The ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the ileocecal junction.
The jejunum and ileum are suspended in the abdominal cavity by mesentery. The mesentery is part of the peritoneum. Arteries, veins, lymph vessels and nerves travel within the mesentery.[12]
Blood supply[edit]
The small intestine receives a blood supply from the coeliac trunk and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the coeliac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and anastomose. The jejunum and ileum receive blood from the superior mesenteric artery.[13] Branches of the superior mesenteric artery form a series of arches within the mesentery known as arterial arcades, which may be several layers deep. Straight blood vessels known as vasa recta travel from the arcades closest to the ileum and jejunum to the organs themselves.[13]
Histology[edit]
Main article: Gastrointestinal wall
Micrograph of the small intestine mucosa showing the intestinal villi and crypts of Lieberkühn.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The parts of the intestine are as follows:
Layer
Duodenum
Jejunum
Ileum
Serosa 1st part serosa, 2nd–4th adventitia Normal Normal
Muscularis externa Longitudinal and circular layers, with Auerbach's (myenteric) plexus in between Same as duodenum Same as duodenum
Submucosa Brunner's glands and Meissner's (submucosal) plexus No BG No BG
Mucosa: muscularis mucosae Normal Normal Normal
Mucosa: lamina propria No PP No PP Peyer's patches
Mucosa: intestinal epithelium Simple columnar. Contains goblet cells, Paneth cells Similar to duodenum ?
Development[edit]
See also: Development of the digestive system
The small intestine develops from the midgut of the primitive gut tube.[14] By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop. The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the large intestine.[14]
Function[edit]
Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter.
Digestion[edit]
The small intestine is where most chemical digestion takes place. Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct. Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates:
Proteins are degraded into small peptides and amino acids before absorption.[15] Chemical breakdown begins in the stomach and continues in the small intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and stored in the gall bladder. Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose, like most poly-saccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large intestine.
Absorption[edit]
Digested food is now able to pass into the blood vessels in the wall of the intestine through either diffusion or active transport. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or folds called plicae circulares, which are considered permanent features in the wall of the organ. They are distinct from rugae which are considered non-permanent or temporary allowing for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as microvilli. The functions of the plicae circulares, the villi, and the microvilli are to increase the amount of surface area available for the absorption of nutrients, and to limit the loss of said nutrients to intestinal fauna.
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The material that remains undigested and unabsorbed passes into the large intestine.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
Iron is absorbed in the duodenum.
Vitamin B12 and bile salts are absorbed in the terminal ileum.
Water and lipids are absorbed by passive diffusion throughout the small intestine.
Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport.
Fructose is absorbed by facilitated diffusion.
Immunological[edit]
The small intestine supports the body's immune system.[16] The presence of gut flora appears to contribute positively to the host's immune system. Peyer's patches, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system.[17]
Gene and protein expression[edit]
Further information: Bioinformatics § Gene and protein expression
About 20,000 protein coding genes are expressed in human cells and 70 % of these genes are expressed in the normal duodenum.[18][19] Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the small intestine. The corresponding specific proteins are expressed in glandular cells of the mucosa, such as fatty acid binding protein FABP6. Most of the more specifically expressed genes in the small intestine are also expressed in the duodenum, for example FABP2 and the DEFA6 protein expressed in secretory granules of Paneth cells.[20]
Clinical significance[edit]
See also: Gastrointestinal disease
The small intestine is a complex organ, and as such, there are a very large number of possible conditions that may affect the function of the small bowel. A few of them are listed below, some of which are common, with up to 10% of people being affected at some time in their lives, while others are vanishingly rare.
Small intestine obstruction or obstructive disorders
Paralytic ileus
Volvulus
Hernia
Intussusception
Adhesions
Obstruction from external pressure
Obstruction by masses in the lumen (foreign bodies, bezoar, gallstones)
Infectious diseases
Giardiasis
Ascariasis
Tropical sprue
Tape worm (Diphyllobothrium latum, Taenia solium, Hymenolepsis nana)
Hookworm (e.g. Necator americanus, Ancylostoma duodenale)
Nematodes (e.g. Ascaris lumbricoides)
Other Protozoa (e.g. Cryptosporidium parvum, Cyclospora, Microsporidia, Entamoeba histolytica)
Bacterial infections
Enterotoxigenic Escherichia coli
Salmonella enterica
Campylobacter
Shigella
Yersinia
Clostridium difficile (antibiotic-associated colitis, Pseudomembranous colitis)
Mycobacterium (Mycobacterium avium paratuberculosis, disseminated Mycobacterium tuberculosis)
Whipple's disease
Vibrio (cholera)
Enteric (typhoid) fever (Salmonella enterica var. typhii) and paratyphoid fever
Bacillus cereus
Clostridium perfringens (gas gangrene)
Viral infections
Rotavirus
Norovirus
Astrovirus
Adenovirus
Calicivirus
Neoplasms (cancers)
Adenocarcinoma
Carcinoid
Gastrointestinal stromal tumor (GIST)
Lymphoma
Sarcoma
Leiomyoma
Metastatic tumors, especially SCLC or melanoma
Developmental, congenital or genetic conditions
Duodenal (intestinal) atresia
Hirschsprung's disease
Meckel's diverticulum
Pyloric stenosis
Pancreas divisum
Ectopic pancreas
Enteric duplication cyst
Situs inversus
Cystic fibrosis
Malrotation
Persistent urachus
Omphalocele
Gastroschisis
Disaccharidase (lactase) deficiencies
Primary bile acid malabsorption
Gardner syndrome
Familial adenomatous polyposis syndrome (FAP)
Other conditions
Crohn's disease, and the more general inflammatory bowel disease
Typhlitis (neutropenic colitis in the immunosuppressed
Coeliac disease (sprue or non-tropical sprue)
Mesenteric ischemia
Embolus or thrombus of the superior mesenteric artery or the superior mesenteric vein
Arteriovenous malformation
Gastric dumping syndrome
Irritable bowel syndrome
Duodenal (peptic) ulcers
Gastrointestinal perforation
Hyperthyroidism
Diverticulitis
Radiation enterocolitis
Mesenteric cysts
Peritoneal Infection
Sclerosing retroperitonitis
Small intestinal bacterial overgrowth
Other animals[edit]
The small intestine is found in all tetrapods and also in teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epitheliu [21]
In tetrapods, the ileocaecal valve is always present, opening into the colon. The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in herbivores, as well as in mammals and birds, which have a higher metabolic rate than amphibians or reptiles. The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi.[21]
The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary.[21]
There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.[21]
In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.[21]
Society and culture[edit]
In traditional Chinese medicine, the small intestine is a yang organ.[22]
Additional images[edit]
Small intestine in situ, greater omentum folded upwards.
Third state of the development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The mode of preparation is the same as in Fig 400
Second stage of development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The liver has been removed and the two layers of the ventral mesogastrium (lesser omentum) have been cut. The vessels are represented in black and the peritoneum in the reddish tint.
First stage of the development of the intestinal canal and the peritoneum, seen from the side (diagrammatic). From colon 1 the ascending and transverse colon will be formed and from colon 2 the descending and sigmoid colons and the rectum.
See also[edit]
Wikimedia Commons has media related to Small intestine.
Large intestine
Stomach
References[edit]
Jump up ^ Physiology: 6/6ch2/s6ch2_30 - Essentials of Human Physiology
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Jump up ^ Tortora, Gerard (2014). Principles of Anatomy & Physiology. USA: Wiley. p. 913. ISBN 978-1-118-34500-9. ..its length is about 3m in a living person and about 6.5m in a cadaver due to loss of smooth muscle tone after death.
Jump up ^ Standring, Susan (2016). Gray's Anatomy. UK: Elsevier. p. 1124. ISBN 978-0-7020-5230-9. ..and has a mean length of 5 metres (3 - 8.5 metres) when measured intraoperatively in the living adult (Tietelbaum et al 2013).
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^ Jump up to: a b Schoenwolf, Gary C.; Bleyl, Steven B.; Brauer, Philip R.; Francis-West, Philippa H. (2009). "Development of the Urogenital system". Larsen's human embryology (4th ed.). Philadelphia: Churchill Livingstone/Elsevier. p. 237. ISBN 9780443068119.
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Jump up ^ "Intestinal immune cells play an unexpected role in immune surveillance of the bloodstream". Massachusetts General Hospital. 13 December 2012.
Jump up ^ Canny, G. O.; McCormick, B. A. (2008). "Bacteria in the Intestine, Helpful Residents or Enemies from Within?". Infection and Immunity. 76 (8): 3360–3373. doi:10.1128/IAI.00187-08. ISSN 0019-9567. PMC 2493210  .
Jump up ^ "The human proteome in small intestine - The Human Protein Atlas". www.proteinatlas.org. Retrieved 2017-09-26.
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Bibliography[edit]
Sherwood, Lauralee (2006). Fundamentals of physiology: a human perspective (Third ed.). Florence, KY: Cengage Learning. p. 768. ISBN 0-534-46697-4.
Solomon et al. (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
Townsend et al. (2004) Sabiston Textbook of Surgery, Elsevier ISBN 0-7216-0409-9
Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 1". Dig Dis Sci. 48 (8): 1546–64. doi:10.1023/A:1024719925058. PMID 12924651.
Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 2". Dig Dis Sci. 48 (8): 1565–81. doi:10.1023/A:1024724109128. PMID 12924652.
External links[edit]
Small intestine at the Human Protein Atlas
[hide]
v
t
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Anatomy of the gastrointestinal tract, excluding the mouth
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peripharyngeal
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Cell theory
From Wikipedia, the free encyclopedia
Jump to: navigation, search
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 Modern version
6 Opposing concepts in cell theory: history and background
6.1 Evolution of the membrane and bulk phase theories
6.2 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]
Modern version
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 Mg2+
. 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.
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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