4.1+Structure+and+cell+theory

​//taken from ;www.infoplease.com/cig/biology/fluid-mosaic-model-membrane-structure-function.html//

Formulation of the Cell Theory

In 1838, [|Theodor Schwann] and [|Matthias Schleiden] were enjoying after-dinner coffee and talking about their studies on cells. It has been suggested that when Schwann heard Schleiden describe plant cells with nuclei, he was struck by the similarity of these plant cells to cells he had observed in animal tissues. The two scientists went immediately to Schwann's lab to look at his slides. Schwann published his book on animal and plant cells (Schwann 1839) the next year, a treatise devoid of acknowledgments of anyone else's contribution, including that of Schleiden (1838). He summarized his observations into three conclusions about cells: 1) The cell is the unit of structure, physiology, and organization in living things. 2) The cell retains a dual existence as a distinct entity and a building block in the construction of organisms. 3) Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).

We know today that the first two tenets are correct, but the third is clearly wrong. The correct interpretation of cell formation by division was finally promoted by others and formally enunciated in Rudolph Virchow's powerful dictum, " Omnis cellula e cellula "... " All cells only arise from pre-existing cells ".

The modern tenets of the Cell Theory include: 1. all known living things are made up of cells. 2. the cell is structural & functional unit of all living things. 3. all cells come from pre-existing cells by division. (Spontaneous Generation does not occur). 4. cells contains hereditary information which is passed from cell to cell during cell division. 5. All cells are basically the same in chemical composition. 6. all energy flow (metabolism & biochemistry) of life occurs within cells. As with any theory, its tenets are based upon previous observations and facts, which are synthesized into a coherent whole via the scientific method. The Cell Theory is no different being founded upon the observations of many. Landmarks in the Study of Cells

Credit for the first compound (more than one lens) microscope is usually given to Zacharias Jansen, of Middleburg, Holland, around the year 1595. Since Jansen was very young at that time, it's possible that his father Hans made the first one, but young Jansen perfected the production. Details about the first Jansen microscopes are not clear, but there is some evidence which allows us to make some guesses about them Jansen microscopes.

Ten years later [|Anton van Leeuwenhoek] (1632-1723), a Dutch businessman and a contemporary of Hooke used his own (single lens) monocular microscopes and was the first person to observe bacteria and protozoa. Leeuwenhoek is known to have made over 500 "microscopes," of which fewer than ten have survived to the present day. In basic design, probably all of Leeuwenhoek's instruments were simply powerful magnifying glasses, not compound microscopes of the type used today. Leeuwenhoek's skill at grinding lenses, together with his naturally acute eyesight and great care in adjusting the lighting where he worked, enabled him to build microscopes that magnified over 200 times, with clearer and brighter images than any of his colleagues at that time. In 1673, Leeuwenhoek began writing letters to the newly formed [|Royal Society of London], describing what he had seen with his lenses. His first letter contained some observations on the stings of bees. For the next fifty years he corresponded with the Royal Society. His observations, written in Dutch, were translated into English or Latin and printed in the [|//Philosophical//] [| //T//][|//ransactions of the Royal Society//]. Leeuwenhoek looked at animal and plant tissues, at mineral crystals, and at fossils. He was the first to see microscopic single celled protists with shells, the foraminifera, which he described as "little cockles. . . no bigger than a coarse sand-grain." He discovered blood cells, and was the first to see living sperm cells of animals. He discovered microscopic animals such as nematodes (round worms) and rotifers. The list of his discoveries is long. Leeuwenhoek soon became famous as his letters were published and translated. In 1680 he was elected a full member of the Royal Society. After his death on August 30, 1723, a member of the Royal Society wrote... " Antony van Leeuwenhoek considered that what is true in natural philosophy can be most fruitfully investigated by the experimental method, supported by the evidence of the senses; for which reason, by diligence and tireless labour he made with his own hand certain most excellent lenses, with the aid of which he discovered many secrets of Nature, now famous throughout the whole philosophical World". No truer definition of the scientific method may be found.
 * In 1663 an English scientist, Robert Hooke, discovered cells in a piece of cork, which he examined under his primitive microscope (figures). Actually, Hooke only observed cell walls because cork cells are dead and without cytoplasmic contents. Hooke drew the cells he saw and also coined the word CELL . The word cell is derived from the Latin word ' cellula ' which means small compartment. Hooke published his findings in his famous work, [|Micrographia]: Physiological Descriptions of Minute Bodies made by Magnifying Glasses (1665).

Between 1680 and the early 1800's it appears that not much was accomplished in the study of cell structure. This may be due to the lack of quality lens for microscopes and the dedication to spend long hours of detailed observation over what microscopes existed at that time. Leeuwenhoek did not record his methodology for grinding quality lenses and thus microscopy suffered for over 100 years.

German natur-philosopher and microscopist, Lorenz Oken had been trained in medicine at Freiburg University. He went on to become a renown philosopher and thinker of the 19th century. It is reported that in 1805 Oken stated that "All living organisms originate from and consist of cells"... which may have been the first statement of a cell theory.

Around 1833 Robert Brown reported the discovery of the nucleus. Brown was a naturalist who visited the "colonies of Australia" from 1801 through 1805, where he cataloged and described over 1,700 new species of plants. BrownBrownBrown's name became inextricably linked. The effect, since described as Brownian Movement, was first noticed by him in 1827. Having worked on the ovum, it was natural to direct attention to the structure of pollen and its Brown interrelationship with the pistil. In the course of his microscopic studies of the epidermis of orchids, discovered in these cells "an opaque spot," which he named the [|nucleus]. Doubtless the same "spot" had been seen often enough before by other observers, but Brown was the first to recognize it as a component part of the vegetable cell and to give it a name. This nucleus (or areola as he called it) of the cell, was not confined to the epidermis, being also found, in the pubescence of the surface and in the parenchyma or internal cells of the tissue. This nucleus of the cell was not confined to only orchids, but was equally manifest in many other monocotyledonous families and in the epidermis of dicotyledonous plants, and even in the early stages of development of the pollen. In some plants, as Tradascantia virginica, it was uncommonly distinct, especially in the tissue of the stig was an accomplished technician and an extraordinarily gifted observer of microscopic phenomena. It was who identified the naked ovule in the gymnospermae. This is a difficult observation to make even with a modern instrument and the benefit of hindsight. But it was with the observation of the incessant agitation of minute suspended particles that ma, in the cells of the ovum, even before impregnation, and in all the stages of formation of the grains of pollen.

It is upon the works of Hooke, Leeuwenhoek , Oken , and Brown that Schleiden and Schwann built their Cell Theory. It was the German professor of botany at the University of Jena, Dr. M. J. Schleiden, who brought the nucleus to popular attention, and to asserted its all-importance in the function of a cell. Schleiden freely acknowledged his indebtedness to Brown for first knowledge of the nucleus, but he soon carried out his own observations of the nucleus, far beyond those of Brown. He came to believe that the nucleus is really the most important portion of the cell, in that it is the original structure from which the remainder of the cell is developed. He called it the cytoblast. He outlined his views in an epochal paper published in Muller's Archives in 1838, under title of "Beitrage zur Phytogenesis." This paper is in itself of value, yet the most important outgrowth of Schleiden's observations of the nucleus did not spring from his own labors, but from those of a friend to whom he mentioned his discoveries the year previous to their publication. This friend was Dr. Theodor Schwann, professor of physiology in the University of Louvain. Schwann was puzzling over certain details of animal histology which he could not clearly explain. He had noted a strange resemblance of embryonic cord material, from which the spinal column develops, to vegetable cells. Schwann recognized a cell-like character of certain animal tissues. Schwann felt that this similarity could not be mere coincidence, and it seemed to fit when Schleiden called his attention to the nucleus. Then at once he reasoned that if there really is the correspondence between vegetable and animal tissues that he suspected, and if the nucleus is so important in the vegetable cell as Schleiden believed, the nucleus should also be found in the ultimate particles of animal tissues. A closer study of animal tissues under the microscope showed, in particular in embryonic tissues, that the "opaque spots" that Schleiden described were found in abundance. The location of these nuclei at comparatively regular intervals suggested that they are found in definite compartments of the tissue, as SchleidenSchwann was convinced that his original premise was right, and that all animal tissues are composed of cells not unlike the cells of vegetables. Adopting the same designation, Schwann CELL THEORY. So expeditious was his o had shown to be the case with vegetables; indeed, the walls that separated such cell-like compartments one from another were in some cases visible. Soon propounded what soon became famous as the bservations that he published a book early in 1839, only a few months after the appearance of Schleiden's paper.

The main theme of his book was to unify vegetable and animal tissues. Accepting cell-structure as the basis of all vegetable tissues, he sought to show that the same is true of animal tissues.

And by cell Schwann meant, as did Schleiden also, what the word ordinarily implies--a cavity walled in on all sides. He knew that the cell might be filled with fluid contents, but he regarded these as relatively subordinate in importance to the nucleus and cell wall.

Their main thesis, the similarity of development of vegetable and animal tissues and the cellular nature of life, was supported almost immediately by a mass of carefully gathered evidence which a multitude of microscopists confirmed. So Schwann's work became a classic almost from the moment of its publication. Various other workers disputed Schwann's claim to priority of discovery, in particular an English microscopist, Valentin, who asserted that he was working closely along the same lines. So did many others, such as Henle, Turpin , Du-mortie r, Purkinje , and Muller , all of whom Schwann himself had quoted in his work. Many physiologists had, earlier than any of the above, foreshadowed the cell theory, including Kaspar Friedrich Wolff around the close of the previous century, and Treviranus in 1807.

But, as we have seen in the scientific method, it is one thing to foreshadow a discovery, it is quite another to give it full expression and make it the cornerstone of future discoveries. And when Schwann put forward the explicit claim that "there is one universal principle of development for the elementary parts, of organisms, however different, and this principle is the formation of cells," he enunciated a doctrine which was for all practical purposes absolutely new and opened up a novel field for the microscopist to enter. A most important era in Cell Biology dates from the publication of his book in 1839.

For the first 150 years, the cell theory was primarily a structural idea. This structural view, which is found in most textbooks, describes the components of a cell and their fate in cell reproduction. Since the 1950's, however, cell biology has focused on DNA and its informational features. Today we look at the cell as a unit of self-control. ie., the description of a cell must includes ideas about how genetic information is converted to structure. The cell doctrine reached its present-day eminence in 1896 with the publication of E. B. Wilson's [|The Cell in Development and Heredity], which was an accumulation of what was known about the roles of cells in embryology and chromosomal behavior. ||

//taken from:http://www.bio.miami.edu/~cmallery/150/unity/cell.text.htm//

//taken from:http://www.infoplease.com/images/cig/biology/02fig03.png//

A living cell is a complex, multi-functional unit. Even the simplest of cells performs a large array of different tasks and functions. Despite our size prejudice, which makes us view cells as very tiny, they are very large places at the level that matters, which is the chemical level. Cells come in two basic types. Read the Prokaryotic and Eukaryotic Cells essay for a discussion of the differences between these cells. Prokaryotic cells are found in bacteria, including both Archaebacteria and Eubacteria, and including the blue-green algae. Eukaryotic cells are found in animals, plants, Fungi and protists. In this essay we are looking at straightforward structure issues. Here's a labeled diagram of a simple prokaryotic cell: The structures shown here are: > > > > Not pictured here are <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__plasmids__. These are much smaller circles of DNA, carrying only a very few genes each. The genes on plasmids are "luxury" genes--the cell doesn't need any of them for normal functions of life. Genes carried on plasmids are often things like antibiotic resistance genes. The<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __F factor__ which determines the role of the cell in sexual reproduction is also often a plasmid gene. Prokaryotic cells trade plasmids easily and frequently, and the population of plasmids in any prokaryotic cell is pretty much constantly changing.
 * ==<span style="color: #ff0000; display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 130%; text-align: center;">Cell Structure == ||
 * 1) The<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __plasma membrane__, which serves as a diffusion barrier between the cell and its environment. All living cells have plasma membranes. A "diffusion barrier" prevents the loss of cellular materials by interfering with the physical tendency of molecules to spread out. It also prevents many substances from diffusing __into__ the cell. And it also controls the movements of various substances and objects into and out of the cell.
 * 1) The<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __cell wall__, which is very different from the plasma membrane. Cells with cell walls also have plasma membranes; the wall can't perform the diffusion barrier tasks of the plasma membrane. One of the primary functions of the cell wall is physical support. Some kinds of bacterial cell walls also have other functions. Prokaryotic cell walls are composed at least partially of an interesting substance called<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __peptidoglycan__ , which is a kind of hybrid between polysaccharide and protein.
 * 1) The __ capsule __, which is not found in all prokaryotic cells. The capsule is composed of rather amorphous polysaccharide material similar to mucous. This material is<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __hygroscopic__, meaning that it has a great capacity to retain water. Water is a vital component of any living cell, and this layer around the cell can be very advantageous for the cell.
 * 1) The<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __nucleoid__, which is an essentially imaginary "structure." This is the central region of the cell, where the DNA is largely located. There is no physical structure enclosing the nucleoid. The name means "imitating a nucleus," implying that this is the region in the cell in which the functions of a eukaryotic cell's nucleus occur.
 * 1) The naked, circular __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">DNA __, which is characteristic of prokaryotic cells, mitochondria and chloroplasts. This DNA is structurally identical to the DNA found in a eukaryotic cell's nucleus, except that instead of being like a long string, with two ends, it's formed into a closed circle. This DNA molecule, which is not properly called a chromosome, carries all of the essential genes for the cell.

> > > >
 * 1) <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__Ribosomes__ are another essential component for any living cell. Each cell contains many, many ribosomes. The function of a ribosome is to make protein, following instructions sent from the DNA's genes.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Cytoplasm __ (not labeled) is the fluid substance that fills the interior of the cell. Cytoplasm is often described as a "rich, organic soup." This analogy seeks to honor the complexity of cytoplasm, which is mostly water, but contains a large assortment of molecular and structural components.
 * 1) There are a couple of additional structures found in some, but not all, prokaryotic cells, but which are not included in this diagram. Both of these structures are actually elaborations of the plasma membrane, not separate, independent structures.
 * 2) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Mesosomes __ are rosette-like clusters of folds in the plasma membrane, protruding toward the interior of the cell. These structures are important in the performance of the aerobic parts of aerobic cellular respiration. A key part of this process requires a lot of membrane surface, and mesosomes greatly increase the membrane surface of the cell.
 * 1) <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__Thylakoids__ are long, strap-like invaginations of the plasma membrane. They're so long the typically wrap multiple times around in the interior of the cell. Thylakoids are found only in photosynthetic prokaryotic cells, such as the cells of blue-green algae and the purple photosynthetic bacteria. Like aerobic respiration, photosynthesis contains a vital function which requires a lot of membrane surface, and the long, thin thylakoid surfaces provide that area.

Eukaryotic cells, while very similar in function to prokaryotic cells, are much larger and more complex. They are typically characterized by the presense of a prominent, more or less central nucleus. Viewed through a light microscope, this nucleus is by far the most striking visible feature of the cell. However, there is a lot more inside a eukaryotic cell than what can be seen through a light microscope. Most serious study of cell structure utilizes an electron microscope, which is capable of much higher resolution (and thus supports much higher magnification) than a light microscope. Here's a labeled diagram of a typical eukaryotic animal cell. Eukaryotic cells are found in animals, plants, Fungi and protists, and there are structural differences among these four groups. We'll be using the animal cell as our basic eukaryotic cell, but will discuss some of these differences down below.

As you can see from comparing these labels, there are several features which are common to both prokaryotic and eukaryotic cells. Recall that these cells perform the same kinds of functions. The eukaryotic cell is much larger than the prokaryotic cell, and this larger size means that there's a lot more space inside the cell. If you think of a prokaryotic cell as something like a studio apartment, a eukaryotic cell is like a gigantic warehouse. In order to make the huge space relatively as efficient as the small space, a lot of compartmentalization and internal specialization is required.

Endoplasmic Reticulum (ER) is a system of membrane-enclosed channels which ramifies throughout the cytoplasm of the cell. It comes in two types--smooth and rough. The difference is that rough ER has ribosomes all over its outer surface. "Endo" means "inside," "plasmic" refers to the cytoplasm, and "reticulum" means "network." ER interconnects with the plasma membrane and the nuclear envelope. Some suspect that all of the ER within a cell is actually interconnected, but this has never been established. ER has several functions. It helps to compartmentalize the cell, and it serves as routes for the transport of materials from one part of the cell to another. It's associated with lipid synthesis and protein synthesis (rough ER only). And smooth ER is responsible for generating new layers for Golgi bodies. > The other kind of Golgi vesicle contains materials to be exported (secreted) from the cell. These materials are __not__ waste products--they are chemicals intentionally manufactured by the cell for export, like hormones and pheromones. > > An interesting observation about centrioles is that there are none in the cells of higher plants, even though these plants perform mitosis and meiosis just fine. The explanation for this turns out to be pretty simple. One of the important functions that centrioles perform is the generation of cilia and flagella for cells. These are surface features that cells use for movement. The cells of higher plants have no centrioles because there's no cell anywhere throughout their life cycles which makes cilia or flagella. A cell can make a spindle without centrioles, but it can't make cilia or flagella. > > > > >
 * 1) <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__Vacuoles__ are small, varied membrane "bubbles" found throughout the cytoplasm. Vacuole is a very general term, and there are quite a few different kinds of vacuoles, including food vacuoles, contractile vacuoles, and the very specialized central vacuoles found only in plant cells.
 * 1) Each cell contains a number of <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__Golgi bodies__. "Golgi" is the name of the person who first described these structures. Golgi bodies are like little stacks of hollow membrane pancakes. Their function is to process materials manufactured by the cell, then package those products into small structures called "Golgi vesicles." The materials arrive at the Golgi bodies from the smooth endoplasmic reticulum. Golgi vesicles come in two general types--microbodies and secretory vesicles. Microbodies are fated to remain in the cell. They contain materials, usually enzymes, which the cell needs, but which must remain packaged away from the cell's other contents. The best known of these microbodies is the __lysosome__. "Lysis" means "breakage," and "some" means "body." Lysosomes contain digestive enzymes which, if released into the cell, would digest the vital components of the cell and kill it. "Break" it, in other words.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Mitochondria __ are very complex, double-membrane-bound organelles. Their function is to perform the aerobic portions of aerobic cellular respiration, the essential energy-producing process of the cell. This is the same function performed by the mesosomes in many prokaryotic cells. Mitochondria contain their own naked, circular DNA and their own ribosomes.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Ribosomes __ perform precisely the same function in eukaryotic cells as they perform in prokaryotic cells. Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes, but they are very similar.
 * 1) The __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">centrioles __ are a pair of structures composed of microtubules. The primary function of centrioles is to generate the cell's __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">cytoskeleton __ (not shown in this diagram). The cytoskeleton is a system of microtubules and microfilaments which runs all through the cell, particularly just under the plasma membrane. Microtubules and microfilaments are responsible for all kinds of movement functions. For example, the contractile part of a muscle cell is composed of two kinds of microfilaments. And the spindle apparatus which moves chromosomes around during mitosis or meiosis is composed of microtubules.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Rough ER __ has ribosomes attached to its outer surface. This is the source of the oft-read claim that "ER performs protein synthesis." This statement is quite misleading. ER never performs protein synthesis. Only ribosomes do that. Protein synthesis is associated with rough ER because rough ER has ribosomes all over it; the ribosomes are doing the protein synthesis, not the ER itself.
 * 1) The<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __nucleus__ of the cell is a chamber specialized in DNA functions. It's enclosed by a double layer of membrane called the <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__nuclear envelope__. The function of the nuclear envelope is to confine the materials necessary for DNA and RNA synthesis inside the nucleus, and controlling movement into and out of the nucleus.
 * 1) The nucleus contains several--generally two to four--dense structures called __nucleoli__ (singlur "nucleolus"). Assembly of ribosomes takes place in nucleoli.
 * 1) The nucleus of a eukaryotic cell contains a number of<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __chromosomes__, which are composed of DNA and histone proteins. The number, shape and arrangement of genes on chromosomes is characteristic of the species from which the nucleus came. Chromosomes carry the information archive of the cell. Each gene is a set of instructions for the construction of a specific protein.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Nuclear pores __ perforate the double nuclear envelope. These pores do have structures in them which open and close to control movement through the pores. The nuclear pores are important for ribosomes to leave the nucleus. Though ribosomes are constructed in the nucleus, they must move to the cytoplasm in order to function.
 * 1) The __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">nucleoplasm __ is the rich organic soup that fills the interior of the nucleus. This is very similar to cytoplasm. It's mostly composed of water, containing a complex assortment of materials. Nucleoplasm would be distinct from cytoplasm due to the high concentration of materials like nucleotides, which are used to make DNA and RNA, and the suite of enzymes which control the DNA and RNA construction reactions.

This diagram and list are not exhaustive. There are other structures which have been identified within cells. Some of them are mentioned in this list, though they are not included in the diagram--the cytoskeleton, cilia and flagella. There are others, but this is a pretty good introduction to the structures in animal cells

Other Eukaryotic Cells
As mentioned above, animals aren't the only kind of creature to have eukaryotic cells. Plants, Fungi and protists are also eukaryotic organisms. There are several important differences between plant and animal cells. The description of the animal cell above points out that at least higher plants have no centrioles in their cells. There are also three kinds of structures found in plant cells and not in animal cells.

> > > Despite common beliefs, <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">__Fungi__ are not plants. Biologically speaking, they are more animal-like than plant-like. They just __look__ like plants to us. They do no photosynthesis and have no plastids. They have cell walls, but they are different from plant cell walls, and are clearly a separate evolutionary development. The cell walls of Fungi are typically composed of a material called __chitin__. Strangely enough, we also find chitin in one of the most successful of all animal phyla, the Arthopoda (which includes spiders, insects, crabs and lobsters). Arthropods have __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">endoskeletons __ made of chitin. Again, a separate evolutionary invention. Another odd aspect of cell structure in many Fungi is that, in some groups, the concept of a "cell" is only very loosely applicable. There are Fungi which are largely composed of a single, huge structure with many, many nuclei and no subdivisions into cellular chambers. And there are Fungi whose bodies are divided by incomplete subdivision, with continuous cytoplasm connecting all of the "cells" into one giant super-cell. Fungi are strange ;^) <span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;"> __Protists__ are very diverse--typical of a more primitive (meaning evolutionarily older) group of organisms. This group contains animal-like cells, plant-like cells, and fungus-like cells, as well as a dizzying assortment of in-betweens and oddities. The task of classifying protists is dreadfully difficult.
 * 1) __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">Plant __ cells have __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">cell walls __; animal cells do not. As noted in the description of the prokaryotic cell wall, the cell must also have a plasma membrane. Plant cells have cell walls //in addition to// plasma membranes, not //instead// of plasma membranes. Plant cell walls are composed all or partially of a carbohydrate called __cellulose__, which is quite different from the peptidoglycan of prokaryotic cell walls. The primary function of the cell wall is support, and inconjunction with the central vacuole, to create turgor (stiffness) in plant structures like leaves.
 * 1) Plant cells contain a specialized vacuole called the __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">central vacuole __. This is a large, membrane bound structure filling most of the interior of the cell. The central vacuole is filled mostly with water, but always with some impurities--mineral or protein--so that the water concentration is always less than 100%. When the cell is surrounded by sufficient water, osmosis causes the central vacuole to swell, and thus causes the cell to press against the inside of the cell wall. This phenomenon in all of the cells of a leave causes the leaf's tissues to be stiff, and keeps this delicate structure spread out so it can serve its vital function as a solar panel.
 * 1) Plant cells contain a family of organelles called __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">plastids __. There are several kinds of plastids, all related to each other and, under appropriate conditions, capable of modifying from one type to another. The best known of these plastids is the __<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif;">chloroplast __, which performs the function of photosynthesis. Chloroplasts are double-membrane-bound, like mitochondria. Also like mitochondria, their inner membrane is very complicated. In fact, it's formed into many thylakoid structures which perform the same function performed by the thylakoids in prokaryotic cells.

Speaking of Taxonomy--The Kingdoms Quandary
As we've seen so many times, we are a species driven to categorize, and biologically speaking, the mother of all classification enterprises is our Linnaean taxonomy--the classification of living things. In principle, it seems like such a simple thing: just group things on the basis of similarity. Back when Linnaeus set out to examine, describe, classify and name all living species, he wasn't even worried about a lot of the things that are so important to us--like evolutionary relationships. He just wanted a nice, useful, organized, easy outline-like list of all those pesky, confusing organisms. It turns out this is a lot easier to plan than to accomplish. You might want to go back and reread the essay on *content*taxonomy.htm:Essay ASLINK=taxonomy*endcontent*. Consider the very fundamental question of Kingdoms. How many kingdoms do we need to subdivide the world of living things into the smallest possible very general categories of "different kinds of things"? For Linnaeus, the answer to this was obvious. Clearly, there were only two kinds of creatures: animals and plants. So that's how many kingdoms he created--two. Plantae and Animalia. Tradition is strong, even in the sciences, and Linnaeus was, after all, the "father of taxonomy." So for a long time, biologists stuck to this two-kingdom system, despite clear evidence that it didn't really work. Linnaeus didn't know anything about things like bacteria and protists (or if he knew about them, he just ignored them). And as more knowledge was gained about Fungi, which Linnaeus just shoved into the plant kingdom because they looked more like plants than animals, the more obvious it was that they were very different from plants. Finally, the two-kingdom system had to be discarded. So then the question was, how many kingdoms __do__ we need? Out of the variety of proposed systems, the one that emerged as the favorite for quite a while was the Whittaker five-kingdom system. Easy. Five kingdoms: Plantae, Animalia, Fungi, Protista, and Monera (bacteria). All of those annoying in-between sorts of eukaryotic organisms were jammed into the Protista, and all of the prokaryotic organisms were shoe-horned into the Monera. The biggest apparent problem with this system was the Kingdom Protista, which contains a whole lot of really different kinds of organisms. But everyone just looked the other way. To a large extent, everyone __still__ looks the other way when it comes to the protists ;^) But things aren't so comfortable with another one of these kingdoms. With the advent of knowledge of DNA and the great ability to compare the DNA sequences of different organisms, we've gained a marvelous kind of telescope into the biological past. Comparing the DNA sequences of different organisms gives us at least a rough idea of how closely related they are. And the better we get at it, the more precise that idea can become. So here's the problem. DNA comparison tells us some really interesting things about these five kingdoms of organisms. The first thing it tells us is that there are two distinct groups of organisms among the Monera (bacteria). Two groups of bacteria which are, genetically speaking, __very__ distant from each other. The second fascinating thing DNA tells us is that the total diversity among __all four__ of the eukaryotic kingdoms is __less__ than the genetic difference between those two groups of bacteria. So just what do we do with these bits of very significant information?

The first thing we have to do is recognize that, at the very least, we need a sixth kingdom for one of those groups of bacteria. So now we have added the kingdom Archaeobacteria for the group of bacteria which seem to be of a very ancient type. "Archaeo" means "ancient." But that really isn't enough, because remember that the difference between the two kingdoms of bacteria is greater than the __total__ difference among all of the other four kingdoms. So it has been suggested that we need to add a new, even more general level to our classification system. This new level has been called the __Domain__. This system names three Domains: Monera (or Eubacteria), Archaeobacteria and Eukarya. Guess where the various Kingdoms go ;^) Even that system seems a bit lop-sided. Some folks have made a really radical suggestion that we should actually have only three __kingdoms__. This would "demote" all of those eukaryotic kingdoms to, at best, sub-kingdoms. Eek! This would do away with even those two original kingdoms Linnaeus devised! So far, this suggestion hasn't gained a lot of ground. As I mentioned, tradition is strong, and this would be a __big__ step.

<span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 80%; text-align: right;">//taken from:http://www.cod.edu/people/faculty/fancher/cellstructure.htm//

<span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 80%; text-align: right;">//taken from:http://sites.google.com/site/hellokittyoh8/Animal-Cell.jpg//




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 * Landmarks in the Study of Cells ||  ||
 * Landmarks in the Study of Cells ||  ||

<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: center;">The **CELL THEORY,** or cell doctrine, states that all organisms are composed of similar units of organization, called cells. The concept was formally articulated in 1839 by Schleiden & Schwann and has remained as the foundation of modern biology. The idea predates other great paradigms of biology including Darwin's theory of evolution (1859), Mendel's laws of inheritance (1865), and the establishment of comparative biochemistry (1940). Ultrastructural research and modern molecular biology have added many tenets to the cell theory, but it remains as the preeminent theory of biology. The Cell Theory is to Biology as Atomic Theory is to Physics. || ||

<span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 80%; text-align: right;">//taken from:http://www.bio.miami.edu/~cmallery/150/unity/cell.text.htm//