Wednesday, 25 June 2014

HISTORY OF BIOLOGY : FORM FIVE.

HISTORY OF BIOLOGY : CELL THEORY AND CELL STRUCTURE

Photo by: Russi & Morelli

All living organisms are composed of cells, and all cells arise from other cells. These simple and powerful statements form the basis of the cell theory, first formulated by a group of European biologists in the mid-1800s. So fundamental are these ideas to biology that it is easy to forget they were not always thought to be true.

Robert Hooke's microscope. Hooke first described cells in 1665.

Early Observations

The invention of the microscope allowed the first view of cells. English physicist and microscopist Robert Hooke (1635–1702) first described cells in 1665. He made thin slices of cork and likened the boxy partitions he observed to the cells (small rooms) in a monastery. The open spaces Hooke observed were empty, but he and others suggested these spaces might be used for fluid transport in living plants. He did not propose, and gave no indication that he believed, that these structures represented the basic unit of living organisms.
Marcello Malpighi (1628–1694), and Hooke's colleague, Nehemiah Grew (1641–1712), made detailed studies of plant cells and established the presence of cellular structures throughout the plant body. Grew likened the cellular spaces to the gas bubbles in rising bread and suggested they may have formed through a similar process. The presence of cells in animal tissue was demonstrated later than in plants because the thin sections needed for viewing under the microscope are more difficult to prepare for animal tissues. The prevalent view of Hooke's contemporaries was that animals were composed of several types of fibers, the various properties of which accounted for the differences among tissues.
At the time, virtually all biologists were convinced that organisms were composed of some type of fundamental unit, and it was these "atomistic" preconceptions that drove them to look for such units. While improvements in microscopy made their observations better, it was the underlying belief that there was some fundamental substructure that made the microscope the instrument of choice in the study of life.
In 1676 the Dutch microscopist Antony van Leeuwenhoek (1632–1723) published his observations of single-cell organisms, or "little animalcules" as he called them. It is likely that Leeuwenhoek was the first person to observe a red blood cell and a sperm cell. Leeuwenhoek made numerous and detailed observations on his microorganisms, but more than one hundred years passed before a connection was made between the obviously cellular structure of these creatures and the existence of cells in animals or plants.

The Development of the Cell Theory

In 1824 Frenchman Henri Milne-Edwards suggested that the basic structure of all animal tissues was an array of "globules," though his insistence on uniform size for these globules puts into question the accuracy of his observations. Henri Dutrochet (1776–1847) made the connection between plant cells and animal cells explicit, and he proposed that the cell was not just a structural but also a physiological unit: "It is clear that it constitutes the basic unit of the organized state; indeed, everything is ultimately derived from the cell" (Harris 1999, p. 29). Dutrochet proposed that new cells arise from within old ones, a view that was echoed by his contemporary François Raspail (1794–1878). Raspail was the first to state one of the two major tenets of cell theory: Omnis cellula e cellula, which means "Every cell is derived from another cell." However, despite this ringing and famous phrase, his proposed mechanism of cell generation was incorrect. Raspail was also the founder of cell biochemistry, making experiments on the chemical composition of the cell and their response to changing chemical environments.
In 1832 Barthelemy Dumortier (1797–1878) of France described "binary fission" (cell division) in plants. He observed the formation of a mid-line partition between the original cell and the new cell, which, Dumortier noted, "seems to us to provide a perfectly clear explanation of the origin and development of cells, which has hitherto remained unexplained" (Harris 1999, p. 66) These observations led him to reject the idea that new cells arise from within old ones, or that they form spontaneously from noncellular material. The discovery of cell division is usually attributed to Hugo von Mohl (1805–1872), but Dumortier proceeded him in this regard. Von Mohl did coin the word "protoplasm" for the material contained in the cell.
The first unequivocal description of the cell nucleus was made by a Czech, Franz Bauer, in 1802 and was given its name in 1831 by Robert Brown (1773–1858) of Scotland, who is best remembered for discovering the random "Brownian" motion of molecules. The first accurate description of the nucleolus was made in 1835.
Schleiden and Schwann, who are usually given credit for elucidating the cell theory, made their marks in 1838 and 1839. In 1838 Matthais Schleiden (1804–1881) proposed that every structural element of plants is composed of cells or the products of cells. However, Schleiden insisted on priority for several ideas that were not his and clung to the idea that cells arise by a crystallization-like process either within other cells or from outside, which Dumortier had dispensed with some years earlier. (In Schleiden's defense, it should be remembered that drawing incorrect conclusions from limited observations is a risk inherent in science, especially when working on the frontier of a new field.)
In 1839 a fellow German, Theodor Schwann (1810–1882), proposed that in animals too every structural element is composed of cells or cell products. Schwann's contribution might be regarded as the more groundbreaking, since the understanding of animal structure lagged behind that of plants. In addition, Schwann made the explicit claim that the fundamental laws governing cells were identical between plants and animals: "A common principle underlies the development of all the individual elementary subunits of all organisms" (Harris 1999, p. 102).
A special word should be said here about the Czech Jan Purkyňe (1787–1869), or Purkinje, as his name is usually given. Purkinje was the premiere cytologist of his day, and one of the most influential formulators of the cell theory. He gave his name to structures throughout the body, including the Purkinje cells of the cerebellum. Purkinje, in fact, deserves much of the credit that usually goes to Schwann, for in 1837 he proposed not only that animals were composed principally of cells and cell products (though he left room for fibers) but also that the "basic cellular tissue is again clearly analogous to that of plants" (Harris 1999, p. 92). Unfortunately, Schwann did not credit Purkinje in his influential publication.

Reproduction and Inheritance

Despite the work of Dumortier, the origins of new cells remained controversial and confused. In 1852 a German, Robert Remak (1852–1865), published his observations on cell division, stating categorically that the generation schemes proposed by Schleiden and Schwann were wrong. Based on his observations of embryos, Remak stated instead that binary fission was the means of reproduction of new animal cells. This view was widely publicized not by Remak but by Rudolf Virchow (1821–1902), unfortunately without crediting Remak. Virchow is also usually given the credit for the phrase Omnis cellula e cellula, indicating the importance of cell division in the creation of new cells.
The understanding of the central importance of chromosomes lagged well behind their discovery. In 1879 Walther Flemming (1843–1905) noted that the chromosomes split longitudinally during mitosis (a term he introduced). Wilhelm Roux (1850–1924) proposed that each chromosome carried a different set of hereditable elements and suggested that the longitudinal splitting observed by Flemming ensured the equal division of these elements. This scheme was confirmed in 1904 by Theodor Boveri (1862–1915). Combined with the rediscovery of Gregor Mendel's 1866 paper on heritable elements in peas, these results highlighted the central role of the chromosomes in carrying genetic material. The chemical nature of the gene was determined in a series of experiments over the next fifty years, culminating in the determination of the structure of deoxyribonucleic acid (DNA) in 1953 by James Watson and Francis Crick.

Modern Advances

The modern understanding of cellular substructure began with the use of the electron microscope. Keith Porter (1912–1997) was a pioneer in this field and was the first to identify the endoplasmic reticulum and many elements of the cytoskeleton . The explosion of knowledge brought about by improvements in microscopy, biochemistry, and genetics has led to a depth of understanding of cell structure and function undreamed of by the earliest cell biologists.

SEE ALSO Cell ; Electron Microscopy ; Leeuwenhoek, Anton Von ; Light Microscopy ; Porter, Keith

Richard Robinson

Bibliography

Harris, Henry. The Birth of the Cell. New Haven, CT: Yale University Press, 1999.
Magner, Lois N. History of Life Sciences, 2nd ed. New York: Marcel Dekker, 1994.

WILSON, EDMUND B. (1865–1939)

Premier cell biologist of the early twentieth century, Wilson described how a fertilized egg divides up into hundreds of cells to form an embryo and which parts of the body develop from which cells. His student Walter Sutton discovered the role of chromosomes as the units of heredity.

CELL THEORY ---- BIOLOGY FORM FIVE.

CELL THEORY---- BIOLOGY FORM FIVE.

INTRODUCTION

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 a scientific theory which describes the properties of cells. These cells are found to be the basic unit of structure in all organisms and also the basic unit of reproduction. With continual improvements made to microscopes, in the 17th century, magnification became strong enough to discover cells. This discovery is largely attributed to Robert Hooke, thus beginning the study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientists. Most of these being how regeneration happens and connecting cells to being the fundamental unit of life. During the mid-19th century, in 1838, cell theory was formulated. This is usually credited to Matthias Schleiden and Theodor Schwann. However, many other scientists like Rudolf Virchow contributed to the theory.
Overtime, cell theory has become the foundation of biology and is a widely accepted explanation of the relationship between cells and living things. The cell theory holds true for all living things, no matter how big or small. Since according to research, cells are common to all living things, they can provide information about all life. And because all cells come from other cells, scientists can study cells to learn about growth, reproduction, and all other functions that living things perform. By learning about cells and how they function, you can learn things like diseases in living things.
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 most basic unit of life.
All cells arise from pre-existing, living cells.

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 to fit over one eye, allowing for a magnification effect to that eye. It was not until the 1590s when a Dutch spectacle maker Zacharias Jansen began to test lenses that progress had been made to microscopes. Jansen was able to obtain about 9x magnification, but the objects appeared to be blurry. In 1595, Jansen and his father built the first compound microscope. While simple glasses were able to magnify objects, they were not considered to be a microscope. A compound microscope was defined by having two or more lenses in a hollow tube.^[1] 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 direct transmitted light, because this allowed for a clearer image.^[2]
However, the first real invention and use of a microscope was by Anton van Leeuwenhoek. He was a Dutch draper that took 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 microscope. His were instead simple powerful magnifying glasses, 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.^[3]
These microscopes could focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than a wavelength. Later in the 1920s, the electron microscope was developed, making it possible to view objects that are smaller than a wavelength, once again, changing the possibilities in science.^[3]

Discovering 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.^[2] 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.^[4] 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" was alive.^[5] 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.^[2]
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.^[5] 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.^[3] Though he had did not have much formal education, 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.^[2]
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.^[6] This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization,” ^[7] 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.^[8] 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. While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it. In 1838, 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. However, this was not an original idea of Schlieden. 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.^[9] In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures. 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.^[6]

All living organisms are composed of one or more cells
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.^[3] In 1855, Rudolf Virchow added the third tenet to cell theory. In Greek, this tenet states Omnis cellula e cellula.^[6] This translated to:

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.^[10] 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^[11]
All living cells arise from pre-existing cells by division.
The cell is the fundamental unit of structure and function in all living organisms.^[12]
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.^[13]
All cells are basically the same in chemical composition in organisms of similar species .^{[citation needed]}

Opposing concepts in Cell Theory: History & 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.^[14] 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.^[15] 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 1930's, 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 principle 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 1940’s, 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.

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

Prokaryote which are plant cells

Eukaryote which are animal cells

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.^[16] 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.^[16] 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).^[17] In addition, they possess organized chromosomes which store genetic material.^{[citation needed]}

References

"History of the Microscope". History-of-the-microscope.org, United Kingdom. Retrieved 24 April 2014.
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. PMID 15209075. edit
Mazzarello, P. (1999). "A unifying concept: the history of cell theory". Nature Cell Biology 1 (1): E13–5. doi:10.1038/8964. PMID 10559875. edit
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.
Becker, Wayne M.; Kleinsmith, Lewis J. and Hardin, Jeff (2003). The World of the Cell. Benjamin/Cummings Publishing Company. p. 1. ISBN 978-0-8053-4854-5.
Robinson, Richard. "History of Biology: Cell Theory and Cell Structure". Advameg, Inc. Retrieved 17 March 2014.
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"
http://www.mathnat.uni-rostock.de/geschichte/kalenderblatt/kalenderblatt-dezember-2013/
Schleiden, Matthias Jakob (1839) "Contributions to Phytogenesis"
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.
Wolfe
Wolfe, p. 5
Wolfe, p. 8
Ling, Gilbert N. (1984). In search of the physical basis of life. New York: Plenum Press. ISBN 0306414090.
Michaelis, L. (1925). "Contribution to the Theory of Permeability of Membranes for Electrolytes". The Journal of General Physiology 8 (2): 33–59. PMC 2140746. PMID 19872189.
Wolfe, p. 11
Wolfe, p. 13

Bibliography

Wolfe, Stephen L. (1972). Biology of the cell. Wadsworth Pub. Co. ISBN 978-0-534-00106-3.

External links

Mallery C (2008-02-11). "Cell Theory". Retrieved 2008-11-25.
"Studying Cells Tutorial". 2004. Retrieved 2008-11-25.

Categories:

CYTOLOGY---- BIOLOGY FORM FIVE.

CYTOLOGY---BIOLOGY FORM FIVE.

INTRODUCTION

Cytology is the study of the cells, especially their appearance and structure. Cells are the small parts that make up all living things, and their effects on each other and their environment.
There are two types of cells. Prokaryotic cells do not have a clear and easy-to-see nucleus, and do not have a membrane, or wall, around them. Eukaryotic cells have an easy-to-see nucleus where all of the cell's functions take place, and a membrane around them. The main organelles of a cell and their uses are:

Mitochondria: produces energy for the cell
Endoplasmic reticulum: makes proteins and carbohydrates for the cell to use
Golgi bodies: store and package products that the cell uses
Plastid (present in plant cells only): contains chemicals needed to photosynthesize (create energy from sunlight); in plants only.
Nucleus: directs the actions of the cell
Centrosomes: guides the cell in mitosis and meiosis, the processes for cell division.

Other pages

Cell biology is mostly about how cells work, and about cell division and molecular biology.
Histology deals with techniques for looking at tissues under a microscope.

This short article about biology can be made longer. You can help Wikipedia by adding to it.

Categories:

Sunday, 8 June 2014

HOW TO SURVIVE FINALS BY MINIMISING STRESS

How To Survive Finals By Minimising Stress.

One day some evil person decided to invent a thing called finals. During this time, most students are united in a state of self-doubt and stress. I have some tips for you to help minimise your stress levels and get through your exams.

1. Make a study timetable, but don’t overload yourself.

Just don’t do this kind of thing^^. Read the rest of this post and fit some of my stress relief tips into your timetable and you will find that your study sessions go a lot better.

2. Start the day properly.

Get up and make your bed (sorry Mum, I do not iron my bedsheets). This will stop you from being tempted to climb back in and sleep all day. You will thank yourself for it when you get into bed at night. Open your curtains and let some natural light in. This will help you wake up and increase your Serotonin levels.

3. Get washed and dressed.

The simple task of personal hygiene can be totally forgotten about during finals, but if you just take 15 minutes to give yourself a quick shower you will feel much more relaxed.

4. Eat some brain food.

No, not literally brains (I know we all look like zombies at this time of year). Lots of sugary snacks will make you crash quickly, so eat healthy nuts and fruits as you study to keep your energy levels up. If your are really craving some chocolate then go for it, but remember that moderation is key. You can try a graze box for free by clicking here.

5. Get some fresh air.

Going for a short walk and getting a bit of sunshine will boost your mood and get you energised for your study session.

6. Abstain from alcohol.

Don’t damage your brain cells at this crucial time. Drinking to get away from your stress is an unhealthy way to cope and can lead to more serious problems. Wait until your exams are finished and you can have a drink to celebrate.

7. Treat yourself.

If you are getting a bit isolated sitting at your desk all the time, go to a cafe and have a wee treat with a friend. Chatting about what is going on in your life will give you a lot of relief. And you deserve that chocolate milkshake after all the nuts and fruit you have been munching!

8. Take a break.

Schedule breaks properly. Don’t sit and play Nintendo for hours on end; Hyrule can wait. Instead reward yourself by reading one article from your favourite magazine. If your energy levels are getting really low then have something sugary to perk you up for your next burst of studying, but don’t go overboard on the sugar!

9. If you must procrastinate, do something productive.

When I just can’t get my studying started, I pick a simple task on my to-do list instead. Today I decided to do laundry before starting my studying. It is nice to have the smell of fresh cotton in my room, and having a shorter to-do list takes some stress away from life in general. Then you can get back to the books knowing you haven’t been wasting your time.

10. If you live in Halls, speak to your RA.

(Yes, I am an RA). If there is too much noise in your corridor, or if you feel like you are really struggling to manage your stress, talk to your RA. We are trained in helping you through these times and we have done plenty of finals ourselves. Don’t suffer in silence.

11. Get a good night of sleep.

I use Badger Sleep Balm every night to help me get off to sleep. My mum has always told me lavender is the best for getting you drift off, and it is definitely worth trying. I got mine for £4.29 here. (And aren’t you glad you made your bed this morning?!).

12. Final tip.
If you are experiencing high levels of stress and are unable to manage it properly, speak to your student support service about help groups and relaxation methods. Also, do not be afraid to speak to your doctor if you feel depressed or are having panic attacks. Becoming ill is nothing to be ashamed of, and your doctor can help you.

GOAL SETTING FOR STUDENTS

Goal Setting for Students: How to Easily Create “The Master Plan” for Exams!

Goal setting for students can be a frustrating process. Why? Because University is supposed to be all about parties and more parties! Studying is for people in high school right? That’s what a lot of University students think, until a couple of days before the big exams start. Why do students not set goals? Because of procrastination. Fail to prepare, prepare to fail springs to mind!

So how can students manage their time effectively and efficiently so that they can enjoy the University experience of socializing and not be overwhelmed by the work that’s ahead of them? Well goal setting for students just requires planning for exams from day one so that procrastination and overwhelm are a thing of the past. Once you set goals, you are halfway to achieving them. This is a big mistakes most students make, they wait until the last minute to plan and then fail their exams or don’t get the results they want.
goal setting for students

Goals are like a GPS navigation system for your life and your exams. When I was in University, my friends and I would sit down at a table and create what we called a “Master Plan”. So what is a Master Plan? A master plan is simply a detailed framework for achieving high results in your exams. It is a framework designed to study the patterns of behaviour of the lecturers and what possible topics and questions will come up on the final exams. It is a framework for success. If you don’t plan or anticipate, you can be sure that a very big surprise will await you in the exams. Suffice to say you won’t know what to expect on the day. Planning will help you.
Would you like to know the 4 EASY steps to creating your very own Master Plan? Ok let’s get started shall we?
Step 1) Buy A Copy of Old Exam Papers: Got to the Student Union and purchase a copy of the past exam papers. Try and get exam papers for the past five years. That should be statistically representative of the patterns of behaviour of the lecturers so that you can start to figure out what topics will come up on your current exam.
Step 2) Plan at least 6 weeks before Exam Time: Get a fresh sheet of paper and write the words “Master Plan” on the top of your page. Writing helps clarify things in our mind. Write the date you start the plan so that all of this is documented. Remember a goal is just a wish until you write it down.
Step 3) Study Patterns of Behaviour: I have provided a picture of what a master plan can look like to help with goal setting for students. It’s important to go back at least three years i.e. 2008, 2009, 2010 etc… and then look at the topics that came up in those years. If it came up then write down an “X” on the year it came up. For example, if Topic A was on the 2008 exam paper and hasn’t come up in the past two years, it is very likely to focus on that topic because it will most likely appear again. Also study the patterns of behaviour of the old exam papers and see when topics came up and when they didn’t.
Step 4) Write up Sample Answers and Practice Writing Them Down: Once you know the topics that are likely to come up. Read the question and then write a sample answer using the books you were told to read by the lecturer. Sample answers are generally five to eight A4 pages long. Once you are happy that you answered the question, it’s just a matter of learning the answer off like a parrot. Keep revising it until you know the answer off by heart.
There you have it, four simple steps to create a Master Plan.