Protoplasm is typically maintained in units called cells. As a unit, the cell is considered by most biologists to represent the level of organization in the continuum of matter and energy which constitutes the simplest living system.
From a structural viewpoint, the cell constitutes a kind of building block for the more complex living systems. Although some organisms consist of only more complete unit of protoplasm, and hence are said to be unicellular, the great majority of organisms are made up of more than one protoplasmic unit and thus are multicellular.
Let us note at this point that the cell is the smallest and least complex unit of matter which can unquestionably be called living. This means that, within limits of specialization, it can carry on all the basic activities which characterize organisms.
In other words, the activities to which we refer as metabolism, growth, reproduction, responsiveness, and adaption are, in the final analysis, carried on by protoplasm. It should be kept in mind as one considers these fundamental activities that a cell is a highly organized entity whose material substance so ordered as to warrant its being considered “living”.
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In its own right, therefore, and not simply by virtue of its association with other such units in a complex organism, the cell holds this unique distinction. More precisely, a cell is a system of supramolecular complexes so organized as to take in free energy and matter. By so doing, it is able to maintain and extend its organization, and exhibit the properties listed above which emerge at this level of organization.
Cells show a great deal of variability in size, shape, structure, and function. This is especially true of the various cells making up a complex multicellular organism, where the association of cells in the formation of tissues is accompanied by a division of labour.
On the other hand, there are many structural and functional features which most cells have in common. Because of these common features, we permit ourselves to speak of a “typical” or “average”, cell, although such a cell is quite mythical.
When viewed with an ordinary light microscope, the typical cell appears as a unit separated from its environment by a plasma membrane enclosing the protoplasm. In plants, a rigid cell wall, composed chiefly of cellulose, typically surrounds the sell and delimits it as a unit from others. This wall is nonliving and is not strictly a part of the cell, having been formed by the cytoplasm during its inception and growth.
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The plasma membrane lies just within this wall. Most animal cells, in contrast, are simply limited by their plasma membranes, although some possess a flexible, nonliving pellicle which corresponds to the plant cell wall. Inside the cell, the nucleus appears to be separated from the cytoplasm by a nuclear membrane. It should be pointed out that in a few types of cells this definite nucleus- cytoplasm relationship does not exist.
This organism is approximately 100 m in diameter, and may represent the lowest organizational level of protoplasm that can be considered a living cell. At the other extreme, one of the largest cells known is that of the giant amoeba, Chaos, which is approximately 100m in diameter and is multinuclear. But the typical nucleus-cytoplasm ratio is one nucleus per cell.
The protoplasm within the nucleus is called nucleoplasm, and its contains one or more dense bodies known as nucleoli, as well as granular mass called the chromatin network, or simply chromatin material. When a cell enters into a divisional cycle, the chromatin assumes the form of discrete chromosomes. The number of chromosomes thus formed is usually constant for a given species of organisms.
When viewed in the living condition with an ordinary light microscope cytoplasm appears as somewhat homogenous, translucent material containing retractile bodies of different sizes. In man}’ cells, the cytoplasm appears to be thicker or more viscous around the periphery and less viscous toward the center of the cell. Whenever such a distinction is made, the outer portion is called the ectoplasm.
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The sol-gel change which occurs as cytoplasm, alternates between these two physical phases is a reflection of its colloidal nature. Although several bodies lie within the cytoplasm, most of them are difficult to see in the living cell with an ordinary light microscope.
These bodies include mitochondria, a structure called Golgi complex, and various granules, yolk bodies, and crystals. Under ideal conditions of observation, the centrospheres of the animal cell can be distinguished from the surrounding cytoplasm. Less difficult to see are the plastids found in the cytoplasm of many plant cells, of which the green chloroplasts are the most common.
In order to observe the more intricate structural details of cells, one must either use certain specialized types of microscopes, or else the cells must be killed and stained.
Sophisticated microscopic techniques employ forms of radiation other than visible light, or else take advantage of special chemical and physical features of the different part of the cell. Staining methods are based on the differential chemical nature of the various parts of the cell, which is reflected by their differential affinities for certain dyes.
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As is generally true in any area of science, our knowledge of giving systems has advanced as techniques for studying them have been developed. An excellent example of this is the invention, and development of the electron microscope, which became generally available in the early 1940’s. By the use of this instrument, extremely intricate details of cellular structure have been observed.
Compared with the ordinary light microscope, the electron microscope is a powerful instrument indeed. It magnifies at 200.000 diameters with clarify, and by using special photographic methods, even greater magnification may be obtained. The source of radiation in the electron microscope is a beam of electrons which is passed through magnetic fields and through the specimen to cast an image on a photographic plate.
This image, per picture, can then be viewed by the human eye. There are some disadvantages inherent in the use of the electron microscope; not only is it an expensive and complicated instrument, but materials to be studied must be sliced ultrathin and dried thoroughly.
Furthermore, since beams of electrons must travel through a vacuum, it is necessary that materials be prepared in such a way that they will not be distorted under these rigorous conditions. Needless to say, cells cannot be studied in a living state with the electron microscope; they must be killed, treated with chemicals, and sliced under the most exacting conditions. Nevertheless, electron microscopy has yielded a wealth of information about cell structure which would not have been attainable otherwise.
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In general, the structural entities in the cytoplasm of the cell can be classified as cytoplasmic inclusions or cytoplasmic organelles. The cytoplasmic inclusions are such structures as glycogen granules, fat droplets, and yolk bodies.
Most of these inclusions are rather passive entities which represent stored food materials in the cell. In contrast, cytoplasmic organelles are functional entities or sites of activity in the cell. .Because of their importance to living systems, we shall list and discuss the more important organelles in turn.
The electron microscope shows the cell membrane to be a doublelayered structure ranging from 65 to 100 A in thicknesses. Chemical analysis has shown it to be composed of lipid and protein. The lipid components are compound lipids and are primarily the phospholipids lecithin and cephalic.
The protein components belong to the fibrous protein group. A theoretical model of molecular arrangement in the cell membrane has been proposed by several investigators.
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According to this concept, the cell membrane is composed of a double layer of lipid molecules sandwiched between two layers of protein. This model is based on electron-microscope studies, X-ray-diffraction studies, chemical analysis, and known physiological properties of the membrane.
The cell membrane serves as a boundary between the external environment and the internal environment of the cell. It thus represents a barrier which all molecules must traverse in entering or leaving the cell.
This includes the molecules supplying the energy that keeps the cell organized and functional, the micro molecules, which constitute the building blocks of the basic macromolecules, and the various by-products or waste materials which result from the activities of the cell.
Thus, the cell membrane is permeable that is, it “permits’ the passage of substances through it. To be more precise, it is selectively or differentially permeable, since some ions or molecules pass through it while others are prohibited from doing so.
Therefore, in regard to permeability, it “selects” some substances and “rejects” others.
The mitochondria are filamentous or granular organelles ranging in size from about 0.2 to 0.7 in diameter. Their size, shape, and distribution are relatively constant in cells of the same type; however, cells of different organisms or tissues show considerable variability in this regard.
Although mitochondria can be seen by using the ordinary light microscope, very little detailed structure is discernible by this method. However, the electron microscope reveals a very intricate and detailed structure.
The mitochondrial membrane, like the cell membrane, is double structure composed of lipid and protein. The inner membrane is thrown into a series of folds forming “shelves” that extend into the matrix. These folds, or shelves, are known as cristae, and they take various forms in the mitochondria of different cell types.
In terms of thermodynamics, mitochondria are more directly involved than any other organelle in the maintenance of the high enthalpy a low entropy characteristic of living systems.
Since energy must be put into a system if it is to resist an increase in entropy, an efficient mechanism for translating this energy into a utilizable form is essential. It is to this particular role that mitochondria are adapted, and this reflected in their structure, number, and distribution within the cell.
Plastids are found in at least some cells of virtually all members of the plant kingdom, except for certain of the least complex forms. They vary in size, number, shape, distribution, and chemical organization, as well as in colour.
On the basis of presence or absence of colour, they may be classified into two major groups: the leucoplasts and the chromoplasts. This distinction is rather artificial, however, since it is known that leucoplasts may change into chromoplasts.
Chloroplasts, like mitochondria, are functional in the transformation of energy. The process of photosynthesis, through which the suns’ energy is transformed into chemical energy, is a first step in the world of life toward reversing the trend to increasing entropy.
Thus, the functional role of the chloroplast is vital not only to the life of the plant cell which contains it, but to living systems in general, since all organisms ultimately benefit from the energy of sunlight.
The electron microscope has shown the cytoplasm to be traversed by a reticulum of strands and vesicle-like bodies. On the basis of available data, it is thought that this network is a three-dimensional continuum of cavities bounded by a membrane system.
According to this view, it divides the cytoplasm into that within the network and that outside the network. The reticulum is not confined to the endoplasm, as its name might imply, but extends into the ectoplasm as well.
In a number of cell types, large numbers of small granules approximately 150 an in diameter are seen to be attached t the outer membrane of the endoplasmic reticulum. These granules are called ribosomes.
Chemical analysis of ribosomes indicates that they are composed largely of RNA and protein. Functionally, they serve as the site where amino acids are joined together in the synthesis of protein molecules.
In this connection, another term should be presented and defined – the term micro some. A micro some is not a structure of the intact and living cell as such. Rather, it is a term which has arisen in connection with the development of a technique for studying cells, the technique of differential centrifugation. In order to study subcellular particles, the biochemist breaks the cell membrane by one means or another and subjects the cell contents to high speed centrifugation.
When particles of various sizes are spun in a centrifuge they migrate toward the bottom of the centrifuge tube at varying rates, depending on their mass. By this means, particles of different sizes can be isolated for further study.
When cell contents are centrifuged at speeds equal to 40,000 or 100,000 times the force of gravity, the endoplasmic reeticulum breaks up into particles of relatively small size, and these particles, the macrodomes, migrate toward the bottom of the tube.
Thus, a micro some is a product of differential centrifugation representing a small portion of the endoplasmic reticulum with one or more ribosome’s attached, and is capable of carrying on protein synthesis under experimental conditions. From a functional standpoint, the terms ribosome and micro some are often used synonymously – but they actually represent different physical entities.
This organelle is a system of smooth membranes which are arranged in parallel fashion and which enclose vesicles, or cavities, of varying sizes. It is particularly conspicuous in animal cells, especially those which are active in secretary functions. This observation, plus other evidence, has led to the belief that the Golgi complex is somehow associated with secretary processes, at least in certain cells. Other lines of evidence suggest that this organelle may have other functions as well, especially in cells which are not secretary in nature. For example, it appears to be physically involved in the formation of new cell wall material in plant cell division.
It is still controversial among cytologists as to whether the Golgi complex has a separate identity from that of the endoplasmic reticulum, or whether it is continuous with it. At least from a functional viewpoint, it may be best to regard it as a part of the endoplasmic reticulum, since it resembles those portions of the reticulum which axe devoid of ribosomes.
However, electron micrographs indicate that it has structural features not characteristic of the endoplasmic reticulum proper, and some cytologists regard this as evidence that the Golgi complex is not a part of the general membrane system within the cell.
In 1955, the cytologist C. de Duve obtained a group of sub cellular particles from differential centrifugation studies, which were intermediate in size between the microtonal and mitochondrial fractions.
Biochemical analysis revealed that these particles were rich in hydrolytic enzymes, that is, enzymes which catalyze the digestion of large macromolecules into micro molecules. Because of their association with digestive activity, these particles were named lissome. Apparently, the lissome with their hydrolytic enzymes are functional in processes of intracellular digestion.
They probably account for the digestion of relatively large micro molecules taken into the cell by special transporting mechanisms. They may also function in the breakdown of cell parts whenever a cell dies. A number of electron microscope studies have confirmed the presence of these organelles in a variety of cells.
In many animal cells a clearly defined region of cytoplasm may be observed to the nucleus. This region is termed the centrospheres or centrosome. Within the centrospheres are located a pair of small granules, the centrioles, which are usually rodshaped and may be seen with an ordinary light microscope. These sections of cells viewed with the electron microscope show each centriole to be a hollow cyliner 300 to 500 m in length and approximately 150m in diameter. The centriole wall around a central cavity is composed of nine separate fibrils.
The cell is the basic unit of living systems, and as a solitary entity, it is capable of carrying out all the activities characteristic of “living” matter.
However, as one surveys the diversity of living forms, it becomes evident that in the competition for energy, higher levels of organization than the solitary cell are extremely efficient, and apparently, this accounts in large part for the successful adaptation or complex organisms in nature.
Thus, cells are associated together in varying degrees of complexity and interdependence, and in many circumstances, a division of labour with regard to the intake and transformation of energy has occurred.
At the highest levels of organization, such as may be found in a complex animal body, the characteristics of living systems are manifestations of highly organized and specialized cell groups. In many of the less complex multicellular organisms, cells have become associated together in the formation of colonies with no subsequent division of labour. In other words, each cell in the colony seems to retain its separate functional identity. Such organisms are still essentially unicellular, but they represent a colonial level of organization.
It may be that these organisms reflect a stage in the development of truly solitary unicellular forms. This colonial level of organization is exemplified by many of the green algae.
The next level of organization is that of a loose association of cells which exhibit a certain degree of cell specialization and division of labour. We might call this the associational level, and it is exemplified by the sponges, where specialized cells function in such activities as digestion and reproduction.
In this case, other cells do not perform these functions; they are either very generalized in this regard, or else they exhibit specialization of their own. At a still higher level of organization, whole blocks or groups of cells differ from other groups both structurally and functionally.
A tissue may be defined, therefore, as group of similar cells which at associated in the performance of a particular function. The level of organization is apparent in most multicellul organisms, where a variety of different tissues may be found in a given organism.
Similarly, an organ system is a group of organs which are associated cellular organism, such as a human being, represents an extremely high level organization involving” integrated organ systems which exhibit the activities characteristic of life. In a special sense, the complexity this organization is extended to groups of organisms, when a division of labour may exist. This is particularly obvious in such an insect society as hive of honey bees.
Perhaps an analogy will serve to clarify the foregoing discussion to some degree. Let us imagine a wilder net area which has just each builds a homestead several miles from any of the others. Any given family is in virtues isolation, and is obliged to grow its own food, manufacturing its own clothing, and in fact, provide for its every necessity.
In time, however, other families move into the area and the picture changes. Families in close proximity to each other realize that they can get along much better if there is a division of labour. Consequently, one man who excel, at blacksmithing does this work, another raises wheat an swaps it for labour and other commodities, and so 01 Eventually, with the influx of more people, an even greater’ degree of specialization is achieved; shops and store concentrate on particular services, clearly define occupations and professions arise, and in time, there is a great division of labour.
Within limits inherent in analogies, something there same thing has apparently happened in the world the life. At one time, evidently, all life existed in the one-cell state. Gradually, in time, the colonial and association; levels of specialization appeared, and after this, the tissue, level.
Eventually, multicellular organisms with organs and organ systems arose, and in general, they became the mo successful organisms on earth. It appears highly probable. that the greater efficiency made possible by complete division if labour played a large part in their success, especially in their adaptation to environments not accessible to the less complex organisms.
We must be careful to avoid a particularly dangerous pitfall in thinking of development in these terms. It is tempting to postulate some inherent, protoplasmic drive toward specialization. While we have no absolute assurance that such an inner force does not exist in protoplasm, we have no evidence that it does.
Consequently, the postulation of such a fore is not a very fruitful conceptual scheme to use in explaining increased complexity and division of labour over time.
It is not unreasonable to assume that under some environmental circumstances a division of labour among cells enhanced survival in a world of one-celled organisms. As a matter of fact, it is possible to demonstrate this at the present time.
Following this line of thought, perhaps we can visualize colonial, tissue-level, and more complex stages of specialization as imparting certain advantages to organisms in their competition with less complex forms.
The question might be raised as to why these less complex forms did not disappear from the earth if natural selection explains the rise of more complex forms. This is an interesting question, and as we proceed through the remainder of the book, perhaps you can supply as a satisfactory answer to it.