Different Types of Cells:
The cell theory defines that all living organisms includes cells as structural and functional units. Few organisms comprise of only one cell and are known as Unicellular organisms. Others are comprised of several cells and are known as Multicellular organisms.
When it comes to levels of cells, these can be categorized according to complexity of cell involved. Organisms which show simples form of complexity in not having any internals membranes are known as prokaryotes (or prokaryotes). These organisms e.g. bacteria Escherichia coli, blue-green algae and mycoplasma lack membrane-bound nucleus. Organisms with discrete nuclei in their cells are known as eukaryotes (or eukaryotes) e.g. amoeba and clhlamydomonas. The aberration in cell Theory exists in viruses. These are biological particles that are composed of outer protein coat (capsid) that encloses a genetic material (DNA or RNA). Virus on its own has no metabolic capabilities until it gets within a appropriate host, takes over host's metabolic processes to produce its own kind. Such structures are explained as acellular like Tobacco Mosaic virus (TMV).
Coarse Structure of the Cell:
What is seen of coarse structure of typical animal cell is the structure which is bounded on outside by limiting membrane, known as cell (or plasma) membrane. Cell membrane is cell's contact point with its environment and through it materials enter or leave cell. Generally, plasma membranes are semi-permeable permitting certain molecules and particles to enter cell while disallowing others. Within eukaryotic cell there is a nucleus that is bounded by membrane the nuclear membrane. Nucleus is centre of control of all cellular activities. Enucleated eukaryotic cell (i.e. one whose nucleus has been removed) can only function for the restricted period; an example is human erythrocyte that has a half life of 60 days (maximum life span is 120 days) Rest of cell is composed of cytoplasm, in which are contained different particulate organelles (membrane systems) and soluble substances. These perform different chemical reaction which is typical for cell.
Fine Structure of a Cell:
Structure of cell observed under electron microscope (E.M.) is known as fine structure (or ultra structure) of cell. EM has capacity to magnify object over 500,000 times and has resolution of 0.05Å. The E.M. therefore can magnify and resolve far better than light microscope (L.M.). Under EM different structures will be seen as membrane -bound entities. These include plasma membrane, Ribosomes, endoplasmic reticulum, mitochondria, Lysosomes, Golgi Body (of Dictyosome), Centrosome, the nucleus and also in plant cells, plastids, and vacuoles.
1) Plasma Membrane:
Under the light microscope plasma membrane is observed as thin line. But under EM it is seen as being composed of three layers; two dark layers separated by the light one. Two dark regions of membrane are composed mostly of protein while light one in-between comprises of lipids. Plasma membrane, thus, consists of thin sandwich of lipids between 2 layers of proteins. This structure is called as unit membrane and it is trait of all cell membranes. There are pores that perforate membrane at regular intervals. These pores are very tiny, and provide membrane in these regions polar properties, with result that water, but not larger polar molecules, penetrates membrane readily.
2) The Endoplasmic Reticulum (ER):
This is the system of membranes found inside cytoplasm. It comprises of cavities of tubes lined with thin membrane. Cavities are interrelated and membrane is continuous with nuclear membrane and plasma membrane. In portions of endoplasmic reticulum, number of granules is joined to membranes on the matrix side. Such is called as rough endplasmic reticulum (RER) or granular ER. These granules are rich in ribonucleic acids and are known as ribosome. The ER is thus a kind of intracellular transport system that makes the transport of materials from one part of the cell to another easy.
3) The Golgi body (Dictyosome):
This is another attribute system of cytoplasmic membrane. It resembles SER but is smaller, more compact and discontinuous. It comprises of stacks of flattened cavities that are connected with SER. Golgi body is thought to be involved in secretion of glycoproteins; these are proteins conjugated with carbohydrate. Proteins are synthesized in channels of rough ER. From here, they are transported in vesicles to fuse to form Golgi Body.
4) Cytoplasmic Particles:
(i) Mitochondria (sing mitochondrion) are found in almost every kind of cell except bacteria, blue-green algae and red blood cells of mammals. Every mitochondrion is enclosed by the double lipoprotein membrane like the plasma and nuclear membranes. Inner membrane is variously folded in projections known as cristae. The Mitochondria are respiratory centers of cell. They are concerned with energy problems of the cell.
ii) Lysosomes: These look like mitochondria externally but are noticeable by their lack of internal cristae. Enzymes contained in them are also dissimilar from those of mitochondria. Lysosomal enzymes are mostly hdroxylases, and their main function is degradation of large molecule by the addition of water (hydro = water, lysis = break). Lysosomes also have significant role to play in destruction of worn-out organelles in cell.
iii) Ribosomes: Ribosomes may also lie free in cytoplasm; it is for this reason they should be included in discussion of cytoplasmic particles. Function, though, remains are related with ER or they lie free in cytoplasm.
iv) Plastids: Plastids are strange characteristics of cytoplasm of plant cells. They are categorized as leucoplasts relying on whether they are colored or not. Leucoplasts are colorless plastids in which starch is deposited. They are found in cells not generally exposed to light e.g. in deep-seated cells, but they may grow green colored plastids and are of 2 types: those which have red orange or yellow pigments. Plastids which have chlorophyll are known as chloroplasts while those which have other colors are termed chromoplasts.
5) The Centrosome (Centriole):
This is cytoplasmic particle that is more characteristic of animal than plant cells. It lies just outside nucleus and functions in cell division. It has high degree of internal organization. It is cylindrical structure with thorough structure that is like to that of cilia and flagella; in fact it is involved in formation of cilia and flagella in certain cells. Centriole has same 2 + 9 tubular structure found in cilia and flagella.
6) The Nucleus: This is the most important feature of cell under microscope. It is controlling centre of cell, in that it gives information to cytoplasm to kept it (the cytoplasm) functioning for indefinite period. A cell in which nucleus has been removed ultimately dies. For example, mature red blood corpuscles (erythrocytes) of man which lack nuclei are short-lived.
The vacuole is fluid-filled space inside a cell. Fluid (or cell sap) is solution of different salts in water. Animal cells generally have many minute vacuoles while in several plant cells a single large vacuole that takes up most of volume of cell is present. Vacuoles are formed at cell surfaces by the process of pinocytosis by which cell takes in substance in liquid form.
This is the solution of several substances, mostly protein in water. It is a transparent somewhat viscuous fluid that fills cell, and in which different structures are suspended.
Cells are extremely varied structures not only in size and shape, but also in structure and function. Therefore, it is relevant to always remember that there is actually o typical cell. Most cell sizes range from approx 1/10μm to 1mm. Bacterial cells are among the smallest and their sizes fall in range of 1/10μm to 1mm. Majority of cells which make up bodies of plants and animals fall in this group. Large cells like eggs and unicellular organisms like Amoeba and paramecium estimate up to 1mm while really large cells like ostrich eggs could measure many inches.
Cells are as uneven in shape as in size. They could be spherical like cells of coccus bacteria, or cubical like cells of cuboidal epitheha. Other shapes of cells are: cylindrical like columnar epithelia, rod bacteria, and parenchyma of palisade mesophyll of leaf; linear like fibres of different kinds - and host of others.
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