Read this article to get information about the structure of Eukaryotic Cell!
Many animal cells can be observed in an isotonic liquid, such as blood serum, aqueous humor or physiological salt solutions or in tissue culture.
They seem to be as irregular, transluscent masses of cytoplasm containing a nucleus.
Structurally, cell consists of mainly protoplasm, which is made of two parts:
(i) Cell wall and
Image Courtesy : web.uaccb.edu//03_04Figure-U.jpg
(ii) Cytosome.
[I] Cell wall:
It is the outermost bounding membrane, a secreted product of the cell, encircling the cytoplasm. Cell wall is absent in animals. In plants, this cell wall is hard, rigid non-elastic, and quite porous.
It consists of an external, thin primary wall and frequently a thicker, inner secondary wall. The primary walls of two adjacent cells are often separated by the middle lamella. The middle lamella is formed first. The middle lamella is secreted by the cell and is composed of viscous, gelatinous material, the pectin.
The primary wall is secreted against the middle lamella, consists of intertwined cellulose fibres, pectin and other substances. Secondary cell wall is secreted inside the primary wall and contains pectins and other noncellulose polysaccharides, and lignin which endows wood with its hardness and mechanical rigidity.
The cell wall gives protection and support to the plasma membrane and the cytoplasm which lies beneath it. The cell walls of some plant cells possess some pit-like or canal-like minute apertures through which delicate strands of cytoplasm, plasmodesmata, form bridges between adjacent cells. Plasmodesmata form much more intimate cellular connections.
In animal cells, the external covering is known as plasmalemma, cell membrane or plasma membrane. It is living, ultra-thin, elastic, porous, and semipermeable. It primarily gives mechanical support and external shape to the protoplasm, checks the entry or exit of undesirable substances and it allows the transmission of necessary materials to and from the cells.
Eukaryotic plasma membrane does not have enzymes of aerobic metabolism and does not possess the small projections on the inner side of the plasma membrane. It is composed primarily of lipids and proteins plus, in some instances carbohydrates. It consists of three layers—outer and inner layers are of proteins and the middle layer of lipids.
The main function of the plasma membrane is to control selectively the entrance and exit of materials. This includes entrance of water and large molecules by the process of endocytosis and the exit of cell products by exocytosis. Plasma membrane is covered and reinforced by the cell coat in animal cells.
Cell coat is made of glycoproteins, glycolipids and polysaccharides that may extend the thickness of cell membrane and continue far beyond it. It also contains enzymes and antigens and is fundamental in the association of cells in a tissue.
[II] Cytosome:
Inner matrix of cell is called the cytosome and constitutes two parts-cytoplasm and nucleus.
1. Cytoplasm:
It is the inner amorphous, transluscent, homogeneous colloid fluid (ground cytoplasm), containing refractile particles of various sizes. Frequently the peripheral layer of cytoplasm, the ectoplasm, also called the cortex, is relatively more rigid and devoid of granules. The internal cytoplasm is called the endoplasm, is less viscous than the ectoplasm and contains different granules.
The ground cytoplasm or matrix behaves, in general, as a reversible sol-gel colloid system. This property is genera thixis, touch + trope, a change.) It is also called the hyaloplasm and contains all the essential organelles of cell among of which the mitochondria are the most conspicuous.
The cytoplasm consists of various inorganic compounds (water, salts of Na, K, etc. and other metals) and various organic compounds such as carbohydrates, proteins, lipids, nucleoproteins, nucleic acids and enzymes.
The cytoplasm contains many protoplasmic strands, called trophoplasm. Besides these, the cytoplasm also contains non-living and living structures. The non-living structures are known as deutoplasm, paraplasm or inclusions and the living structures are called organoids (organelles) which are membrane bounded.
(a) Cytoplasmic inclusions:
The cytoplasmic inclusions are oil drops, yolk granules, pigments, secretory granules and glycogen granules. All these are found suspended in the cytoplasm in the form of refractile granules.
(b) Cytoplasmic organelles:
These are living structures, performing biosynthetic and metabolic activities. The cytoplasmic organelles are given as below:
(i) Endoplasmic reticulum:
It constitutes the bulk of the endomembrane system. It is made of tubules and flattened sacs. It is in the form of a single large inter-connected cavity bounded by a membrane. The membrane is believed to be originated from the inpushings of plasma membrane within the cytoplasm.
Thus, it has the same three layers as found in cell membrane. The endoplasmic reticulum membrane may either be smooth or rough due to the presence of ribosomes attached with the membrane. Ribosomes synthesize protein molecules which are delivered into the lumen or cavity of the reticulum.
The smooth ER is in continuity with the rough and is engaged along with the rough, in the transport of products within their cavities. Both parts of ER are involved primarily in secretion. The endoplasmic reticulum is found to be continuous with the plasma membrane and nuclear membrane.
It forms the ultrastructural skeletal framework of the cytoplasm, giving mechanical support to the cytoplasm. It also acts as an intracellular circulatory system, through it various substances flow into and out of the cells. Synthesis of lipids, cholesterol, glycerides, glycogen, etc., also occurs in the endoplasmic reticulum.
(ii) Golgi complex:
It is the most important constituent of cytoplasm which is especially developed in high metabolically active cells. It is a stack of flattened parallely arranged sacs and vesicles found in association of endoplasmic reticulum. It is composed of many lamellae, tubules, vesicles, and vacuoles.
Its membrane is made of lipoproteins and supposed to be originated from the membranes of endoplasmic reticulum. In plant cells, the Golgi complex is called dictyosome and it secretes necessary materials for the formation of cell wall at the time of cell division. It helps in the formation of acrosome of sperms, release of enzymes, hormones and other synthetic materials.
(iii) Lysosomes:
These are recently discovered tiny spheroid, cylindrical or irregular-shaped vesicles enclosed by a single membrane. They are originated from the ER and the Golgi complex and contain many digestive enzymes. They help in intracellular digestion of food material entering the cell by pinocytosis and phagocytosis, and also for the digestion of parts of the cytoplasm.
Peroxisomes are also bound by a single membrane. They contain enzymes related to the production and breakdown of peroxides (H2O2), and they fulfill a protective function in the cell.
(iv) Mitochondria:
These are rod-like or globule-shaped particles scattered in the cytoplasm. They are found singly or in groups. The mitochondrion is bounded by two membranes of lipoproteins—outer forms bag-like structure around the inner one and the inner membrane gives off many finger-like folds in the lumen, known as cristae.
These cristae partially subdivide the inner chamber of the mitochondrion, and their inner surface is covered with mushroom-like projections that are related to phosphorylation, a portion of the process of cell respiration.
Its lumen and the space between the two membranes is filled with a viscous mitochondrial matrix. Both the membranes and the matrix contain many oxidative enzymes and coenzymes. The mitochondria contain DNA molecules and ribosomes and hence they synthesize certain proteins. They are an important source of energy needed for respiration which they reserve in the form of adenosine triphosphate (ATP).
(v) Plastids:
These are characteristic of plant cells which are not found in case of animal cells. They may be coloured green like chloroplasts, or colourless like leucoplasts. The leucoplasts store and metabolize starch, and lipids and are hence called amyloplasts and lipoplasts respectively. The chloroplasts contain DNA, ribosomes and complete protein synthetic machinery.
The chloroplasts help in photosynthesis because they contain the green pigment chlorophyll, and protein storage. The chloroplast has a double outer membrane, a stroma pilled with many soluble enzymes, and a complex system of membrane-bound compartments. Chloroplasts contain a kind of semiautonomous genetic system with its own DNA. They also contain ribosomes and RNA molecules, and are able to synthesize some of their own proteins.
(vi) Metaplasm:
It represents other passive particles like vacuoles, granules and other cytoplasmic bodies such as ribonucleoprotein molecules.
(vii) Сentrosome:
It is the centre of the cell discovered by Van Benden in 1887. It is a clear zone around centrioles and also called microcentrum. It is found near the nucleus and includes a specialized portion of cytoplasm, called centrosphere. Its matrix is called as kinoplasm, in which two rounded bodies or centrioles are embedded.
Each centriole is open at both ends and each centriole consists of nine fibrillar units, and each fibrillar unit is found to contain three microtubules arranged in a circle. Both the centrioles are arranged at right angles to one another. The function of centrioles is to form the spindle of microtubules at the time of cell division. Plant cells lack centrioles, but the spindle is formed without their aid.
(viii) Cilia, basal bodies and flagella:
Cell may be covered by minute outgrowths called cilia, which are connected with cytoplasm by basal bodies or blepharoplasts. The cilia and flagella consist of nine outer fibrils encircling the two large central fibrils, and each outer fibril consists of two microtubules. The flagella also originate from the basal granules embedded in the cytoplasm.
(ix) Microtubules:
These are the ultra-fine tubules of protein (tubulin) traversing the cytoplasm of plant and animal cells. The walls of tubules are made of 13 individual filaments. Microtubules can change rapidly by a process of polymerization or depolymerization of the tubulin subunits. Microtubules facilitate changes of cell shape and displacement of macromolecules and organelles throughout cytoplasm. Their functions are to transport water and ions (small molecules), cytoplasmic streaming (cyclosis) and the formation of spindle fibres during cell division.
(x) Basal granules:
These are spherical bodies found at the base of cilia and flagella. Each is composed of nine fibrils and each fibril consists of three microtubules, out of which two enter the cilia or flagella.
(xi) Ribosomes:
These are minute spherical structures found attached with the membrane of the endoplasmic reticulum and in the cytoplasm. These originate in the nucleolus and consist of mainly the ribonucleic acids (RNA) and protein. Protein synthesis occurs in the ribosomes.
2. Nucleus:
Most conspicuous and lying usually in the centre of the cell is a rounded or ovoid body, called nucleus. It is delimited from the cytoplasm by its extremely thin membrane, called karyotheca or nuclear membrane, composed of two membranes having the nuclear pores.
Both these membranes merge at the pores, openings of which allow the transfer of materials between nucleus and cytoplasm. Inner membrane is in contact with the chromatin fibres, while the outer membrane is continuous with the membranes of endoplasmic reticulum and plasma membrane. The two nuclear membranes of the nuclear envelope are separated by a space known as perinuclear space.
The ‘pores’ of the nuclear envelope are filled with electron-dense proteinaceous substance called the annular material. Beneath the nuclear membrane is the nucleoplasm or nuclear sap which fills most of the nuclear space. It represents uncondensed regions of chromatin, i.e., nucleoproteins where the chromosomes are largely dispersed.
Embedded in the matrix (nucleoplasm) of nucleus (i.e., karyolymph) are chromatin, nucleolus, endosomes, etc. Nucleus controls all the vital activities of the cytoplasm and it carries the hereditary material, the DNA.
Nucleoplasm (karyolymph) contains dissolved phrbsphorus, ribose sugars, proteins, nucleotides and nucleic acids, and certain thread-like elongated chromosomes (Gr., chrome, colour + soma, body) which are in the form of ultra-thin chromatin threads before and after cell division. During cell division chromosomes become visible. The chromosomes consist of large molecules of deoxyribonucleic acid (DNA) wrapped in RNA and nucleoproteins.
Within the nucleoplasm also occurs a conspicuous spherical body, the nucleolus. It is very large in nerve cells, pancreatic cells and other cells and are very active in protein synthesis. The nucleoli are either single or multiple and usually acidophilic. It is composed of ribosomal proteins and ribosomal RNA.