The following points highlight the three main components of animal cell. The components are: 1. Plasmalemma 2. Cytoplasm 3. Nucleus.
Animal Cell: Component # 1. Plasmalemma:
This is the outermost limiting membrane of the cell. The concept of plasmalemma developed long before it was actually seen. The structure of plasmalemma was revealed only after the use of electron microscope.
It is very thin, nearly 75 Å and contains a double layer of lipid molecules which remain sandwiched between two layers of protein (Fig. 4.4). The two lipid layers are 35 Å thick and each protein layer is of 20 Å in thickness.
The plasmalemma is extremely elastic and this elasticity is due to the folded nature of its protein molecules. The appearance of the cell surface depends upon the nature of the cell, i.e., whether it is free or in contact with other cells. Even when the cells are in intimate contact, they remain a few angstroms apart and the gap is filled up with extracellular material (ECM).
The plasmalemma is extremely unstable and is always reformed. The cell surface is perforated and the pores lead into canals. The lining of these canals is in continuation with the plasmalemma. These canals go deep into the cytoplasm and are associated with other components of the cell which also are lined by plasmalemma like lipoprotein.
The plasmalemma performs several’ functions which are listed below:
A. It separates the inner contens of the cell from the outer environment.
B. It acts as a selective semipermeable membrane to control the entrance and exit of different molecules. Sometimes the membrane, in addition to selection, helps in the entrance and exit by a system of active pumping.
The two functions, namely, the absorption and excretion of molecules are carried out in the membrane by the action of enzymes and an energy-rich substance called Adenosine triphosphate (ATP). The permeability of membrane depends upon several factors—age and metabolic state of the cell and the condition of external environment.
C. It helps to maintain the isotonic condition by regulating osmosis.
D. In addition to selective absorption of molecules, the plasmalemma may ingest solid or liquid substances by the following two processes—(1) Phagocytosis and (2) Pinocytosis (Fig. 4.5).
(1) Phagocytosis:
It involves ingestion of solid particles. This is done by the extension of plasmalemma and cytoplasm around the particle in the form of a pouch. The pouch finally detaches it from the outer membrane and is drawn inside as a vacuole, the wall of which is formed by the plasmalemma.
Immediately with the intake of such a vacuole in the cytoplasm, the plasmalemma of the projected ends unit and the membrane becomes continuous. Phagocytosis is common in certain protozoans and in the amoeboid cells of higher animals.
(2) Pinocytosis:
The phenomenon involves in the ingestion of liquid droplets in the same process as phagocytosis, the only difference is that the pouch is much smaller.
E. Plasmalemma is extremely sensitive and is responsible for the movement of the cell.
F. Plasmalemma may have specialised substances around it for giving mechanical support to the individual, e.g., chitin, keratin and celluose.
G. It is responsible for maintaining the shape of the cell.
Animal Cell: Component # 2. Cytoplasm:
In the light microscope, cytoplasm appears as a semi-fluid, complex, elastic and heterogeneous material. In some cells the cytoplasm is distinctly divided into an outer stiff ectoplasm and an inner fluid and actively mobile endoplasm.
The endoplasm contains various inclusions or organelles having diverse functions. The following structures are generally seen in the cytoplasm of all cells though it may be that some organelles are the characteristics of certain cells only.
(i) Ground Substance:
The cytoplasm contains a ground substance or hyaloplasm in which various organelles are dispersed. It includes, either in dissolved or suspended state, various reserve food granules, oil droplets and numerous thin fibrils.
(ii) Endoplasmic Reticulum or Ergastoplasm:
It is present as a system of membranous network in the ground substance of cytoplasm. These membranes begin from the plasmalemma pore and are structurally similar to plasmalemma. The membranes occur in pairs and thus form canals.
These canals are in continuation with various organelles and nucleus. The endoplasmic reticulum is found to occur in three forms—cisternae, vesicles and tubules. The cisternae are elongated, flattened and parallelly arranged. The vesicles are circular and the tubules have variety of shapes.
(iii) Ribosomes:
These are very small round bodies, which either remain scattered in the ground substance or are arranged in a linear fashion along the wall of the endoplasmic reticulum (Fig. 4.6). Ribosomes contain a special type of RNA.
Some of the ribosomes remain attached to a single messenger RNA molecule and are called polyribosome, on which proteins are synthesised. When isolated the ribosomes are often accompanied by bits of endoplasmic reticulum. This fraction is called micro-some.
(iv) Mitochondrial or Chondriosomes:
These small bodies are present in all cells excepting bacteria. The size varies from 0.2 µ to 8 µ. The mitochondria may be filamentous, rod-shaped, dot-like or oval. These mobile enzyme packets are usually evenly distributed throughout the cell. In certain cells, during synthesis, the mitochondria in a particular cell type is always constant.
The mitochondria often multiply but the exact mode of division is not known. In a living cell, these bodies can be stained by Janus green B. The study with electron microscope reveals that each mitochondrion is covered by a double membrane of protein and lipid.
The outer membrane forms the smooth covering and the inner membrane gives rise to ruffled projections towards the interior to give a shelf-like appearance called cristae (Fig. 4.7).
There are spherical granules in the cavity of the mitochondria which collect calcium and possibly magnesium needed for mitochondrial activity. The inner surface is filled up with a liquid, containing enzymes, mainly for cellular respiration.
In addition, it contains RNA, vitamins and co-enzymes. Recently, it has been discovered that mitochondria also contain some amount of DNA. Mitochondria remain in close association with other components of the cell and exchange necessary substances.
It is regarded as the power house of the cell and is involved in doing two important functions—oxidation and phosphorylation. Through oxidation it breaks down the carbohydrates, proteins and fats.
The energy thus released passes through a number of reactions into other phosphate containing molecules. This stored energy remains bound in Adenosine triphosphate or ATP. In this form it is transported by the mitochondria to a particular site of the cell where energy exchange is going on.
(v) Golgi Apparatus:
The Golgi complex was discovered by Camillo Golgi an Italian physician. It is best seen in cells having secretory functions. It is visible only after special treatment with osmic acid and silver salts. Its electron microscopic structure is like a collection of paired smooth surfaced membranes and small sacs with a fluid in their inner space (Fig. 4.8).
The Golgi vesicles are in contact with the cisternae of endoplasmic reticulum. Its function is not properly understood, but it is believed to be involved in the synthetic process and also involves in “packaging” of proteins produced in cells.
(vi) Vacuole:
The vacuoles appear as round bodies of different sizes. It is bounded by a plasmalemma like membrane. It contains water and various water-soluble substances. Some vacuoles may have special excretory functions and some may contain food particles. The general function of vacuoles is to regulate the osmotic pressure.
(vii) Centrosome:
This is seen only in the animal cells. Under a light microscope, it is seen to contain two small round central bodies called centrioles. Electron microscope reveals that each centriole is made up of nine groups of filaments which form a circle. Each group includes three filaments (Fig. 4.9). The centrosome plays an important part in cell division by organising the formation of spindle.
(viii) Lysosome:
Lysosomes are small mitochondrion-like bodies with densely packed granules and concentric membranous covering (Fig. 4.10). It contains certain enzymes like phosphatase which take part in the breakdown of different cell substances. These enzymes are collectively known as cathepsin.
These are released generally at the time when it is necessary to break down the cell itself. It is abundantly seen in the cells of amphibian larval tail, where considerable cellular breakdown occurs at the time of metamorphosis. These enzymes also help in the breaking of ingested cellular material.
(ix) Plastids:
These orgtanelles are the features of plant cells, but many lower animals, e.g., Euglena also possesses plastids (Fig. 4.11). The plastids which contain green colouring pigment, chlorophyll (chloroplasts), are responsible for trapping energy from sunlight to synthesise carbohydrate food.
Animal Cell: Component # 3. Nucleus:
The nucleus is the most important structure of the cell. When the nucleus is distinct, the cell is called eukaryotic cell, but in forms like bacteria where definite nucleus is lacking it is regarded as prokaryotic cell.
The nucleus is usually spherical but may be of other shapes too (Fig. 4.12). The size of the nucleus is usually proportional to the size of the cell. The nucleo-cytoplasmic ratio lies between 1 : 7 and 1 :10. Generally, one nucleus is present in each cell, but there are cells with more than one nucleus.
The nucleus consists of following parts— (A) nuclear membrane, (B) nuclear sap, (C) chromatin bodies and (D) nucleolus (Fig. 4.13).
(i) Nuclear Membrane:
Electron microscopy has revealed that nuclear membrane possesses structures identical to the structure of plasmalemma and other membranous structures of the cell. There are a number of pores on the surface of the nuclear membrane through which different substances may pass in and out of the nucleus. Each pore has a central knob like diaphragm, which regulates the expansion and contraction of the pore.
(ii) Nuclear Sap:
The nuclear sap or nucleoplasm is made up of a gel-like substance in, which different other structures of the nucleus remain suspended. It acts as a pool of various substances which are either used or produced by the different components of the nucleus. During division, dehydration of the nuclear sap occurs.
(iii) Chromatin Bodies:
These are present in the nuclear sap as beaded or thread-like structures. In a cell which is not dividing these bodies are seen only after suitable staining with the basic dyes. These bodies contain DNA, some amount of RNA, a protein called histone and certain enzymes such as alkaline phosphatase. During division the chromatin bodies condense and thicken to thread-like structures called chromosomes (Fig. 4.14).
The chromosomes are paired structures and are evenly distributed during ordinary cell division. The chromosomes contain genes which bear hereditary traits (Fig. 4.15).
The gene is not visible as a discrete body. The dark bands of the chromosomes (euchromatic regions), represent the sites of the genes. The chemical analysis of chromosomes has now established that it is the DNA part which works as bearer of hereditary characters.
(iv) Nucleolus:
Nucleolus appears as a dense spherical mass inside the nucleus. The number may be one or two. It is rich in RNA. The number of nucleolus is fixed per nucleus and for a given type of cell. The nucleolus contains network of dense material called nucleolonema and coarse granules called chromatin.
The nucleolus is visible only when the cell is not dividing and is believed to play an important role in the synthesis of ribosomal RNA. At the onset of division it dissolves. After division one particular chromo somal region organises it again.
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