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The eukaryotic cells are found in most of the algae, fungi, protozoa and all the higher plants and animals.
Irrespective of their variable shapes and sizes, these cells display the following fundamental components (Fig 1.15):
1. Cell wall and Plasmalemma or cytoplasmic membrane,
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2. Cytoplasm and
3. Nucleus.
The eukaryotic cells contain extensive internal membrane systems such as endoplasmic reticulum (E.R.), as well as organelles surrounded by membranes, as for example, the mitochondria, plastids, lysosomes etc.
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These systems are incapable of internal movement and contain, in addition, a number of energy transducing systems such as mitotic apparatus, multi-stranded flagella and so on. The nuclei are delimited by nuclear membranes and contain nucleoli and more than one chromosome made up of DNA and histones.
The Cell Wall and Plasma Membrane:
There are two important structural components of the surface boundary of the cell:
(i) The outer layer, the glycocalyx, and
(ii) Inner component, the plasma membrane or cytoplasmic membrane. The glycocalyx is the general name applied to any structure which lies outside the plasma membrane.
Glycocalyx contains polysaccharides which are associated sometimes with some other substances as aminoacids, lipids, lignins, proteins, nucleotides, calcium phosphate and so on. Glycocalyx of plant and bacteria are familiarly known as cell wall. Plasma membrane, the outer boundary of cytoplasm i the living portion of the cell.
The Cell Wall:
The plant cell is always surrounded by a cell wall. The cell wall which differs from plasma membrane in being non-living is the additional layer synthesized by the living cytoplasm just on the external surface of plasma membrane. In most of the plant cells, the cell wall is made up of cellulose, hemicellulose and pectin.
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In many fungi the wall is formed of fibrous polysaccharides and chitin. In animal cells, the non-living cell wall is lacking, but in some a thin layer of mucopolysaccharide and cuticle is deposited just outside the plasma-membrane in the form of pellicle.
The cell wall is perforated at one or more places through which the cytoplasm of a cell is connected with that of adjacent cells. These cytoplasmic bridges interconnecting any two adjacent cells are termed as plasmodesmata.
These are of great physiological importance because they allow flow of cytoplasmic materials from one cell to the other. Between the walls of any two adjacent cells I intercellular substance is deposited in the form of middle lamella (Fig. 1.15).
In plants the middle lamella is formed of calcium pectate. The intercellular matrix cements the cells together cell wall to cell wall or pellicle to pellicle as the case may be. The middle lamella and outer cell wall or pellicle are emphasized as non-living because of the fact that they do not play important role in the life of cell.
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The cell wall or pellicle can be removed from a number of plant and animal cells by microsurgery and other techniques. The naked cells thus obtained after the removal of cell wall or pellicle are surrounded by plasma membrane only and are more liable to mechanical injuries. Though they remain alive yet they cannot retain their original shape and size.
In plants the cell wall can be differentiated into primary, secondary and sometimes tertiary layers. The secondary layer is deposited on the inner face of the primary membrane and the tertiary layer below the secondary one (Fig. 1.16).
Electron microscopy has revealed that the primary cell wall is composed of sub-microscopic strands of cellulose which in turn can be split into even finer threads by means of ultrasonic irradiations. These micro-fibrils of the primary wall layer are interwoven to form a very dense network (Fig. 1.17).
The micro-fibrils of cellulose in the secondary cell wall tend to lie in parallel fashion forming successive lamellae instead of forming a network. In the successive layers of secondary wall the orientation of the parallel microfibrils is changed.
At certain places the microfibrils of secondary wall materials are not deposited, resulting thereby depressions or perforations. Such characteristic perforations in the secondary walls are known as pits.
There are microcapillary spaces between the microfibrils of cellulose in the secondary wall. In these microcapillary spaces lignin, cutin, suberine, hemicellulose, minerals and some other wall materials are deposited which make the secondary wall tensile and sometime impermeable to water and gases (Fig. 1.18).
In general, the structure and the chemical constitution of cell walls will vary according to the physiology of plant tissue in which they are found. The cell wall provides rigidity and support to the cells. It also protects the protoplasm from mechanical injury and maintains the characteristic shape of cell.
The Plasma Membrane:
The plasma membrane is exceedingly thin and not visible as a separate layer. It appears merely as the surface layer of the cytoplasm. Earlier biologists doubted its appearance Existence of plasma membrane was proved when phenol-red dye was microinjected into the cytoplasm.
In the experiment it was seen that the dye coloured the whole of the cytoplasm and did not pass out c the cell. Further, when some colourless cells were placed in the dye, it was found that the dye could colour the cell wall only and not the cytoplasm.
The two observations suggest that there exists boundary layer in between the outer non-living cell wall and internal cytoplasm. This is called plasma; membrane or plasma lemma.
The plasma membrane is a living semi-permeable membrane covering the cell cytoplasm. It provide mechanical supports and definite shape to the cell. It delimits the protoplast from the outside environment and due to its semi-permeable nature it transmits necessary material into the cell and checks the entry of undesirable substances into the cell.
It is found to contain small pores through which exchange of molecules occurs.
