Plant Cell: An Overview (With Diagram)

The below mentioned article provides an overview on the plant cell with diagram.

Living things arc composed of material units called cells. A cell is defined as a unit of structure and function.

A typical plant cell consists of the essential living substance, proto­plasm with the surrounding cell wall.

The latter is a secretory product of the protoplasm itself. Structu­rally, plant cells are more distinct, in contrast to animal cells, due to the presence of rigid non-living cell wall (Fig. 482). Protoplasm, the mysterious fluid, has been aptly described as the physical basis of life. The organised mass of protoplasm of a cell is known as protoplast. A plant cell may be considered separable into two parts—the protoplast and the cell wall.

But this separation is, by no means, complete. Very fine fibrils or threads of protoplasm pass through the minute perforations of the cell wall and connect the protoplasm of adjoining cells. These threads are called plasmodesmata. The organic continuity of protoplasm in a living body is thus established.

The use of the term cell has been associated with an interesting account of anatomical studies since a long time. Serious attempts to explore the internal organisation of the plants and animals started with the invention of the compound microscope by Jansens in the later part of sixteenth century, though there are evidences that even early ancients in India and Greece had some knowledge about tissues and organs. In 1665 an Englishman Robert Hooke, observed a thin slice of bottle cork under a microscope improved by himself and saw some compartments.

As each of them looked like a chamber of a honeycomb Hooke called it a cell (Latin cellula—a small apartment). His observations were published in his classical book ‘Micrographia’ in 1665 (Fig. 483). Obviously Hooke noted the more distinct cell wall.

Though the mystery of living protoplasm remained unknown, he men­tioned of some sort of ‘nourishing juice’ present in the cells. In the same century Italian professor Marcello Malphigi and English physician Nehemiah Grew carried on observations on the internal structures of the plants.

Protoplasm escaped their notice as well. Corti (1772) and Fontana (1781) saw something like living sap in plants. Dutrochet (1824), a French biologist, also worked on cellular structures and laid emphasis on the cells as individuals. Sometime in 1835 Dujardin in France appreciated the importance of the living substance in the cells of lower animals, to which he applied the term ‘sarcode’. Hugo von Mohl was the first man to recognise the importance of the living contents of the cell and the term protoplasm, meaning first-formed substance, was proposed by him.

Robert Brown observed some round bodies at the central regions of the cells in the skin of orchid leaves. He suggested that each of these centrally located bodies be called a nucleus. After the discovery of the nucleus two biologists, Matthias J. Schleiden, a German botanist, and Theodor Schwann, a German zoologist, postulated a theory known as the cell theory in 1838, which claimed that all living bodies are essentially cellular in nature, i.e. they are composed of cells.

They recog­nised the cells as the units and thus ‘primary agents of organisation. Von Mohl’s obser­vations about the importance of protoplasm certainly put the cell theory on a strong foundation. The announcement of cell theory was an epoch-making incident in the history of biological studies. Schultze in 1861 put forward the protoplasm doctrine, stating that all masses of protoplasm are the units of organisation and that protoplasm in general, is similar in all organisms.

The cell wall, on which the early workers put so much emphasis, practically shrank to the background and was considered of secondary importance. The cell was thought to be the mass of protoplasm with a nucleus, which would originate from a pre-existing cell, another generalisation made by Virchow. To this organised mass of protoplasm Hanstein in 1880 applied the term protoplast.

The protoplasm doctrine received support from many biologists. Huxley in his famous essay very aptly described protoplasm as ‘the physical basis of life’.

In the mean time the concept of cell had undergone considerable modifications and some biologists like de Bary, Sachs and Others severely criticised cell theory. In fact, they proposed a new theory called organismal theory.

The idea that a multicellular body is an aggregate of cells, did not appeal to the protagonists of the new theory. They were of opinion that a multicellular body was a continuous mass of protoplasm which was in­completely subdivided into small centres of activities—the cells, during the development of the body.

Thus according to this theory the cells are not to be regarded as the ultimate units or seats of activities, but the whole are to be considered the ‘primary agent of organi­sation’.

Then growth is not due to multiplication of elementary units, but due to formation of cells by differentiation within the entire mass of protoplasm.

It is really difficult to explain the coenocytic bodies like the members of the green algae Siphonales in terms of cell theory, according to which a nucleus with the surround­ing bit of cytoplasm is to be treated as a cell; but in the light of organismal theory the whole coenocytic body would be a single cell.

The term cell has been rather loosely used, some people included the wall and some excluded the outer cover. The present-day conception of a cell is that it is essentially a mass of protoplasm, the protoplast, with or without the cell wall.

In modern terms cell may be defined as “a unit of biological activity delimited by a selectively permeable membrane and capable of self-reproduction in a medium free of other living organisms” —Loewy and Siekevitz.

It has even been suggested that the term cell be abandoned and let it be replaced by protoplast. In plants cells like tracheids and sclerenchyma lose pro­toplasm with maturation and thus become dead. Here cell practically means the dead wall.

The compartments observed by Robert Hooke from bottle cork were also nothing but cell wall. It is, at any rate, desirable to retain the term cell which would include the protoplast and the surrounding wall, particularly in view of the intimate relationship existing between the two and establishment of protoplasmic connections between adja­cent cells through plasmodesmata.

Though vast majority of the plant cells are walled, cells without walls are not altogether absent. The reproductive cells are as a rule naked bodies. In lower groups of plants very often naked masses of protoplasm are noticed.

The cell is regarded as the basic unit—the common denominator of the innumerable variety of living forms. Recent studies with electron microscope revealed two plans of cellular organisation—-the simpler procaryotic and the more complex eucaryotic.

The procaryotes including the bacteria and the blue-green algae are small ones (0.05 to 3 µm) usually have primitive nucleus without membrane and also lacking in organelles like chloroplasts, mitochondria, Golgi bodies and lysosomes.

The genetic information here is located mostly in the single chromosome consisting of a double strand of DNA and that also without basic proteins—histones. Mitotic apparatus and nucleolus are also absent.

The procaryotic cells are surrounded by cell wall made of carbohydrates and amino acids. The plasma membrane lying directly beneath the cell wall often forms intrusions into cytoplasm—the mesosomes.

The eucaryotes are the cells of remaining groups from the algae to higher plants showing high degree of organization. They possess extensive membrane system—endoplasmic reticulum, Golgi bodies, well-formed nuclei, plastids, mitochondria and lysosomes. Nucleoli and more than one chromosome with DNA and histones are present.

The starting point in the life of every individual is a single cell. If it continues as such the organism remains unicellular, as in many lower plants like yeasts, diatoms, desminds, etc.; but in large majority of plants the cell goes on dividing, so that ulti­mately a distinct multicellular body is formed.

In fact, the cellular complexity of an organism is based on the systematic position of the plants in the line of evolution. The cells exhibit wide diversities as regards their shape, size and arrangement. As a mass of jelly-like protoplasm tends to become round due to physical reasons, free cells are usually spherical in shape (Fig. 484A).

With growth and differentiation cells attain diff­erent shapes. Two chief types, however, are polyhedral with diameters more or less same, and elongate. All transitional forms between the two extremes are possible (Fig. 484).

In case of young cells of same size and age the shape becomes polyhedral due to mutual
contact and compression. They may be fourteen-sided (Fig. 485) approaching in shape the mathematically determined form tetrakaidecahedron.

The size is closely related to shape. The average diameter of a common polyhedral cell ranges between 0.1 to 0.01 mm though there are cells much larger and much smaller than that, so much so that it may be even 1 mm in diameter.

Very small cells are not common in higher plants. The fibres are often of unusual length. In some monocotyledons and dicotyledons of family Urticaceae the fibres may have length as much as 550 mm. On the average, the length of the fibres in higher plants ranges between 1 to 8 mm.

It has been stated at the outset that a plant cell has two parts—the living protoplast and the surrounding non-living cell wall. Protoplasm is the seat of all physiological activities of a cell. The wall is of secondary importance, as it is a secretory product of the protoplasm. Nevertheless, there exists an intimate relationship between the two.

The presence of the wall not only makes the plant cells structurally more distinct, but has other advantages too. Apart from serving as a protective membrane against loss of water, interne heat, attacks of insects, fungi, bacteria, it is positively helpful in delimiting the physiological units and adding to the mechanical strength of the organs.

The shapes of the cells are determined by the walls. At any rate, protoplast is much more important part of the cell, and it should be considered first.