Essay on Plant Cells | Botany

Here is an essay on the plant cell.

If a thin slice is taken from any part of a plant and observed under a microscope, it is found to be composed of many chambers or compartments resembling the chambers of a honeycomb. Each of these chambers is called a cell. A plant cell has usually an outer firm non-living boundary called the cell wall and a cavity which contains granular jelly-like protoplasm.

The organised mass of protoplasm of each cell is called protoplast. Protoplasm, as we knew, is the essence of life. It has been very aptly described by the famous nineteenth century biologist Thomas Henry Huxley as the ‘physical basis of life’.

Due to the presence of the distinct non-living wall the plant cells appear to be separate from one another. But this separation is by no means complete. Proto­plasm is a continuous living system. Through the small pores on the cell wall fine fibres of protoplasm, called plasmodesmata, pass to the contiguous cells, establishing the organic continuity of protoplasm.

A cell is defined as the unit of structure and function. It is a unit of structure more or less in the same sense as a piece of brick is a unit in a building. Plant-body is built up structurally of cells-. It is called a unit of function, because a cell contains a unit mass of proto­plasm which is solely responsible for all the vital activities.

In 1665 an Englishman Robert Hooke observed a thin slice of bottle cork under a microscope improved by himself. He noted many chambers and suggested the term ‘cell’, on the basis of their resemblance with the so-called cells or chambers of a bee-hive.

As bottle cork is a dead thing Hooke certainly noticed only the cell wall and not protoplasm. Italian Professor Marcello Malpighi (1628-94) and English physician Nehemiah Grew (1641-72) also carried on studies on cellular structures.

A theory, known as ‘cell theory’, was postulated by Matthias Jacob Schleiden (1804-81). a German botanist, and Theodor Schwann (1810-82), a German zoologist (Fig. 115), in 1838-39, who claimed that all living things, plants and animals, are cellular in nature, i.e. they are essentially made up of cells.

The announce­ment of cell theory was an incident of paramount import­ance in the history of our knowledge about cell. Early workers concentrated their attention practically on the distinct cell wall, possibly because the transparent proto­plasm escaped their notice.

A Frenchman Felix Dujardin appreciated the importance of cell contents to which he applied the term ‘sarcode’. Hugo von Mohl, a German botanist, was, however, the first man to recognise the importance of the living contents in the middle of the nineteenth century, and he gave it the name protoplasm.

Thomas Henry Huxley in 1868 described it as ‘the physical basis of life’. Hanstein in 1880 proposed the term protoplast for a cell.

Though even today we use the term cell as proposed by Hooke, present-day conception of a cell is that it is essentially a mass of protoplasm (protoplast) with or without the wall. In contradistinction to animal cells, the plant cells are usually walled.

Even in plant kingdom ‘naked’ cells, i.e. cells without walls are not altogether absent. Reproductive cells are usually naked. A cell is called dead, when it has lost its protoplasm like the cells of bottle- cork noted by Hooke.

Cells are so numerous and diverse that it is rather difficult to give a precise account of their shape and size. All shapes between round or circular on one hand and ‘elongate needle-like on the other, are possible.

The majority of them, however, are more or less isodiametric—round, elliptical, square, pentagonal, hexagonal, etc. (Figs. 116, 117). The average diameter of these cells ranges between 0.01 mm. to 0.1 mm., though there are cells much smaller and much larger in size; and average length of elongate cells ranges between 8 to 10 mm.

Some lower organisms are made up of single cells. They are called unicellular. But the vast majority of the plants have bodies built of innumerable cells. They are multicellular organisms.

A plant cell is considered separable into two distinct parts, viz.:

(i) The protoplast or the organised mass of protoplasm, and

(ii) The surrounding non-living cell wall.

(i) The Protoplast:

The organised mass of protoplasm present in a cell is called protoplast. Protoplasm is really the essence of life. It has very aptly been described by Huxley as the physical basis of life. The manifestations of life, such as metabolism, irritability, growth and reproduction are all due to mysterious activities of proto­plasm.

Physically protoplasm is a colourless, transparent jelly-like fluid, always saturated in a high percentage of water. Under the microscope it looks granular, due to the fact that a large number of granules of varying shape and size remains suspended in the watery background.

The amount of water may be as high as 98%. With the decrease of water protoplasmic activities also become more and more feeble; ultimately it comes to a standstill. Protoplasm contracts on application of heat, strong chemicals, electric shock, etc., and may again expand on removal of those external influences.

This power of contraction and expansion, referred to as ‘contractility’, is an inherent property of protoplasm. It is a colloidal system.

Chemical composition of protoplasm is very illusive. It can­not be analysed in the living condition. During the process of analysis it is killed. The problem becomes still more complicated, because protoplasm always undergoes chemical changes.

It is not a chemical compound; it can only be called a wonderful organi­zation in which various substances of diverse chemical nature remain dispersed in watery medium and constantly undergo chemical changes. It is, however, mainly proteinaceous in nature.

Proteins are composed of carbon, hydrogen, oxygen, nitrogen, usually sulphur and often phosphorus. Besides proteins, carbo­hydrates, fats and many inorganic salts enter into the composition of protoplasm. The protoplast usually possesses a highly specialised, dense, round or elliptical body called the nucleus; the remaining granular portion is termed cytoplasm.

a. Cytoplasm:

The extremely delicate outermost layer of cytoplasm lining the cell wall is known as plasma membrane (ectoplasm). It is of ultra-microscopic fineness and is semi-permeable in nature. This layer, which may be compared to the surface film of a drop of water, is of utmost physiological importance, because it controls the entrance and exit of fluids to and from the cells.

The remain­ing granular portion of cytoplasm is called endoplasm. In young 110 outlines of biology cells cytoplasm almost fills up the volume, but with the growth of the cells some distinct cavities, called vacuoles, are noticed in the cytoplasm.

Even young cells have vacuoles, but they are very small. In adult cells a number of vacuoles may be present or they may coalesce to form a large central vacuole, pushing cytoplasm with the nucleus towards the cell wall (Fig. 118).

In that case the lining protoplasm is called primordial utricle. Vacuoles are not empty but contain watery fluid with various matters in solution. This fluid is called cell sap. Thus vacuoles often serve as store­house of reserve foods and different by-products of metabolic changes of protoplasm.

Many vacuoles possess water-soluble pigments or colouring matters called anthocyanin. They give red, blue and purple colourations to many young leaves, flowers and fruits. The thin layer of cytoplasm lining a vacuole is called tonoplasm.

b. Nucleus:

The nucleus is the most highly organised part of the proto­plast. It controls or regulates all the activities and is thus of utmost importance. The nucleus is universally present in all plant cells excepting some lower plants where true well-formed nuclei may be absent, but they have corresponding nuclear materials. Nuclei are never formed de novo or afresh, but always originate from the pre-existing ones.

They are usually round or elliptical in shape. The shape of the cell has influence on the shape of the nucleus to some extent, so that in long cells the nuclei are also correspondingly elongated. The nucleus is quite prominent in young cells where it usually occupies the central position; but in an adult cell it is not so con­spicuous, because the nucleus does not correspondingly increase with the growth of the cell.

In an adult vacuolated cell it be present in the centre or pushed towards the cell wall with primordial utricle. Sometimes nucleus remains suspended by delicate cytoplasmic threads in the midst of a large central vacuole (Fig. 119). Whatever may be the position, the nucleus always remains associated with cytoplasm. It has a special protein, nuclein, in its composition which is rich in phosphorus.

Usually the cells of the higher plants are uninucleate, i.e. have only one nucleus in each cell. The presence of more than one nucleus or multinucleate condition, as it is called, is common in some lower plants. The size of the nucleus is quite variable.

The average diameter, however, ranges between 10 to 15 miora. (1 micron, written as µ, is the unit of micro-measurement. It is equal to 1/25000th part of 1 mm. or roughly 1/1000th part of an inch).

The nucleus has an outer delicate membrane, known as nuclear membrane, which demarcates it from the surrounding cytoplasm. The main-body of the nucleus is filled up with a dense fluid called nuclear sap or karyolymph.

Many crooked threads, called chromonemata, remain suspended in the karyolymph. Chromonemata are made of chromatin matters which have a strong affinity for taking up stains. Thus they are quite distinct in fixed prepara­tions.

They often anastomose to form a network referred to as chromatin reticulum. Chromonemata ultimately are trans­formed into the most important bodies, called chromosomes, which play a prominent role in cell division.

They are main­ly composed of a complex chemical deoxyribose nucleic acid (DNA) and a protein, and are chiefly concerned with heredity. It also encloses one or more highly stainable refractive bodies called nucleoli or plasmosomes (Fig. 120), which are largely the storage places of another nucleic acid—ribonucleic acid (RNA)

RNA also present in cytoplasm is concerned with protein synthesis. Functionally, the nucleus is of the highest importance. It is the controlling centre of all vital activities and is, therefore, called the brain of the cell.

It plays a prominent part in cell division which is indispensable for growth as well as reproduction. The chromosomes are regarded as the bearers of heredity character which are transmitted from the parents to the offspring.

c. Plastids:

Plastids are living inclusions of cytoplasm. They have distinct protoplasmic basis. In size they are much smaller than the nucleus, and a fairly good number of them occur in a cell.

They are uni­versally present in all plant groups with the exception of some lower plants like fungi and bacteria. Plastids are not formed anew but originate from pre-existing ones. The colourless ground sub­stance of the plastid is called stroma. Plastids are the colour- bearers of the plants.

According to the colours they bear, plastids are of three types, viz.

(i) Chloroplasts or green plastids,

(ii) Chromoplasts or plastids coloured other than green, and

(iii) Leucoplasts or colourless plastids.

Chloroplasts or the green plastids (Fig. 122) are present in the aerial parts of the plants which are exposed to sunlight. The green colour is due to the pigment, chlorophyll. Chloroplasts are usually small bodies, round, elliptical or disc-like in shape ranging between 4 µm and 6 µm in length. The peculiar star-shaped, spiral (Fig. 119) or reticulate plastids are found in some algae.

Chloroplasts carry on the most important function, photosynthesis or manufacture of carbohydrate food in the presence of sunlight, out of water and carbon dioxide gas. Thus they occupy the most strategic position in the living world. At first simple soluble sugar is formed which is converted into insoluble starch grains (assimilatory starch), only to be reconverted into sugar, with nightfall.

Chromoplasts (Fig. 121) are colour­ed other than green. The colour, ranging between red and yellow, is imparted by the pigments, xanthophyll and carotin. Chromoplasts are irregular in shape—rod-like, angular, forked, etc.

The function of chromoplasts in flowers and fruits is colour manifestation for attracting insects and animals for pollination and dispersal of fruits, but their function in some underground organs is rather obscure.

Leucoplasts or the colourless plastids are present in the organs which are not exposed to sunlight. They are small, often irregular in shape. Some of the leucoplasts, called amyloplasts, are con­cerned with the storage of food. They convert the simple soluble sugar manufactured by chloroplasts into their complex insoluble forms, starch grains (reserve starch), for future use.

Plastids are present even in the embryonic cells, when they are too small to be seen under the microscope. They are called plastid-primordia or pre-plastids, which multiply and mature with the growth of the cell to develop into different types of plastids. These are the precursors of all the plastids. Thus all the plastids are essentially same, as they have the same origin.

This is quite evident from the fact that one type of plastid is readily converted into another. The potato tuber has leucoplasts which are often converted into chloroplasts when exposed to sunlight. Some fruits nicely show conversion of plastids. When very young they have leucoplasts, with growth they become green possessing chloroplasts which are converted into chromoplasts with the ripening of the fruit.

d. Chondriosomes or Mitochondria:

These are small bodies universally present in the cytoplasm of plant cells. They are smaller than plastids and occur as rod-lets or gianules. The function of chondriosomes was not clearly known for a long time. Now it has been definitely established that they are responsible for cellular oxidation and liberation of energy; so they are aptly called ‘power-house’ of the cell.

Movement of Protoplasm:

Protoplasm is always in a state of motion.

Naked masses of protoplasm show movements of two types, viz.:

(a) Amoeboid movement, in which the protoplast moves slowly by alternate contraction and expansion like the small unicellular animal, amoeba;

(b) Ciliary movement, in which naked masses of protoplasts are provided with whip-like projections, called cilia, by means of which they can freely move, e.g. zoospores of algae, male gametes of fern, moss.

The cells of higher plants with distinct cell wall often exhibit movement of protoplasm. Ribbon-like leaves of Vallisneria (B.Patashaola) have elongated cells with large central vacuoles. Protoplasm lining the celling movements along the cell wall in a definite direction.

This is called rotating (fig 122).Circulatory is another kind of cells with more than one vacuole where the movements is irregular, i.e. not in a definite direction. Circulatory movements is founds in stamina hairs of Rhoeo (fig 123).

(ii) The Cell Wall:

Most of the plant cells have distinct rigid walls. Cell wall is regarded as a secretory product of the protoplast. Naked protoplast always runs the risk of injuries from external influences. For self- protection a thin delicate layer is formed, which is the primary cell wall. Besides protection, the cell wall delimits the proto­plasts, and gives definite shape, requisite strength and rigidity to the cell.

The cell wall is made up primarily of an insoluble carbohydrate, cellulose, with the formula (C_gH₁₀O₅)_n, where the value of V is not known. The thin delicate primary wall has pectin substances which ultimately change into calcium pectate.

New layers deposited by the protoplast on the primary wall are mainly of cellulose. The primary walls of the living cells are continuous, but for some minute breaks through which fine cytoplasmic fibrils, plasmodesmata, pass from cell to cell, connecting thereby the proto­plasts of the adjacent cells (Fig. 130).

During the growth of the cell wall generally the primary wall gets stretched and new cell wall materials are deposited on the inner-side of the primary wall in layers one after another. So the wall becomes considerably thick and stratified in appearance. This is called growth by apposition. Growth of the cell wall in surface area may be due to intercalation of new materials in between the parts of the original wall.

This type is known as growth by intussus­ception. The primary cell wall, lying bet­ween the secondary thickening of the adjacent cells, is clearly noticed and known as middle lamella. This middle lamella, composed of calcium pectate, is the cementing material which binds the cells in a multicellular structure (Fig. 132).

Secondary walls made of cellulose or modifications of cellulose are deposited either uniformly all over the primary wall or may be localised here and there. Due to localised thickenings the follow­ing patterns are formed, particularly in the water-conducting elements of plants which look very interesting under the micros­cope (Fig. 131).

(1) Annular or ring-like, when secondary matters are deposited in the form of rings on the inner side of the original wall.

(2) Spiral, when they are present in the form of spiral bands,

(3) Scalariform or ladder-like, when secondary matters are deposited in the form of bars, one above another, so that they look like so many rungs of a ladder.

(4) Reticulate or in form of a network, and

(5) Pitted, when the cell wall is almost uniformly thickened leaving some small un-thickened areas, called ‘pits’, here and there. Pits are pas­sages for facilitating movement of fluids, and so they are usual­ly formed in pairs on the two adjacent cells lying side by side. After the formation of pits the primary wall lying between them is called the closing membrane.

Pits are of two types:

Simple pits and bordered pits. Simple pits have equal deposition and look like so many holes on the wall in surface view. Bordered pits are those where the secondary depositions are unequal and bulge both sides, forming dome-shaped bodies.

Each is represented by two circles, smaller circle to indicate the opening at the mouth and the larger (which, in fact, is the border), indicates the cavity inside. The closing membrane in a bordered pit often swells up at the middle forming what is known as torus. Torus can change its position and thus regulate diffusion of fluids (Fig. 132).

It has been mentioned that the cell wall is mainly composed of insoluble carbohydrate, cellulose. Cellulose is thin, delicate, to some extent elastic and permeable to water. It is of much commercial value as linen, paper, artificial silk like rayon, etc., are manufactured from cellulose.

It turns blue when treated with chlorine-iodine solution and dissolves in strong sulphuric acid. According to the necessities of plants, cellulose often undergoes modifications when it is either transformed into other substances or those matters are deposited on existing cellulose walls.

Lignification:

Lignin is profusely deposited on the walls of the cells, specially of the woody parts of plants. During lignification of the wall protoplasm usually vanishes and the cells become dead. Lignin gives the necessary strength and rigidity to the plant organs to stand against external injuries. Lignified walls turn bright yellow when treated with acid aniline sulphate solution.

Cutinisation and Suberisation:

Cutin and suberin are fatty substances deposited on the wall to make them impermeable to water. Thus they serve as water-proofs. Outer walls of the epidermal cells are cutinised for checking evaporation of water. The layer formed by deposition of cutin is called cuticle. Suberin is present in the cork cells. The skin of potato tuber has suberised walls.

Mucilaginous Change:

Some seeds like Plantago (B. Ishabgul) have mucilaginous hairs on the walls which swell up quickly by absorbing water and can retain that water. Mucilage is also present in plants like China-rose, lady’s finger, Indian aloe.

Mineralisation:

Mineral matters, like silicon particles, oxa­lates and carbonates of calcium, are frequently deposited on the walls. Leaves of grasses and bamboos have rough surface due to strong impregnation of silica. Calcium carbonate crystals in form of cystolith are present in the leaves of India rubber and banyan. Calcium oxalate crystals are found in many plants.

Intercellular Spaces:

Young cells usually remain compactly arranged, so that spaces in between them are absent. During the growth of the cells some re-adjustments occur, and as a result, small spaces arise at the corners of the cells. These are called inter­cellular spaces (Fig. 136). Though usually small, often some of them become quite big. They are known as cavities. Intercellular spaces may originate by two methods.

In some cases during the growth the common walls between the adjacent cells split up and spaces appear. These are said to be formed schizogenously (schizo = split; genesis = beginning). Resin ducts of pine and secretory ducts present in the members of sunflower family are examples of schizogenic spaces.

The second type of intercellular spaces arises due to the complete dissolution of a number of cells. These are lysigenous (lysis = loosening; beginning) spaces. Large air-spaces of the aquatic plants and some monocotyledonous roots are the examples.