Definition of a Plant Cell: A cell is defined as the basic structural and functional unit of a living organism. According to Lowey and Siekevitz (1963, ‘69) cell is a “unit of biological activity delimited by a semipermeable membrane and capable of self-reproduction in a medium free of other living systems”.
According to Esau (1965) the cell may be defined as “a protoplast with or without a nonliving envelope, the cell wall, and consisting of protoplasmic components and of non-protoplasmic materials, the latter intimately connected with the vital activities of the protoplast”. For convenience the term cell, in plants, is also applied to the dead remains of a cell consisting mainly of cell wall.
Types of cellular organization
Living organisms i.e. plants and animals are composed of cells; some of which consists of only one cell, they are called unicellular. But the vast majority of plants and animals have bodies built of many cells—hence they are multicellular.
The recent development of electron microscopy has made it clear that two basic types of cellular organisation occur e.g. prokaryotic and eukaryotic. Organisms having prokaryotic cell structure i.e. prokaryotes include the bacteria and the blue-green algae—their cells are small (0.5-3u), lacking a membrane around the nucleus and clearly defined membrane-limited organelles such as mitochondria, plastids, Golgi bodies etc.
The eukaryotes are the cells of remaining groups of algae, fungi, and of all higher plants and animals. They contain extensive internal membrane systems such as endoplasmic reticulum as well as organelles surrounded by membranes such as the distinct nucleus, Golgi bodies, plastids, mitochondria etc.
The flowering plants, with which this portion is mainly concerned are multicellular and eukaryotes. This multicellular body is formed as a result of the process of differentiation of the zygote cell.
Diversity of cell types (shape and size) in higher plants (eukaryotes)
Mature cells of multicellular higher plants vary greatly in size and shape—it has been found that the mechanical forces determine form of cells to some extent, but the function a cell performs is also another factor in the shaping process. Cells may be spherical, oval or oblong, polygonal, star-like stellate, cylindrical, fusiform, elongated fibre-like etc.
As the cells grow there is continuous formation of cellulose. Cells become of various forms by irregularity of growth due to internal causes bringing about greater possibility of stretching. When the growth is more or less uniform in all parts of the cell wall, we get a spherical or rounded cell ; but when it is greater at the two extremities than at the sides, the form is elliptic or oblong.
In both the cases, the cells are almost or entirely free from external pressure. But under other circumstances, in consequence of the mutual pressure of the surrounding cells, they assume a polygonal form; the number of the angles or faces depending upon the number and arrangement of the contiguous cells. The basic shape of cells is a 14-faced polyhedron (Matzke, 1946), but plant cells with 12, 13, 15, 16 or more faces are also found. According to Matzke, most of the faces of the cell wall are pentagonal, but tetragonal and hexagonal faces can also be found.
When growth takes place on all sides of the cell wall, but certain spots are more extensible than the rest, the internal pressure causes protrusions at those regions so that cells acquire a more or less regular star-like shape—such cells are called stellate.
When the growth takes place chiefly in one direction, the cells become elongated, either horizontally or vertically. Vertically elongated cells may be cylindrical and fusiform in form; by the mutual pressure of contiguous cells, the latter often become prismatic.
When the cell has attained its full size or in some cases while it is still growing in surface, its wall becomes thickened by the deposition of successive layers over those already formed. A transverse section of many cell walls shows a traces of this mode of thickening, the successive layers appearing as shells of substance lying one upon another. Such a cell wall is said to be stratified.
Viewed longitudinally, the walls often appear covered with delicate oblique striations which may run regularly in one direction only, or may be crossed by others. Such striations can be well observed in many of the elongated cells or fibres which form part of the soft portions of the vascular bundles of higher plants.
Figure 1.1 shows a number of cell shapes from various parts of higher plants—each type varies a great deal in structure and function. These are the undifferentiated isodiametric parenchyma cells of the growing regions of the plant body, the vertically elongated tube like sieve tubes of phloem with their special properties for food transport, the vertically elongated pipe-like broad tracheae (vessels) of the xylem for the transport of water and mineral salts, and the laterally elongated root hairs adapted for water and mineral absorption.
The pointed stinging hair cells, glandular hair cells etc. are defensive mainly; the semilunar guard cells are photosynthetic which are related to the closing and opening mechanism of the stomatal pore for gaseous exchange.
It is to be mentioned, therefore, that the shapes of cells are related both to the body plan of the plant and to the varied activities the plant performs in order to live.
The size of cells, like their shape, varies widely. Among higher plants, extremely small cells do not occur. Parenchyma cells with normal protoplasm have a transverse diameter of 0.01 to 0.1 mm. In pith and fleshy fruits, the diameter of parenchyma cells may be upto 1 mm or more.
In angiosperms, phloem and wood fibres range in length from 1 to 3 mm. Fibres of extensive length i.e. 20 to 550 mm occur in Urticaceae and in some monocots. The largest cells known are latex cells of the type which form branching system throughout the plant body.
Plant cell structure
Does a plant cell have a cell wall?
A typical plant cell consists of a centrally situated separable mass or unit—the protoplast and the surrounding membrane or wall known as the cell wall. In plants therefore, the term cell includes the protoplast together with the wall.
(a) CELL WALL
It is the non-living boundary wall of a cell, mainly composed of cellulose but often chemically altered by other protoplasmic secretions. Cell wall encloses a space known as cell cavity or cell lumen in which protoplast occurs.
Plant cell wall may be a primary wall only or may comprise both primary and secondary walls; depressions or pits may be present in the cell wall and the wall may be traversed by cytoplasmic strands known as plasmodesmata.
Functions:- i) Cell wall gives a definite shape to the cell, (ii) It protects the protoplasm from external injury and (iii) allows water and mineral salts to pass through it.
(b) THE PROTOPLAST
The protoplast is the organised mass that lies within the wall. The constituents of the protoplast are divided into two groups, viz. (I) protoplasmic components i.e. protoplasm and (II) non-protoplasmic components.
To the protoplasmic component belongs the cytoplasm, the living basic substance of the cell in which other specialised protoplasmic organelles such as endoplasmic reticulum (ER), nucleus, plastids, mitochondria, ribosomes, spherosomes, Golgi apparatus or complex (i.e. dictyosomes), microbodies, microtubules, centrosomes (in the cells of some thallophytes) etc. are located as living cell inclusions. To the non-protoplasmic component belongs the (i) ergastic substances i.e. non-living cell inclusions like reserve food materials, excretory materials, secretory materials etc. and (ii) the vacuole (Esau, 1965).
I. Protoplasmic components in plant cell
Cytoplasm comprises the living, hyaline, jelly-like and viscous transparent semi-fluid portion of the protoplast. It forms a granular ground substance in which nucleus, plastids and other cell inclusions, both living and non-living, are embedded.
Cell plasm i.e. cytoplasm is the substance which fills up the space between the nucleus and the cell wall. Sometimes, the cytoplasmic portion of the protoplast is called cytosome which literally means the ‘cell body’.
Cytoplasm is a very complex structure both from physical and chemical point of view. It contains various organic and inorganic substances; those substances may occur in various states such as colloidal condition, true solutions and crystals.
The hyaline ground substance of the cytoplasm is called hyaloplasm. The non-granular outermost membrane-like layer of cytoplasm is called ectoplasm, plasma membrane or plasmalemma. The plasma membranes are characterised by their selective permeability to the passage of different substances through them. The granular general mass of cytoplasm is known as endoplasm. The layer of cytoplasm surrounding or bounding the vacuole is termed tonoplasm or vacuolar membrane.
On the basis of the difference in viscosity, the cytoplasm is often differentiated into two regions, e.g. (1) plasmagel — it is the much more viscous portion of the cytosome forming a cortical layer of variable thickness just beneath the plasma membrane or ectoplasm and (2) plasmasol — it is the more fluid portion lying further inside.
The cavities or vacuoles formed within the cytoplasm are filled up with liquid, called the cell-sap. The cell-sap is watery and non-protoplasmic liquid containing various substances either in solution or in colloidal condition or in crystals viz, inorganic salts, carbohydrates, proteins, colouring matters etc.
In a young cell, cytoplasm completely fills up the space. As the cell grows, the growth of the cytoplasm does not compete with the growth of the cell wall, thereby small cavities are formed within the cytoplasm—these are called vacuoles.
Finally in adult condition, all the vacuoles fuse with one another forming a big vacuole in the centre of the cell. Thereby the cytoplasm together with all inclusions is pushed towards the inner surface of the cell wall forming a thin lining layer—this thin lining layer of cytoplasm is called primordial utricle.
(ii) Endoplasmic reticulum (ER)
Previously the cytoplasm was considered to be structureless, but with the help of electron microscope an intricate membranous structure within the cytoplasm has been discovered. That structure is known as endoplasmic reticulum which consists of two lipoproteinaceous unit-membranes forming a canal-like anastomosing system.
Generally, three main types of endoplasmic reticulum are found e.g. long flattened units called cisternae, round units called vesicles and tube-like irregular units called tubules. The endoplasmic reticulum may be smooth (non-granular) or rough (granular)—the latter type bears ribosomes. Sometime the ER occurs in the cytoplasmic strands, plasmodesmata traversing the cell wall of neighbouring cells. The portion of ER situated in tubular form in the centre of the plasmodesmata is called desmotubule.
The main function of endoplasmic reticulum is to increase surface area within the cytoplasm for various metabolic activities, it also helps in translocation of metabolites and their storage within the vacuoles. It also plays an important role in cell wall formation, and also establishes the plane of cell division by the provision and transport of materials to be incorporated in the cell plate (Cutter, 1978). There are also views that the ER gives rise to membranes of Golgi bodies and microbodies.
Membranes of more or less similar structure are also found on the external surface of the cytoplasm i.e. ectoplasm and on the layer bordering the vacuoles i.e. tonoplasm.
The nucleus is a highly organised globular, ellipsoidal, spherical or disc-shaped protoplasmic body enclosed within the cytoplasm. The density of nucleus is greater than that of cytoplasm.
The nucleus generally lies in the central position of the cell and occupies a considerable proportion of the volume of the cell, sometimes as much as 2/3rd or 3/4th—but this proportion becomes less during differentiation. Due to the development of a big vacuole in the centre of the cell the nucleus moves to the periphery. Nucleus is always formed from the pre-existing one.
According to the number of nucleus present in a cell, a cell may be uninucleate, binucleate or multinucleate. (a) Uninucleate — One nucleus is present in a cell, it is very common. (b) Binucleate — Two nuclei are present in a cell e.g. some tapetum cells of anther. (C) Multinucleate — Many nuclei are present in a cell, e.g. some cells of fungi.
A. Structure of Nucleus
(a) The nucleus is surrounded by a thin fine protoplasmic membrane called nuclear membrane or nuclear envelope which separates the nucleus from the cytoplasm. Under electron microscope, the nuclear membrane appears as a double membrane structure having pores at regular intervals. The nuclear membrane is connected with the endoplasmic reticulum. The contents of the nucleus merge with the surrounding cytoplasm through the pores of the nuclear membrane.
(b) The cavity of the nucleus is filled up with a colourless, jelly-like and non-staining or slightly chromophilic fluid (often referred to as electron-dense chromatic material) called nuclear gel, nuclear sap, karyolymph or nucleoplasm. Nucleoplasm resembles the cytoplasm but is often of a different density. Nucleoplasm consists of a pattern of irregularly shaped particles or granules and is essentially protein in character.
(c) Within the nucleoplasm, a network or reticulate structure known as nuclear reticulum, karyotin network or chromonemata is present in dispersed state. The delicate threads of the nuclear reticulum i.e. chromonemata are the constituents of the chromosomes.
This thread-like reticulum of the chromosomes at the interphase stage (i.e.at the resting stage) of the nucleus is also referred to as chromatin which means an intensely staining substance, because of the affinity of the complex of DNA and protein (present in the chromosomes) to the basic dyes. At this stage condensed and deeply stained chromatin masses are often seen in the nucleoplasm—this chromatin is called heterochromatin, but the chromatin which stains less deeply than heterochromatin is called euchromatin.
Chromosomes consist of nucleoproteins in which the nucleic acid component is mainly DNA (deoxyribonucleic acid)—the carrier of genetic information in the form of segments called genes. In addition to DNA, chromosomes also contain proteins including histones, as well as lipid substances.
(d) One or more spherical, thick, prominent and highly refractory bodies are also found in dispersed conditions within the nucleoplasm of each nucleus, those bodies are called nucleoli (singular: nucleolus). Nucleoli are formed during the telophase stage of nuclear division in association with specific regions of specific chromosomes of the complement.
Nucleoli are not bounded by membranes. Nucleolus generally stains differently from the chromatin and is composed of a mixture of ribonucleic acid (RNA), protein and lipid. The function of nucleolus is not yet definitely known.
B. Function of Nucleus
The nucleus controls the development of the cell by means of its DNA which can regulate the synthesis of a type of RNA known as messenger RNA. Therefore nucleus controls all the vital functions of the cell. Experiments involving nuclear transplants have shown that the nucleus is not autonomous, but has an intimate relationship with the cytoplasm. Through the nucleus parent characters are transferred to the offspring.
(iv) Plastids in plant cell
These are also protoplasmic bodies, denser than cytoplasm, variously shaped and are separated from the cytoplasm by definite membrane system. Plastids develop from similar primordial organelles. One type of plastids may change into other.
On the basis of the presence or absence of pigments, plastids are classified into two major types such as non-pigmented and pigmented. Non-pigmented i.e. colourless plastids are called leucoplasts, found mainly in the cells of underground organs and in the meristematic cells. Leucoplasts include amyloplasts (which synthesize starch), elaioplasts (which synthesize fats and oils) and proteinoplasts (which synthesize protein).
Pigmented plastids consist of:
(a) Chloroplasts or green plastids — bear green pigment chlorophyll, found in the cells of green plant parts e.g., stem, leaves etc.
(b) Chromoplasts or coloured plastids other than green i.e. yellow, orange, red etc. and contain the pigment carotene, found in the cells of flowers, fruits etc.
(v) Mitochondria or Chondriosomes
Do plant cells have mitochondria?
Several protoplasmic organelles smaller than plastids are found to occur in the spaces of the ground cytoplasm—these are known as mitochondria (singular, mitochondrion) or chondriosomes (grain-like bodies).
Mitochondria vary greatly in shape. Through the light microscope, they appear as small granules, rods or filaments and sometimes lobed. They are about 0.5 µm in diameter and upto 6 µm in length. In living cells, mitochondria may be identified easily by means of the Janus-green β-stain.
Each mitochondrion has a double layered membrane, an outer limiting one and an inner one. Such membranes are often provided with numerous granular structures. On the outer membrane, stalkless globose granules are present, while inner membrane bears stalked granules. The inner membrane-layer is extended forming tubular projections or microvilli, or infolded to form finger-like intrusions into the matrix called cristae (singular, crista).
The matrix of a mitochondrion is mainly composed of protein. Ribosomes are present and are slightly smaller than those present in the cytoplasm. In some regions of the matrix DNA fibrils occur.
The origin of mitochondria takes place by fission of existing mitochondria (Cutter, 1978). The mitochondria, according to many authors, are proplastids. Some authors suggest that the origin of mitochondria takes place either from the surface layer of plasma membrane or from the endoplasmic reticulum.
Mitochondria are present abundantly in young cells and are always formed from pre-existing ones. They are produced by division and are passed on from generation to generation through the gametes.
Proteins and lipids are the main chemical constituent of mitochondria; mitochondria also contain enzymes which play important role in respiration. Mitochondria contain some DNA and apparently also a small amount of RNA (Bell, 1965).
Function of mitochondria
Now-a-days it is believed that mitochondria are concerned with processes of energy conversion and therefore they are called the powerhouses of the cell. They are the locus of many enzymes in the cells, especially those of Krebs cycle, thus mitochondria are concerned with enzyme action and respiration.
(vi) Dictyosomes (Golgi apparatus in animal cells)
Golgi body-like cytoplasmic inclusion i.e. Golgi apparatus have been reported in plant cells with the help of electron microscope, e.g. root tip cells of Allium sp., trichoblast of Hydrocharis morsusranae (Cutter, 1978), root cells of Pisum sativum (Wilson and Morrison, 1967) etc. The number of Golgi apparatus in each cell may vary from one to many.
Golgi apparatus of plant cells consists of a system of dictyosomes occurring throughout the cytoplasm (Cutter, 1978). A dictyosome consists of a small stack (from 2-20) of smooth double membrane-bound flattened sacs (cisternae) of varying size, which are often dilated at ends; these are arranged in parallel rows, and are associated at the margins with a number of spherical, larger and smaller vesicles (microvesicles).
Functions — The dictyosomes are now known to function in the secretion of sugar (in nectar secretion), polysaccharides (cell wall materials), and polysaccharide-protein complexes (Fahn, 1982); the function of dictyosomes in the synthesis and transport of polysaccharides has also been demonstrated experimentally by combining autoradiography with electron microscopy (Cutter, 1978). Dictyosomes are also related to wall formation (Mollenhauer et al, 1961).
In animal cells some minute cytoplasmic bodies are universally present near the nucleus called centrosomes. Centrosomes are also found to occur in cells of some lower plants e.g. Dictyota, Fucus, Yeast cells, etc. (Sharp, 1943 ; White, 1973), but not yet found in seed plants.
In the centre of the centrosome a deeply stained granule is present called centride or centriole. This centride is embedded within a mass of hyaline substance known as centrosphere. During cell division, around the cytoplasm, some radiating rays are formed—these rays are known as astral rays.
(i) to take part in spindle formation during cell division and (ii) to help in cilia formation.
Ribosomes are small, sub-spherical particulate components of the cytoplasm; they are often found connected to the outside of the endoplasmic reticulum. Ribosomes also occur free in the cytoplasm and in the nucleus, chloroplasts and mitochondria (Cutter, 1978).
These organelles have a diameter of about 10-15 nm and are composed of 15% RNA and 50% protein (mainly histone)—for this composition ribosomes are often called ribonucleoprotein (RNP) particles.
Functions — Protein synthesis and fatty acid metabolism are generally carried out by ribosomes. Clusters of ribosomes known as polyribosomes or polysomes may be the actual structures associated with protein synthesis.
These are small granular organelles which occur free in the cytoplasm and are highly mobile in living cells.
Spherosomes have a diameter of about 0.25-1 micron and consist of lipids and protein (Esau, 1965). According to Gunning and Steer (1975), the majority of spherosomes consists of lipid droplets which are not bounded by a membrane.
According to some investigator the spherosomes are surrounded by a membrane, whereas according to others a surface skin consisting of an outer layer of oriented lipid molecules is formed in response to aqueous cytoplasm surrounding them (Fahn, 1982).
Spherosomes were called formerly microsomes (Perner 1958), these organelles are probably associated with lipid synthesis and they originate as oil containing vesicles detached from the ER.
These are small bodies, 0.5-1.5 µm in diameter; microbodies occur in the cytoplasm of a variety of tissues, they are bounded by a single membrane and their matrix appears fibrillar or granular.
There are two types of microbodies, such as peroxisomes and glyoxysomes—they differ mainly in the enzymes present in them. Peroxisomes occur in leaves of higher plants, often closely associated with chloroplast envelope; they contain most of the enzymes for the glycolate pathway from the photosynthetic carbon cycle.
Glyoxysomes occur in the storage tissues of fatty seeds, they contain enzymes necessary for the breakdown of fatty acids to acetyl-CoA and the synthesis of succinate from acetyl-CoA.
(xi) Microtubules and Microfilaments
Microtubules are straight, elongated, hollow structures having a diameter of 23-27 nm; these are composed of globular protein subunits called tubulin.
Microtubules are present in the peripheral cytoplasm close to cell wall still growing in area and thickness, in mitotic and meiotic spindles, in the phragmoplast arising between the daughter nuclei at the telophase stage etc. It is supposed that the microtubules might be responsible for orientation of developing microfibrils.
Microfilaments occur in some plant cells, especially components of vascular tissues. They are involved in the control of cytoplasmic streaming.
II. Non-protoplasmic components in plant cell
Vacuoles (derived from a Latin word vacuus which means empty) are cavities present within the cytoplasm; they occupy more than 90% of the volume of most mature plants.
Each vacuole is filled with a liquid i.e. the cell sap and is surrounded by a membrane called tonoplast. The vacuoles appear colourless or pigmented in sections of living tissues, but in well-fixed material they look as clear areas bounded by the stained cytoplasm i.e. tonoplasm.
The composition of cell sap may vary in different cells and even in different vacuoles of the same cell. The main component of the cell sap is water—various substances either in true solution or in colloidal state are present in such water. Sugars, salts, proteins, organic acids and other soluble compounds, phosphatides, tannins, flavonoid pigments, calcium oxalate etc. are present in plant vacuoles. Some substances (e.g. tannins, protein bodies etc.) in the vacuole may occur in solid form and may even be crystalline.
It is to be noted that, the materials present in the vacuoles are classified as ergastic—they are either by products of metabolism or they are reserve substances which may be utilised by the protoplast for vital functions. The vacuolar sap is more or less viscous—this viscosity is probably due to colloidal nature of the sap which sometimes may appear in the form of true gels.
There are two types of vacuoles with regard to pH—the relatively alkaline types of vacuoles stain reddish orange with neutral dye, while acid types stain bluish-magenta with the same dye. The concentration of the vacuolar sap varies, a substance may crystallize out if that accumulates beyond its saturation point.
The size and shape of vacuoles vary according to the stage of development and the metabolic state of the cell. Meristematic cells possess often numerous and small vacuoles. With growth and differentiation of a cell the vacuoles enlarge and fuse forming one single vacuole which occupies the central part of the protoplast—as a result the protoplasmic materials are restricted to a peripheral position i.e. next to the cell wall.
There are various views regarding the origin of vacuoles: according to one vacuoles arise (1) from pre-existing vacuoles which multiply by fission, and after cell division can daughter cell receives a number of vacuoles : (2) some workers think that vacuoles arise from Golgi vesicles; (3) another view is that vacuoles originate by a de novo process, by attraction of water to a certain localised region in the cytoplasm and the formation of membrane around it ; (4) there is another hypothesis according to which vacuoles arise by dilation of endoplasmic reticulum, cisternae or vesicles derived from endoplasmic reticulum.
The vacuoles function in storage, in digestion and in regulation of the water and solute content of the cell (i.e. osmoregulation). The vacuoles contain digestive enzymes that can break down cytoplasmic components and metabolites (Fahn, 1982), although some vacuoles may completely lack digestive enzymes.
Ergastic matters i.e., Non-living cell contents
Besides the protoplasmic substances i.e. organelles, several other non-protoplasmic i.e. non-living matters as by products of metabolism are also found within the cell. Various kinds of solid organic and inorganic materials, including food materials, mineral crystals, oils, gums, resins, mucilages tannins, alkaloids, rubber etc. often occur in the cytoplasm or in the vacuoles. Those subtances are termed as ergastic matters. They are found mainly either in the form of granules, droplets or crystals.
Ergastic matter occurs in the vacuoles and in the cell wall, they may be also associated with the protoplasmic components of the cell. When present in the vacuole, these substances are held in solution in the sap.
Ergastic matters are divided into three groups, viz. (i) Reserve materials, (ii) Secretory materials and (iii) Excretory materials.