Let us make an in-depth study of the ultrastructure and functions of a typical eukaryotic plant cell.
A typical eukaryotic plant cell is usually spherical, polyhedral, box-like or elongated in shape with a diameter of about 0.01 mm to 0.1 mm.
It consists of three parts:
(A) Cell Wall
(B) Cytoplasm, and
(C) Nucleus.
Cytoplasm and the nucleus together are called as protoplasm.
(A) Cell Wall:
The cell wall gives a definite shape and provides protection to the protoplasm. It is nonliving in nature and is permeable.
Cell wall consists of three parts:
(i) Middle-lamella
(ii) Primary wall and
(iii) Secondary wall.
(Cell wall formation begins at the telophase stage of the cell division. Fine granular structures called as phragmoplasts appear at the equatorial plane which condense to form cell-plate or middle-lamella. After the formation of middle lamella which is common between the two newly produced cells, primary and secondary and sometimes tertiary wall layers are laid down inner to middle lamella. These wall layers increase by apposition and intussusception methods. Some places on the cell wall remain un-thickened and are called as pits (Fig. 1.1). Through the pits pass cytoplasmic strands which are known as plasmodesmata.)
Each plasmodesma contains a narrow tubule called desmotubule which connects endoplasmic reticulum (ER) of adjacent cells. Continuity of protoplasm through plasmodesmata forms the symplast. Cell wall space outside the protoplast constitutes apoplast. The latter includes the cell walls, intercellular spaces and non-living mature vascular tissue such as xylem vessels and tracheids.
(i) Middle Lamella:
It chiefly consists of pectipoly galacturonicc acid in the form of the Ca and Mg salts. Pectic acid is long chain acid compound in which α-D-galacturonic acid units are joined together by (1: 4) glycosidic linkages. (The latter have not been elaborated in the structural formula that follows due to convenience). It is hydrophilic in nature.
(ii) Primary Wall:
It chiefly consists of cellulose, a long straight chain polysaccharide in which β-D-Glucose units are jointed together by (1: 4) glycosidic linkages. The cellulose chains which form the crystalline region of the cell wall are associated together forming a network of cellulose strands in the cell wall. An association of about 100 cellulose chains is termed as a micelle, 20 micelles constitute a micro fibril, while an aggregation of 250 micro fibrils is called as a fibril (Fig 1.2).
In the primary wall, the cellulose strands are oriented at right angles to the longitudinal axis of the cell (Fig. 1.3A). Cellulose is strongly hydrophilic in nature. Besides cellulose, the primary wall also contains lignin, hemicellulose, some pectic substances and proteins which form the amorphous matrix. Some inorganic salts such as those of silica and calcium are sometimes present in the primary wall.
(iii) Secondary Wall:
Secondary walls are more pronounced in dead cells such as tracheids and sclerenchyma. It chiefly consists of cellulose and lignin. Three distinct layers have been observed in secondary wall, each having different but definite orientation of the cellulose strands (Fig. 1.3B).
The structure of lignin is obscure but it seems to be a polymer of some phenolic compound e.g., phenyl propane (Fig. 1.4). For details, see Chapter 24.
(There are many other wall materials such as cutin, suberin, wax, mucilage etc. usually secreted outside the cell. Cutin, suberin, wax etc. make the cell wall impervious to water. While cutin is found on exposed parts of the plants, suberin is found chiefly in endodermal and cork cells. Blue-green algae usually have a secretion of mucilage on their outer walls. Chitin is found in cell walls of fungi and bacteria). 
(B) Cytoplasm:
The cytoplasm is a fluid material of varying viscosity and is usually present in the form of a thin film just below the cell wall and completely encloses the vacuole. Most of the physiological activities of the plants take place in cytoplasm.
It consists of following parts:
Plasma Membrane:
Previously known as the plasma lemma or ectoplast, it forms the outermost boundary of the cytoplasm and acts as barrier between protoplasm and the outer environment. It consists of phospholipids and proteins and measures about 75 Å in thickness. It is a unit membrane i.e., it consists of two lipid layers bounded on either side by monomolecular layers of proteins The hydrophilic or polar regions of the lipid layers (‘heads’) project towards the protein layer side while their hydrophobic regions or non-polar regions (‘tails’) lie towards the inner side of the membrane (Fig. 1.5).
The plasma membrane is selectively or differentially permeable in nature. The concept of a lipid bilayer as the basic feature of cell membranes structure which is now widely recognised was an essential feature of Danielli-Davson’s model proposed in 1935. Characteristic appearance of cell membranes after very thin sectioning and heavy metal staining led Robertson (1959) to propose a modified version of Danielli-Davson’s model which is known as the Unit-Membrane model. For more than a decade this model was considered to represent a universal structure for all biological membranes.
More recent investigations have, however, shown that the unit-membrane hypothesis is not truly universal and it does not account for the dynamic changes in membrane permeability. In 1960s many other models alternative to the unit-membrane model were proposed by different workers and their view points were brought together by Singer and Nicolson (1972) in a model which is known as Fluid Mosaic Model. This model is by far the most satisfactory model of membrane structure and is most generally accepted at present.
The Fluid Mosaic Model:
According to this model (Fig. 1.6), the membrane consists of a bilayer or double layer of amphipathic lipids (i.e. lipids with hydrophilic or polar and hydrophobic or non-polar regions e.g., phospholipids, glycolipids, sterols) which form a liquid lipid matrix with globular integral proteins dispersed in it. The proteins and components of the membrane are not fixed but may float in and on the phospholipids, thus creating a mosaic of substances there.
In the bilayer the lipid molecules are arranged in such a way that their hydrophobic ‘tails’ point towards each other and may also intermingle at their tips. Both the surfaces of the bilayer are composed of hydrophilic ‘heads’.
The globular integral proteins are also amphipathic molecules. Their hydrophilic ends protrude into the aqueous phase on either side of the bilayer and their hydrophobic ends remain embedded in the non-polar regions of the lipids bilayer. These proteins may pass right through the bilayer or penetrate the latter from either side. Sometimes, they may be completely embedded in the bilayer and in such situation their hydrophobic regions remain in touch with the non- polar core of the lipid bilayer.
The peripheral proteins although not included in this model are considered by Singer (1974) and others to be present on the polar surfaces of the lipids bilayer. The protein components of the membrane may be structural or enzymatic and there may be considerable variation in their type and amount in different membranes or in different sections of the same membrane.
The fluid mosaic model also makes provision for the presence of other membrane components such as protein derivatives, carbohydrates etc. and satisfactorily explains most of the properties of the natural membranes.
Tonoplast :
It forms the innermost boundary of the cytoplasm and surrounds the vacuole and hence also known as vacuolar membrane. It is also selectively permeable and lipo-protein in nature. It acts as barrier between cytoplasm and the vacuole.
Endoplasmic Reticulum:
Extending throughout the cytoplasm there is a network of paired membranes folded in different ways and enclosing spaces called as cisternae. These membranes are lipo-protein in nature and whole of this membranous system is called as endoplasmic reticulum (ER) or ergastoplasm.
Endoplasmic-reticulum is connected with nuclear membrane and is frequently associated with ribosomes (Fig. 1.7). When associated with ribosomes, the endoplasmic reticulum is called as rough endoplasmic reticulum and when without ribosomes, it is called as smooth endoplasmic reticulum.
The ground phase of the cytoplasm is known as cytosol. The cytosol contains a number of living and non-living substances in it. The living substances inside the cytosol which are membrane bound are called as cell-organelles while the non-living substances are called as ergastic substances or cell-inclusions. The latter are not membrane bound.
Cell Organelles:
(i) Plastids:
These are small membranous discoid structures which arise from small vesicular organelles called pro-plastids and are of three types:
(a) Leucoplasts:
These are colourless. Larger leucoplasts are called as amyloplasts and store starch. Some specialised amyloplasts in root cap also serve as gravity sensors and play important role in gravitropism. On exposure to light, the amyloplasts may develop chlorophyll and converted into chloroplasts e.g., in potato tubers.
(b) Chromoplasts:
These are coloured plastids and contain carotenoid pigments only. They are not photosynthetic due to lack of chlorophyll. Chromoplasts may sometimes be converted into chloroplasts (e.g., in carrots and oranges) or the chloroplasts into chromoplasts (e.g., in ripening tomato fruits and autumn leaves).
(c) Chloroplasts:
These are green plastids. The process of photosynthesis takes place in them. Typically the chloroplasts of higher plants are discoid or ellipsoidal in shape, 4-6 µ in length and 1 to 2 µ thick. The chloroplast is bounded by two membranes each app. 50Å thick and consisting of lipid bilayer and proteins. The thickness of the two membranes including the space enclosed by them is app. 300Å. Internally, the chloroplast is filled with a hydrophilic matrix called as stroma in which are embedded grana.
Each granum has a diameter of 0.25-0.8µ and consists of 5-25 disk-shaped grana-lamellae placed one above the other like the stack of coins (Fig 1.8. A). In cross section these lamellae are paired to form sac-like structures and have been called as thylakoids. Each grana lamella or thylakoid encloses a space, the loculus. The ends of disk-shaped thylakoid are called as margins (which are fused to form sac-like structure) while the contiguous membranes between two thylakoids form the partition.
Some of the grana-lamellae or thylakoids of granum are connected with thylakoids of other grana by somewhat thinner stroma-lamellae or fret-membranes. These also enclose spaces which are called as fret-channels (Fig. 1.8 B). Grana, which are the sites of primary photochemical reaction, contain chlorophylls and other photosynthetic pigments. The dark reaction of photosynthesis takes place in stroma.
The chloroplast membranes, stroma and grana lamellae are lipo-protein in nature and are especially rich in glycosylglycerides. Ribosomes (which are rich in RNA), some DNA and necessary enzymes have also been observed in chloroplasts which give them (chloroplasts) a partial genetic autonomy. The ribosomes and DNA of chloroplasts resemble those of prokaryotes. (DNA of chloroplasts is in the form of double stranded circular molecules while ribosomes have a sedimentation coefficient of 70 S as in prokaryotes).
(In Blue-Green algae the plastids are absent. The lamellae with photosynthetic pigments are dispersed as such in cytoplasm. In other algae although definite chloroplasts are found, they are not internally well organised and lack definite grana).
Etioplasts:
In absence of light, the pro-plastids destined to form chloroplasts, instead form etioplasts. Etioplasts contain semi-crystalline tubular arrays of membranes called as pro-lamellar bodies. The latter contain a pale yellow green precursor pigment which is called as protochlorophyllide or protochlorophyll.
When exposed to light, the etioplasts are converted into chloroplasts. The protochlorophyll is converted into chlorophyll while the prolamellar bodies undergo a reorganization to form internal membranes of the chloroplasts i.e., thylakoids and stroma lamellae. Mature chloroplasts may revert back to etioplasts during prolonged period of darkness.
(ii) Mitochondria (Chondriosomes):
Mitochondria which are sites of cellular respiration are very small usually spherical or rod shaped structures ranging from 0.5 – 1 mµ in diameter and 1mµ to 3.0 mµ in length. Each mitochondrion (fig. 1.9A) has got an envelope consisting of two double-layered lipoprotein membranes.
The space enclosed in between the two membranes is called as inter membrane space. The inner mitochondrial membrane is invaginated into plate-like on finger-like folds which are called as cristae (Fig. 1.9A, B). Aqueous ground phase of the mitochondrion is called as matrix. All the enzymes necessary for Krebs’ cycle are found in matrix.
The outer mitochondrial membrane is readily permeable to small molecules and ions. Transmembrane channels composed of the protein porin allow most molecules of molecular weight less than 5000 to pass through it easily. The inner mitochondrial membrane is selectively permeable.
It is impermeable to most small molecules and ions including H+ ions. However, there are specific transporter proteins in inner mitochondrial membrane for passage of selected metabolites through it. The components of electron transport chain are found in inner mitochondrial membrane.
On the inner sides of the cristae are found small knob-shaped structures which are called as phosphorylating complexes (Fig. 1.9B) or sometimes as respiratory assemblies. Each phosphorylating complex consists of two subunits (Fig. 1.9B, C), a basal subunit or base piece called as F0 which transverses across the inner mitochondrial membrane, and a terminal spherical subunit or head piece called as F1 which projects into matrix.
The F0— F1 complex is in-fact proton conducting ATP-synthase which catalyses the formation of ATP from ADP and Pi and vice versa. The head piece (F1) is a peripheral protein complex consisting of 5 different sub- units (3α, 3β subunits and one copy each of three other subunits called 
, δ and ɛ).
The base piece (F0) is an integral protein complex and consists of cluster of many copies of at least three different small polypeptides which form a membrane channel for protons. Oxidative phosphorylation takes place on phosphorylating complexes, the catalytic sites for phosphorylation being situated on head piece (F1).
Mitochondria also contain ribosomes, some DNA and RNA and like chloroplasts are semi- autonomous genetically. Mitochondria are found in more number in actively dividing cells. Mitochondria proliferate through division by fission of pre-existing mitochondria and not through de novo biogenesis of the organelle.
The mitochondria are often called as Power Houses of the Cell because huge amount of energy liberated during aerobic respiration is trapped inside the mitochondria in the form of energy-rich ATP molecules (Fig. 1.10) which is then utilised in driving off various metabolical processes of the cell.
Like chloroplasts, the mitochondrial ribosomes and DNA resemble those of prokaryotic cells. (Kolliker (1880) was the first to recognise mitochondria in muscle which he called as sarcosomes. W. Flemming (1882) described these as thread like structures while R. Altman (1890) described them as granular structures and called them as bio-blasts. It was C. Benda who in 1897 gave the name ‘mitochondria’ to these thread like granules (mitos = thread; chondrion = granule).
(iii) Ribosomes:
Ribosomes are the cell-organelles which are not membrane bound and are found in both eukaryotic and prokaryotic cells. Ribosomes were first observed under electron microscope by G.E. Palade (1955). A. Claude (1941) called them as microsomes. Ribosomes are very rich in RNA and proteins; hence they have also been called as ribonucleoprotein (RNP) particles or RNP-granules. They have also been referred to as Claude’s particles. (George E. Palade and Albert Claude, both Nobel Laureates of 1947 in Physiology or Medicine category).
Ribosomes are very minute spherical structures having an average diameter of about 23 mµ (230Å) and are sites of protein synthesis. Ribosomes are frequently associated with endoplasmic reticulum (ER) but may be present freely in cytoplasm. They have also been reported from nucleus, mitochondria and chloroplasts.
During protein synthesis, a cluster or a group of ribosomes is often seen attached to a single m-RNA molecule, each ribosome of the cluster being engaged in formation of a separate polypeptide chain. Such a group or cluster of ribosomes is called as polyribosome or polysome.
In bacteria (prokaryotes) partial removal of Mg+- ions bound in the ribosomal structures results in dissociation of ribosomes with a sedimentation coefficient of 70 S into two sub- units with sedimentation coefficient of 50 S and 30 S (Fig. 1.11 A). But in higher plants (eukaryotes) the cytoplasmic ribosomes which have sedimentation coefficient of 80 S dissociate into two sub-units of 60 S and 40 S. (Fig. 1.11B). These sub-units play specific roles in pro- tem synthesis. The ribosomes found in chloroplasts and mitochondria of eukaryotes, however, resemble those of prokaryotic cells.
The ribosomes are usually drawn with smooth outlines with smaller sub-unit fitting as a cap over a sphere which represents the larger sub-unit. However, X-Ray analysis and electron microscopic studies of ribosomes in Escherichia coli (a prokaryote bacterium) have shown 30 S and 50 S subunits to be of remarkably irregular shapes (Fig. 1.11C). These two oddly shaped sub-units fit together in such a way that a cleft is formed through which m-RNA passes as the ribosome moves along it during protein synthesis.
(iv) Golgi-Bodies (Dictyosomes):
The Golgi body (so named after an Italian, Camillo Golgi+ who first discovered them in 1898 with a light microscope) consists of a stack of 3-6 flattened membrane sacs or cisternae which are slightly curved and are dilated (swollen) at the margins (Fig. 1.12). Each cisterna encloses a space or lumen. The cisternae in Golgi body are not contiguous but are located about 10nm apart.
There are present intercisternal elements- protein cross links which hold the cisternae together. Near the dilated ends of the cisternae are found small spherical vesicles which have probably been pinched off from the dilated ends. The number of cisternae in Golgi body may sometime be fewer. In many algal cells Golgi bodies are known to consist of up to 20-30 cisternae.
The Golgi body is also called as Golgi apparatus or Golgi complex or dictyosome and is a dynamic structure. Dictyosomes are constantly changing as some of the cisternae grow while others shrink and disappear. In animals, Golgi bodies tend to cluster in one part of the cell and are inter connected with one another through tubules. But, in plants large number of separate Golgi bodies are dispersed throughout the cytoplasm. Golgi bodies are found in abundance in most secretory cells.
The Golgi body has a polarized structure and is asymmetric structurally and functionally. The face of cisternae closest to the plasma-membrane is called as trans face while the face closest to the centre of cells is cis face. In between the trans and cis cisternae are present medial cisternae.
The proteins synthesized on polysomes of rough ER are first transferred across the ER membrane and glycosylated (bound to sugars). These glycoproteins are then carried in small vesicles (budding from ER) to the cis side of cisternae and fuse with the latter. Within the lumen of cisternae, the glycoproteins are enzymatically modified.
Glycosylation also occurs inside Golgi body. The modified proteins are released in small vesicles budding from trans side of the cisternae. One of the main functions of this modification of newly formed protein is to “address” it to (or confer on it a ‘tag’ or ‘marker’) its proper destination inside or outside the cell.
This process is also called as protein targeting. Giinter Blobel has done pioneer and extensive research work on it and he was awarded Nobel Prize of 1999 in Physiology or Medicine category for his discovery that “proteins have intrinsic signals that govern their transport and localization in the cell”.
In plants, the main function of Golgi body is in cell wall formation. Besides modifying newly formed proteins, Golgi bodies also synthesize non-cellulosic cell wall polysaccharides such as hemi-cellulose and protein. These polysaccharides and glycoproteins are then carried in small vesicles/to the plasma-membrane, where they fuse with the latter and releasing these substances into the region of the cell wall.
(v) Lysosomes:
Sometimes reported from plant cells, they are minute spherical structures bounded by a membrane and containing some enzymes. When the membrane is ruptured the enzymes are released into the cytoplasm causing its disintegration (lysis). Lysosomes are usual features of animal cells and were first discovered in liver cells by Christian de Duve in 1949.
(vi) Spherosomes:
These are usually minute spherical structures about 0.4-3µ in size and surrounded by a membrane. According to some workers it is unit membrane while others regard it as a half unit membrane. Spherosomes are also called as oleosomes or oil-bodies because they store fats (triglycerides).
These structures are called wax bodies in jojoba (Simmondsia chinensis) cotyledons as here they store wax esters instead of triglycerides. Specific proteins called oleosins are found in membrane that surrounds the oleosome. These proteins probably maintain each oleosome as a discrete organelle by preventing their fusion. They may also facilitate to bind other proteins to organelle’s surface.
(vii) Glyoxysomes:
These organelles were first discovered by Beevers (1961) from castorbean endosperm, These are usually spherical in shape, about 0.8µ in diameter and consist of fine granular stroma surrounded by a single membrane. These contain key enzymes of glyoxylate cycle (i.e., isocitratase and malate synthetase) and therefore, are usually found in those plant tissues where fats are being actively converted into sugars (carbohydrates) such as in germinating fatty seeds.
(viii) Peroxisomes:
These particles or organelles which are similar in shape and size to glyoxysomes were first discovered by Tolbert et al (1968) from spinach leaf homogenate and were obtained by density gradient centrifugation. These also consist of a dense stroma surrounded by a single membrane. These contain typical enzymes which are involved in glycolate metabolism and photorespiration during photosynthesis. (The glyoxysomes and peroxisomes are often called as microbodies)
Cytoskeleton:
There is an interlocking three dimensional network of protein filaments throughout the cytoplasm of eukaryotic cell which is called as cytoskeleton and is visible under the electron microscope. Cytoskeleton provides structure and organisation to the cytoplasm and shape to the cell. It also helps to produce motion of organelles or of the whole cell. Besides this, it also plays fundamental roles in mitosis, meiosis, cytokinesis, wall deposition and cell differentiation.
There are three general types of cytoskeletal components in plants and animals:
(i) Actin filaments or microfilaments,
(ii) Microtubules and
(iii) Intermediate filaments.
These components differ in width, composition and specific function, each having a fixed diameter but variable length up to several micrometers. These components of cytoskeleton are not static but constantly disassemble into their monomeric subunits and reassemble into protein filaments. The location of cytoskeletal components also is not fixed in the cell but may change according to the cell’s requirement. Cytoskeletal components are modulated by Ca++ and by a variety of proteins.
(1) Actin Filaments (Microfilaments):
The actin filaments are ubiquitous in all eukaryotic cells. These are composed of special type of protein called actin. Each actin molecule consists of a single polypeptide chain with molecular weight of approximately 42000. These actin molecules are polymerised to form actin subunits. Two chains of polymerised actin subunits intertwine in a helical fashion to form a solid actin filament or microfilament with a diameter of 6-7 nm. (Fig. 1.13).
The actin filaments lying beneath the plasma membrane provide rigidity and shape to the cell surface. In addition to this, actin filaments participate in movements such as muscle contraction, cytoplasmic streaming, amoeboid movements and growth of pollen tubes down the styles.
2. Microtubules:
Microtubules are non-membranous elongated hollow cylinders, several microns in length and with an outer diameter of 24-25nm. (Fig. 1.14). The diameter of the hollow lumen is about 12 nm. Sometimes, the microtubules are surrounded by a ‘clear halo’ which is about 5-10 nm. wide.
Microtubules are macromolecules and made up of a globular protein which is known as ‘α and β-tubulin’.
Microtubules in plants may be grouped into 3 categories:
(a) Microtubules constituting the nuclear spindle in mitosis;
(b) Microtubules which are structural components of flagella and cilia of motile cells such as gametes of lower land plants and algae;
(c) Microtubules found in cytoplasm.
The role of microtubules of the first two categories is obvious. The microtubules of the third category are believed to be concerned with cell wall formation by directing the alignment of the cellulose micro fibrils as the latter are deposited. Probably, these microtubules also direct the Golgi vesicles (which carry polysaccharides found in cell wall matrix) through the cytoplasm to those regions of plasma membrane in the near vicinity of which active cell wall synthesis is taking place.
3. Intermediate Filaments:
Intermediate filaments are a diverse group of helically wound structures with dimensions (diameter 8-10 nm) intermediate between actin filaments and microtubules. Intermediate filaments are composed of several different types of monomeric protein subunits. These filaments are not found in all the cells. In some cells, these may be present in larger amounts than in others while some cell types may lack these filaments altogether.
Intermediate filaments are well known in animals but relatively little known in plants. In animals, lamins in the inner surface of inner nuclear membrane, vimentin in endothelial cells lining the blood vessels and in adipocytes and keratins prominently occurring in certain epidermal cells of vertebrates are common examples of proteins which constitute the intermediate filaments. Hairs, fingernails and feathers are chiefly composed of keratins.
Main function of intermediate filaments appears to provide internal mechanical support for the cell and to position its organelles.
Ergastic Substances:
Ergastic substances or the cell-inclusions may be of different types e.g., reserved food materials tannins, resins, gums, oils, latex, alkaloids, mineral salts (usually in the form of crystals such as cystolith) etc.
The Vacuole:
In young cells the vacuole is absent. Mature cells may have a large central vacuole (Fig. 1.15). The vacuole is surrounded by tonoplast or vacuolar membrane. Sometimes, the cytoplasm may traverse the vacuole as strands or sheets and subdivide the vacuole into many different compartments.
In such cases the cell may have one large and two or more smaller vacuoles, each vacuole being bounded by tonoplast. The vacuole is filled with a watery solution of many inorganic, organic substances and gases which is known as cell-sap. Cell-sap maintains the osmotic relations of the cell. Colours of vast majority of flowers are chiefly due to the presence of anthocyanin pigments dissolved in cell sap.
(C) Nucleus:
The nucleus (See Fig. 1.7) which is usually a spherical structure consists of a double layered membrane made up of phospholipids and proteins. This is called as nuclear-membrane. The space between the two layers is called as peri-nuclear space. The nuclear membrane is at some places continuous with the endoplasmic-reticulum.
It is interrupted at some places by nuclear pores. Inside the nucleus there is a granular substance, the nucleoplasm in which is present chromatin network. At the time of nuclear division the chromatin network breaks up and becomes differentiated into chromosomes. Chromatin network consists chiefly of DNA (Deoxy-Ribose Nucleic Acid) and nucleoproteins, the histones.
About half of the mass of chromatin is DNA and half is histones. The histones and DNA associate in complexes called nucleosomes in which DNA strands wind around a core of histone molecules. The nucleus also contains one or more dense spherical bodies called as nucleoli which are very rich in RNA and proteins. Ribosomes are also found in the nucleus, the nucleoli being the sites of their synthesis.
The nucleus controls almost all the physiological activities of the cell through the formation of specific enzymic proteins. Besides being involved in reproduction directly, the nucleus encodes all the genetic information which produces heredity characters in living organisms.
Physical Nature of Protoplasm:
Many theories were put forward to account for the physical nature of protoplasm during old past. According to granular theory, the protoplasm consisted of numerous tiny granules. Fibrillar theory regarded the protoplasm to be made up of a number of fine fibres. The reticulate theory assumed the protoplasm to form a sort of network while according to alveolar theory the protoplasm is foam-like or alveolar in nature. But according to the modern view, the protoplasm is a colourless, semi-transparent and viscous substance and is considered as complex colloidal system of many phases.
Chemical Nature of Protoplasm:
It is very difficult rather impossible to know the exact chemical nature or composition of the protoplasm without destroying it. In fact all the studies on its chemical composition have been done after it has been killed during chemical analytical methods.
However, it is now well established that water is the chief component of all physiologically active protoplasm and in such cases may constitute up to 90% of the protoplasm and in cases of hydrophytes even more. In dry seeds where the protoplasm is rather inactive, water may be less than 10% of the total protoplasm.
Myxomycetes (slime molds) have proved to be very good material for the chemical analysis of protoplasm because they consist of naked masses of protoplasm (plasmodium) without cell walls at certain stages of their life history. Results of chemical analysis of the plasmodium of a Myxomycete are given in Table 1.2.
It is quite evident from the above table that the proteins and other nitrogenous compounds constitute the bulk of the organic matter of the protoplasm (after water has been removed). The lipids (simple fats or oil, phospholipids and sterols) constitute a smaller fraction. Then come the carbohydrates. The inorganic compounds, i.e., the mineral matter chiefly consist of phosphates, chlorides, sulphates, and carbonates of Mg, K, Na, Ca etc.