In this article we will discuss about the structure of cytoplasm with the help of suitable diagrams.
The substance, except nucleus, surrounded by the plasma lemma is called cytoplasm. Electron micrographs reveal a numbers of membranous and other structures in the cytoplasm; the portion of cytoplasm other than these structures is called hyaloplasm. Of the various structures present in the cytoplasm, mitochondria and plastids contain DNA (the genetic material); as a result they are autonomous to a limited degree.
Therefore, the remaining cytoplasmic structures do not possess DNA and they are specified exclusively by nuclear genes. The cytoplasm may contain the following structures- Endoplasmic reticulum (ER), Ribosomes, golgi bodied, Lysosomes, Spherosomes, Vacuoles, Centrioles (in animals only), Microtubules, Mitochondria, and Plastids (in green plants only).
Endoplasmic Reticulum:
The cytoplasm possesses an extensive network of membrane-enclosed spaces; these spaces along with them membranes enclosing them are called endoplasmic reticulum (ER).
ER of possesses of three types of membrane-enclosed elements:
(a) Vesicles of 25-500 mµ diameter,
(b) Tubules of 50-100 mµ diameter, and
(c) 40-50 mµ thick cisterns of variable length and width.
The tubules may or may not be extensively branched, and the cisterns may or may not be connected with each other. ER is usually present in differentiated cells; it is not found in RBC’s, eggs, undifferentiated cells and prokaryotic cells. ER is connected with plasma lemma as well as nuclear envelope.
It is believed that ER originates from the outer membrane of nuclear envelope. The ultrastructure (structure revealed by electron microscopy) of ER membranes is the same as that of a unit membrane, that is, it has two osmiophilic layers separated by as osmiophobic layer.
ER is divided into two categories:
(a) Smooth ER, and
(b) Rough ER.
In the smooth ER elements, both outer and inner surfaces are regular and smooth. In those cells where little or no protein synthesis takes place, only smooth ER is observed.
In the rough ER elements, the outer surfaces of membranes have a rough appearance because of the attachment of ribosomes onto the outer surface. Rough ER is chiefly composed of cisterns (membrane-enclosed plate-like elements) and is found in cells actively involved in protein synthesis. Smooth and rough ER change into each other as per the requires of cells.
Ribosomes:
Ribosomes are particles of about 200 Å diameter; they are made up of RNA and protein. Usually, ribosomes are attached to the outer surfaces of ER membranes; this converts smooth ER elements into rough ER. But certain ribosomes are present in the hyaloplasm unattached to any membrane.
The weight of ribosomes and other macromolecules is determined by ultracentrifugation (centrifugation at very high speeds of 50.000 rpm or more). The sedimentation rate of macromolecules is expressed as s (sedimentation coefficient), which is the average velocity per unit of acceleration and is expressed as cm/sec2. The size of weight of a macromolecule is expressed in S units or Svedberg (the inventor of ultracentrifuge) units, which has a reasonably reliable relationship with S.
Based on their sedimentation rate, ribosomes are classified into three groups:
(a) 80 S size animal ribosoms,
(b) 80 S plant ribosomes, and
(c) 70 S eukaryotic organelle (mitochondria and plastids) and prokaryotic ribosomes.
In the conditions of low Mg++ concentration, every plant ribosome (80 S) dissociates into a larger subunit of 60 S and a smaller subunit of 36 S; the 80 S animal ribosomes dissociate into 60 S and 45 S subunits; while 70 S prokaryotic and organelle ribosomes dissociate into 50S and 30S subunits.
The smaller subunit of a ribosome is placed onto the larger subunit much like a cap- on-the-head. Animal and plant ribosomes are made up of 55- 60% protein and 40-45% RNA, while prokaryotic and organelle ribosomes have only 37% protein but 63% RNA.
Although organelle ribosomes have been classified with the prokaryotic ones, they differ from the latter both in the size and the type or RNA molecules they possess. Organelle ribosomes are themselves seen to constitute a separate, but highly variable, category of ribosomes.
In E coli, each 50S larger subunit has one molecule each of a 23S and a 5S (about 120 uncleotides) RNA but the 30S smaller subunit has one molecule of 16S RNA (about 1600 nucleotides). The larger subunit has 34 different proteins (L1 to L34), while the smaller subunit has 21 different proteins (S1to S21; Fig. 2.8). One protein of the smaller subunit (S22) is observed in the larger subunit also (L26), while one protein of the larger subunit exists in two different forms (L7 and L12) which differ only by acetylation.
In mammalian ribosomes, the larger subunit has one copy each of three types of RNA molecules (28S, 5-8S and 5S), but the smaller subunit has one RNA molecule of 18S size. Plant ribosome larger subunits have a 25S and a 5S RNA molecule, while the smaller subunits have one copy each of a 16S and 5S RNA.
Ribosomes are synthesised and assembled in nucleolus, within the nucleus, from where they migrate into the cytoplasm. Ribosomes are essential for protein synthesis as mRNA’s may support protein synthesis only when they are attached to ribosomes. Several (5-8) ribosomes can bind to a single mRNA molecule during protein synthesis; these ribosomes together with the mRNA constitute a polysome.
Chloroplasts: (functions)
Chloroplasts possess the single most important pigment on earth i.e., chlorophyll. They impact the characteristic green colour to plants and carry out photosynthesis, the ultimate source of all organic compounds. Chloroplasts are typically biconvex lens-shaped of about 5µ diameter and 3µ thickness. Therefore, they exhibit a large variation is size and shape. An average cell can contain 20-40 chloroplasts, but some algae, viz., Chlamydomonas, have a single chloroplast per cell.
The average chemical composition of chloroplasts can be as follows; protein 50-59%, lipid 21-34%, chlorophyll 5-8%, carotenoids 0.7-1.1%, RNA 1-7.5%, and DNA 0.2-1%. Chlorophyll and carotenoid molecules are associated with chloroplast membranes (grana and stroma lamellae), and certain (about 5%) chlorophyll molecules appear to be complexed with protein molecules.
Chloroplasts are enclosed by two concentric membranes, each being about 50 Å thick. The membrane-free space enclosed by these two membranes is referred to as stroma. Within the stroma are embedded about 40-80 grana of 0.2-0.8µ diameter (Fig. 2.1). Every granum contains 5-25 flat cisternae (membrane-enclosed spaces) stacked on top of each other. Most cisternae are confined to a single granum such cisternae are called grana loculi. But certain cisternae interconnect two or more grana; they are known as fret channels.
The portion of cisternae that passes through stroma between grana constitutes the stroma lamellae. Within grana, most of the membrane area is formed by fusion of two membranes of the two contiguous cisternae; as a result, these membranes appear twice as thick as stroma lamellae.
But the margins of grana loculi are only as thick as stroma lamellae, which supports the above conclusion (Fig. 2.9). Grana and stroma lamellae are made up of several thousand units, called quantasomes, of about 200 × 100 Å size. Å group of 3-8 quantasomes is able of electron transport and photophosphorylation. But the enzymes of dark reaction of photosynthesis are situated in the stoma of chloroplasts.
Several copies of a ring-shaped DNA molecule are present in the stroma of each chloroplast; the size of DNA molecule is somewhat variable depending on the plant species under consideration. Chloroplasts possess ribosomes, tRNA, and carry out protein synthesis. Chloroplasts differentiate from small vesicles enclosed by two concentric membranes; these vesicles are known as proplastids. Proplastids divide by fission to produce more proplastids (Fig. 2.10). Chloroplasts can also arise through fission of pre-existing and differentiated chloroplasts.
In addition to chloroplasts, plant cells contain two more types of plastids known as:
(a) Leucoplasts, and
(b) Chromoplasts, Leucoplasts are colourless plastids and function in the storage of starch, proteins and fats.
Chromoplasts, on the other hand possess pigments other than chlorophyll, e.g., phucoxanthin, phycocyanin etc.
The different types of plastids are achieved through differentiation from proplastids. In addition, plastids of one type readily transform into the other two kinds as per the needs of cells (Fig. 2.11).
Chloroplasts differentiation from proplastids is a light- dependent phenomenon. In the presence of light, the inner membranes of proplastids invaginate and a large numerous small vesicles bud-off from these invaginations into the stroma. Later, these vesicles become oriented in regular arrangements; the vesicles lying in one plane fuse together to generate grana and stroma lamellae.
The sequence of, events in the presence of high and low light intensities are markedly different, but the final outcome of the two sequences is quite comparable (Fig. 2.11). A numerous mutant nuclear genes block the differentiation of proplastids into chloroplasts at various stages of differentiation; structures generated by several of the mutant genes are comparable to those observed during chloroplast differentiation under low light intensities (Fig. 2.11).
The main function is chloroplasts are photosynthesis. Light energy liberates electrons from chlorophyll molecules; these electrons enter through the electron transport chain situated in chloroplast membranes and the energy derived from this transport is utilized to produce ATP (photophosphorylation).
This ATP is utilized for the fixation and reduction of CO2; this is accomplished by the soluble enzymes situated in chloroplast stroma. In addition, chloroplasts contribute to heredity in the form of cytoplasmic inheritance since they possess DNA.
Lysosomes:
Lysosomes are vesicles of 400-800 mµ and possess several hydrolytic enzymes. The main enzyme present in lysosomes is acid phosphatase; other enzymes are, acid DNAase, acid RNAase. β-galactosidase etc. The membrane surrounding a lysosome has a unit membrane organization.
Lysosomes are of two kinds:
(a) Primary, and
(b) Secondary.
(a) Primary lysosomes are produced by Golgi bodies and possess hydrolytic enzymes only. They fuse with food vacuoles produced through phagocytosis and pinocytosis to generate secondary lysosomes.
(b) Secondary lysosomes possess both hydrolytic enzymes as well as food materials.
The food particles are ultimately digested by the hydrolytic enzymes and absorbed into the hyaloplasm; the undigested portion of food materials remains in the secondary lysosomes, which are now called residual bodies.
The enzymes present in lysosomes are capable of digesting any living organism. When, a white blood cell (WBC) ingests (by phagocytosis) a bacterium or certain other organism, all the lysosomes of the WBC fuse with the food vacuole hence produced. As a result, the bacterium as well as the WBC itself is lysed.
In certain situations, the enzymes present in lysosomes are released into the cytoplasm, which leads to a lysis of the concerned cell (autolysis). Therefore, the function of lysosomes is digestion (lysis) of food particles and microorganisms ingested by a cell and also to cause autolysis of the cell, if needed.
Spherosome:
Spherosomes are vesicles of 0.5-1µ diameter and passes; upto 98%, lipid; they also contain some acid phosphatase. Spherosomes are observed in plant cells only, and they are absent in animal and prokaryotic cells. Spherosomes are surrounded by a single unit membrane. According to some scientists, spherosomes function in lipid storage, but certain others do not agree with this view.
Vacuole:
Plant cells have one or more vacuoles of variable size. In mature and differentiated plant cells, the main part of cytoplasm is occupied by a large vacuole, and the cytoplasm is pushed to the periphery of cells. The material possessed in the vacuoles is referred to as cell sap. Cell sap is relatively less dense than the surrounding cytoplasm; it contains sugars, salts, proteins, phenols etc., as well as certain specific pigments, e.g., anthocyanin. Usually, vacuoles are surrounded by a unit membrane; this membrane is referred to as tonoplast. Tonoplasts show differences in permeability as compared to the plasma lemma.
Centriole:
Centrioles are cylindrical structures of about 1200-1500 Å and of about 3000-5000 Å in length. Centrioles are always present in pairs, one centriole being oriented vertical to the other (Fig. 2.12 a). They are confined to animal cells, and are not observed in plant cells. However, basal bodies found at the base of flagella of plant cells are identical with centrioles in their ultrastructure. Usually, new centrioles arise in association with preexisting centrioles; they organize at an interval from and at right angles to the two preexisting centrioles in a cell (Fig. 2.12 a). But at least in certain cases, preexisting centrioles are not essential for the organization of new centrioles, which is arise de novo.
Electron micrographs of transverse sections of centrioles reveal nine fibrils arranged at the periphery of a centriole (Fig. 2.12 b). Each fibril is composed of three micro fibrils arranged in a single plane so that each fibril has a flat topography. The fibrils of a centriole are oriented at an angle on the periphery.
The inner micro fibril of each of the nine fibrils is associated with the outer micro fibril of a neighbouring fibril; at the same time, it is also connected with a central micro fibril situated in the centre of the centriole. Thus the transverse ultrastructure of a centriole is somewhat similar to the wheel off a bullock-cart (or any other wheel with spokes) (Fig. 2.12 b).
In animal cells, centrioles are included in the organization of spindle apparatus. In fact, a pair of centrioles lies at each of the two poles in a cell from which spindle fibres radiate toward the equatorial plate. In both plant and animal cells, centrioles serve as the basal bodies of flagella.
Microtubules:
Microtubules are tubules of 150-260 Å in diameter; their wall is about 45-70 Å in thickness. In plants, the walls of microtubules are made up of 13 fibres made up of protein molecules. It is likely that lipid molecules provide stability to the structure of microtubule walls.
In centrioles and flagella, each micro fibril is essentially a microtubule; microtubules also constitute the fibres of spindle apparatus, that is, spindle fibres. Microtubules are primarily found in dividing cells in the form of spindle fibres, and are responsible for chromosome movement during cell division. But they are also observed in non-dividing cells, and can provide some degree of stability to the various structures present in the cytoplasm.
Golgi Body:
It consists of 2-7 flat cisternae stacked close to each other (Fig. 2.1).
A network of 300-500 Å tubules emerges from around the margins of the cisternae (Fig. 2.13). In addition, vesicles of 200-800 Å diameter are also present on the margins of cisternae.
The number of golgi bodies in a cell depends on the synthetic activity of the cell. In a synthetically active cell, many well differentiated and developed golgi bodies are present, while synthetically inactive cells have few poorly evolved golgi bodies. Golgi bodies originate from ER elements. Membranes of golgi bodies have an ultrastructure similar to that of a unit membrane.
It is believed that materials synthesized in association with ER (proteins, lipids, phospholipids etc.) are transported to golgi bodies, where they are packed into vesicles cut off from them. Therefore golgi bodies function as packaging plants of the cell. Ordinarily, these materials are then exported to other cells and tissues where they are utilized in different ways.
Mitochondria:
The term mitochondria (mitos = thread + chondrion = granule) was first utilized by Benda in 1897, but they were first found about 20 years earlier by Holliker, Mitochondria are cylindrical bodies with an average diameter of 0.2 to1µ, and ordinarily 3-10µ in length. But in certain cells, they may be upto 40µ long. An average cell may have 200 to 800 mitochondria. But in some protozoa, viz., Chaos chaos, there can be as many as 500,000 mitochondria in a cell. The average composition of mitochondria is as follows- protein 70%, lipids 25.30% RNA, approximately 1%, and DNA, less than 1%.
Mitochondria are surrounded by two concentric unit membranes. The outer membrane is about 60 Å thick and regular in outline. The inner membrane is located about 20-60 Å away from the outer one, is about 60 Å thick, and is infolded at many places. The infolds of inner membrane are called cristae; each crista is about 140-180 Å in thickness. The space outside the cristae, i.e., on the inside of the inner membrane, is called matrix (Fig. 2.1).
In electron micrographs, mitochondrial membranes may show one of the following three ultrastructures:
(a) Typical unit membrane organization,
(b) Crossbar ultrastructure, and
(c) Fur-coated structure.
The unit membrane ultrastructure includes two typical osmiophilic layers separated by an osmiophobic layer (Fig. 2.14 a).
In the cross-bar ultrastructure, number of osmiophobic spheres of 40-50 Å diameters is, embedded in an osmiophilic layer of 75-80 Å thickness. This gives the impression as if the two osmiophilic layers of a unit membrane are associated with each other by several osmiophilic cross-bars; such structure resembles a metal or bamboo ladder (Fig. 2.14 b).
The fur-coated ultrastructure is achieved by negative staining of crista membranes with agents like phosphotungstic acid. A large number of particles of 75- 100 Å in diameter are attached to the outer surface of crista membranes by means of 45-50 Å long stalks (Fig. 2.14 c). It is not clear which of the three organizations is of common occurrence.
The outer membrane has about 50% lipid and 50% protein; cholesterol constitutes an appreciable portion of the lipid fraction. The outer membrane is considerably more permeable than the inner membrane; it allows molecules of upto 10,000 daltons molecular weight to enter through.
The inner mitochondrial membrane, in contrast, is much less permeable and only very small uncharged molecules like water and pyruvic acid are able to enter through it. Larger molecules need special transport systems to pass through the inner membrane. The inner membrane has about 75% protein and 25% lipid; about 60 different kinds of proteins are present in the inner membrane.
The inner membrane is made up of numerous identical units referred to as oxysomes. Each oxysome is capable of complete electron transport to O2 as well as coupled oxidative phosphorylation. Clearly, the components of electron transport and coupled phosphorylation are integral parts of the inner membrane, and the precise sequence of interactions among these components is most likely on the basis of their precise arrangement in the cover.
Mitochondria arise by fission of preexisting mitochondria, but it is just possible that they can arise from some other membranous structures like plasma lemma and nuclear envelope. Growth and fission of mitochondria has been filmed by time-lapse photography of living cells.
In other word, two or more smaller mitochondria may fuse together to produce a larger one. Each mitochondrion contains several copies of a ring-shaped DNA molecule; the size of such molecule varies considerably from one group of organisms to the other. Mitochondria contain ribosomes which are distinct from those present in the cytoplasm; certain of the properties of these ribosomes are similar to those of prokaryotic ones, but they along with ribosomes of chloroplasts appear to constitute a distinct category of ribosomes (organelle ribosomes).
Mitochondrial matrix contains enzymes for the oxidation of amino acids and fatty acids, and the enzymes of Kreb’s cycle. The enzymes of electron transport and oxidative phosphorylation are situated in the cristae membranes.
Thus the main function of mitochondria is the oxidation of carbohydrates, amino acids and fatty acids, and the production of ATP using the energy obtained from transport of electrons so produced. The ATP produced in mitochondria provides the necessary energy for various biochemical reactions of cells. Because mitochondria contain DNA, they contribute to heredity by way of cytoplasmic inheritance.
Nucleus:
Nuclei take up relatively deep stain with basic dyes, and are usually spherical in shape. But in differentiated tissues, they can exhibit considerable variation in shape. The size of nuclei varies with the physiological state and the degree of differentiation of cells.
In physiologically active and undifferentiated cells, nuclei are relatively larger in size, while in physiologically inactive and differentiated cells they are relatively smaller. Nuclear size is also influenced by the chromosome number (or the amount of DNA) of a species, as well as by the ploidy level of a cell or individual.
In general, a cell possesses a single nucleus, but in some tissue, e.g., early stages of endosperm development, and in certain organisms, e.g., some protozoa, some fungi etc., two or more nuclei are present within a single cell. In contrast, some specialized cells, e.g., RBC’s and companion cells of phloem do not possess any nucleus.
Nucleus is the store-house of almost all genetic information required for the functioning of a cell/organism. It produces ribosomes, tRNA (transfer RNA) and mRNA, which (together with the protein synthesis machinery of cells) form the various structural and enzymatic proteins necessary for cellular organization and biochemical reactions. In essence, nucleus governs the development of almost all the traits of an organism by providing the information essential for the syntheses of various structural and functional proteins.
The significance of nuclei in the evolution was first demonstrated by a German scientist, Hammerling in 1934, through transplantation studies in the unicellular alga Acetabularia. He clearly demonstrated that the development of characters in such alga was governed by the nucleus and not by the cytoplasm.
Pea nuclei possess 14% DNA, 12% RNA and 74% protein on dry weight basis.
In electron micrographs, three distinct types of structures are discernible in interphase nuclei:
(a) Nuclear envelope,
(b) Nucleolus, and
(c) Chromatin fibres. (Fig. 2.1 and 2.15 a).
(a) Nuclear Envelope:
Nucleus is enclosed by two concentric membranes, each being 70-80 Å in thickness. There is a space of about 200-300 Å between the two membranes; this space is called the perinuclear space. The two membranes, together with the perinuclear space, are called nuclear envelope.
In the nuclear envelope, nuclear pores of 200-400 Å in a diameter are arranged in a hexagonal pattern at an interval of 800 Å from each other and from the centre of the hexagons hence formed. In fact, nuclear pores or annuli are not real pores in that the membrane is continuous throughout the area of an annulus.
The ultrastructural details of annuli are relatively much more complex than that of the nuclear membrane itself. The two membranes of a nuclear envelope and fused with each other at the peripheri of each annulus (Fig. 2.15 a). At the peripheri of an annulus, eight annular granules are embedded in each of the two nuclear membranes; the granules are situated at equal distance from each other. In the centre of the annulus, another annular granule in located.
The eight annular granules within a membrane (in an annulus) are connected with each other and with the central granule (Fig. 2.15 a). The annulus is filled with a material known as annular material. The annuli provide the main channel for transport of materials from and into the nucleus. But some materials can be transported out of the nuclei by means of vesicles formed from nuclear membranes.
(b) Nucleolus:
Interphase nuclei contain relatively dense spherical bodies known as nucleolus. Usually, nucleoli disappear during cell division, in the late prophase of mitosis and meiosis, and the nucleolar material is distributed onto the chromosomes. At the end of cell division, the nucleolar materials dissociate from chromosomes, and fuse together to organize a new nucleolus. Nucleolus is ordinarily linked to a specific region (nucleolar organizer region; NOR) of a specific chromosomes (nucleolar organizer chromosome; NOC) of the genome. Ordinarily, two nucleoli are present in an interphase nucleus, but during meiotic prophase a single nucleolus may be seen (since the two homologous nucleolus organizer chromosomes pair to form a single bivalent). But in some special situations, one interphase nucleus may contain more than two nucleoli.
The average chemical composition of nucleoli is as follows- protein, 70%, RNA 30%, and a small amount of DNA.
The ultrastructure of nucleoli reveals four regions:
(a) Amorphous region,
(b) Granular region,
(c) Fibrillar region, and
(d) Chromatin fibres.
The central region of a nucleolus is amorphous and is composed chiefly of protein.
The amorphous region is surrounded by nucleolonema, which includes a granular portion mostly containing ribosomes of 150-200 Å dimeter, and a fibrillar region primarily possessing RNA fibres of 50-150 Å diameter. In addition, chromatin fibres from the nucleolar organizer region also pass through nucleoli.
The major function of nucleoli is the production and organization of ribosomes. Ribosomal RNA (rRNA) is formed by genes located in the nucleolar organizer region of the NOC’s.
(c) Chromatin Fibres:
In transverse sections, transversely and obliquely-cut ends of chromatin fibres are visible throughout the interphase nucleus, except for the main part of nucleolus.
The average diameter of chromatin fibres is about 230 Å. In interphase nuclei, chromatin fibres are relatively much more condensed in heterochromatin than those in euchromatin. As a result, heterochromatic regions appear denser than euchromatic regions (Fig. 2.1). During interphase, one end (telomore) of each chromosome is linked at the peripheri of an annulus. The rest of the chromosome dangles free in the nucleus in a highly, but loosely, folded state (Fig. 2.15 b). Therefore Chromatin fibres are the basic units of chromosome structure.
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