Tuatara: Volume 11, Issue 3, September 1963
The Cytoplasm of Plant Cells — A review
The Cytoplasm of Plant Cells — A review
In the preceding issue of Tuatara, Dr. M. C. Probine reviewed recent advances in our knowledge of the structure of the plant cell wall. The present paper is an attempt to do the same for the cytoplasm and in forthcoming issues of this journal, Mr. G. K. Richards will review the nucleus of the cell.
It is somewhat unfortunate that a multiplicity of terms has often been used to describe a single part of the cytoplasm. In other cases, recent work has revealed that two different cytoplasmic components, with different names, are actually different phases of a single component. Where possible I have endeavoured to indicate synonyms for terms.
In recent years, sections of the cytoplasm have been examined up to magnifications of 300,000 times with the electron microscope. With improved methods of biochemical analysis it has become increasingly obvious that each of the various organelles in the cytoplasm is a compartment in which specific physiological activities occur, different from but interrelated with those occurring in other organelles. It is also becoming increasingly apparent that many cytoplasmic components which seemed clearly distinct to the optical microscopist, are in fact not entirely independent. For page 144 example, extensions of the nuclear envelope* into the cytoplasm have been found in plant and animal cells. These blend with the endoplasmic reticulum — a discontinuous membrane system which is spread through the ground substance of the cytoplasm.
In a review of this scope, space does not permit a discussion of some cytoplasmic components, for example pyrenoids, fat bodies and phragmosomes (see Manton, 1961, for information about this last component). In general only the cytoplasm of higher plants is considered and thus the chloroplasts of algae, which in many cases are unlike those of higher plants, will not be discussed. All of the electron micrographs which are reproduced here are from animal cells, but they have been chosen to show components which are similar to those found in plants.
The following will be discussed in turn:— plasmalemma and tonoplast; vacuoles; ground substance; mitochondria; chloroplasts; lysosomes; endoplasmic reticulum. Golgi bodies, ribosomes; the nuclear envelope and its fate during cell division.
The Plasmalemma and Tonoplast
In contrast to animal cells, most plant cells are surrounded by a cellulose wall. Internal to this is the cell membrane or plasmalemma (plasma membrane). Minerals in the soil water surrounding roots, freely move into intercellular spaces and into the meshwork of the cell wall. There is evidence that there are charged sites within the wall which are able to bind certain ions, before they are transported into the cytoplasm (see Epstein, 1960). The chemical elements within cells are present in quite different proportions from those in the water and soil surrounding the roots, and from those in the sea in which plant life first began.
‘Cells, tissues, and organisms are microregions of the world containing atoms derived from the environment in proportions differing characteristically from any found in the non-living world. Almost any grouping of atoms can be identified as being part of non-living nature or having been assembled by living cells, by mere determination of the proportions of carbon, oxygen, hydrogen, nitrogen, potassium, phosphorus and sulfur that it contains. Membranes are the boundaries where the living cells abut on the environment.’ (Epstein, 1960).
* This term is preferred to the older ‘nuclear membrane’, because there is actually a double membrane delimiting the nucleus.
Fig. 1 : Diagram showing how the nucleus dictates the structure of proteins synthesised by ribosomes in the cytoplasm. Fig. 2 : Model of the plasmalemma as proposed by Danielli (1954). Fig. 3 : Longitudinal section of a mitochondrion showing membrane reversal; after Chandra (1962). Fig. 4: 3-D diagram of a mitochondrion cut in half.
* A few years ago, with the inception of the ‘Apparent Free Space’ theory, it was considered that the plasmalemma was not a barrier to free diffusion. Experiments revealed that there appeared to be a larger volume within plant tissues open to free diffusion than that occupied by the cell wall and intercellular spaces. It was suggested that only the tonoplast (vacuolar membrane) might be selectively permeable. Subsequent work has revealed experimental errors, e.g. insufficient drying of tissues, which gave values that were too high.
* An Angstrom unit (abbreviated here as ‘A’) is 1/10,000 of a micron, which in turn is 1/1000 of a millimetre.
The tonoplast and plasmalemma possess different permeabilities. Ions passing through the tonoplast into the vacuole can be stored there until required for cellular metabolism.
Vacuoles are membrane-bound bodies which are filled with liquid. The vacuole has the highest water content of the cell's components and they contain up to 98% water. Vacuoles also contain sugars, organic and inorganic salts, organic acids, pigments, proteins, lipoids and other compounds. Many solutes are concentrated and stored in the vacuoles until required for cellular metabolism.
Manton (1962) has studied the growth of vacuoles. She noted that in immature cells, vacuoles may escape recognition under the electron microscope and in fact have been erroneously identified as ‘lipoid bodies’ and ‘dense masses of unknown nature’, by some workers. In Anthoceros (a Bryophyte) in young epidermal cells of the sporophyte, the small vacuoles are star shaped with numerous narrow tubular projections. When the tentacle-like outgrowths of adjacent vacuoles meet they fuse to form a network of tubules, and eventually the adjacent vacuoles become completely fused together.
The Ground Substance
Part of the cytoplasm appears structureless even at the highest magnification of the electron microscope. This is known as the ground substance or matrix. Some authors include the endoplasmic reticulum within the term ‘ground substance.’ Separation of the organelles in the cytoplasm from the ground substance has not been achieved with enough certainly to permit any firm conclusions about its specific chemical composition (Whaley et al, 1960). Porter (1961) has commented that with improved methods there is no reason to believe that this apparently structureless part of the cytoplasm will not in time be shown to contain complex organisations of macromolecules.
Mitochondria (chondriosomes) have been called the ‘powerhouses’ or ‘furnaces’ of the cell. It is within these ‘packets’ that the potential energy in foods, manufactured in photosynthesis within the chloroplasts, is released for metabolic processes by respiration. For example, energy is released for the formation of page 148 new cell wall material, the production of enzymes and the movement of sugars, manufactured in the leaves, to other parts of the plant.
Plant and animal mitochondria are very similar. They have been found in all groups of plants except the blue-green algae, red algae and photosynthetic bacteria (Novikoff, 1961a). However, it would seem likely that in these three groups there are simple membrane systems which perform the functions of mitochondria.
Mitchondria may be rod-shaped (chondriochonts), oval or spherical and range in length from 0.2 to 3.0 microns. There may be hundreds of them within a single cell —- approximately 1000 mitochondria were counted in a rat liver cell. In animals, they are present in greatest numbers in cells which undergo the greatest respiratory activity, e.g. insect flight muscles. There has been little investigation of the number of mitochondria per cell in various plant tissues. According to Mercer (1960) a few observations suggest that they are present in high numbers in the companion cells of the phloem. The companion cells are considered to have a high rate of metabolism because they supply energy for the unknown mechanism by which foods, mainly sucrose, are transported in the sieve tubes.
In most cells the mitochondria are continually moving and changing their shape.
From a number of observations, it seems that mitochondria may fuse and divide. These observations were made from a study of living tissue culture cells, seen with the phase contrast microscope. Novikoff (1961) in his excellent and detailed review, stresses that these mitochondria might well be exhibiting abnormal behaviour when fragmenting or fusing, since tissue, culture cells may be living under stress. Mitochondria are known to be very sensitive to changes and it has been discovered that merely holding a tissue between forceps may cause mitochondria to break into granules.
As seen under the electron microscope, a mitochondrion is bounded by an outer membrane about 40-60A thick which is separated by a less electron dense region about 60-90A thick from an inner membrance as thick as the outer. This inner membrane has many folds which project as cristae into the body or ground substance of the organelle. (Fig. 4). In most plant cells the cristae are flat plates; in some they are more tubular in section. In many animal cells and in those of some plants, e.g. Elodea canadensis (Buvat, 1958), the mitochondria appear to have at least some cristae which seem to extend right across the body of the mitochondrion. The infolded inner membrane provides a large surface area of membranes along which chemical reactions can occur. Each of the two mitochondrial membranes is considered to have a broadly similar structure to that of the plasmalemma. page 149 Like the latter, the mitochondrial membranes are selectively permeable, although their permeability to many compounds differs from that of the plasmalemma.
It is remarkable that mitochondria can swell 4-5 times in volume without losing their internal solutes. It has been suggested that this property is due to the convoluted nature of the inner membrane. Recently Chandra (1962) published photographs which appear to show continuity, or rather a reversion, between inner and outer membrances (Fig. 3). Such an interchange of membranes (which I find difficult to visualise in three dimensions) would more readily explain the considerable swelling of mitochondria which can occur before there is a thinning out and rupturing of the membranes.
20-30% of the dry weight (which is about 33% of the ‘wet’ weight) is lipoid and 65-70% is protein. Some of this protein occurs as the major component of enzymes. Mitochondria do not have DNA (deoxyribose nucleic acid) the hereditary material in chromosomes. There has been controversy as to how much, if any, RNA (ribose nucleic acid) is present. Recent chemical methods indicate that about 5% dry weight of the mitochondrion is RNA and this component (see on) would be important in any protein synthesis which may occur in the mitochondria.
The following processes occur primarily or exclusively in mitochondria and all play a part in respiration.
The Krebs cycle (Citric acid cycle; Tricarboxylic acid cycle). This cycle is the terminal phase in respiration.
The electron transfer chain in respiration.
Fatty acid oxidation. Fatty acids are oxidised in several distinct steps in which the final product is acetyl Coenzye-A.
The overall equation for respiration in which, for example, a gram-molecule of a simple sugar is decomposed to water and carbon dioxide with a release of energy can be depicted as:—
C6 H12 O6 + 6 O2 ——— 6 CO2 + 6 H2 O + about 690,000 calories
The above reaction does not occur in a single step, but in a series of about 18 reactions, each catalysed by its own specific enzyme. There are also additional steps in which hydrogen is combined with oxygen to produce water. There are scores of other chemical reactions associated with respiration, e.g. the production of enzymes, and molecules which accept and transfer electrons and which store the energy released in respiration.
The first series of reactions in respiration is known as glycolysis. These steps are common to anaerobic (respiration in the absence of available oxygen) and aerobic respiration. The end product page 150 is pyruvic acid (CH2 COCOOH). In anaerobic respiration (fermentation), the pyruvic acid is transformed into lactic acid or ethyl alcohol, and sometimes other compounds.
In the Krebs cycle terminating respiration, the pyruvic acid loses carbon dioxide and the degradation product is combined with oxaloacetic acid, with the aid of enzymes and acetyl Coenzyme-A (the final product of fatty acid oxidation). This combination forms citric acid. The citric acid is gradually decomposed with the aid of enzymes, to a series of other acids. During some of these reactions carbon dioxide is released and hydrogen is removed from substrates by DPN (diphosphopyridine nucleotide), forming DPNH (reduced diphosphopyridine nucleotide). At the close of the cycle, oxaloacetic acid is again formed. During the glycolysis steps and Krebs cycle, all of the carbon and oxygen in the original sugar molecule is released as carbon dioxide and the hydrogen has been transferred to DPN. Now in the electron transport chain the DPNH is oxidised back to DPN by a flavine compound which is reduced in the process. The reduced flavine is in turn oxidised by a member of a class of compounds known as cytochromes (cytochrome b)* which in turn is oxidised by a different cytochrome and so on, along a chain of cytochromes. The final cytochrome in the chain (cytochrome A3 or cytochrome C oxidase) takes up oxygen which combines with hydrogen ions (formed earlier in the electron transport system) to form water.
The most important step in respiration for the plant is the storing and utilisation of the energy released. Whether or not the chemical energy released is needed immediately for cellular metabolism, it is first stored as an energy-rich bond in a compound known as ATP (adenosine triphosphate). When this decomposes to ADP (adenosine diphosphate) it releases energy. ATP is formed from ADP and inorganic phosphate at a number of places during the steps in respiration. This process whereby energy-rich ATP is formed is called oxidative phosphorylation. The energy obtained from respiration is available for metabolism when ATP is decomposed to ADP and inorganic phosphate, with the aid of enzymes.
ATP——ADP + inorganic phosphate + about 10,000 calories.
* Recent work has indicated that this particular cytochrome may not be on the main pathway of the electron transport system (Novikoff, 1961 a).
Fragmentation of Mitochondria
Mitochondria have been fragmented into smaller parts by various methods to find out how many of their chemical functions can be performed by isolated cristae for example. Much of this work was done by D. E. Green and co-workers. Their results indicated that particles apparently derived from the external mitochondrial membrane were only capable of electron transport. Other particles which were considered to be derived from cristae, were also able to perform oxidative phosphorylation. Only complete mitochondria were able to carry out the Krebs cycle and fatty acid oxidation and it has been suggested that many of the enzymes needed for these processes are located in the ground substance of the mitochondrion. Using a new techniue of negative staining, Parsons (1963) and Stoeckenius (1963) obtained electron micrographs showing what were apparently one or more enzyme molecules about 85A diameter attached by a narrow stalk (40-50A long) to the membranes of the cristae and probably also on the side of the mitochondrial envelope which faces the matrix.
As I have stated, it seems that mitochondria are able to divide. Manton (1961) states that multiplication of mitochondria by division is ‘most certainly so’ in the small flagellates where the single mitochondrion can be traced throughout a cell division. If this is the only way in which they originate, they resemble nuclei in being self-perpetuating bodies. On the other hand, there have been a number of suggestions that they are formed from other protoplasmic components. It has been suggested that they (1) arise from ‘microbodies’ in the cytoplasm; (2) are formed from Golgi bodies; (3) originate in the nucleus; (4) are formed from the nuclear membrane; (5) are formed from the plasmalemma. This last possibility was suggested to me by Dr. S. G. Wildman, University of California, in 1961. Others too have considered this a possibility and Novikoff comments, ‘It is likely that more attention will be given to the presently unorthodox view that in higher cells mitochondria may arise from infoldings of the cell membrane’.
Only the chloroplasts of higher plants will be discussed. For details of other types of plastids see Granick's (1961) detailed review.page 152
Chloroplasts contain the pigments and enzymes necessary for photosynthesis. In this process light energy from the sun is converted to potential energy when carbon dioxide and water are combined to form sugar. All life depends on photosynthesis for its continuing existence. The overall equation for photosynthesis, in which a simple sugar is formed, is:—
Chloroplasts are considerably larger than mitochondria and vary in size, shape and number per cell. Unlike mitochondria, they are relatively immobile in most plant cells. The chloroplasts of green algae are among the most variable in shape e.g. spiral (Spirogyra), net-like (Oedogonium) and star-shaped (Zygnema). In most higher plants they are disc-like with convex ends. They are about 5 microns in diameter and 2-3 microns thick. The chloroplast is bounded by a differentially permeable double-layered membrane about 100A thick, in the centre of which is an area of low electron density. Little is known of the detailed structure of the membrane. It has been suggested that it consists or two layers of protein separated by a bimolecular lipid layer as in the plasmalemma.
When examined under high magnification with the light microscope, chloroplasts appeared to contain a number of small discs which were called grana. Electron microscope studies have shown that each granum consists of a pile of flattened vesicles (lamellae) arranged one on top of the other, like a pile of pennies. They are embedded in a ground substance called the matrix or stroma. There are lamellae (stroma lamellae) or tubules present in low density in the matrix interconnecting the grana (Fig. 9).
There are densely staining bodies, 20A to 0.2 microns in the stroma (Mercer, 1960). McLean has tentatively identified them are a carotenoid lipid phase. They have been called ‘osmiophilic droplets’. Starch grains, manufactured in photosynthesis, also occur in the stroma.
The double membrane components in a granum have been called discs. Grana are about 0.3-1.0 microns in diameter. There have been different opinions about the fine structure of the chloroplast. Hodge. McLean and Mercer (1955) consider that chlorophyll occurs on all lamellae, but Thomas (1958) and Frey-Wyssling (1957) believe it to be localised in the lamellae of the grana.
* All of the oxygen evolved is derived from water molecules.
Fig. 5: Sectional view of the fine structure of chloroplast lamellae according to Steinmann and Sjostrand (1955). Fig. 6: Sectional view of fine structure of chloroplast lamellae according to Hodge, McLean and Mercer (1955). Fig. 7: Sectional views of development of the chloroplast from a proplastid; after Granick (1961). Fig. 8: 3-dimensional view of the fine structure of chloroplast lamellae according to Weier and co-workers (1963). fr = fretwork; gr = granum. Fig. 9: Longitudinal section of a chloroplast from maize from an electron micrograph made by Abbott Laboratories. gr = grana; st = stroma; od = osmiophilic droplet; bm = outer membrane.
Opinions also differ as to the structure of the grana discs. Steinmann and Sjostrand (1955) from a study of Aspidistra chloroplasts consider the grana are flat hollow discs interleaved between lamellae which are continuous with the stroma lamellae (Fig. 5). Hodge, McLean and Mercer (1955, 1956) studied maize (Zea mays) chloroplasts and concluded grana and stroma lamellae were identical and grana are simply regions where the stroma lamellae have divided (bifurcated) and become more highly oriented (Fig. 6).
When considerable swelling occurs during fixation of material the intergrana membrane system becomes fragmented into small, round, closed vesicles and the grana are seen as disjunct ‘piles of pennies’.
Recent work by Weier. Stocking, Thomson and co-workers (1963) on tobacco (Nicotiana rustica) and bean (Phaseolus vulgaris) chloroplasts has given evidence for a different pattern of chloroplast structure. Their results indicate that the stroma does not invade between the discs of a granum so that discs do not alternate with interdisc space. Thus the discs (membrane bounded loculi) are tightly appressed together. They also concluded that adjacent grana are connected not by intergrana lamellae but by a network of flattened tubular channels which they called frets (Fig. 8). Their photos indicated there are connections between discs in a granum by means of these channels. Their model seems to more readily explain the ‘pile of pennies’ configuration, when the membrane system (tubules) in the stroma is ruptured by swelling treatments.
Chemical Composition of Chloroplasts of Higher Plants
The chief components are, proteins 35-55% dry weight; lipid 20-30%; pigments (chlorophyll a, chlorophyll b, xanthophyll and carotene) 13.5%; RNA 2-3% (from Granick, 1961). It is not yet certain whether DNA is present. However Ri and Plaut (1962) using new techniques revealed ‘microfibrils’ which appeared to be DNA macromolecules in the chloroplast of Chlamydomonas, a green alga. They have undertaken preliminary studies on chloroplasts of higher plants, e.g. maize, which indicate that there are small (25A) fibrils which are DNA macromolecules. They suggest that these fibrils represent the genetic system of the chloroplast.
As in all cytoplasmic organelles, structure and function are closely related. The molecular structure of the membrane system (at present not known in detail) is such that there is an efficient transfer of energy absorbed by the pigments from the sunlight. All of the pigments absorb energy from the sun and this is page 155 transferred to chlorophyll a. This energy from the ‘excited’ chlorophyll ‘a’ molecules is efficiently used as chemical energy to combine carbon dioxide and water to produce starch in a complicated series of enzymatically controlled steps not fully known. Thomas (1958) suggests that the lamellae (1) provide the required complex of pigments with their associated lipoproteins to permit their functioning in the aqueous medium of the cell; (2) ensure maintenance of intermolecular distances at which an efficient energy transfer is possible; (3) may facilitate energy transfer by orientation of the photosynthetic pigments; (4) carry enzymic centres involved in the first steps of the photosynthetic chain and thus guarantee a close connection between these enzymic centres and chlorophyll molecules; (5) the closely packed lamellae may give capillary channels enabling the photosynthetic products to be quickly transported from sites of synthesis.
The pigments are bound into lipoprotein complexes in the lamellae. It appears that the enzymes which ‘fix’ CO2 and convert it finally to starch are localised in the stroma. The photodecomposition of water occurs in the grana. For further details, see Granick's review (1961).
J. W. Lyttelton (1962) of the New Zealand D.S.I.R. isolated ribosomes from chloroplasts. Although most protein synthesis occurs in the endoplasmic reticulum and in ribosomes free in the cytoplasm, it was known that mitochondria and chloroplasts could synthesise proteins from amino acids. Ribosomes have also been isolated from mitochondria and nuclei.
‘Mature plastids in many pigmented algae can multiply by fission and habitually do this to keep pace with normal cell division. In higher plants and in algae with specialised growing points this capacity seems to have been lost’ (Manton, 1961).
Chloroplasts of higher plants arise by the growth of small bodies called proplastids and multiplication occurs at this proplastid stage. Young proplastids are amoeboid, colourless bodies, 0.4-0.9 microns in diameter and are surrounded by a double membrane (Fig. 7a). They divide by elongating and ‘pinching’ in half. When they are about 1 micron in diameter. the inner membrane buds off spherical or elongate vesicles (Fig. 7b). These increase in number, fuse, widen and in some areas the vesicles thicken and a pale green colour develops. The proplastids continue enlarging and become lens shaped. Vesicles are still formed from the inner membrane (Fig. 7c). Numerous double membraned lamellae now extend the length of the plastid with slight differentiation into grana and non-grana regions (Fig. 7d). Then in regions where the grana are becoming differentiated. the vesicles increase in thickness and become arranged close together, forming grana (Fig. 9).page 156
When seedlings are grown in the dark, the vesicles accumulate to form a dense body, the primary granum or prolamellar body which has a three-dimensional lattice of beaded or tubular strands. When the seedling is placed in the light. normal grana are formed. It has been considered that the formation of the primary granum was typical of any developing chloroplast, but most workers now believe that it is an atypical structure formed only when seedlings are kept in the dark.
Less than 10 years ago a new cytoplasmic component was found by de Duve and co-workers, using improved centrifugation techniques. They discovered spherical particles, about 0.4 microns in diameter which they named ‘lysosomes’ because of their richness in hydrolytic enzymes (Novikoff, 1961). It has been deduced that they are bounded by a lipoprotein membrane. There is also evidence for the existence of other particles of similar size but not chemically identical to lysosomes. Most of the research has been on mammalian tissues, especially liver cells, but recent work indicated that plants too apparently have acid phosphatase located in granules (Novikoff. 1961b). At present any granules which stain for acid phosphatase are considered to be lysosomes and considerable work is being undertaken to clarify the nature of them.
It has been suggested that together with Golgi bodies, lysosomes play a part in the formation of many kinds of secretion products. Brachet (1961) states that de Duve has shown that the lysosome, ‘contains the digestive enzymes that break down large molecules, such as those of fats, proteins and nucleic acids, into smaller constituents that can be oxidised by the oxidative enzymes of the mitochondria.’
Endoplasmic Reticulum, Golgi Bodies and Ribosomes
Extending throughout the ground substance of the cytoplasm is a network of membrane-bound vesicles — the endoplasmic reticulum. This endoplasmic reticulum (ER) or ergastoplasm, is a more or less labile (changing) structure which in many meristematic cells is an elaborate network of tubules. In older cells it may be a less extensive system of membrane-bound vesicles. The bounding membrane of the ER is lipoprotein and is considered to be of a broadly similar structure to the plasmalemma. The material enclosed by the membranes appears structureless under the electron microscope. The membranes provide a large surface area within the cytoplasm which would allow an ordered distribution of enzymes and substrate. The ER probably represents, ‘various and varying packets of metabolites and enzymes’ (Porter, 1961).page 157
Fig. 10: Composite cross section of young outer rootcap cells of maize showing development of particles formed from Golgi bodies. Drawn from electron micrographs of Mollenhauer and co-workers (1961). Fig. 11: Cross section of older rootcap cell of maize, after vacuolation, showing development of cell wall. Drawn from electron micrograph of Mollenhauer and co-workers (1961). ga = Golgi apparatus; er = endoplasmic reticulum; gv = Golgi vesicle; gs = ground substance; gp = particle formed from the Golgi apparatus; m = mitochondrion; p = plasmalemma; cw = cell wall; t = tonoplast; v = vacuole.
There are two types of ER, a smooth or agranular form and a rough or granular one. The smooth form consists of a complex of tubules with a diameter of 500-1000A. The rough type which is especially prominent in cells undergoing considerable protein synthesis, has vesicles which are flat rather than tubular. There are always small round particles (sometimes called Palade granules) on the outer surface of the membranes. They also occur on the outer side of the nuclear envelope. These sperical particles which are rich in RNA seem similar to or identical with ribosomes. Ribosomes are small spherical cytoplasmic bodies. 150-200A in diameter, which are involved in protein synthesis. Continuity between the smooth and rough forms of the ER has been repeatedly demonstrated (Porter, 1961).
When the ER is centrifuged. it is broken up into small phospholipidribonucleoprotein particles. 500-2000A diameter, to which Claude gave the name ‘microsomes’. For a time, it was believed that ‘microsomes’ were integral components of the cytoplasm but it has now been established that they represent a breakdown of smooth and rough ER.
The Golgi bodies (Golgi apparatus, dictyosomes) of plants each consist of a ‘stack’ of about six flattened plate-like sacs at the edges of which are associated small spherical vesicles, apparently budded off from the edges of the Golgi-sacs. The Golgi apparatus resembles smooth ER except that the sacs are somewhat smaller, more closely appressed together and may not stain with the same density. There are many Golgi bodies in a meristematic cell. It is not clear how they reproduce, but daughter cells have as many or more Golgi bodies as their parent cell and thus multiplication must occur at about the time of division (Whaley et al., 1960).
At present it is not clear just how closely related the ER and Golgi system are. Periodic continuity has occasionally been shown between them but it is not yet clear whether the Golgi apparatus represents a special differentiation of the ER. ‘Stacks’ of rough endoplasmic reticulum have also been reported in some cells.
In plant and animal cells it has been found that the outer membrane of the nuclear envelope (i.e. the outer layer of what is also called the nuclear membrane) is continuous with the ER. In places, this outer membrane bounding the nucleus extends out into the cytoplasm as part of the ER. Porter (1961) comments, ‘this striking fact … makes it proper to regard the nuclear envelope as part of the endoplasmic reticulum.’
Functions of the Endoplasmic Reticulum
Fig. 12: Generalised diagram of the plant cell. cw = cell wall; p = plasmalemma; gs = ground substance; ga = Golgi apparatus; ch = chloroplast; m = mitochondria; er = endoplasmic reticulum; ri = ribosomes; yv = young vacuole; ly = lysosome; nl = nucleolus; ne = nuclear envelope; np = nuclear pore.
It has also been suggested that the ER may function in the transport of metabolites, e.g. from sites of synthesis to sites of breakdown. In meristematic cells, elements of the ER extend to the cell surface and occasionally at least through the wall into neighbouring cells (Whaley et al., 1960).
The smooth form of the ER is common in cells engaged in the synthesis of lipoids and there is an elaborate development of this reticulum along surfaces where cell walls are being formed (Porter, 1961).
Porter (1961) noted that a membrane enclosed space would allow development of electrical membrane potentials of possibly great significance in life processes.
Functions of Golgi Bodies
Fig. 13: Electron micrograph of weta spermatocytes. Material fixed in KMnO4, embedded in Araldite and stained in KMnO4. Further details in text.
The Nuclear Envelope and Cell Division
This double membraned structure delimits the nucleus. The space between these two membranes (perinuclear space) is 200-400A wide, ‘a sort of moat around the nucleus’ (Porter, 1961) and each membrane is 50A thick. It has been clearly shown that there are pores (annuli) in the nuclear envelope 500-1000A in diameter and larger. Their position seems to coincide with places where the nucleoplasm extends to the nuclear surface, i.e. pores are not found where the chromatin of the chromosomes abuts onto the membrane. Current evidence suggests that the pore is an opening which allows the passage of relatively large particles between nucleus and cytoplasm. Feldherr (1962) injected small gold particles of up to 55A diameter into the cytoplasm of an amoeba (Chaos chaos) and obtained electron micrographs showing the particles in the nucleus and within the pore of the nuclear envelope. The inner and outer membranes of the envelope join to form the circumference of the pore. The extensions of the outer membrane into the cytoplasm which become part of the ER do not have pores.
Fate of the Nuclear Envelope During Cell Division
* Mazia (1961) comments that lysosomes might be a more likely source of such enzymes.
Fig. 14: Electron micrograph of weta spermatid. Material fixed in OsO4 embededd in Methacrylate and stained in KMnO4.
Porter and Machado recently studied division in onion root tip cells and also concluded that the nuclear envelope was reformed from the ER. In contrast to these conclusions, Manton (1960) from a study of cell division in the meristem of Anthoceros cautiously suggested that the new nuclear envelope appeared to be derived by fusion of tubular elements formed from Golgi bodies.
Our knowledge of plant and animal cells has been greatly extended over the last few decades. There has been an ever increasing volume of literature published and it is becoming common for research projects to be duplicated, especially with research on animal tissues. Techniques are still rapidly improving and within a few years our knowledge of the cytoplasm will be greatly extended. Recent work emphasises the following points. (a) There are many detailed similarities between the cytoplasm of plants and animals. (b) A close relationship between structure and function of cytoplasmic organelles. (c) Many organelles are more closely interrelated than was once thought.
Fig. 12 is a diagrammatic representation of plant cell structure, which summarises the ultrastructure of the plant cell.
Explanation of Electron Micrographs
Fig. 13 is an electron micrograph of a section through part of two spermatocytes of the weta, Pachyrhamma fascifer. Magnification about 20,000. N = nucleus; CY = cytoplasm; PL = plasmalemma, separating the two cells: NE= nuclear envelope: P = pore in nuclear envelope (to the left of the letter); C = continuity between nuclear envelope and endoplasmic reticulum (below the letter); M = mitochondrion.
Fig. 14 shows a section through part of a weta spermatid. Magnification about 50,000. G = Golgi body; N = nucleus; C = cytoplasm: E = nuclear envelope.
Fig. 15 was chosen to show both rough endoplasmic reticulum (R) and smooth endoplasmic reticulum (S) in a single cell. P = the plasmalemma (plasma membrane), separating two cells; N = nucleus; M = mitochondrion. The section is of sheep liver hepatic cells, magnified about 20,000 times.
I wish to thank Mr. W. S. Bertaud, Electron Microscope Section, Dominion Physical Laboratory, Lower Hutt, and Dr. G. W. Ramsay, Entomology Division, D.S.I.R., Nelson, for allowing me to publish their electron micrographs.page 165
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