The chloroplast is a non-living structure
The epidermis is the barrier tissue of all above-ground organs from their environment. The cells of the onion epidermis are standard objects on the first day of every botanical-microscopic beginner's internship. Since they do not contain any chlorophyll, they should not actually be used as "typical" plant cells.
Cells in associations are elongated, the length / width ratio can fluctuate within quite wide limits. Each cell is surrounded by a self-contained wall. Especially in the area of the cell poles (tips) or where three cells meet, an extracellular intercellular space is left out. Otherwise, a pectin-containing central lamella is formed between adjacent cells, which ensures the cohesion of the cells and thus the formation of tissues. The cell wall is broken through (perforated) at regular intervals so that the cell contents can make contact with one another. The recesses in the wall are called pits, the strands of plasma running through them are called plasmodesmata. The top of the epidermal cells looks irregularly wrinkled. The reason for this is a water-repellent, waxy deposit, the cuticle.
The "living" cell content surrounded by a membrane (plasma membrane or plasmalemma) is the plasma (= protoplasm). It is also called the protoplast. Usually this is so close to the cell wall that the surrounding membrane cannot be perceived. To prove their existence, the cells are expediently transferred to a medium with a high salt (or sugar) concentration. The protoplast shrinks and becomes detached from the wall. The process is reversible and is called plasmolysis; the reverse of the process is deplasmolysis. The cause of the changes in shape is to be found in the membrane and plasma properties. Only the keywords semipermeability, osmosis and turgor (osmotic pressure) should be noted here. A substance that causes plasmolysis is called a plasmolytic, and depending on its chemical composition (e.g. potassium ions or calcium ions) the protoplast takes on different but characteristic shapes. From this in turn it follows that the plasmolytic affects the membrane properties. It is also clear that the properties of the plasma lemma differ from those of the tonoplast. The tonoplast is the membrane that encloses the vacuole. The difference is particularly evident when cells with stained vacuole contents are used. The vacuole is often traversed by numerous strands of plasma, the occurrence of which indicates that we must not regard the plasma as a simple solution which only obeys the rules of hydrodynamics. Rather, it contains viscous, structure-determining components, the chemical, physical-chemical and structural properties of which have only been recognized in the last few years, and only in fragments.
The cell nucleus is a striking component of almost all living plant cells. The structure of the core is separated from the rest of the plasma, it is surrounded by a shell (according to electron microscopic examinations a double membrane). The term caryoplasm was coined for the core content and the term cytoplasm for the rest of the plasma. However, such terms only apply during certain phases in the life cycle of a cell. In the course of cell and nucleus division (mitosis), the nuclear membrane dissolves, whereby the structure of the nucleus is lost, and the chromosomes take its place. Of course, during this phase it does not make sense to speak of karyo- and cytoplasm.
The nuclei of plant cells are usually round or elliptical, sometimes also spindle-shaped. There is usually one nucleus per plant cell, but cells with two nuclei are not too rare an exception. The cells of some algae, e.g. those from the genus Cladophora are polynuclear or polyenergid. One or more nucleoli are noticeable as substructures of the cell nucleus (often only after staining). They too disintegrate during cell and nucleus division and only form again after a renewed nucleus formation.
Plastids are typical organelles of plant cells. These include the chloroplasts already mentioned. In addition, there are also others, such as the colored chromoplasts and the colorless leukoplasts, as well as transition stages (proplastids). These, in turn, are rudimentary (regressed) forms that arise, for example, during egg cell formation through the degeneration of plastids. During the plant embryonic development, they can differentiate into complete plastids again. In most of the higher plants, however, greening, and thus the conversion into chloroplasts, only takes place after exposure to light.
Transitions between the individual plastid types. Light is required for the formation of the chloroplasts; the regression to proplastids takes place via several intermediate stages in the course of the differentiation of certain cell types (e.g. egg cells) (according to T. BUTTERFAß, 1970).
© M. Knee
Chloroplasts are carriers of the green plant pigment chlorophyll. The photosynthesis reactions take place in them. They enable the plants to convert solar energy into chemical energy. Plants are therefore considered to be primary producers, on whose existence that of the consumers (mainly animals) depends. Chloroplasts are found in many types of cells in aboveground organs. They can be found particularly easily in single-layer fabrics, such as the leaflets of some mosses, such as Funaria hygrometrica or Mnium hornum or a water plant (Vallisneria) observe. Here they are relatively large, their shape is lenticular. In diffuse daylight, they are mainly on the upper and lower surface of the leaves. You can therefore see them from above, and they appear as round structures. With strong exposure they shift and take a position parallel to the side walls, whereby they appear narrower in profile.
Starch is formed and stored in chloroplasts. It can be easily detected with iodine-iodine potassium (Lugol's reagent). The starch - iodine complex is colored blue.
The starch formation in the course of the photosynthesis process can, as J. v. SACHS - probably the most prominent plant physiologist of the last century, showed for the first time, in an experiment, in which a leaf is partially covered by a template so that it is only partially exposed to sunlight. After a day has elapsed, the leaf is first bleached in order to eliminate the disturbance of the starch detection by chlorophyll and other pigments and then placed in an iodine-iodine potassium solution. The image of the stencil is obtained - comparable to a photograph - as a distribution pattern of starch in the sheet.
J. v. SACHS assumed that starch was the primary product of photosynthesis. This assumption turned out to be wrong, however, because simple sugars (glucose, etc.) are initially formed, only a part of which is polymerized into starch.
The shape of the chloroplasts in cells of higher plants is largely similar to that in mosses. Their diameter is on average 4-8 µm. Their number is on the order of 10-50 per cell. The chlorophyll is unevenly distributed in them. At high resolution, areas rich in chlorophyll and poor in chlorophyll (grana and stroma) can be distinguished from one another. Excitation with short-wave light (e.g. blue or violet light) leads to an intense, bright red self-fluorescence (autofluorescence) of the chlorophyll, which can be impressively demonstrated in a fluorescence microscope. The differences between the grana and the stroma are particularly evident. The uniformity of the chloroplast structure in all higher plants indicates that the optimal shape was found quite early in the course of evolution and has not been changed since then. It looks different with the algae. Chloroplasts of green algae (Chlorophyceae) are diverse. Many species have only one chloroplast, which often fills almost the entire plasma of the cell. He is at Spirogyra-Types of helically wound, at Zygnema and Zygnemopsis star-shaped and at Oedogonium structured like a net.
The plate-shaped MougeotiaDepending on the amount of light, chloroplasts can be observed in flat or edge positions. Its rotation is an experimentally well-analyzed example of induced chloroplast movement. In contrast to most of the higher plants, the chloroplasts of many algae have clearly visible pyrenoids in which starch is formed and stored.
Chromoplasts are colored (yellow, orange, or red) plastids. The color is usually based on the presence of yellow xanthophyll and yellow to red carotenoids. Both classes of substances also occur in the chloroplasts, but are covered by chlorophyll there. The fact that chlorophyll is broken down faster than the carotenoids can be seen in autumn from the color of the leaves. There are smooth transitions between chromoplasts and chloroplasts, just as there are transitions between chromoplasts and leucoplasts. Typical chromoplasts cause the orange color of the carrot, the red color of the ripe paprika and tomato fruits and the color of numerous, but by no means all, flowers. The carotenoids are sparingly water-soluble and therefore often crystallize out in the chromoplasts, whereby the crystals can assume different shapes: plate or needle-shaped, jagged, sickle-shaped, etc.
In many cases the color of the flowers and / or the color of the leaves is caused by colored vacuole contents. The color of the vacuole content and the color of the plastids can lead to mixed colors. The typical example: the leaves of the copper beech (the vacuole content is red, the chloroplasts green). According to tradition, the plastids of red and brown algae are also classified as chromoplasts, although they contain chlorophyll. The green color is covered by the red phycoerythrin (in red algae, Rhodophyceae) or the brown fucoxanthin (in brown algae, Phaeophyceae).
Leukoplasts are widespread, colorless plastids. They arise from proplastids, but do not represent a homogeneous group of organelles. A subpopulation can differentiate into chloroplasts or chromoplasts when exposed to light, while this does not apply to others. In the secondary cells of the guard cells, leukoplasts are regularly found that are constantly exposed to light without converting to chloroplasts. Leukoplasts also occur in colorless leaves or leaf parts (= variegated leaves). There are a number of examples which show that they mutated from chloroplasts, which have lost the ability to form chlorophyll. There are even some species, such as the Nestwurz (Neottia, an orchid), which can no longer form any chlorophyll at all, therefore cannot photosynthesize and are dependent on a parasitic or saprophytic way of life. (Saprophy: Dependence on the presence of dead organic material.)
A second class of leukoplasts occurs regularly in non-green tissues of otherwise green plants. They are particularly evident in the roots. These leukoplasts can turn green, but this usually does not happen because there is no light as a trigger. The leukoplasts of the root cap (calyptra) are starchy and are therefore classified as amyloplasts (starchy leukoplasts). As explained elsewhere, they have a function of statoliths (statholite strength), which play a decisive role in the perception of the earth's gravity (geotropism).
In the previous section, we got to know starch as an ingredient in chloroplasts and leucoplasts (amyloplasts). It arises from the polymerisation of glucose residues, which in turn arise as products of photosynthesis. Since sugar can be transported in the plant, e.g. from leaves into the roots or from leaves in seeds and fruits, starch is also formed in these storage organs. Depending on the type of plant, starch grains of different shapes are formed in the plastids. Since one can deduce the origin from their shape, they are suitable for the identification of seeds and other starchy parts of plants. The following values illustrate the size variation of their diameters: starch from potato tubers: 70-100 µm from wheat endosperm: 30-45 µm and from corn endosperm 12-18 µm. Their shape reflects the type of education. The starch molecules are elongated and not very branched. In the plastids, they are deposited radially, starting from a so-called educational center. Layer upon layer is applied, the thickness of the layer depending on the average length of the molecule. A starch grain is therefore crystal-like (semicrystalline), which can be impressively demonstrated with a polarizing microscope. In moistened preparations, a stratification based on different degrees of hydration (water content) of the individual molecule segments can be seen.
Model for the structure of a starch grain. The individual lines symbolize starch molecules. Due to their arrangement, a radial pattern is created. Stratification of the starch grains. a. Educational core and stratification boundaries, b. Diagram of the light refraction ratios. The refractive index is indicated on the ordinate (after A. FREY-WYSSLING, 1938).
The more tightly the molecules are packed, the less water is stored. In turn, water-poor layers are more refractive than water-rich layers. After the preparations have dried out, the stratification disappears. Depending on whether the educational center is centrally or peripherally located, starch grains are created with concentric or eccentric layers. The starch grains of the Gramineae (grasses: wheat, maize, etc.) are usually concentric, those of the potato are always eccentric. Occasionally there are two to three centers of formation in one plastid of the species mentioned, which leads to the formation of several starch grains. In the course of the increase in size, it can happen that such twin or triple grains are finally enveloped by common layers (semi-composite starch grains).
Compound starch grains are characteristic of oat starch, they consist of a large number of partial grains.
Two more examples: starch granules in the seeds of the bean (Phaseolus vulgaris) are very large, their shape is round or oval, the distances between the layers (lamellae) are very even. The center can be easily hollowed out by adding water. In the microscopic image, radial cracks appear - starting from the center. In the milky sap of euphorbias (milkweed plants such as B. Euphorbia splendens) you can find dumbbell-shaped starch grains.
Many plant cells contain crystalline inclusions of varying chemical composition and shape. Crystal aggregates are called drusen, bundles of needle-shaped crystals raphids. a Calcium oxalate druse in a mesophyll cell of an oleander leaf (Nerium oleander). Typical drusen form of the dicotyledons. b Calcium oxalate needles (Raphiden) from the root of Vanilla (Orchidaceae). Typical monocotyledon raphid bundles. e Dissected silica bodies from the silica cells of the epidermis of Schizachyrium sanguineum [a gramineae (grass) species of the ancient world tropics. Characteristic mineralization of a gramineous cell. [W. BARTHLOTT, MARTENS, 1979 (c) W. BARTHLOTT, unpublished. (a, b)]
With very few exceptions, plant cells are surrounded by a cell wall containing cellulose. During growth, it is plastic, i.e. it can be stretched and deformed. After completion of this phase, it assumes elastic properties, i.e. the elasticity remains (within limits), but the deformability is lost. Due to these changed properties, a distinction is made between primary and secondary walls. As we shall see when discussing electron micrographs of the wall, the main difference between the two forms is the different arrangement of cellulose fibrils. In the primary wall they are disordered (scattered texture), in the secondary wall they are directed and stacked in layers (ring texture, screw texture). Secondary walls of many cells (especially those of the strengthening and conductive tissue) are encrusted by additional wall-strengthening substances. Details of their chemistry in another section, here are just the keywords
Lignin (basic substance of wood) and
Called suberine (basic substance of cork).
In addition, secondary walls that have been modified in this way often contain phenolic oxidation products, which give them a dark color (reddish-black with various intermediate tones).
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