| It is the purpose of this series of articles to serve as a tutorial or aid to those
students at the university level who are taking botanical courses. This series will address each topic
on several different levels. The basics will be presented for those taking elementary level courses, followed
by advanced concepts for advanced levels. There will be a section at the end of each topic that will discuss
current relevant research for theose graduate students needing the latest research on the topic. It is also our desire to present this material in such a way thayt the basic concept becomes apparent from the onset. In doing this, we may take a non-traditional approach in the order of material presentation. We hope however, that in doing this, we can save some confusion about the concept being discussed. |
| The Cell | ||||||
| The cell is the smallest unit of a plant that is capable of regeneration of the entire plant.
This is to say, a cell is "totipotent". Cells may be either living or dead and both perform valuable
functions.
The dead cells, serve in some instances, as transport vehicles, such as the tracheids, which transport the
necessary water and nutrients to the various sites within the plant.
Distinguishing living cells from dead cells is not always simplistic. We say that a cell is living if the cell does work. A cell that grows leaves no doubt as to its livelihood, but what about the non-growing cell? Cells perform mechanical work, as is evident, when the protoplasm constanly moves in streams carrying important nutrients. This streaming is referred to as cytoplasmic streaming and requires a constant supply of energy. This constant supply of energy is interrupted, the work stops, and the streaming does the same when the cell dies. Does this mean this cell was never alive? Both living and non-living cells show another type of movement called brownian movement.This type of movement occurs due to kinetic energy exerted almost equally on all sides, and therefore results in no net gain in any one direction, though it does move in all directions about a central point. This type of movement; however is no proof of life. Living cells have a semipermeable membrane, which is to say that different membranes will let differing amounts of an aqueous solution pass through their walls. There are two ways in which semipermeability can be determined. The first is through vital staining, which involves using basic dyes which are composed of salts of a dye and organic acids. Examples of these would be neutral red and methylene blue. These dyes pass through the protoplasm and into the vacuole where they are converted into an ionic form. Since the membranes of the vacuole are impermeable to the ionic solution, the dyes cannot escape, thereby leaving the vacuole stained. When the cell dies, the membrane becomes extremely permeable and therefore the dyes readily escape leaving the vacuole uncolored. The second method of identification by staining is know as estimation of germination potential. This method utilizes tetrazolium dyes which can change from colorless to colored when exposed to freshly cut cell surfaces. The cell; however, are not killed in this type of staining thereby allowing a chemical reaction of the stain to take place when exposed to a freshly cut cell surface. This type of reaction does not take place in the case of a dead cell. Cellular Components The outermost surface of a cell is referred to as the primary cell wall. This structure is basically a container with which to hold the internal structures and to give it shape. The cell wall is composed of cellulose, hemicellulose, or pectins. cellulose is a polyglucose chain composed of hundreds of glucose residues which are covalently linked, forming a ribbon-like structure stabilized by hydrogen bonds. It is the intermolecular hydrogen bonds that cause these chains to adhere together forming bundles which are referred to as cellulose microfibrils. These microfibrils are then arranged in layers bound by hydrogen molecules to form the primary cell wall. It is also the cellulose that is responsible for the cells tensile strength. Hemicellulose is a heterogenous group of polysaccharides that bind to the surface of the microfibrils, and to each other to form a network. Pectins are also a group of cross-linked polysaccharides, which are negatively charged. When Ca++ is added to the pectins, a gel is formed providing an adhesive to hold the cell walls together with the lamella between them. Another substance is found within the cell wall, more predominately in younger cells, than older cells. This substance is called extension and it is a protein that is very rich in proline and hydroxyproline. By contrast, there are substances that are more predominant in older plant cells. These substances include lignin in wood cells, cutin in epidermal cells, suberin in cork cells, and tannin in bark cells. Lignin is a complex organic compound occurring only in the cell walls of vascular plants containing secondary cell walls. This substance adds rigidity to the wall for increased support. Cutin is a polyester derived from hydroxyl and hydroepoxy fatty acids as a result of cuticle synthesis by the epidermal cells. The function of cutin is to restrict the loss od water and solutes from the interior tissues. Suberin is a substance in cork tissues of the roots and stems that is responsible for the protection against excessive gas exchange. The last of these substances is tannin that is found in the bark of many trees. Tannin reduces proteins to an insoluable product that resists decomposition. It is widely used in tanning leather products and processing coagulants for rubber. The cell wall is a secretory product of the cytoplasm and is permeable to water. It has been regarded as being elastically and plastically extensible. This means that when it is stretched out of shape by outside forces that it will return to its original shape when conditions return to normal. There are many types of cells, which serve many specialized functions. Likewise, the cell walls of these cells are as diverse as are the specialized types. This gives us a method for identifying and classifying them. There are also differences in the cell walls of higher plants and lower plants, each having different compositions and organizations. Earlier it was mentioned that cellulose microfibrils were responsible for the tensile strength of cells. This enables the cells to develop turgor pressure, which is the pressure that is built up inside the cell to counterbalance the pressure exerted on the cell from the outside, thereby preventing the cell from collapsing. This turgor pressure, as will be explained later in more detail, plays a significant role in the growth of the shape of the plant. Current Research [Abstract] Liginin, the second most abundant biopolymer, gives the cell wall both strength and hyarophobocity. A lignified cell wall is also resistant to fungal attacks and serves to aid water flow. It has been known for sometime that in the biosynthesis of lignin, phenylalanine ammomalyse (PAL) catalyzes the conversion of phenylalanine to cinnamic acid, and the cinnamyl alcohol dehydrogenase reduces the cinnamyl aldehydes to their corresponding alcohols. If these reactions are suppressed by gene manipulation or through inhibitors, then the lignification of cell walls becomes impossible control. In the experiment a specific inhibitor of PAL@-aminoindan-2-phosphonic acid (AIP) was used, and produced a striking effect on the lignification. In the control specimen there was a strong UV-absorption in the secondary walls. In the test sample after the treatment, there was very weak absorption. In the control specimen the developing walls showed afibrillar texture and that of the AIP treatment showed this fibrillar texture even after the final cell wall formation. From this conclusion, we decerned that while the AIP treatment did not affect the thickening of the cell walls, it did effectively inhibit the lignification process. Control of Lignification In Plant Cell Wall, Takabe Keiji, Keiji, Nakashima, Jin Hibino,Takashi, Daisuke, Saiki, Hiroshi (1955)
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