CHAPTER 1 PART 2

Within the cell wall there are small openings or pores referred to as pits. They may appear in clusters or as individuals spread around the periphery of each cell. These pits are passageways between cell, which link the cells together. Through these pits runs another structure, that is a hollow protoplasmic strand known as the plasmodesmata. The plasmodesmata serves as a conduit through whuch large molecules, which are unable to pass through the tissue can move into adjoining cells. In the higher plants this structure is found in every cell. Lining the plasmodesmata is a continuous membrane running, from cell to cell, called the plasma membrane. Inside the plasma membrane is another cylindrical structure called the desmotuble, which essentially is a liner or supportive structure in the plasmodesmata. Thid desmotubule in turn connects to another structure inside the cell known as the endoplasmic reticulum. We will discuss this structure in more depth shortly. In between the outer surface of the desmotubule and the plasmodesmata lies a space which is referred to as the annulus of cytosol. It is through this area that the smaller molecules pass from cell to cell, and the larger molecules pass through the demotubuleitself.(From this point on, we shall refer to the desmotubule as part of the plasmodesmata).In order to pass through the plasmodesmata, the molecules must have a molecular weight of less than 800. Unfortunately, this passageway is sufficiently large enough to allow viruses to pass from cell to cell. Within the cell wall lies an extremely important area known as the protoplasm. This substance is a viscous, semi- transparent, elastic fluid. (To avoid later confusion, it should be understood that there are two separate terms to describe this area). The protoplasm refers to the area of viscous liquid including the nucleus.The second term cytoplasm refers to the same area excluding the nucleus. Ageneral analysis of this protoplasmic material shows that it is about 85-90% water,7- 10% proteins, 1-2% fatty substances, 1-1.5% organic substances, and 1-1.5% unorganic ions. The protoplasm is one of the most important areas of the cell since this is the site where all the chemical reactions take place. Though this site has so many things happening at one time, most of the studies on the protoplasm have concentrated on the proteins. As a result of this there are many unanswered questions regarding the remainder of the protoplasmic constituents. Suspended within this viscous protoplasm, are numerous other components e.g. the nucleus, nucleoli, vacuole, mitochondria, golgi bodies, lipisa, ribosomes, and the endoplasmic reticulum, which we will discuss, separately begining with the latter. The endoplasmic reticulum is a system of interconnecting membranous canals and sacs containing a homogenous liquid. These membranes consists of both lipids and proteins and occupy about half of the intraceelular space.It is connected at one end to the plasmodesmata and at the other end to the nuclear membrane. There are two basic forms of ER, the rough which is characterized by the presence of ribosomes lining the exterior surface of the membrane. The second form is the smooth, characterized by theabsence of these ribosomes. The rough form; however is the predominant form. There will be further discussion on these two forms in future chapters. The ER occupies about 50% of the internal space of the cell. It is the site in which the transmembrane proteins and lipids for the productionkof the organelles eg.golgi apparatus, lysosomes, endosomes, new ER tissues, secretory vessels, and plasma membrane are formed. It also plays a major role in the synthsis of the mitochondria and peroxisomal membranes. In fact most of the proteins that will be secreted to the exterior of the ER, in addition to those which will be retained by the ER lumen are initially staged witin the ER lumen. It is for this reason that it is considered to be the principal site of protein sysnthesis and most of the cellular metabloism. It also explains why there are so many ribosomes attached to the endoplasmic reticulum. There is a difference in the manner in which the proteins are sent to the endoplasmic reticulum and to the other organelles. Proteins are sent to the organelles only after completion of synthesis where in the case of the endoplasmic reticulum the proteins are sent during the protein synthesis. The ER also serves as a type of quality control area in that any protein which is not folded correctly or assembled properly is retained in the ER. Transitional elements, those vesicles from a specialized area in the ER and destined for the golgi apparatrus must fulfill this requirement in order to exit. If it is unable to do so, it is retained in the Er and eventually broke down and its components re-used. These transitional elements membranes lack bound ribosomes and are non-selective thus necessitating a monitoring quality control area such as the ER. Those proteins that are properly configured and folded do not need a signal to transport them out of the ER. Those that are to remain in the Er lumen however do need a signal to be retained. This signal is usually in the form of a four amino acid signal. As a result if through mechanical manipulation, this signal is removed the protien would be allowed to pass out of the ER. If a protein, not having this signal originally, were to have one placed on it, through the same manipulation, that signal would cause it to be retained. The retention however would not actually be done in the ER but at the cis Golgi network whose receptor has picked up the signal attached in the ER. Those proteins which are caught are then transported back to the ER. The Golgi Apparatus, of which the cis Golgi network is a part, is a series of stacked compartments known as Golgi cisternae. Each one of these cisternae are composed of these compartments in stacks of four to six depending on the cell type. On the outside surface of the golgi apparatus are small vessels called golgi vessels. These vessels receive lipids and protein from the Er and then covalently modify them on the way to their destination. Each golgi cisternae stack is organized as a series of processing compartments which has a Cis face and a Trans face. The proteins from the ER are transported to the cis face which is the entry face and then move through the cisternae to the trans face which is the exit face where glycosylation is completed.Both faces are connected by a network of cisternal structures called the cis golgi netowrk and the trans golgi network. Both networks are responsible for the sorting of proteins and either of them can send proteins back to the ER or send them on to the lysosomes, secretory vessels, or on to the cell surface. It is during the period between the cis and trans networks that oligosaccharides are processed and proteoglycans are assembled. Oligosaccharides are of two types, high mannose and complex. The high mannose form has no sugars added to it in the golgi apparatus. It does; however, contain two N- acetylglucosamines and many mannose residues. the complex form on the other hand contains more than two N- acetylglucosamines and a variable quantity of galactose, sialic acid, and occasionally frucose. Of these sialic acid is the only negatively charged sugar added to the glycoprotein. Complex oligosaccharides are formed by trimmiong the original form of oligosaccharide formed in the Er, and adding sugars to it. These proteins are modified in stages as they move from cisternae to cisternae across the stack and each oligosaccharide must be processed by an enzyme prior to being accepted as a substrate. Transportation of these proteins from palce to place is accomplished by buds which are formed in each cisternae and then conect to the one next to it in the stack. The manner in which an oligosaccharide is determined to be a high-mannose, or a complex oligosaccharide by its position on the protein and by the availability of processing enzymes. If the oligosaccharides and the enzymes are available then a complex oligosaccharide will be formed. If on the other hand they are not available together, a high mannose version will be formed. While N-linked oligosaccharide chains found on proteins are altered while passing through the golgi apparatus, other proteins are modified in other ways. Some of these proteins have sugars added to the OH groups of serine or thronine side chains. This process is called O-linked glycosylation. O-linked glycosylation is catalyzed by a series of glycosyl transferase enzymes which use sugar nucleotides in the lumen of the golgi apparatus to add sugar one at a time. N-acetylgalactosamine is usually the first sugar added; however, as many as ten or more may be added afterward. The heaviest glycosylation is on the core proteins which are modified to form proteoglycans. This process involves the polymerization of one or more of the glycosaminoglycan chains by way of a xylose link into the serines on the core protein. Many proteoglycans become components of the extracellular mix while others remain anchored to the plasma membrane. Then the glycosaminoglycans and some of the tyrosine residues are heavily sulfated helping to give them their negative charges. The production of an oligosaccharide differs from that of the synthesis of a macromolecule of DNA, RNA, or proteins. Nucleic acids are copied from templates, with each component being the same and being applied in the same manner. Complex carboyhydrates; however, require a different enzyme at each step with each end product being the exclusive substrate to link to the next enzyme in the series. It would appear that with the complicated production pathways for oligosaccharides, glycolipids, and glycoproteins each would have an important function; however, none is know at this time. © BOTRESEARCH USA 1998-2008