Biological Sciences
Structural Carbohydrates
Structural carbohydrates are complex carbohydrates that provide support and structure to cells and tissues in living organisms. They are primarily composed of polymers of glucose, such as cellulose and chitin, and are essential for maintaining the structural integrity of plant cell walls and the exoskeletons of certain organisms. These carbohydrates play a crucial role in maintaining the physical form and function of biological structures.
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11 Key excerpts on "Structural Carbohydrates"
- Hari G. Garg, Mary K. Cowman, Charles A. Hales(Authors)
- 2011(Publication Date)
- Elsevier Science(Publisher)
Chapter 1 The Development of Carbohydrate Chemistry and Biology DEREK HORTON Department of Chemistry, American University, Washington, DC 20016, USA I. Early History A major proportion of the organic matter on Earth is plant tissue (“biomass”) and is composed of carbohydrates, principally cellulose. This is the structural support polymer of land plants and the material used since ancient times in the form of cotton and linen textiles, and later as paper. Chitin is a polymer related to cellulose that has skeletal function in arthropods and fungi. Other polymeric carbohydrates constitute the structural support framework for marine plants and the cell walls of microorganisms. The sweet carbohydrate of sugar cane, now termed sucrose, has been a dietary item for at least 10 millennia. Ranking alongside cellulose in abundance is starch, a biopolymer that is the food-reserve carbohydrate of photosynthetic plants, and the closely related glyco-gen, the storage carbohydrate in the animal kingdom. Starch occurs as microscopic granules in plant storage tissue (cereal grains, tubers), and the process for its isola-tion was clearly described by Cato the Elder (1): steeping the grains in cold water to swell them, straining off the husks, and allowing the milky suspension to settle to afford, after drying, a white powder. This procedure is essentially the same as the modern corn wet-milling process, and the resultant starch powder is essentially pure carbohydrate whose molecular formula can be expressed as C 6 (H 2 O) 5 , hence the term carbohydrate. It was used as early as 4000 BCE in Egypt as an adhesive with the cellulosic fiber of the papyrus plant to make writing material, and early in the first millennium CE for sizing paper and to stiffen cloth (2).
eBook - PDFProduce Degradation
Pathways and Prevention
- Olusola Lamikanra, Syed H. Imam(Authors)
- 2005(Publication Date)
- CRC Press(Publisher)
On the other hand, structural polysaccharides provide the rigidity and elasticity needed to protect cells. Such structural polysaccharides are characterized by the β -linkages between glucopyranosyl units and, like storage polysaccharides, their macromolecular structure varies with variation in life form. Unmodi fi ed cellulose is the primary structural polysaccharide of plants. The β -linkage produces a nearly linear, extended macromolecular conformation that permits close packing of polymer chains; this close packing in turn encourages intermolecular hydrogen bonding (and crystallinity) between adjacent chains to produce the rigidity required in a structural material. The chemistry, structure, and functional properties of plant polysaccharides will be covered in greater detail in the subsequent sections. Also included is a brief discussion on enzymes, which are speci fi c biological catalysts used in the synthesis and assembly of carbohydrate polymers that provide useful functionalities to plants. Structure and Function of Complex Carbohydrates in Produce 565 The role of enzymes in the syntheses, degradation, and modi fi cation of polysaccha-rides has been the subject of several excellent reviews [1–7]. In addition to enzymes, plants also synthesize and/or utilize any required cofactors and, eventually, inhibitors to terminate reactions once the polymer synthesis is complete or not needed anymore. Because nature assembles these structures, it has both the inherent ability and the capacity to degrade and depolymerize these complex structures into simple mole-cules or polymer building blocks (precursors) via specialized enzymes. Once the polymer degrades back to simple molecules, naturally occurring microbes and/or their enzymes can utilize them as a carbon source, recycling these materials back into the biosphere.
eBook - ePub- Donald Voet, Judith G. Voet(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
They are often associated with proteins (glycoproteins) and lipids (glycolipids) in which they have both structural and regulatory functions (glycoproteins and glycolipids are collectively called glycoconjugates). Polysaccharides consist of many covalently linked monosaccharide units and have molecular masses ranging well into the millions of daltons. They have indispensable structural functions in all types of organisms but are most conspicuous in plants because cellulose, their principal structural material, comprises up to 80% of their dry weight. Polysaccharides such as starch in plants and glycogen in animals serve as important nutritional reservoirs. The elucidation of the structures and functions of carbohydrates has lagged well behind those of proteins and nucleic acids. This can be attributed to several factors. Carbohydrate compounds are often heterogeneous, both in size and in composition, which greatly complicates their physical and chemical characterization. They are not subject to the types of genetic analysis that have been invaluable in the study of proteins and nucleic acids because saccharide sequences are not genetically specified but are built up through the sequential actions of specific enzymes (Section 15-8B). Furthermore, it has been difficult to establish assays for the biological activities of polysaccharides because of their largely passive roles. Nevertheless, it is abundantly clear that carbohydrates are essential elements in many, if not most, biological processes. In this chapter, we explore the structures, chemistry, and, to a limited extent, the functions of carbohydrates, alone and in association with proteins. Glycolipid structures are considered in Section 11-1D. The biosynthesis of complex carbohydrates is discussed in Section 15-8. 1 Monosaccharides Monosaccharides or simple sugars are aldehyde or ketone derivatives of straight-chain polyhydroxy alcohols containing at least three carbon atoms
eBook - ePubAnnual Plant Reviews, Plant Polysaccharides
Biosynthesis and Bioengineering
- Peter Ulvskov(Author)
- 2010(Publication Date)
- Wiley-Blackwell(Publisher)
Chapter 14 PLANT CELL WALL BIOLOGY: POLYSACCHARIDES IN ARCHITECTURAL AND DEVELOPMENTAL CONTEXTS14.1 IntroductionMaureen C. McCann1 and J. Paul Knox21 Biological Sciences, Purdue University, West Lafayette, IN 47907, USA2 Centre for Plant Sciences, Biological Sciences, University of Leeds, Leeds LS2 9JT, UKAbstract:Land plant cell walls contain in the region of 11 major polysaccharides that are grouped as cellulose, hemicelluloses and pectins. Methodologies involving the use of molecular probes and advanced spectroscopies are revealing considerable diversity and complexity as to how these polysaccharides are configured within cell wall structures. Our knowledge of the integration of selected sets of polysaccharides into diverse and multifunctional primary and secondary cell walls is currently increasing although many challenges remain. In addition to understanding the structure and function of polysaccharides in the architectures of individual cell walls and in varied developmental contexts there is also a taxonomic dimension to cell wall polysaccharides that is likely to reveal insights into functions. Challenges for future studies include the understanding of aspects of possible polysaccharide redundancy during cell wall assembly and function, and of how specific polysaccharides are integrated with the mechanical attributes of cell walls and the placing of specific polysaccharide dynamics within physiological frameworks of regulation.Keywords: architecture; carbohydrate-binding modules; cell wall biology; microstructure; monoclonal antibodies; plant cell adhesion; spectroscopy.Cell walls are central to many cell processes that are fundamental to plant growth and survival. Cell walls are also significant materials in the contexts of food, paper, polymers, textiles and fuel. Many of the sets of plant polysaccharides discussed elsewhere in this book are quantitatively the most important macromolecular components of cell walls. Our understanding of how these sets of polysaccharides are assembled to provide functionally diverse, mechanically flexible but robust sets of cell walls is the basis of this chapter. Cell wall polysaccharides are currently separated into cellulose, hemicelluloses and pectic polymers – groupings largely derived from both structural features and solubilization properties. The latter two groups contain polysaccharides of considerable structural diversity and complexity and, in addition, contain polymers that are subject to structural modulation within cell walls.
eBook - PDF- Monica Ek, Göran Gellerstedt, Gunnar Henriksson, Monica Ek, Göran Gellerstedt, Gunnar Henriksson(Authors)
- 2009(Publication Date)
- De Gruyter(Publisher)
However, cellulose, as well as different types of hemicelluloses, belongs to the large group of biomolecules, the carbohydrates , which play central roles in all forms of life. Before the struc-ture, biosynthesis and properties of cellulose are discussed, an introduction to carbohydrate chemistry is given. QKO `~êÄçÜóÇê~íÉ `ÜÉãáëíêó The term 'carbohydrate' (French 'hydrate de carbone') was originally applied to the large group of biomolecules which formula can be expressed as C n (H 2 O) n . However, this definition is not ideal, since many biomolecules with similar functions and properties as carbohydrates do not fulfill this formula due to substitution with for instance amines or phosphate, or a reduction or oxidation. Furthermore, the simplest chemical fulfilling the formula ( n = 1) is formaldehyde (H 2 C=O) – a gas that has neither the physical or biological properties of other carbohydrates. A more useful definition is: Carbohydrates are polyhydroxic carbon chains with at least one aldehyde- or keto group. It follows the definition that a simple carbohydrate (i.e., with one single carbon chain), a monosaccharide , must have at least three carbons ( n 3). Monosaccharides with up to six car-bons are common in nature and there exist also larger molecules, although they are rare. The monosaccharides have excellent possibilities to couple to each other by the hydroxyl groups. If two monosaccharide residues are covalently connected a disaccharide is created. Three con-nected monosaccharide residues give a trisaccharide etc. Oligosaccharide has a more vague definition and can contain from 3 up to approximately 10 monosaccharide units, and if it is even more monosaccharide residues it is a polysaccharide . The term 'sugar' is frequently applied to monosaccharides, disaccharides, and sometimes to short oligosaccharides.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
CHAPTER 22 Carbohydrates LEARNING OBJECTIVES After studying this chapter, you should be able to: 22.1 define carbohydrates 22.2 describe monosaccharides using aldose/ketose terminology 22.3 understand and describe the cyclic structure of monosaccharides 22.4 describe the chemical reactions of monosaccharides 22.5 explain disaccharides and oligosaccharides 22.6 define polysaccharides and describe starch, glycogen and cellulose. Carbohydrates is probably the chemical term that is most widely used by the general public. Commonly referred to as ‘carbs’, it seems everyone has an idea of how much, or how little, or what type we should be consuming in our diets. Carbohydrates are in fact a major class of organic molecules that are important not only in food, but more broadly in biochemistry, medicines, agriculture and even as structural materials. Carbohydrates act as storehouses of chemical energy (glucose, starch, glycogen) and are components of supportive structures in plants (cellulose), crustacean shells (chitin) and connective tissues in animals (polysaccharides). Carbohydrates are also essential components in the nucleic acids RNA(d-ribose) and DNA (2-deoxy- d-ribose), and they play crucial roles in cell surface and membrane recognition that are necessary for cell function. Small carbohydrate molecules, such as glucose, are readily soluble in water and so can be transported through the vascular system to meet a plant’s or animal’s energy requirements. Chemists are increasingly interested in carbohydrates as a potential solution for many of the problems caused by the burning of fossil fuels for energy. Increasing research efforts are being focused on ‘biofuels’, largely ethanol, derived from cellulose. Cellulose accounts for approximately three-quarters of the dry weight of the plant, where it is used to provide plant cell walls with strength and rigidity.
eBook - PDF- J P Felix D'Mello, Carol M. Duffus, J H Duffus(Authors)
- 1991(Publication Date)
- Woodhead Publishing(Publisher)
Their action as toxicants are minimal but their effect ' H. Trowell. D. A. T. Southgate, T. M. S. Wolever. A. R. Leeds. M. A. Gassull. and D. J . A. Jenkins. Lancer, 1976. i, 967. ' M. Eastwood, in 'Diseases of the Colon Rectum and Anal Canal'. ed. J. P. Kirsner and R. G. Shorter, Williams and Wilkins, Baltimore, 1988, p. 133. Report of the British Nutrition Foundations Task Force, 'Complex Carbohydrates in Foods', Chapman and Hall, London, 1990. M. Eastwood and C. A . Edwards 259 on the environment of the intestine and hence on the metabolism of other dietary and biliary secreted substances is profound. 2 Polysaccharides of the Plant Cell Walls (Chemical Structure and Role in the Plant) Complex carbohydrates classified as dietary fibre that are indigestible by mammalian gastrointestinal enzymes are largely derived from and involved in the structure of the plant cell wall. In addition gums, mucilages, and seed galactomannans are used in the food industry and arabinoxylans from seed husks are extracted for therapeutic purposes, e.g. ispaghula. There are also storage polysaccharides which behave in a similar manner, e.g. resistant starch and inulin.”’ The amounts and types of polysaccharides in each cell wall are characteristic, not only of the plant but also the type of cell involved. The complex carbohydrates forming dietary fibre are varied in chemical and physical structure. An important aspect of the plant cell wall is that water soluble polysaccharides interlock to form biological barriers which are water resistant. Primary cell wall, which is the principal contributor to dietary fibre, is 90 % polysaccharides and 10 % protein plus glycoprotein. The backbone of the plant cell wall, cellulose, is a polymer of linear p-( 1,4)-linked glucose molecules, several thousand molecules in length. It occurs largely in a crystalline form in microfibrils, coated with a monolayer of more complex hemicellulosic polymers held tightly by hydrogen bonds.
No longer available |Learn more- Gustavo Blanco, Antonio Blanco(Authors)
- 2017(Publication Date)
- Academic Press(Publisher)
Fig. 4.16 ). This allows the formation of long straight chains, stabilized by hydrogen bonds. Instead, the α-1→ 4 bonds of amylose favor a helical conformation. Cellulose chains cluster in parallel strands that form strong microfibers. This structure is maintained by numerous hydrogen bonds, established between neighboring cellulose chains.Figure 4.16 Diagram of a cellulose molecule segment. Each glucose unit turns 180° with respect to the preceding. Note the H bonds that stabilize the polymer strand.Human digestive secretions do not contain enzymes capable of catalyzing the hydrolysis of β-glycosidic linkages. Therefore, the cellulose ingested with plant foods cannot be modified during its transit through the gastrointestinal tract.Chitin
Chitin is a polysaccharide abundant in nature, which constitutes the exoskeleton of arthropods, such as insects and crustaceans. It consists of units of N -acetyl-D -glucosamine, linked by β-1→4 glycosidic bonds.Heteropolysaccharides
When hydrolyzed, these compounds give more than one type of monosaccharide or monosaccharide derivatives. Often they associate with proteins to form large molecular complexes.Glycosaminoglycans
Formerly called mucopolysaccharides, these compounds are of great biological interest. They are linear polymers, formed by a succession of a disaccharide, generally composed of an uronic acid and a hexosamine. They usually contain sulfate groups. Glycosaminoglycans behave as polyanions thanks to the existence of many ionizable groups of uronic acid (COO− ) and sulfate (S) in the molecule. Except for heparin, which is an intracellular compound, the other members of this group are found in the extracellular space, especially in the ground substance or extra fibrillar matrix of connective tissue. The structure of several types of glycosaminoglycans will be analyzed.O 3 −Hyaluronic acid . The structural unit of this compound is a disaccharide composed byD -glucuronic acidlinked toN-acetyl-D -glucosamineby β-1→3 glycosidic bond. Each unit is attached to the next by a β-1→4 bond (Fig. 4.17 ).Figure 4.17Structural unit of hyaluronic acid (disaccharide on pink box ).D -Glucuronic acid-β-1→3-N -acetyl-D -glucosamine. At the body pH, the carboxyl function is ionized (COO−
- George Rothblat(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The reasons are similar to the reasons for usefulness of these systems for the study of other macromolecules such as nucleic acids. They include the advantages of working with defined, homogeneous populations of cells, growing under defined conditions, with many of the complexities of the animal removed. Provision of radio-active precursors results in very high specific-activity products, compared to those achieved by in vivo labeling. In some cases, metabolism can be studied in synchronized populations of cells or cells in the process of reaggregating or expressing differentiated functions. There is no doubt that these in vitro systems may yield some results which reflect the peculiarities of the mode of cultivation; nevertheless, many basic biologi-cal mechanisms appear to be similar to those operative in vivo. II. General Structural Features of Mammalian Complex Carbohydrates There are a large number of recent reviews on the structure of complex carbohydrates, and a detailed outline will not be attempted here. The reader is referred to glycoprotein reviews by Montgomery (1970), R. G. Spiro (1970), and Winzler (1972); glycolipid reviews by McCluer (1968), McKibbin (1970), and Svennerholm (1970); glycosaminoglycan reviews by Meyer (1966), Cifonelli (1968), and Jeanloz (1970); and 374 Paul M. Kraemer general complex carbohydrate reviews by Ginsburg and Neufeld (1969) and Kraemer (1971a). A rudimentary synopsis of general structural features follows. Native mammalian complex carbohydrates consist of carbohydrate chains covalently linked to protein (glycoproteins and glycosamino-glycans) or to lipid (glycolipids).
- Michael EJ Lean(Author)
- 2006(Publication Date)
- CRC Press(Publisher)
The building-up of carbohydrate molecules by plants is accomplished by photosynthesis. Energy is required to transform the carbon dioxide and water into carbohydrates and this is supplied by the action of sunlight on chlorophyll in the leaves. Consequently, photosynthesis does not take place in the dark. Animals, including man, are unable to synthesize carbohydrates, and this is one of the fundamental differences between animals and plants. The sugars formed by photosynthesis are transported within the plant as sucrose, which is soluble in water. Sucrose is subsequently converted by the plant to polysaccharides, the most important of which are starch and cellulose. Starch is the principal energy reserve of most plants stored in tubers and elsewhere, whereas cellulose, which is the main component of plant walls, provides structural support and synthesized in growing plants of plants.The solar energy used in photosynthesis is stored as chemical energy and this may later be drawn upon by the plant, which, by oxidizing the carbohydrate back to carbon dioxide and water, is able to make use of the energy liberated. Alternatively, animals, by eating the plant, may utilize the chemical energy stored in the carbohydrate molecules.Carbohydrates contain only carbon, hydrogen and oxygen and, except in rare cases, there are always two atoms of hydrogen for every one of oxygen. Accordingly, carbohydrates have the general formula Cx(H2 O)ywhere x and y are whole numbers, and it is from this formal representation as hydrates of carbon that the name carbohydrate is derived. There are, of course, no water molecules as such present within the molecule of a carbohydrate. However, starch usually exists in nature is usually in hydrated form (i.e. associated with water molecules); for example, the storage of carbohydrate and glycogen always involves increased weight from water.Many of the carbohydrates known, particularly the simple ones, do not occur naturally but have been obtained by synthesis in the laboratory. The naturally occurring carbohydrates containing six, or multiples of six, carbon atoms are particularly important. Familiar examples are glucose C6 H12 O6 , sucrose C12 H22 O11 and starch, the very large molecules which are represented by the formula (C6 H10 O5 )n
eBook - ePub- P. M. Dey, J. B. Harborne(Authors)
- 1997(Publication Date)
- Academic Press(Publisher)
Nor do we understand fully the molecular alterations to the wall networks which accompany and control the process of cell expansion. These problems will be approached more closely when it is possible to alter the structures of hemicelluloses and pectin within the wall and observe the mechanical and developmental consequences of the structural changes. To achieve this it is desirable, if not essential, to gain an understanding of the pathways of biosynthesis and metabolic turnover of individual cell wall components. These topics are discussed in sections 5.5 and 5.6. 5.5 BIOSYNTHESIS OF STRUCTURAL POLYSACCHARIDES The biosynthesis of structural cell wall carbohydrates must be seen in the overall context of the carbohydrate metabolism of the cell. The photosynthetic process (Chapter 2), and the processes of breakdown of starch and other carbohydrates (Chapter 4) are relevant. It is these processes which generate sucrose, the main transported carbohydrate in higher plants. The pathways of sucrose catabolism (Chapter 4) are relevant. They are the main energy-generating process in most plant cells, and they are responsible for the generation of phosphorylated sugar intermediates which are in turn required for the synthesis of the sugar nucleotides (Chapter 12) which serve as direct precursors for the formation of cell wall polysaccharides. In this section, only the synthesis of polysaccharides from sugar nucleotide precursors will be discussed. It should, however, be remembered that the pathways of sugar nucleotide formation are important in the regulation of intracellular sugar nucleotide levels, which may themselves participate in the regulation of cell wall polysaccharide biosynthesis. The sugar nucleotides which have been demonstrated to be, or are believed to be, precursors of cellulose, hemicellulose and pectin are listed in Table 5.1
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