8 Key excerpts on "Carbohydrates in nature"

• 

  eBook - ePub

  Medical Biochemistry

  • Antonio Blanco, Gustavo Blanco(Authors)
  • 2017(Publication Date)
  • Academic Press

    (Publisher)

  Carbohydrates are another important component of living beings. They have a structural role, forming the fibrous components of plants and serve as nutrient reserve, stored in roots, seeds, and fruits. Carbohydrates are also widely distributed in animals, where they form molecules of diverse structural and functional relevance.

  Plants synthesize carbohydrates from CO2 and H2 O by capturing the energy from light in the process of photosynthesis . These carbohydrates are ingested by animals, and largely used as fuel. In humans, carbohydrates are the main source of energy. In a balanced diet, they provide 50%–60% of the total calories needed by an individual.

  Carbohydrates are composed of carbon, hydrogen, and oxygen and are defined as polyhydroxy-aldehydes or polyhydroxy-ketones. They have an aldehyde or ketone and various alcoholic functions. Substances that render these polyhydroxy-aldehydes or polyhydroxy-ketones when subjected to hydrolysis are also considered carbohydrates.

  Classification . Depending on their complexity, carbohydrates are classified into monosaccharides, oligosaccharides, or polysaccharides.

  1. Monosaccharides , also known as simple sugars, they consist of only one polyhydroxy-aldehyde or polyhydroxy-ketone. They are obtained as water soluble white crystals and many of them are sweet. Glucose is the most important member of this group.

  2. Oligosaccharides are polymers formed of 2–10 monosaccharides that can be separated by hydrolysis. According to the number of molecules that constitute them, they are designated disaccharides, trisaccharides, tetrasaccharides, etc. Representatives of greater interest within this group are the disaccharides. They are water soluble, can be obtained in crystalline state, and generally have a sweet taste.

  3. Polysaccharides are large molecules, formed by the assembly of monosaccharides, arranged in linear or branched chains. In general, they are water insoluble, tasteless, and amorphous.

  Monosaccharides

  Simple sugars can be defined as polyhydroxy-aldehydes (polyols-aldehydes) or polyhydroxy-ketones (polyols-ketones). In general, carbohydrates are distinguished with the suffix “ose.” When they have an aldehyde function, the monosaccharides are called aldoses ; if they contain a ketone function, they are named ketoses

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  eBook - ePub

  Chemistry and Biochemistry of Food

  • Jose Perez-Castineira(Author)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  3  Carbohydrates

  3.1  Definition, terminology, and classification

  Carbohydrates are a family of biomolecules composed, in principle, by carbon (C), hydrogen (H), and oxygen (O) atoms whose basic general formula is Cn (H2 O)m . The fact that many carbohydrates have two atoms of hydrogen per atom of oxygen in their molecules is responsible for the somewhat misleading name that these molecules have, as “hydrate” means “containing water.” Carbohydrates have no water in their chemical composition, as we shall see in this Chapter, although they are usually hydrated to different degrees both in vivo and in vitro.

  Carbohydrates are usually constituted by an indeterminate number of basic units linked forming polymers. The connections among these units may occur by means of different linkage types, thereby allowing many structural variations [1 ].

  There are several important terms related to carbohydrates:

  • Monosaccharides: They are the basic non-hydrolyzable units of carbohydrates. Monosaccharides can be chemically altered yielding derivatives that may also form polymers. These alterations may involve the addition of elements such as nitrogen (N), sulfur (S), or phosphorus (P).
  • Oligosaccharides. Molecules composed of 2 to 12 (20 for some authors) linked units (residues) of monosaccharides by a specific type of chemical bond known as glycosidic bond. Oligosaccharides are denoted according to the number of monosaccharide residues they have: disaccharides (2 units), trisaccharides (3), tetrasaccharides (4), and so on.
  • Sugar: Many monosaccharides and disaccharides are sweet, hence their trivial name sugars, although table sugar is only composed of sucrose, a disaccharide. Carbohydrates are also known as glycids (from the Greek glykys, glykeros: sweet) or saccharides (from the latin saccharum: sugar).
  • Polysaccharides: Polymers composed of more than 12 (or 20) residues of monosaccharides. Polysaccharides are named as homo- or heteropolysaccharides

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  eBook - ePub

  Biochemistry

  An Organic Chemistry Approach

  • Michael B. Smith(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  13   Carbohydrates

  Another important class of molecules that are critical to an understanding of biological processes are carbohydrates, commonly known as sugars. Carbohydrates are key components of glycosides and cells, and they comprise the backbone of nucleotides, including DNA and RNA. This chapter will introduce the fundamentals of carbohydrate structure and nomenclature.

  13.1 (Poly)hydroxy Carbonyl Compounds

  Most carbohydrates are (poly)hydroxylated aldehydes or ketones, although there are other (poly)hydroxylated derivatives. There are two fundamental ways to classify carbohydrates. The first is based on the type of functional group that accompanies the hydroxyl units. The second is based on the number individual units that make up the carbohydrate. Carbohydrates are classified by the nature of the functional groups X1 and X2 . In a carbohydrate there are several repeating CHOH units defined by the integer “n ,” where n = 3, 5, or 6, and so on. For example, when n = 3 in a carbohydrate, the structure is X1 —CHOH-CHOH-CHOH—X2 . Most carbohydrates are defined by making X1 and/or X2 = CH2 OH, CHO, COR (a ketone), or COOH. A glycose has a CH2 OH and an aldehyde or ketone unit and a glycitol has two CH2 OH units (sometimes called an alditol ). When one group is a carboxylic acid and the other is CH2 OH, it is a glyconic acid (sometimes called an aldonic acid ). A glycaric acid has two carboxyl units and is a hydroxy-dioic acid (sometimes called an aldaric acid), and a uronic acid has a carboxyl group and an aldehyde group.

  The other way to categorize carbohydrates is by the number of sugar units the carbohydrate contains. The general carbohydrate structure shown above contains one carbohydrate unit and it is categorized as a monosaccharide . If two monosaccharides are coupled together, the resulting molecule is a disaccharide and a molecule with three monosaccharide units is a trisaccharide . The structures shown for the disaccharide show both the open-chain form (A ) and the pyranose form (B ; see Section 13.2 and Figure 13.4), as discussed in Section 14.2. Likewise, the trisaccharide is shown as the open-chain form (C ) and the pyranose form (D ), as discussed in Section 14.2. Linking 5–15 monosaccharides yields an oligosaccharide . A polysaccharide

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  eBook - ePub

  Biermann's Handbook of Pulp and Paper

  Volume 2: Paper and Board Making

  • Pratima Bajpai(Author)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)

  The field of carbohydrate chemistry has seen many advances in the past 30 years. The interest in carbohydrate chemistry is not surprising, considering the importance of carbohydrates in most aspects of our environment. It is only in the last few decades that the significance of carbohydrates, in their many forms, has truly been recognized. Carbohydrates have long been recognized for their roles as structural materials and sources of energy in the biological world, but their role as informational molecules has only relatively recently been understood, and we have much more to learn. Some interesting facts regarding carbohydrates only hint at their remarkable properties.

  • 1. Cellulose is the most abundant organic chemical on the face of the earth.
  • 2. Some 400 billion tons of carbohydrates are produced annually by photosynthesis.
  • 3. The typical diet consists of more than 60% (dry weight) of carbohydrates.
  • 4. The major blood group types are determined by sequences and branching of carbohydrates.
  • 5. Clearance of erythrocytes from the blood stream by the spleen is determined by the structure of oligosaccharides on the erythrocyte membrane.

  Generally, carbohydrates may be defined as polyhydroxy aldehydes or polyhydroxy ketones, occurring in their open chain forms or in their heterocyclic ring forms (the acetal or ketal forms). The simplest types of carbohydrates are called neutral sugars . Sugars may be chemically modified to form derivatives. Some examples include reduction of the carbonyl group to give alditols and oxidation of either terminal carbon to form sugar acids, acetylation or methylation of hydroxy 1 groups. The chemistry of carbohydrates is presented here to the extent that it is of importance to the understanding of cellulose and hemicelluloses during pulping and papermaking. For further information, introductory biochemistry, wood chemistry, and organic chemistry texts should be consulted. Many of these texts contain chapters devoted to carbohydrate chemistry.

  17.2. Nomenclature

  The monosaccharides , simple sugars that cannot be easily hydrolyzed into smaller units, are classified according to the number of carbon atoms in the molecule. This classification is used for carbohydrates with three to seven carbon atoms; that is, with trioses, tetroses, pentoses, hexoses , and heptoses. Aldoses are monosaccharides that have an aldehyde when in the acyclic form (in the absence of the hemiacetal form); ketoses are monosaccharides with a ketone when in the acyclic form (absence of the hemiketal bond). Glucose is an example of an aldohexose, and fructose is an example of a ketohexose or hexulose, a six-carbon ketose, as shown in Fig. 17.1

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  eBook - ePub

  Bancroft's Theory and Practice of Histological Techniques E-Book

  • Kim S Suvarna, Christopher Layton, John D. Bancroft(Authors)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)

  13

  Carbohydrates

  Christopher Layton, and John D. Bancroft

  Introduction

  This is a large group of compounds with the general formula Cn (H2 O)n . The role of carbohydrates in cellular metabolism has been known for many years but carbohydrates have more recently been implicated in a wide range of cellular functions including protein folding, cell adhesion, enzyme activity and immune recognition (Varki et al., 1999 ). Histochemical techniques for the detection and characterization of carbohydrates and carbohydrate-containing macromolecules (glycoconjugates) are used regularly in the histology laboratory. These techniques may provide invaluable information assisting the pathologist in diagnosing and characterizing various pathological conditions including neoplasia, inflammation, autoimmune and genetic disorders, and infectious diseases.

  Classification of carbohydrates

  Carbohydrates are divided into two broad categories. Firstly, simple carbohydrates, molecules composed purely of carbohydrates, and secondly glycoconjugates composed of carbohydrates and other molecules such as protein or lipid. These are further categorized as in Table 13.1 . Although lipid carbohydrate complexes are widely distributed in cells and tissues, these types of molecule are not discussed here as they are not detectable by the routine histochemical techniques described in this chapter.

  Monosaccharides

  These are the simplest form of a carbohydrate. Typical monosaccharides have the empirical formula (CH2 O)n , where n is a value between 3 and 9. Glucose (Fig. 13.1 ) is a six-carbon simple carbohydrate which is not charged or ionized and referred to as a neutral sugar. Other neutral sugars include mannose, galactose and fructose. Monosaccharides contain asymmetric carbons referred to as chiral centers and the letters D or L at the beginning of a name refer to the structure of one of the chiral carbons within the molecule. D

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  eBook - ePub

  Chemistry of Biomolecules, Second Edition

  • S. P. Bhutani(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Carbohydrates are the major source of energy. We usually think of carbohydrates as “quick energy”. Our body can mobilise carbohydrates more easily than fat, even though fats contain more energy on a gram-for-gram basis. Free glucose in the blood stream is quickly depleted, but we have stored glucose in readily accessible form as the polymer-glycogen. When we require energy glycogen breaks down to glucose in a quick response to the need for energy. Oligosaccharides play a key role in processes that take part on the surfaces of the cells. Polysaccharides such as cellulose are essential components of grass and trees and other polysaccharides are major components of bacterial cell walls. Starch is the prinicipal food reserve of plants.

  1.3    DEFINITIONS

  Originally, the name carbohydrate was given to all such compounds having the general formula Cx (H2 O)y i.e., they were considered to be hydrates of carbon. Now, we know that carbohydrates are not simply hydrates of carbon but have a variety of other structural features. They are usually defined as polyhydroxy aldehydes and ketones or substances that hydrolyse to yield polyhydroxy aldehydes and ketones. We shall see later that this definition is not entirely satisfactory as they exist primarily as hemiacetals and acetals or as hemiketals and ketals.

  Polyhydroxy aldehyes are also called aldoses and polyhydroxy ketones are called ketoses. Here ald- is taken for aldehydes and ket- for ketones and -ose is the common suffix used for these.

  Low molecular weight carbohydrates are also called as sugars or saccharides, e.g., glucose and sucrose are simple sugars. More common sugars like glucose and fructose, and some less common sugars also occur in the combined state with various hydroxy compounds. Such derivatives are called glycosides and the non-sugar component is called ‘aglycone’. When we have glucose as the sugar component, the compound is called glucoside, if fructose is present, it is a fructoside and so on.

  Glycosides are widely distributed in plants and animals. Structurally, these compounds are related to simple methyl glucosides. When the sugar moiety forms an ether linkage with the aglycone, it is called O-glycoside and when there is a formation of a C—C bond, it is called C

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  eBook - ePub

  Science of Food

  • K. B. Sherrington, P. M. Gaman(Authors)
  • 2010(Publication Date)
  • Routledge

    (Publisher)

  5    Carbohydrates

  Carbohydrates are a group of nutrients important in the diet as a source of energy. They contain the elements carbon, hydrogen and oxygen and are produced in plants by the process of photosynthesis, which may be represented by the following equation:

  Chlorophyll is a green pigment which absorbs energy from sunlight and enables plants to build up carbohydrates from carbon dioxide and water. There are various different carbohydrates but they may be divided into three main groups according to the size of their molecules:

  Sugars 1 Monosaccharides

  The monosaccharide sugars commonly found in food contain six carbon atoms and have the general formula C6 H12 O6 . The three most important members of this group are:

  (A) Glucose (Dextrose)

  The structure of a molecule of glucose is shown in Figure 5.1 . In the conventional representation the carbon atoms in the ring are omitted.

  Figure 5.1   Structure of glucose.

  Glucose is found in varying amounts in fruits and vegetables. Large amounts are found in fruits such as grapes and smaller quantities in vegetables such as young peas and carrots. It is also found in the blood of animals.

  Glucose syrup or commercial glucose is not pure glucose but a mixture of glucose, other carbohydrates and water (see page 65). (B) Fructose (Laevulose) This is chemically similar to glucose except that the arrangement of the atoms within the molecule is slightly different. Fructose is found, together with glucose, in many fruits and in honey. (C) Galactose This is also chemically similar to glucose. It does not exist as such in foods but is produced when lactose, a disaccharide, is broken down during digestion. 2 Disaccharides

  These sugars have the general formula C12 H22 C11 . They are formed when two monosaccharide molecules combine with the elimination of a water molecule.

  This is an example of a condensation reaction

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  eBook - ePub

  Biochemistry

  • Raymond S. Ochs(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  4 Carbohydrates

  Carbohydrates are most commonly encountered as the sweetener sucrose and as starch, the nutrient of bread and pasta. The carbohydrates are literally hydrates of carbon, having the empirical formula (CH2 O)n . Because they possess both hydroxyl and carbonyl groups, they are a step up in complexity from lipids. Unlike lipids, carbohydrates are generally very soluble in water. The Latin word for sugar is saccharide; this is commonly used in the term polysaccharides. Sugars are nutrients for both animals and plants. Animals use sugars for rapid (i.e., minute-to-minute) energy production and lipids for long-term storage. Plants use sugars for energy exclusively, except at the seed stage. Sugars have greater weight than lipids, making them less suitable for long-term energy storage for organisms with mobile lifestyles (i.e., animals and seeds). The reason for the greater mass of sugars is two-fold. Sugars contain more oxygen atoms and are thus denser than lipids. Sugars also associate with water (through hydrogen bonding), further increasing their weight.

  Aside from providing energy, sugars have a wide variety of roles once they are chemically modified, such as cell–cell recognition and signaling. The specificity of blood group types derives from sugars attached to membrane lipids of red blood cells. Proteins secreted from cells are usually covalently linked to sugars. Thus, sugars are integral to biological function. We begin our study of sugars with the simplest ones: the monosaccharides.

  4.1 Monosaccharides

  Monosaccharides have a single carbonyl group, which occurs at the first or second carbon atom. All remaining carbons have an attached hydroxyl group. There are two classes of monosaccharides: aldoses and ketoses. Aldoses (aldehydes) have the carbonyl at carbon one, whereas ketoses (ketones) have the carbonyl at carbon two.

  The “-ose” suffix (as in aldose and ketose) usually indicates that a compound is a sugar. The general category name for sugars is shown in Table 4.1 , starting with the smallest, the triose

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