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Basics of Carbohydrates

Carbohydrates are one of the 3 macronutrients, along with fats Fats The glyceryl esters of a fatty acid, or of a mixture of fatty acids. They are generally odorless, colorless, and tasteless if pure, but they may be flavored according to origin. Fats are insoluble in water, soluble in most organic solvents. They occur in animal and vegetable tissue and are generally obtained by boiling or by extraction under pressure. They are important in the diet (dietary fats) as a source of energy. Energy Homeostasis and proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis, serving as a source of energy to the body. These biomolecules store energy in the form of glycogen and starch, and play a role in defining the cellular structure (e.g., cellulose). Carbohydrates can be broadly classified into simple and complex carbohydrates based on the number of sugar units and chemical bonds. Carbohydrates are the most abundant organic molecules in nature and are the structural basis of many organisms. These biomolecules have a carbon:water ratio of 1:1 and a general molecular formula that can be represented as Cn(H2O)n.

Last updated: Jul 7, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Nomenclature and Structure

Nomenclature

Carbohydrates are composed of carbon, hydrogen, and oxygen; they are defined as aldehydes or ketones of polyalcohols and have a general molecular formula of Cn(H2O)n. Carbohydrates are named based on the number of polymers (number of units) present in the molecule: 

  • Sugars: composed of 1 or 2 units
    • Monosaccharides: molecules containing only 1 actual or potential sugar group; cannot be further hydrolyzed into smaller units
    • Disaccharides: combination of 2 monosaccharides with the elimination Elimination The initial damage and destruction of tumor cells by innate and adaptive immunity. Completion of the phase means no cancer growth. Cancer Immunotherapy of a water molecule
  • Oligosaccharides: combination of 3–10 sugar units
  • Polysaccharides: combination of > 10 sugar units
  • Glycolipids Glycolipids Lipid attached to carbohydrate, outward-facing. The Cell: Cell Membrane: a saccharide molecule linked to a lipid
  • Glycosaminoglycans (GAGs):
    • Previously known as mucopolysaccharides
    • Repeating disaccharide units containing uronic acid and an amino sugar
  • Proteoglycan: carbohydrate chain attached to a polypeptide chain

Structure

  • Carbohydrates consist of carbon, hydrogen, and oxygen.
  • The basis of all carbohydrates is a monosaccharide unit. 
  • A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose).
  • The general empirical structure is Cn(H2O)n.
  • Carbohydrates are organic compounds organized in the form of aldehydes or ketones with multiple hydroxyl groups attached to a carbon chain.
  • Carbohydrates can be structurally represented in both the straight-chain and ring form:
    • Open-chain structure: a long, straight-chain form of carbohydrates
    • Hemiacetal structure: The 1st carbon of glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance condenses with the hydroxyl group of the 5th carbon to form a ring structure (6-membered rings like glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance are known as pyranoses).
    • Haworth structure: a common 3D representation using the pyranose ring structure
Haworth structure

Haworth projection ( glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance being the reference molecule)

Image by Lecturio. License: CC BY-NC-SA 4.0

Classification

Carbohydrates are classified into simple and complex carbohydrates based on their polymerization.

Monosaccharides

  • A simple sugar that forms the most basic carbohydrate unit
  • Cannot be further hydrolyzed into smaller units
  • Monosaccharides are further classified depending on the number of carbon atoms:
    • Triose (C3)
    • Tetrose (C4)
    • Pentose (C5)
    • Hexose (C6)
    • Heptose (C7), and so on
  • Monosaccharides can also be classified based on the primary functional group:
    • Aldose: contains an aldehyde group
    • Ketose: contains a ketone group
  • Glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance is the most well-known monosaccharide: 
    • A hexose aldose
    • Most abundant monosaccharide
    • Plays a pivotal role in metabolism as a source of energy to the body
    • Often stored as starch in plants Plants Cell Types: Eukaryotic versus Prokaryotic and glycogen in animals Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, animalia was one of the kingdoms. Under the modern three domain model, animalia represents one of the many groups in the domain eukaryota. Cell Types: Eukaryotic versus Prokaryotic

Disaccharides

  • 2 monosaccharides combined by a glycosidic linkage
  • Disaccharides are water-soluble simple sugars.
  • The glycosidic linkage is formed by a dehydration Dehydration The condition that results from excessive loss of water from a living organism. Volume Depletion and Dehydration reaction (i.e., by the elimination Elimination The initial damage and destruction of tumor cells by innate and adaptive immunity. Completion of the phase means no cancer growth. Cancer Immunotherapy of a water molecule).
  • Disaccharides can be hydrolyzed to 2 monosaccharides further by enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes known as disaccharidases.
Disaccharides

Disaccharides:
Sucrose, lactose, and maltose are common disaccharides. The image highlights the composition of these carbohydrates.

Image by Lecturio. License: CC BY-NC-SA 4.0

Oligosaccharides

  • Saccharide polymer containing 3–10 monosaccharide units
  • Commonly present as glycans, i.e., linked to other molecules such as lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids or compatible amino acid Amino acid Amino acids (AAs) are composed of a central carbon atom attached to a carboxyl group, an amino group, a hydrogen atom, and a side chain (R group). Basics of Amino Acids chains
  • The main functions of oligosaccharides include cell recognition and cell adhesion Adhesion The process whereby platelets adhere to something other than platelets, e.g., collagen; basement membrane; microfibrils; or other ‘foreign’ surfaces. Coagulation Studies.

Polysaccharides

  • Polymerized products containing > 10 monosaccharide units
  • Further categorized into homopolysaccharides and heteropolysaccharides
  • Homopolysaccharides: 
    • Composed of a single type of monosaccharide
    • Example: glycogen (made up entirely of glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance molecules and serves as a carbohydrate reserve in animals Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, animalia was one of the kingdoms. Under the modern three domain model, animalia represents one of the many groups in the domain eukaryota. Cell Types: Eukaryotic versus Prokaryotic)
  • Heteropolysaccharides: 
    • Polysaccharides containing > 1 type of sugar unit
    • Example: peptidoglycans (saccharide connected to a polypeptide)

Glycosaminoglycans (GAGs)

  • Previously referred to as mucopolysaccharides
  • Present in animals Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, animalia was one of the kingdoms. Under the modern three domain model, animalia represents one of the many groups in the domain eukaryota. Cell Types: Eukaryotic versus Prokaryotic and bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology
  • GAGs are long, linear polysaccharides consisting of repeating disaccharide units. 
  • The repeating disaccharide units consist of a galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance or uronic sugar and an amino sugar.
  • GAGs are highly polar (negatively charged) and hydrophilic Hydrophilic Aminoglycosides; therefore, they function as lubricants or shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock absorbers.
  • The 4 main categories of GAGs include:
    • Hyaluronic acid Hyaluronic acid A natural high-viscosity mucopolysaccharide with alternating beta (1-3) glucuronide and beta (1-4) glucosaminidase bonds. It is found in the umbilical cord, in vitreous body and in synovial fluid. A high urinary level is found in progeria. Connective Tissue: Histology
    • Heparin sulfate
    • Chondroitin sulfate Chondroitin sulfate Derivatives of chondroitin which have a sulfate moiety esterified to the galactosamine moiety of chondroitin. Chondroitin sulfate a, or chondroitin 4-sulfate, and chondroitin sulfate c, or chondroitin 6-sulfate, have the sulfate esterified in the 4- and 6-positions, respectively. Chondroitin sulfate B is a misnomer and this compound is not a true chondroitin sulfate. Connective Tissue: Histology
    • Keratan sulfate

Protein-carbohydrate molecules

  • Proteoglycans (also called mucoproteins): 
    • Proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis covalently bound to 1 or more GAGs (sugar >> protein)
    • Bind BIND Hyperbilirubinemia of the Newborn cations Cations Positively charged atoms, radicals or groups of atoms which travel to the cathode or negative pole during electrolysis. Electrolytes and water
    • Functions:
      • Regulate the movement of molecules through the extracellular matrix Extracellular matrix A meshwork-like substance found within the extracellular space and in association with the basement membrane of the cell surface. It promotes cellular proliferation and provides a supporting structure to which cells or cell lysates in culture dishes adhere. Hypertrophic and Keloid Scars
      • Joint lubricants in synovial fluid
      • May affect Affect The feeling-tone accompaniment of an idea or mental representation. It is the most direct psychic derivative of instinct and the psychic representative of the various bodily changes by means of which instincts manifest themselves. Psychiatric Assessment signaling molecules Signaling molecules Second Messengers
      • Components of bacterial cell walls 
  • Glycoprotein: 
    • Proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis attached to oligosaccharides (protein >> sugar)
    • Involved in:
      • Cellular identity
      • Immune system Immune system The body’s defense mechanism against foreign organisms or substances and deviant native cells. It includes the humoral immune response and the cell-mediated response and consists of a complex of interrelated cellular, molecular, and genetic components. Primary Lymphatic Organs functions: antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions, MHC
      • ABO blood types
      • Some hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types (e.g., follicle-stimulating hormone ( FSH FSH A major gonadotropin secreted by the adenohypophysis. Follicle-stimulating hormone stimulates gametogenesis and the supporting cells such as the ovarian granulosa cells, the testicular sertoli cells, and leydig cells. Fsh consists of two noncovalently linked subunits, alpha and beta. Within a species, the alpha subunit is common in the three pituitary glycoprotein hormones (TSH, LH, and FSH), but the beta subunit is unique and confers its biological specificity. Menstrual Cycle), erythropoietin Erythropoietin Glycoprotein hormone, secreted chiefly by the kidney in the adult and the liver in the fetus, that acts on erythroid stem cells of the bone marrow to stimulate proliferation and differentiation. Erythrocytes: Histology)
  • Protein-carbohydrate bonds (types of glycosidic bonds):
    • N-linked glycosylation Glycosylation The chemical or biochemical addition of carbohydrate or glycosyl groups to other chemicals, especially peptides or proteins. Glycosyl transferases are used in this biochemical reaction. Post-translational Protein Processing: sugar attaches via the anomeric carbon to nitrogen Nitrogen An element with the atomic symbol n, atomic number 7, and atomic weight [14. 00643; 14. 00728]. Nitrogen exists as a diatomic gas and makes up about 78% of the earth’s atmosphere by volume. It is a constituent of proteins and nucleic acids and found in all living cells. Urea Cycle in asparagine Asparagine A non-essential amino acid that is involved in the metabolic control of cell functions in nerve and brain tissue. It is biosynthesized from aspartic acid and ammonia by asparagine synthetase. Synthesis of Nonessential Amino Acids (Asn)
    • O-linked glycosylation Glycosylation The chemical or biochemical addition of carbohydrate or glycosyl groups to other chemicals, especially peptides or proteins. Glycosyl transferases are used in this biochemical reaction. Post-translational Protein Processing: sugar attaches via the anomeric carbon to the hydroxyl group of serine Serine A non-essential amino acid occurring in natural form as the l-isomer. It is synthesized from glycine or threonine. It is involved in the biosynthesis of purines; pyrimidines; and other amino acids. Synthesis of Nonessential Amino Acids, threonine, or tyrosine Tyrosine A non-essential amino acid. In animals it is synthesized from phenylalanine. It is also the precursor of epinephrine; thyroid hormones; and melanin. Synthesis of Nonessential Amino Acids
    • Other types:
      • Phosphoglycosylation: Sugar is linked to a phosphoserine.
      • Glypiation: Protein is linked to glycosylphosphatidylinositol (GPI).
      • C-linked glycosylation Glycosylation The chemical or biochemical addition of carbohydrate or glycosyl groups to other chemicals, especially peptides or proteins. Glycosyl transferases are used in this biochemical reaction. Post-translational Protein Processing (rare): Sugar is linked to tryptophan.

Overview

Table: Types of carbohydrates
Type of carbohydrate Examples
Monosaccharides
  • Glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance
  • Fructose
  • Galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance
Disaccharides
  • Sucrose
  • Lactose
  • Maltose
Oligosaccharides
  • Raffinose
  • Stachyose
Homopolysaccharides
  • Amylose
  • Glycogen
  • Cellulose
Heteropolysaccharides
  • Agar
  • Agarose
  • GAGs
  • Glycoproteins
Mucopolysaccharides (GAGs)
  • Hyaluronic acid Hyaluronic acid A natural high-viscosity mucopolysaccharide with alternating beta (1-3) glucuronide and beta (1-4) glucosaminidase bonds. It is found in the umbilical cord, in vitreous body and in synovial fluid. A high urinary level is found in progeria. Connective Tissue: Histology
  • Heparin
  • Chondroitin sulfate Chondroitin sulfate Derivatives of chondroitin which have a sulfate moiety esterified to the galactosamine moiety of chondroitin. Chondroitin sulfate a, or chondroitin 4-sulfate, and chondroitin sulfate c, or chondroitin 6-sulfate, have the sulfate esterified in the 4- and 6-positions, respectively. Chondroitin sulfate B is a misnomer and this compound is not a true chondroitin sulfate. Connective Tissue: Histology
  • Keratan sulfate
Glycoproteins
  • Immunoglobulins Immunoglobulins Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions
  • Hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types
  • Histocompatibility antigens
Proteoglycans (mucoproteins)
  • Found in saliva Saliva The clear, viscous fluid secreted by the salivary glands and mucous glands of the mouth. It contains mucins, water, organic salts, and ptyalin. Salivary Glands: Anatomy
  • Gastric juice
  • Mucous secretions
  • Synovial fluid

Properties

Physical properties

Stereoisomerism: 

Compounds having the same structural formula but differing in spatial configuration are called stereoisomers:

  • Different spatial arrangements of H and OH groups around asymmetrical carbon molecules create different stereoisomers, which can have different properties.
  • The number of possible stereoisomers depends on the number of asymmetric carbon atoms and can be determined using the formula 2n, where n = number of asymmetric carbon atoms.

Enantiomers: refers to two stereoisomers that are mirror images of one another

  • The 2 mirror forms are denoted as D and L.
  • The reference molecule is glyceraldehyde.
  • D: The OH group is on the right of the next-to-the-last carbon in the chain (the reference carbon, which is C5 in glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance).
  • L: The OH group is on the left of the reference carbon.
Stereoisomers

Stereoisomers with glyceraldehyde as the reference molecule

Image by Lecturio. License: CC BY-NC-SA 4.0

Diastereoisomers: 

Diastereoisomers are nonsuperimposable stereoisomers that are not mirror images of each other.

  • That is, stereoisomers that are not enantiomers
  • Configurational differences around the other carbons present in the chain or ring
  • Three primary hexose diastereoisomers are found in the human body: glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance, galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance, and mannose.
  • An epimer is a diastereoisomer that differs by only 1 stereocenter.
Diastereoisomers

Diastereoisomer:
Glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance and gulose are both 6-carbon monosaccharides (known as hexoses) and diastereoisomers. Although they have the same chemical structure, the spatial orientation Orientation Awareness of oneself in relation to time, place and person. Psychiatric Assessment of the OH and H groups around the 2 middle, asymmetric carbons are different, which gives the molecules slightly different properties.

Image by Lecturio. License: CC BY-NC-SA 4.0

Epimers: 

Epimers are compounds that differ from each other only in configuration with regard to a single carbon atom, other than the reference carbon atom. Glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance and galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance are epimers.

Epimer

Epimer:
Image demonstrating epimers: compounds that differ in configuration around only 1 stereocenter

Image by Lecturio. License: CC BY-NC-SA 4.0

Anomerism: 

Anomerism refers to different spatial configurations around the anomeric carbons.

  • The anomeric carbons:
    • C1 in aldoses (the carbon in the aldehyde group)
    • C2 in ketoses (the carbon in the ketone group)
    • The term is used when the molecule is in the ring form.
  • Different anomers of the same molecules are referred to as α and β configurations (e.g., α-D-Glucose vs. β-D-Glucose)
Α and β anomers of d-glucose

α and β anomers of D-glucose

Image: “ Glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance anomers in Haworth and Fischer projections” by miguelferig. License: Public Domain

Optical activity: 

Optical activity is the ability of stereoisomers to rotate plane-polarized light. The molecules are designated dextrorotatory (+) or levorotatory (-) depending on the direction of light rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays.

  • D and L notations are unrelated to optical activity.
  • An equimolecular mixture of optical isomers has no net rotation Rotation Motion of an object in which either one or more points on a line are fixed. It is also the motion of a particle about a fixed point. X-rays and is known as a racemic mixture.

Chemical properties

Glycosidic bonds:

  • Connect a sugar molecule to something else
  • Covalent bonds between:
    • The OH group on the anomeric carbon (C1)
    • The OH group of another compound (may be another sugar molecule or something else)
  • Bond formation results in loss of H2O
  • Named based on the carbons joined together and the anomerism of the anomeric carbon (e.g., α-1,4-glycosidic bonds join the C1 (anomeric) carbon of the 1st saccharide (with a hydroxyl group oriented in the α configuration) to the hydroxyl group on the C4 carbon of the next saccharide)
  • Glycosides: sugars with a glycosidic bond to another molecule

Oxidation and reduction reactions:

  • Reduction to alcohols:
    • The C=O groups in the open-chain forms of carbohydrates can be reduced to alcohols using sodium Sodium A member of the alkali group of metals. It has the atomic symbol na, atomic number 11, and atomic weight 23. Hyponatremia borohydride or via catalytic hydrogenation.
    • The end products are called alditols.
  • Oxidation:
    • Most monosaccharides (and some disaccharides) can be readily oxidized, which causes other molecules that react with them to be reduced. 
    • These monosaccharides are known as “reducing sugars.”
    • Monosaccharides are reducing sugars if their carbonyl groups can be oxidized to yield carboxylic acid.
  • Benedict’s reaction:
    • Benedict’s reagent is commonly used to detect the presence of reducing sugars (typically glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance) in urine.
    • Principle: reducing sugars reduce Cu2+ ions in Benedict’s reagent (Cu2+ is normally blue in solution) to Cu+, readily forming bright orange Cu2O → the color change from blue to orange indicates a reducing sugar is present

Nutrition and Functions

Carbohydrates play many roles in the sustenance of life. Primarily, carbohydrates serve as a source of nutrition. Additionally, they are structural components of cells, and partake in immune defense and intracellular communication Communication The exchange or transmission of ideas, attitudes, or beliefs between individuals or groups. Decision-making Capacity and Legal Competence.

Nutrition

  • Carbohydrates are 1 of the 3 primary macronutrients ( proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis and fats Fats The glyceryl esters of a fatty acid, or of a mixture of fatty acids. They are generally odorless, colorless, and tasteless if pure, but they may be flavored according to origin. Fats are insoluble in water, soluble in most organic solvents. They occur in animal and vegetable tissue and are generally obtained by boiling or by extraction under pressure. They are important in the diet (dietary fats) as a source of energy. Energy Homeostasis are the others)
  • Carbohydrates are the primary source of energy in the body:
    • Brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification cells and RBCs RBCs Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology are almost completely dependent on carbohydrates as a source of energy.
    • Carbohydrates serve as an instant source of energy (e.g., breakdown of glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance).
    • The recommended carbohydrate intake per day in a 2000-calorie diet is 275 grams.
    • Carbohydrates contribute to 45%–65% of the total calories.
    • 1 gram of carbohydrate provides 4 calories.
  • Sources:
    • Vegetables, fruits, whole grains, legumes, and milk products are major sources of carbohydrates.
    • Grains and certain vegetables, including corn and potatoes, are rich in starch. Sweet potatoes mainly contain sucrose and not starch.
    • Fruits and dark-green vegetables contain little or no starch but provide sugars and dietary fibers.
  • Storage:
    • Stored carbohydrates can be used as an energy source instead of proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis.
    • Carbohydrates can be stored as: 

Functions

  • Serve as intermediates in the biosynthesis Biosynthesis The biosynthesis of peptides and proteins on ribosomes, directed by messenger RNA, via transfer RNA that is charged with standard proteinogenic amino acids. Virology of fats Fats The glyceryl esters of a fatty acid, or of a mixture of fatty acids. They are generally odorless, colorless, and tasteless if pure, but they may be flavored according to origin. Fats are insoluble in water, soluble in most organic solvents. They occur in animal and vegetable tissue and are generally obtained by boiling or by extraction under pressure. They are important in the diet (dietary fats) as a source of energy. Energy Homeostasis and proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis
  • Carbohydrates associate with lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids and proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis:
    • Form surface antigens, receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors molecules, vitamins, and antibiotics.
    • Important for cell-to-cell communication Communication The exchange or transmission of ideas, attitudes, or beliefs between individuals or groups. Decision-making Capacity and Legal Competence and in interactions between cells and other elements in the cellular environment.
  • Important constituent of connective tissues in animals Animals Unicellular or multicellular, heterotrophic organisms, that have sensation and the power of voluntary movement. Under the older five kingdom paradigm, animalia was one of the kingdoms. Under the modern three domain model, animalia represents one of the many groups in the domain eukaryota. Cell Types: Eukaryotic versus Prokaryotic
  • Fiber-rich carbohydrates help prevent constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation
  • Help form the structural framework of RNA RNA A polynucleotide consisting essentially of chains with a repeating backbone of phosphate and ribose units to which nitrogenous bases are attached. RNA is unique among biological macromolecules in that it can encode genetic information, serve as an abundant structural component of cells, and also possesses catalytic activity. RNA Types and Structure and DNA DNA A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). DNA Types and Structure (the ribose Ribose A pentose active in biological systems usually in its d-form. Nucleic Acids and deoxyribose Deoxyribose Nucleic Acids backbones)

Metabolism

Carbohydrates are central to many metabolic pathways in the body, which include the formation, breakdown, and interconversion of carbohydrates.

Key metabolic pathways:

  • Glycolysis Glycolysis Glycolysis is a central metabolic pathway responsible for the breakdown of glucose and plays a vital role in generating free energy for the cell and metabolites for further oxidative degradation. Glucose primarily becomes available in the blood as a result of glycogen breakdown or from its synthesis from noncarbohydrate precursors (gluconeogenesis) and is imported into cells by specific transport proteins. Glycolysis: conversion of glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance to pyruvate Pyruvate Derivatives of pyruvic acid, including its salts and esters. Glycolysis in aerobic conditions and to lactate in anaerobic conditions
  • Gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis: conversion of noncarbohydrate carbon substrates to glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance
  • Glycogenolysis Glycogenolysis The release of glucose from glycogen by glycogen phosphorylase (phosphorolysis). The released glucose-1-phosphate is then converted to glucose-6-phosphate by phosphoglucomutase before entering glycolysis. Glycogenolysis is stimulated by glucagon or epinephrine via the activation of phosphorylase kinase. Glycogen Metabolism: breakdown of glycogen to individual glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance molecules in liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy, muscles, and kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy
  • Glycogenesis Glycogenesis Glycogen Metabolism: binding of individual glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance molecules together to form glycogen
  • Pentose phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes pathway:
    • Alternative pathway Alternative pathway Complement activation initiated by the interaction of microbial antigens with complement C3b. When complement factor B binds to the membrane-bound C3b, complement factor d cleaves it to form alternative C3 convertase (c3bbb) which, stabilized by complement factor p, is able to cleave multiple complement C3 to form alternative C5 convertase (c3bbb3b) leading to cleavage of complement C5 and the assembly of complement membrane attack complex. Innate Immunity: Barriers, Complement, and Cytokines to oxidize glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance
    • Anabolic pathway 
    • Mainly occurs in erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology 
    • Generates pentose and nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide A coenzyme composed of ribosylnicotinamide 5′-diphosphate coupled to adenosine 5′-phosphate by pyrophosphate linkage. It is found widely in nature and is involved in numerous enzymatic reactions in which it serves as an electron carrier by being alternately oxidized (NAD+) and reduced (NADH). Pentose Phosphate Pathway phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes ( NADPH NADPH Nicotinamide adenine dinucleotide phosphate. A coenzyme composed of ribosylnicotinamide 5′-phosphate (nmn) coupled by pyrophosphate linkage to the 5′-phosphate adenosine 2. Pentose Phosphate Pathway)
  • Fructose metabolism: phosphorylated to fructose-1-phosphate → converted to glyceraldehyde-3-phosphate Glyceraldehyde-3-phosphate An aldotriose which is an important intermediate in glycolysis and in tryptophan biosynthesis. Pentose Phosphate Pathway (enters glycolysis Glycolysis Glycolysis is a central metabolic pathway responsible for the breakdown of glucose and plays a vital role in generating free energy for the cell and metabolites for further oxidative degradation. Glucose primarily becomes available in the blood as a result of glycogen breakdown or from its synthesis from noncarbohydrate precursors (gluconeogenesis) and is imported into cells by specific transport proteins. Glycolysis/ gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis pathways)
  • Galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance metabolism: phosphorylated to galactose-1-phosphate → converted to glucose-6-phosphate Glucose-6-phosphate An ester of glucose with phosphoric acid, made in the course of glucose metabolism by mammalian and other cells. It is a normal constituent of resting muscle and probably is in constant equilibrium with fructose-6-phosphate. Gluconeogenesis (enters glycolysis Glycolysis Glycolysis is a central metabolic pathway responsible for the breakdown of glucose and plays a vital role in generating free energy for the cell and metabolites for further oxidative degradation. Glucose primarily becomes available in the blood as a result of glycogen breakdown or from its synthesis from noncarbohydrate precursors (gluconeogenesis) and is imported into cells by specific transport proteins. Glycolysis/ gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis pathways)
Glucose metabolism

Diagram highlighting the pathways of carbohydrate metabolism

Image by Lecturio. License: CC BY-NC-SA 4.0

Clinical Relevance

Several conditions are associated with disorders in carbohydrate metabolism:

  • Glycogen-storage disorders: there are multiple types of glycogen-storage disorders that all are related to an inability to process glycogen. The most commonly tested disorders from this family of conditions are von Gierke’s disease von Gierke’s Disease An autosomal recessive disease in which gene expression of glucose-6-phosphatase is absent, resulting in hypoglycemia due to lack of glucose production. Accumulation of glycogen in liver and kidney leads to organomegaly, particularly massive hepatomegaly. Increased concentrations of lactic acid and hyperlipidemia appear in the plasma. Clinical gout often appears in early childhood. Glycogen Storage Disorders, Pompe’s disease Pompe’s Disease An autosomal recessively inherited glycogen storage disease caused by glucan 1, 4-alpha-glucosidase deficiency. Large amounts of glycogen accumulate in the lysosomes of skeletal muscle; heart; liver; spinal cord; and brain. Three forms have been described: infantile, childhood, and adult. The infantile form is fatal in infancy and presents with hypotonia and a hypertrophic cardiomyopathy. The childhood form usually presents in the second year of life with proximal weakness and respiratory symptoms. The adult form consists of a slowly progressive proximal myopathy. Glycogen Storage Disorders, and McArdle’s disease McArdle’s disease Glycogenosis due to muscle phosphorylase deficiency. Characterized by painful cramps following sustained exercise. Glycogen Storage Disorders. Treatment depends upon the condition and may include dietary changes or treatment with medications including allopurinol Allopurinol A xanthine oxidase inhibitor that decreases uric acid production. It also acts as an antimetabolite on some simpler organisms. Gout Drugs, and/or liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy transplantation.
  • Galactosemia Galactosemia Galactosemia is a disorder caused by defects in galactose metabolism. Galactosemia is an inherited, autosomal-recessive condition, which results in inadequate galactose processing and high blood levels of monosaccharide. The rare disorder often presents in infants with symptoms of lethargy, nausea, vomiting, diarrhea, and jaundice. Galactosemia: an autosomal recessive Autosomal recessive Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal recessive diseases are only expressed when 2 copies of the recessive allele are inherited. Autosomal Recessive and Autosomal Dominant Inheritance condition that causes defective galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance metabolism. Galactosemia Galactosemia Galactosemia is a disorder caused by defects in galactose metabolism. Galactosemia is an inherited, autosomal-recessive condition, which results in inadequate galactose processing and high blood levels of monosaccharide. The rare disorder often presents in infants with symptoms of lethargy, nausea, vomiting, diarrhea, and jaundice. Galactosemia presents in infants with symptoms such as lethargy Lethargy A general state of sluggishness, listless, or uninterested, with being tired, and having difficulty concentrating and doing simple tasks. It may be related to depression or drug addiction. Hyponatremia, nausea Nausea An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various enteroviruses. Antiemetics, vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia, diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea, and jaundice Jaundice Jaundice is the abnormal yellowing of the skin and/or sclera caused by the accumulation of bilirubin. Hyperbilirubinemia is caused by either an increase in bilirubin production or a decrease in the hepatic uptake, conjugation, or excretion of bilirubin. Jaundice. Dietary modification to eliminate lactose and galactose Galactose An aldohexose that occurs naturally in the d-form in lactose, cerebrosides, gangliosides, and mucoproteins. Deficiency of galactosyl-1-phosphate uridyltransferase causes an error in galactose metabolism called galactosemia, resulting in elevations of galactose in the blood. Lactose Intolerance is a treatment option. Serious neurologic complications include speech and motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology deficits.
  • Disorders in fructose metabolism: 3 major conditions ( essential fructosuria Essential fructosuria Essential fructosuria is a benign asymptomatic metabolic disorder caused by deficiency in fructokinase, leading to decreased conversion of fructose to fructose-1-phosphate and alimentary hyperfructosemia, but with no clinical dysfunction; may produce a false-positive diabetes test. Disorders of Fructose Metabolism, hereditary fructose intolerance Hereditary fructose intolerance An autosomal recessive fructose metabolism disorder due to deficient fructose-1-phosphate aldolase activity, resulting in accumulation of fructose-1-phosphate. The accumulated fructose-1-phosphate inhibits glycogenolysis and gluconeogenesis, causing severe hypoglycemia following ingestion of fructose. Prolonged fructose ingestion in infants leads ultimately to hepatic failure and death. Patients develop a strong distaste for sweet food, and avoid a chronic course of the disease by remaining on a fructose- and sucrose-free diet. Disorders of Fructose Metabolism, and deficiency of fructose-1,6-biphosphatase) are considered important disorders of fructose metabolism Disorders of fructose metabolism Fructose metabolism is a complex cascade involving several enzymes. The enzymes may be deficient or cause abnormal processing and disease. Essential fructosuria, hereditary fructose intolerance, and intestinal fructose intolerance are 3 of the distinct disorders. The presentation may range from asymptomatic to complaints of vomiting, bloating, flatulence, and diarrhea. Disorders of Fructose Metabolism. Depending on the condition, patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship may be asymptomatic or exhibit symptoms of vomiting Vomiting The forcible expulsion of the contents of the stomach through the mouth. Hypokalemia, bloating Bloating Constipation, flatulence, and diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea. The fundamental treatment is dietary modification.
  • Lactose intolerance Lactose intolerance Lactose intolerance (LI) describes a constellation of symptoms due to lactase deficiency (LD), the enzyme located in the brush border of the absorptive cells in the small intestine. Lactose is the disaccharide present in milk and requires hydrolysis by lactase to break it down into its 2 absorbable constituents, glucose and galactose. Lactose intolerance typically presents with bloating, abdominal cramping, diarrhea, and flatulence. Lactose Intolerance: a condition caused by a lack of the enzyme lactase Lactase An enzyme which catalyzes the hydrolysis of lactose to d-galactose and d-glucose. Defects in the enzyme cause lactose intolerance. Digestion and Absorption of Carbohydrates in the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy. There are 4 classifications of this disease: primary, secondary, developmental, and congenital Congenital Chorioretinitis. Lactose intolerance Lactose intolerance Lactose intolerance (LI) describes a constellation of symptoms due to lactase deficiency (LD), the enzyme located in the brush border of the absorptive cells in the small intestine. Lactose is the disaccharide present in milk and requires hydrolysis by lactase to break it down into its 2 absorbable constituents, glucose and galactose. Lactose intolerance typically presents with bloating, abdominal cramping, diarrhea, and flatulence. Lactose Intolerance leads to a decreased ability to digest lactose. The symptoms include abdominal pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, bloating Bloating Constipation, diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea, flatulence, and nausea Nausea An unpleasant sensation in the stomach usually accompanied by the urge to vomit. Common causes are early pregnancy, sea and motion sickness, emotional stress, intense pain, food poisoning, and various enteroviruses. Antiemetics.

References

  1. The Medical Biochemistry Page. (2020). Biochemical Properties of Carbohydrates. Retrieved June 24, 2021, from https://themedicalbiochemistrypage.org/biochemistry-of-carbohydrates/
  2. Eufic. (2020). The functions of carbohydrates in the body. Retrieved June 24, 2021, from https://www.eufic.org/en/whats-in-food/article/the-basics-carbohydrates
  3. Slavin, J., Carlson, J. (2014). Carbohydrates. Advances in Nutrition. 5(6), 760-761. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4224210/
  4. Aryal, S. (2018). Carbohydrates – definition, function, types, examples, functions. Retrieved June 24, 2021, from https://microbenotes.com/carbohydrates-structure-properties-classification-and-functions/
  5. Murray, R.K., Granner, D.K., Rodwell, V.W. (2006). Harper’s Illustrated Biochemistry 27th edition (pp. 112–120).

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