1- Carbohydrates - Ross University

Learning Objectives

  • Understand the functions of carbohydrates in the cell and their physiological relevance.
  • Describe the structure of carbohydrates.
  • Understand what are isomers and enantiomers.
  • Describe the glycosidic bond.
  • Understand how carbohydrates can be linked to noncarbohydrates molecules.
  • Describe the digestion and absorption of dietary carbohydrates.

Terminology and Abbreviations

  • Amylase
  • Amylose
  • Disaccharides
  • Enantiomers
  • Enterocytes
  • Glucosamine
  • Glycogen
  • Glycoside
  • Glycosidic bonds
  • Hepatocyte
  • Herbivorous
  • Hexoses
  • Hydrolyses
  • Intestinal microvilli
  • Isomaltase
  • Isomers
  • Lactase
  • Lumen
  • Maltase
  • Monosaccharides
  • Mucosa
  • Oligosaccharides
  • Omnivores
  • Polysaccharides
  • Ptyalin
  • Ruminants
  • Small intestine (duodenum, jejunum and ileum)
  • Sucrase
  • Trisaccharide

Carbohydrates

  • Carbohydrates have the formula (CH2O)n where n ranges from 3 to 9. These are also known as saccharides.
  • They are the most abundant organic molecules in nature.
  • Functions:
    • Main energy source for most animals.
    • Storage form of energy (in animals = glycogen).
    • Cell membrane components (glycoproteins for cell communication).
    • Structural components (cell walls of plants, bacteria, fungi, algae; exoskeleton of insects).

Carbohydrate Classification

  • All carbohydrates consist of monosaccharides (simple sugars).
  • Can be organized according to the number of carbon atoms (C).

Carbohydrate Classification: Carbonyl Group

  • Monosaccharides can be organized depending on the type of carbonyl group they contain:
    • Aldo-
    • Keto-

Monosaccharide Isomers

  • Isomers = same chemical formula; different structures/spatial arrangement of atoms
  • Examples: fructose, glucose, mannose and galactose (C6H{12}O_6)
  • Glucose Alpha and Beta isomers
    • Difference on the orientation of the hydroxyl group
  • Fructose (C6H{12}O_6)

Monosaccharide Enantiomers

  • Enantiomers are special types of isomers:
    • Mirror images of each other.
    • Two forms are assigned as D- and L-sugar.
    • Most sugars found in nature are D-isomers.
      • D-isomers: the –OH group on the asymmetric carbon farthest from the carbonyl group (CHO) is on the right; in L-isomers, it is on the left.
    • Most enzymes responsible for carbohydrate degradation are specific for either isomer.
  • Isomerases are enzymes capable of interconverting D- and L-isomers.
  • Asymmetric carbons are shown in green

Monosaccharide Joining

  • Monosaccharides can be joined to form disaccharides, oligosaccharides, and polysaccharides.
  • The bonds that link sugars are glycosidic bonds.
    • Disaccharides = 2 monosaccharides.
    • Oligosaccharides = 3-10 monosaccharides.
    • Polysaccharides: > 10 monosaccharides (up to hundreds!).
  • Example: Lactose

Monosaccharide Joining: Important Disaccharides and Polysaccharides

  • Important disaccharides:
    • LACTOSE (galactose + glucose)
    • SUCROSE (glucose + fructose)
    • MALTOSE (glucose + glucose)
  • Important polysaccharides:
    • Branched Glycogen (animal)
    • Starch (plant amylose, amylopectin)
    • Cellulose

Carbohydrates Linked to Non-Carbohydrates

  • Glycosides are carbohydrates attached to non-carbohydrates molecules via glycosidic bonds.
    • Carbs + purine and pyrimidine → Nucleosides
    • Carbs + Lipids → Glycolipids
    • Carbs + proteins → Glycoproteins/proteoglycans
  • Glucosamine = aminosugar → common monosaccharide in many polysaccharides (cartilage, chitin)
  • Sugar derivatives = replacement of a single OH group by another group (Amino group)
  • Mucin (glycoprotein)
    • O-glycosidic bond: polysaccharide attached to a protein backbone, bind to a hydroxyl group
    • N-glycosidic bond: bind to an amine group

Digestion and Absorption

  • Digestion is the process of breaking down complex nutrients into simple molecules.
  • Absorption is the process of transporting those simple molecules across the intestinal epithelium.
    • Absorption cannot occur if food is not digested.
    • Digestion is fruitless if the digested nutrients cannot be absorbed.

Dietary Carbohydrate Digestion

  • Mastication helps break down material (starch and glycogen).
  • Main dietary carbohydrates:
    • Starch (plant) and glycogen (animal).
    • Lactose, saccharose (sucrose), fructose.
  • Mammals generally do not have the enzymes necessary to break down cellulose β (1→4) glycosidic bonds.
  • Ruminants & other herbivores have bacteria in their digestive tracts, which produce enzymes to digest cellulose.

Dietary Carbohydrate Digestion: Omnivores

  • In most omnivores (i.e., humans, pigs, rats) digestion of carbohydrates begins in the mouth.
    • Enzyme: salivary alpha-amylase = ptyalin. Plays a small role overall.
  • Carbohydrate digestion happens mainly in the small intestine (duodenum).
  • Pancreatic and enterocyte enzymes (glycosidases) break glycosidic bonds.

Dietary Carbohydrate Digestion: Salivary and Pancreatic Amylase

  • Salivary amylase starts digestion.
  • Further digestion of carbohydrates is achieved by pancreatic enzymes.
  • Luminal phase of digestion (occurs in the lumen of the small intestine).

Dietary Carbohydrate Digestion: Mucosal Lining

  • Final digestive processes and absorption occur at the mucosal lining (membranous-phase digestion) in the distal duodenum and upper jejunum
  • Digestion is finished by enzymes synthesized and secreted by the enterocytes:
    • maltase
    • isomaltase
    • lactase
    • sucrase

Dietary Carbohydrate Absorption

  • The final products of carbohydrate digestion are monosaccharides:
    • glucose
    • fructose
    • galactose
  • Monosaccharide absorption occurs in the enterocytes of the duodenum and upper jejunum, facilitated by membrane transporters:
    • Sodium (Na^+) dependent transport mechanisms (SGLT1)
    • Facilitated transport (GLUT5 and GLUT2)

Glucose Absorption into Enterocytes

  • GLUT = glucose transporter, SGLT-1 = (Na^+)-dependent glucose cotransporter
  • Glucose transport into the enterocytes:
    • Glucose cannot diffuse into cells (it’s hydrophilic); it must be transported by membrane transporters:
      1. GLUT transporters
        • Passive system (do not require ATP) → transport happens downwards the concentration gradient.
        • GLUT-1 to GLUT-14 (tissue specific).
        • GLUT 4 is insulin dependent.
      2. SGLT-1 transporter
        • Na^+- gradient dependent (cotransporter).
        • Can transport glucose against its concentration gradient.
    • Na^+/K^+- ATPase (pump)
      • Necessary to maintain the Na^+ gradient (intra and extracellular) to facilitate SGLT cotransport.
      • Energy-requiring process (ATP).

Dietary Carbohydrate Utilization

  • After being absorbed through the intestinal lining, monosaccharides are transported to the liver via the hepatic portal system.
  • In the liver:
    • Processed (energy metabolism).
    • Stored for later use (glycogen, fatty acids).
    • Released into the systemic circulation to be utilized by cells throughout the body.

Summary - Dietary Carbohydrate Digestion

  • Luminal-phase digestion:
    • Starch, glycogen --(Pancreatic amylase)--> Glucose polymers
      • Amylase digest to Maltose and oligosaccharides
  • Membranous-phase digestion:
    • Lactose, Maltose, Isomaltose, Maltotriose, Sucrose --(Contact Digestion)--> Glucose
      • Uses:
        • Lactase
        • Maltase
        • Isomaltase
        • Sucrase
      • Maltase, dextrinase, and glucoamylase
  • Absorption:
    • Glucose --(SGLT-1)-->
    • Galactose --(SGLT-1)--> Circulation
    • Fructose --(GLUT-5)-->
  • Na-K ATPase
    • ATP -> ADP + P_1

Abnormal Degradation of Disaccharides

  • Abnormal degradation of disaccharides leads to passage of disaccharides into the large intestine, increasing osmotic activity and bacterial fermentation, resulting in abdominal cramps, diarrhea, and flatulence.
  • Reasons:
    • genetic
    • intestinal diseases
    • pharmaceuticals/drugs that injure the mucosa
    • lactose intolerance
    • age-related reductions in enzyme activities

Carbohydrates Digestion Recall

  1. Salivary alfa amylase (ptyalin) – oral cavity
  2. Pancreatic alfa amylase – lumen of small intestine (duodenum)
  3. Enterocytes cell enzymes – small intestine brush border:
    • Maltase, isomaltase, lactase and sucrase
  4. Absorption of monosaccharides – enterocytes of small intestine (mucosal lining) - transporters GLUT and SGLT

Supplemental Videos

  • Carbohydrate digestion and absorption process and end products: https://www.youtube.com/watch?v=zkden107w9o
  • Carbohydrate Structure and Metabolism, an Overview, Animation: https://www.youtube.com/watch?v=VzAjOPzUIP4
  • Carbohydrate digestion 3D: https://www.youtube.com/watch?v=xIIVUAAn1zM