Introduction to Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, including their derivatives, and substances yielding these compounds.
• They are composed of carbon, hydrogen, and oxygen.
• Hydroxyl groups are present as functional groups.
• The suffix "-ose" indicates a sugar.
• These are found in foods like pasta and bread, and provide energy.

Abundance and Sources

• Carbohydrates are the most abundant organic compounds in nature, and about 65% of the food in diets consist of carbohydrates
• Plants use energy from the Sun to convert carbon dioxide and water into the carbohydrate glucose.
• Glucose molecules are made into long-chain polymers of starch to store energy.
• Common foods like bread, pasta, potatoes, and rice contain carbohydrates.
• Disaccharides include sucrose (table sugar) and lactose in milk.
• During digestion and cellular metabolism, carbohydrates convert into glucose, which oxidizes in cells, providing energy and carbon for proteins, lipids, and nucleic acids.
• In plants, cellulose is a glucose polymer that builds structural frameworks, and is also used for furniture, paper, and clothing.

Functions in Cells

• Carbohydrates serve as a main source of energy for cells.
• They are major structural components of plant cells.
• They provide immediate energy in the form of glucose, and act as a reserve in the form of glycogen.
• Carbohydrates provide energy, especially for the brain and cells.
• They spare proteins and are involved in the synthesis of non-essential amino acids, glycoproteins, and glycolipids.
• They facilitate complete lipid metabolism and provide bulk fiber in diet.
• Carbohydrates are essential for fat oxidation and protein conservation, and also provide carbon for synthesizing non-essential amino acids.
• They add flavor, and are a component of connective tissue matrixes and nerve tissue galactosides.
• Carbohydrates are needed for central nervous system function.
• Adequate hepatic glycogen storage supports liver detoxification.

Dietary Sources

• Sugars come from honey, fruits, soft drinks, and milk.
• Starches come from cereals, pasta, flour, bread, potatoes, root vegetables, and pulses.
• Fiber comes from cereals, bran, the outer skin of fruits and vegetables, rice, and oatmeals.
• Pectin is found in fruits.
• Sources for Glucose include fruits, honey and corn syrup.
• Fructose can be found in fruits and honey.
• Galactose is sourced from milk.
• Maltose comes from backed starch.
• Sucrose comes from cane and beet sugar.
• Lactose comes from milk products.
• Starch and Dextrin are sourced from grains, roots, tubers and legumes.
• Glycogen comes from meat products and seafood.
• Cellulose is found in vegetables and the outer coats of seeds.
• Pectin & Gums come from fruits, plant secretion and seeds.

Classification

• They are classified by the number of subunits they contain.
• The three types of Carbohydrates include Monosaccharides, Oligosaccharides, and Polysaccharides.
  • Oligosaccharides are Disaccharides, Trisaccharides, and Tetrasaccharides
• Carbohydrates are energy foods composed of starch and sugar.
• They are a comparatively inexpensive and readily available source of nutrients
• They are organic compounds with the formula CnH2nOn, and are easily digested.
• Carbohydrates consist of a single sugar unit (glucose), and combined sugars (starch).
• Fiber does not provide energy and cannot be digested by humans, is known as cellulose and roughage, and consists of pectin from fruits and jam.

Classification by Structure

• Carbohydrates are classified as simple or complex.
• Simple carbohydrates are divided into monosaccharides and disaccharides.
• Complex carbohydrates include starch and glycogen.
• Monosaccharides are further classified as aldoses, which include trioses, tetroses, pentoses, and hexoses, or ketoses, such as fructose.
• Disaccharides include sucrose, lactose, and maltose.
• Complex carbohydrates are divided into Homopolysaccharides such as starch and dextrin or Hetropolysaccharides such as hyaluronic acid.

Monosaccharides

• They are single-sugar units and include glucose, fructose, and galactose.
• Glucose (dextrose) is a source of energy and the primary fuel for cells.
• Glucose is typically not found in diets but is produced from starch and found in corn syrup.
• Fructose is derived from fruit and honey, and is the sweetest simple sugar.
• Fructose can be found in soft drinks, cereals, and desserts.
• Galactose is a milk sugar.
• Monosaccharides are simple sugars that possess a free aldehyde (CHO) or ketone (C=O) group with two or more hydroxyl (OH) groups; they are the simplest sugars that cannot be hydrolyzed into smaller units.
• They are classified as aldoses or ketoses, based on the number of carbon atoms.

Monosaccharides Chart

• Triose Structure : C3H6O3: Aldoses: Glyceraldehydes , Ketoses: Dehydroxyacetone
• Tetroses Structure : C4H8O4: Aldoses: Erythrose, Threose Ketoses: Erthrulose
• Pentoses Structure : C5H10O5: Aldoses: Xylose, Ribose, Arabinose Ketoses: Ribulose
• Hexoses Structure : C6H12O6 Aldoses: Glucose, Galactose, Mannose Ketoses: Fructose

Hexoses

• Glucose is the essential energy source for the body and its other names are dextrose and blood sugar.
• Galactose rarely occurs freely in nature, but combines with glucose to form lactose in milk.
  • After absorption, Galactose converts to Glucose to provide Energy.
• Fructose is the sweetest of all sugars being (1.5x sweeter than sucrose), and occurs naturally in fruits and honey referred to as "the fruit sugar".
• Monosaccharides are simple single sugars, which do not require digestion, and are quickly absorbed from the intestine.
  • Once absorbed from the intestine they move to the Liver and are converted into Glycogen and stored.

Diassacharides

• These are simple double sugars, that link with two single sugars.
• Important disaccharides are Sucrose, Lactose & Maltose.
• Sucrose = Glucose + Fructose; for example sugar, brown sugar, sugarcane, sugar beet.
• Oligosaccharides produce 2 to 10 molecules of the same or different monosaccharides on hydrolysis
• They are compound sugars.
• Disaccharides yield 2 molecules of monosaccharide on hydrolysis
• Trisaccharides yield 3 molecules of monosaccharide on hydrolysis

Composition of Oligosaccharides

• Disaccharides: Sucrose, Lactose, Maltose, Cellobiose, Trehalose, Gentiobiose, Melibiose
• Trisaccharides: Rhamninose, Gentianose, Raffinose (= Melitose), Rabinose, Melezitose
• Tetrasaccharides: Stachyose, Scorodose
• Pentasaccharide: Verbascose
• Raffinose: α-Galactose (1-6) a-Glucose (1-2) β-Fructose
• Stachyose: α-Galactose (1-6) α-Galactose (1-6) α-Glucose (1-2) B-Fructose
• Verbascose: α-Galactose (1-6) α-Galactose (1-6) α-Galactose (1-6) α-Glucose (1-2) β-Fructose
• Disaccharides are composed of 2 monosaccharides
  • Cells can make disaccharides by joining two monosaccharides by biosynthesis.
• Glucose + fructose = sucrose (table sugar)
  • It is found naturally in sugar cane, sugar beets, honey, and maple syrup.
  • It can be purified from plant sources.
• Glucose + galactose = lactose
  • It is the primary sugar in milk and milk products.
  • Many people have problems digesting lactose.
• Glucose + glucose = maltose
  • It is produced when starch breaks down
  • It is used naturally in fermentation reactions of alcohol and beer.

Tri and Tetra saccharides and relative sweetness

• Trisaccharides have three monosaccharide ex: Raffinose (Formed by one mole of each i.e. glu, fruc, galac)
• Tetrasaccharides are composed of two moles of galactose one mole of glu & one mole of fruct)
• Relative Sweetness to Sucrose:
  • Monosaccharides:
    • Galactose 30%, Glucose 75%, Fructose 175%
  • Disaccharides:
    • Lactose 16%, Maltose 33%, Sucrose 100% (reference Standard)
• Sugar Alcohols:
  • Sorbitol 60%, Maltitol 80%, Xylitol 100%
• Artificial Sweeteners: Aspartame 18,000%, Saccharin 45,000%, Sucralose 60,000%, Neotame 1,000,000%

Polysaccharides

• Starch = (Glucose)n, such as Cereals, Legumes, Tubers
• Glycogen = (Glucose)n which is an animal food stored in the liver & muscles
• Fiber: Plant foods (Vegetable, Fruits, Grains) not digested by Humans such as Cellulose, Hemicellulose, Pectins, Gums, Lignin.
• Polysaccharides contain 10 or more monosaccharide units attached together.
  • Starch is digestible.
  • Glycogen is digestible.
  • Fiber is indigestible.
• Long glucose chains make up polysaccharides.
• Cellulose gives structure to plants and is a fiber in diets.
• Glycogen is an energy-storage sugar produced by animals.
• Liver cells synthesize glycogen after a meal to maintain blood glucose levels.
• Polysaccharides are either homopolysaccharides (yielding a single product on hydrolysis or heteropolysaccharides (yielding a mix).
• Nutrient polysaccharides act as a metabolic reserve, and Structural polysaccharides serve as more rigid support structures.

Starch

• It is the main digestible polysaccharide in human diet.
• Starch is a storage form of carbohydrate in plants.
• Sources include wheat, rice, corn, rye, barley, potatoes, tubers, and yams.
• Two types of plant starch are amylose and amylopectin.
• Amylose forms straight chains with α-1-4 linkages
  • Amylose contains 300 to 5,500 glucose units per molecule with a molecular weight ranging from 10^5 to 10^6.
  • It is typically water soluble and gives a blue color with iodine.
• Amylopectins contain both straight and branched chains with α-1-4 and ß-1-6 linkages
  • A single molecule of amylopectin includes 50,000 to 500,000 glucose molecules and a molecular weight ranging from 10^7 to 10^8.
  • It is insoluble in water and stains purple with iodine.

Cellulose

• It forms cell walls in plant cells, is also named fiber, and is indigestible by humans.
• Glycogen is the storage form of glucose in the body and is found in the liver and muscles, also in small quantities in meat sources.
  • It is not found in plants and is not a major food source.
• Complex starches are large glucose molecules.
  • They require longer digestion times.
  • Ex: Cereal grains, corn, peas, potatoes, squash, legumes.

Fiber

• Insoluble fiber examples are Cellulose, Hemicellulose & Lignin.
  • Cellulose is from cell walls; sources include wheat flour and bran.
  • Hemicellulose is from secretion & Cell; sources include Bran and Gram.
  • Lignin comes from woody parts of plants; sources include vegetables, fruits, and wheat.
• Soluble fiber examples are gums and pectins.
  • Gums are from Special Cell Secretion; sources include Oats, Legumes, Guar, Barley.
  • Pectins are from woody parts of plants; sources include Apple, Guavas, Citrus Fruits.

Action of Fiber

• Insoluable fiber: Accelerates GI Transit, Increases Fecal weight, Promotes bowel movement,Sslows Starch Hydrolysis, Delays Glucose Absorption.
• Soluable fiber: Delays GI Transit, Lowers blood Cholesterol, Delays Glucose Absorption.

Caloric Value of Carbohydrates

• Carbohydrates yields 4 kcal/gm which is needed for Physical Activity, Work of cell, Brain Function
• Recommended Carbohydrate Intake: 40% of Total energy, India 60-80% and Developed Country 30-40%.

Digestion and Metabolism

• Digestion begins in Mouth with Chewing releasing - Saliva (Amylase).
• Dietary Carbohydrate/ Starch combines with Amylase to produce Maltose + Isomaltose
• Carbohydrate Digestion further occurs in Small Intestine.
• Reactions: Maltose + Maltase produces Glucose, Lactose + Lactase produces Glucose + Galactose, and Sucrose + Surcease produces Glucose + Fructose.
• Glucose may proceed to Glycogen in the Liver & Muscle.
• Anabolism includes Glucose to Glycogen, and Glucose to Fat (Lipogenesis)
• Catabolism includes Glucose to ATP, and Glycogen to Glucose.

Malnutrition

• Deficiency: results in Low body weight and accumulation of ketones in the body.
• Overconsumption results in increased dental caries, obesity, sugar-related gastric ulcers, appetite suppression and elevating blood triglyceride levels, leading to the potential for heart diseases.

Summary Chart

• Monosaccharides form Glycosidic Bonds and are categorized as reducing sugars.
  • Examples: Glucose, galactose, fructose
• Disaccharides: include maltose lactose and sucrose
• Polysaccharides are polymers of glucose.
  • Examples found in plants are Amylose, Amylopectin, Cellulose, and in animals as Glycogen.

Clinical Concepts

• Carbohydrates are the main energy source for mammals
• Glucose is the end product.
• Fructose is plentiful in semen and serves energy
• There are several diseases associated with carbohydrates: diabetes mellitus, glycogen storage diseases, galactosemia.
• Non-digestible cellulose is needed in human nutrition
• Heparin is an example of anticoagulant for blood
• Streptomycin is an example of glycoside employed for treatment of tuberculosis