Carbohydrate Chemistry Overview

Carbohydrate Chemistry

• Carbohydrates are widely distributed in nature, found in both animal and plant tissues.
• They are composed of carbon, hydrogen, and oxygen, commonly with the formula [CH2O]n, where n is 3 or more.
• Carbohydrates are also called saccharides, derived from the Greek word "saccharon" meaning sugar.
• Some carbohydrates can also contain nitrogen, phosphorus, or sulfur.

Carbohydrate Definition

• Carbohydrates are chemically defined as aldehyde or ketone derivatives of polyhydroxy alcohols.
• They can also be defined as compounds that yield these derivatives upon hydrolysis.

Digestion and Metabolism

• Digestion refers to the chemical breakdown of large food molecules into smaller molecules that cells can use.
• Specific enzymes are involved in the breakdown process, facilitating the conversion of food into usable components.
• Metabolism represents the biochemical pathways through which cells obtain energy.

Main Components of Food

• Carbohydrates
• Proteins
• Lipids

Classification of Carbohydrates

• Carbohydrates are classified depending on the number of monomeric units present in their structure.

Monosaccharides

• Simple sugars that cannot be further hydrolyzed into simpler carbohydrates.
• Classified based on the number of carbon atoms (trioses, tetroses, pentoses, hexoses, or heptoses).
• Further categorized into aldoses (containing an aldehyde group) and ketoses (containing a ketone group).
• D-glucose is the most abundant monosaccharide in nature, a six-carbon sugar.

Disaccharides

• Condensation products of two monosaccharide units.
• Examples: maltose and sucrose.

Oligosaccharides

• Condensation products of three to ten monosaccharide units.
• Most are not digested by human enzymes.
• Raffinose, a trisaccharide found in legumes, whole grains, cabbage, and broccoli, is an example.
• Recent evidence suggests that raffinose oligosaccharides have a beneficial effect on the gut microflora.

Polysaccharides

• Polymers consisting of hundreds or thousands of monosaccharide units.
• Also called glycans or complex carbohydrates.
• Can be linear (e.g., cellulose) or branched (e.g., glycogen) in structure.
• Possess high molecular weight and are sparingly soluble in water.
• Do not exhibit the sweetness or aldehyde/ketone properties of simpler sugars.
• Cellulose, a non-starch polysaccharide, is not digested by human enzymes and constitutes a major component of dietary fiber.

Types of Polysaccharides

• Homopolysaccharides (homoglycans):

  • Composed of several units of the same type of monosaccharide.
  • Common examples:
    • Starch: A homopolymer of glucose, the most important dietary carbohydrate in plants.
    • Dextrin: Intermediates in the hydrolysis of starch.
    • Glycogen: The storage polysaccharide in animals, primarily found in the liver and muscles, serving as a readily available source of glucose for energy within muscle tissue. Liver glycogen is involved in storage and maintenance of blood glucose levels.
    • Inulin: A polysaccharide of fructose, readily soluble in water but not hydrolyzed by intestinal enzymes.
    • Cellulose: The primary constituent of plant cell walls (dietary fiber).

• Heteropolysaccharides (heteroglycans):

  • Contain two or more different types of monosaccharide units or their derivatives.
  • Important heteropolysaccharide in humans: Glycosaminoglycans (mucopolysaccharides):
    • Complex carbohydrates containing amino sugars and uronic acid.
    • Can be attached to a protein molecule to form a proteoglycan.
    • Examples: Heparin, Hyaluronic acid, and Blood group polysaccharides.

Functions of Carbohydrates

• Energy Production:

  • Primary role of carbohydrates is to supply energy to all cells in the body.
  • Many cells prefer glucose as an energy source over other compounds like fatty acids.
  • Some cells, like red blood cells, can only produce cellular energy from glucose.
  • The brain is highly sensitive to low blood-glucose levels as it primarily uses glucose for energy (except under extreme starvation conditions).
  • Approximately 70% of the glucose entering the body from digestion is redistributed back into the blood for use by other tissues.

• Energy Storage:

  • Excess glucose beyond the body's energy requirements is stored as glycogen, mainly in the muscles and liver.
  • Glycogen is a highly branched molecule containing over fifty thousand single glucose units, enabling rapid release of glucose when needed for energy production.

• Building Macromolecules:

  • Some absorbed glucose is converted to ribose and deoxyribose, essential building blocks for important macromolecules such as RNA, DNA, and ATP.
  • Glucose is also used to synthesize NADPH, crucial for protection against oxidative stress and involved in numerous cellular reactions.
  • If all energy, glycogen storage, and building needs are met, excess glucose can be converted into fat.
  • Carbohydrates also serve as structural components, such as glycosaminoglycans in humans, cellulose in plants, and chitin in insects.

• Sparing Protein:

  • When the body's glucose demand exceeds available supply, amino acids are used to create glucose.
  • This process involves protein degradation, particularly from muscle tissue, as there is no storage molecule for amino acids.
  • Sufficient glucose intake prevents the body from breaking down proteins to produce glucose.

• Lipid Metabolism:

  • Rising blood-glucose levels inhibit the use of lipids as an energy source.
  • Glucose has a "fat-sparing" effect, as increased blood glucose stimulates insulin release, signaling cells to utilize glucose rather than lipids for energy.
  • Adequate glucose levels in the blood also prevent ketosis, a metabolic condition characterized by elevated ketone bodies in the blood.
  • Ketone bodies are an alternative energy source but are acidic, and high levels in the blood can lead to acidosis.
  • The minimum daily carbohydrate intake required to inhibit ketosis in adults is 50 grams.

• Digestion and Intestinal Movement:

  • Dietary fiber, a type of carbohydrate not digested by humans, aids in digestion and promotes intestinal movement.

• Detoxification:

  • Carbohydrates are involved in detoxification processes, particularly through glucuronic acid.
  • Glucuronic acid, derived from glucose, plays a role in removing foreign substances from the body.
  • It forms complexes with certain drugs and hormones, enhancing their solubility and excretion.

Structure of Glucose

• Fisher Projection: Straight-chain structural formula representation.
• Haworth Projection: Cyclic formula representation, also known as ring structure.
• Chair Form: Three-dimensional representation depicting the chair conformation of glucose.

Monosaccharide Solutions

• In solution, monosaccharides are mainly present in the ring form.

Cyclization of Monosaccharides

• The aldehyde (CHO) or ketone (C=O) group of a monosaccharide reacts with a hydroxyl (OH) group within the same molecule, forming a hemiacetal or hemiketal bond, respectively, leading to ring formation.

Isomerism

• Isomers are compounds with the same molecular formula but different structures.
• The phenomenon of isomers is called isomerism.

Types of Isomerism in Sugars

• Ketose-aldose Isomerism: Isomers differ in the position of their functional groups (keto group vs. aldehyde group).
• Optical Isomerism: Presence of asymmetric carbon atoms in a molecule, leading to the rotation of plane-polarized light (dextrorotatory or levorotatory).
• Enantiomers (D and L isomerism): Mirror image isomers determined by the orientation of the H and OH groups around the asymmetric carbon adjacent to the terminal primary alcohol carbon.
• Epimerism: Isomers differing in the configuration around a single asymmetric carbon atom, excluding the anomeric carbon.
• Anomerism (α and β Anomerism): Cyclic forms of monosaccharides that differ in the orientation of the -OH group on the anomeric carbon.

Pyranose and Furanose Ring Structures

• Monosaccharide ring structures resemble either pyran (six-membered ring) or furan (five-membered ring).
• Glucose forms glucopyranose (six-membered ring) as the more stable form.
• Fructose forms fructofuranose (five-membered ring) as the more stable form.

This quiz explores the essential concepts of carbohydrate chemistry, including their structure, definition, and role in digestion and metabolism. Gain insight into how these crucial biomolecules function in both plant and animal systems. Test your knowledge on the composition and breakdown of carbohydrates.