Overview
This lecture reviews the fundamentals of carbohydrates as presented in Lippincott's Biochemistry Chapter 7, including their classification, structure, digestion, and absorption.
Carbohydrate Basics
- Carbohydrates are the most abundant organic molecules in nature, serving as energy sources, energy storage, and cell membrane components.
- Monosaccharides are the simplest carbohydrates; classified by carbon number (triose = 3, hexose = 6 carbons).
- Monosaccharides are further classified by carbonyl group: aldose (aldehyde group) or ketose (keto group).
Carbohydrate Structure and Classification
- Disaccharides consist of two monosaccharides; oligosaccharides have 3–10; polysaccharides exceed 10 monosaccharides.
- Isomers share the same chemical formula but have different structures (e.g., glucose, fructose, mannose, galactose).
- Epimers are isomers differing at one specific carbon atom (e.g., glucose and galactose at C4).
- Enantiomers are mirror images (D and L forms) of a molecule; enzymes are usually specific to one form.
- Anomers arise when a sugar cyclizes, creating different orientations (α or β) at the anomeric carbon.
Chemical Properties of Carbohydrates
- Reducing sugars have a free anomeric carbon not involved in a glycosidic bond; all monosaccharides are reducing sugars.
- Glycosidic bonds join sugars and require glycosyl transferase enzymes; can connect to other sugars, proteins, or lipids.
- Glycosidic bonds are classified as O- or N-linkages depending on the group attached.
Digestion and Absorption of Carbohydrates
- Carbohydrate digestion occurs in the mouth (salivary amylase) and small intestine (pancreatic amylase), but not in the stomach.
- Humans digest starch (plants) and glycogen (animals), but not cellulose (plants).
- Disaccharidases in the intestinal lining further break down disaccharides into monosaccharides for absorption.
- Glucose and galactose are absorbed via active transport (SGLT-1), fructose via facilitated diffusion (GLUT5), all exit enterocytes via GLUT2.
Carbohydrate Digestion Disorders
- Lactose intolerance results from lactase deficiency, leading to GI symptoms; common in adults.
- Sucrase-isomaltase deficiency is a congenital disorder affecting sucrose digestion; managed by dietary reduction and enzyme therapy.
- Diagnostic tests include oral sugar tolerance and breath hydrogen analysis.
Key Terms & Definitions
- Monosaccharide — simplest carbohydrate unit.
- Disaccharide — two monosaccharides linked together.
- Polysaccharide — many monosaccharides joined.
- Isomer — molecules with same formula, different structures.
- Epimer — isomers differing at one carbon atom.
- Enantiomer — mirror-image isomers (D or L form).
- Anomer — form of a sugar differing at the anomeric carbon.
- Reducing sugar — sugar with a free anomeric carbon.
- Glycosidic bond — linkage joining carbohydrates or to other molecules.
- Disaccharidase — enzyme breaking down disaccharides.
Action Items / Next Steps
- Review summary and questions at the end of the chapter.
- Complete oral tolerance or breath tests if diagnosing carbohydrate digestion disorders.
- Read the next chapter on dietary carbohydrate digestion.