Carbohydrate

Topics:

1. Definition 
2. Classification 
3. Digestion & Absorption of Carbohydrate 
4. The Transport of Carbohydrate in body 

DEFINITION

Carbohydrates are one of the main types of nutrients. They are the most important source of energy for your body. Your digestive system changes carbohydrates into glucose (blood sugar). Your body uses this sugar for energy for your cells, tissues and organs. It stores any extra sugar in your liver and muscles for when it is needed.



CLASSIFICATION OF CARBOHYDRATE

Monosaccharides are those sugars that cannot be hydrolyzed into simpler carbohydrates.

Disaccharides are condensation products of two monosaccharide units, for example, lactose, maltose, isomaltose, sucrose, and trehalose.

Oligosaccharides are condensation products of three to ten monosaccharides. Most are not digested by human enzymes.
Polysaccharides are condensation products of more than ten monosaccharide units; examples are the starches and dextrins, which may be linear or branched polymers

DIGESTION & ABSORPTION OF CARBOHYDRATE

• The dietary polysaccharides starch and glycogen are converted into the disaccharide maltose through the action of salivary and pancreatic amylase.
• Maltose and the dietary disaccharides lactose and sucrose are converted to their respective monosaccharides by the disaccharidases (maltase, lactase, and sucrase-isomaltase) located in the brush borders of the small-intestine epithelial cells.
• The monosaccharides glucose and galactose are absorbed into the epithelial cells by Na+- and energy-dependent secondary active transport (via the symporter SGLT) located at the luminal membrane.
• The monosaccharide fructose enters the cell by passive facilitated diffusion via GLUT-5. • Glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2.
• These monosaccharides enter the blood by simple diffusion.

TRANSPORT OF CARBOHYDRATE

The transport of most hexoses is dependent on Na+ in the intestinal lumen
– SGLT-1 is responsible for uptake of dietary glucose from the gut.
– SGLT 2, is responsible for glucose transport out of the renal tubules