• It is the breakdown &

degradation of polysaccharides &

oligosaccharides into their

simplest units (Monosaccharides)

Digestion of Carbohydrates

1. Digestion

Digestible

Monosaccharides

Do not need

digestion

Starch

Hexoses

Classification of Carbohydrates

Glycogen

Lactose

Sucrose

Pentoses

Non-Digestible

Cellulose

1

2

3

The Digestive System

PH 6.4 – 6.9. Activated

by Cl-, digests starch to

dextrins, maltose &

isomaltose

Acidic PH of stomach (1-2) is

unsuitable for salivary

amylase

3. Brush border (Intestinal juice,

Succus Entericus) containing:

Maltase, Lactase & Sucrase

(Activated by Cl-).

4.

Carbohydrate Digestion in Small Intestine

• Salivary amylase stops working in acidic

stomach (if pH 4.5)

• 50% of dietary starch digested before it

reaches small intestine

• Brush border enzymes act upon

oligosaccharides, maltose, sucrose & lactose

• lactose indigestible after age 4 in most

humans (due to lack of lactase)

Carbohydrate Digestion

in Small Intestine

& Isomaltose

Monosaccharides Absorption

• Pentoses are absorbed by facilitated diffusion then

excreted through the kidney

• Fructose is absorbed by facilitated diffusion then

converted to glucose inside the cell

• Sodium-glucose transport proteins (SGLT) in

membrane help absorption of glucose & galactose

2. Absorption

Absorption of Pentoses & Hexoses

Pentoses absorbed

by Passive Diffusion

Hexoses absorbed

by Active Transport

According

concentration

gradient

Against

concentration

gradient

Types of Transport Carriers

Monosaccharides Absorption

Liver

GLUT1 in RBCs membrane is Facilitated Diffusion

Digestion of Lactose

(Lactase)

Lactose Intolerance• Some individuals (90% of adult blacks & Orientals)

have a defect in lactase enzyme

• Undigested lactose (osmotically active compound)

passes to the bowel (large intestine), acted upon by

bacteria of large intestine producing short chain fatty

acids & CO2 gas, leading to:

1. Abdominal distention (cramping)

2. Abdominal pain

3. Nausea

4. Bloating

5. Watery diarrhea

Lactase deficiency

Fate of Absorbed Glucose

• Glucose enter Liver & Brain by Passive

diffusion

• Glucose enter other tissues (skeletal muscle,

adipose tissue, etc…) by Active transport

• Active transport is enhanced by Insulin

Utilization of Glucose

Anabolic

Reactions Catabolic

Reactions

1) Storage in the form of

Glycogen (Glycogenesis)

2) Storage in the form of Lipid

(Lipogenesis)

3) Synthesis of sugar alcohols

and amino sugars

4) Interconversion between

monosaccharides

1) Glycolysis

2) Kreb’s Cycle

3) Pentose Shunt

4) Formation of Uronic acids