IV. CARBOHYDRATES A. Two Types 1. Simple Carbohydrate a. Monosaccharides i. Glucose-mild sweetness blood sugar for the body i. Fructose-intensely sweet found in fruits and honey iii. Galactose-hardly sweet not found in many foods

b. Disaccharides i. Maltose (glucose + glucose) produced when starch breaks down. ii. Sucrose (glucose + fructose) table sugar iii. Lactose (glucose + galactose) milk sugar

2. Complex Carbohydrate a. Polysaccharides-chains of monosaccharides i. Glycogen- the human body stores glucose as glycogen ii. Starches- plant cells store glucose as starches. All starches come from plants. Grains and legumes are the richest food source.

iii. Fibers- structural parts of plants derived from fruits veg, whole grains and legumes. The bands cannot be broken down by hydrolysis like starches so fiber passes through the body providing no calories.

There are two types of fiber: aa.Soluble-fiber that dissolves in water and is easily digested by bacteria in the colon. Food sources include oats, legumes and citrus fruits.

bb. Insoluble-fibers that do not dissolve in water. Found in whole grains, and vegetables. B. Digestion 1. Digestion occurs when the sugars and starches are broken down into small molecules (glucose) so the body can absorb it and use it. 2. The initial breakdown occurs in the mouth and the final in the sm. intestine along with absorption. 3. The conversion to glucose takes place in the liver.

4. Small Intestine- most carb. Digestion occurs here. a.Pancreatic amylase breaks down the polysaccharides to shorter glucose chains and maltose. b. Disaccharides are broken down by specific enzymes: 1. Maltase-breaks maltose into two glucose molecules 2. Sucrase-breaks sucrose into one glucose molecule and one fructose molecule 3. Lactase-breaks lactose into one glucose molecule and one galactose molecule

5. Large Intestine – fibers attract water which soften the stool for passage. Bacteria ferments some of the fibers producing gas.

C. Carbohydrate Absorption 1. Glucose and Galactose – by way of active transport enter the cells lining the sm. intestine 2. Fructose – by way of facilitated diffusion enters the cells of the small intestine 3. Blood from the sm. intestine circulate through the liver and the liver cells take up fructose and galactose and convert them to glucose.

D. Carbohydrate Metabolism 1. The liver stores 1/3 of the body’s total glycogen. 2. After a meal blood glucose levels rise and liver cells link the excess glucose molecules to form glycogen. 3. When blood glucose levels fall the liver cells break glycogen by hydrolysis into single molecules of glucose and release them into the bloodstream.

4. Muscle cells can also store glucose as glycogen (the other 2/3), but they take most of this supply using it for themselves during exercise. 5. The brain maintains a small amount of glycogen which is thought to provide an emergency energy reserve during times of glucose deprivation.

6. Glycogen holds water and the body can only store enough glycogen to provide energy for less than a day during rest and a few hours during exercise. 7. Glucose fuels the body’s cells by being broken down in half and then further broken down completely to carbon dioxide and water yielding energy 8. Glucose is the preferred energy source for the brain, nerve cells and RBC

E. Blood Glucose Homeostasis 1. When a person eats blood glucose rises. 2. High blood glucose stimulates pancreas to release insulin. 3. Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscles. Insulin also stimulates the con- version of excess glucose into fat.

4. As the body’s cells use glucose, blood levels decline. 5. Low blood glucose stimulates the pan- creas to release glucagon into the bloodstream. 6. Glucagon stimulates liver cells to break down glycogen and release glucose into the blood. 7. Blood glucose begins to rise. ***See p.114 Fig. 4-12 Illustrate***