Carbohydrate metabolism

Metabolism of Carbohydrates

Carbohydrates are made up of carbon, hydrogen and oxygen. They act as source of body heat and energy. Carbohydrates are found in sugar, jam cereals, fruits, potatoes, yams, vegetables and milk. 

There are three major groups of carbohydrates are

Metabolism of Carbohydrates

1. Monosaccharide 
2. Disaccharides
3. Polysaccharides

The major groups of carbohydrates. These are:

 1. Monosaccharide 

These are simple sugars that can be absorbed directly from the intestine. Monosaccharide includes the hexoses (which contain 6 carbon atoms) such as glucose, fructose and galactose, and the pentoses (which contain 5 carbon atoms) such as xylose, ribose and arabinose.

2. Disaccharides 

these are not directly absorbed from the intestine. On hydrolysis with specific enzymes, they yield two molecules of monosaccharide. This group includes sugars such as sucrose, lactose and maltose. Table 3.1 shows the specific enzymes acting on these sugars and the monosaccharide components.

Table 3.1 Monosaccharide components formed by disaccharides by the action of enzymes
Enzyme	Disaccharide	Monosaccharide
Lactase	Lactose	Galactose + glucose
Sucrase	Sucrose	Fructose + glucose
Maltase	Maltose	Glucose + glucose

3. Polysaccharides 

Many carbohydrates in nature exist in the form of polysaccharides. They are more complex and are not easily absorbed in the body. The most common polysaccharides are starch and glycogen which are hydrolysed by amylase to mainly maltose and glucose.

• When carbohydrate which has been ingested is converted into glucose by various enzymes, and the glucose is absorbed from the small intestine, then the metabolism of carbohydrate in the body begins. 

• The glucose is transported through the bloodstream to the liver where it is used or stored depending on the need of the body at that time.

There are three ways in which the glucose is either utilised or stored in the body:

1. Glycolysis 

Glycolysis is the breakdown of glucose to pyruvic acid or lactic acid (or both) for the production of energy in the form of the compound adenosine triphosphate (ATP). It is used to control the level of glucose in the blood. 

• Glucose is taken from the blood by the tissues and is converted into pyruvic acid by glycolysis. The heat and energy produced is used for cellular activity.

 

• Glycolysis occurs through aerobic or anaerobic pathway.

 

2.glycogenolysis  

glycogenolysis describes the breakdown of glycogen. Glucose is converted to glycogen and stored in the liver and skeletal muscles when it is not needed for immediate use.

 

• When glucose is required, the stored glycogen is converted back to glucose.

 

A number of enzymes are involved in glycogenesis and glycogenolysis, as shown below. 

3. Conversion to fat 

 Excess glucose which is not immediately needed by the body and which cannot be stored as glycogen in the liver, is oxidised to fatty acids and is stored as fat in the tissues.

 

Glucose, when needed by the body can be recovered from fat and protein. This process of reconversion is known as gluconeogenesis.

 

1. Anaerobic pathway 

occurs when the energy requirement exceeds the oxygen available. This process continues only for a limited period till enough oxygen is made available for the cellular activity. 

 

2. Glycogenesis 

 The process of glycogen formation is called glycogenesis, while the term