Digestion and metabolism

21 Digestion and metabolism



Learning objectives


After reading this chapter you should understand:


• the digestion of carbohydrates, fats and proteins

• the main features of metabolism

The anatomy of the digestive tract was considered in Chapter 19. The aim of the present chapter is to consider the chemical breakdown (digestion) of individual foodstuffs and their subsequent use in metabolism to provide energy and structural components for the body.


The digestion of the major sources of nutrition – carbohydrate, fat and protein – is dealt with in the following sections, and the interrelated nature of their metabolism in a subsequent section (Figs 21.1 and 21.2).


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Fig. 21.1 Digestion and absorption of carbohydrate, fat and protein.


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Fig. 21.2 Metabolism. ATP, adenosine triphosphate; CO2, carbon dioxide; CoA, coenzyme A; H2O, water.



Digestion of carbohydrates


Carbohydrate is present in the diet in several forms: as polysaccharides, disaccharides and monosaccharides. The polysaccharides that can be metabolized are starch (from plants) and glycogen (from animal sources). Both these polysaccharides are polymers (long-chain molecules) of glucose. The major polysaccharide is starch and the digestion of this begins in the mouth where salivary amylase begins to break it down into shorter polymers of glucose such as maltose and dextrin.


The action of amylase on starch is probably very short as food in the mouth is quickly swallowed and, when the food reaches the stomach, the hydrochloric acid inactivates the amylase. Some starch digestion may continue inside balls of food but, by the mixing action of the stomach movements, all the amylase is eventually inactivated.


There is no digestion of disaccharides and monosaccharides in the mouth or in the stomach. The main disaccharides present in the diet are sucrose, maltose and lactose (from milk). The composition of these disaccharides is:


• sucrose: glucose–fructose

• maltose: glucose–glucose

• lactose: glucose–galactose.

The monosaccharides in the diet are glucose and fructose.


Once the contents of the stomach have passed into the duodenum the digestion of carbohydrates continues. In the duodenum, pancreatic amylase continues to break starch down into smaller polysaccharides and eventually into the disaccharide maltose. The intestinal juice contains three enzymes, maltase, lactase and sucrase, which complete the digestion of disaccharides into glucose, fructose and galactose. All the monosaccharides are subsequently absorbed in the small intestine from where they enter the blood stream and can then be metabolized in the body.

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Jul 18, 2016 | Posted by in NURSING | Comments Off on Digestion and metabolism

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