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border; they vastly increase the surface area for absorption. They also contain enzymes to complete the enzymatic digestion of macromolecules. The main difference in function of the cells lining the small intestine and the stomach is that the stomach cells are not as involved in absorption. Their main function is to breakdown food making it more manageable for the small intestine and to destroy any potentially harmful microbes.
The glands contained in the mucosal epithelium of the stomach are formed by invagination and the three types of specialised cell present there are called; the parietal cell, chief cell and goblet cell (see diagram 2). Each of these cells has a specific function. The parietal cells produce the protein intrinsic factor and hydrochloric acid (HC1). Intrinsic factor is a glycoprotein essential for the absorption of vitamin B12 in the small intestine and HC1 dissolves food into smaller molecules and kills bacteria. There are also immune system cells in the small intestine should any pathogens survive the HC1 in the stomach.
To protect and lubricate the stomach lining from the strong acid, goblet cells secrete mucus which create a less acidic microenvironment in the lumen. Goblet cells also produce gastrin which is a hormone and is secreted into the blood vessels in order to activate other epithelial cells in the gut. Finally, pepsinogen is produced by the chief cells and is converted into pepsin by HC1 to breakdown polypeptide chains and peptides. To complete the breakdown of peptides, the small intestine secretes amino peptidase. Together with caboxypeptidase secreted by the pancreas they convert them into free amino acids by each
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removing one end of the peptide chain. For the amino acids to then be absorbed into the epithelial cells they require active transport (ATP) and use a process called cotransport to cross the epithelial membrane.
Amino peptidase is just one of the enzymes produced by the specialised cells of the small intestine to break down protein. Others include dextrinase, lactase, sucrase and maltase and have their role in the breaking down of carbohydrates into monosaccharides; glucose galactose and fructose. Glucose and galactose are transported across the epithelial cells by glucose transporter protein. Using energy they transport sodium ions with the monosaccharide into the epithelial cells. Fructose however does not require energy as its concentration gradient is very low so is transported via passive diffusion. Once absorbed by the epithelial cells the monosaccharides leave the cell membrane and enter into the blood and lymph vessels via facilitated diffusion.
Lipids and fats are not water soluble so are digested differently to carbohydrates and monosaccharides. They are broken down into smaller droplets and dispersed by bile salts in a process called emulsification. In order to be absorbed into the epithelial cells, micelles provide a reservoir of fatty acids by constantly breaking down and reforming. Fatty acids are then able to diffuse the brush border by moving into the solution in the lumen. It is the same process for fat soluble vitamins such as vitamins A, E, D and K. Instead of entering the blood stream, they leave the epithelial cells into the lymph vessels in a process called
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exocytosis after being converted into chylomicrons. From the lymph vessels they can be transported into the blood stream. Most minerals are absorbed by active transport and water soluble vitamins by facilitated or passive diffusion. Water is absorbed by osmosis via transcellular and paracellular routes.
In conclusion, the types of specialised stomach and small intestine cells are dynamic. Each one has a specific function contributing to the whole process of digestion. The stomach cells are chiefly responsible for the production of gastric secretions to breakdown food into smaller molecules to be further broken down and absorbed by the small intestine. The small intestine has much more involvement in the digestion and absorption process with cells producing or containing enzymes specific to this role.
The structure of the cells varies in that the stomach does not have a brush border as the small intestine does. This is because the structure of the cells serves their function and as the stomach is not involved in absorption, it does not need to increase surface area in this way. Instead, the stomach expands and contracts depending what is in it. The lining of the stomach also does not contain lymph nodes, blood vessels and enteric ganglia as the small intestine does. However, they are both invaginated to form glands containing secretery epithelial cells. In this way and the thick layer of mucus, they are protected in the stomach from the hydrochloric acid produced by the parietal cells. In contrast some enzymes are also located on the brush border of the small intestine which is exposed to the lumen.
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The structure and function of the stomach cells, although different to the small intestine cells, are complementary. They are both crucial in the digestive process and are structured appropriately to primarily produce either gastric secretions or enzymes to digest food and absorb nutrients into the body.
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References:
SK277 (2006 ed). Digestion and Absorption of Nutrients, in Book 1 Cells and Nutrition. The Open University, Milton Keynes.
