Digestion and Distribution

Published: 2019-10-02 08:00:00
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Dry weight of chicken meat contains about 82% of proteins and 3% of fats. Other 5% include carbohydrates, vitamins, ions and other chemical substances (Wattanachant, S. 2004, 125). Sandwich bread contains starch and saccharose, which are the most important polysaccharides for human body. All these nutrients are essential for energy production. The process of energy molecules synthesis is mediated by combined action of digestive, cardiovascular and endocrine systems.

Proteins, fats and carbohydrates are the main nutrients we need. The main role of the digestive system is to excrete these molecules from food and to break down them into smaller molecules. The cardiovascular system transports these small molecules to cells of the whole body. The endocrine system produces hormones, which regulate gastric digestion and mediate transport through cell membrane. And then inside the cell energy production passes through a series of stages and results in ATP synthesis.

Digestion of different nutrients takes place in different parts of the gastrointestinal tract. There is an alternation of acidic and alkaline environments in the intestine. It begins in mouth and ends in colon.

Epithelial cells of the intestine can absorb only monosaccharides. Therefore, the process of digestion is a process of enzymatic hydrolysis of glycosidic bonds in oligo- and polysaccharides.

Starch hydrolysis begins in mouth. Enzyme amylase (alpha-1,4-glucosidase) in saliva cleaves alpha-1,4-glycosidic bonds and this leads to formation of dextrins (low molecular weight carbohydrates). Time of the amylase action is not enough to break down the starch completely. Therefore, further hydrolysis of starch and dextrins continues in the upper intestine by the action of pancreatic amylase, which also cleaves alpha-1,4-glycosidic bonds. As a result, starch and dextrins are hydrolyzed by this enzyme to yield maltose (two residues of glucose linked by alpha-1,4-glycosidic bound) and isomaltose (alpha-1,6-glycosidic bound). There is no beta-amylase in animal cell, only alpha-amylase is synthesized. For this reason we cannot break down cellulose, which consists of glucose molecules linked by beta-1,4-glycosidic bonds. Amylases require alkaline environment, so digestion of carbohydrates is suspended in stomach.

Hydrolysis of disaccharides (such as maltose or saccharose) occurs on the surface of intestinal cells, which are covered by villi. It is catalyzed by specific enzymes: sucrase, lactase, maltase and isomaltase. These glycosidases are synthesized in the intestinal cells.

Absorption of produced monosaccharides from intestine to blood is carried out by facilitated diffusion. If glucose concentration in the intestine is not high, its transport is mediated by concentration gradient of sodium ions generated by Na+/K+-ATPase. Glucose and other monosaccharides are the key molecules for energy production.

Protein degradation begins in stomach, continues in duodenum and ends in small intestine. In the stomach food mixes with gastric juice, which consists of hydrochloric acid and few types of proteolytic enzymes called pepsines. Optimum pH value of each pepsin type is different. These enzymes destroy peptide bonds and break down proteins into smaller peptides.

Regulation of gastric juice secretion is mediated by hormones gastrin and histamine. Gastrin increases hydrochloric acid secretion in two ways: (1) acts directly at parietal cells and (2) binds to receptors of ECL cells (enterochromaffin-like cell), which then release histamine. Histamine acts through adenylyl cyclase mechanism, increasing secretion of hydrochloric acid. This provides optimum pH level for pepsin catalysis. Gastrin also increases pepsin production by chief cells.

The next step of protein digestion takes part in the duodenum and the small intestine. Enzymes trypsin, chymotrypsin, carboxypolypeptidase and proelastse break down proteins and big peptides into smaller peptides and amino acids. Finally inside intestine cells (enterocytes) specific peptidases destroy peptide bonds to yield single amino acids. Then these amino acids are transported into blood.

Most fat in human diet is in the form of triacylglycerols (TAG). They consist of glycerol and three fatty acids. TAG digestion is completed in small intestine by enzyme lipase. It is activated by hormone cholecystokinin, which is secreted by enteroendocrine cells of duodenum.

Lipase is water soluble, but lipids are hydrophobic. Therefore lipids form emulsion droplets to achieve the biggest surface area for the reaction. When digestion is finished, acylglycerols and fatty acids associate with bile salts and phospholipids to form micelles. They are about two hundred smaller than emulsion droplets. Their size is important to have ability to pass through intestine cell membrane. Inside enterocytes acylglycerols and fatty acids are re-synthesized into triacylglycerols to associate with transport proteins and form chylomicrons (lipoproteins). Then they are released by exocytosis and enter cardiovascular system.

As a result, monosaccharides (glucose), amino acids and lipoproteins are transported to the body cells by the cardiovascular system. Monosaccharides and amino acids in the blood are in dissolved form. Blood maintains a constant concentration of each of these chemical substances. Glucose concentration is maintained at a level of 3.35.5 mmol/l during a day. After ingestion its concentration increases to 8.2 mmol/l. Normal amino acid concentration in blood is about 1.93.6 mmol/l. The maximum is reached after 30-50 min after ingestion of protein foods.

Transport of these molecules across the cell membrane is carried out by specific transport proteins and channels. Hormone insulin increases glucose transport, because glucose is the most important molecule for the production of energy storage molecules. Insulin interacts with its receptor on the cell membrane and this leads to activation of signaling mechanism, mediated by phosphoinositide 3-kinase/Akt cascade. As a result, there is increasing of glucose transporters affinity (encoded by GLUT, especially GLUT4) to cell membrane of insulin-responsive tissues. On the other hand, glucocorticoids slow down glucose transport into endothelial cells of blood vessels. Amino acid transport across the cell membrane is also stimulated by insulin. Somatotropin (growth hormone) increases amino acid transport into muscle cells.

The most common energy storage molecule is adenosine triphosphate (ATP). It is synthesized by enzyme ATP synthase. Two molecules of ATP are produced as a result of glycolysis in cytoplasm. Glycolysis is anaerobic metabolic pathway of glucose conversion to pyruvate by series of 10 enzyme-catalysed reactions. Most monosaccharides can be converted to some phosphated intermediates of glycolysis. Pyruvate is the last intermediate of glycolysis. It is actively transported into mitochondrion matrix and here it can be decarboxylated to acetyl coenzyme A (acetyl CoA) by the pyruvate dehydrogenase complex. Acetyl CoA is an important intermediate of many biochemical pathways. But its main function is to convey the carbon atoms within the acetyl group to the Krebs cycle to be oxidized for energy production. Krebs cycle is also known as citric or tricarboxylic acid (TCA) cycle and takes place in mitochondrion. TCA is a series of 10 chemical reactions, which result in production of reducing agent NADH, as well as energy molecule guanosine triphosphate (GTP) and some precursors of amino acids. NADH produced by TCA is involved in the action of electron transport chain (ETC) in the mitochondrion as a co-enzyme of oxidoreductase (. It is oxidized, providing energy to power ATP synthase. Oxidation of one acetyl-CoA molecule leads to production of 36 ATP molecules. This process is peculiar only to aerobic organisms, because ETC requires oxygen.

Amino acids and lipids can be involved in these processes at various stages.

There are 20 main proteinogenic amino acids in the standard genetic code. They all are divided into three groups, according to their ability to be converted to intermediates, important for energy production. These groups are: (1) glucogenic amino acids that can be converted to glucose through gluconeogenesis, (2) ketogenic amino acids that can be degraded directly into acetyl-CoA and (3) amino acids that are both ketogenic and glucogenic.

Involvement of lipids in the energy production is mediated by beta-oxidation of fatty acids. It is a process by which acetyl-CoA, NADH and FADH2 are produced to be used in TCA and ETC, respectively.

Thus, all main nutrients from food we eat take part in the process of ATP synthesis one way or another. The most essential substance and the key molecule of this process is glucose. Its catabolism results in production of 38 molecules of ATP in aerobic organisms.

In conclusion, it should be said, that three body systems described above are not only consistently involved in the process of energy molecules production, but regulate and complement each other.


Wattanachant, S., Benjakul, S., & Ledward, D. A., 2004. Composition, Color, and Texture of Thai Indigenous and Broiler Chicken Muscles. Poultry Science, 83, pp.123128.


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