Upon completion of this section, you will have the ability to:
- Recognize the primary locations and secretions involved in the chemical breakdown of carbohydrates, proteins, lipids, and nucleic acids
- Differentiate between the absorption of hydrophilic and hydrophobic nutrients
As previously discussed, the process of mechanical digestion is straightforward and involves the physical breakdown of food without changing its chemical composition. In contrast, chemical digestion is a complex process that breaks down food into its essential chemical components, which are then absorbed to fuel the body’s cells. This section will delve deeper into the intricate processes of chemical digestion and absorption.
Figure 1. Digestion commences in the oral cavity and continues as food travels through the small intestine, where most absorption takes place.
In addition to breaking down food into essential chemical components, the digestive system also plays a crucial role in maintaining the body’s overall health. For example, the gut microbiota present in the intestines help in the digestion and absorption of nutrients, as well as in the synthesis of certain vitamins and amino acids. Furthermore, the digestive system is closely linked to the immune system and plays a key role in defending the body against pathogens that enter through the oral route.
Overall, understanding the intricate processes of chemical digestion and absorption is essential for maintaining optimal health and ensuring that the body receives the necessary nutrients for its proper functioning.
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Large food molecules, such as proteins, lipids, nucleic acids, and starches, need to be broken down into smaller subunits that can be absorbed by the lining of the digestive tract. Enzymes facilitate this breakdown process through hydrolysis. Table 1 summarizes the various enzymes involved in chemical digestion.
- Aminopeptidase: acts on amino acids at the amino end of peptides
- Dipeptidase: breaks down dipeptides
- Ribonuclease: digests ribonucleic acids
- Deoxyribonuclease: hydrolyzes deoxyribonucleic acids
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About half of the average American diet consists of carbohydrates, which can be categorized based on the number of simple sugar units they contain (monosaccharides and disaccharides) or complex sugar units (polysaccharides). Glucose, galactose, and fructose are commonly consumed monosaccharides that are easily absorbed. Your digestive system can also break down disaccharides like sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose), as well as the polysaccharides glycogen and starch (chains of monosaccharides). Enzymes in the body are generally unable to break down fibrous polysaccharides like cellulose. Although indigestible polysaccharides offer no nutritional value, they do provide dietary fiber that aids in the movement of food through the digestive tract.
The initial stages of starch digestion occur in the mouth, as previously mentioned.
In the small intestine, pancreatic amylase plays a crucial role in the breakdown of starch and carbohydrates (Figure 2). Following the action of amylases, the brush border enzyme α-dextrinase begins breaking down α-dextrin into individual glucose units. Three brush border enzymes are responsible for hydrolyzing sucrose, lactose, and maltose into monosaccharides. Sucrase cleaves sucrose into fructose and glucose, maltase breaks down maltose and maltotriose into glucose molecules, and lactase breaks down lactose into glucose and galactose units. Lactose intolerance can result from insufficient lactase production.
Figure 2. Carbohydrates undergo stepwise breakdown into their simple sugar components.
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Proteins are chains of amino acids linked by peptide bonds and are reduced to individual amino acids during digestion. Protein typically contributes 15 to 20 percent of your total caloric intake.
Protein digestion commences in the stomach, where HCl and pepsin fragment proteins into smaller polypeptides that proceed to the small intestine. Pancreatic enzymes like chymotrypsin and trypsin continue the chemical breakdown of proteins in the small intestine, targeting specific bonds within amino acid sequences. Additionally, enzymes from the brush border like aminopeptidase and dipeptidase further break down peptides into smaller molecules for absorption into the bloodstream.
Figure 3. Protein digestion initiates in the stomach and concludes in the small intestine.
Figure 4. Proteins are progressively broken down into their amino acid constituents.
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A healthy diet restricts lipid consumption to 35 percent of total caloric intake. Triglycerides, composed of a glycerol molecule linked to three fatty acid chains, are the most prevalent dietary lipids. Smaller amounts of cholesterol and phospholipids are also consumed.
Lipid digestion relies on three lipases: lingual lipase, gastric lipase, and pancreatic lipase. However, the majority of lipid breakdown occurs in the small intestine due to the substantial presence of pancreatic lipase. This enzyme breaks down each triglyceride into two free fatty acids and a monoglyceride, encompassing both short-chain (fewer than 10 to 12 carbons) and long-chain fatty acids.
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DNA and RNA, nucleic acids present in many foods, are broken down by two forms of pancreatic nuclease: deoxyribonuclease, which targets DNA, and ribonuclease, which acts on RNA. The resulting nucleotides are further degraded by brush border enzymes like nucleosidase and phosphatase into pentoses, phosphates, and nitrogenous bases for absorption through the digestive tract lining. Table 2 summarizes the large food molecules that must be broken down into smaller components.
| List of Food Substances that Can Be Absorbed by the Body | |
|---|---|
| Origin | Element |
| Carbohydrates | Consist of monosaccharides such as glucose, galactose, and fructose |
| Proteins | Contain single amino acids, dipeptides, and tripeptides |
| Triglycerides | Composed of monoacylglycerides, glycerol, and free fatty acids |
| Nucleic acids | Consist of pentose sugars, phosphates, and nitrogenous bases |
Absorption
The main objective of mechanical and digestive processes is to break down food into small molecules that can be absorbed by the epithelial cells in the intestinal villi. The absorbent capacity of the digestive system is vast, processing around 10 liters of food, liquids, and GI secretions each day, with less than one liter reaching the large intestine. The small intestine plays a crucial role in absorbing almost all ingested food, 80 percent of electrolytes, and 90 percent of water, with the jejunum being the primary site for absorbing carbohydrates and proteins. Bile salts and vitamin B12 are absorbed in the terminal ileum. By the time chyme enters the large intestine, it consists of indigestible food residue, water, and bacteria.
Figure 5. The process of absorption is complex, involving the extraction of nutrients from digested food.
Absorption can occur through five mechanisms: active transport, passive diffusion, facilitated diffusion, co-transport, and endocytosis. Active transport involves the movement of substances across a cell membrane from an area of lower to higher concentration, powered by cellular energy. Passive diffusion is the movement of substances from higher to lower concentration, while facilitated diffusion uses carrier proteins. Co-transport moves molecules in opposite directions across the membrane, and endocytosis engulfs material needing energy, typically ATP.
Water-soluble nutrients require transport molecules for entry into cells due to the hydrophobic nature of the plasma membrane. Lipid-soluble nutrients can diffuse through the membrane and are transported through the lacteals of the villi to the systemic circulation. The absorption of nutrients in the intestinal villi relies on active transport fueled by ATP. Table 3 summarizes the absorption routes for different food categories.
| Table 3. Digestion and Absorption Process in the Digestive System | ||||
|---|---|---|---|---|
| Food | Digestive breakdown products | Process of absorption | Transport into the bloodstream | Final destination |
Carbohydrate Absorption
Monosaccharides are the only form of carbohydrates that the small intestine absorbs. Glucose and galactose are transported with sodium ions, while fructose is absorbed through facilitated diffusion. In terms of proteins, amino acids are co-transported with sodium ions for absorption. As for lipids, long-chain fatty acids and monoacylglycerides combine with proteins to form chylomicrons that enter the systemic circulation via the lymphatic system.
Protein Absorption
In the duodenum and jejunum, breakdown products of proteins, mainly amino acids, are absorbed through active transport mechanisms. Amino acids are transported together with sodium ions, and shorter amino acid chains are actively absorbed. Once inside cells, amino acids are released through diffusion.
Lipid Absorption
A significant portion, approximately 95%, of lipids are absorbed in the small intestine. Bile salts assist in the digestion and absorption of lipids. Short-chain fatty acids directly enter absorptive cells as they are water-soluble. Long-chain fatty acids and monoacylglycerides form micelles for absorption through simple diffusion. The transportation of lipids is done through chylomicrons via the lymphatic system.
During absorption, lipids undergo changes to reform free fatty acids and monoacylglycerides into triglycerides, which are then packaged into chylomicrons for transport via the lymphatic system.
Nucleic Acid Absorption
The breakdown products of nucleic acids are transported across the villus epithelium through carriers via active transport before entering the bloodstream.
Mineral Absorption
Electrolytes present in gastrointestinal secretions and foods are absorbed in the small intestine through active transport mechanisms. Sodium ions are accumulated inside cells through co-transport mechanisms, while potassium concentrations are reduced via anti-port mechanisms. However, iron and calcium are partially absorbed based on the body’s needs, mostly in the duodenum.
Vitamin Absorption
Vitamins are absorbed in the small intestine, with fat-soluble vitamins absorbed alongside lipids. Water-soluble vitamins are absorbed through simple diffusion, except for vitamin B12, which binds to intrinsic factor for absorption in the ileum via endocytosis.
Water Absorption
The small intestine absorbs around 90% of the water that enters it through concentration gradients. Water moves from the chyme into cells due to higher concentrations in the chyme. Water absorption continues in the colon.
Chapter Review
The small intestine serves as the primary location for chemical digestion and absorption, facilitated by brush border and pancreatic enzymes, along with bile for fat breakdown.
In addition to its role in digestion and absorption, the small intestine also plays a crucial role in the immune system, as it contains specialized immune cells that help defend against pathogens and maintain immune balance.
The small intestine is divided into three sections: the duodenum, jejunum, and ileum. Each section has unique functions, with the duodenum being responsible for receiving partially digested food from the stomach, the jejunum for further absorption of nutrients, and the ileum for absorption of bile salts and vitamin B12.