COOH
7 Digestion and absorption
Contents
Key terms 133
Chemical basis related to digestion and absorption 133
• Hydrolysis and condensation 133
• Enzymes: the biological catalysis 134
The digestive system: an overview 136
• Organization of the gastrointestinal tract 138
• Gastrointestinal motility and secretions 139
• Regulation of gastrointestinal motility and secretions 140
Digestion and absorption processes 141
• The mouth 141
• The esophagus 141
• The stomach 142
• The small intestine 143
• The large intestine 147
• Paths of absorbed nutrients 148
Factors affecting food intake and choice 150
• Hunger and appetite 150
• Role of hypothalamus 150
• Psychological stress 151
Immune function of the digestive system 151
Common problems with digestion and absorption 152
• Lactose intolerance 153
• Ulcers 153
• Heartburn 154
• Constipation 154
• Hemorrhoids 155
• Diarrhea 155
• Irritable Bowel Syndrome 156
• Gallstones 156
Summary 157
Case study 158
Review questions 158
Suggested reading 159
Glossary 159
Digestion and absorption 133 Key terms
• Allergic reaction • Antigen
• Appendix • Appetite
• Arteries • Arterioles
• Bolus • Cecum
• Cephalic phase • Chemoreceptors
• Cholecystokinin (CCK) • Chyme
• Coenzymes • Colon
• Condensation • Constipation
• Diarrhea • Duodenum
• Energy of activation • Enteric endocrine system
• Enteric nervous system • Enzymes
• Epiglottis • Gallstones
• Gastric inhibitory protein • Gastric phase
• Gastrin • Gastrointestinal tract
• Heartburn • Hemorrhoids
• Hepatic portal circulation • Hunger
• Hydrolysis • Ileum
• Immunoglobulins • Intestinal phase
• Irritable bowel syndrome • Jejunum
• Lacteals • Lactose intolerance
• Lower esophageal sphincter • Lumen
• Mechanoreceptors • Microvilli
• Pepsin • Peptic ulcers
• Peristalsis • Probiotic
• Pyloric sphincter • Rectum
• Satiety • Secretin
• Segmentation • Sphincter
• Substrates • Ulcer
• Veins • Venules
• Villi
Chemical basis related to digestion and absorption
Proper food intake provides an uninterrupted supply of energy and tissue- building chem-icals to sustain life. For exercise and sports participants, the ready availability of specific nutrients takes on added importance because physical activity increases energy expendi-ture and the need for tissue repair and synthesis. Nutrient uptake by the body involves complex physiological and metabolic processes that usually progress unnoticed for a life-time. Hormones and enzymes work in concert throughout the digestive tract, at proper levels of acidity–alkalinity, to facilitate the breakdown of complex nutrients into simpler and absorbable subunits. Substances produced during digestion are absorbed through the thin lining of the small intestine and pass into blood and lymph. Self- regulating processes within the digestive tract usually move food along at a slow rate to allow for its complete absorption, yet rapidly enough to ensure timely delivery of its nutrient.
Hydrolysis and condensation
Hydrolysis reactions digest or break down complex molecules such as carbohydrates, lipids, and proteins into simpler forms that the body absorbs and assimilates. During the
decomposition process, chemical bonds split by the addition of hydrogen ions (H+) and hydroxyl ions (OH–), the constituents of water, to the reaction by- products. Examples of hydrolysis reactions include the digestion of starches and disaccharides to monosac-charides, protein to amino acids, and lipids to glycerol and fatty acids. A specific enzyme catalyzes each step in the breakdown process. For breaking down disaccharides, the enzymes are lactase, sucrase, and maltase for lactose, sucrose, and maltose, respectively.
The enzyme lipase degrades the triglycerides molecule by adding water, thereby cleaving the fatty acids from their glycerol backbone. During protein degradation, protease enzymes accelerate amino acid release when the addition of water splits the peptide bonds. All of these examples represent catabolism, which in some cases may result in a release of energy. Figure 7.1a illustrates the hydrolysis reaction for the disaccharide sucrose to its end- product molecules of glucose and fructose.
The reactions illustrated for hydrolysis also occur in the opposite direction known as condensation. In this reverse process (shown in Figure 7.1b), a hydrogen atom is cleaved from one molecule and a hydroxyl group is removed from another. As a result, while a compound of maltose is synthesized, a water molecule is also formed. The condensation reactions are also referred to as an anabolic process during which individual compon-ents of the nutrireactions are also referred to as an anabolic process during which individual compon-ents bind together in condensation reactions to form more complex molecules. Condensation reactions also apply to protein synthesis. In this process, as a peptide bond is formed from two amino acids, a water molecule is created from a hydroxyl ion cleaved from one amino acid and hydrogen ion from the other amino acid.
For lipids, water molecules form when a glycerol binds with three fatty acids to form a triglyceride molecule.
Enzymes: the biological catalysis
The speed of cellular chemical reactions is regulated by catalysts called enzymes.
Enzymes are proteins that play a major role in digestion as well as in the regulation of metabolic pathways in the cell. Enzymes do not cause a reaction to occur, but simply
&+2+
+ + +2&+
&+2+
+ 2
2+
6XFURVH D +\GURO\VLV
E &RQGHQVDWLRQ + 2+
+
+2
+ 2
&+2+
+ +
2+
2+
*OXFRVH + 2+
+
+2
+ 2 &++2+ +
2+
2+
*OXFRVH + 2+
+
+2
+ 2
+
:DWHU
:DWHU +2
+
2+
2 &+2+
+ +
2+ +2 +2&+
&+2+
+
2+
)UXFWRVH
*OXFRVH
1HZERQGFUHDWHG +
2+
+
+2
+ 2
+ +2 +
2+
2
&+2+
+ +
2
2+
+ 2+
+
+2
+ 2 &++2+ +
2+
2+
0DOWRVH
+ 2+
+
+ 2
Figure 7.1 Hydrolysis reaction of the disaccharide sucrose to the end-product molecules glucose and fructose (a) and condensation reaction of two glucose molecules forming maltose (b)
Digestion and absorption 135 regulate the rate or speed at which the reaction takes place. The great diversity of protein structures enables enzymes to perform highly specific functions. Enzymes only affect reactions that would normally take place but at a much slower rate. Enzymes do not change the nature of the reaction nor its final result.
Chemical reactions occur when the reactants have sufficient energy to proceed. The energy required to initiate chemical reactions is called the energy of activation. Enzymes work as catalysts by lowering the energy of activation. By reducing the energy of activa-tion, enzymes increase the speed of chemical reactions and therefore increase the rate of product formation.
Enzymes possess the unique property of not being altered by the reactions they affect.
Consequently, the turnover of enzymes in the body remains relatively slow and they are continually reused. A typical mitochondrion may contain up to 10 billion enzyme mol-ecules, each carrying out millions of operations within a brief time. During exercise, enzyme activity increases enormously within the cell owing to an increase in energy demand. Enzymes make contact at precise locations on the surfaces of cell structures (e.g., mitochondria); they also operate within the structure itself. Many enzymes func-tion outside the cell – in the bloodstream, digestive mixture, or fluids of the small intestine.
Although there is a standardized naming system for enzymes, most textbooks use common names that generally reflect the mode of operation or substance with which it interacts. Except for older enzymes such as rennin, trypsin, and pepsin, almost all enzyme names end with the suffix “ase.” For example, hydrolase adds water during hydrolysis reactions, protease interacts with protein, oxidase adds oxygen to a substance.
In addition, kinases are a group of enzymes that add phosphate groups to the reactants or substances with which they react. Further, dehydrogenases are enzymes that remove hydrogens from substances they catalyze. In the realm of human biology, those sub-stances that are acted upon by enzymes are referred to as substrates.
The ability of enzymes to lower the energy of activation results from unique structural characteristics. In general, enzymes are large protein molecules with a three- dimensional shape. Each type of enzyme has characteristic ridges and grooves. The pockets formed from the ridges or grooves located on the enzyme are called active sites. These active sites are important because it is the unique shape of the active site that causes a specific enzyme to adhere to a particular reactant molecule or substrate. The concept of how enzymes fit with a particular substrate molecule is analogous to the idea of a lock and key (Figure 7.2). The shape of the enzyme’s activity site is specific to the shape of a par-ticular substrate, which allows the two molecules, enzyme and substrate, to form a complex known as the enzyme–substrate complex. Following the formation of the enzyme–substrate complex, the energy of activation needed for the reaction to occur is lowered, and the reaction is more easily brought to completion. This is followed by disso-ciation of the enzyme and product.
The “lock- and-key” mechanism offers a protective function so that only the correct enzyme activates the targeted substrate. Consider the enzyme hexokinase, which acceler-ates a chemical reaction by linking with a glucose molecule. As a result of the action of this enzyme, a phosphate group transfers from adenosine triphosphate (ATP) to a spe-cific binding site on one of the glucose’s carbon atoms. Once the two binding sites join to form a glucose–hexokinase complex, the substrate begins its stepwise degradation (controlled by other specific enzymes) to form less complex molecules during energy metabolism.
The temperature and hydrogen ion concentrations of the reactive medium dramatic-ally affect enzyme activity. Each enzyme performs its maximum activity at a specific pH.
The optimum pH of an enzyme usually reflects the pH of the body fluids in which it
bathes. For some enzymes, optimal activity requires a relatively high acidity level. For example, the protein- splitting enzyme pepsin released by the stomach is most active in hydrochloric acid, whereas trypsin released by the pancreas functions more effectively on the alkaline side of neutrality. Increases in temperature generally accelerate enzyme reactivity. As the temperature rises above 40 to 50°C, enzymes may become denatured and therefore lose their function permanently.
Some enzymes require activation by additional ions and/or smaller organic mol-ecules termed coenzymes. These complex non- protein substances facilitate enzyme action by binding the substrate with its specific enzyme. The metallic ions iron and zinc function as coenzymes, as do the B vitamins or their derivatives. Oxidation- reduction reactions use the B vitamins riboflavin and niacin, while other vitamins serve as transfer agents for groups of compounds in metabolic processes. A coenzyme requires less specificity in its action than an enzyme because the coenzyme affects a number of different reactions. It can serve as a temporary carrier of intermediary products in the reaction. For example, the coenzyme nicotinamide adenine dinucleo-tide (NAD) forms NADH to transport hydrogen atoms and electrons that split from food fragments during energy metabolism. The electrons then pass to special trans-porter molecules in another series of chemical reactions that ultimately deliver the electrons to molecule oxygen.
The digestive system: an overview
The foods and beverages we consume, for the most part, must undergo extensive altera- tion by the digestive system to provide us with usable nutrients. The digestive system pro-vides two major functions: (1) digestion, the physical and chemical breakdown of food, and (2) absorption, the transfer of nutrients from the digestive tract into the blood or lymphatic circulatory systems. Carbohydrates, lipids, and proteins are digested and absorbed as sugars, fatty acids, and amino acids, respectively. Some substances, such as water, can be absorbed without digestion, whereas others, such as dietary fibers, cannot be digested by humans and therefore cannot be absorbed. These unabsorbed substances pass through the digestive tract and are excreted in the feces.
The digestive system is made up of the digestive tract and accessory organs (Shier et al. 2010). The digestive tract, more commonly known as the gastrointestinal tract or alimentary tract, may be thought of as a hollow tube that runs from the mouth to the anus (Figure 7.3). Organs that make up the gastrointestinal tract include the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The inside of the tube that these organs form is called the lumen. Food within the lumen of the
(Q]\PH (Q]\PHVXEVWUDWH
3URGXFWV
(Q]\PH 6XEVWUDWH
$FWLYH VLWH
Figure 7.2 Sequences and steps in the “lock and key” mechanism of enzyme action
Digestion and absorption 137 gastrointestinal tract has not been absorbed and is therefore technically still outside of the body. Only after food is transferred into the cells of the intestine by the process of absorption is it actually inside the body. The accessory organs participate in digestion but are not part of the gastrointestinal tract, and include the salivary glands, pancreas, liver, and gall- bladder (Figure 7.3). The accessory organs release their secretions needed for the process of digestion into ducts, which empty into the lumen of the gastro intestinal tract.
Parotid salivary gland Mouth
Tongue Tooth Sublingual salivary gland
Esophagus
Stomach
Pancreas
Rectum Anal canal
Small intestine Large intestine Duodenum (of small intestine) Gallbladder
Liver
Pharynx Submandibular salivary gland
ge ti ode
small intestine) lbla
enum all intestine) adder
)
all stine s
all stine
ectum canal
Re
Sto
Pa
Figure 7.3 Gastrointestinal tract and accessory organs of the digestive system Source: Shier et al. (2010). Used with permission.
The amount of time between the consumption of food and its elimination as solid waste is called transit time. It takes approximately 24 to 72 hours for food to pass from mouth to anus. Many factors affect transit time, such as composition of diet, illness, certain medications, physical activity, and emotions. Bands of smooth muscle called sphincters act like one- way valves, regulating the flow of the luminal contents from one organ to the next. The gastrointestinal tract has several sphincters, which are often named according to their anatomical locations. For example, the ileocecal sphincter is between the ileum, the last segment of the small intestine, and the cecum, the first portion of the large intestine.
Organization of the gastrointestinal tract
The digestive tract contains our major tissue layers: the mucosa, submucosa, muscular layer, and serosa. Each tissue layer contributes to the overall function of the gastroin-testinal tract by providing secretions, movement, communication, and protection.
Mucosa
The innermost lining of the digestive tract, called the mucosa, consists mainly of epithe-lial cells and carries out a variety of digestive functions. The mucosa, often called the mucosal lining, produces secretions needed for digestion such as enzymes, hormones, and mucus. The digestive system produces and releases a variety of substances and secre-tions referred to collectively as digestive juices, some of which are relatively acidic.
Because mucosal cells are continuously exposed to harsh digestive secretions within the gastrointestinal tract, their life span is a mere two to five days. Once the mucosal epithe-lial cells wear out, they slough off and are replaced by new cells.
Submucosa
A layer of connective tissue called the submucosa surrounds the mucosal layer. The mucosal layer contains a rich supply of blood- vessels, which nourish the inner mucosal layer and the next outer muscular layer. In addition to blood- vessels, the submucosa con-tains lymphatic vessels, which are filled with fluid called lymph. Lymph transports fluid away from the body tissues and aids in the circulation of fat. The submucosa also con-tains a network of nerves called the submucosal plexus, which regulates the release of gastrointestinal secretions from cells making up the mucosal lining.
Muscular layer
Moving outward from the submucosa, the next layer in the gastrointestinal tract is the two layers of smooth muscle organized as an outer longitudinal and an inner circular layer. Located between these two muscle layers is the myenteric plexus, a network of nerves that control the contraction and relaxation of the muscle. Such contraction and relaxation promotes mixing of the food mass with digestive secretions and keeps food moving through the entire length of the gastrointestinal tract.
Serosa
The serosa is the outermost layer that encloses the gastrointestinal tract and consists of connective tissue and provides overall support and protection. In particular, the serosa
Digestion and absorption 139 secretes a fluid that lubricates the digestive organs, preventing them from adhering to one another. In addition, much of the gastrointestinal tract is anchored within the abdominal cavity by mesentery, a membrane that is continuous with the serosa.
Gastrointestinal motility and secretions
The term motility refers to the mixing and propulsion of material by muscular contrac-tions in the gastrointestinal tract. These movements result from the contraction and relaxation of circular and longitudinal muscle in the muscular layer. There are two types of movement in the gastrointestinal tract: segmentation and peristalsis. Segmentation occurs when circular muscles in the small intestine move the food mass back and forth, thereby increasing the contact between food particles and digestive secretions. Peristalsis involves rhythmic, wave- like muscle contractions that propel food along the entire length of the gastrointestinal tract. The contraction of circular muscles behind the food mass causes the longitudinal muscle to shorten. When the longitudinal muscles lengthen, the food is propelled forward. Peristalsis is similar to the motion as exhibited when an earthworm moves.
Gastrointestinal secretions are important for digestions and protections of the gastroin-testinal tract, and include water, acid, electrolytes, mucus, salts, enzymes, bicarbonate, and other substances (Table 7.1). For example, mucus forms a protective coating that lubri-cates the mucosal lining. Digestive enzymes are biological catalysts that facilitate chemical reactions which break down complex food particles. More specifically, digestive enzymes catalyze hydrolysis reactions as mentioned above, which break down chemical bonds by adding water. As a result, molecules such as starch and protein are broken down into smaller components so that they may be absorbed across the mucosal lining. Organs that release digestive secretions include the salivary glands, stomach, pancreas, gall- bladder, small intestine, and large intestine. In fact, approximately 7 liters of secretions, most of which is water, are released daily into the lumen of the gastrointestinal tract. Fortunately, the body has a “recycling” system that enables much of this water to be reclaimed.
Table 7.1 Important gastrointestinal secretions and their functions
Secretion Source Function
Saliva Mouth Partially digesting starch with salivary amylase, lubricating food for swallowing
Mucus Mouth, stomach, small intestine, large intestine
Protecting GI tract, lubricating food as it travels through the GI tract
Enzyme Mouth, stomach, small intestine, pancreas
Breaking down complex foods into smaller particles for absorption
Acid Stomach Promoting digestion of protein among other functions Bile Liver (stored in
gall-bladder) Assisting fat digestion in the small intestine by suspending fat in water
Bicarbonate Pancreas, small
intestine Neutralizing stomach acid when food mix reaches the small intestine
Hormones Stomach, small
intestine, pancreas Stimulating production of acid, enzyme, bile, and bicarbonate, regulating peristalsis and food movement, and influencing the desire to eat
Regulation of gastrointestinal motility and secretions
Gastrointestinal motility and secretions are carefully regulated by neural and hormonal signals. These involuntary regulatory activities ensure that complex food particles are physically and chemically broken down and food mass moves along the gastrointestinal tract at the appropriate rate. The gastrointestinal tract has three regulatory control systems. The intestinal and the central nervous system provide neural control, and the intestinal endocrine system provides hormonal control.
Intestinal nervous system
The gastrointestinal tract has its own local nervous system called the enteric nervous system. The enteric nervous system receives information from other nerves called sensory receptors located within the gastrointestinal tract. There are two kinds of sensory receptors, chemoreceptors and mechanoreceptors, each monitoring conditions and changes related to digestive activities. Chemoreceptors detect changes in the chemical composition of the luminal contents, whereas mechanoreceptors detect stretching or distension in the walls of the gastrointestinal tract. The presence of food in the tract can stimulate both chemo- and mechanoreceptors. Information from both kinds of sensory receptors is relayed to the enteric nervous system, which responds by communicating with a variety of muscles and glands. In return, muscles and glands carry out the appro-priate response to help with digestion, such as an increase in peristalsis and/or release of digestive secretions.
Central nervous system
The intestinal nervous system controls digestive functions at the local level. However, the gastrointestinal tract also communicates with the central nervous system. The central nervous system consists of the brain and spinal cord, which receive and respond to sensory input from the gastrointestinal tract. The function of both the enteric and central nervous systems keeps the digestive system and the brain in close communica-tion. This is why sensory and emotional stimuli can affect one’s digestive function. For example, the sight, smell, or thought of food stimulates gastrointestinal motility and secretion. Similarly, emotional factors such as fear, sadness, anger, anxiety, and depres-sion can cause gastrointestinal distress.
Intestinal endocrine system
The gastrointestinal tract consists of many different types of cell, some of which are hormone- producing cells referred to collectively as the enteric endocrine system. Hor-mones produced by these cells are important in providing communication in the body.
Enteric hormones, which act as chemical messengers, are released into the blood in response to chemical and physical changes in the gastrointestinal tract. This information is then communicated to other organs, alerting them to the impending arrival of food.
Similar to neural signals, hormones also influence the rate at which food moves through the gastrointestinal tract and the release of gastrointestinal secretions. In addition to regu-lating gastrointestinal motility and secretion, some enteric hormones communicate with appetite centers in the brain, and thus influence the desire to eat. The four major enteric hormones are gastrin, secretin, cholecystokinin, and gastric inhibitory protein. These hor-mones are released from different digestive organs. As such, the specific role of each of these hormones will be discussed later in the chapter as each organ is introduced.