3. DIGESTION DIGESTION ABSORPTION DIGESTION. Refers to the process of breaking down large nutrient particles into their basic components

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3. DIGESTION T incrprate nutrients cntained in fds, rganisms have t be able t reduce particles size t sugars, fatty acids, amin acids r small peptides (Fig. 3-1). This prcess is called digestin.

1 3. DIGESTION T incrprate nutrients cntained in fds, rganisms have t be able t reduce particles size t sugars, fatty acids, amin acids r small peptides (Fig. 3-1). This prcess is called digestin. Once the materials have reached these absrbable sizes they have t be translcated frm the lumen f the intestine t the extracellular fluid f the gut fr further prcessing and distributin thrughut the bdy. This prcess f is called absrptin (Figs. 3-2, 3-3). T fllw a smehw structured sequence in this cmplex subject, a general view f the digestive prcesses will be discussed first, fllwed by a review f sme anatmical pertinent details, and then the types f fdstuff used by animals. Finally a review f the digestive prcess thrughut the digestive tract and then a mre detailed view fr each type f material thrughut the digestive system will be dne. DIGESTION Refers t the prcess f breaking dwn large nutrient particles int their basic cmpnents Figure 3-1. Prcess f digestin ABSORPTION Refers t the prcess f translcating basic mlecules frm the intestinal lumen thrugh epithelial cells f the intestine The prcess f digestin is carried ut with the supprt f mechanical reductin in particle size within the ral cavity and stmach and by enzymatic separatin f individual elements in the stmach and small intestine. T sustain life, the rganism als has t absrb micr and macrnutrients, as well as, water. Figure 3-2. Prcess f absrptin The digestive prcess starts in the ral cavity where masticatin reduces particle size and incrprates saliva int the ingesta. Saliva cntains enzymes that will cmmence the degradatin f sme f the carbhydrates DIGESTION Oral cavity Masticatin Salivatin Stmach Grinding and mixing Acid, enzymatic Small intestine Pancreatic enzymes Entercyte bund enzymes Figure 3-4. Steps invlved in digestin Figure 3-3. Changes in particle size as fd underges digestin and lipids. The stmach cntributes t the grinding and mixing f the fd and starts the digestin f prteins with the help f acid and enzymes. Once the chyme passes int the small intestine, it receives pancreatic enzymes that almst cmplete the prcess f digestin fr mst materials. In the entercytes f the small intestine, there are membrane bund enzymes which cmplete the digestin prcess and makes available fr absrptin the basic units f nutrients (Fig. 3-4). V BS 122 Physilgy II 23 Class f 2012

2 DIGESTIVE PHASES There are tw phases t the digestive prcess ging n simultaneusly in the rganism (Fig. 3-5). The luminal phase, which takes place in the lumen f the digestive tract, is dedicated t the initial hydrlysis f large particles int smaller cmpnents. This phase is carried ut by the acid secreted in the stmach and by a variety f enzymes secreted either by the stmach, pancreas r intestine. The ther phase is called the membranus phase and it is cnducted by enzymes which are bund t the membranes f the entercytes in the villi f the small intestine. This is the final step in the digestin f carbhydrates and prteins befre their basic cmpnents are absrbed. FUNCTIONAL ANATOMY TYPES OF DIGESTION Luminal phase Takes place in the lumen f the digestive tract Is the initial hydrlysis f fd Is carried ut by secreted enzymes and acid Membranus phase Cnducted by enzymes attached t membranes f entercytes Figure 3-5 Phases f digestin The small intestine is cvered with villi, which cntain mucusprducing gblet cells and entercytes (Fig. 3-6). Between the villi are fund the crypts f Lieberkühn where the entercytes and gblet cells are generated befre starting a migratin pattern twards the tip f the villi where they reach the extrusin zne and are disldged frm the villi. As shwn in the previus class, while in the crypts, the entercytes are fluid secreting cells and when they reach the walls f the villi they are fluid and nutrient absrbing cells (Fig. 3-6). In a mre detailed view f adjacent entercytes (Fig. 3-7), it can be seen that each cell has micrvilli in the apical regin t increase the absrptive area f the apical Figure 3-6. Transversal cut f the intestinal villi. Between villi are the crypts f Lieberkühn, in the middle entercytes and gblet cells and, at the tip the extrusin zne where these aged cells are slughed ff membrane. These micrvilli are immersed in a mucus-like substance that reduces the speed f mvement f the fd particles, s they can be digested by the membrane bund enzymes assciated with the membranus phase f digestin. The micrvillus and the glyccalix cnstitute the brush brder f the cells. Between cells there are tight junctins that prevent large mlecules frm infiltratin. These junctins are nt as tight as t prevent the escape f water frm V BS 122 Physilgy II 24 Class f 2012

3 the intracellular space which is als called lateral space. The sides f the cells are cvered by the baslateral membrane and the base by the basement membrane, which cntains fenestratin amng cells in the area f the lateral space. NUTRIENTS Carbhydrates There are many types f carbhydrates (Fig. 3-8). They can be classified by size and degree f cmplexity r by their type f structure. Fllwing the first criteria, carbhydrates can be gruped as cmplex plysaccharides, disaccharides and mnsaccharides. The secnd classificatin criteria appear t be mre useful t understand cmpsitin (Fig. 3-9). Figure 3-7. Magnificatin f the entercytes Fibres CARBOHYDRATES Cmplex plysaccharides Disaccharides Mnsaccharides Figure 3-8. Types f carbhydrates by cmplexity The mst abundant carbhydrates are the fibers such as cellulse, which can nt be digested by mammals. TYPES OF CARBOHYDRATES Fibers (cellulse) Indigestible by mammals β-acetal bnd between carbns 1 and 4 Digested by fermenting bacteria Sugars Shrt mlecules (sucrse, lactse) Starches (vegetal rigin) Large mlecules (α-acetal bnd between carbns 1 and 4) Glycgen (animal rigin) Large mlecule Ruminants, with bacterial fermentatin can utilize these materials. Fibres cntain a β-acetal bnd between carbns 1 and 4 fr which mammals d nt prduce the enzyme t hydrlyze it. Sugars Figure 3-9. Types f carbhydrates by structure The secnd types f carbhydrates are the sugars, such as sucrse and lactse, which are cmbinatins f mnsaccharides as glucse, fructse r galactse. Figure Types f plysaccharides V BS 122 Physilgy II 25 Class f 2012

Starches Starches are plysaccharides made f chains f glucse mlecules that are linked by α-acetal (glycsidic) bnds between carbns 1 and 4. They are prduced by vegetables.

Amylse is made f a single straight chain f glucses jined by α-acetal bnd between carbns 1 and 4 while amylpectin has the same shrter chains with branches generated by 1-6 α-acetal bnds.

4 Starches Starches are plysaccharides made f chains f glucse mlecules that are linked by α-acetal (glycsidic) bnds between carbns 1 and 4. They are prduced by vegetables. The enzyme t hydrlyze this bnd is available in mammals. There are several types f starches. The mre cmmn starches are amylse and amylpectin (Fig. 3-10). Amylse is made f a single straight chain f glucses jined by α-acetal bnd between carbns 1 and 4 while amylpectin has the same shrter chains with branches generated by 1-6 α-acetal bnds. Glycgen A mlecule similar t starches, but f animal rigin. The mlecule f glycgen is started with a nucleus called glycgenin that serves as the initial anchr fr the grwing chains (Fig. 3-9). Lipids The diet f animals cntains a variety f lipids (Fig. 3-11). These lipids can range frm simply emulsified fats t triglycerides cmplexes made f a glycerl mlecule with three fatty acids. The fatty acids by themselves cnstitute a huge variety f cmpunds, which are classified accrding t the number f carbns and psitin f multiple bnds. LIPIDS Emulsified fats Triglycerides Fatty acids Glycerl Chlesterl Figure Types f dietary lipids Chlesterl is anther imprtant lipid which is essential fr the prductin f vital hrmnes, detergents and vitamins. Althugh an excess f chlesterl can be harmful t the rganism, a lack f chlesterl is undubtedly fatal. Prteins Prteins are sequences f amin acids. They range frm very shrt t extremely lng. The basic unit is an amin acid, which when jined with thers becmes a small peptide. Larger chains f amin acids are called plypeptides and much lnger nes are called prtein chains (Fig. 3-12). Structure f prteins Prteins have primary, secndary, tertiary and quaternary structures. The sequence f amin acids in the mlecule determines the primary structure. The spatial arrangement that the prtein adpts, based n ther frces r bnds, determine the secndary structure. The term shape serves t visualize this. Amng these a prtein may adpt a fibrus arrangement r a glbular arrangement. The final rigid cmpact structure that the prtein adpts after ther chains have bund determines the tertiary structure f the prtein. The term cnfrmatin is used t describe the cmbined secndary and tertiary structure. The quaternary structure depends n jining f identical r different chains in space. Figure Cmpsitin f prteins Figure Digestin in the ral cavity V BS 122 Physilgy II 26 Class f 2012

The main enzyme released in saliva is α-amylase, which reduces carbhydrates t smaller plysaccharides leaving chains as small as disaccharides, such as maltse, t units f 3-9 mnsaccharides.

5 OVERALL DIGESTIVE PROCESS The digestive prcess cmmences as sn as the fd enters the ral cavity (Fig. 3-13). Initially, masticatin reduces the size f particles increasing the surface area t allw the enzyme digestin. The main enzyme released in saliva is α-amylase, which reduces carbhydrates t smaller plysaccharides leaving chains as small as disaccharides, such as maltse, t units f 3-9 mnsaccharides. In the muth, nly abut 5% f the ingested carbhydrates get digested. This enzyme cntinues wrking in the stmach, but nly fr a shrt time until the drp in ph inactivates it. By this time abut % f the carbhydrates has been partially digested, maltse being the predminant prduct. Carnivres d nt prduce α-amylase in saliva. Calves and ther lactating animals prduce lingual lipase, an enzyme which starts the prcess f lipid digestin (Fig. 3-13). Once the fd enters the stmach, there is significant mixing and maceratin f the particles t further reduce particle size and t increase the surface area. The acid prduced in the stmach lwers the ph and starts hydrlyzing prteins. Lw ph als cntributes t the activatin f prtelytic enzymes, thus, furthering the digestin f prtein matter (Fig. 3-14). In the intestine, the main digestive activity takes place. With the incrpratin f bile, the fat is emulsified, its drplet size is reduced and packaged t be water Figure Events taking place during intestinal digestin sluble (Fig. 3-15). CARBOHYDRATE DIGESTION Figure Events taking place during gastric digestin (Luminal digestin) Oral cavity α-amylase Hydrlyze starch t smaller plysaccharides 2 (maltse), 9 glucse lng Stmach Cntinues digestin (bdy and fundus) fr 1 hur Neutralized by acid with mixing 30-40% f starches cnverted t maltse Smaller intestine Pancreatic α-amylase Mre pwerful than salivary min all starches t di r plysaccharides Maltse Ismaltse Malttrise Dextrins Figure Luminal digestin f carbhydrates V BS 122 Physilgy II 27 Class f 2012

The large variety f prtelytic enzymes secreted by the pancreas cntributes t further reduce the size f peptides prducing small peptides and amin acids.

6 The triglycerides are hydrlyzed t fatty acids and mnglycerides. These smaller mlecules frm micelles. At the same time the luminal phase f the carbhydrate digestin takes place prducing a variety f sugars (Fig. 3-15, 3-16). The large variety f prtelytic enzymes secreted by the pancreas cntributes t further reduce the size f peptides prducing small peptides and amin acids. The final step in digestin takes place in the membranus phase f digestin fr carbhydrates and prteins. As a result the mnsaccharides glucse, fructse, galactse, etc are generated, as well as, amin acids, di and tripeptides. Carbhydrate digestin Figure Substrates and prducts f the luminal digestin f carbhydrates The initial digestin takes place in the luminal phase. As it shuld be clear by nw, α-amylase secreted in the saliva hydrlyzes starches int smaller plysaccharides. This generates material ranging frm disaccharides (maltse) t ligsaccharides f up t 9 glucses. This prcess cntinues in the stmach fr abut ne hur, specifically in the bdy and fundus f the stmach. The digestin ceases when the enzyme is degraded by the acidic envirnment. By nw 30-40% f the starches have been cnverted t maltse and ther ligsaccharides (Figs. 3-15, 3-16). CARBOHYDRATE DIGESTION (membranus phase) Enzymes bund t entercyte apical membrane Lactase Sucrase Maltase a-dextrinase Malttriase Ismaltase In the intestine, the enzyme pancreatic α-amylase, which is significantly mre pwerful than the salivary versin, cnverts the reminder f the starches and ligsaccharides in nly 15 minutes int di r plysaccharides, such as maltse, ismaltse, malttrise, dextrin (Fig. 3-16). This cmpletes the luminal phase f carbhydrate digestin. Figure Enzymes invlved in the membranus phase f digestin Figure Entercyte-bund enzymes as part Frm here n the membranus phase f digestin takes place f the membranus phase f digestin (Fig. 3-17). The small plysaccharides mve frm the lumen f the intestine twards the surface f the villi epithelium where the intestinal mucus and the gyccalyx in the brush brder have V BS 122 Physilgy II 28 Class f 2012

7 created a zne f very little r n flux f water called the unstirred water layer. This layer permits mlecules f plysaccharides t be captured and the membrane bund enzymes (lactase, sucrase, maltase, α-dextrinase, malttriase, ismaltase) digest them further (Figs. 3-18, 3-19). The final prduct being nly mnsaccharides, such as glucse fructse and galactse (Fig. 3-20). Prtein digestin The prcess f digestin f prteins is similar t that used fr carbhydrates. They bth have a luminal phase which generates smaller plypeptides and a membranus phase which cmpletes the digestin. In the case f prtein there are a much larger number f enzymes invlved and they are all secreted as prenzymes in rder t prtect the tissue that synthesizes them, and are then activated t perfrm digestin (Fig. 3-21). PROTEIN DIGESTION Similar t carbhydrates Luminal phase Prduce sme single amin acids Membranus digestin Cmpletes digestin Many mre enzymes invlved Many different amin acids All enzymes secreted as prenzymes Prevents self destructin Figure Digestin f prteins is mre cmplex than that f carbhydrates The digestin f prteins nly starts in the stmach. In the ral cavity the particle size is reduced, but n specific prtelytic enzyme is added. The first expsure t a digestive envirnment that prteins encunter is the HCl in the stmach. HCl can hydrlyze sme f the bnds in prteins, but its mre imprtant rle is t prvide an acidic envirnment in which ther prtelytic enzymes may functin. As pinted ut previusly all prtelytic enzymes are secreted as zymgens, that is, inactive enzymes. The HCl secreted in the stmach activates pepsin and chymsin Figure Substrates and prducts f the membranus digestin f carbhydrates PROTEIN DIGESTION Oral Cavity Mechanical size reductin Stmach Mechanical size reductin Acid hydrlysis Pepsin (endpeptidase) Chymsine (endpeptidase) Small intestine Trypsin (endpeptidase) Chemtrypsin (endpeptidase) Elastase (endpeptidase) Carbxypeptidase A (expeptidase) Carbxypeptidase B (expeptidase) Figure Sequence f events during prtein digestin frm pepsingen and chymsingen respectively. These are tw endpeptidases, r enzymes which hydrlyze bnds in the middle f the prtein mlecule, generating smaller plypeptides (Fig. 3-22). Once the chyme mves t the dudenum the acidity activates enterkinase which in turn activates the zymgen trypsingen t trypsin and trypsin activates the ther pancreatic prduced zymgens t chemtrypsin, elastase, carbxypeptidase A and B. V BS 122 Physilgy II 29 Class f 2012

Trypsin, as well as, chemtrypsin and elastase are all endpeptidases, while the tw carbxypeptidases are expeptidases, r enzymes that hydrlyze the last amin acid in a peptide.

8 Trypsin, as well as, chemtrypsin and elastase are all endpeptidases, while the tw carbxypeptidases are expeptidases, r enzymes that hydrlyze the last amin acid in a peptide. The membranus digestin fr prteins is again similar t that f carbhydrates with the difference being that the prteases bund t the entercytes d nt always divide the chains t single amin acids and di r tripeptides can be absrbed by entercytes. Fat digestin There is a minimal amunt f digestin f fat by lipase in lactating animals. In adults, digestin starts with the preparatin f the material in the stmach. Here it is emulsified and the slid fat glbules start t liquify frming lipid drplets. Entering the small intestine, the bile acids frm the liver r gallbladder cmpletely emulsify the fat generating small drplets cntaining triglycerides, chlesterl, chlesterl esters as well as phsphlipids (Fig. 3-23). FAT DIGESTION Oral cavity Minimal initial digestin lipase Stmach Initial emulsificatin Fat glbules liquify Lipid drplets Small intestine Emulsificatin is cmpleted Bile acids and phsphlipids Emulsified drplets are digested Pancreatic lipase and c-lipase Prduce free fatty acids Mnglycerides Chlesterl esterase and phsphlipase Free fatty acids Chlesterl Lys phsphlipids Micelle frmatin All digestin prducts surrunded by bile acids Figure Sequence f events in the digestin f fats Once the drplets are frmed these are attacked by tw pancreatic enzymes which wrk in assciatin. The pancreatic c-lipase displaces the bile acids cating the drplet, thus generating an area where the ther enzyme, the pancreatic lipase, can access the triglycerides and hydrlyze tw f the fatty acids leaving a mnglyceride. At the same time, ther cmpnents f the drplet are hydrlyzed by ther types f pancreatic lipases, particularly chlesterl esterase and phsphlipase. The results f this hydrlysis are chlesterl, nnesterified fatty acids and lysphsphlipids. All f these cmpunds are then released int the lumen f the intestine where they are rapidly gruped and cated with bile salts, and phsphlipids t frm micelles in preparatin fr absrptin (Fig. 3-24). Figure Actin f lipase and c-lipase n a lipid drplet, and frmatin f a micelle V BS 122 Physilgy II 30 Class f 2012

3. DIGESITION. Absorption. Digestion. Process of digestion

3. DIGESITION. Absorption. Digestion. Process of digestion 3. DIGESITION T incrprate the nutrients cntained in the ingested material, the rganisms have t reduce the size f the particles and the cmplex nutrients have t be cnverted t sugars, fatty acids, amin acids