3. DIGESITION. Absorption. Digestion. Process of digestion

Size: px

Start display at page:

Download "3. DIGESITION. Absorption. Digestion. Process of digestion"

Mary Griffin
5 years ago
Views:

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

1 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 and / r small peptides (Fig. 3-1). This prcess is called digestin. Digestin Refers t the prcess f breaking dwn large particles int small nes in rder t cnvert cmplex nutrients int their basic cmpnents Figure 3-1. Prcess f digestin Figure 3-3. Changes in particle size as fd underges digestin Once the materials have reached an absrbable size, 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 is called absrptin (Figs. 3-2, 3-3). Absrptin Refers t the prcess f translcating basic mlecules frm the intestinal lumen thrugh the epithelial cells f the intestine Figure 3-2. Prcess f absrptin Oral cavity Stmach Prcess f digestin Masticatin Salivatin Grinding and mixing Acid, enzymatic Small intestine Pancreatic enzymes Entercyte bund enzymes 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. 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 3-4. Steps invlved in digestin 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 and lipids. The stmach cntributes t the grinding and mixing f the fd. It 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. Membrane bund enzymes in the entercytes f the small intestine cmplete the digestin prcess and V BS 122 Physilgy II 23 Class f 2015

make the basic units f nutrients available fr absrptin (Fig. 3-4). DIGESTIVE PHASES There are tw simultaneus phases t the digestive prcess ging n in the rganism (Fig. 3-5).

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.

2 make the basic units f nutrients available fr absrptin (Fig. 3-4). DIGESTIVE PHASES There are tw simultaneus phases t the digestive prcess ging n 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, 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 ANTOMY The small intestine is cvered with villi, which cntains Types f digestin 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 the walls f the villi they are bth fluid and nutrient absrbing cells (Fig. 3-6). In a mre detailed view f adjacent entercytes (Fig. 3-7), 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 it can be seen that each cell has micrvilli in the apical regin t increase the absrptive area f the apical membrane. These micrvilli are immersed in a mucus-like substance that reduces the speed f mvement f the fd particles, Figure 3-5 Phases f digestin mucus-prducing gblet cells and entercytes (Fig. 3-6). The crypts f Lieberkühn are fund between the villi where entercytes and gblet cells are generated. These cells then migrate twards the tip f the villi where they reach the extrusin zne frm where they are disldged. As shwn in the previus class, while in the crypts, the entercytes are fluid secreting cells and when they reach Figure 3-7. Magnificatin f the entercytes s that they can be digested by the membrane bund enzymes assciated with the membranus phase f V BS 122 Physilgy II 24 Class f 2015

3 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 tight enugh t prevent the escape f water frm 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 the 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 Carbhydrates Cmplex plysaccharides Disaccharides Mnsaccharides Figure 3-8. Types f carbhydrates by cmplexity by their type f structure. Fllwing the first criteria, carbhydrates can be gruped as cmplex plysaccharides, disaccharides and mnsaccharides. The secnd classificatin criterin appears t be mre useful t understanding cmpsitin (Fig. 3-9). mre cmmn starches are amylse and amylpectin (Fig. 3-10). Amylse is made f a single straight chain f glucses jined by -acetal bnd between carbns 1 and 4 while amylpectin has the same shrter chains with branches generated by 1-6 -acetal bnds. Glycgen. Glycgen is 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 Types f carbhydrates Fibres (cellulse) Sugars Indigestible by mammals β-acetal bnd between carbns 1 and 4 Digested by fermenting bacteria Shrt mlecules (sucrse, lactse) Starches (vegetal rigin) Large mlecules (α-acetal bnd between carbns 1 and 4) Glycgen (animal rigin) Large mlecule Figure 3-9. Types f carbhydrate based n their structure Fibres. The mst abundant carbhydrates are the fibres such as cellulse, which cannt be digested by mammals. Fibres cntain a β-acetal bnd between carbns 1 and 4 fr which mammals d nt prduce the enzyme t hydrlyze it. Ruminants, with bacterial fermentatin can utilize these materials. Sugars. The secnd types f carbhydrates are the sugars, such as sucrse and lactse, which are cmbinatins f mnsaccharides such as glucse, fructse r galactse. Starches. Starches are plysaccharides made f chains f glucse mlecules 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 Figure Types f plysaccharides V BS 122 Physilgy II 25 Class f 2015

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.

4 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. 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 Emulsified fats Triglycerides Fatty acids Glycerl Chlesterl Lipids Figure Types f dietary lipids rganism, a lack f chlesterl is undubtedly fatal. 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, determines 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 t it 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 the jining f identical r different chain in space. 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 enzyme digestin. The main enzyme released in saliva is α-amylase, which Prteins Prteins are sequences f amin acids. They range frm very shrt t extremely lng. The basic unit is an amin Figure Digestin in the ral cavity Figure Cmpsitin f prteins reduces carbhydrates t smaller plysaccharides. This leaves 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 t wrk in the stmach, but nly fr a shrt time until the drp in ph inactivates it. By the time V BS 122 Physilgy II 26 Class f 2015

that α-amylase is inactivated abut 30-40% f the carbhydrates have been partially digested, maltse being the predminant prduct. Carnivres d nt prduce α- amylase in saliva.

Once the fd enters the stmach, there is significant mixing and maceratin f the particles t further reduce particle size and t increase activatin f prtelytic enzymes, thus, furthering the digestin f

5 that α-amylase is inactivated abut 30-40% f the carbhydrates have 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 activatin f prtelytic enzymes, thus, furthering the digestin f prtein matter (Fig. 3-14). The intestine is where the main digestive activity takes place. With the incrpratin f bile, the fat is emulsified, as drplet size is reduced and packaged t be water sluble (Fig. 3-15). The triglycerides are hydrlyzed t fatty acids and mnglycerides. These smaller mlecules frm micelles. At the same time the luminal phase f the carbhydrate's digestin takes place prducing a variety f sugars (Fig. 3- Carbhydrate digestin (Luminal digestin) Figure Events taking place during gastric digestin Figure Events taking place during intestinal digestin the surface area. The acid prduced in the stmach lwers the ph and starts hydrlyzing prteins. Lw ph als cntributes t the Oral cavity Stmach 15, 3-16). α-amylase Smaller intestine Hydrlyzes starch t smaller plysaccharides 2 (maltse), 9 glucse lng Cntinues digestin (bdy and fundus) fr 1 hur Neutralized by acid with mixing 30-40% f starches cnverted t maltse 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 2015

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

are generated, as are, amin acids, di and tripeptides. Carbhydrate digestin The initial digestin takes place in the luminal phase.

6 The large variety f prtelytic enzymes secreted by the pancreas cntributes t the further reductin f the size f peptides, prducing smaller 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 are, amin acids, di and tripeptides. Carbhydrate digestin 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 then 30-40% f the starches have been cnverted t maltse and ther ligsaccharides (Figs. 3-15, 3-16). In the intestine, the pancreatic α-amylase enzyme, which is significantly mre pwerful than the salivary versin, needs nly 15 minutes t cnvert the reminder f the starches and ligsaccharides int di- r plysaccharides, such as maltse, ismaltse, malttrise, dextrin (Fig. 3-16). This cmpletes the luminal phase f carbhydrate digestin. Frm here n the membranus phase f digestin takes place (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 glyccalyx in the brush brder have created a zne f very little r n flux f water, called the unstirred water layer. The unstirred water layer permits mlecules f plysaccharides t be captured and the membrane bund enzymes (lactase, sucrase, maltase, α-dextrinase, malttriase, ismaltase) t digest them further (Figs. 3-18, 3-19). Carbhydrate digestin (membranus phase) Enzymes bund t entercyte s apical membrane Lactase Sucrase Maltase a-dextrinase Malttriase Ismaltase Figure Enzymes invlved in the membranus phase f digestin Figure Substrates and prducts f the luminal digestin f carbhydrates Figure Entercyte-bund enzymes as part f the membranus phase f digestin V BS 122 Physilgy II 28 Class f 2015

7 The final prducts f carbhydrate digestin are nly mnsaccharides, such as glucse fructse and galactse (Fig. 3-20). Figure Substrates and prducts f the membranus digestin f carbhydrates Prtein digestin The prcess f prtein digestin 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 Prtein digestin 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 digestin there is a much larger number f enzymes invlved and they are all secreted as prenzymes in rder t prtect the tissue that synthesizes them. They are then activated t perfrm digestin (Fig. 3-21). 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. A prteins first expsure t a digestive envirnment 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 frm pepsingen and chymsingen, respectively. These are tw gastric endpeptidases, r enzymes prduced in the stmach, 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 Oral Cavity Stmach Prtein digestin Mechanical size reductin 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 activates enterkinase which in turn activates the zymgen V BS 122 Physilgy II 29 Class f 2015

8 trypsingen t trypsin. Trypsin activates the ther pancreatic prduced zymgens t chemtrypsin, elastase, and carbxypeptidase A and B. Trypsin, 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 int single amin acids and di r tripeptides can be absrbed by entercytes. Fat digestin There is a minimal amunt f fat digestin by lipase in lactating animals. In adults, fat digestin starts with the preparatin f the material in the stmach, where it is emulsified and the slid fat glbules start t liquify by 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 and phsphlipids (Fig. 3-23). Oral cavity Stmach Small intestine Fat digestin Minimal initial digestin, lipase Initial emulsificatin Fat glbules liquify Lipid drplets Micelle frmatin 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 All digestin prducts surrunded by bile acids Once the fat drplets are frmed these are attacked by tw pancreatic enzymes which Figure Sequence f events in the digestin f fats wrk in assciatin. The pancreatic clipase 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 2015

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