GIT 3: Metabolism of Nutrients PDF

PHYSIO-LEC: LE 4 | TRANS 3 GIT 3: Metabolism of Nutrients MARIE FRANZE C. PAMATMAT, MD, DPPS | JAN/13/2024 OUTLINE LEARNING OBJECTIVES I.​ Introducti...

PHYSIO-LEC: LE 4 | TRANS 3 GIT 3: Metabolism of Nutrients MARIE FRANZE C. PAMATMAT, MD, DPPS | JAN/13/2024 OUTLINE LEARNING OBJECTIVES I.​ Introduction to VI.​ Digestion and ✔​ Discuss an overview of digestion and absorption in Digestion and Absorption of Proteins the gastrointestinal tract Absorption A.​Dietary Proteins ✔​ Discuss the digestion, absorption, metabolism of II.​ Digestion and B.​Storage of Proteins major nutrients in the body Absorption of C.​Functional Role of o​ Carbohydrates Carbohydrates Proteins o​ Fats or Lipids III.​ Metabolism of VII.​ Metabolism of Proteins Carbohydrates VIII.​Regulation of Proteins o​ Proteins A.​ Glycolysis IX.​ Digestion and ✔​ Discuss the absorption of the following substances in B.​ Citric Acid Cycle Absorption of Water the gastrointestinal tract C.​ Oxidative and Ions o​ Water Phosphorylation A.​Water Absorption o​ Ions D.​ Regulation of B.​Sodium Absorption o​ Vitamins and Minerals Energy C.​Chloride Absorption ✔​ Discuss how absorption occurs in the large intestine Metabolism D.​Bicarbonate E.​ Summary of ATP Absorption I. INTRODUCTION TO DIGESTION AND ABSORPTION Formed from X.​ Digestion and ​ The gastrointestinal tract has major physiological Glucose Absorption of Vitamins processes which include - Motility, Secretion, Digestion, F.​ Anaerobic A.​Fat-Soluble Vitamins Absorption Glycolysis (A, D, E, K) ​ It is important for excretion of the food that we intake in our G.​ Glycogenesis, B.​Folate or Folic Acid daily lives Glycogenolysis, Absorption ​ Since it is in contact with the body’s external environment, and C.​Vitamin B12 it is vulnerable to infectious organisms, hence to protect Gluconeogenesis Absorption itself the gastrointestinal tract (GI tract) has a complex H.​ Pentose D.​Calcium Absorption system of defense consisting of immune cells Phosphate E.​Magnesium →​That's why it represents the largest immune organ of Pathway Absorption the body IV.​ Digestion and F.​Iron Absorption of Fats XI.​ Digestion and V.​ Metabolism of Fats Absorption in the Large A.​Hormonal Intestine Regulation of Fat Utilization B.​Fat Deposits SUMMARY OF ABBREVIATIONS SGLT1 Sodium/glucose cotransporter 1 GLUT Glucose Transporter PFK Phosphofructokinase PPP Pentose Phosphate Pathway ACTH Adrenocorticotropic hormone Figure 1. Overview of General Mechanisms of Digestion and Na-AA Na/Amino Acid Cotransporters Absorption[Lecturer’s PPT] DMT1 Divalent Metal Transporter 1 ​ There are substances (e.g. glucose) that does not require FP1 Ferroportin 1 digestion. However, there are substances that need to be digested first into the lumen into its constituent monomers ❗️ Must know 📣 Lecturer 📖 Book 📋 Previous Trans by pancreatic enzymes before it can be absorbed →​Big polymers of glucose like disaccharides, need to be hydrolyzed first to their monomers so they can be processed by the brush border enzymes to be further absorbed. ▪​ For big polymers to be functional, they need to be resynthesized inside the cell →​Others may need to be hydrolyzed inside the cell before it is absorbed. ​ Some may need to be digested by brush border enzymes instead of pancreatic enzymes before they can be directly absorbed in the intestinal epithelium LE 1 TRANS 1 VER 3 TG-19&20: R. Mariano, E. Marigmen, E. Martin, R. Martin, TE: I. Martinez, J. Matta AVPAA: Jarabata, J Page 1 of 16 L. Martinez, C. Masangkay, J. Mapute, J. Mateo, Z. Matta, J. Maynigo, J. Medina, A. Mendoza, C. Mendoza  ​ Others may be broken down first into its constituent II. DIGESTION AND ABSORPTION OF components or smaller components before it can be fully CARBOHYDRATES absorbed inside the cell and then inside the cell they need ​ The food that we eat in our daily lives, need to be digested ​📣to be resynthesized again to be fully metabolized Digestion of carbohydrates starts at the mouth with the breakdown of starch and absorbed for us to have energy ​ Carbohydrates exist in different types: →​Sucrose →​Using salivary amylase and ptyalin which are secreted →​Lactose or sugar in milk ​📣 by the parotid gland Only 10% of carbohydrates are digested in the mouth →​Starches which can be found in grains and potatoes ​ The three big parts of carbohydrates are absorbed and ​📣and stomach Only monosaccharides (glucose, galactose and fructose) can be absorbed by the GIT digested in different ways ​ Other types of carbohydrates: amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins and dextrins Figure 3. Overview of Digestion and Absorption of Carbohydrates[Lecturer’s PPT] ​ Starches once ingested in our mouth mix with saliva which contains the enzyme ptyalin which is secreted by the parotid gland. Ptyalin will hydrolyze starch into Figure 2. Thickness of the arrows indicate the relative disaccharide like maltose and smaller polymers of glucose. magnitude of total absorption[Lecturer’s PPT] ​ The contact time of food inside the mouth is very short, only about 5% of the starches are hydrolyzed before it is ​ Carbohydrates, proteins and lipids, they are mostly swallowed. Digestion will continue in the fundus of the absorbed at the duodenum stomach for 1 hour before it is mixed with stomach →​The entire small intestine is responsible for absorbing secretions. From the mouth up to the fundus of the 📣 carbohydrates, proteins and fats stomach only about 10% are digested. →​ Pancreatic amylase is responsible digestion of ​ Once in the stomach, the salivary amylase is blocked by 📣 carbohydrates in the duodenum and jejunum/ileum the acidity of the gastric secretion because it becomes →​ Absorption of macromolecules, vitamins and inactive if the ph falls below 4.0. 📣 minerals occur at the duodenum ​ After digestion, the chyme empties from the stomach →​ Duodenum has large absorptive surface area due to through the duodenum, mixes with pancreatic juices which valvulae conniventes (mucosal folds) and villi. contain pancreatic amylase. After it comes in contact with ​ Moderately absorbed in the jejunum area pancreatic amylase, 50-80% or virtually almost all of the ​ Very low absorption up to the ileum area carbohydrates were already digested. ​ Some substances are actively absorbed only in the ​ By the time that the chyme is passed on beyond the duodenum in the case of Calcium duodenum, expect them to totally become disaccharide or ​ Bile acids on the other hand are absorbed along the entire into smaller glucose polymers to fully digest them. small intestine, but active absorption only occurs in the ​ Once in the small intestine, the lining or the villus contains ileum area specific important enzymes that will metabolize further. ​ Bile acids are mostly absorbed in the ileum area. →​Maltase ​ Cobalamin or vitamin B12 is only absorbed in the ileum ▪​ For maltose area →​A-dextrinase ▪​ Split into one molecule of glucose →​Lactase ▪​ For lactose and split into galactose + glucose →​Sucrase ​📣 ▪​ For sucrose and split into fructose + glucose Most common problem in terms of digestion- Lactase deficiency *space left intentionally blank* ​ Lactose intolerance shows symptoms of diarrhea, persistent passage of flatus, abdominal cramping due to the body's inability to digest lactose. Food containing lactose such as milk, cheese and yogurt are not advisable. However, yogurt is tolerable since it uses a probiotic component that is not affected or does not require lactase. ​ These four enzymes are the ones digesting the disaccharides once they come into contact with the enterocytes lining the villus of the small intestines. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 2 of 16  ​ The product of the digestion will become monosaccharides lumen to move through the brush border of the epithelial because they are the only ones that can be absorbed in cells to the cell interior by secondary active transport. the portal blood. They are water soluble and readily ​ Once this sodium ion is transported inside, glucose will be absorbed. attached to it via the transport protein sodium glucose co-transporter, or SGLT1. Figure 6. Ways of different monosaccharides absorption in small intestine[Lecturer’s PPT] ​ SGLT1 will transport sodium along with glucose. Once Figure 4. Food movement through the small intestine sodium is inside the epithelial cell already, glucose will be [Lecturer’s PPT] acted upon by GLUT2, or will be transported by GLUT2 ​ Absorption of nutrients occurs in the small and large through the cell’s lateral basal membrane into the intestine. ​ Structures in the small intestine that increase the absorption area: ​📣paracellular space to the blood circulation already. There must be a gradient inside and outside the cell to push sodium out and create a negative charge inside the →​Valvulae conniventes (folds of Kerckring) cell. ▪​ Increases the intestinal absorption by 3 fold. Well ​ The two monosaccharides: galactose and fructose. developed along the duodenum and jejunum area. →​Galactose is transported in a manner almost identical to →​Villi glucose. It uses SGLT1 along with sodium to enter the ▪​ Enhances total absorption by 10 fold. cell, and once inside, it is transported out by GLUT2 →​Intestinal epithelial cell with brush border into the paracellular space. However, fructose uses a ▪​ Inside the villi increasing the absorption area by 20 different transport protein. Instead of SGLT1, fructose fold. relies on GLUT5 to enter the cell. Despite this ​ Each day, several hundred grams of carbohydrates are difference, its transport out of the cell into the absorbed in the small intestine. It depends on how much paracellular space is the same, facilitated by the GLUT2 you consume per day. But they should be already in the transporter. form of monosaccharides. These monosaccharides, 80% ​ Glucose can also pass by facilitated diffusion of them are in the form of glucose and the other 20% is in →​If glucose concentration is greater on one side of the the form of galactose and fructose. membrane, more glucose will be transported from high ​ All of those monosaccharides, the glucose, galactose, to low concentration area fructose, are absorbed by a secondary active transport, →​Glucose is one of the examples that can be absorbed meaning they should occur in a cotransport mode with the directly without further digestion active absorption or active transport of sodium. SUMMARY The absorption of carbohydrate monosaccharides occurs via secondary active transport, starting with sodium leaving the cells, which decreases the sodium concentration inside the cell. This decrease prompts sodium from the interstitial lumen to enter the cell with the help of SGLT1. SGLT1 will only transport sodium when it is coupled with a glucose molecule. The same process applies to galactose, which also uses SGLT1 to transport it from the lumen into the cell. In contrast, fructose is transported into the cell by GLUT5. Once inside the cell, all three monosaccharides—glucose, galactose, and fructose—are transported out via GLUT2, moving into the paracellular space and then into the blood circulation. Figure 5. Secondary active transport of different molecules[Lecturer’s PPT] Question: What is the energy currency of the body? ​ In this image, the first step is the active transport of sodium →​ATP, the end goal of all nutrients absorbed is to be through the basal lateral membrane of endothelial cells digested and metabolized into ATP into the interstitial fluid. The sodium inside the cell will be transported out of the cell, going into the interstitial fluid. ​ There will be a decrease or a depletion of sodium ions inside the cell causing the sodium from the interstitial PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 3 of 16  III. METABOLISM OF CARBOHYDRATES B. CITRIC ACID CYCLE ​ Final products of carbohydrate digestion: Glucose (80%), ​ Pyruvic acid → 2 moles of acetyl CoA Galactose, and Fructose, all of which will undergo →​The two Acetyl CoA can enter the Citric Acid Cycle or glycolysis Krebs cycle ​ Interconversion of monosaccharides occur in the liver ​ The Krebs or Citric Acid Cycle occurs in the matrix of ​ Insulin mitochondria →​Increases rate of glucose transport by as much as 10x ​ For every cycle, only 1 molecule of ATP is formed during 📣 →​Secreted by the pancreas →​ In diabetes, glucose is not absorbed. conversion of ɑ-ketoglutaric acid to succinic acid ​ From glycolysis, you form two molecules of pyruvic acid. So for one glucose molecule, the Krebs cycle will undergo two cycles because there are two pyruvic acids that can enter the cycle. →​In total, we will have 2 ATPs formed after one glucose molecule. Figure 7. Entry of galactose, glucose, & fructose into glycolysis [Lecturer’s PPT] A. GLYCOLYSIS ​ After absorption, glucose can go into different pathways. One of them is that it will be used immediately for the release of energy or it can be stored in the form of glycogen. ​ The use of glucose for the release of energy is through Glycolysis. ​ Upon entry into the cells, glucose is phosphorylated to form energy. ​ Glycolysis is almost completely irreversible except in liver cells, renal tubular epithelial cells, intestinal epithelial cells ​ Glycolysis is the process of splitting glucose to form two Figure 9. Citric Acid Cycle[Lecturer’s PPT] molecules of pyruvic acid. C. OXIDATIVE PHOSPHORYLATION ​ 10 successive chemical reactions ​ From those two cycles of TCA, not much of ATP is formed. ​ NET REACTION: 1 glucose molecule = 4 moles of ATP ​ Almost 90% of ATP or energy formed through glucose formed BUT to be able to metabolize that 1 glucose metabolism is by oxidation of hydrogen or oxidative molecule, you need to use 2 moles of ATP used to phosphorylation. phosphorylate glucose. ​ In the two previous aforementioned pathways– Glycolysis →​In the end, you will only have 2 moles of ATP formed at and Krebs cycle– included in the net reaction is the the end of every phosphorylation of glucose. formation of hydrogen atoms. →​Aside from that there are other substances like NADH and FADH, of which are important in the oxidative phosphorylation of hydrogen. Figure 8. Glycolysis[Lecturer’s PPT] ​ The end product of glycolysis is the formation of two Figure 10. Oxidative Phosphorylation[Lecturer’s PPT] pyruvic acid. ​ For the first step, you split each hydrogen atom into a hydrogen ion and an electron. Those electrons that are released will be combined to dissolve oxygen of the fluids with water molecules to form hydroxyl ions. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 4 of 16  ​ As these electrons pass through the electron transport ▪​ When ATP levels are sufficient, the body naturally chain, large amounts of energy are released, which are decreases the formation of ATP. Without the used to pump hydrogen ions from inner mitochondria into substrate ADP available for phosphorylation, ATP the outer chamber. production slows down. ​ For each molecule of glucose metabolized, 24 hydrogen ▪​ Conversely, when energy is low, the levels of ADP atoms are extracted. and AMP increase, signaling the need for ATP →​Enzyme dehydrogenases remove hydrogen atoms in production. pairs during various stages of carbohydrate metabolism →​The extracted hydrogen atoms are transferred to NAD⁺, E. SUMMARY OF ATP FORMED FROM GLUCOSE a coenzyme, forming NADH. ​ From glycolysis, a total of 4 ATP is formed but 2 ATP were ​ The release of CO₂ by decarboxylases is to be dissolved in used in the phosphorylation of glucose. body fluids (primarily blood) and is transported to the lungs →​Hence, only a net of two ATP is produced. for exhalation. ​ In the Krebs cycle, there are a total of 2 ATP formed. ​ A total of 24 hydrogen atoms released from those two cycles. →​20 hydrogen atoms can be oxidized and for every two hydrogen atoms oxidized, there is a release or →​📣formation of 3 ATP. The four other hydrogen atoms remaining will undergo the chemiosmotic oxidative scheme, where in two hydrogen atoms oxidize will form two ATP molecules. Hence, resulting in 4 net ATP. ​ For every 1 molecule of glucose, 38 molecules of ATP are formed. For Physio, the reference for the conversion of 1 molecule glucose to ATP is Guyton. Hence, 1 molecule of glucose = 38 atp ATP Figure 11. Oxidative Phosphorylation[Lecturer’s PPT] CYCLE YIELD ​ Hydrogen atoms removed from the food substrate during earlier stages of metabolism are ionized. Glycolysis 2 ATPs ​ These hydrogen ions are initially carried by NAD⁺, forming NADH. NADH acts as an electron carrier, transporting TCA 2 ATPs, 24 H+ atoms electrons to the electron transport chain (ETC). ​ Essentially, during energy metabolism, the two electrons ETC 30 ATPs*** released from NADH enter the inner membrane of the mitochondria. These electrons undergo a series of 4 ATPs** transfers via electron acceptors in the electron transport chain, being accepted and released repeatedly, until they Total 38 ATPs reach cytochrome a3 (cytochrome oxidase). ​ Cytochrome oxidase plays a critical role as it can donate two electrons, which then combine with oxygen to form NOTE: water. This process essentially recycles the hydrogen From the 24 H+ atoms released during Glycolysis and atoms formed during glycolysis and the Krebs cycle. TCA: ​ ATP synthase, a knob-like structure located in the inner *** 20 H+ are oxidized in conjunction with the mitochondrial membrane, allows hydrogen ions (protons) chemiosmotic mechanism. 3 ATPs are produced for to pass through, facilitating the synthesis of ATP. The ATP every 2 H+ metabolized hence a total of 30 ATPs produced exits the mitochondria through facilitated produced. diffusion across the outer membrane. ** Remaining 4 H+ are released into the chemiosmotic oxidative schema in the mitochondrion D. REGULATION OF ENERGY METABOLISM beyond the first stage of figure 9. 2 ATPs are ​ Energy metabolism is tightly regulated because the body produced for every 2 H+ hence giving 4 ATPs does not constantly require energy. Excess carbohydrates that are not immediately used are converted into glycogen, the storage form of carbohydrates. F. ANAEROBIC GLYCOLYSIS ​ The regulation of energy metabolism involves several ​ Anaerobic glycolysis is the process of metabolizing mechanisms: glucose in the absence of oxygen. →​Inhibition of Phosphofructokinase (PFK): ​ Remember: glycolysis results in the production of two end ▪​ Phosphofructokinase, the enzyme responsible for a products which are pyruvic acid and hydrogen atoms, key rate-limiting step in glycolysis, can be inhibited to which are temporarily stored with NAD (forming NADH). If stop energy production these products accumulate excessively, glycolysis will ▪​ High levels of citrate ions in the body can inhibit PFK, stop. thereby halting glycolysis and energy formation. ​ In cases of excessive buildup, pyruvic acid and hydrogen →​ATP-ADP-AMP System: atoms react to form lactic acid. Lactic acid has two ▪​ This system helps regulate energy production based possible fates: on the body's energy needs: →​It can be reconverted to glucose in the liver →​It can be used directly as a source of energy. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 5 of 16  ​ The heart (cardiac muscle) can use lactic acid directly as H. PENTOSE PHOSPHATE PATHWAY an energy source because it contains enzymes to efficiently metabolize it into ATP. ​ The two organs that can only use glucose as their source of energy are the RBC and brain →​That’s why it is important for those undergoing ketogenic diet to still consume carbohydrates because the brain and RBC only glucose from carbohydrates as a source of their energy →​Other organs (e.g. skeletal muscle, kidneys, etc.) can use other sources of energy aside from glucose, such as proteins and lipids G. GLYCOGENESIS, GLYCOGENOLYSIS, AND GLUCONEOGENESIS Figure 13. Pentose Phosphate Pathway [Lecturer’s PPT] ​ Another important mechanism for breakdown and oxidation of glucose is the pentose phosphate pathway or PPP →​This is responsible for as much as 30% of glucose Figure 12. Pathways of Glycogen Metabolism →​📣breakdown in the liver and fat cells PPP can provide energy independently of all the Krebs cycle enzymes. This means that it doesn’t need 📣the enzymes for the Krebs cycle for it to function. [Lecturer’s PPT] ​ After absorption, glucose can either be released for →​ This is the reason why PPP is mainly used to break energy, or they can be stored in the form of glycogen - also down any excess glucose to help convert acetyl-CoA known as glycogenesis. into long chain fatty acids. ​ Liver cells can store up to 5-8% of their weight as glycogen IV. DIGESTION AND ABSORPTION OF FATS ​ Muscle cells can store up to 1-3% of their weight. ​ Fats or Lipid ​ Glycogenesis is the formation of glycogen. The start of this →​may exist in our diet as triglycerides, small process would be from glucose-6-phosphate that will be phospholipids, cholesterol, cholesterol esters phosphorylated into glucose-1-phosphate and then to ▪​ The basic moiety of triglycerides and phospholipids ​📣uridine diphosphate glucose and then finally to glycogen. The glycogenesis and the glycogenolysis, or the breakdown of stored glycogen to glucose, are somewhat are fatty acids. ▪​ Cholesterol, on the other hand, does not contain fatty acid, but its sterol nucleus is actually synthesized reverse of each other's pathways. from portions of fatty acid molecules. 📣 ​ The breakdown of glycogen into glucose is necessary. →​ For instance when one person is undergoing fasting, glucose molecules are needed to sustain energy for ​ Like the carbohydrates, only about 10% or less of fat digestion occurs in the mouth up to the stomach area. ​ In the mouth, lingual lipase digest initially the fats and daily activities. lipids, but essentially all fat digestion occurs in the small ​ Another way of getting glucose during fasting is through intestine the process gluconeogenesis. →​Formation of new glucose from a different substrate (i.e. amino acids or glycerol portion of fat) →​60% of amino acids in body proteins can be converted to glucose through the process of deamination ​ Role of liver in maintaining blood glucose levels during fasting Figure 14. Lipid Emulsification →​Decreased blood glucose or carbohydrates in the body ​ The first step in the digestion of the fats and lipids is the are detected by the liver and stimulates the process of process of emulsification of the fat. either glycogenolysis or gluconeogenesis →​involves physically breaking the fat globules into smaller ​ Another hormone that can contribute to metabolism of sizes, so that water-soluble enzymes can act on the carbohydrates aside from insulin is Adrenocorticotropic globule surface. hormone (ACTH) →​Most of this process occurs in the duodenum, where →​ACTH stimulates adrenal cortex to produce large there is the presence of bile, containing bile salts and →​ 📣 amounts of glucocorticoid hormones (ex. Cortisol) The decreased blood sugar will stimulate the adenohypophysis to secrete your adrenocorticotropic lecithin which are important substances for the emulsification of fat. ​ Once fat is emulsified, through the action of pancreatic hormone or ACTH that will then stimulate the adrenal lipase, it will be digested into their counterpart fatty acids cortex to produce glucocorticoid hormones, such as and monoglycerides cortisol. This will mobilize the protein in the liver to provide substrates for conversion into glucose. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 6 of 16  →​Apolipoproteins are synthesized in the rough endoplasmic reticulum, and will move into the smooth endoplasmic reticulum where they will associate with the lipid droplets ​ The chylomicron will be transported out through the thoracic duct which will then empty into the circulating venous blood at the juncture of the jugular and subclavian vein ​ Once the chylomicrons pass through the capillaries of the adipose tissue, heart, and skeletal muscle, an enzyme called lipoprotein lipase will act on this chylomicron to hydrolyze the triglycerides ​ It will release the fatty acids and glycerol from the Figure 15. Schematic representation of fat digestion chylomicron. The remaining chylomicrons will be [Lecturer’s PPT] recirculated into the intestinal epithelial cells to be reused ​ During emulsification, bile salts act like a detergent which for the next cycle decreases the interfacial tension of fat making them highly ​ Once the fatty acids are released, they will be absorbed by soluble in water. fat cells and muscle cells to be used as fuel or synthesized ​ Bile salts are also important in removing the free fatty into triglycerides acids and monoglycerides from the fat globules for an ​ In the post-absorptive state, all the chylomicrons are easier process of digestion removed from the blood, and more than 95% of lipids are ​ Once the bile salts coat the lipid droplets it forms micelles in the form of lipoproteins. These lipoproteins are smaller →​Micelles contain a hydrophobic tail and a hydrophilic than chylomicrons but also contain triglycerides, head, meaning inside the micelle is the fat which the cholesterol, phospholipids, and protein. polar bile salts surrounding that fat globule →​Types: →​The polar outward projections of the micelles are highly ▪​ VLDL soluble in the aqueous nature of the digestive fluids. ▪​ IDL The fat inside the micelles will remain stable until it is ▪​ LDL absorbed into the blood circulation →​Micelles also transport other molecules (ex. Fat-soluble 📣▪​ HDL →​ The difference among the four types of lipoproteins lies in their cholesterol and triglyceride contents. VLDL vitamins) into the intestinal cell →​Bile salts also act as a transport medium, carrying the (Very Low-Density Lipoprotein) contains a high free fatty acids and monoglycerides to the brush border triglyceride content along with moderate levels of of the intestinal epithelial cells cholesterol and phospholipids. As you move down the types of lipoproteins, the ratio of triglycerides decreases. HDL (High-Density Lipoprotein), in contrast, has a high protein content with low levels of cholesterol and phospholipids. ​ Most lipoproteins are synthesized in the liver. There are small amounts of HDL synthesized in the intestinal epithelium, but most of them are synthesized in the liver. V. METABOLISM OF FATS Figure 16. Fat Absorption[Lecturer’s PPT] ​ Inside the small intestine 100 or more grams of fat are absorbed each day ​ The formed bile micelles consist of polar exterior and they are soluble in the aqueous solution of the digestive juices ​ The monoglycerides and free fatty acids that were carried to the surfaces of the microvilli will penetrate the recesses Figure 17. Beta-oxidation of fatty acids[Lecturer’s PPT] of the microvilli until they diffuse out of the micelles and ​ 1st step - transport fatty acids inside the mitochondria into the interior of epithelial cells using carnitine as carrier ​ With the help of bile micelles, about 97% of fat is absorbed ​ Degradation occurs by progressive release of two carbon as compared to 40-50% without the help of micelles segment acetyl Coa ​ Once the free fatty acids and monoglycerides are inside ​ There is also release of hydrogen atoms the cell, they will be taken up by the smooth endoplasmic reticulum to be taken up to form new triglycerides ​ The newly formed triglycerides will enter the lymph as chylomicrons →​Chylomicrons are the free fatty acids and monoglycerides with an addition of an apolipoprotein PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 7 of 16  Figure 19. Digestion of Proteins[Lecturer’s PPT] ​ The products of protein digestion and absorption in GIT are almost entirely amino acids. ​ Each day, about 50-100g of amino acids are absorbed ​ The increase of amino acid concentration after a meal is Figure 18. Beta oxidation of Fatty acids to Kreb’s only minimal since protein digestion and absorption is Cycle[Lecturer’s PPT] usually extended over a period of 2-3 hours ​ 1 molecule of FA will combine with 1 CoA, forming Fatty ​ After entering the blood additional amino acids are acyl-CoA. absorbed within 5-10 minutes by cells, especially by liver ​ Fatty acyl-carnitine will combine with 1 molecule of CoA to cells. form Fatty acyl-CoA. ​ Once protein is ingested, the enzyme Pepsin, which is a ​ Oxidation: The Beta carbon of Fatty acyl-CoA will bind peptic enzyme of the stomach, is used to digest proteins. with Oxygen, releasing Acetyl CoA. ​ Enzymes Trypsin, Chymotrypsin, →​Release of Acetyl CoA means there is another available Carboxypolypeptidase, Elastase will further hydrolyze Acetyl CoA that can be used for binding in the polypeptides into smaller molecules until it becomes remaining portion of FA. Amino acids alone to be absorbed. ​ The alternating process of oxidation, hydration, and ​ Pepsin - is selective with the environment/medium where oxidation will continue. But the substrate will always be 2 it will act upon. carbon atoms shorter. Along with the release of Acetyl →​Most active at acidic environment (pH 2-3) CoA, there is also a release of Hydrogen atom. →​The gastric glands in the stomach need to secrete large ​ Until the end product would be Acetyl CoA, which will quantities of HCl to achieve this acidic environment. enter the Krebs Cycle to be broken down to form more ▪​ If the environment is not acidic, Pepsin will not work energy. and digest proteins ​ Each H+ atom released each time a molecule of acetyl →​Initiates the process of digestion even if only about CoA is split from fatty acid chain will be released in the 10-20%, it can already convert proteins into Proteoses, forms of FADH2, NADH, and H+ Peptones, and Polypeptides. →​1 molecule of FADH: 1.5 ATP →​Can digest Collagen, which is a major constituent of →​1 molecule of NADH: 2.5 ATP intercellular connective tissues of meats. ​ Liver - Initial degradation of fatty acids for energy →​Most digestion occurs in the upper small intestine →​Early stages of starvation particularly in duodenum and jejunum →​In Diabetes Mellitus ▪​ Polypeptides will be acted upon by peptidases to →​Any condition where fat is used for energy form amino acids ​ Ketosis - increased levels of acetoacetic acid , beta A. DIETARY PROTEINS hydroxybutyric acid, acetone ​ Fat Synthesis - as storage of excess of carbohydrate A. HORMONAL REGULATION OF FAT UTILIZATION ​ Heavy Exercise →​Release of epinephrine and norepinephrine →​Stress ▪​ Releases ACTH to stimulate secretion of triglyceride lipase →​Thyroid hormone B. FAT DEPOSITS ​ Adipose tissue and liver ​ Major functions: Figure 20. Absorption of Dietary Proteins [Lecturer’s PPT] →​Storage of triglycerides ​ Dietary proteins in the stomach with the help of HCl, →​Heat insulation pepsin will digest about 10-20% of dietary proteins to form →​Secretion of hormones proteoses, peptones, and polypeptide ​ In the small intestine, it’s already denatured and partially VI. DIGESTION AND ABSORPTION OF PROTEINS hydrolyzed ​ ¾ of our body solids are proteins →​Proteolytic enzymes from pancreas: trypsin, →​Structural proteins, enzymes, nucleoproteins, proteins chymotrypsin, carboxypeptidase and elastase that transport oxygen, proteins for muscle contraction ▪​ Trypsin & Chymotrypsin: split proteins into smaller ​ Basic moiety: amino acid with an acidic group and an polypeptide attached nitrogen atom, linked by peptide linkages ▪​ Carboxypeptidase: cleave individual amino acids from the carboxyl ends PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 8 of 16  ▪​ Proelastase: converted to into its active form which is →​Deamination of proteins: mainly involves elastase to digest your elastin fibers transamination which means transferring an amino ​ Last part of digestion of protein occurs at the enterocytes group to an acceptor substance that lines the villi →​Microvilli contains peptidases e.g. aminopeptidase and dipeptidase which can cleave further the proteins until it forms only a single amino acids since it can only be actively transported then into the bloodstream ​ Elastase - initially secreted as a proenzyme (Proelastase), thus, it needs to be degraded to its active form to act upon as a partially hydrolyzed protein. ​ Peptidase - so that small peptides will be hydrolyzed into monomers of amino acids. ​ Once they are in the form of amino acids they can now be Figure 22. Deamination of Proteins [Lecturer’s PPT] actively transported in the bloodstream. ​ α-Ketoglutaric acid and amino acid, the amino group will be transferred to α-Ketoglutaric acid forming Glutamic acid. ​ In the process, ammonia will be released during deamination and it’s removed from the blood by conversion into urea. ​ The resulting α-Keto acid, as a product of the amino acid deamination can be oxidized to release energy by changing it into a chemical substance that can enter the Krebs cycle to be degraded as an energy so it can be reused. ​ Urea (formed during deamination): transported into the blood → kidneys for excretion ​ If proteins are not used for energy, it will be stored. VIII. REGULATION OF PROTEINS ​ Essential vs Nonessential amino acids →​PVT TIM HALL – essential amino acids Figure 21. Oligopeptide Absorption[Lecturer’s PPT] ▪​ All amino acids not in this mnemonic are ​ There are sodium transporters that also transport amino nonessential amino acids acids aside from glucose. →​Essential: Can not be synthesized by the body, so ​ There are also specific transporters in the intestinal should be taken in a food component epithelium that are responsible for transporting amino ​ Obligatory degradation of proteins - 20 to 30 grams of acids. protein each day (degraded so that the body will have a ​ Amino acids will enter and then acted upon by peptidases supply of protein) towards the interstitial space. →​The body itself will act on the structural proteins you have to degrade proteins B. STORAGE OF PROTEINS →​It is important that you take in at least 20 to 30 grams of ​ Amino acids are stored mainly in the form of actual protein each day. proteins but, rapidly decomposed again under intracellular ​ Starvation ❌ lysosomal digestive enzymes ​ EXCEPT proteins in the chromosomes of nucleus, structural proteins →​Body will look for other substrates that can be used for energy. After fatty acids, proteins are next. Proteins will be used by the body to metabolize energy. C. FUNCTIONAL ROLE OF PROTEINS Table 2. Regulatory Hormones Table 1. Function of Proteins Hormones Functions Proteins Functions Growth Take up proteins. Increase utilization of hormone proteins in the body. Albumin Provide colloid osmotic pressure to prevent plasma loss from capillaries Insulin Absence of insulin causes an increase in mobilization of proteins for energy use. Globulins Enzymatic functions Natural and Acquired immunity Testosterone Increase mobilization of proteins. Fibrinogen Polymerizes into long fibrin threads during blood coagulation Thyroid Also same with fats, increase hormone mobilization of proteins VII. METABOLISM OF PROTEINS ​ Proteins can move either inward or outward by facilitated transport or by active transport using carriers ​ Excess amino acids — degraded into other products and used for energy which occurs in the liver and begins with deamination. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 9 of 16  What’s PVT TIM HALL (essential amino acids) ​ Not all inflow goes through outflow. Most of them are →​Phenylalanine, Valine, Threonine, Tryptophan, reabsorbed by the small intestine; water and electrolytes. Isoleucine, Methionine, Histidine, Arginine, Leucine, While in the colon, it also reabsorbs fluids and electrolytes. Lysine ​ Until the only amount excreted is about 0.1 L/day in the ​ Do glucose and lactose both compete for SGLT1 in form of fecal material. order to get transported inside the cell? ​ Take note what specific ion is reabsorbed at a certain →​No, because most monosaccharides formed are location actually glucose →​Small intestine: all ions involved; H2O, Na+, K+, Cl →​The body takes up glucose more than galactose. →​Colon: H2O, Na+, Cl ​ Does the consumption of alkaline water instead of ▪​ Secretes K+ and HCO3 normal drinking water affect digestion and absorption? −​ This is because of bacterial action inside the →​Minimally. intestine that forms the acid. In order to neutralize →​Body can detect differences in the ion-gradient the acid that formed by the bacteria, secretion of formed with whatever you ingest. bicarbonate is needed. There are enzymes →​Alkaline water pH shouldn’t be that much of different specific to a medium where it will act upon. with the plasma pH →​Body is always striving for homeostasis B. SODIUM ABSORPTION ​ How is the digestion of carbohydrates present in the stomach if the acidic environment inactivates salivary amylase? →​Tricky but the duration of stay by the carbs inside the mouth and stomach, is only minimal →​Carbohydrates get hydrolyzed inside the stomach and the mouth because the transit time of the carbs to the stomach is quick in order for it to be fully absorbed in the small intestine. With this, salivary amylase is not that much ended IX. DIGESTION AND ABSORPTION OF WATER AND Figure 24. Na/Glucose or Na/Amino acid cotransporters. IONS [Lecturer’s PPT] A. WATER ABSORPTION ​ SGLT-1 (Sodium-Glucose Transporter), Na/Amino Acid Cotransporters ​ Each day, about 7-8 liters of water is absorbed by the →​Secondary active transporters body. →​SGLT-1: facilitate absorption of glucose, galactose →​Transported entirely by diffusion, obeying laws of →​Na-AA: facilitate absorption of amino acid osmosis. →​Rely on the sodium ion concentration gradient brought ▪​ The osmotic gradient formed because of ions and about by primary active transport by Na+ K+ pump −​ 📣 sodium (Na), and chloride will cause flow of water. Na goes H2O follows and Cl follows →​When your chyme or the food you digest with the located on the basolateral membrane. →​Primary found on jejunum and proximal ileum gastric juices is dilute enough, water is absorbed through the intestinal mucosa into the blood of the villi almost entirely by osmosis. ▪​ Osmotic gradient - created by the elevated concentration of ions in the paracellular space. ▪​ Can also happen in a reverse manner, when you have hyperosmotic solutions, they discharge from the stomach into the duodenum meaning it is another way of water absorption. Figure 25. Na-H exchanger. [Lecturer’s PPT] ​ Na-H exchanger →​Primarily found on the duodenum and jejunum on both the apical and basolateral portions of the enterocytes →​This transporter would pump out hydrogen ions in exchange for sodium. So, it maintains their electron neutrality →​It pumps out positively-charged hydrogen ions in exchange for positively-charged sodium ions Figure 23. Intestinal Fluid and Electrolyte Movement [Lecturer’s PPT] PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 10 of 16  →​An average person eats about 5-8 grams of sodium each day so to prevent the net loss of sodium into the feces, the intestines must absorb 25-35 grams of sodium daily. ​ Sodium absorption from inside the cell through the basal and lateral walls is by active transport meaning it requires energy that is catalyzed by ATPase. →​Some of these sodium ions are absorbed along with chloride ions →​With the negatively charged chloride ions being passively dragged by the positive electrical charges of sodium Figure 26. Parallel Na-H exchanger and CI-HCO3 ▪​ Where sodium goes, aside from water, chloride also Exchangers. [Lecturer’s PPT] follows. ​ Parallel Na-H exchanger and CI-HCO3 →​Primarily found on the ileum and ascending colon →​Important in the maintenance of acid-base balance →​Carbonic anhydrase produces carbonic acid from H20 and C02 in which the carbonic acid will dissociate into H+ and Bicarbonate. →​Exchangers can facilitate the maintenance of the balance of these ions Figure 29. Sodium Transport in Brush Border Membrane[Lecturer’s PPT] ​ Sodium is also transported through the brush border membrane by SGLT1, Na-H exchanger, Na-K ATPase pump, and Secondary absorption of glucose and amino acids. Figure 27. Epithelial Na+ Channel. [Lecturer’s PPT] ​ As mentioned in the carbohydrate absorption, sodium is ​ Epithelial Na+ Channel actively transported through the basolateral membrane →​Located on the distal colon, which is located on the causing a decrease or diffusion of sodium concentration apical side of your enterocytes inside the cell which is about 50 nets/L as compared to the →​Affected by ALDOSTERONE facilitate the absorption of sodium concentration in the chyme or intestinal lumen sodium which is about 142 nets/L. →​Eventually all of the sodium that goes to the interstitial →​The movement of sodium from high to low fluid will enter the portal circulation. concentration gradient is from the intestinal lumen through the brush border of epithelial cells (inside its cytoplasm). ▪​ Because you actively transported the sodium out of the cell to the paracellular space, the high sodium concentration gradient of chyme will move the sodium from the intestinal lumen inside to the epithelial cells. C. CHLORIDE ABSORPTION ​ Absorbed at the upper part of the small intestine. →​Mainly by diffusion along the electrical gradient to follow the sodium ions. →​Also via a membrane chloride-bicarbonate exchanger. D. BICARBONATE ABSORPTION ​ Absorbed at the upper part of small intestine →​Absorbed indirectly →​On the surface of the ileum and large intestine, HCO3- Figure 28. Interaction of Ions between Paracellular Space is secreted in exchange for Chloride ions to neutralize and Intestinal Lumen[Lecturer’s PPT] acid products formed by bacteria. ​ The osmotic gradient formed is due to the concentration of ▪​ When sodium ions are absorbed, moderate amounts ions inside the paracellular space so on average there’s of H atoms are secreted into the lumen in exchange about 20-30 grams of sodium secreted in the intestinal for sodium. secretions each day. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 11 of 16  −​ These H atoms will combine with bicarbonate ions into the lymph to go into the systemic blood circulation to form carbonic acid which will then dissociate to to enter the liver via receptor-mediated endocytosis of form water and carbon dioxide. chylomicrons / their remnants o​ The water remains as part of chyme while CO2 ▪​ Basically, the pathway of the fat-soluble vitamins will be absorbed into the blood then expired follow the pathway of the digestion and absorption of through the lungs. the lipids / fats →​Epithelial cells on the surfaces of the villi of ileum and ▪​ That’s for fat-soluble vitamins, for water-soluble large intestines have the special capability of secreting vitamins; they will just diffuse along the intestinal bicarbonate in exchange for chloride ions epithelial cells because they are water-soluble ▪​ This is important because this will neutralize the acid vitamins. products formed by the bacteria in the large intestine B. FOLATE OR FOLIC ACID ABSORPTION ▪​ Even though it is indirectly absorbed, it is important that bicarbonate is reabsorbed because large amounts of bicarbonate have been secreted into the duodenum; in both the pancreatic secretions and the bile secretions. X.DIGESTION AND ABSORPTION OF VITAMINS A. FAT-SOLUBLE VITAMINS (A, D, E, K) Figure 31. Folate absorption [Lecturer’s PPT] ​ Folate or Folic Acid: essential for the synthesis of thymine and purines for DNA formation ​ Absorbed via an apical membrane exchange process, linked to the efflux of OH- ​ In the clinical setting, it is important especially for women in their reproductive age Figure 30. Absorption of Vitamins in the GI Tract[Lecturer’s PPT] →​Because it is needed for DNA formation →​Women in the reproductive age are advised to consume ​ At the smooth ER, associate with lipid droplets to form a higher amount of folic acid as compared to a woman chylomicrons and VLDLs to golgi apparatus for exocytosis who is not planning to get pregnant / a male individual into lymph to systemic blood circulation to enter liver →​Since they are fat-soluble, they rely on lipid absorption C. VITAMIN B12 ABSORPTION →​After they are ingested, the fat-soluble vitamins are released from their association with proteins via the acidity of gastric juices / proteolysis →​In the proximal small intestine, they will now incorporate with other lipid products (emulsion droplets, vesicles, mixed micelles) →​Remember from the digestion-absorption of fats, once the fat droplets have been formed into micelles; they can now be absorbed into the intestinal epithelial cells (because they are already water-soluble) ▪​ Fat-soluble vitamins would “ride” with the micelle formed so they can be “ferried up” into the enterocyte surface for uptake −​ The enterocytes will take up these fat-soluble vitamins either by simple diffusion or via transporters Figure 32. Vitamin B12 Absorption[Lecturer’s PPT] −​ Once inside the epithelial cell, they will diffuse into ​ Vitamin B12, also known as cobalamin, is primarily the smooth endoplasmic reticulum EITHER as ingested from animal products like meat, fish, shellfish, free molecules/vitamins OR they are attached to eggs, and milk. carrier proteins →​Necessary for RBC formation and maturation o​ Example: cellular-retinol binding protein in the ​ Once cobalamin reaches the stomach, it is initially bound case of Vitamin A to proteins from the ingested food. The low pH of the −​ In the smooth endoplasmic reticulum they will now stomach and the action of pepsin help to release the associate with lipid droplets that form your new cobalamin from these proteins. Then, cobalamin binds to chylomicrons and the Very-Low Density haptocorrin, a glycoprotein secreted by salivary and gastric Lipoproteins (VLDLs) glands, which protects the vitamin from the harsh →​Like the path of uniform triglycerides, they will go to the environment of the gastric mucosa. golgi apparatus and secretory vesicles for exocytosis PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 12 of 16  ​ As the haptocorrin-cobalamin complex travels to the E. MAGNESIUM ABSORPTION duodenum, pancreatic proteases degrade the haptocorrin, ​ Important intracellular ions because it is required as an releasing the cobalamin, which then binds to the intrinsic enzyme cofactor for some metabolic pathways in the body. factor, secreted by parietal cells in the stomach. The ​ Linked with calcium absorption but different in a few ways: cobalamin-intrinsic factor complex is highly resistant to →​Active transport exists in ileum →​📣 enzyme degradation. For cobalamin to be absorbed, it needs intrinsic factor. Once absorbed, intrinsic factors dissociate for →​Vitamin D does not consistently increase absorption →​Patients with increased calcium absorption have normal magnesium absorption recycling. ​ This complex travels to the terminal ileum, where it binds F. IRON to specific receptors on the apical membrane of the ​ Iron - Part of heme groups in cytochromes and also a key enterocytes. This binding is calcium-dependent, and the component of the oxygen-carrying heme moieties of internalization of the complex by the enterocytes requires hemoglobin and myoglobin energy. ​ Iron absorption is low ​ Once inside the enterocyte, cobalamin dissociates from →​About 10-20% of ingested iron is absorbed the intrinsic factor and binds to transcobalamin 2. The ​ Exists in 2 major forms: cobalamin-transcobalamin 2 complex exits the enterocyte →​Part of heme moiety via the basolateral membrane and enters the portal ▪​ Absorbed more efficiently circulation, going to the liver for storage and secretion into →​Nonheme iron (Not part of heme moiety) bile. ▪​ Exists as either ferric or ferrous ​ Cobalamin is taken up by all cells for DNA synthesis and is →​📣 needed for converting homocysteine to methionine. For patients with Vitamin B12 deficiency, they commonly have anemia. ▪​ Thus, for patients with GIT problems or who have undergone GIT surgery, always check if intrinsic factor secretion was affected. If yes, you might expect the patient to have manifestations of anemia. D. CALCIUM ABSORPTION Figure 34. Iron Absorption[Lecturer PPT] ​ Iron requires one or more proteins to facilitate its movement into and out of the cells and for intracellular binding ​ Iron absorption is tightly regulated by the size of the existing body iron stores ​ Iron can be absorbed in two forms: →​Nonheme Iron →​Heme Iron NON HEME IRON ​ In the absorption of nonheme iron in the duodenum, the enterocytes take up the nonheme iron through the Divalent Metal Transporter 1 (DMT1) which will transport the Figure 33. Calcium Absorption[Lecturer’s PPT] ferrous form of iron and hydrogen into the cell. ​ Calcium ions are actively absorbed into the blood ​ Inside the cytoplasm, the enterocytes will translocate the →​Duodenum ferrous iron across the basal lateral membrane through the ▪​📣 ▪​ Controlled by the parathyroid hormone and vitamin D Can still be passively absorbed to distal parts of the small intestine. Ferroportin 1 (FP1). From there on, this will exit the enterocyte ​ Once the ferrous iron exits the enterocyte, it will be ​ Can also be absorbed by passive paracellular diffusion oxidized from ferrous to ferric by the enzyme Ferroxidase throughout the small intestine hephaestin. Once it is in its ferric form it will bind to plasma ​ Vitamin D enhances calcium absorption transferrin for carriage into the blood circulation ​ Non heme iron will bind to transferrin to deposit the iron in ACTIVE TRANSPORT OF CALCIUM all of the tissues in the body. Some will also bind to the 1. Calcium is absorbed through calcium channels on the protein apoferritin to form ferritin–which is the major apical membrane of enterocytes, driven by electrochemical storage form of iron. gradients. ​ The presence of the Duodenal cytochrome B reduces the 2. Once inside the cell, calcium binds to calbindin, which ferric form of iron to the ferrous form at the extracellular buffers intracellular calcium, maintaining low levels of surface of the apical membrane before the uptake by the unbound calcium. DMT1 receptor 3. Calcium is then moved out of the cell into the interstitial space by: HEME IRON ​ A calcium pump that actively transports calcium. ​ Heme iron is also absorbed by the duodenal epithelial cells ​ Sodium-calcium exchanger that exchanges calcium ions ​ They enter the cells either by binding to a brush border for sodium ions. protein or through a endocytic mechanism PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 13 of 16  ​ Once inside the cytosol of the cell, the Heme oxygenase c. Monosaccharides will split the heme iron releasing free ferric iron, carbon d. Starches monoxide, and biliverdin. 7.​ What is the role of intrinsic factor in the absorption of ​ The cell reduces biliverdin to bilirubin which the liver Vitamin B12? excretes in bile form a. It helps with the digestion of B12 XI.DIGESTION AND ABSORPTION IN THE LARGE b. It binds to B12 in the stomach and protects it from INTESTINE degradation c. It transports B12 across the basolateral membrane ​ After all the food is digested and absorbed, the fecal d. It converts B12 into its active form material must be excreted 8.​ What is the role of carnitine in fatty acid metabolism? ​ Composition of fecal material - about ¾ water and ¼ solid a) To synthesize fatty acids matter b) To transport fatty acids into the mitochondria ​ Brown color - stercobilin and urobilin c) To break down fatty acids into acetyl CoA ​ Odor - bacterial action d) To store fatty acids in adipose tissue →​Bacteria in the large intestine secretes substances like 9.​ What is the main function of the large intestine in indole, scatole, and hydrogen sulfide 📣 Question: Why was it stated that most nutrient absorption happens in duodenum, and not in the digestion and absorption? a. Absorption of most nutrients b. Digestion of complex carbohydrates jejunum? c. Absorption of water and electrolytes, and excretion of →​Most nutrients are absorbed in the small intestine fecal matter because of its large absorptive surface area. In d. Secretion of digestive enzymes addition it possesses structures like the valves of 10.​Unlike carbohydrates and fats, the absorption of kerckring/conniventes that further increase the proteins is not significantly increased immediately absorptive surface by three times. These structures after a meal. Why? are most developed in the DUODENUM and a. Protein digestion is slower and more extended JEJUNUM area, according to Guyton. b. Proteins are actively transported into cells →​Villis from the small intestine going towards the c. The products of protein digestion are not absorbed by ileocecal valve also increases the surface area the portal blood →​Inside the villi of intestinal epithelial cells, there are d. Proteins are initially absorbed by the lymphatic system. also at least 1000 microvilli that contribute more to the absorptive surface area of the small intestine ANSWER KEY 1. C The end goal of absorbed nutrients is to be XII. REVIEW QUESTIONS metabolized into ATP, the body’s main energy 1.​ What is the end goal of all nutrients absorbed in the source for essential functions. body? 2. B Bile micelles aid fat absorption by emulsifying a.​ To be stored as fat lipids and transporting them to the intestinal b.​ To be digested into glucose lining. c.​ To be metabolized into ATP 3. A SGLT-1 actively absorbs glucose and galactose d.​ To be converted into proteins in the small intestine by co-transporting them 2.​ What is the main function of bile micelles in the small with sodium, using the sodium gradient for intestine? efficient uptake. a.​ Absorb glucose 4. C Sodium ions facilitate glucose absorption via b.​ Facilitate absorption of fats SGLT-1 by driving active transport using the c.​ Digest proteins sodium gradient. d.​ Neutralize Gastric acid 5. B Iron absorption efficiency is about 10-20%, 3.​ Which transporter facilitates the absorption of influenced by the form of iron and various glucose and galactose? enhancers or inhibitors. a.​ SGLT-1 6. C Only monosaccharides like glucose, galactose, b.​ Na/Amino Acid Cotransporters and fructose can be absorbed directly into the c.​ Facilitated Diffusion Transporters bloodstream. Larger carbohydrates must be d.​ Calcium Pumps broken down first. 4.​ What is the effect of sodium ion concentration on 7. B Intrinsic factor, secreted by parietal cells in the glucose absorption? stomach, binds to cobalamin (Vitamin B12) in a.​ It decreases absorption the small intestine, forming a complex that is b.​ It has no effect needed for absorption in the ileum c.​ It facilitates absorption 8. B To transport fatty acids into the mitochondria. d.​ It inhibits absorption Carnitine acts as a carrier for fatty acids across 5.​ What is the absorption efficiency of ingested iron? the mitochondrial membrane a.​ About 50-60% 9. C Absorption of water and electrolytes, and b.​ About 10-20% excretion of fecal matter c.​ About 30-40% 10. A Protein digestion is slower and more extended. d.​ About 70-80% The process of protein digestion and absorption 6.​ Which of the following is the only form of is longer, with amino acids being absorbed over carbohydrates that can be directly absorbed by the 2-3 hours. gastrointestinal tract? a. Disaccharides b. Polysaccharides PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 14 of 16  XIII. REFERENCES ​ Batch 2027. (2024). GIT 3: Digestion, Absorption & Nutrition. [Transcription] ​ Hall, J., Hall, M. (2021). Guyton and Hall Textbook of Physiology. 14th edition. PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 15 of 16  APPENDIX Table 3. Nutrient Absorption Across the GI Tract [Lecturer’s PPT] Carbohydrates Proteins Lipids Water Electrolytes Mouth Ptyalin, salivary None None None None amylase Stomach Salivary amylase Pepsin, HCl Lingual lipase, gastric None None (fundus of the lipase; emulsification stomach) Duodenum Pancreatic Pancreatic enzymes, Pancreatic lipase, Reabsorbed Reabsorbed - amylase, maltase, trypsin, chymotrypsin, colipase, bile salts, milk Na, K, CI- sucrase, lactase elastase, lipase carboxypeptidase Jejunum/Ileum amylase, maltase, Peptidases Pancreatic lipase, Reabsorbed Secreted - sucrase, lactase colipase, bile salts, milk НС03- lipase Colon None None None Reabsorbed Reabsorbed - Na, CI- Secreted - K, C03- Enzyme, SGLT1 Na/AA cotransporter, Fatty acid translocase Aquaporins, Na/K pump, Channels GLUT2 H/oligopeptide (FAT or CD36); Fatty transcellular and NaGlu/NaAA GLUT5 cotransporter PepT1 acid binding protein; paracellular cotransporter, Fatty acid transport pathways NaH+ proteins exchanger, parallel NaH and Cl-HCO3- exchanger, epithelial Na channel NOTE: ​ SGLT1 - inserts monosaccharides inside the cell; responsible for glucose and galactose ​ GLUT2 - removes monosaccharides outside the cell; responsible for glucose, galactose, and fructose ​ GLUT5 - inserts monosaccharide inside the cell; responsible for fructose PHYSIOLOGY GIT 3: Metabolism of Nutrients Page 16 of 16