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Energetics and Products of Pepsin/HCl Protein Digestion

Energetics and Products of Pepsin/HCl Protein Digestion


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What are the energetics of protein digestion during which the enzyme pepsin is "activated" (whatever that means) by HCl? I've looked and been unable to find anything like a chemical equation that includes an energy term.

Of course, pepsin, being an enzyme, is not used up in that sought-for equation. Is HCl used up? If so, what are the products? How and in what form is the chlorine removed, assuming the HCl is consumed in the equation?


This is well-explained on the Wikipedia page for pepsin. You are misinterpreting the use of the word activated. The protein is secreted by chief cells in the gastric glands in the form of pepsinogen, an inactive pro- form which has an extra ~40 amino acids at its N-terminus (the propeptide). The propeptide binds at the catalytic site of the enzyme and keeps it inactive. At low pH (this is where the HCl comes in - HCl secreted by parietal cells of the gastric glands acidifies the stomach) the protein is able to cleave off its own propeptide, making it fully active.


Digestion is the conversion of large food molecules ( polymers ) into smaller ones ( monomers ) by means of hydrolysis , This process is catalyzed by an enzymatic action .

The importance of digestion : The breaking down of large and complex food substances into a simpler and smaller molecules which are easily absorbed by the cells ( either by the diffusion or active transport ) , The cell will then use these simple compounds as a source of energy in the building of new tissues .

    Amino acids ( breaking down )
  • StarchGlucose ( Monosaccharide )Fatty acids + Glycerol

Enzymes

Enzymes is a protein substance which has the properties o f catalyst and has the ability to activate a particular chemical reaction .

The enzyme ‘s ability to activate a particular chemical reaction depends on t he structure of reacting molecules ( reactants ) & t he name of enzyme .

When the reaction is completed , the resulting molecules break away from the enzymes , leaving it in the same form as it was before the reaction .

Characteristics of enzymes

  1. Specific , as each enzyme can accelerate only one type of reactions .
  2. Do not affect the products of reaction , as they work as catalyst , inc reasing the rate of reaction , until it reaches the equilibrium .
  3. Most have a reversible effect , as the same enzyme may catalyze the decomposition of a complex molecule into two simpler ones and may recombine these two small molecules to give rise to the same complex molecule .
  4. Their activity depends on the temperature and pH of the medium .
  5. Some are secreted from the cells in an inactive state , so , they need certain substances to activate them .

Examples : Pepsin enzyme is secreted by the stomach as an inactive pepsinogen w hich is changed into the active pepsin in the p resence of HCl in the stomach .

pepsinogen ( Inactive )pepsin ( Active ) , ( HCl acid )

Structure of the digestive system in man

Digestive ( Alimentary ) canal which consists of : mouth , pharynx , oesophagus , stomach , small intestine , large intestine , rectum and anus opening .

Accessory ( Associated ) glands w hich are salivary glands , liver and pancreas .

Buccal digestion

Mouth : The digestive canal starts with the mouth which contains teeth , tongue and salivary glands .

Teeth : that are differentiated into :

  • Incisors : in the front of jaw for cutting food .
  • Canines : follow the incisors to tear food .
  • Premolars and molars : at the back for crushing and grinding food .

Tongue : helps to manipulate the food to be chewed by the teeth and it serves as an organ of taste .

Salivary glands : There are three pairs of salivary glands which open into the mouth cavity through ducts , The saliva secreted by the salivary glands contains :

  1. Mucus that softens the food to be easily swallowed .
  2. Amylase enzyme ( Ptyalin enzyme ) which works in a weak alkaline medium of pH = 7.4 .

Function of amylase : it catalyzes the hydrolysis of starch to disaccharide maltose .

Starch + WaterMaltose sugar ( disaccharide ) , ( Amylase enzyme & weak alkaline medium )

Pharynx : It is a cavity a t the back of mouth which leads to two tubes :

Swallowing process is an organized reflex action which pushes the food from the mouth to the oesophagus and during this , the top of trachea with the larynx is elevated together , causing the epiglottis to close over the glottis ( the entrance to air passage ) .

Oesophagus : It extends from the pharynx downward through the neck and into the chest cavity , It is about 25 cm long , It lies parallel to the vertebral column , It is lined with glands to secrete mucus , Food is carried through the oesophagus to the stomach by a phenomenon known as peristalsis .

Peristalsis : A series of rhythmical muscular contractions and relaxations of the circular muscles of alimentary canal to sweep any food contained within the canal , It is also responsible for churning the food and mixing it with the digestive juices .

Gastric digestion ( digestion in stomach )
Stomach

Stomach is a dilated muscular sac which lies in the abdominal cavity , It is joined with the oesophagus by a constricted circular muscle which is called the cardiac sphincter , It is connected to the small intestine by a muscular valve of circular smooth muscle which is called the pyloric sphincter .

Stomach secretes a gastric juice ( a colourless acidic liquid ) which consists of 90 % water , HCl acid , Pepsin enzyme which is secreted in an inactive form called pepsinogen .

Function of HCl :

It creates an acidic medium ( 1.5 – 2.5 pH ) which :

  1. Stops the action of ptyalin enzyme .
  2. Kills the harmful bacteria that may enter with the food .
  3. Activates the pepsinogen enzyme into active pepsin .

Pepsinogen ( Inactive )Pepsin ( Active ) , ( HCl acid )

Protein digestion : Pepsin catalyzes the hydrolysis of protein by breaking a certain peptide linkages in the long chain of protein to yield smaller fragments called polypeptides .

Protein + WaterPolypeptides ( Pepsin & HCl )

Proteins are the only food substances which are affected by the gastric juice , Although the stomach is made up of protein , the gastric juice does not affect the cells which line it , this due to the presence of mucus secretions which protect the cells against the effect of digestive enzymes , The presence of pepsinogen in an inactive form which is activated only when it is mixed with HCl in the cavity of stomach .


Digestion

Digestion of food is a form of catabolism, in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. It begins in the mouth and ends in the small intestine. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: mechanical digestion and chemical digestion. Figure (PageIndex<2>) summarizes the roles played by different digestive organs in mechanical and chemical digestion, both of which are described in detail in the text.

Figure (PageIndex<2>): Both, mechanical and chemical digestion take place throughout the gastrointestinal tract as indicated in this diagram, but absorption takes place only in the stomach and small and large intestines.

Mechanical Digestion

Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming changed chemically. It begins with your first bite of food and continues as you chew food with your teeth into smaller pieces. The process of mechanical digestion continues in the stomach. This muscular organ churns and mixes the food it contains, an action that breaks any solid food into still smaller pieces.

Although some mechanical digestion also occurs in the intestines, it is mostly completed by the time food leaves the stomach. At that stage, food in the GI tract has been changed to the thick semi-fluid called chyme. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the surface area of food particles so they can be acted upon more effectively by digestive enzymes.

Chemical Digestion

Chemical digestion is the biochemical process in which macromolecules in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates must be broken down into simple sugars, proteins into amino acids, lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine (duodenum).

Digestive Enzymes

Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze or speed up biochemical reactions. Digestive enzymes are secreted by exocrine glands or by the mucosal layer of the epithelium lining the gastrointestinal tract. In the mouth, digestive enzymes are secreted by salivary glands. The lining of the stomach secretes enzymes, as does the lining of the small intestine. Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. Table (PageIndex<1>) lists several important digestive enzymes, the organs and/or glands that secrete them, and the compounds they digest. You can read more about them in the text.

Digestive Enzyme

Organ, Glands That Secretes It

Compound It Digests

Chemical Digestion of Carbohydrates

About 80 percent of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch. Additional dietary carbohydrates include the animal polysaccharide glycogen, along with some sugars, which are mainly disaccharides.

To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus, which lubricates the food, and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.

Other sugars are digested with the help of different enzymes produced by the small intestine. For example, sucrose, or table sugar, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose, which are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.

Chemical Digestion of Proteins

Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin, secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas. The stomach also secretes hydrochloric acid, making the contents highly acidic, which is required for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidases, also secreted by the pancreas.

Chemical Digestion of Lipids

The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas as well as bile secreted by the liver. Bile is required for the digestion of lipids because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies, or breaks up, large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase and also point the hydrophilic (&ldquowater-loving&rdquo) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

Chemical Digestion of Nucleic Acids

Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

Chemical Digestion by Gut Flora

The human gastrointestinal tract is normally inhabited by trillions of bacteria, some of which contribute to digestion. Here are just two of dozens of examples:

  1. The most common carbohydrate in plants, which is cellulose, cannot be digested by the human digestive system. However, tiny amounts of cellulose are digested by bacteria in the large intestine.
  2. Certain bacteria in the small intestine help digest lactose, which many adults cannot otherwise digest. As a byproduct of this process, the bacteria produce lactic acid, which increases the release of digestive enzymes and the absorption of minerals such as calcium and iron.

Chapter Review

Digestion of proteins begins in the stomach, where HCl and pepsin begin the process of breaking down proteins into their constituent amino acids. As the chyme enters the small intestine, it mixes with bicarbonate and digestive enzymes. The bicarbonate neutralizes the acidic HCl, and the digestive enzymes break down the proteins into smaller peptides and amino acids. Digestive hormones secretin and CCK are released from the small intestine to aid in digestive processes, and digestive proenzymes are released from the pancreas (trypsinogen and chymotrypsinogen). Enterokinase, an enzyme located in the wall of the small intestine, activates trypsin, which in turn activates chymotrypsin. These enzymes liberate the individual amino acids that are then transported via sodium-amino acid transporters across the intestinal wall into the cell. The amino acids are then transported into the bloodstream for dispersal to the liver and cells throughout the body to be used to create new proteins. When in excess, the amino acids are processed and stored as glucose or ketones. The nitrogen waste that is liberated in this process is converted to urea in the urea acid cycle and eliminated in the urine. In times of starvation, amino acids can be used as an energy source and processed through the Krebs cycle.


Methods

Label four zippered plastic bags with a permanent marker as shown in Table 1. Line four 250-mL beakers with one each of the plastic bags. Follow the chart in Table 1 to fill the bag-lined beakers with the proper solutions. Use the graduated cylinders to measure the pepsin and HCl solutions into the bags. Do not add the sodium bicarbonate (NaHCO3) to the artificial duodenum yet.

Amounts of pepsin, HCl, and NaHCO3 in the bags each bag was labeled as shown in the column listing the contents.

Contents of Bags . pH . Pepsin (mL) . HCl (mL) . NaHCO3(g) .
Pepsin 4.5 100 0 0
HCl 1.0 0 100 0
Artificial stomach 1.0 50 50 0
Artificial duodenum 8.0 50 50 2.5
Contents of Bags . pH . Pepsin (mL) . HCl (mL) . NaHCO3(g) .
Pepsin 4.5 100 0 0
HCl 1.0 0 100 0
Artificial stomach 1.0 50 50 0
Artificial duodenum 8.0 50 50 2.5

The foods I usually test are steak or stew meat, cooked egg white, broccoli, and apple, all cut on a cutting board into approximately 1-cm pieces with a knife or razor blade. Make the food pieces as close to the same size as possible. The broccoli can be broken or cut into a piece that includes the floret and part of its stalk. A set of sample foods to ““feed the stomach”” will include one piece from each food being tested. Prepare one set of sample foods for each of the beakers. Add the 2.5-g sodium bicarbonate into the bag labeled ““artificial duodenum”” at this time.

Deposit one set of foods into each bag –– this constitutes ““feeding”” the stomach. Seal the bags and place the beakers with their contents into a 37°°C water bath. Put some of the warm water into the beaker (not into the bag) so that the temperature of the foods and solutions are brought to and maintained at 37°°C. Also, adding warm water to the beakers prevents them from floating and tipping in the water bath.

Once the artificial stomach is in the water bath, the instructor can bring out a previously prepared artificial stomach made the day before for students to observe and record results (Table 2). Otherwise, the beakers will need to incubate in the water bath for 24 hours for digestion to take place, and the results can be observed the next day.

Example of a completed data sheet. Students can record their results on the data sheet by describing the foods before and after 24 hours in the 37°°C water bath.

. Food Sample .
Treatment . Beef . Egg White . Apple . Broccoli .
Pepsin alone
0 hours All foods intact and firm
24 hours No change
HCl alone
0 hours All foods intact and firm
24 hours No change
Pepsin and HCl (artificial stomach)
0 hours All foods intact and firm
24 hours Tiny, shredded, or disappeared Disappeared or nearly so Intact, firm possible slight color change Intact, firm green may fall into pieces
Pepsin, HCl, and NaHCO3 (artificial duodenum)
0 hours All foods intact and firm
24 hours No change
. Food Sample .
Treatment . Beef . Egg White . Apple . Broccoli .
Pepsin alone
0 hours All foods intact and firm
24 hours No change
HCl alone
0 hours All foods intact and firm
24 hours No change
Pepsin and HCl (artificial stomach)
0 hours All foods intact and firm
24 hours Tiny, shredded, or disappeared Disappeared or nearly so Intact, firm possible slight color change Intact, firm green may fall into pieces
Pepsin, HCl, and NaHCO3 (artificial duodenum)
0 hours All foods intact and firm
24 hours No change

Digestion and Absorption of Protein | Biochemistry

The proteolytic enzymes secreted in gastric juice, pancreatic juice and also present in the intestinal mucosa cause the hydrolysis of pro­tein in the gastrointestinal tract.

Pepsin, the endopeptidase, is present in gastric juice and hydrolyzes the peptide bonds in the interior of the protein molecule.

Pepsin hydrolyzes the dietary protein into a mixture of polypeptides:

Renin has a strong clotting action on milk. This is very important in the digestion of milk pro­teins in infants. The pH of the gastric juice becomes low in achlorhydria, achylia gastrica (both pepsin and HCl absent) and in pernicious anemia. Then dietary protein will not be digested in the stomach.

The polypeptides formed in the stomach are digested in the intestine by trypsin, chymotrypsin and carboxy-peptidases secreted in pancreatic juice and amino-peptidases present in the intestinal mucosa.

Trypsin hydrolyzes peptide linkages containing arginine or lysine and chymo­trypsin hydrolyzes peptide linkages containing tyrosine or phenylalanine:

Carboxypeptidase A hydrolyzes the end group of peptides containing aromatic or aliphatic amino acid and releases free amino acids. Carboxypeptidase B hydrolyzes peptides containing arginine and lysine residues.

The intestinal mucosa also con­tains tripeptidase, di-peptidase etc., which hydrolyze tri- and dipeptides:

The final products of digestion of proteins are amino acids which are absorbed.

Absorption of Protein:

1. Three different active processes are in­volved in the transport of amino acids. One process involves cystine and the ba­sic amino acids, another the amino acids proline and hydroxyproline and the third the neutral (L-) amino acids.

2. D-amino acids are absorbed by simple dif­fusion. But the neutral (L-) amino acids require a carrier system in the absorption. Na + is also required. This is similar to that of active transport of glucose. Vitamin B6 (pyridoxal phosphate) is also involved in the process. The amino acid associates with the carrier and Na + in the microvilli and the complex travels to the inner side of the membrane where it dissociates, re­leasing the amino acid Na into the cytosol. The carrier returns back and func­tions repeatedly. Na + is then actively trans­ported out of the cell.

3. If one amino acid is fed in excess, it re­tards the absorption of another. This is similar to those made with respect to reabsorption of amino acids by the renal tu­bules.

4. Sometimes the whole protein is absorbed into the blood. A protein is antigenic and accounts for food allergies. In the young mammal, the permeability of the mucosa, in this respect, is greater than that in the adult.

5. Food proteins are generally readily di­gested (90 to 97 per cent) under normal conditions, very little escapes in the fae­ces. The insoluble fibrous protein, kera­tin, is not hydrolyzed by enzymes of the human digestive tract.

These are altered by heating to coagulation and hydrolyzed by superheated steam. The biological val­ues of these proteins are not affected by such procedures. Cooked egg albumin is digested more readily than raw. The nutri­tional value of cereal proteins is lowered by overheating or toasting.


Urea Cycle

The urea cycle is a set of biochemical reactions that produces urea from ammonium ions in order to prevent a toxic level of ammonium in the body. It occurs primarily in the liver and, to a lesser extent, in the kidney. Prior to the urea cycle, ammonium ions are produced from the breakdown of amino acids. In these reactions, an amine group, or ammonium ion, from the amino acid is exchanged with a keto group on another molecule. This transamination event creates a molecule that is necessary for the Krebs cycle and an ammonium ion that enters into the urea cycle to be eliminated.

In the urea cycle, ammonium is combined with CO2, resulting in urea and water. The urea is eliminated through the kidneys in the urine (Figure 2).

Figure 2. Nitrogen is transaminated, creating ammonia and intermediates of the Krebs cycle. Ammonia is processed in the urea cycle to produce urea that is eliminated through the kidneys.

Amino acids can also be used as a source of energy, especially in times of starvation. Because the processing of amino acids results in the creation of metabolic intermediates, including pyruvate, acetyl CoA, acetoacyl CoA, oxaloacetate, and α-ketoglutarate, amino acids can serve as a source of energy production through the Krebs cycle (Figure 3).

Figure 3. Click for a larger image. Amino acids can be broken down into precursors for glycolysis or the Krebs cycle. Amino acids (in bold) can enter the cycle through more than one pathway.

Figure 4 summarizes the pathways of catabolism and anabolism for carbohydrates, lipids, and proteins.

Figure 4. Click for a larger image. Nutrients follow a complex pathway from ingestion through anabolism and catabolism to energy production.

Disorders of the Metabolism: Pyruvate Dehydrogenase Complex Deficiency and Phenylketonuria

Pyruvate dehydrogenase complex deficiency (PDCD) and phenylketonuria (PKU) are genetic disorders. Pyruvate dehydrogenase is the enzyme that converts pyruvate into acetyl CoA, the molecule necessary to begin the Krebs cycle to produce ATP. With low levels of the pyruvate dehydrogenase complex (PDC), the rate of cycling through the Krebs cycle is dramatically reduced. This results in a decrease in the total amount of energy that is produced by the cells of the body. PDC deficiency results in a neurodegenerative disease that ranges in severity, depending on the levels of the PDC enzyme. It may cause developmental defects, muscle spasms, and death. Treatments can include diet modification, vitamin supplementation, and gene therapy however, damage to the central nervous system usually cannot be reversed.

PKU affects about 1 in every 15,000 births in the United States. People afflicted with PKU lack sufficient activity of the enzyme phenylalanine hydroxylase and are therefore unable to break down phenylalanine into tyrosine adequately. Because of this, levels of phenylalanine rise to toxic levels in the body, which results in damage to the central nervous system and brain. Symptoms include delayed neurological development, hyperactivity, mental retardation, seizures, skin rash, tremors, and uncontrolled movements of the arms and legs. Pregnant women with PKU are at a high risk for exposing the fetus to too much phenylalanine, which can cross the placenta and affect fetal development. Babies exposed to excess phenylalanine in utero may present with heart defects, physical and/or mental retardation, and microcephaly. Every infant in the United States and Canada is tested at birth to determine whether PKU is present. The earlier a modified diet is begun, the less severe the symptoms will be. The person must closely follow a strict diet that is low in phenylalanine to avoid symptoms and damage. Phenylalanine is found in high concentrations in artificial sweeteners, including aspartame. Therefore, these sweeteners must be avoided. Some animal products and certain starches are also high in phenylalanine, and intake of these foods should be carefully monitored.


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From the Stomach to the Small Intestine

The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. The cells that line the small intestine release additional enzymes that finally break apart the smaller protein fragments into the individual amino acids. The muscle contractions of the small intestine mix and propel the digested proteins to the absorption sites. In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Recall that amino acids contain nitrogen, so further catabolism of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it a good choice for transporting excess nitrogen out of the body. Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.


Digestion and Absorption

Select the correct match of the digested products in humans given in column I with their absorption site and mechanism in column II.

  • Column IColumn II
    Glycine and glucoseSmall intestine and active absorption
  • Fructose and Na +Small intestine passive absorption
  • Glycerol and fatty acidsDuodenum and move as chilomicrons
  • Cholesterol and maltoseLarge intestine and active absorption

Column I Column II
Glycine and glucose Small intestine and active absorption

Amino acids monosaccharides like glucose, electrolytes like Na are absorbed into the blood by active transport. Fructose and some amino acids are absorbed with the help of the carrier ions like Na by facilitated transport. Fatty acid and glycerol cannot be absorbed into the blood. They are first incorporated into small droplets called micelles, which move into the intestinal mucosa.

Marasmus in children is caused by deficiency of

Marasmus is a protein energy malnutrition disorder. It is found in infants due to lack of proper diet of protein and calories. It is also in case of less breast feeding diet. Due to this body become lean and weak, eyes depressed and skin wrinkled.

Assertion : Small intestine is the principal organ for absorption of nutrients.

Reason : Absorption of water, simple sugars and alcohol etc. takes place in small intestine.

If both assertion and reason are true and reason is the correct explanation of assertion.

If both assertion and reason are hue but reason is not the correct explanation of assertion.

If assertion is true but reason is false.

If both assertion and reason are false.

If assertion is true but reason is false.

Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine. Therefore, small intestine is the principal organ for absorption of nutrients. Digestion is completed here and the final products of digestion that is glucose, fructose, fatty acids etc are absorbed through the mucosa into the blood stream and lymph.

However, absorption of sugar, water, alcohol etc takes place in stomach. It mainly takes place in large intestine where absorption of minerals and drugs take place too.


Amino Acids Are Recycled

Just as some plastics can be recycled to make new products, amino acids are recycled to make new proteins. All cells in the body continually break down proteins and build new ones, a process referred to as protein turnover. Every day over 250 grams of protein in your body are dismantled and 250 grams of new protein are built. To form these new proteins, amino acids from food and those from protein destruction are placed into a “pool.” Though it is not a literal pool, when an amino acid is required to build another protein it can be acquired from the additional amino acids that exist within the body. Amino acids are used not only to build proteins, but also to build other biological molecules containing nitrogen, such as DNA, RNA, and to some extent to produce energy. It is critical to maintain amino acid levels within this cellular pool by consuming high-quality proteins in the diet, or the amino acids needed for building new proteins will be obtained by increasing protein destruction from other tissues within the body, especially muscle. This amino acid pool is less than one percent of total body-protein content. Thus, the body does not store protein as it does with carbohydrates (as glycogen in the muscles and liver) and lipids (as triglycerides in adipose tissue).

Figure 6.8 Options For Amino Acid Use In The Human Body

Image by Allison Calabrese / CC BY 4.0

Amino acids in the cellular pool come from dietary protein and from the destruction of cellular proteins. The amino acids in this pool need to be replenished because amino acids are outsourced to make new proteins, energy, and other biological molecules.

Learning Activities

Technology Note: The second edition of the Human Nutrition Open Educational Resource (OER) textbook features interactive learning activities. These activities are available in the web-based textbook and not available in the downloadable versions (EPUB, Digital PDF, Print_PDF, or Open Document).

Learning activities may be used across various mobile devices, however, for the best user experience it is strongly recommended that users complete these activities using a desktop or laptop computer and in Google Chrome.

The molecules from which proteins are built, each protein being composed of a specific sequence of linked amino acids.

The system of organs that are responsible for ingestion, digestion, and absorption of food and the discharge of residual wastes.

A class of compounds composed of linked amino acids. They contain carbon, hydrogen, nitrogen, oxygen, and sometimes other atoms in specific configurations.

A protein-digesting enzyme found in the stomach.

A mixture of partially digested food and gastric secretions in the stomach.

A protein molecule that speeds up or accelerates specific chemical reactions without changing itself.


Watch the video: Protein Digestion and Absorption (May 2022).


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