This post contains likely questions in Metabolism of digestion and absorption of macromolecules for students in the College of Medicine, UNEC.
Table of contents
CARBOHYDRATE METABOLISM
Write on carbohydrate digestion and absorption
Answer:
DIGESTION OF CARBOHYDRATE
Outline:
The Mouth (Salivary Digestion)
The Stomach (Gastric Digestion)
Digestion in The Small Intestine (Pancreatic Digestion)
- Duodenum
- Jejunum
Products of Carbohydrate Digestion
Brush Border Enzymes
General Classes of Transport of Carbohydrate
The Mouth (Salivary Digestion)
The mechanical and chemical digestion of carbohydrate (CHO) begins in the mouth. The digestive event in the mouth only takes place for a short period of time.
The mechanical digestion of carbohydrate is by the process of mastication using the teeth.
The chemical digestion of carbohydrate in the mouth is carried out by an enzyme secreted by the salivary gland called salivary α – amylase (ptyalin).
This enzyme acts on glycogen and starch to produce glucose, maltose, isomaltose, maltotriose and dextrin.
The action of salivary α – amylase is to break the α (1 → 4) glycosidic bond but not the α (1 → 4) bond near the branches.
Fructose and Galactose are not digested in the mouth.
At the end of digestion in the mouth, the end products of carbohydrate are; Glucose, Fructose, Galactose, Maltose, Maltotriose, Isomaltose, Trehalose, Raffinose and Lactose.
The Stomach (Gastric Digestion)
Carbohydrate digestion does not take place in the stomach because of the presence of hydrochloric acid. The acidic environment of the stomach stops the action of the amylase enzyme.
The role of the stomach in carbohydrate digestion is to make the ingested food substance to be a homogenous mass called chyme.
Digestion in The Small Intestine (Pancreatic Digestion)
The final process of digestion of carbohydrate takes place in lumen of the small intestine.
From the stomach, the food substances are released into the duodenum. Here, digestion of carbohydrate continues by the action of the pancreatic amylase contained in the pancreatic juice.
The pancreatic amylase is produced by pancreas which breaks down dextrin to shorter carbohydrate chains.
In the jejunum, the final process of carbohydrate takes place. It contains specific enzymes that breaks down carbohydrate.
- Maltose (Maltase) → Glu + Glu
- Maltotriose (Maltase) → Glu + Glu + Glu
- Lactose (Lactase) → Glu + Gal
- Sucrose (Sucrase) → Glu + Fru
- Isomaltose (Isomaltase) → Glu + Glu
- Dextrin (dextrinase) → Glucose units
- Trehalose (Trehalase) → Glu + Glu
- Raffinose → Undigested (no appropriate enzyme)
Products of Carbohydrate Digestion
The end product of carbohydrate digestion is Glucose, Fructose and Galactose. In addition, Cellulose and Raffinose are not digested in the gastrointestinal tract of humans.
Cellulose is not digested because of lack of the enzyme to hydrolyze β (1 → 4) glycosidic bond.
Raffinose is not digested due to lack of the enzyme necessary to hydrolyze the α configuration of galactose in the molecule.
Brush Border Enzymes
The bulk of carbohydrate digestion and absorption takes place in the small intestine. Digestion takes place in duodenum and jejunum while absorption takes place in the jejunum.
The epithelial lining of the lumen of the small intestine have a brush border. These lining have villi that is thrown into folds to increase the surface area of the lining cells.
ABSORPTION OF CARBOHYDRATE
The absorption of carbohydrate
DIGESTION OF PROTEINS
Write on protein digestion and absorption
Protein digestion (hydrolysis) starts in the stomach. No form of digestion takes place in the mouth.
The stomach acts as a reservoir and helps in the churning of food. It also secretes the gastric juice containing;
Hydrochloric acid (pH 1-3) – secreted by oxyntic or parietal cells of the gastric mucosa.
Pepsin/Renin – secreted by peptic or chief cells.
Mucin and inorganic salts – secreted by goblet cells.
Water.
Carbon dioxide from plasma enter the parietal cell where it reacts with water to from carbonic acid. The reaction is catalyzed by the enzyme carbonic anhydrase.
The carbonic anhydrase hydrolyzes to bicarbonate and hydrogen ion.
The bicarbonate moves out from the parietal cell to the plasma.
The hydrogen ion moves out to the lumen of the stomach by H/K ATPase. It results to a K ion from the stomach lumen to move into the cell.
Chloride ion moves from the plasma into the parietal cell and further moves out into the lumen of the stomach.
The hydrogen ion combines with chloride ion to form HCl.
The numerous mitochondria help in the production of ATP for the process.
Write on nucleic acid digestion and absorption
Nucleic acid digestion takes place in the stomach and small intestine. The mouth is not involved in the digestion process.
In the stomach, the HCl act on the nucleoprotein to release the nucleic acid and protein.
The nucleic acid involved are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
The nucleic acid is denatured by the low pH of the stomach.
In the small intestine the pancreatic enzyme nucleases hydrolyze DNA/RNA to oligonucleotide by the action of the enzyme nucleases.
Oligonucleotide is further hydrolyzed to mononucleotide by the enzyme phosphodiesterases.
Mononucleotide is hydrolyzed to nucleoside by the enzyme nucleotidases with the release of a phosphate group.
The nucleoside is acted upon by the enzyme nucleosidases to give a pentose sugar (Ribose/Deoxyribose) and free bases (purine and pyrimidine).
Briefly describe the following
- Mechanism of lipid absorption
- Carbohydrate digestion at the brush border
- Protein digestion at the brush border
Mechanism of lipid absorption
Lipid absorption is the process by which the products of lipid digestion (fatty acids, monoglycerides, glycerol, fat-soluble vitamins and cholesterol) are absorbed into the blood circulation.
Bile which is made by the liver and stored in the gall bladder helps in the absorption of lipids from the intestinal lumen.
Bile salts cluster around the products of fat digestion to form micelles, which helps the fats get close to the microvilli of intestinal cells so that they can be absorbed.
The products of fat digestion diffuse across the membrane of the intestinal cells and bile salts are recycled back to do more work emulsifying fat and forming micelles.
Once inside the intestinal cell, short- and medium-chain fatty acids and glycerol can be directly absorbed into the bloodstream, but larger lipids such as long-chain fatty acids, monoglycerides, fat-soluble vitamins, and cholesterol need help with absorption and transport to the bloodstream.
Long-chain fatty acids and monoglycerides reassemble into triglycerides within the intestinal cell, and along with cholesterol and fat-soluble vitamins, are then incorporated into transport vehicles called chylomicrons.
Chylomicrons are large structures with a core of triglycerides and cholesterol and an outer membrane made up of phospholipids, interspersed with proteins (called apolipoproteins) and cholesterol.
This outer membrane makes them water-soluble so that they can travel in the aqueous environment of the body.
Chylomicrons from the small intestine travel first into lymph vessels, which then deliver them to the bloodstream.
Carbohydrate digestion at the brush border
Carbohydrate digestion of monosaccharide conclude in microvilli of the small intestine, in brush border epithelial cells
The major brush border enzymes and disaccharidases, located in the brush border of the intestinal lumen are:
- α Dextrinase – breaks down – α dextrin chains by removing or clipping off glucose units
- Maltase – hydrolyzes maltose to two glucose units.
- Sucrase-isomalase – hydrolyzes sucrose and isomaltose
- Lactase- hydrolyzes lactose.
Cellulose and other polysaccharides of plant origin with β- 1,4 linkages are not digested by humans owing to the lack of β- 1,4 glucosidase. They constitute the fiber in the human diet.
During early stages following a meal, the concentration of monosaccharides in intestinal fluids may exceed that in the body; thus sugar transport may be passive and facilitated.
Protein digestion at the brush border
The digestion of protein starts in the stomach by the action of pepsin which hydrolyzes the peptide bond present in protein.
Digestion is completed in the brush border by two active enzymes;
- Aminopeptidase: Breaks peptide bond that attaches terminal amino acids to the amino end of the peptide
- Dipeptidase: splits dipeptides into single amino acids.
- End products of protein digestion amino acids, dipeptides and tripeptides.
- End products are absorbed at the intestinal villus.
- Absorption depends on 3 mechanism
- Active transport, Na+ – dependent secondary active transport and H+ – dependent secondary active transport.
Discuss the process of digestion of oil and yam in the mouth and stomach
Mouth
The digestion of oil and yam begins in the mouth by the action of the enzyme salivary amylase and lingual amylase.
Five (5) percent of the yam will be digested in the mouth by the ptyalin enzyme. Mastication of the yam by the teeth also aids the digestion of yam.
Thirty (30) percent of the oil will be digested in the mouth by the lingual enzyme.
Stomach
The digestion of yam by amylase enzyme is inhibited in the stomach by the HCl. It means that no digestion will take place here.
In the stomach, gastric lipases hydrolyze a small amount of triglycerides end product fatty acid and monoglyceride.
Describe fully the occurrence and digestive role(s) of the following (i) Ptyalin (ii) Rennin (iii) Bile acids (iv) Pepsin (v) Bile
- Ptyalin:
Ptyalin also called salivary amylase is an enzyme secreted by the salivary gland in the mouth.
The digestive process of carbohydrate begins with salivary amylase, which randomly cleaves the α 1,4 linkage of starch.
- Rennin
Rennin is secreted in the stomach of infants. In the presence of Ca2+ ions.
Rennin converts casein (milk protein) to paracasein which is then acted upon by pepsin (coagulation of milk). This prevents the rapid passage of milk from the stomach.
Make a list of the final products of complete digestion of nucleoprotein in the GIT
- Pentose sugar (ribose/deoxyribose)
- Free bases (purines, pyrimidines)
- Phosphate group
- Amino acids
Give the composition of the pancreatic juice and discuss briefly, their role in the digestion of food.
- Water
- Digestive enzymes (α-amylase, lipase, trypsin, chymotrypsin, elastase, carboxypeptidase & nucleases).
- Bicarbonate
During digestion, the pancreas makes pancreatic juices called enzymes.
These enzymes break down sugars, fats, proteins and starches.
Pancreatic hormones help regulate blood sugar levels and appetite, stimulate stomach acids, and tell the stomach when to empty.
Explain the term lactose intolerance, symptoms and discuss the enzymatic reaction involved.
Lactose intolerance also called milk intolerance arises from a deficiency of the enzyme lactase.
In this condition, lactose passes undigested into the colon (large intestine). Bacterial fermentation occurs giving 2- and 3- carbon metabolites, CO2 and H2. Lactose and the fermentation products also osmotically absorb water from the tissues.
Symptoms of Lactose Intolerance
- Dehydration
- Diarrhea
- Flatulence (distension of swollen abdomen due to gas)
- Abdominal cramps (caused by the bacterial metabolites)
Treatment
Treatment of lactose intolerance is by the removal of lactose from diet. The disorder could be hereditary or due to low activity of the enzyme which could be caused by intestinal disease (e.g. peptic ulcer), malnutrition, physiological decline with age, drugs and injuries to the mucosa.
Distinguish between passive diffusion and facilitated diffusion.
| SN | Passive | Facilitated |
| 1. | It is the net movement of molecules from region of high concentration to lower concentration. | it involves the movement of materials by a carrier-molecule across the cell membrane from the region of high to low concentration |
| 2. | The speed of simple diffusion is relatively low. | The speed of simple diffusion is relatively higher. |
| 3. | Simple diffusion is mostly involved in the passage of small non-polar molecules. | Facilitated diffusion is commonly involved in the movement of large and polar molecules across a biological membrane. |
| 4. | The driving force for simple diffusion is the concentration gradient across the membrane. | The difference in concentration of solute across the membrane is the driving force for facilitated diffusion |
| 5. | The process of simple diffusion is not solute specific. | Facilitated diffusion is directed by the specificity between solute and carrier molecules. |
Distinguish between active transport and facilitated diffusion
| SN | Active | Facilitated |
| 1. | It the movement of molecules and ions against a concentration gradient. | it involves the movement of materials by a carrier-molecule across the cell membrane from the region of high to low concentration |
| 2. | It requires the use of ATP | It does not require the use of ATP |
| 3. | Active transport is a directional process | There is often a reversible interaction between the molecule and the transported material. |
What are the advantages of active transport mechanism over facilitated diffusion in the absorption of nutrients in the GIT.
- The absorption of nutrients via active transport is faster than facilitated diffusion.
- Active transport is a directional process.
Briefly explain the term auto digestion and the role of pancreatic trypsin inhibitor.
Auto digestion is the self-digestion of an organ by enzyme(s) produced by the same organ.
To prevent auto digestion, enzymes are generally secreted in their inactive forms know as zymogens.
Pancreatic trypsin inhibitor is a specific inhibitor produce by the pancreas against its self-digestion.
It is a protein that plays the role of preventing the premature production of proteolytic enzymes within the pancreas cells. This will inhibit the auto-digestion of the pancreas for pancreatitis not to occur.
Make a list of the end products of digestion of RNA in the GIT
- Ribose sugar
- Phosphate group
- Nitrogenous base (purine/pyrimidine)
Comment on nucleic acid digestion in the small intestine and give the end products of nucleic acid absorption.
The end product of nucleic acid digestion in the small intestine are;
- Inorganic phosphate
- Ribose or deoxyribose sugar
- Nitrogenous base (Purine or Pyrimidine)
