Digestion and Absorption of Carbohydrates and Proteins
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Questions and Answers

What is the primary mechanism of protein absorption in adults?

  • Luminal proteases and brush border peptidases
  • Direct uptake of amino acids through the apical membrane
  • Absorption of dipeptides, tripeptides, or tetrapeptides through a H+-driven cotransporter (correct)
  • Intact protein uptake through a cellular route

    • The absorption of amino acids across the small intestine requires only movement across the apical membrane.

    False

    What is the characteristic of lipids that distinguishes them from other substances?

    Preferential solubility in organic solvents, compared with water

    Amino acids exit enterocytes through _______________ transporters and enter through _______________ transporters.

    <p>Na+-independent, Na+-dependent</p> Signup and view all the answers

    Match the following protein absorption mechanisms with their descriptions:

    <p>Intact protein uptake = Occurs in neonates, may not have nutritional value in adults H+-driven cotransporter = Absorbs dipeptides, tripeptides, or tetrapeptides Luminal proteases and brush border peptidases = Releases amino acids in the lumen of the small intestine Amino acid transporters = Mediates amino acid movement across the apical and basolateral membranes</p> Signup and view all the answers

    What is the process by which glucose is absorbed in the small intestine?

    <p>Active transport</p> Signup and view all the answers

    SGLT1 can transport fructose.

    <p>False</p> Signup and view all the answers

    What is the protein responsible for the apical step of fructose absorption?

    <p>GLUT5</p> Signup and view all the answers

    The basolateral membrane uses the facilitated sugar transporter, _______________, to exit all three monosaccharides.

    <p>GLUT2</p> Signup and view all the answers

    Match the protein digestion pathway with its description:

    <p>1 = Luminal enzymes hydrolyse proteins to peptides and amino acids 2 = Luminal enzymes digest proteins to peptides, and brush border enzymes digest peptides to amino acids 3 = Luminal enzymes digest proteins to peptides, which are taken up as oligopeptides by enterocytes 4 = Proteins are absorbed directly by enterocytes</p> Signup and view all the answers

    Amino acids can be absorbed directly without digestion.

    <p>False</p> Signup and view all the answers

    What is the purpose of luminal enzymes in protein digestion?

    <p>To hydrolyse proteins to peptides and amino acids</p> Signup and view all the answers

    What is the result of protein digestion in the small intestine?

    <p>Oligopeptides and amino acids</p> Signup and view all the answers

    In adults, what is the primary route of protein absorption?

    <p>Dipeptide, tripeptide, or tetrapeptide uptake</p> Signup and view all the answers

    Whole protein uptake in adults has nutritional value.

    <p>False</p> Signup and view all the answers

    What is the role of luminal proteases and brush border peptidases in protein digestion?

    <p>Releasing amino acids in the lumen of the small intestine</p> Signup and view all the answers

    Amino acids appear in the cytosol of intestinal villous cells as the result of either their uptake across the _______________ membrane or of the hydrolysis of oligopeptides.

    <p>apical</p> Signup and view all the answers

    Match the following membrane transport processes with their descriptions:

    <p>Apical membrane transport = Uptake of amino acids from the lumen Basolateral membrane transport = Uptake of amino acids from the blood for cell nutrition</p> Signup and view all the answers

    How many distinct transport systems are present at the apical membrane for amino acid uptake?

    <p>7</p> Signup and view all the answers

    Lipids are soluble in water.

    <p>False</p> Signup and view all the answers

    What is the importance of the chemical structure of lipids in their biological fate?

    <p>It determines their interactions with water and other lipids in aqueous body fluids.</p> Signup and view all the answers

    What is the primary function of the stomach in the gastrointestinal system?

    <p>Temporary food storage and mechanical breakdown</p> Signup and view all the answers

    The small intestine is the primary site for nutrient absorption.

    <p>True</p> Signup and view all the answers

    What is the function of the liver in the gastrointestinal system?

    <p>Bile secretion, stored in the gallbladder between meals</p> Signup and view all the answers

    The pancreatic juices are secreted into the _______________ via the major and minor duodenal papilla.

    <p>duodenum</p> Signup and view all the answers

    What is the primary function of the salivary glands in the gastrointestinal system?

    <p>Lubrication of food</p> Signup and view all the answers

    Match the following structures with their functions in the gastrointestinal system:

    <p>Mouth and Oropharynx = Mechanical breakdown and lubrication of food Oesophagus = Conduit to the stomach Small intestine = Primary site for nutrient absorption Large intestine = Reabsorbs fluids and electrolytes</p> Signup and view all the answers

    The small intestine can directly absorb polysaccharides.

    <p>False</p> Signup and view all the answers

    What is the process of uptake of nutrients into cells or across tissues and organs through diffusion or osmosis?

    <p>Absorption</p> Signup and view all the answers

    What happens to starch in the presence of gastric acid?

    <p>It is inactivated</p> Signup and view all the answers

    Disaccharides can be absorbed by the small intestine.

    <p>False</p> Signup and view all the answers

    What is the name of the membrane protein responsible for glucose and galactose uptake at the apical membrane?

    <p>sodium/glucose transporter 1 (SGLT1)</p> Signup and view all the answers

    The three brush border oligosaccharidases in the small intestine are lactase, maltase, and ___________________.

    <p>sucrase-isomaltase</p> Signup and view all the answers

    What is the process of carbohydrate digestion in the small intestine?

    <p>Membrane digestion and hydrolysis of starch to monosaccharides</p> Signup and view all the answers

    The small intestine absorbs disaccharides directly.

    <p>False</p> Signup and view all the answers

    Match the following brush border oligosaccharidases with their substrates:

    <p>Lactase = Lactose Maltase = Maltose Sucrase-isomaltase = Sucrose</p> Signup and view all the answers

    What are the three monosaccharide products of carbohydrate digestion?

    <p>glucose, galactose, and fructose</p> Signup and view all the answers

    What is the purpose of hydrolysis in carbohydrate digestion?

    <p>to break down carbohydrates into monosaccharides</p> Signup and view all the answers

    Glycogen is the storage form of carbohydrates in plants.

    <p>False</p> Signup and view all the answers

    What are the two steps involved in the digestion of carbohydrates?

    <p>Intraluminal Hydrolysis and Membrane Digestion</p> Signup and view all the answers

    Dietary fibre is composed of ______________ polymers found in fruits, vegetables, and cereals.

    <p>non-digestible</p> Signup and view all the answers

    Which enzyme is responsible for initiating starch digestion in the mouth?

    <p>Salivary amylase</p> Signup and view all the answers

    All polymers are digestible.

    <p>False</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Soluble fibre = Can be dissolved in water Non-soluble fibre = Cannot be dissolved in water Glycogen = Storage form of carbohydrates in plants Alpha-amylase = Enzyme responsible for breaking down proteins</p> Signup and view all the answers

    What is the role of brush border disaccharidases in carbohydrate digestion?

    <p>Break down oligosaccharides into monosaccharides</p> Signup and view all the answers

    Study Notes

    Carbohydrate Absorption

    • Carbohydrate digestion produces monosaccharides, which are absorbed by the small intestine in a two-step process
    • Glucose, galactose, and fructose are absorbed through the sodium/glucose transporter 1 (SGLT1) at the apical membrane
    • Fructose absorption occurs by facilitated diffusion through GLUT5
    • Exit of all three monosaccharides across the basolateral membrane uses the facilitated sugar transporter, GLUT2

    Protein Digestion

    • Proteins are digested into oligopeptides and amino acids before being taken up by enterocytes
    • Digestion-absorption of proteins occurs through four major pathways
    • Luminal enzymes (proteases) from the stomach and pancreas hydrolyze proteins to peptides and then to amino acids, which are then absorbed
    • Brush border peptidases fully digest some oligopeptides to amino acids, whereas cytosolic peptidases digest oligopeptides that directly enter the enterocyte
    • Both gastric and pancreatic proteases are secreted as proenzymes that require conversion to their active form for protein hydrolysis to occur

    Protein, Peptide, and Amino Acid Absorption

    • In adults, proteins are almost exclusively digested to their constituent amino acids and dipeptides, tripeptides, or tetrapeptides before absorption
    • Substantial amounts of protein are absorbed from the intestinal lumen, via a H+-driven cotransporter, as dipeptides, tripeptides, or tetrapeptides and are then hydrolyzed to amino acids by intracellular peptidases
    • Amino acids appear in the cytosol of intestinal villous cells as the result of either their uptake across the apical membrane or of the hydrolysis of oligopeptides that had entered the apical membrane
    • Movement of amino acids across the basolateral membrane is bi-directional; the movement of any one amino acid can occur through one or more amino acid transporters

    Lipid Digestion

    • Lipids are typified by their preferential solubility in organic solvents, compared with water
    • The biological fate of lipids depends critically on their chemical structure, as well as on their interactions with water and other lipids in aqueous body fluids

    Organisation of the Gastrointestinal System

    • The gastrointestinal tract is a tube that is specialized along its length for sequential processing of food
    • It consists of a series of hollow organs (mouth to anus) and accessory glands and organs that add secretions to the hollow organs
    • Each hollow organ serves a specialized function, separated at key locations by sphincters

    Carbohydrate Digestion and Absorption

    • Carbohydrates are classified into three major groups: monosaccharides (monomers), disaccharides (short polymers), and polysaccharides (long polymers)
    • The small intestine can directly absorb monomers, but not polymers, which require hydrolysis to monosaccharides before absorption
    • Dietary fibre is a non-digestible polymer found in fruits, vegetables, and cereals, and can be either soluble or nonsoluble
    • Glycogen is the storage form of carbohydrate in animals, equivalent to starch in plants

    Intraluminal Hydrolysis

    • Starch is broken down to oligosaccharides by salivary and pancreatic enzymes
    • Salivary amylase initiates starch digestion in the mouth, which ceases at around six months of age
    • Pancreatic amylase continues starch digestion in the small intestine

    Membrane Digestion

    • Oligosaccharides are broken down to monosaccharides by brush border disaccharidases in the small intestine
    • The small intestine has three brush border oligosaccharidases, each with a different hydrolytic function: lactase, maltase, and sucrase-isomaltase

    Carbohydrate Absorption

    • The three monosaccharide products of carbohydrate digestion (glucose, galactose, and fructose) are absorbed by the small intestine in a two-step process
    • Uptake of glucose and galactose occurs across the apical membrane into the epithelial cell via the sodium/glucose transporter 1 (SGLT1)
    • Fructose is absorbed through facilitated diffusion via GLUT5
    • All three monosaccharides exit the epithelial cell through facilitated diffusion via GLUT2

    Protein, Peptide, and Amino Acid Absorption

    • Proteins are digested into oligopeptides and amino acids before being taken up by the enterocytes
    • Four major pathways of protein digestion and absorption occur in the gastrointestinal tract
    • Amino acids are absorbed across the small intestine through a combination of facilitated and active transport processes
    • Amino acids exit the enterocytes through Na+-independent transporters and enter through Na+-dependent transporters

    Organisation of the Gastrointestinal System

    • The gastrointestinal tract is a tube that is specialised along its length for sequential processing of food
    • The tract consists of a series of hollow organs (mouth to anus) and accessory glands and organs that add secretions to the hollow organs
    • Each hollow organ serves a specialised function, separated at key locations by sphincters

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    Description

    This quiz covers the process of digestion and absorption of carbohydrates and proteins in the small intestine, including the role of transport proteins and facilitated diffusion.

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