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Questions and Answers
What characterizes facilitated diffusion in the absorption of sugars across membranes?
What characterizes facilitated diffusion in the absorption of sugars across membranes?
Which statement accurately describes active transport in the transport of glucose?
Which statement accurately describes active transport in the transport of glucose?
In the context of glucose transport, what role does sodium play?
In the context of glucose transport, what role does sodium play?
Which process is incorrectly described in the context of absorption of monosaccharides?
Which process is incorrectly described in the context of absorption of monosaccharides?
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What is the main difference between facilitated diffusion and simple diffusion?
What is the main difference between facilitated diffusion and simple diffusion?
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What is the primary consequence of lactose accumulation in the gut due to lactose intolerance?
What is the primary consequence of lactose accumulation in the gut due to lactose intolerance?
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Which of the following is NOT a fate of absorbed sugars in the body?
Which of the following is NOT a fate of absorbed sugars in the body?
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In glycolysis, which products are generated from the breakdown of glucose?
In glycolysis, which products are generated from the breakdown of glucose?
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Which enzyme deficiency presents symptoms similar to those of lactose intolerance?
Which enzyme deficiency presents symptoms similar to those of lactose intolerance?
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Which metabolic pathway must all carbohydrates enter for catabolism?
Which metabolic pathway must all carbohydrates enter for catabolism?
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Study Notes
Carbohydrate Digestion and Absorption
- Two mechanisms are responsible for the absorption of sugars: facilitated transport (with a concentration gradient) and active transport (against a concentration gradient).
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Passive transport is the movement of molecules with the concentration gradient, from a higher concentration (intestinal lumen) to a lower concentration (mucosal cells). It does not require ATP, and is bidirectional.
- Simple diffusion: molecules cross membranes without the assistance of membrane proteins.
- Facilitated diffusion: the membrane allows only selective molecules and ions to pass through with the assistance of membrane proteins. All monosaccharides are absorbed by facilitated diffusion.
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The transport of glucose and galactose across the brush border of mucosal cells is by active transport. This process requires energy, transport proteins, and the presence of sodium ions.
- A sodium-dependent glucose transporter (SGLT-1), binds both glucose and sodium at separate sites and transports them into the cell.
- The sodium transport down its concentration gradient powers glucose transport against its concentration gradient. This process is called cotransport or symport.
- The energy for this reaction is provided by ATP linked to the sodium pump.
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Disorders of digestion and absorption of carbohydrates:
- Lactose intolerance is caused by a deficiency of lactase enzyme, leading to the accumulation of lactose in the gut. Bacteria ferment the lactose producing H2, CO2, and low molecular weight acids (acetic, propionic, butyric). These acids osmotically draw water from intestinal cells into the lumen of the GIT causing diarrhea and dehydration.
- Sucrase deficiency presents with similar symptoms to lactose intolerance.
Fate of Absorbed Sugars
- Uptake by tissues: Absorbed sugars are taken up by various tissues throughout the body.
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Utilization by tissues:
- Oxidation: Sugars undergo glycolysis and the tricarboxylic acid cycle (TCA). These processes produce ATP, NADH, and other essential molecules.
- Conversion to biologically active substances: Sugars are converted into ribose, fructose, galactose, glucuronic acid, amino sugars, and amino acids.
- Storage: Sugars are stored as glycogen or triglycerides.
Glycolysis
- Glycolysis is a 10-step metabolic pathway that occurs in the cytosol. This pathway converts glucose into two molecules of pyruvate, two molecules of NADH, and two molecules of ATP.
- All carbohydrates that are catabolized must enter the glycolytic pathway.
Carbohydrate Digestion
- Digestion in the mouth: Carbohydrate digestion begins in the mouth. Salivary glands secrete saliva, which contains salivary amylase (ptyalin). This enzyme requires a neutral pH (6.7) to function and hydrolyzes α-(1-4) glycosidic bonds at random within polysaccharides (starch, glycogen), producing dextrins, maltose, maltotriose, and glucose.
- Digestion in the stomach: Carbohydrate digestion temporarily stops in the stomach due to the high acidity of gastric juice. There are no carbohydrate-splitting enzymes present in gastric juice.
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Digestion in the intestine: Further digestion of carbohydrates occurs in the small intestine by pancreatic enzymes. Food bolus enters the small intestine from the stomach and mixes with pancreatic juice, which contains pancreatic amylase (amylopsin) similar to salivary amylase.
- Pancreatic amylase activity: Pancreatic amylase hydrolyzes dextrins into a mixture of maltose, isomaltose, and limit dextrin (oligosaccharides).
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Intestinal brush border enzyme activity: These enzymes are responsible for the final digestion of carbohydrates and their reactions are as follows:
- Maltase: Maltose → 2 Glucose
- Isomaltase: Isomaltose → 2 Glucose
- Sucrase: Sucrose → Glucose + Fructose
- Lactase: Lactose → Glucose + Galactose
- Dextrinase: Limit dextrin → Glucose + Maltose + Maltotriose
Why Cellulose Is Not Digested
- Cellulose is a polysaccharide found in plants and humans lack the enzyme necessary to break its glycosidic bonds. As a result, cellulose is not digested by humans.
- Although not digested, cellulose is important in the diet as it provides bulk as fiber. Fiber helps with intestinal motility and softens stools.
Carbohydrate Absorption
- Glucose, fructose, and galactose, the end products of carbohydrate digestion, are readily absorbed through the intestinal mucosal cells into the bloodstream.
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Description
This quiz covers the key mechanisms of carbohydrate absorption, including facilitated and active transport. You'll explore how sugars are absorbed in the intestines through passive and active processes, with a focus on glucose and galactose transport. Test your understanding of these essential concepts in digestion!
