BIOC 460 - DR. TISCHLER - LECTURE 24

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BIOC 460 - DR. TISCHLER - LECTURE 24
DIGESTION ABSORPTION OF CARBOHYDRATES
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OBJECTIVES

• 1. Principal dietary monosaccharides and
  disaccharides.
• 2. General structural features of amylose,
  amylopectin, glycogen and cellulose.
• 3. Products of starch digestion by pancreatic
  a-amylase.
• Four glycoprotein brush-border enzyme complexes,
  their roles in digestion of carbohydrate,
  products of their actions.
• How the primary nutritional monosaccharides
  (glucose, fructose, and galactose) are moved from
  the intestinal lumen to capillaries.
• 6. Tissue where each facilitative glucose
  transporters (GLUT 1
• to GLUT 5) is found describe their basic
  function

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Figure 1. Structures of the most common dietary
monosaccharides.
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Figure 1. Structures of the most common dietary
disaccharides.
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Figure 2. General structure of amylopectin
(principal plant polysaccharide) and glycogen
(principal animal polysaccharide)
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TYPES OF POLYSACCHARIDES
Glycogen animal CHO storage similar to
amylopectin except more branches
Cellulose linear chain with ?1,4 glycosidic
bonds
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Figure 3. Digestion of carbohydrates Top
Early digestion depicting the intake of dietary
carbohydrates and the role of salivary amylase
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Figure 3. Digestion of carbohydrates Middle
Luminal digestion showing the combined roles of
the stomach, pancreas and intestine.
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Figure 4. ?-Amylase, of salivary or pancreatic
origin, cleaves amylopectin to produce all of the
products shown
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Figure 4. ?-Amylase, of salivary or pancreatic
origin, cleaves amylopectin to produce all of the
products shown
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Figure 4. ?-Amylase, of salivary or pancreatic
origin, cleaves amylopectin to produce all of the
products shown
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Figure 3. Digestion of carbohydrates Bottom
Summary of the brush border glycoprotein
complexes containing disaccharidases and
oligosachharidases.
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Table 1. Brush border complexes of the luminal
membrane (brush border).
 
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• Primary
• defective lactase enzyme at birth
• late onset appears gradually in late teens/adults

• Secondary (acquired)
• consequence of diseases that damage brush border
• tropical sprue bacterial infection
• celiac sprue gluten-sensitive enteropathy

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Patient with celiac sprue
Normal intestinal villi
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Na
Fructose


Glucose
Galactose
Lumen of
also glucose
intestine

Na
Intestinal
SGLT-1
GLUT-5

Epithelial cell
Brush border

Na
Fructose (glucose)




Na
2K
ATP


3Na
2K
contraluminal membrane


ADP Pi
2K
3Na


to capillaries
2K
3Na


facilitated diffusion
Na,K-ATPase


Na
-
dependent co
-
transport
Figure 5. Absorption of monosaccharides
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Table 2. Types of Facilitative Glucose
Transporters
Transporter Tissue Function
GLUT-1 many mammalian tissues (e.g., red blood cell, brain, kidney) glucose uptake
GLUT-2 Liver pancreas ?-cell, contraluminal membrane of intestinal cell high capacity low affinity regulates insulin release by pancreas liver removes glucose after a meal
GLUT-3 many mammalian tissues especially brain, kidney glucose uptake
GLUT-4 skeletal muscle, heart, adipose insulin-stimulated glucose uptake helps prevent hyperglycemia through insulin action
GLUT-5 small intestine primarily fructose absorption