Carbohydrate Digestion

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Central lacteal
Capillaries
Brush border ?absorption Membrane bound enzymes
Vein
Artery
Lymph duct
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Digestion of carbohydrate, protein and fats by
catalytic hydrolysis enzymes are either luminal
(e.g. from salivary glands or pancreas) or
membrane bound
Digested nutrients/fluids absorbed through the
brush border by - active transport diffusion
- passive facilitated solvent drag
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Carbohydrate Digestion

• Mouth
• 1. Mastication

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Carbohydrate Digestion

• Mouth
• 1. Mastication
• 2. Saliva moistens feed, together with
  mastication disrupts structure

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Carbohydrate Digestion

• Mouth
• 1. Mastication
• 2. Saliva moistens feed, together with
  mastication disrupts structure
• 3. Salivary amylase attacks 1,4 linkages
  within starch

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Carbohydrate digestion - initiated by salivary
a amylase from salivary glands - majority by
pancreatic a amylase in small intestine - pH
optimum 7, activated by Cl- ions
? 1,4 bonds give straight chains ? 1,6 bonds
give branched chains Amylase can only hydrolyse
1,4 bonds - branched chains cannot be broken
down by amylase
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Carbohydrate Digestion

• Stomach
• 1. Low pH denatures amylase

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Carbohydrate Digestion

• Stomach
• 1. Low pH denatures amylase
• 2. Acid helps disrupt H-bonds within complex
  carbohydrates

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Carbohydrate Digestion

• Stomach
• 1. Low pH denatures amylase
• 2. Acid helps disrupt H-bonds within complex
  carbohydrates
• 3. N0 CHO digesting enzymes

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Induced Fit Model of Enzyme Function

• The active site is in the induced conformation

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Carbohydrate Digestion

• Duodenum
• Pancreatic amylases break down starches to
  dextrins, maltotriose, maltose
• Amylase hydrolyzes -1,4 linkages
• Amylose -gt-gt-gt maltose
• Amylopectin -gt-gt-gt maltose, dextrin

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Carbohydrate Digestion

• Duodenum
• Pancreatic amylases break down starches to
  dextrins, maltotriose, maltose
• The major brush border carbohydrases are
• Maltase (maltose, maltotriose) gt -gt glucose
• Isomaltase (dextrins) -gt -gt glucose
• Lactase (lactose) -gt -gt galactose, glucose in the
  enterocyte
• Sucrase (sucrose) -gt-gt fructose, glucose

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Carbohydrate Digestion

• Duodenum
• Pancreatic amylases break down starches to
  dextrins, maltotriose, maltose
• The major brush border carbohydrases are
• Maltase (maltose, maltotriose) gt -gt glucose
• Isomaltase (dextrins) -gt -gt glucose
• Lactase (lactose) -gt -gt galactose, glucose in the
  enterocyte
• Sucrase (sucrose) -gt-gt fructose, glucose

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Carbohydrate Digestion

• Enterocytes
• Only absorb monosaccharides
• Final breakdown products are glucose,
  galactose, fructose
• Fructose -gt-gt-gt lactic acid, glucose
• Galactose -gt-gt-gt glucose

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Carbohydrate digestion
Non-ruminants do not have cellulase - cellulose
makes up most of undigested fibre in diet
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Absorption of simple sugars
Limiting step on simple sugar availablity is rate
of absorption- large excess in small
intestine Majority absorbed in duodenum and
jejunum Digested at membrane so available for
transport Fructose absorbed by facilitated
diffusion Deficiencies of brush border enzymes
cause osmotic diarrhoea Human SI can absorb up to
10kg sucrose per day
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Absorption of Glucose Across the Intestinal
Epithelium

• The transporter for glucose and galactose into
  the enterocyte is the SODIUM DEPENDENT HEXOSE
  TRANSPORTER
• (SGLUT-1).

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Absorption of Glucose Across the Intestinal
Epithelium

• The transporter for glucose and galactose into
  the enterocyte is the SODIUM DEPENDENT HEXOSE
  TRANSPORTER
• (SGLUT-1).
• The molecule transports both glucose and sodium
  into the cell (wont transport either alone).

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Glucose Transporter

• The transporter is initially oriented facing the
  lumen can only bind sodium, not glucose.
• After sodium is bound, a conformational change
  occurs, opening a glucose pocket.
• After glucose is bound, the transporter reorients
  in the membrane and is moved inside the cell.

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Glucose Transport contd

• Sodium dissociates inside the cytoplasm and this
  destabilizes the glucose transporter bond.
• Glucose dissociates within the cytoplasm and the
  free transporter is reoriented outward facing the
  lumen.
• Glucose is transported out of the enterocyte via
  a different transporter (GLUT 2) in the
  basolateral membrane.

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Absorption of glucose
Low Na
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Disruption of carbohydrate absorption
1. Deficiency in brush border enzymes lactase
deficiency is either congenital or acquired later
in life - most common maltase deficiency is not
known 2. GI infection/disease
coeliac disease, bacterial infections,
protazoan infections can all cause inflammation
and interference with brush border absorption
Consequence of poor carbohydrate absorption -
osmotic diarrhoea
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Important Carbohydrates
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Dietary Carbohydrate Composition

• Monosaccharides glucose from the hydrolysis of
  starch, fructose (keto-hexose) is 140 sweeter
  than sucrose and is very reactive with amino
  acids (maillard reactivity)

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• Disaccharides sucrose is the major disaccharide
  in most diets (in foods, is often responsible for
  functional qualities including sweetness,
  mouth-feel, textural characteristics).

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• Disaccharides sucrose is the major disaccharide
  in most diets (in foods, is often responsible for
  functional qualities including sweetness,
  mouth-feel, textural characteristics).
• Decreased use in soft drinks because of
  replacement with high-fructose corn syrup
  (increased availability and lower cost).

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• Disaccharides sucrose is the major disaccharide
  in most diets (in foods, is often responsible for
  functional qualities including sweetness,
  mouth-feel, textural characteristics).
• Decreased use in soft drinks because of
  replacement with high-fructose corn syrup
  (increased availability and lower cost).
• Lactose (galactose-glucose) milk products

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• Oligosaccharides galactosucroses and
  fructo-oligosaccharides
• Galactosucroses range from 5-8 DM
• Fructo-oligosaccharides 60-70 DM of Jerusalem
  artichoke, do not undergo maillard reactivity
  (non-reducing)

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Polysaccharides

• Starch the most important, abundant
  polysaccharide. It is the reserve polysaccharide
  in leaves, stems, roots (tuber), seeds, fruit.

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Polysaccharides

• Starch the most important, abundant
  polysaccharide. It is the reserve polysaccharide
  in leaves, stems, roots (tuber), seeds, fruit.
• It occurs as discrete, crystalline granules whose
  size, shape, and gelatinization temperature are
  dependent on plant species.

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Starch contd

• Seed starches wheat, corn, rice, barley are
  located within protein matrices (poor quality
  protein sources for non-ruminants)
• Root starch potato, casava

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Starch contd

• Starch is a homopolysaccharide (repeating glucose
  units) with a mixture of two polymers, amylose
  and amylopectin

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• Amylose is largely a linear polymer (1, 4
  linkages) and a small percentage of
• 1, 6 branch points
• Amylopectin highly branched (5-6 1,6 branch
  points

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• Most common cereal starches contain
• 20-30 amylose
• Waxy starches (corn, rice, sorghum, barley) have
  no amylose and are essentially 100 amylopectin

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Starch Gelatinization

• Starch granules hydrate in aqueous environments,
  swelling about 10. Addition of heat takes the
  granules from an organized to disorganized state,
  this is the gelatinization temperature .
  Digestion of starch by amylase is greatly
  enhanced if it has been gelatinized.

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Starch Gelatinization

• Retrograde starch realignment of starch
  granules within amylose and amylopectin, not
  normal and may lead to decline in digestive
  efficiency.