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AHS 2013 Digestion and Metabolism 2
Sarah Harney
2. Digestion & Absorption
• How are water and ions absorbed?
• How are the main nutrient groups - carbohydrates,
proteins, fats – digested and absorbed?
• What happens in disorders of digestion/absorption? –
lactose intolerance and coeliac disease
Reading: Stanfield, pp 580-589
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Digestion & Absorption of Nutrients
GENERAL PRINCIPLES
• Breakdown of complex molecules
– Enzymes (pH)
• Absorption into gut cells
Mechanisms of absorption:
• Passive -not energy-dependent
Diffusion
Facilitated diffusion –carrier mediated
• Active - requires energy
Carrier-mediated
Primary active transport – transporter protein directly
hydrolyses ATP for energy
Secondary active transport – couples energy released
from movement of one substance to co-transport
another
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Bulk flow of liquid in gut
• Input
– Ingestion ~ 2 litres per day
– Secretion (gut) ~ 7 litres/day
• Output
– Faeces ~100 ml/day
• Conclude ~ 9 litres/day absorbed
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Absorption of Water
•
•
Passive
Driven by an osmotic gradient across mucosal epithelium
– Due to Na+ [actively pumped from gut lumen into lateral spaces]
•
Water can move either through the epithelial cells
(transcellular) or through the ‘tight junctions’
H 2O
(paracellular)
Na+
~95% absorption in small intestine
Na+
•
Absorptive cell
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Carbohydrate (CHO) Digestion
Amylase digestion of carbohydrates produces the disaccharide
maltose or short polysaccharides known as limit dextrins
Starch (polysaccharide)
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Carbohydrate digestion and
Absorption
• Carbohydrates
are initially
digested by
amylases
• Further digestion
occurs at the
brush border /
microvilli on
intestinal
epithelial
(absorptive) cells
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Carbohydrate (CHO) Digestion
of microvilli
Disaccharides are further digested to monosaccharides by brush border enzymes
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Digestion of Carbohydrates to Disaccharides is
done by brush border enzymes
• CHOs must be monosaccharides to be absorbed
• Brush border enzymes hydrolyse disaccharides to
monosaccharides
Dextrins
Sucrose
Lactose
Maltose
Dextrinase
Sucrase
Lactase
Maltase
Glucose
Glucose + Fructose
Glucose + Galactose
Glucose X 2
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
CHO Digestion — Summary
• Polysaccharides to disaccharides (gut amylases)
• Disaccharides to monosaccharides (brush border)
– Glucose, galactose enter gut cells by
energy-dependent secondary active transport
• i.e. ‘piggy-back on Na+ gradient (active)
– Fructose enters gut cells by
facilitated diffusion (passive)
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CHO Absorption into GI Epithelial Cells
Glucose + galactose are
co-transported with Na+
across apical membrane
Fructose absorbed by
facilitated diffusion
Glucose molecules cross basolateral membrane by facilitated diffusion
and diffuse into capillaries
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
CHO Absorption into Bloodstream
• Glucose, galactose
enter capillaries down
concentration
gradient
• Fructose enters
capillaries via passive
carrier-mediated
transport
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Lactose Intolerance
• Lactose intolerance is due to deficient production of
lactase
• Lactase normally hydrolyses lactose (sugar present in
milk) to glucose and galactose
• Insufficient lactase means lactose is not digested and
absorbed in the small intestine
• Digestion of lactose by bacteria in colon results in
production of gas and other bi-products leading to
bloating and diarrhoea
• Incidence ~5 % in northern Europe, up to 90 % in Asian
populations
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Protein Digestion
Essentially similar to CHO
• Proteins converted to peptides
• Protein digestion begins in stomach with
gastric pepsinogen
• Pepsinogen is activated by HCl to form active
pepsin
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Protein Digestion
• Protein digestion is continued in small intestine by
pancreatic enzymes
• Proenzymes (zymogens) secreted by the pancreas
are converted to active forms following activation of
trypsin by a brush border enzyme, enterokinase
Precursor
Trypsinogen
Chymotrypsinogen
Procarboxypeptidase
Active form
Trypsin
Chymotrypsin
Carboxypeptidase
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Peptidase activation
• Pancreatic trypsinogen
• Activated by
enterokinase
• Trypsin then activates
other zymogens
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Protein absorption
similar to CHO
• Peptides broken down to amino acids (brush border)
• Amino acids are actively transported by carriers into
gut epithelial cells
• Amino acids then transported across basolateral
membrane by facilitated diffusion and diffuse into
capillaries
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Digestion and Absorption of Fat
• Most ingested fats are triglycerides
• They are broken down to monoglycerides
and free fatty acids for absorption
• Digestion of fat is done by lipases secreted in
the mouth and stomach but mostly by
pancreatic lipases in small intestine
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Digestion of Fat
• Hydrophobic fat globules
are broken up into
smaller droplets or
emulsified by bile salts
• Bile salts contain polar
–OH and –COOH groups
that are hydrophilic
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Digestion of Fat
• Bile salts + lecithin
aggregates called
micelles
•As fats are absorbed
by epithelial cells,
micelles release more
fatty acids and
monoglycerides for
absorption
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Micelles deliver fats to the
absorptive cells
• Bile salts, lecithin and
cholesterol aggregate to
form micelles with a
hydrophilic outside and
hydrophobic lipid inside
• Fats can be carried
through the water based
intestinal fluids dissolved
in the lipid core
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Absorption of Fat
• Monoglycerides and FFA enter cells
by diffusion
• Once absorbed they are
reassembled as triglycerides
• They are then complexed with
proteins in Golgi apparatus to form
lipoproteins known as chylomicrons
• Chylomicrons are secreted by
exocytosis into the interstitial fluid
and from there enter the lymphatic
vessels
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Summary of Fat Digestion &
Absorption
• Fat to triglycerides (pancreatic lipase)
• Bile salts emulsify (surface area)
• Bile salts — micelles containing
monoglycerides and free fatty acids (FFA)
• Enter passively (fat-soluble)
• Triglyceride synthesis — chylomicrons
• Exocytosis and entry into lacteals
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AHS 2013 Digestion and Metabolism 2
Sarah Harney
Coeliac Disease
• An autoimmune disease, high incidence in
Ireland, up to 1 in 100
•Intolerance to gluten –a protein present in
wheat and other grains such as rye, barley,
oats
• Ingestion of gluten triggers an immune
response, resulting in inflammation of the
intestinal mucosa
• Autoimmune response results in damage to
intestinal villi causing malabsorption
From: www.uhbristol.nhs.uk
Endoscopy
Coeliac
• Reduced absorption leads to bloating,
diarrhoea and vitamin deficiencies
• Disease is managed by following a glutenfree diet
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