Lactose Intolerance (AQA Biology Unit 1)

In depth document on lactose intolerance. I have used a wide variety of creditbale data including AS Level textbooks. I hope this helps!

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Lactose intolerance (also known as acquired/primary lactase
deficiency and is normal for most mammals):
Lactose intolerance - The inability to digest the lactose found in milk
and other dairy products into its constituent monosaccharides, glucose
and galactose. Lactose is a disaccharide and as such cannot pass
through the plasma membrane of the cells of the gut epithelium. They
must first be hydrolysed by enzymes bound to the membrane of
microvilli. Digestion of lactose is carried out by the enzyme lactase
present in all young mammals. The monosaccharides produced are
transported across the plasma membranes of the gut epithelial cells
into the bloodstream and then metabolised by tissues to yield ATP or
are stored as glycogen in the liver and muscle for subsequent use.
(Sodium ions move out of the base of the epithelial cell by active transport
which establishes a concentration gradient between the lumen of the gut
and the epithelial cell. Glucose and Galactose are co-transported into the
epithelial cells with sodium)
Milk provides carbohydrate, protein, fat, mineral and vitamins and is
produced by the mammary glands of all female mammals after giving
birth. When mammals stop breast feeding their young, their young lose
the ability to make lactase, this is because after weaning most
mammals never drink milk again so it is not required. Continuing to
synthesise an enzyme in the gut and hydrolyse a nutrient that won't
appear in the diet would be a waste of energy.
Lactose intolerance can be considered to be a physiological response
to the intake of lactose in the diet by an individual who has undergone
a genetically programmed loss of the enzyme lactase after weaning.
The lactase gene fulfils a critical role in directing the synthesis of
lactase at birth as a major source of carbohydrate in the baby's
nutrition is the lactose in mother's milk. In the UK due to modern
storage and distribution methods mean that adults now consume milk
and milk products in greater quantities. In many parts of the world milk
products don't form a part of the adult diet and switching off the
lactase gene is a normal phenomenon that goes unnoticed. So their
lactose remains undigested and can't be absorbed into the
Lactose therefore passes unchanged into the colon bacteria living here
switch their metabolism and begin to ferment lactose, producing lactic
acids and large amounts of gases including carbon dioxide and

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These lower the water potential of the lumen of the gut,
water enter the gut via osmosis from the blood. This gas causes
bloating, cramps, flatulence and diarrhoea, vomiting and general
discomfort. The faeces produced are therefore are also explosive due
to the gases, watery due to osmosis and acidic due to the lactic acid and
also contain lactose.
Lactose intolerance can be diagnosed by giving person lactose and
monitoring the blood for an increase in glucose.…read more


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