00:00 / 00:00
Amino acid metabolism
Nitrogen and urea cycle
Citric acid cycle
Electron transport chain and oxidative phosphorylation
Pentose phosphate pathway
Physiological changes during exercise
Fatty acid oxidation
Fatty acid synthesis
Ketone body metabolism
Maple syrup urine disease
Ornithine transcarbamylase deficiency
Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Hereditary fructose intolerance
Pyruvate dehydrogenase deficiency
Glycogen storage disease type I
Glycogen storage disease type II (NORD)
Glycogen storage disease type III
Glycogen storage disease type IV
Glycogen storage disease type V
Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)
Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)
Fabry disease (NORD)
Gaucher disease (NORD)
Metachromatic leukodystrophy (NORD)
Niemann-Pick disease type C
Niemann-Pick disease types A and B (NORD)
Tay-Sachs disease (NORD)
Disorders of amino acid metabolism: Pathology review
Disorders of carbohydrate metabolism: Pathology review
Disorders of fatty acid metabolism: Pathology review
Dyslipidemias: Pathology review
Glycogen storage disorders: Pathology review
Lysosomal storage disorders: Pathology review
0 / 9 complete
0 / 3 complete
lactase deficiency p. 79
lactose intolerance p. 390
The three most common forms of sugar are glucose, fructose, and galactose, and these are all types of monosaccharides, meaning they’re made of just one sugar molecule, molecules like this are called carbohydrates, because they’re made up of carbon, hydrogen, and oxygen, usually with a hydrogen-oxygen ratio of 2:1.
If you link two of these guys together, you get a disaccharide because “di” means two, and this is also a carbohydrate.
Now our body uses these sugar molecules for energy, right?
For us humans, glucose is our gasoline, our energy source, we’ll take galactose and fructose...but ultimately we need to use glucose, so almost all the fructose and galactose we ingest is converted to glucose, and then we use that glucose for energy.
Alright, but usually carbohydrates aren’t in monosaccharide form when we ingest them, and a lot of what we take in are in the disaccharide form, and one notorious disaccharide that tends to cause serious gastrointestinal distress for a lot of people, is lactose.
Lactose is a disaccharide that’s made up of a glucose molecule and a galactose molecule.
For us to use it as energy, though, we have to first break it down to those two monosaccharides.
In the milk of most mammals, lactose is generally the major carbohydrate, so when you have a glass of milk, and it gets through your stomach to the small intestine, that lactose gets chopped into glucose and galactose by an enzyme that’s fittingly called lactase.
The gene responsible for production of the lactase enzyme is expressed exclusively in the enterocytes lining the small intestine, which are cells that help absorb nutrients from stuff that we eat.
Once produced, the enzyme makes it’s way to the cell’s surface along the cell’s microvilli, these little tentacles that help increase surface area and absorb nutrients.
K, once lactose gets chopped by lactase, we’re good to go, and we absorb the glucose and galactose and all is well.
Now, as mammals, we’re wired to be able to ingest milk after birth, right?
So it makes sense that when we’re young we have a whole bunch of lactase enzyme, since that’s pretty much all we drink.
After weaning, in most mammal species, expression of the gene responsible for lactase is way down-regulated, and so production of lactase also goes way down.
The majority of humans actually follow this protocol as well, and down-regulate lactase production around 3-5 years of age.
Interestingly, though, the majority of caucasians, mainly those from northern european background, continue to have elevated lactase activity all the way into adulthood, and so they exhibit “lactase persistence”.
Copyright © 2023 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
Cookies are used by this site.
USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.