The failure to effectively use these molecules accounts for the majority of the inborn errors of human carbohydrates metabolism.

[citation needed] Glucose-6-phosphate dehydrogenase deficiency affects the degradation of glucose in the pentose phosphate pathway, which is especially important in red blood cells.

[citation needed] Lactose is a disaccharide sugar composed of galactose and glucose that is found in milk.

[citation needed] In most mammals, production of lactase diminishes after infants are weaned from maternal milk.

However, 5% to 90% of the human population possess an advantageous autosomal mutation in which lactase production persists after infancy.

Long term disabilities include poor growth, mental retardation, and ovarian failure in females.

A milder form of galactosemia, called Galactokinase deficiency, is caused a lack of the enzyme uridine diphosphate galactose-4-epimerase which breaks down a byproduct of galactose.

Early identification affords prompt treatment, which consists largely of eliminating dietary galactose.

[citation needed] Three autosomal recessive disorders impair fructose metabolism in liver cells.

The most common is caused by mutations in the gene encoding hepatic fructokinase, an enzyme that catalyzes the first step in the metabolism of dietary fructose.

HFI is caused by a deficiency of fructose 1,6-biphosphate aldolase in the liver, kidney cortex and small intestine.

[citation needed] Deficiency of hepatic fructose 1,6-biphosphate (FBPase) causes impaired gluconeogenesis, hypoglycemia and severe metabolic acidemia.

[citation needed] Essential fructosuria is a clinically benign condition characterized by the incomplete metabolism of fructose in the liver, leading to its excretion in urine.

[citation needed] The metabolic pathway glycolysis is used by cells to break down carbohydrates like glucose (and various other simple sugars) in order to extract energy from them.

During glycolysis ATP, NADH (both an energy transport form used inside cells) as well as pyruvate are produced.

[citation needed] Glycolysis is taking place in the cytosol where, under anaerobic conditions, pyruvate is converted to lactate.

Under aerobic conditions, the pyruvate is transported from the cytosol to the mitochondrion, where further energy can be extracted through the citric acid cycle (CAC) [citation needed] The liver can also create glucose (gluconeogenesis, see below); during times of low carbohydrate supply from the digestive system, the liver creates glucose and supplies it to other organs.

While the pentose phosphate pathway does involve oxidation of glucose, its primary role is anabolic rather than catabolic.

(Leigh necrotizing encephalopathy due to pyruvate carboxylase deficiency, Ataxia with lactic acidosis II) Glycogenesis is the metabolic pathway in which glycogen is created.

Errors in glycogenesis can have different consequences on a cellular level: Depending on the affected cells and the extent of the deficiency, a wide range of symptoms and severities are the result.

Muscle cells in contrast do not have the enzyme glucose 6-phosphatase, so they cannot share their glycogen stores with the rest of the body.