Disorders of the Lens

A cataract is a condition defined as painless clouding or opacity of the lens. It causes visual impairment, as the lens provides part of the eye’s refractive power. Although all age groups can be affected, the age-related or senile type of cataract is the most common. Aside from age, there are multiple risk factors, including systemic diseases, medications, or trauma. Patients present with blurry vision, glare sensitivity, and color vision change. Ophthalmologic inspection often shows darkening of or opacities in the red reflex. Slit-lamp examination will show the extent and location of the cataract. The treatment is surgery, which is indicated when loss of vision function interferes with daily function.

Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis and glycogenolysis are the 2 major ways the body produces glucose. Key enzymes for gluconeogenesis are pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. Thus, gluconeogenesis becomes the main source of glycemic maintenance after glycogen stores are depleted.

Galactosemia is a disorder caused by defects in galactose metabolism. Galactosemia is an inherited, autosomal-recessive condition, which results in inadequate galactose processing and high blood levels of monosaccharide. The rare disorder often presents in infants with symptoms of lethargy, nausea, vomiting, diarrhea, and jaundice. Serious neurologic complications such as speech and motor deficits (e.g., ataxia) may occur. Diagnosis is made through blood testing, which detects an absence or low level of the enzymes necessary to process galactose. Treatment is avoidance of lactose and galactose in the diet.

Glycolysis is a central metabolic pathway responsible for the breakdown of glucose and plays a vital role in generating free energy for the cell and metabolites for further oxidative degradation. Glucose primarily becomes available in the blood as a result of glycogen breakdown or from its synthesis from noncarbohydrate precursors (gluconeogenesis) and is imported into cells by specific transport proteins. Glycolysis occurs in the cytoplasm and consists of 10 reactions, the net result of which is the conversion of 1 C6 glucose to 2 C3 pyruvate molecules. The free energy of this process is harvested to produce adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide hydride (NADH), key energy-yielding metabolites. The overall stoichiometry of the pathway is: glucose + 2 Pi + 2 ADP + 2 NAD+ > 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O (H+: hydrogen ion, Pi: phosphate ion, NAD+: nicotinamide adenine dinucleotide).