Classification and Physiological Role
Carbohydrates are ingested primarily through diet. During gastrointestinal digestion, complex molecules are broken down, and ultimately all forms of carbohydrates are converted into glucose.
The basic classification of carbohydrates includes three groups:
- Monosaccharides: Simple sugars (glucose, fructose, pentoses).
- Disaccharides: Molecules consisting of two monosaccharides (e.g., sucrose, composed of glucose and fructose).
- Polysaccharides: Complex compounds divided into plant-based (starch, cellulose) and animal-based (glycogen). Hyaluronic acid and heparin also belong to this group.
The primary function of carbohydrates is energetic, acting as the fastest source of energy. However, they also fulfill an important plastic (structural) function: participating in lipogenesis (conversion to lipids), and the formation of glycoproteins and glycolipids. Intermediate oxidation products—pentoses—are incorporated into the structure of nucleotides and nucleic acids. Additionally, glucose serves as a metabolic precursor for several amino acids. Notably, vitamin C can be synthesized from carbohydrates, but this pathway does not occur in humans, making ascorbic acid an essential dietary requirement.
Intracellular Metabolism and Transport
For carbohydrates to be utilized, glucose must enter the cell. Transport is mediated by carrier proteins via facilitated diffusion. This process is tightly regulated: the hormone insulin significantly enhances glucose uptake by skeletal muscle and liver cells.
Immediately upon entry, the glucose molecule is phosphorylated by specific enzymes (glucokinase or hexokinase). Subsequent metabolism proceeds in stages:
- Cytoplasmic Glycolysis: Under the influence of intracellular enzymes, glucose is transformed into pyruvic acid (pyruvate) via aerobic glycolysis.
- Mitochondrial Stage: Pyruvate is converted into acetyl-coenzyme A (acetyl-CoA), which then enters the mitochondria for complete oxidation and energy production.
Glucose Homeostasis
Blood sugar concentration is a vital constant that also helps maintain blood osmotic pressure. Glucose distribution among organs follows approximate proportions: in addition to the brain (12%), significant amounts are taken up by the intestines (9%) and skeletal muscles (7%).
Primary sources of blood glucose:
- Dietary: Absorption from the GI tract following digestion.
- Glycogenolysis: Release from the liver via the breakdown of glycogen (animal starch).
- Gluconeogenesis: Synthesis from proteins and lipids. This pathway rescues the body during prolonged starvation or marked dietary carbohydrate deficiency.
Glucose is consumed via tissue metabolism (aerobic or anaerobic glycolysis, pentose phosphate pathway). Excess carbohydrates are stored as glycogen (glycogenesis) or converted into fats (lipogenesis). If blood glucose exceeds the renal threshold, it is excreted in the urine (glucosuria).
A drop in blood sugar leads to hypoglycemia, manifested by generalized weakness and rapid fatigue. A critical drop causes coma accompanied by loss of consciousness, convulsions, delirium, and profuse sweating.
Regulation of Carbohydrate Metabolism
Maintenance of blood glucose is governed by a self-regulatory functional system. The main neural center is the hypothalamus-pituitary axis. Efferent signals are transmitted via the sympathetic and parasympathetic divisions of the autonomic nervous system, as well as the endocrine system.
Hormonal balance relies on the interaction of two antagonistic groups:
- Insulin: The only hormone that lowers blood glucose. It accelerates cellular glucose transport, stimulates glycogen synthesis (glycogenesis), and simultaneously inhibits sugar-producing pathways (glycogenolysis and gluconeogenesis).
- Counterregulatory Hormones: Elevate blood glucose levels. These include epinephrine, glucocorticoids, glucagon, and pituitary hormones—adrenocorticotropic hormone (ACTH) and growth hormone (somatotropin, GH).