Biología e Introducción a la Biología Celular (054): Org. molec.de las células: Hidratos de carbono

The seminar introduces the molecular organization of a cell, emphasizing how carbohydrates and other biomolecules are crucial for its structure and function. It compares a human cell to a water droplet to illustrate that life's complexity stems from molecular organization, not just elemental composition. The seminar outlines the chemical components of cells: water, inorganic ions, and carbon-containing organic molecules.

Water is the most abundant molecule in living beings, making up 75-85% of cell volume. Its polar structure allows for hydrogen bonds, leading to key properties like cohesion, adhesion, high specific heat, and high heat of vaporization, all essential for life. Water's polarity makes it an excellent solvent for hydrophilic (polar and ionic) molecules. Non-polar molecules are hydrophobic and tend to clump together. Amphipathic molecules, with both polar and non-polar parts, are fundamental for cell membranes. Water can also ionize, creating hydrogen and hydroxyl ions, which determine pH. Cellular pH must be tightly regulated by buffers to maintain molecular function.

Inorganic ions constitute 2-3% of cellular composition and are unevenly distributed. Cations like potassium and magnesium, and anions like phosphate and bicarbonate, are prevalent inside cells, while sodium and chloride are more concentrated extracellularly. These ions regulate osmotic and electrical balance, participate in electrical signaling, and active transport. Some ions, like calcium, iron, and magnesium, have structural roles or act as enzymatic cofactors. Trace elements like copper, zinc, and iodine are also essential for specific biological processes.

Six elements (carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur) account for 99% of living organisms. Carbon's unique ability to form four stable covalent bonds in a tetrahedral geometry allows for a vast array of complex organic structures, including linear, branched, and cyclic chains, and can form single, double, or triple bonds. This versatility makes carbon the structural backbone of all organic biomolecules.

The addition of specific atoms or groups to the carbon skeleton forms functional groups that dictate the chemical behavior of biomolecules. Examples include alcohols, carboxylic acids, amines, aldehydes, and ketones. These combinations give rise to four main classes of biomolecules: carbohydrates, lipids, proteins, and nucleic acids, each performing essential global functions across all cell types.

Carbohydrates, or glucids, are biomolecules composed of carbon, hydrogen, and oxygen (CH2O)n. Chemically, they feature a carbonyl group (aldehyde or ketone) and multiple hydroxyl groups. Their primary function is energetic, serving as quick fuel for cells, but they also have structural roles, like cellulose in plants.

Monosaccharides are the basic units of carbohydrates and a rapid energy source. They are classified by the number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses) and the position of the carbonyl group (aldoses or ketoses). Monosaccharides are sweet, crystalline, and water-soluble, exhibiting optical isomerism (D or L series) due to chiral carbons. The body primarily uses D-series sugars like D-glucose. They can exist in linear or cyclic forms, with the cyclic form involving an anomeric carbon that determines alpha or beta anomers. This interconversion is reversible in aqueous solution.

Monosaccharides link via glycosidic bonds, formed when a hydroxyl group reacts with an anomeric carbon, releasing water. The naming indicates the anomeric configuration (alpha or beta) and the carbons involved. The orientation and participating carbons are crucial for the resulting carbohydrate's structure, flexibility, and function. These bonds enable the formation of disaccharides, oligosaccharides, and complex polysaccharides.

Disaccharides consist of two monosaccharides joined by a glycosidic bond. Examples include sucrose (glucose + fructose, alpha 1-2 bond), maltose (two glucoses, alpha 1-4 bond, intermediate in starch digestion), and lactose (galactose + glucose, beta 1-4 bond, primary milk sugar). They are soluble, crystalline, and sweet, common in our diet. Human enzymes hydrolyze these bonds for absorption as energy sources. Lactose intolerance stems from lactase deficiency.

Oligosaccharides (3-20 monosaccharides) are vital for cell communication, often found on cell surfaces as glycoproteins and glycolipids. They act as identity markers, as seen in blood groups, where a single monosaccharide difference determines the type. Certain oligosaccharides in legumes and vegetables are indigestible by human enzymes, fermenting in the colon and causing digestive issues.

Polysaccharides (over 20 monosaccharides) have high molecular weights, lack sweetness, are often insoluble, and serve as energy stores or structural support. They are divided into homopolysaccharides (single sugar type) and heteropolysaccharides (different sugar types). Homopolysaccharides include starch (plant energy reserve) and glycogen (animal energy reserve), and structural ones like cellulose (plant cell walls) and chitin (insect/fungi exoskeletons). Heteropolysaccharides like glycosaminoglycans are important in connective tissues, providing hydration, elasticity, and mechanical support.

Starch, the plant glucose storage, consists of amylose (linear, alpha 1-4 glucose chain, helical shape) and amylopectin (branched, alpha 1-4 and alpha 1-6 bonds). Glycogen is the highly branched animal glucose storage, similar to amylopectin but with more frequent alpha 1-6 branches, allowing rapid glucose release. Cellulose, the most abundant natural polysaccharide, is a linear chain of glucose units with beta 1-4 bonds, forming strong fibrils for plant cell wall rigidity. Humans cannot digest cellulose but it provides essential dietary fiber. Glycosaminoglycans, complex heteropolysaccharides, form proteoglycans in connective tissues (skin, cartilage, tendons), providing structural support, elasticity, and hydration through water absorption.