Carbohydrates, Fats, and Proteins: Essential Biomolecules

Posted on Dec 17, 2024 in Biology

Carbohydrates

Carbohydrates are biomolecules formed by C (Carbon), H (Hydrogen), and O (Oxygen). There is always a carbonyl group, which may be an aldehyde or a ketone.

Classification

  • Monosaccharides: Single-chain polyhydroxy aldehydes or polyhydroxy ketones. They can have 3-7 carbons.
    • Physical Properties: White, crystalline solids, soluble, sweet flavor.
    • Chemical Properties: Reduce Fehling’s reagent, ability to aminate.
  • Oligosaccharides: 2-10 monosaccharides.
  • Polysaccharides: More than 10 monosaccharides.

Carbohydrates can combine with lipids (glycolipids) or proteins (glycoproteins).

Trioses

There are two trioses: dihydroxyacetone and glyceraldehyde. The second carbon (C2) is asymmetric.

Tetroses

There are two aldotetroses (erythrose and threose), with two asymmetric carbons, and one ketotetrose (erythrulose).

Pentoses

  • D-ribose: Found in RNA.
  • D-2-deoxyribose: Found in DNA.
  • D-ribulose: Reacts with CO2.

Hexoses

  • Glucose: Provides most of the energy needed by cells. In solution, the molecules form a hexagon (1-5) and are called glucopyranose. It joins to form polymers with reserve functions (polysaccharides). Also called dextrose because it is dextrorotatory.
  • Fructose: Has a pentagonal shape and is called fructofuranose. It forms sucrose with glucose. Also called levulose because it is levorotatory.

Disaccharides

Disaccharides are formed by the union of two monosaccharides. The aldehyde group of one monosaccharide meets any radical of another monosaccharide. A water molecule (H2O) is released, and the two monosaccharides are joined by an oxygen atom (O-glycosidic bond).

There are two types of glycosidic bonds:

  • Monocarbonyl: Between the carbonyl carbon of one monosaccharide and a non-carbonyl carbon of another monosaccharide. The disaccharide retains the reducing capacity of Fehling’s reagent. Examples: maltose, cellulose, lactose.
  • Dicarbonyl: Between the carbonyl carbons of both monosaccharides. The disaccharide loses the reducing capacity of Fehling’s reagent. Example: sucrose.
  • Maltose: Two molecules of D-glucopyranose with an α(1-4) bond.
  • Cellobiose: Two molecules of D-glucopyranose with a β(1-4) bond.
  • Lactose: One molecule of D-galactopyranose with a β(1-4) bond.
  • Sucrose: One molecule of D-glucopyranose with an α(1-2) bond.

Polysaccharides

Polysaccharides are formed by many monosaccharides. They have a high molecular weight, are amorphous, have no sweet taste, and do not reduce Fehling’s reagent.

There are two types:

  • Homopolysaccharides: Polymers with one type of monosaccharide.
  • Heteropolysaccharides: Polymers with more than one type of monosaccharide.

If they have β-glycosidic bonds, they have a structural function. Examples: cellulose (provides consistency and rigidity to the cell wall of plants), chitin (forms the exoskeleton of arthropods).

If they have α-glycosidic bonds, they serve as energy reserves. Examples: starch (vegetable reserves), glycogen (animal reserves).

Fats

Fats are soluble in organic solvents but not in polar solvents. They are constituted by C and H, and some also contain O, P, N, and S.

  • Saturated Fatty Acids: Have simple bonds. Hydrocarbon chains are linear.
  • Unsaturated Fatty Acids: Have one or more double bonds between carbons in the hydrocarbon chain. The molecules are not straight. If they only have one double bond, they are monounsaturated.

Chemical Properties

  • Esterification Reaction: Reaction of one fatty acid with one alcohol, yielding one ester and one water molecule.
  • Saponification Reaction: Reaction of one fatty acid with NaOH or KOH, yielding soap.

Physical Properties

  • Solubility: Fatty acids with 4-6 carbons are soluble in water; those with more than 8 carbons are not. Bipolar molecules have one hydrophilic part and one hydrophobic part. They can form micelles, monolayers, or bilayers, depending on whether they have water inside. When micelles trap air, they have a sparkling effect; if they contain lipid droplets, they have an emulsifying or detergent effect.
  • Melting Point: The more carbons, the more energy is needed to break the bonds, and the higher the melting point.

Proteins

Proteins are polymers of amino acids. They have a low molecular weight. They contain a carboxyl group (-COOH) and an amine group (-NH2).

Peptide Bond

A peptide bond is established between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule.

Primary Structure

The primary structure is the amino acid sequence of the protein. The first amino acid has the free amino group (N-terminal). The last amino acid has the free carboxyl group (C-terminal).

Secondary Structure

The secondary structure is the spatial arrangement of the primary structure. There are three types: α-helix, collagen helix, and β-conformation.

Tertiary Structure

The tertiary structure is the spatial arrangement of the secondary structure when it folds on itself, resulting in a globular conformation.

Quaternary Structure

The quaternary structure is present in proteins composed of two or more polypeptide chains, with identical or different tertiary structures, linked by bonds.

Properties

  • Solubility: Higher proportion of amino acids with polar radicals than with nonpolar radicals.
  • Denaturation: Loss of the tertiary and secondary structures.
  • Renaturation: Some proteins can restore their initial conformation.
  • Specificity.
  • Buffering Capacity: Can behave as an acid, releasing protons, or as a base, releasing hydroxyl groups.