Carbohydrates and Lipids: Structure, Classification, and Functions

1.2. Carbohydrates and Lipids: Concept, Classification, and Functions

Carbohydrates

Carbohydrates are biomolecules formed fundamentally by C, H, and O in the proportion C_nH_2nO_n. Hence, H and O are in the same proportion as in water (H₂O), also called hydrates of carbon. Carbohydrate molecules are chains formed by carbon atoms attached to radical groups and hydroxyl (OH) groups. There may be an alcoholic or a carbonyl group; this group may be an aldehyde (gluc aldose) or a ketone (gluc ketonic). For this reason, they are defined as polyhydroxyaldehydes or polyhydroxyketones.

Classification:

Carbohydrates are classified in two forms:

1. According to the functional group they possess:
   • Ketoses: if they have ketone groups.
   • Aldoses: if they have aldehyde groups.
2. According to their complexity:
   1. Monosaccharides: are simple, non-hydrolyzable carbohydrates.
   2. Oligosaccharides: are the union of 2 to 10 monosaccharides. They are classified as:
      1. Holosides: composed only of monosaccharides. They can be:
         • Oligosaccharides: formed by 2 to 10 monosaccharides.
         • Polysaccharides: formed by more than 10 monosaccharides.
           • Homopolysaccharides: formed by only one type of monosaccharide.
           • Heteropolysaccharides: formed by different monosaccharides.
      2. Heterosides: composed of monosaccharides and other substances that are not carbohydrates. They may be glycolipids and glycoproteins.

Functions:

1. Energy Reserve: Carbohydrates are metabolic fuel; the cell uses energy to perform cellular work.
2. Structural Links: Deoxyribose and ribose are part of the structure of DNA and RNA. Cellulose forms the cell wall in plants. Chitin forms the cell wall in fungi. Peptidoglycans form the bacterial cell wall. Mucins intervene in the formation of the extracellular matrix of connective tissue, bone, and cartilage. Hyaluronic acid forms the coat of oocytes.
3. Antibiotics and antitumor agents.
4. Anticoagulants.
5. Markers in cell biology and molecular recognition sites; they behave as antennas and molecular carriers able to recognize other molecules, serving as an identity card to differentiate various cells of the body.

Lipids

Lipids are organic biomolecules formed by C, H, O, N, and sometimes P and S. They all have in common the property of being "oily to the touch." They are insoluble in water but soluble in organic solvents such as ether, gasoline, or benzene. They present fatty acids. Lipids are amphipathic molecules, having a polar (hydrophilic) and a nonpolar (hydrophobic) area, which causes them in an aqueous solvent to form lipid monolayers, bilayers, and micelles. There are two types: saponifiable and unsaponifiable.

Saponifiable Lipids:

Lipid derivatives that are esters of fatty acids. They possess ester linkages that can lead to hydrolysis and the formation of soaps by alkaline reaction. There are three types:

• Fatty Acids: Organic monocarboxylic acids formed by an open carbon chain. They can be:
  • Saturated: No double bonds; the hydrocarbon chain is straight.
  • Unsaturated: Double bonds, forcing the chain to bend.
• Simple Lipids:
  • Fats: Glycerol esters, formed because they are saponified from fatty acids by esterification reactions.
  • Waxes: Simple lipids resulting from the ester bond between a fatty acid and a monoalcohol (e.g., beeswax).
• Complex Lipids:
  • Glycerolipids: Consisting of glycerol. There are two types:
    • Glycerophospholipids (e.g., phosphatidic acid).
    • Glyceroglycolipids, where glycerol is linked via two fatty acids and by an O-glycoside to a monosaccharide.
  • Sphingolipids: Two types, sphingophospholipids and sphingoglycolipids.

Unsaponifiable Lipids:

Not possessing ester linkages, they cannot give rise to soaps by alkaline hydrolysis. There are three types:

• Terpenes: Polymers of isoprene molecules.
• Steroids: Gonane-derived compounds.

Functions:

• Energy: Unlike carbohydrates, there is no need to transform them; they are stored as fat. When necessary, they are hydrolyzed into glycerol and fatty acids.
• Structure: Part of biological membranes, forming the lipid bilayer, thus separating different aqueous compartments inside cells.
• Vitamins: Vitamin A, K, E, and D.
• Hormonal: Sex hormones, adrenal cortex hormones, prostaglandins.
• Heat Production: Their combustion does not result in ATP, but heat.