Lipids and Proteins: Structure, Types, and Functions

Posted on Apr 1, 2025 in Biology

Lipids: Structure and Functions

Waxes

Waxes result from the esterification of a long-chain alcohol with a fatty acid. They primarily function to prevent dehydration.

Esterification Reaction Example:
CH₃-(CH₂)_n-CH₂OH + HOOC-(CH₂)_m-CH₃ → CH₃-(CH₂)_n-CH₂-OOC-(CH₂)_m-CH₃ (Wax) + H₂O

An example is beeswax.

Unsaponifiable Lipids

These lipids cannot be hydrolyzed by saponification.

Steroids

Steroids are lipids derived from the cyclopentanoperhydrophenanthrene structure. Their structure consists of four fused carbon rings, distinguished by the number and location of substituents.

Key types of steroids include:

• Sterols: Cholesterol is the most biologically significant sterol. It is a crucial component of biological membranes, providing rigidity, and serves as a precursor for most other steroids.
• Bile Acids: These emulsify dietary fats, aiding their digestion and intestinal absorption.
• Steroidal Hormones: Examples include:
  • Hormones from the adrenal cortex, such as cortisol (stimulates glycogen synthesis, fat and protein degradation) and aldosterone (regulates water and mineral excretion by the kidney).

Terpenes

Terpenes are formed by the polymerization of isoprene units. These molecules are abundant in plants and are classified by the number of isoprene units they contain.

• Monoterpenes (2 isoprene units): Include essential oils found in many plants, such as menthol.
• Diterpenes (4 isoprene units): Notable examples include phytol (a component of chlorophyll), vitamins A, E, and K, and carotenes.
• Polyterpenes (many isoprene units): Natural rubber is a significant example.

Functions of Lipids

• Energy Reserve: Lipids constitute body fat. One gram of fat yields 9.4 kcal, whereas carbohydrates yield only 4.1 kcal/gram.
• Structural: Lipids form the bilayers of biological membranes and cell organelles.
• Transport: Lipid transport from the intestine to adipose tissue involves emulsification by bile acids, allowing transport via blood and lymph (as lipoproteins).
• Hormonal: Steroids function as hormones.
• Vitamin Precursors: Vitamins A, D, K, and E are lipids or derived from them.
• Protective: Lipids provide thermal insulation (especially in animals in cold climates), mechanical protection (shock absorption), and prevent dehydration.

Proteins: Essential Biological Molecules

Proteins are essential chemical constituents of living matter. They act as the “molecular tools” through which genetic information is expressed. They are also structural (“plastic”) substances for living beings, and many possess significant biological activity.

Protein Composition and Classification

Proteins are organic macromolecules with high molecular weight, primarily composed of Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N). They may also contain Sulfur (S) and Phosphorus (P).

Amino Acid Classification

Amino acids, the building blocks of proteins, are classified based on their R group (side chain):

• Nonpolar: The R group is hydrophobic. These amino acids interact with others primarily through Van der Waals forces.
• Polar Uncharged: The R group is hydrophilic but carries no net charge. They interact via hydrogen bonds.
• Polar Charged:
  • Acidic: The R group contains a carboxyl group (COO^–) and carries a negative charge at physiological pH.
  • Basic: The R group contains an amino group (NH₃⁺) and carries a positive charge at physiological pH.

Protein Functions

Proteins perform a vast array of functions in living organisms:

• Structural: Glycoproteins form parts of cell membranes, acting as receptors or facilitating transport. Histones are structural proteins within chromosomes that help regulate gene expression.
• Enzymatic: Enzymes are proteins that act as biological catalysts, speeding up chemical reactions in cellular metabolism. This is the largest and most specialized group of proteins.
• Hormonal: Some hormones are proteins, such as insulin and glucagon (regulating blood glucose), pituitary hormones, and growth hormone.
• Defensive:
  • Immunoglobulins (antibodies) defend against antigens.
  • Thrombin and fibrinogen participate in blood clotting to prevent hemorrhage.
  • Mucins protect mucosal surfaces.
  • Some toxins produced by organisms (e.g., botulinum toxin, snake venom) are proteins with defensive roles.
• Transport:
  • Hemoglobin transports oxygen in the blood of vertebrates.
  • Hemocyanin transports oxygen in the blood of some invertebrates.
  • Cytochromes transport electrons in cellular respiration and photosynthesis.
  • Lipoproteins transport lipids in the blood.
• Contractile: Actin and myosin are proteins responsible for muscle contraction.
• Reserve: Proteins like ovalbumin (in egg white) and lactalbumin (in milk) serve as a reserve supply of amino acids for developing embryos or newborns.