Biological Macromolecules: Lipids, Nucleic Acids, and Proteins

Lipids

Triglycerides

Triglycerides are a major class of lipids that function primarily as energy storage molecules in living organisms.

• Structure: Composed of one glycerol molecule and three fatty acid chains.

• Formation: Glycerol + 3 fatty acids → Triglyceride + 3 H2O (via dehydration synthesis)

• General Formula: $\text{Triglyceride} = \text{Glycerol} + 3 \times \text{Fatty Acid}$

Types of Triglycerides

• Saturated Fats: Contain only single bonds between carbon atoms in the fatty acid chains.

  • Solid at room temperature

  • Common in the animal kingdom (e.g., bacon)

• Unsaturated Fats: Contain one or more double bonds in the fatty acid chains.

  • Liquid at room temperature

  • Common in the plant kingdom (e.g., vegetable oil)

Common name for liquid lipids: oil

Example: Margarine is made by hydrogenating unsaturated triglycerides to make them solid at room temperature (by creating single bonds instead of double bonds).

Phospholipids

Phospholipids are essential components of cell membranes, characterized by their amphipathic nature.

• Structure: Contain a phosphate functional group, two fatty acid tails, and a glycerol backbone.

• Amphipathic: Have both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Function: Form the phospholipid bilayer of cell membranes, with hydrophilic heads facing outward and hydrophobic tails facing inward.

Example: The cell membrane consists of two layers of phospholipids (phospholipid bilayer).

Steroids

Steroids are a class of lipids with a characteristic four-ring structure.

• Functions:

  • Regulatory: Many hormones are steroids (e.g., sex hormones, adrenal cortical hormones such as aldosterone).

  • Defense and other roles (e.g., in cell membranes).

Nucleic Acids

Functions of Nucleic Acids

Nucleic acids are macromolecules responsible for the storage, transmission, and expression of genetic information.

• Information storage: DNA (deoxyribonucleic acid)

• Information transfer: RNA (ribonucleic acid)

• Energy transfer: ATP (adenosine triphosphate)

Structure of Nucleic Acids

• Polymers of nucleotide monomers: Nucleotides are the building blocks of nucleic acids.

• Nucleotide Structure:

  1. Five-carbon sugar (pentose): deoxyribose in DNA, ribose in RNA

  2. Nitrogenous base: can be a single or double ring, rich in nitrogen (e.g., adenine, guanine, cytosine, thymine, uracil)

  3. Phosphate functional group

Example: DNA and RNA are polymers of nucleotides linked by phosphodiester bonds.

Proteins

Functions of Proteins

Proteins are versatile macromolecules that perform a wide range of functions in living organisms.

• Structural: Provide support (e.g., collagen in cartilage, tendons, skin, and hair)

• Mechanical: Involved in muscle contraction (e.g., actin, tubulin)

• Receptor: Serve as receptors for signals (e.g., receptor proteins in tongue and nose)

• Regulatory: Many hormones are proteins (e.g., insulin, growth hormone)

• Defense: Antibodies are proteins that defend against pathogens

• Transport: Hemoglobin transports oxygen; channel proteins move ions and molecules across membranes

• Venomous: Some proteins function as venoms for immobilizing prey

• Catalysts: Enzymes are proteins that speed up chemical reactions

  • Organic catalysts made by living organisms are called enzymes

  • 99% of enzymes are proteins

Levels of Protein Structure

Proteins have four levels of structural organization, each contributing to their final shape and function.

• Primary Structure: The unique sequence of amino acids in a polypeptide chain.

  • Peptide bonds (strong covalent bonds) link amino acids.

  • The order of amino acids determines the protein's properties.

• Secondary Structure: Local folding of the polypeptide chain into structures such as:

  • Alpha helix: Resembles a coiled spring (like an old telephone cord); held together by hydrogen bonds.

  • Beta pleated sheet: More wavy, sheet-like structure; also stabilized by hydrogen bonds.

Amino Acid Structure

• Each amino acid has a central carbon (alpha carbon) bonded to:

  • A hydrogen atom

  • An amino group ($-NH_2$)

  • A carboxyl group ($-COOH$)

  • An R group (side chain) that is unique for each amino acid

• The R group determines the properties of the amino acid (polar, nonpolar, or ionic).

• There are 20 standard amino acids used by all living things to make proteins.

Example: The presence of the same 20 amino acids in all living organisms is strong evidence for a common ancestor.

Table: Comparison of Lipid Types

Type	Structure	Function	Example
Triglyceride	Glycerol + 3 fatty acids	Energy storage	Butter, oil
Phospholipid	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure	Phospholipid bilayer
Steroid	Four fused carbon rings	Hormones, membrane component	Cholesterol, testosterone

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology curriculum.