Lipids and Proteins: Structure and Function in General Biology

Lipids: Structure, Types, and Biological Roles

Overview of Lipids

Lipids are a diverse group of hydrophobic biological molecules that include fats, oils, and phospholipids. They play essential roles in energy storage, membrane structure, and signaling within cells.

• Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon, forming the backbone of many lipids.

• Fats and oils are triglycerides, composed of glycerol and three fatty acids.

• Phospholipids are major components of cell membranes, containing a phosphate group in place of one fatty acid.

• Examples: Butter (solid fat, mostly saturated), vegetable oil (liquid fat, mostly unsaturated).

Components of Fats and Oils

Fats and oils are primarily made up of fatty acids and glycerol. Fatty acids are long hydrocarbon chains with a terminal carboxyl group.

• Fatty acids are classified by the length of their hydrocarbon chain and the presence or absence of double bonds.

• Glycerol is a three-carbon alcohol that serves as the backbone for triglycerides.

• Triglyceride formation: Three fatty acids are esterified to one glycerol molecule.

Table: Common Fatty Acids in Fats and Oils

The following table summarizes the structure and sources of several important fatty acids:

Name of Fatty Acid	Structure	Source
Butyric acid	CH3(CH2)2COOH	Butter
Caproic acid	CH3(CH2)4COOH	Butter, palm oil, coconut oil
Caprylic acid	CH3(CH2)6COOH	Palm oil, coconut oil
Capric acid	CH3(CH2)8COOH	Palm oil, coconut oil
Lauric acid	CH3(CH2)10COOH	Plants of lauric acid, coconut oil
Myristic acid	CH3(CH2)12COOH	Seed fats, butter, coconut oil
Palmitic acid	CH3(CH2)14COOH	Plant fats, peanut oil
Stearic acid	CH3(CH2)16COOH	Plant and animal fats

Structure of Triglycerides

Triglycerides are formed by the esterification of three fatty acids to a glycerol molecule. This process involves the removal of water molecules.

• Glycerol structure: $\mathrm{HOCH_2-CHOH-CH_2OH}$

• Triglyceride formation: $\mathrm{Glycerol + 3 Fatty\ Acids \rightarrow Triglyceride + 3 H_2O}$

• Example: Palmitic acid is a common fatty acid found in triglycerides.

Saturated vs. Unsaturated Fatty Acids

The physical and chemical properties of fats depend on the saturation of their fatty acid chains.

• Saturated fatty acids have no double bonds between carbon atoms; all carbons are saturated with hydrogen. They tend to be solid at room temperature (e.g., butter).

• Unsaturated fatty acids contain one or more double bonds, causing kinks in the chain and making them liquid at room temperature (e.g., vegetable oil).

• Monounsaturated fats have one double bond; polyunsaturated fats have two or more.

• Example: Oleic acid (monounsaturated), linoleic acid (polyunsaturated).

Phospholipids and Membrane Structure

Phospholipids are essential for forming biological membranes. They have a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

• Structure: Glycerol backbone, two fatty acid tails, and a phosphate group attached to a polar head (e.g., choline).

• Function: Form bilayers in cell membranes, creating a barrier between the cell and its environment.

• Example: The phospholipid bilayer is the fundamental structure of the plasma membrane.

Functions of Lipids

Lipids serve multiple biological functions beyond energy storage.

• Energy storage: Lipids provide long-term energy reserves, while carbohydrates are used for short-term energy needs.

• Membrane structure: Phospholipids and cholesterol are key components of cell membranes.

• Signaling: Some lipids act as hormones or signaling molecules.

Proteins: Structure and Function

Overview of Proteins

Proteins are polymers of amino acids and perform a vast array of functions in living organisms, including catalysis, transport, and structural support.

• Amino acids are the building blocks of proteins, each containing an amino group, a carboxyl group, and a unique side chain (R group).

• Polypeptides are chains of amino acids linked by peptide bonds.

• Example: Titin is a large protein (>34,000 amino acids); average proteins are 400-470 amino acids long.

Structure of Amino Acids

Each amino acid has a central carbon atom (alpha carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain.

• General formula: $\mathrm{NH_2-CHR-COOH}$

• Peptide bond formation: $\mathrm{Amino\ Acid_1 + Amino\ Acid_2 \rightarrow Dipeptide + H_2O}$

• Side chains determine the chemical properties and function of each amino acid.

Levels of Protein Structure

Proteins have up to four levels of structure, each contributing to their final shape and function.

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

• Secondary structure: Local folding patterns such as alpha helices and beta pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: The overall three-dimensional shape of a single polypeptide, determined by interactions among side chains (hydrophobic interactions, hydrogen bonds, ionic bonds, van der Waals forces, and disulfide bridges).

• Quaternary structure: The assembly of multiple polypeptide subunits into a functional protein complex (e.g., hemoglobin).

Protein Folding and Function

The folding of a protein determines its function. Misfolded proteins can lead to loss of function or disease.

• Hydrogen bonds stabilize secondary and tertiary structures.

• Disulfide bridges (covalent bonds between cysteine residues) stabilize tertiary and quaternary structures.

• Example: Hemoglobin is a quaternary protein with four subunits, each binding oxygen.

Summary Table: Levels of Protein Structure

Level	Description	Stabilizing Forces
Primary	Amino acid sequence	Peptide bonds
Secondary	Alpha helix, beta sheet	Hydrogen bonds
Tertiary	3D folding of polypeptide	Hydrophobic interactions, hydrogen bonds, ionic bonds, van der Waals forces, disulfide bridges
Quaternary	Assembly of multiple polypeptides	Same as tertiary, plus subunit interactions

Additional info:

• Phospholipids are amphipathic, meaning they have both hydrophilic and hydrophobic regions, which is critical for membrane formation.

• Cholesterol modulates membrane fluidity and is a precursor for steroid hormones.

• Glycoproteins and glycolipids are important for cell recognition and signaling.