Storage Lipids and Membrane Lipids: Lipids play a crucial role in biological systems, serving as energy reserves (storage lipids) and structural components of cell membranes (membrane lipids).

1. Storage Lipids

Definition: Storage lipids are neutral lipids that serve as energy reserves in plants and animals. They are primarily composed of triacylglycerols (TAGs), which consist of three fatty acids esterified to a glycerol backbone.

Structure of Storage Lipids (Triacylglycerols – TAGs)

• Glycerol backbone (3-carbon molecule).
• Three fatty acid chains attached via ester bonds.
• They are nonpolar, hydrophobic, and water-insoluble, making them ideal for energy storage.

Functions of Storage Lipids

• Energy Storage → Provide 9 kcal/g (higher than carbohydrates and proteins).
• Thermal Insulation → Stored in adipose tissue to regulate body temperature.
• Protection of Organs → Fat deposits cushion vital organs (kidneys, heart).
• Metabolic Fuel → Mobilized during fasting or energy-demanding conditions.

Examples of Storage Lipids

• Animal fats → Butter, lard.
• Vegetable oils → Olive oil, coconut oil.
• Adipose tissue → Main energy reserve in animals.

2. Membrane Lipids

Definition: Membrane lipids are amphipathic (having both hydrophilic and hydrophobic parts) and form biological membranes that regulate the movement of substances in and out of cells.

Types of Membrane Lipids

1. Phospholipids → Glycerophospholipids & Sphingophospholipids.
2. Glycolipids → Carbohydrate-attached lipids found in cell membranes.
3. Sterols (Cholesterol) → Provides membrane stability and fluidity.

Structure of Membrane Lipids

• Polar (hydrophilic) head group → Interacts with water.
• Nonpolar (hydrophobic) fatty acid tails → Faces inward in bilayer formation.
• Forms lipid bilayers in aqueous environments.

Functions of Membrane Lipids

• Structural Integrity → Forms lipid bilayer of cell membranes.
• Selective Permeability → Regulates transport of molecules (via passive/active transport).
• Signal Transduction → Lipids participate in cell signaling (e.g., phospholipids in signal cascades).
• Cell Recognition & Communication → Glycolipids are important for cell-cell interactions.

Examples of Membrane Lipids

Functions of Lipids

Lipids perform various essential functions in biological systems, including:

1. Energy Storage → Triacylglycerols serve as long-term energy reserves.
2. Cell Membrane Structure → Phospholipids and cholesterol help form the lipid bilayer.
3. Insulation & Protection → Adipose tissue helps in thermal regulation and cushions organs.
4. Hormone Synthesis → Steroid hormones (testosterone, estrogen, cortisol) are derived from cholesterol.
5. Signaling Molecules → Lipid-derived molecules (prostaglandins, leukotrienes) play roles in inflammation & immunity.
6. Transport of Fat-Soluble Vitamins → Lipids help in the absorption of vitamins A, D, E, and K.
7. Nerve Function → Myelin sheath (rich in sphingolipids) protects neurons and facilitates nerve conduction.