Major Classes and Properties of Lipids
Chemically, lipids are hydrophobic derivatives of higher fatty (carboxylic) acids. They are completely insoluble in water, but readily soluble in organic solvents such as alcohol, ether, or benzene. The human body contains the following key lipid classes:
- Neutral fats (triglycerides) and their derivatives (mono- and diglycerides).
- Phospholipids.
- Cholesterol.
- Fat-soluble vitamins (A, D, E, K).
Normally, fat accounts for 10% to 20% of total body weight, though in pathological obesity this figure can reach 50%. Depending on their physical state at room temperature, lipids are classified as fats (containing saturated fatty acids) or oils (containing unsaturated fatty acids).
Essential Fatty Acids
Certain fatty acids are termed essential because they cannot be synthesized de novo by human cells and must be obtained exclusively from the diet. These include:
- Oleic acid (Note: oleic acid is conditionally essential/non-essential depending on context, but included per source list)
- Linoleic acid
- Linolenic acid
- Arachidonic acid
Plant-based fats serve as the primary source of linoleic and linolenic acids, whereas animal fats provide arachidonic acid. Arachidonic acid plays a critical metabolic role as a precursor to potent bioactive mediators (prostaglandins, prostacyclins, thromboxanes, and leukotrienes). Dietary deficiency of essential fatty acids leads to growth retardation, infertility, and severe dermatological lesions.
Metabolism and Energy Function
Lipids act as a slowly mobilizable energy substrate. During prolonged starvation, major fat depots can sustain vital body functions for over a month.
Biochemistry of Oxidation:
- Fatty acids undergo $\beta$-oxidation to form acetyl-CoA (acetyl-coenzyme A).
- Acetyl-CoA enters the citric acid cycle (Krebs cycle).
- Final oxidation yields carbon dioxide ($CO_2$) and water ($H_2O$).
Tissues with high metabolic demands (e.g., the brain and skeletal muscle) actively utilize not only fatty acids but also ketone bodies (acetone, acetoacetate, and $\beta$-hydroxybutyrate). Triglycerides may enter the bloodstream from the intestine (exogenous), from adipose tissue via lipolysis (endogenous), or be synthesized in the liver from proteins and carbohydrates (lipogenesis).
Physiological Functions
Beyond energy production, lipids perform numerous vital tasks:
- Structural. Phospholipids form the framework of plasma membranes. The nervous system is rich in sphingomyelins, which act as electrical insulators within myelin sheaths.
- Transport. Dietary fats are absolutely required for the intestinal absorption of fat-soluble vitamins.
- Protective and Thermal Insulation. Subcutaneous adipose tissue and brown fat depots preserve core body temperature, while fat pads surrounding internal organs provide a mechanical shock-absorbing cushion.
- Regulatory. Lipids serve as precursors for steroid hormone synthesis, directly participating in the regulation of male and female reproductive functions.
Regulation of Lipolysis and Lipogenesis
The interplay between lipid and carbohydrate metabolism is tightly regulated by the endocrine system, with acetyl-CoA acting as a key metabolic intersection.
- Upregulation of Catabolism. Fat breakdown (lipolysis) is stimulated by adrenal hormones (epinephrine, glucocorticoids), thyroid hormones, and growth hormone (somatotropin). Consequently, chronic stress involving activation of the sympathoadrenal system leads to the depletion of fat reserves.
- Upregulation of Anabolism. Hyperglycemia (elevated plasma glucose) triggers insulin release, which stimulates the storage of fatty acids in adipose tissue. Conversely, insulin deficiency, as seen in diabetes mellitus, is frequently associated with dyslipidemia and fat redistribution due to systemic metabolic disruption.