Main classes and properties of lipids
By chemical nature, lipids are hydrophobic derivatives of higher fatty (carboxylic) acids. They are completely insoluble in water, but readily soluble in organic liquids such as alcohol, ether, or benzene. The following key classes are distinguished in the human body:
- 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 body mass, but in pathological obesity, this figure can reach 50%. Depending on their physical state, a distinction is made between solid fats (containing saturated acids) and liquid oils (based on unsaturated fatty acids).
Essential fatty acids
A number of fatty acids are called essential because they cannot be synthesized by the body's cells and must be obtained exclusively from food. These include:
- Oleic acid
- Linoleic acid
- Linolenic acid
- Arachidonic acid
Plant fats serve as the main source of linoleic and linolenic acids, while animal fats are the primary source of arachidonic acid. The latter plays a critically important role in metabolism: it is a precursor to potent biologically active substances (prostaglandins, prostacyclins, thromboxanes, and leukotrienes). A deficiency of such acids in the diet leads to growth retardation, infertility, and severe skin diseases.
Metabolism and energy function
Lipids act as a slowly mobilizable energy substrate. During total starvation, a significant depot reserve is able to maintain the body's viability for more than a month.
Biochemistry of oxidation:
- Fatty acids are broken down to form acetyl-CoA (acetyl-coenzyme A).
- Acetyl-CoA enters the Krebs cycle (tricarboxylic acid cycle).
- The final stage involves breakdown into carbon dioxide ($CO_2$) and water ($H_2O$).
Tissues with a high metabolic rate (e.g., the brain and skeletal muscles) actively utilize not only fatty acids, but also ketone bodies (acetone, acetoacetic acid, and $\beta$-hydroxybutyric acid). Triglycerides can enter the blood from the intestines (exogenous), from adipose tissue via lipolysis (endogenous), or be synthesized in the liver from proteins and carbohydrates (lipogenesis).
Physiological Functions
In addition to energy production, lipids perform a number of vital tasks:
- Plastic. Phospholipids form the framework of plasma membranes. The nervous system has a high content of sphingomyelins, which act as electrical insulators within myelin sheaths.
- Transport. Fats are absolutely essential for the absorption of fat-soluble vitamins in the intestine.
- Protective and thermal insulation. Subcutaneous adipose tissue and brown fat depots retain heat, while fat layers around organs create a shock-absorbing mechanical "casing".
- Regulatory. Lipids serve as the basis for the synthesis of steroid hormones, directly participating in the regulation of sexual function in men and women.
Regulation of lipolysis and lipogenesis
The interrelation between fat and carbohydrate metabolism is strictly regulated by the hormonal system, with acetyl-CoA acting as the main connecting link.
- Activation of catabolism. Fat breakdown (lipolysis) is enhanced by adrenal hormones (adrenaline, glucocorticoids), thyroid hormones, and growth hormone (somatotropin). This is precisely why chronic stress, accompanied by activation of the sympathoadrenal system, leads to the depletion of fat reserves.
- Activation of anabolism. In hyperglycemia (excess plasma glucose), insulin is released. It stimulates the deposition of fatty acids in adipose tissue. Interestingly, insulin deficiency, observed in diabetes mellitus, is also frequently combined with pathological obesity due to a complex systemic metabolic disruption.
Briefly: answers to topical questions
What is the difference between reserve fat and protoplasmic fat?
The amount of protoplasmic fat, which is a structural component of cells, remains unchanged in the body. Reserve (neutral) fat is stored in adipose tissue (including in the form of brown fat) and can range from 10–20% of body mass under normal conditions to 50% in pathological obesity, acting as a nutrient reserve and a slowly mobilizable energy substrate consumed during starvation.
In what transport forms are lipids carried through the blood?
In blood plasma, lipids are transported by plasma proteins and lipoproteins. The transport forms of lipids are:
- Albumins — transport fatty acids.
- Chylomicrons — transport particles for exogenous fats.
- VLDL (very low-density lipoproteins) — the main lipid component: triacylglycerols (~55%).
- LDL (low-density lipoproteins) — the main lipid component: cholesterol and its esters (~50%).
- HDL (high-density lipoproteins) — contain about 50% fats; mature HDL serve as donors of apoE and apoC-II for chylomicrons and VLDL.
What specific biochemical reactions are included in the beta-oxidation cycle of fatty acids?
The $\beta$-oxidation cycle of fatty acids consists of four sequential reactions, resulting in the shortening of the fatty acid by two carbon atoms with the cleavage of acetyl-CoA:
- Dehydrogenation.
- Hydration.
- Dehydrogenation.
- Cleavage of acetyl-CoA.
This cycle repeats until the entire acid is converted into acetyl-CoA residues.
An important physiological rule: "Fats burn in the flame of carbohydrates." This means that for the complete oxidation of ketone bodies, the adequate presence and breakdown of carbohydrates in the body are necessary.
Class and exam questions
What is the difference between fats and oils from a biochemical perspective?
Fats contain saturated fatty acids and solidify at room temperature. Oils consist of unsaturated fatty acids and remain in a liquid state.
What are the main pathways of triglyceride utilization?
Triglycerides undergo catabolism to produce energy, are stored in fat depots, or participate in gluconeogenesis — the process of converting non-carbohydrate compounds into glucose.
Where in the body is the majority of endogenous phospholipids synthesized?
The bulk of endogenous phospholipids is synthesized in the liver, with only hepatocytes capable of secreting them directly into the systemic circulation.
What else to study on this topic
- Mechanisms of absorption and transport of fat-soluble vitamins
- Biosynthesis and functions of eicosanoids from arachidonic acid
- Features of thermogenesis in brown adipose tissue
- Biochemistry of the tricarboxylic acid cycle (Krebs cycle)
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