Understanding Lipids: Types and Functions

• Lipids encompass fats, waxes, sterols, fat-soluble vitamins (A, D, E, and K), mono-, di-, and triglycerides, phospholipids, and more.
• Lipids function in energy storage, signaling, and cell membrane structure.

Lipid Types

• Fatty acids are the simplest lipids.
• Glycerides consist of glycerol attached to one, two, or three fatty acids via ester linkages, forming mono-, di-, and triglycerides.
• Phospholipids resemble triglycerides but have a phosphate group with a polar head instead of a fatty acid.
• Sterols have a four-ring structure.
• Waxes are fatty acid esters with long-chain alcohols.

Fatty Acids

• Fatty acids, with long aliphatic tails, are either saturated (no double bonds) or unsaturated (one or more double bonds).
• Fatty acids are integral to complex lipids.
• Hydrocarbon chain length and unsaturation affect fatty acid properties like melting point.
• Naming uses carbon number (e.g., palmitic acid: 16-carbon saturated).
• Omega-3 and -6 fatty acids are essential and diet-derived.
• Fatty acids act as fuel and building blocks for complex lipids.

Glycerides

• Glycerides consist of glycerol linked to one, two, or three fatty acids via ester bonds.
• One fatty acid forms monoglycerides, two form diglycerides, and three form triglycerides.
• Triglycerides (triacylglycerols) are the major body and vegetable fat constituents.
• Triglycerides primarily store energy.
• Esterification synthesizes glycerides by attaching fatty acids to glycerol.
• Lipases hydrolyze glyceride ester bonds, releasing fatty acids and glycerol.

Phospholipids

• Phospholipids contain a phosphate group and one/two fatty acids on a glycerol backbone.
• They are amphipathic, possessing hydrophobic (fatty acid tails) and hydrophilic (phosphate head) regions.
• Biological membranes are largely composed of phospholipids, forming lipid bilayers.
• Polar head group variation yields different phospholipids like phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine.
• Phospholipids participate in cell signaling, molecular recognition, and act as emulsifiers.
• Synthesis starts from glycerol-3-phosphate and fatty acids, modified by adding the head group.

Sterols

• Sterols feature a four-ring steroid nucleus.
• Cholesterol is the most prevalent animal sterol, a precursor to steroid hormones and bile acids.
• Sterols in cell membranes modulate fluidity.
• Cholesterol synthesis proceeds from acetyl-CoA via a complex, multi-enzyme pathway.
• Steroid hormones (testosterone, estrogen, cortisol) come from cholesterol, regulating physiological processes.
• Lipoproteins transport sterols in blood.

Waxes

• Waxes are long-chain fatty acid esters with long-chain alcohols.
• They are hydrophobic with high melting points.
• Waxes protect leaves, fruits, and skin, preventing water loss and pathogen entry.
• Natural waxes include beeswax, carnauba wax, and lanolin.
• Uses include candles, polishes, and cosmetics.

Lipid Functions

• Energy Storage: Triglycerides are the main energy reserve in animals and plants.
• Structural Components: Phospholipids and cholesterol are key in cell membranes.
• Signaling Molecules: Steroid hormones and eicosanoids act as signaling molecules, regulating physiology.
• Insulation: Subcutaneous fat provides thermal insulation.
• Protection: Waxes coat surfaces, preventing water loss and pathogen entry.
• Digestion and Absorption: Lipids aid fat-soluble vitamin use.
• Enzyme Cofactors: Some lipids serve as enzyme cofactors.

Lipid Metabolism

• Lipid digestion involves lipases breaking down triglycerides into fatty acids and glycerol.
• Lipoproteins (chylomicrons, VLDL, LDL, HDL) transport fatty acids in blood.
• Beta-oxidation breaks down fatty acids into acetyl-CoA for the citric acid cycle, yielding energy.
• Lipogenesis synthesizes fatty acids from acetyl-CoA, mainly in the liver and adipose tissue.
• Cholesterol metabolism manages cholesterol synthesis, transport, and excretion.

Lipid Transport

• Lipoproteins (lipid-protein complexes) transport lipids in the blood.
• Chylomicrons transport dietary triglycerides from the intestine.
• VLDL transports liver-synthesized triglycerides.
• LDL carries cholesterol from the liver.
• HDL transports cholesterol from tissues to the liver for excretion.
• Dyslipidemia (abnormal lipid levels) elevates cardiovascular disease risk.

Essential Fatty Acids

• Essential fatty acids (linoleic acid [omega-6] and alpha-linolenic acid [omega-3]) cannot be synthesized and must be dietary.
• They are precursors to arachidonic acid, EPA, and DHA.
• They support brain development, immune function, and inflammation regulation.
• Deficiency leads to skin disorders, stunted growth, and neurological problems.