1.5 Lipids

Lipids include triglycerides, phospholipids, waxes and steroids. They are grouped mainly by shared physical behavior rather than by one universal monomer or structure: most are dominated by non-polar hydrocarbon regions, are insoluble in water and dissolve in non-polar organic solvents. Fats are triglycerides that are solid at a stated temperature, while oils are liquid. The terms describe physical state, not separate chemical families.

Triglycerides are the most abundant storage lipids. One glycerol molecule has three hydroxyl groups. Each can undergo condensation with the carboxyl group of a fatty acid, creating an ester bond. Complete assembly therefore joins one glycerol to three fatty acids by three ester bonds and produces three water molecules. The fatty acids may be identical or different, so organisms can synthesize a large range of triglycerides with different melting points and metabolic properties.

A fatty acid has a carboxyl group at one end and a hydrocarbon chain ending in a methyl group at the other. Common biological fatty acids often contain 14–22 carbons and usually have an even carbon number because synthesis adds two-carbon units. The carboxyl group is polar, but the much larger hydrocarbon region is non-polar. Once fatty acids are esterified into a triglyceride, the molecule has no strongly charged head and is overwhelmingly hydrophobic.

A saturated fatty acid contains no carbon–carbon double bonds, so its carbon chain carries the maximum number of hydrogen atoms. An unsaturated fatty acid contains at least one C=C bond. One double bond makes it monounsaturated; two or more make it polyunsaturated. Double bonds restrict rotation and their geometry matters. In a cis double bond, the continuing carbon chains lie on the same side; in a trans bond, they lie on opposite sides.

A cis double bond introduces a fixed bend or kink. Kinked chains cannot pack as closely, so intermolecular attractions are weaker and less thermal energy is needed to separate the molecules. Lipids rich in cis-unsaturated fatty acids therefore tend to have lower melting points and are often oils at room temperature. Saturated and trans-unsaturated chains remain straighter, pack more closely and tend to form solids at the same temperature.

Omega names locate the first double bond by counting from the methyl end. In an omega-3 fatty acid the first double bond begins at the third carbon from that end; in omega-6 it begins at the sixth. Humans cannot synthesize particular fatty acids in sufficient quantities and must obtain them from food, so they are described as essential. Essential does not mean that unlimited intake is harmless; it means a dietary source is required.

Lipids contain a high proportion of reduced carbon–hydrogen bonds and relatively little oxygen. Oxidizing their fatty acids transfers many high-energy electrons to respiratory carriers, so lipid yields about 39 kJ per gram compared with roughly 17 kJ per gram for carbohydrate. A lipid reserve holding the same chemical energy as a carbohydrate reserve can therefore have less than half the dry mass.

Hydrophobicity provides a second storage advantage. Triglycerides accumulate as droplets without dissolving, so they do not contribute many osmotically active particles and are stored without associated hydration water. Glycogen binds substantial water, increasing the mass of the reserve. Migrating animals, flying animals and animals fasting for long periods benefit from the high energy-to-mass ratio of lipid. Glycogen remains valuable because it can release glucose rapidly and support anaerobic respiration.

Fatty acids released by lipolysis are transported to cells and oxidized mainly in mitochondria. Carbon fragments enter aerobic respiration, generating ATP and ultimately carbon dioxide and water. Lipid is therefore a long-term store rather than an inert deposit. When energy intake persistently exceeds expenditure, surplus dietary lipid and carbohydrate can increase triglyceride storage; when demand exceeds intake, hormonal signals promote mobilization.

Body mass index is calculated as mass in kilograms divided by height in metres squared. It is a population screening measure, not a direct measurement of body fat or an individual diagnosis. Muscular people may have a high BMI without excessive fat, while older people with reduced muscle may have a lower BMI despite substantial fat. Ethnic, age and physiological differences affect interpretation. A biological conclusion should distinguish the numerical index from evidence about tissue distribution, metabolism and health.

A phospholipid commonly contains glycerol joined to two fatty acids and a phosphate-bearing group. The phosphate head is polar and hydrophilic, while the fatty-acid tails are non-polar and hydrophobic. A molecule with both regions is amphipathic. In water, phospholipids spontaneously arrange so heads interact with the aqueous environment and tails cluster away from it.

Two layers form a bilayer with hydrophobic tails facing inward and hydrophilic heads contacting water on both sides. This organization creates the basic barrier of every cell membrane. The bilayer is self-sealing because an exposed edge would force hydrophobic tails into water. Fatty-acid composition affects fluidity: cis-unsaturated tails create space and preserve movement at lower temperatures, while saturated tails pack tightly. Cholesterol modifies packing and stabilizes membrane fluidity across temperature changes.

The hydrophobic core restricts ions and most polar molecules but permits some small non-polar molecules to diffuse through. Membrane proteins provide selective routes and communication functions. Phospholipid architecture therefore does more than surround a cell: it creates controlled internal chemistry, supports concentration gradients and allows compartments with different conditions. These functions depend on amphipathic self-assembly rather than on phospholipids being rigid building blocks.

Adipocytes store triglycerides in lipid droplets. White adipose tissue is specialized for energy storage, cushioning around organs and thermal insulation beneath the skin. Lipid conducts heat poorly, so a thick subcutaneous layer reduces heat transfer. Marine mammals living in cold water rely on blubber because water removes body heat much faster than still air. Lipid is also less dense than water and contributes to buoyancy.

Brown adipose tissue contains many mitochondria and numerous smaller lipid droplets. It can oxidize fatty acids while allowing much of the energy to be released as heat rather than captured in ATP, contributing to thermogenesis. Adipose tissue is also endocrine: it releases signaling molecules including leptin and cytokines. It should therefore be understood as metabolically active tissue, not merely passive packing material.

Steroids share a framework of four fused carbon rings. Cholesterol is a membrane component and a precursor for steroid hormones, vitamin D and bile compounds. Small changes to side groups attached to the common ring skeleton produce molecules with different signaling roles. Unlike peptide hormones, steroid hormones are largely non-polar and can diffuse through the phospholipid bilayer.

After entering a target cell, a steroid hormone binds a specific intracellular receptor. The hormone–receptor complex changes conformation, associates with particular DNA regulatory sequences and influences transcription. Only cells expressing the appropriate receptor respond strongly. Lipid solubility explains membrane entry, but receptor specificity explains the targeted biological effect.

Epidemiological studies can reveal correlations between dietary patterns, blood-lipid profiles and cardiovascular disease. A positive correlation means that higher values of one variable tend to occur with higher values of another; it does not by itself show that one causes the other. Diet, exercise, smoking, age, genetics, socioeconomic conditions and medical treatment can covary, making causal interpretation difficult.

Industrial partial hydrogenation can convert some cis double bonds to trans configurations and make oils more solid and shelf-stable. High intake of industrial trans fats is associated with an unfavorable lipoprotein profile and increased cardiovascular risk. Saturated-fat intake can also affect blood lipoproteins, but the effect of changing a diet depends on what replaces that energy. Replacing one fat with refined carbohydrate is not biologically equivalent to replacing it with unsaturated fat.

Strong conclusions draw on converging evidence: controlled feeding studies can test intermediate markers, mechanistic research can examine lipid transport and arterial changes, and long-term population studies can compare disease outcomes. Ethical limits prevent decades-long experiments that impose potentially harmful diets. Responsible evaluation therefore reports uncertainty and confounding rather than classifying every lipid as simply healthy or unhealthy.

Dietary triglycerides cannot cross the intestinal epithelium intact. Bile salts emulsify large fat droplets, increasing surface area for lipase. Lipase hydrolyses ester bonds, and the products associate with bile salts in micelles that deliver them to the epithelial surface. After absorption, fatty acids and monoglycerides are reassembled into triglycerides and packaged with proteins into transport particles.

Most newly packaged dietary lipid enters lymphatic lacteals before reaching the bloodstream. Because hydrophobic molecules cannot circulate freely in plasma, lipoproteins surround a lipid-rich core with an amphipathic surface. LDL and HDL refer to particle classes with different densities, compositions and transport roles; cholesterol itself is one molecule and is not intrinsically 'good' or 'bad'.

Waxes are esters formed from long-chain fatty acids and long-chain alcohols. Their hydrophobicity makes them useful waterproof coatings. Plant cuticular wax reduces uncontrolled evaporation from leaves, beeswax provides a moldable structural material, and waxes protect feathers, fur and skin. The same water-repelling behavior supports different functions because the material is deployed in different structures.