Lipids

Lipids are a vital class of biological molecules found in all cells. They are commonly known as fats or oils and have a variety of properties and functions. Unlike carbohydrates and proteins, lipids are generally not polymers formed from long chains of repeating monomers. Instead, they are made from a variety of different components, primarily containing carbon (C), hydrogen (H), and oxygen (O), though with a much smaller proportion of oxygen compared to carbohydrates. All lipids inherently contain hydrocarbons (molecules with only hydrogen and carbon atoms). A key property of lipids is their insolubility in water, making them hydrophobic (water-repelling). They are, however, soluble in organic solvents like ethanol.

The main types of lipids include triglycerides and phospholipids. Steroids and waxes are also considered lipids.

Triglycerides

• Structure: A triglyceride molecule consists of one molecule of glycerol (an alcohol with three hydroxyl groups) with three fatty acids attached to it. Fatty acid molecules have long hydrocarbon tails made of carbon chains with hydrogen atoms, typically 14 to 22 carbon atoms long. The hydrocarbon tail is often represented as an 'R' group.

• Formation: Triglycerides are formed from their components by condensation reactions. This process involves the formation of an ester bond between the hydroxyl group of the glycerol and the carboxyl group of each fatty acid, with the elimination of a molecule of water for each bond formed (so, three water molecules are released for one triglyceride). The reverse process, hydrolysis, breaks these ester bonds by adding water.

• Types of Fatty Acids: Fatty acids can be:

  • Saturated: They have no double bonds between their carbon atoms in the hydrocarbon tails. They are "saturated" with hydrogen. Saturated fatty acids form straight chains and tend to have higher melting points. Animal lipids are often saturated and exist as fats (solid at room temperature).

  • Unsaturated: They have one or more double bonds between carbon atoms in their hydrocarbon tails. These double bonds cause the chain to kink, which prevents close packing and results in lower melting points. If there's one double bond, it's monounsaturated; if two or more, it's polyunsaturated. Plant lipids are often unsaturated and exist as oils (liquid at room temperature). Trans-fatty acids, where hydrogen atoms are on opposite sides of the double bond, are not metabolised by enzymes and are linked with coronary heart disease.

• Functions: Triglycerides are primarily used as energy storage molecules. The long hydrocarbon tails contain abundant chemical energy, meaning lipids contain about twice as much energy per gram as carbohydrates and proteins (approx. 39.4 kJ/g compared to 15.8-17.0 kJ/g). They are stored as insoluble droplets (e.g., in adipose tissue in mammals and oils in plant seeds/fruits). Other functions include:

  • Insulation: Adipose tissue (containing triglycerides) provides insulation against heat loss, notably as blubber in marine mammals. Myelin lipid (part of the myelin sheath around nerve fibers) acts as electrical insulation, speeding up nerve impulses.

  • Buoyancy: Their low density increases the ability of aquatic animals to float.

  • Metabolic Water Source: Complete oxidation of fats and oils produces a large amount of water. Desert animals and bird/reptile embryos benefit from this "metabolic water".

  • Protection: Adipose tissue also forms a protective layer around organs, like the kidneys.

Phospholipids

• Structure: Phospholipids are similar to triglycerides, but one of the fatty acid molecules is replaced by a phosphate-containing group. This gives them a distinct structure with a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails. This dual nature makes them amphipathic. The phosphate group is polar and can dissolve in water.

• Role in Cell Membranes: Phospholipids are the major components of all cell membranes. In aqueous environments, they automatically arrange themselves into a bilayer (a double layer), with their hydrophilic heads facing outwards towards the watery solutions and their hydrophobic tails facing inwards to form a non-polar interior. This hydrophobic core acts as a barrier to water-soluble substances (like ions, sugars, amino acids, proteins), preventing them from easily diffusing across the membrane. The fluidity of the membrane is influenced by the proportion of saturated versus unsaturated fatty acid tails and tail length. Phospholipids also contribute to compartmentalization within cells by forming organelle membranes. They can influence membrane protein orientation and may act as signaling molecules when chemically modified.

Other Lipids

• Cholesterol: A type of lipid found in all cell membranes (except bacterial ones). It fits between phospholipids, binding to their hydrophobic tails, which restricts their movement, making the membrane less fluid and more rigid. Cholesterol helps maintain the shape of animal cells (which lack cell walls) and creates a further barrier to polar substances. It also helps stabilize membranes at both low and high temperatures. Cholesterol can be transported in the blood by lipoproteins (HDLs and LDLs). Excess blood cholesterol may cause harmful deposits in artery walls, leading to atherosclerosis.

• Waxes: Esters formed from a fatty acid and a complex alcohol. They provide waterproofing, for instance, in plant cuticles, insect exoskeletons, and on animal fur and feathers.

• Steroids: Characterized by a set of complex rings of carbon atoms. Most are hydrophobic, but can have a polar hydroxyl group. Steroid hormones (e.g., testosterone, oestrogen, progesterone) are lipid-soluble and can pass directly through the phospholipid bilayer of cell membranes to bind to internal receptors.

Digestion and Absorption of Lipids

Large lipid molecules in food are too big to cross cell membranes and must be broken down during digestion.

• Emulsification: Lipids are first emulsified by bile salts (produced by the liver) into tiny droplets in the small intestine. This process increases the surface area for enzymatic action.

• Hydrolysis: Lipase enzymes (mainly from the pancreas) then catalyze the hydrolysis of ester bonds in lipids, breaking them down into monoglycerides and fatty acids.

• Micelle Formation: Monoglycerides and fatty acids combine with bile salts to form small structures called micelles. Micelles help transport these lipid digestion products to the epithelial cells of the ileum.

• Absorption: Monoglycerides and fatty acids are lipid-soluble and can diffuse directly across the epithelial cell membranes into the cells. Within the intestinal cells, they are often re-formed into triglycerides and then combine with proteins to form lipoproteins (chylomicrons) which enter the lymphatic system and then the bloodstream.

Biochemical Test for Lipids

The presence of lipids in a sample can be tested using the emulsion test.

• The test substance is shaken with ethanol for about a minute. Any lipids present will dissolve in the ethanol.

• This solution is then poured into water.

• If lipids are present, a milky white emulsion will form. The more lipid, the more noticeable the milky color will be. If no lipid is present, the mixture remains clear. This occurs because lipid molecules, being insoluble in water, form tiny droplets throughout the liquid when the ethanol is diluted, scattering light and appearing cloudy.