Lipids are a class of organic molecule composed primarily of hydrocarbon chains (fatty acids) or rings (steroids)
Hydrocarbons are non-polar (they lack charged regions) and will not dissolve in polar substances like water
Hence lipids are considered to be hydrophobic molecules (they are water-repelling)
The hydrophobic properties of lipids have many important biological consequences
Waxes are used to prevent water loss from leaves while birds coat their feathers with oil to render them waterproof
Phospholipids provide a structural framework for cells by forming spontaneous membranes in aqueous solutions
Lipids in foods help the body to absorb certain fat-soluble micronutrients, including vitamins A and D
However, the hydrophobic properties of lipids make them difficult to transport around the body
Digested fats are packaged within a protein coat to become water soluble lipoproteins
Steroid hormones are bound to carrier proteins (such as albumin) to facilitate their movement through the bloodstream
Certain lipids have polar components, becoming amphipathic (having both hydrophobic and hydrophilic regions)
Phospholipids possess a polar phosphate head, cholesterol has a polar hydroxyl group and glycolipids can have polar carbohydrate chains
These amphipathic molecules all have a limited capacity to interact with water, helping them to function in maintaining membrane integrity