lac operon
The bacterial operon that contains the genes coding for enzymes involved in lactose degredation.

ligands
Ligands are molecules that bind to specific receptor sites on other molecules. They are important in cell-cell signalling.

light reactions
The reactions of photosynthesis that capture the energy of light and transfer it into the chemical bonds of ATP and NADPH.

limiting resources
Resources that organisms must compete for, because the demand for them exceeds their supply.

linkage
Genes that are inherited together more often than would be expected due to chance. Linked genes are found relatively close together on the same chromosome.

lipid
Lipids are fatty acid esters, a class of water-insoluble organic molecules, that are used as basic building blocks of biological membranes, as well as for energy storage (e.g., triglycerides). Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by water) tails. The hydrophobic tail consists of one or two (in triglycerides, three) fatty acids. These are unbranched chains of carbon atoms (with the correct number of H atoms), which are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.