lipid raft   Small region of a membrane enriched in sphingolipids and cholesterol. (Figure 10–13) liposome   Artificial phospholipid bilayer vesicle formed from an aqueous suspension of phospholipid molecules. (Figure 10–9) local mediator Extracellular signal molecule that acts on neighboring cells.
long noncoding RNA (lncRNA) One of a large group  (≈8000 in humans) of RNAs longer than 200 nucleotides and not coding for protein. The functions, if any, of most lncRNAs is unknown but individual lncRNA are known to play important roles in the cell, for example, in telomerase function and genomic imprinting. In a general sense, lncRNAs are believed to act as scaffolds, holding together proteins and nucleic acids to speed up a wide variety of reactions in the cell.
long-term depression (LTD) A long-lasting (hours or more) decrease in the sensitivity of certain synapses in the brain triggered by NMDA receptor activation. As the opposing process to long-term potentiation, it is thought to be involved in learning and memory.
long-term potentiation (LTP) Long-lasting increase (days to weeks) in the sensitivity of certain synapses in the brain, induced by a short burst of repetitive firing in the presynaptic neurons. (Figure 11–44) loss of heterozygosity   The result of errant homologous recombination that uses the homolog from the other parent instead of the sister chromatid as the template, converting the sequence of the repaired DNA to that of the other homolog.
low-density lipoprotein (LDL) Large complex composed of a single protein molecule and many esterified cholesterol molecules, together with other lipids. The form in which cholesterol is transported in the blood and taken up into cells. (Figure 13–51) lumen   The space inside a hollow structure. In cells: the cavity enclosed by an organelle membrane. In tissues: the cavity enclosed by a sheet of cells.
lymphocyte   White blood cell responsible for the specificity of adaptive immune responses. Two main types: B cells, which produce antibody, and T cells, which interact directly with other effector cells of the immune system and with infected cells. T cells develop in the thymus and are responsible for cell-mediated immunity. B cells develop in the bone marrow in mammals and are responsible for the production of circulating antibodies.
lymphoid organ An organ containing large numbers of lymphocytes. Lymphocytes are produced in primary lymphoid organs and respond to antigen in peripheral lymphoid organs. (Figure 24–12) lymphoma Cancer of lymphocytes, in which the cancer cells are mainly found in lymphoid organs (rather than in the blood, as in leukemias).
lysosomal storage diseases   Genetic diseases resulting from defects in or a lack of one or more functional hydrolases in lysosomes of some cells, leading to accumulation of undigested substrates in lysosomes and consequent cell pathology.
lysosome Membrane-enclosed organelle in eukaryotic cells containing digestive enzymes, which are typically most active at the acid pH found in the lumen of lysosomes. (Figure 13–37) lysozyme Enzyme that catalyzes the cutting of polysaccharide chains in the cell walls of bacteria.
M-Cdk (M-phase Cdk) Cyclin-Cdk complex formed in vertebrate cells by an M-cyclin and the corresponding cyclindependent kinase (Cdk). (Figure 17–11 and Table 17–1, p. 969)
M-cyclin   A cyclin found in all eukaryotic cells that promotes the events of mitosis. (Figure 17–11)