lagging strand One of the two newly synthesized strands of DNA found at a replication fork. The lagging strand is made in discontinuous lengths that are later joined covalently.  (Figure 5–7) lamellipodium (plural lamellipodia) Flattened, sheetlike protrusion supported by a meshwork of actin filaments, which is extended at the leading edge of a crawling animal cell. (Figures 16–77 and 16–79) laminin   Extracellular matrix fibrous protein found in basal laminae, where it forms a sheetlike network. (Figures 19–52 and 19–53) lampbrush chromosome   Huge chromosome paired in preparation for meiosis, found in immature amphibian eggs; consisting of large loops of chromatin extending out from a linear central axis. (Figure 4–47) late endosome   Compartment formed from a bulbous, vacuolar portion of early endosomes by a process called endosome maturation; late endosomes fuse with one another and with lysosomes to form endolysosomes that degrade their contents.
LDL-receptor-related protein (LRP) Co-receptor bound by Wnt proteins in the regulation of β-catenin proteolysis.
leading strand One of the two newly synthesized strands of DNA found at a replication fork. The leading strand is made by continuous synthesis in the 5′-to-3′ direction. (Figure 5–7) lectin   Protein that binds tightly to a specific sugar. Abundant lectins from plant seeds are used as affinity reagents to purify glycoproteins or to detect them on the surface of cells.
Legionnaire’s disease   Type of pneumonia resulting from infection with Legionella pneumophila, a parasite of freshwater amoebae that is spread to humans by air-conditioning systems that harbor infected amoebae and produce microdroplets of water that are easily inhaled.
lethal factor One of the two A subunits of anthrax toxin; a protease that cleaves several activated members of the MAP kinase kinase family and causes a large fall in blood pressure and death on entry into the bloodstream of an animal.
leucine-rich repeat (LRR) receptor kinases Common type of receptor serine/threonine kinase in plants that contains a tandem array of leucine-rich repeat sequences in its extracellular portion.
leukemia Cancer of white blood cells.
leukocyte General name for all the nucleated blood cells lacking hemoglobin. Also called white blood cells. Includes lymphocytes, granulocytes, and monocytes. (Figure 22–27) ligand   Any molecule that binds to a specific site on a protein or other molecule. From Latin ligare, to bind.
light microscope   One of a class of microscopes that uses visible light to create the image.
lignin   Network of cross-linked phenolic compounds that forms a supporting network throughout the cell walls of xylem and woody tissue in plants.
limit of resolution   In microscopy, the smallest distance apart at which two point objects can be resolved as separate. Just under 0.2 μm for conventional light microscopy, a limit determined by the wavelength of light.
linkage In ligand binding, the conformational coupling between two separate ligand-binding sites on a protein, such that a conformational change in the protein induced by binding of one ligand affects the binding of a second ligand.
lipid bilayer (phospholipid bilayer) Thin double sheet of phospholipid molecules that forms the core structure of all cell membranes. The two layers of lipid molecules are packed with their hydrophobic tails pointing inward and their hydrophilic heads outward, exposed to water. (Figure 10–1 and  Panel 2–5, pp. 98–99) lipid droplets Storage form in cells for excess lipids; comprised of a single monolayer of phospholipids and proteins that surrounds neutral lipids that can be retrieved from droplets as required by the cell.