bivalent   A four-chromatid structure formed during meiosis, consisting of a duplicated chromosome tightly paired with its homologous duplicated chromosome.
blastomere One of the many cells formed by the cleavage of a fertilized egg.
blastula Early stage of an animal embryo, usually consisting of a hollow ball of epithelial cells surrounding a fluid-filled cavity, before gastrulation begins.
blebbing   Membrane protrusion formed when the plasma membrane detaches locally from the underlying actin cortex, allowing cytoplasmic flow and hydrostatic pressure within the cell to push the membrane outward.
bone   Dense and rigid connective tissue comprising a mixture of tough fibers (type I collagen fibrils), which resist pulling forces, and solid particles (calcium phosphate as hydroxylapatite crystals), which resist compression.
brassinosteroids   Class of steroid signal molecules in plants that regulate the growth and differentiation of plants throughout their life cycle via binding to a cell-surface receptor kinase to initiate a signaling cascade.
bright-field microscope   Normal light microscope in which the image is obtained by simple transmission of light through the object being viewed.
buffer   Solution of weak acid or weak base that resists the pH change that would otherwise occur when small quantities of acid or base are added.
C3   The pivotal complement protein that is activated by the early components of all three complement pathways (the classical pathway, the lectin pathway, and the alternative pathway). (Figure 24–7)
Ca2+ pump (calcium pump, Ca2+ ATPase) Transport protein in the membrane of sarcoplasmic reticulum of muscle cells (and elsewhere). Pumps Ca2+ out of the cytoplasm into the sarcoplasmic reticulum using the energy of ATP hydrolysis.
Ca2+-activated K+ channel   Opens in response to the raised concentration of Ca2+ in nerve cells that occurs in response to an action potential. Increased K+ permeability makes the membrane harder to depolarize, increasing the delay between action potentials and decreasing the response of the cell to constant, prolonged stimulation (adaptation).
Ca2+/calmodulin-dependent kinase (CaM-kinase) Serine/ threonine protein kinase that is activated by Ca2+/calmodulin. Indirectly mediates the effects of an increase in cytosolic Ca2+ by phosphorylating specific target proteins. (Figure 15–33) cadherin Member of the large cadherin superfamily of transmembrane adhesion proteins. Mediates homophilic  Ca2+-dependent cell–cell adhesion in animal tissues. (Figure 19–3 and Table 19–1, p. 1037) cadherin superfamily Family of classical and nonclassical cadherin proteins with more than 180 members in humans.
calmodulin Ubiquitous intracellular Ca2+-binding protein that undergoes a large conformation change when it binds Ca2+, allowing it to regulate the activity of many target proteins. In its activated (Ca2+-bound) form, it is called Ca2+/calmodulin. (Figure 15–33) calnexin   Carbohydrate-binding chaperone protein in the endoplasmic reticulum (ER) membrane that binds to oligosaccharides on incompletely folded proteins and retains them in the ER.
calreticulin   Carbohydrate-binding chaperone protein in the endoplasmic reticulum (ER) lumen that binds to oligosaccharides on incompletely folded proteins and retains them in the ER.
CaM-kinase II Multifunctional Ca2+/calmodulin-dependent protein kinase that phosphorylates itself and various target proteins when activated. Found in most animal cells but is especially abundant at synapses in the brain, and is involved in some forms of synaptic plasticity in vertebrates. (Figure 15–34) cancer stem cells    Rare cancer cells capable of dividing indefinitely.