membrane potential   Voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other. A typical membrane potential for an animal cell plasma membrane is –60 mV (inside negative relative to the surrounding fluid). (Figure 11–23) membrane protein   Amphiphilic protein of diverse structure and function that associates with the lipid bilayer of cell membranes. (Figure 10–17) membrane transport protein   Membrane protein that mediates the passage of ions or molecules across a membrane. The two main classes are transporters (also called carriers or permeases) and channels. (Figure 11–4) membrane-associated protein   Membrane protein not extending into the hydrophobic interior of the lipid bilayer but bound to either face of the membrane by noncovalent interactions with other membrane proteins. (Figure 10–17) membrane-bending proteins Attach to specific membrane regions as needed and act to control local membrane curvature and thus confer on membranes their characteristic three-dimensional shapes.
membrane-bound ribosome Ribosome attached to the cytosolic face of the endoplasmic reticulum. The site of synthesis of proteins that enter the endoplasmic reticulum. (Figure 12–38) memory cell In immunology: a T or B lymphocyte generated following antigen stimulation that is more easily and more quickly induced to become an effector cell or another memory cell by a later encounter with the same antigen. (Figure 24–17) mesoderm   Embryonic tissue that is the precursor to muscle, connective tissue, skeleton, and many of the internal organs. (Figure 21–3) messenger RNA (mRNA) RNA molecule that specifies the amino acid sequence of a protein. Produced in eukaryotes by processing of an RNA molecule made by RNA polymerase as a complementary copy of DNA. It is translated into protein in a process catalyzed by ribosomes. (Figure 6–20) metabolism   The sum total of the chemical processes that take place in living cells. All of catabolism plus anabolism. (Figure 2–14) metabotropic receptors   Neurotransmitter receptors that regulate ion channels indirectly through the activation of second-messenger molecules.
metaphase plate Imaginary plane at right angles to the mitotic spindle and midway between the spindle poles; the plane in which chromosomes are positioned at metaphase. (Panel 17–1, pp. 980–981) metaphase-to-anaphase transition   Transition in the eukaryotic cell cycle preceding sister-chromatid separation at anaphase. If the cell is not ready to proceed to anaphase, the cell cycle is halted at this point. (Figure 17–9, and Panel 17–1, pp. 980–981) metastases Secondary tumors, at sites in the body additional to that of the primary tumor, resulting from cancer cells breaking loose, entering blood or lymphatic vessels, and colonizing separate environments.
metastasis   The spread of cancer cells from their site of origin to other sites in the body. (Figures 20–1 and 20–16)
MHC complex (major histocompatibility complex) Cluster of genes in one vertebrate chromosome (chromosome 6 in humans) that code for a set of highly polymorphic cell-surface glycoproteins (MHC proteins). (Figure 24–37) microbiome The combined genomes of the various species of a defined microbiota.
microbiota   The collective of microorganisms that reside in or on an organism.
microelectrode A piece of fine glass tubing, pulled to an even finer tip, that is used to inject electric current into cells or to study the intracellular concentrations of common inorganic ions (such as H+, Na+, K+, Cl–, and Ca2+) in a single living cell by insertion of its tip directly into the cell interior through the plasma membrane.