G-14 glossary
macroevolution The large-scale patterns, trends, and rates of
change among groups of species.
macrophage A phagocytic white blood cell. It engulfs any-
thing detected as foreign. Some also become antigen-
presenting cells that serve as the trigger for immune
responses by T and B lymphocytes. Compare
antigen-presenting cell.
mandible The lower jaw; the largest single facial bone.
mass extinction An abrupt rise in extinction rates above the
background level; a catastrophic, global event in which large
groups of organisms are wiped out simultaneously.
mast cell A type of white blood cell that releases enzymes
and histamine during tissue inflammation.
matrix In connective tissue, fiberlike structural proteins
together with a “ground substance” of polysaccharides that
give each kind of tissue its particular properties.
mechanoreceptor Sensory cell or cell part that detects
mechanical energy associated with changes in pressure, posi-
tion, or acceleration.
medical imaging Any of several diagnostic methods includ-
ing PET scanning, magnetic resonance imaging (MRI), X-ray,
ultrasound, and CT (computed tomography) scanning.
medulla oblongata (meh-dull-uh ahb-lawn-gah-tuh) Part
of the brain stem with reflex centers for respiration, blood
circulation, and other vital functions.
meiosis (my-oh-sis) [Gk. meioun, to diminish] Two-stage
nuclear division process in which the chromosome number of
a germ cell is reduced by half, to the haploid number. (Each
daughter nucleus ends up with one of each type of chromo-
some.) Meiosis forms gametes.
melanocytes Cells in the deepest layer of epidermis
that produce the brown-black pigment melanin found in
keratinocytes.
membrane attack complexes Structures that form pores in
the plasma membrane of a pathogen, causing disintegration.
memory The storage and retrieval of information about pre-
vious experiences; underlies the capacity for learning.
memory cell Any of the various B or T cells of the immune
system that are formed in response to invasion by a foreign
agent and that are available to mount a rapid attack if the
same type of invader reappears.
menarche A female’s first menstruation.
meninges (meh-nin-jeez) Membranes of connective tissue
that are layered between the skull bones and the brain and
cover and protect the neurons and blood vessels that service
the brain.
menopause (men-uh-pozz) [L. mensis, month, and pausa,
stop] End of the reproductive period of a human female’s life
cycle.
menstrual cycle The cyclic release of oocytes and priming
of the endometrium (lining of the uterus) to receive a fertil-
ized egg; the complete cycle averages about 28 days in female
humans.linkage The tendency of genes located on the same chromo-
some to end up in the same gamete. For any two of those
genes, the probability that crossing over will disrupt the link-
age is proportional to the distance separating them.
lipid A greasy or oily compound of mostly carbon and
hydrogen that shows little tendency to dissolve in water, but
that dissolves in nonpolar solvents (such as ether). Cells use
lipids as energy stores and structural materials, especially in
membranes.
lipid bilayer The structural basis of cell membranes, consist-
ing of two layers of mostly phospholipid molecules. Hydro-
philic heads force all fatty acid tails of the lipids to become
sandwiched between the hydrophilic heads.
lipoprotein A protein that has a lipid attached to it. Molecule
that forms when proteins circulating in blood combine with
cholesterol, triglycerides, and phospholipids absorbed from
the small intestine.
liver Organ with roles in storing and interconverting car-
bohydrates, lipids, and proteins absorbed from the gut;
disposing of nitrogen-containing wastes; and other tasks.
local signaling molecule A molecule that alters chemical
conditions in the immediate vicinity where it is secreted, then
is swiftly broken down.
locus (low-cuss) The location of a particular gene on a
chromosome.
loop of Henle The hairpin-shaped, tubular region of a neph-
ron that functions in reabsorption of water and solutes.
loose connective tissue Flexible fibrous connective tissue
with few fibers and cells.
lymph (limf) [L. lympha, water] Tissue fluid that has moved
into the vessels of the lymphatic system.
lymphatic system An organ system (such as lymph nodes
and the thymus) with vessels that take up fluid and solutes
from interstitial fluid and deliver them to the bloodstream; its
lymphoid organs have roles in immunity.
lymph capillary A small-diameter vessel of the lymph
vascular system that has no obvious entrance; tissue fluid
moves inward by passing between overlapping endothelial
cells at the vessel’s tip.
lymph node A lymphoid organ that serves as a battleground
of the immune system; each lymph node is packed with mac-
rophages and lymphocytes that cleanse lymph of pathogens
before it reaches the blood.
lymphocyte A T cell or B cell.
lymph vascular system [L. vasculum, a small vessel] The
vessels of the lymphatic system, which take up and transport
excess tissue fluid and reclaimable solutes as well as fats
absorbed from the digestive tract.
lysosome (lye-so-sohm) A cell organelle that contains
enzymes that can break down polysaccharides, proteins,
nucleic acids, and some lipids.
lysozyme Present in mucous membranes that line body sur-
faces, an infection-fighting enzyme that attacks and destroys
various types of bacteria.
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