light microscope Acommon laboratory instrument
that uses optics to bend visible light to magnify images
ofspecimens placed on an attached platform, or other
viewing area.
light reactions A major component of photosynthe-
sis in which a group of chemical reactions occur in the
thylakoid membranes of chloroplasts that harvest ener-
gy from the sun to produce energy-packed chemical
bonds of ATPand NADPHand that give off oxygen as a
by-product.
lignin A complex amorphous polymer in the sec-
ondary cell wall (middle lamella) of dead woody plant
cells that cements or naturally binds cell walls to help
make them rigid. Highly resistant to decomposition by
chemical or enzymatic action, it is the major source
material for coal. It also acts as support for cellulose
fibers. Cells that contain lignin are fibers, sclerids, ves-
sels, and tracheids.
limbic system The limbic system underlies the cor-
pus callosum, an area that provides communication
and links to the two cerebral hemispheres, and is a col-
lective term referring to several brain parts, an inter-
connected neural network or collection of bodies
including the hippocampus, cingulate cortex, and the
amygdala. The limbic structures are important in the
regulation of visceral motor activity and emotional
expression. The amygdala deals with emotion, while
the cingulate cortex plays a role in emotional behavior,
and the hippocampus deals with spatial and memory
functions.
linkage Refers to the tendency for two genes that
reside next to each other on a chromosome to remain
together, “linked,” during reproduction. The closer
they are to each other, the lower the chance of separa-
tion during DNA replication, and therefore the greater
the chance that they will be inherited with each other.
Linked genes are an exception of Mendel’s laws of
inheritance, specifically the law of independent assort-
ment, which states that pairs of genes segregate inde-
pendently of each other when germ cells are formed.
linkage map A map that shows the relative positions
of genetic loci on a chromosome, determined by how
often the loci are inherited together. Linked genes are
usually inherited together and are located close to each
other on the chromosome. The distance is measured in
centimorgans (cM). Genes that are located on different
chromosomes follow Mendel’s principle of independent
assortment, while genes that are on the same chromo-
some do not sort independently. The closer the two
genes are to each other on a chromosome, the greater
the chance that they will remain together during meio-
sis instead of crossing over. Researchers can construct a
genetic map showing the relative positions of the genes
on the chromosomes by comparing appearance of a
trait to appearance of marker phenotypes. The distance
between closely spaced genes can be expressed in “map
units” reflecting relative frequency of recombination.linked genes Genes that are located close to each
other on the same chromosome and do not show inde-
pendent assortment. This results in parental allele com-
binations occurring greater than recombinant
arrangements. A set of closely linked genes that are
inherited together are called a haplotype. Some diseases
are a result of abnormalities in certain gene combina-
tions. X-linked diseases such as hemophilia, color
blindness, and some muscular dystrophies are caused
by genes located on the X chromosome. Sex-linked dis-
eases are caused by a mutant gene on part of the X
chromosome and affects men, since they do not have a
Y chromosome of the XY pair that has a compensating
normal gene.lipid A large group of hydrophobic (water insoluble)
molecules that are the building blocks of cell mem-
branes and liposomes (lipid vesicles) and contain fatty
acids; the principal components of fats, oils, waxes,
triglycerides, and cholesterol. They are insoluble in
water but soluble in solvents such as alcohol and ether.
The phospholipid bilayer of the plasma membrane is a
double layer of phospholipid molecules arranged so
that the hydrophobic “tails” lie between the
hydrophilic “heads.” Also known as fat, they easily
store in the body and are an important source of fuel
for the body.
See alsoFAT.206 light microscope