L
Leucine
see amino acids
L-flagella
Lateral flagella
see Vibrio.
L-form (L-phase variant)
A defective bacterial cell of
spherical or irregular shape, formed either spontaneously (e.g. by Streptobacillus
moniliformis) or as a result of various stimuli (e.g.temperature shock,
osmotic shock, or antibiotics which inhibit cell wall biosynthesis); in an
L-form the cell wall is either partly or totally absent. On removal of the
stimulus, an L-form may resume cell wall synthesis and revert to the condition
of the original strain, or it may continue to reproduce as an
L-form (stable L-form). L forms have been observed in various bacteria –
including species of Bacillus, Proteus, Streptococcus and Vibrio.
L-form colonies often resemble those of Mycoplasma spp.
lac operon (lactose operon)
An operon containing genes which encode
proteins involved in the uptake and utilization of b galactosides such as lactose.
The lac operon occurs e.g. in Escherichia coli – in which it is
located at ca. 8 minutes on the chromosome map. A simplified scheme for the lac
operon is shown in the figure. The lacZ gene encodes b-galactosidase,
lacY encodes a transport protein, ‘b-galactoside permease’, and lacA encodes
thiogalactoside transacetylase. (Lac A catalyses the transfer of an acetyl
group from acetyl-CoA to a b-galactoside. Its function in vivo is
unknown; it has been suggested that it may be involved in the detoxification of
non-metabolizable sugars by bringing about their acetylation and, hence,
excretion.)
lacteal
(lak-teel)
A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
lactic acid (CH3.CHOH.COOH)
A
compound present e.g. in many fermented dairy products, and also widely used as
a food additive. Only the L (+)-isomer can be readily assimilated by man and
animals. L(+)-Lactic acid is obtained commercially by the lactic acid
fermentation of sucrose by lactic acid bacteria; it is used primarily in the
food industry, being added e.g. to confectionary, pickles, beverages, etc.
lactic acid bacteria
A (non-taxonomic) group of
Gram-positive, non-sporing bacteria which carry out a lactic acid fermentation of
sugars; it includes species of Lactobacillus, Lactococcus, Leuconostoc
and Pediococcus. (Bifidobacterium is sometimes included.)
lagging
strand
A
discontinuously synthesized DNA strand that elongates in a direction away from
the replication fork.
lamella
(lah-mell-ah)
[L. dim. of lamina, plate or leaf]
Layer, thin sheet.
Layer, thin sheet.
larva pl. larvae
(lar-vuh)
[L. ghost]
A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
lateral
line system
A mechanoreceptor system consisting of a series of pores and receptor
units (neuromasts) along the sides of the body of fishes and aquatic
amphibians; detects water movements made by an animal itself and by other
moving objects.
lateral
meristem
(mare-eh-stem)
[L. latus, lateris, side + Gk. meristos, divided]
The vascular and cork cambium, a cylinder of dividing cells that runs most of the length of stems and roots and is responsible for secondary growth.
The vascular and cork cambium, a cylinder of dividing cells that runs most of the length of stems and roots and is responsible for secondary growth.
law of
independent assortment
Mendel's
second law, stating that each allele pair segregates independently during
gamete formation; applies when genes for two traits are located on different
pairs of homologous chromosomes.
law of
segregation
Mendel's
first law, stating that allele pairs separate during gamete formation, and then
randomly re-form pairs during the fusion of gametes at fertilization.
leaching
The
dissolving of minerals and other elements in soil or rocks by the downward
movement of water.
leading strand
The new
continuous complementary DNA strand synthesized along the template strand in
the mandatory 5' to 3' direction.
leaf
The main
site of photosynthesis in a plant; consists of a flattened blade and a stalk (petiole) that joins the leaf to the stem.
learning
The
process that leads to modification in individual behavior as the result of
experience.
leukocyte
(loo-koh-site)
[Gk. leukos, white + kytos, vessel]
A white blood cell; typically functions in immunity, such as phagocytosis or antibody production.
A white blood cell; typically functions in immunity, such as phagocytosis or antibody production.
leukotriene
A type of
prostaglandin produced by various white blood cells involved in the
inflammatory and immune responses and in allergic reactions.
lichen
(ly-ken)
An organism formed by the symbiotic association between a fungus and a photosynthetic alga.
An organism formed by the symbiotic association between a fungus and a photosynthetic alga.
life
cycle
The
entire sequence of stages in the life of an organisms, from the adults of one
generation to the adults of the next.
life-history
pattern
A group
of traits, such as size and number of offspring, length of maturation, age at
first reproduction, and the number of times reproduction occurs, that affect
reproduction, survival, and the rate of population growth.
life
table
A table
of data summarizing mortality in a population.
ligament
A type of
fibrous connective tissue that joins bones together at joints.
ligand
(lig-und)
A molecule that binds specifically to a receptor site of another molecule.
A molecule that binds specifically to a receptor site of another molecule.
ligand-gated
ion channel receptor
A signal
receptor protein in a cell membrane that can act as a channel for the passage
of a specific ion across the membrane. When activated by a signal molecule, the
receptor either allows or blocks passage of the ion, resulting in a change in
ion concentration that usually affects cell functioning.
light-dependent
reactions
The
reactions of the first stage of photosynthesis, in which light energy is
captured by chlorophyll molecules and converted to chemical energy stored in
ATP and NADPH molecules.
light-independent
reactions
The
carbon-fixing reactions of the second stage of photosynthesis; energy stored in
ATP and NADPH by the light-dependent reactions is used to reduce carbon from
carbon dioxide to simple sugars; light is not required for these reactions.
light
microscope (LM)
An
optical instrument with lenses that refract (bend) visible light to magnify
images of specimens.
light
reactions
The steps
in photosynthesis that occur on the thylakoid membranes of the chloroplast and
convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen
in the process.
lignin
(lig-nin)
A hard material embedded in the cellulose matrix of vascular plant cell walls that functions as an important adaptation for support in terrestrial species.
A hard material embedded in the cellulose matrix of vascular plant cell walls that functions as an important adaptation for support in terrestrial species.
limbic
system
(lim-bik)
[L. limbus, border]
A group of nuclei (clusters of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining emotions; includes the
A group of nuclei (clusters of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining emotions; includes the
hippocampus
and the amygdala.
linkage
The
tendency for certain alleles to be inherited together because they are located
on the same chromosome.
linked
genes
Genes
that are located on the same chromosome.
linkage
group
A pair of
homologous chromosomes.
linkage
map
A genetic map based on the frequencies
of recombination between markers during crossing over of homologous
chromosomes. The greater the frequency of recombination between two genetic
markers, the farther apart they are assumed to be.
lipid
(lih-pid)
[Gk. lipos, fat]
One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.
One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.
lipoprotein
[Gk. lipos, fat
+ proteios, primary]
A protein bonded to a lipid; includes the low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) that transport fats and cholesterol in blood.
A protein bonded to a lipid; includes the low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) that transport fats and cholesterol in blood.
locus pl. loci
(loh-kus)
[L. place]
A particular place along the length of a certain chromosome where a given gene is located.
A particular place along the length of a certain chromosome where a given gene is located.
logistic
population growth
A model
describing population growth that levels off as population size approaches
carrying capacity.
long-day
plant
A plant
that flowers, usually in late spring or early summer, only when the light
period is longer than a critical length.
loop of
Henle
(after F.
G. J. Henle, German pathologist)
The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.
The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.
lumen
[L.
light]
The cavity of a tubular structure, such as endoplasmic reticulum or a blood vessel.
The cavity of a tubular structure, such as endoplasmic reticulum or a blood vessel.
lungs
The
invaginated respiratory surfaces of terrestrial vertebrates, land snails, and
spiders that connect to the atmosphere by narrow tubes.
luteinizing
hormone (LH)
(loo-tee-in-eye-zing)
A protein hormone secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.
A protein hormone secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.
lymph
(limf)
[L. lympha, water]
The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate animals.
The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate animals.
lymph
node
[L. lympha, water
+ nodus, knot]
A mass of spongy tissues, separated into compartments; located throughout the lymphatic system, lymph nodes remove dead cells, debris, and foreign particles from the circulation; also are sites at which foreign antigens are displayed to immunologically active cells.
A mass of spongy tissues, separated into compartments; located throughout the lymphatic system, lymph nodes remove dead cells, debris, and foreign particles from the circulation; also are sites at which foreign antigens are displayed to immunologically active cells.
lymphatic
system
(lim-fat-ik)
A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid and protein to the blood.
A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid and protein to the blood.
lymphocyte
[L. lympha, water
+ Gk. kytos, vessel]
A white blood cell. The lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.
A white blood cell. The lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.
lymphokine
A
chemical, released by an activated cytotoxic T cell, that attracts macrophages
and stimulates phagocytosis.
lysis
[Gk. lysis, a
loosening]
Disintegration of a cell by rupture of its plasma membrane.
Disintegration of a cell by rupture of its plasma membrane.
lysogenic
bacteria
(lye-so-jenn-ick)
[Gk. lysis, a loosening + genos, race or
descent]
Bacteria carrying a bacteriophage integrated into the bacterial chromosome. The virus may subsequently set up an active cycle of infection, causing lysis of the bacterial cells.
Bacteria carrying a bacteriophage integrated into the bacterial chromosome. The virus may subsequently set up an active cycle of infection, causing lysis of the bacterial cells.
lysogenic
cycle
A type of
phage replication cycle in which the viral genome becomes incorporated into the
bacterial host chromosome as a prophage.
lysosome
(ly-so-some)
[Gk. lysis, loosening + soma, body]
A membrane-enclosed bag of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.
A membrane-enclosed bag of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.
lysozyme
(ly-so-zime)
An enzyme in perspiration, tears, and saliva that attacks bacterial cell walls.
An enzyme in perspiration, tears, and saliva that attacks bacterial cell walls.
lytic
cycle
(lit-ik)
A type of viral replication cycle resulting in the release of new phages by death or lysis of the host cell.
A type of viral replication cycle resulting in the release of new phages by death or lysis of the host cell.
