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Label (in assays)

The label is an atom or molecule which is attached to either the ligand or binding protein and which, by itself or as part of an enzyme system, is capable of generating a signal which can be quantitated.


State of having notable shifts in emotional state (e.g., uncontrolled laughing or crying).


The production, secretion, and shedding of tears. From the Indo-European dakru meaning a tear (from a weeping eye) via the Latin lacrima.


A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.


Type of prebiotic/probiotic found in yogurt and some other dairy products.

Lactose intolerance

A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the main sugar in milk and other dairy products.

Lagging strand

A discontinuously synthesized DNA strand that elongates in a direction away from the replication fork.

Lag phase

During the lag phase, cells increase in size but not in number because they are adapting to a new environment, and, synthesis and repair are taking place.
The length of the lag phase depends on the current environment as well as the previous physiological state of the cells. Cells that are from a very different environment or are damaged from their previous physiological state may require more time to adjust. In some foods a lag phase does not exist which results in cells that are ready for immediate growth.

Lag screw

A screw inserted across a fracture that provides compression to the fracture. Lag screws can provide static or dynamic compression. The term "lag" refers to the function of a screw. The physical design of the screw, the type of bone the screw is inserted into, and the manner in which it is inserted determine whether a screw functions as a lag screw. In the diaphyses of long bones, cortical screws are used as lag screws while cancellous screws function as lag screws for cancellous bone fractures.

Lag time

The initial period in a bacterial population life when cells are adjusting to a new environment before commencing growth.

Landau reflux

An infantile, automatic posture in which the head and trunk extend when the baby is held in a prone position with the examiner's hands under the trunk.

Language delay

A term used when the normal rate of language development is interrupted, but the developmental sequence remains intact.
Contrast to Language disorder.

Language disorder

A term used when the sequence of language development is seriously disrupted.
Contrast to Language delay.

Lagrangian Measurement

Measurement of the motion of a fluid by tracing the path traveled by that fluid.

Large for gestational age (LGA)

When the weight of a newborn is greater than the 90th percentile acceptable norms for the particular gestational age.

Large  intestine

The region of the digestive system after the small intestine and ending in the anal canal that voids through the anus.
It consists of five sections:

  • The caecum is a dead-end pouch at the beginning of the large intestine, just below the ileocecal valve.
  • The appendix (vermiform appendix) is an 8 cm (3 in) long fingerlike attachment to the cecum that contains lymphoid tissue and serves immunity functions.
  • The colon, the greater part of the large intestine, consists of four sections: the ascending, transverse, descending, and sigmoid colons. At regular distances along the colon, the smooth muscle of the muscularis layer causes the intestinal wall to gather, producing a series of pouches called haustra. The epithelium facing the lumen of the colon is covered with openings of tubular intestinal glands that penetrate deep into the thick mucosa. The glands consist of absorptive cells that absorb water and goblet cells that secrete mucus. The mucus lubricates the walls of the large intestine to smooth the passage of feces.
  • The rectum is the last 20 cm (8 in) of the large intestine. The mucosa in the rectum forms longitudinal folds called anal columns.
  • The anal canal, the last 3 cm (1 in) of the rectum, opens to the exterior at the anus. An involuntary (smooth) muscle, the interior anal sphincter, and a voluntary (skeletal) muscle, the external anal sphincter, control the release of the feces through the anus.

The functions of the large intestine include

  • Mechanical digestion. Rhythmic contractions of the large intestine produce a form of segmentation called haustral contractions in which food residues are mixed and forced to move from one haustrum to the next. Peristaltic contractions produce mass movements of larger amounts of material.
  • Chemical digestion. Digestion occurs as a result of bacteria that colonize the large intestine. They break down indigestible material by fermentation, releasing various gases. Vitamin K and certain B vitamins are also produced by bacterial activity.
  • Absorption. Vitamins B and K, some electrolytes (Na+ and Cl−), and most of the remaining water is absorbed by the large intestine.
  • Defecation. Mass movement of feces into the rectum stimulates a defecation reflex that opens the internal anal sphincter. Unless the external and sphincter is voluntarily closed, feces are evacuated through the anus.

(see Small Intestine)

Larva (plural Larvae)

A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
Among invertebrates, an immature stage in the life cycle which usually is much smaller than, and morphologically different from, the adult. In insects with metamorphosis, the larva must become a pupa before reaching adulthood.

Laryngeal webbing

When a membrane grows between the true vocal cords.
Its presence results in a voice disorder that not only affects phonation, but can cause a blockage of the airway. These people experience shortness of breath and a higher-pitched than usual voice with some hoarseness. These webs must be surgically removed, followed by subsequent voice therapy.


Visual examination of the inside of the larynx to detect tumors, foreign bodies, nerve or structural injury, or other abnormalities.

Latent heat

The amount of energy required to change a solid to a liquid or liquid to a gas.

Latent schizophrenia

Having clear symptoms of schizophrenia, but no history of psychotic schizophrenic episodes.
Sometimes designated as incipient, prepsychotic, pseudoneurotic, pseudopsychopathic or borderline schizophrenia.

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.
It detects water movements made by an animal itself and by other moving objects.

Lateral ventricle

One of the fluid filled spaces inside the brain, containing cerebrospinal fluid that acts to cushion the brain.

Latin square design

A randomisation schedule for drug tests in which each animal receives each treatment once, in a predetermined random sequence. Also used in randomising doses in some isolated tissue experiments. An example for four drugs (A,B,C,D) in a four-day study is shown below.
GIF version of table


Drug on day number:

























Latus rectum

The chord through a focus of an ellipse perpendicular to the major axis.
The chord through the focus of a parabola parallel to the directrix.

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.

Laws of Themodynamics

Two laws that govern the flow of energy.
1.    The amount of energy in the universe is fixed.  It cannot be created or destroyed only changed from one state to another.
2.    Heat cannot pass from a cold to a hot body.  The opposite condition where heat always flows from a hot to a cold body is valid for the whole universe.

Laws of Trignometry

Law of cosines:    c2 = a2 + b2 - 2abCosC
Law of sines:   a/sin A = b/sin B = c/sin C


Non-essential amino acid that permits the passage of fatty acids into the mitochondria

LC50 (LC50 )

The concentration of air contaminant that will kill 50% of the test animals in a group during a single exposure (inhalation exposure). Generally expressed in units of parts per million or mg of contaminant m3 of air.

LD50 (LD50 )

The LD50 value is the quantity of chemical that is lethal to 50% of the tested animals and is usually expressed as the dose in milligrams, per kilogram of the animal’s body weight. The animal species is given, as is the administration route.
A typical toxicity citation may read: LD50 (mus orl) = 154 mg/kg, meaning the LD50 orally in mice is 154 mg/kg. Toxicity is occasionally expressed as the lethal dose (LD) which is the dose observed to cause death in all animals tested or the minimum lethal dose (MLD) which is the minimum dose, in milligrams/kilogram of body weight, observed to cause death.

Leading strand

The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' to 3' direction.

Leak channels or passive channels

Ion channels that are always open in a cell membrane, thereby allowing the movement of the appropriate ion(s) down their concentration gradients.
The presence of these channels makes the cell membrane “leaky” with respect to current flow even at rest.  The leakiness of these channels varies markedly from one cell type to another: e.g., neuroglia are very much more leaky to K+ than neurons; photoreceptors are very much more leaky to Na+ than neurons, etc., and this influences their resting membrane potential.

Lean body mass

The mass of the body minus the fat: heterogeneous mass composed of body water (> 70 % of the total), muscle proteins and constituents of the skeleton.
There are a number of methods for determining the lean body mass. Some of these methods require specialized equipment such as underwater weighing (hydrostatic weighing), BOD POD (a computerized chamber), and DEXA (dual-energy X-ray absorptiometry). Other methods for determining lean body mass are simple such as skin calipers and bioelectric impedance analysis (BIA).


the process by which soluble materials in the soil, such as salts, nutrients, pesticide chemicals or contaminants, are washed into a lower layer of soil or are dissolved and carried away by water.


The process that leads to modification in individual behavior as the result of experience.

Learning disability (LD)

A disorder in one or more of the basic psychological processes involved in understanding or using language, spoken or written, which may manifest itself in an imperfect ability to listen, think, speak, read, write, spell, or do mathematical calculations.  The term includes, but is not limited to conditions such as perceptual handicaps, brain injury, minimal brain dysfunction, dyslexia, and developmental aphasia. The term does not include children who have learning problems which are primarily the result of visual, hearing, or motor handicaps; mental retardation; emotional disturbance; or environmental, cultural, or economic disadvantages.

Least common denominator

The least common denominator of two fractions, a/b and c/d, is the smallest number that contains both b and d as factors.

Least common multiple

The least common multiple of two numbers, and b, is the smallest  number that contains both a and b as factors.


Phosphatides naturally occurring in oil from both plants and animals; A major component of cell membranes containing equal amounts of saturated and monounsaturated fatty acids, phosphate, and choline. Lecithin is a member of the lipid group called phospholipids. Its biochemical name is phosphatidylcholine (PC). Lecithin is capable of forming colloidal solutions in water and possesses emulsifying, wetting and antioxidant properties.

Leg Bag

A small, thick plastic bag that can be tied to the leg and collects urine. It is connected by tubing to a catheter inserted into the urinary bladder.


Fastidious gram-negative rod is isolated from surface water, mud, and thermallypolluted lakes and streams. There is no known soil or animal source. It is pathogenic for humans, causing pneumonia (Legionnaires disease) or a mild, febrile disease (Pontiac fever).


Atheorem that is proved mainly as an aid in proving another theorem.


a transparent, colorless, firm structure of the eye, enclosed in a capsule, located between the iris and the vitreous humor; refracts light to focus images onto the retina at the back of the eye; in old age the lens becomes flattened, more dense, slightly opaque, and amber-tinted.

Lentic water

ponds or lakes (standing water).


A hormone produced mainly by the fat cells of the adipose tissue in the body, that helps regulate food intake, metabolism and reproduction. Leptin has also been shown to promote and sustain the bodys immune response by binding to T lymphocytes. In this regard it appears to actively contribute to the pathogenesis of experimental autoimmune encephalomyelitis (EAE) in mice, influencing both its onset and clinical severity, a disease that is being used as a model of human multiple sclerosis (MS). MS is characterized by the production of autoreactive T lymphocytes that turn against the body and attack cells within the brain and spinal cord, first inducing weight loss and ultimately resulting in paralysis.

Leptokurtic Distribution

A leptokurtic distribution is a gaussian distribution having a positive kurtosis and a narrow shaped peakedness.


Fundamental particles that are relatively non-reactive and capable of an independent existence: electrons, muons, tau particles and neutrinos.

Leucine zipper

A motif found in certain proteins in which Leu residues are evenly spaced through an -helical region, such that they would end up on the same face of the helix.
Dimers can form between two such proteins. The Leu zipper is important in the function of transcription factors such as Fos and Jun and related proteins.


A white blood cell, an important component of the body's immune system, such as , such as in phagocytosis or antibody production..


A condition in which there is an abnormal increase in the number of white blood cells present in the blood.


The operation of cutting the white matter in the oval center of the frontal lobe of the brainAlso known as Lobotomy.


A type of prostaglandin produced by various white blood cells involved in the inflammatory and immune responses and in allergic reactions.
They are lipid mediator molecules (synthesized from arachidonic acid via 5-lipoxygenase enzyme) that are synthesized & released by certain inflammatory cells (i.e., macrophages, polymorphonuclear leukocytes (PMN), mast cells, T cells). They "signal" leukocytes (white blood cells) during the initial stages of an infection or an allergic reaction. Their primary mode of action is through certain G-protein-coupled receptors. When thus activated, the leukocytes migrate to the site of infection to combat the pathogens (or allergens), and mediate the inflammation.

Leuprolide Stim Test

Specialized blood test used for diagnosing early or delayed puberty.A leuprolide stimulation test is done to determine a child's stage of puberty. The test may be done when it is suspected that a child may soon be entering puberty, or when a child has late puberty. The leuprolide stim test is done by placing an IV catheter in a vein and drawing a small amount of blood to measure hormone levels. Then a medication called Leuprolide Acetate is given by injection into the fatty tissue just under the skin. Blood is drawn 1-3 times after the medication is given to measure hormone levels again. The IV catheter is then removed and the child goes home. Placement of the IV catheter and the injection of medication is slightly painful, but is done quickly.


a natural or manmade earthen barrier along the edge of a stream, lake, or river. Land alongside rivers can be protected from flooding by levees.


Rigid structure that moves at a fixed point called the fulcrum;
anatomically, the bone is the lever and the joint is the fulcrum.
First-class lever: Fulcrum is placed between the applied force and the resistance.
common example: seesaw
anatomical example
fulcrum: atlanto-occipital joint
force: splenius capitis
weight: head
Second-class lever: The resistance is between the applied force and the fulcrum.
common example: wheelbarrow
anatomical example
fulcrum: metatarsophalangeal joints (ball of foot)
force: gastrocnemius
weight: body mass on middle of foot during plantar flexion while standing
Third-class lever: The applied force is between the resistance and the fulcrum.
common examples: baseball bat, tweezers
anatomical example
fulcrum: elbow
force: biceps brachii
weight: forearm and hand


A library might be either a genomic library, or a cDNA library. In either case, the library is just a tube carrying an unordered mixture of thousands of different clones - bacteria or l phages. Each clone carries an "insert" - the cloned DNA from a particular organism.  The relationship of the clones to each other can be established by physical mapping.
cDNA library is usually just a mixture of bacteria, where each bacteria carries a different plasmid. Inserted into the plasmids (one per plasmid) are thousands of different pieces of cDNA (each typ. 500-5000 bp) copied from some source of mRNA, for example, total liver mRNA. The basic idea is that if you have a large enough number of different liver-derived cDNAs carried in those bacteria, there is a 99% probability that a cDNA copy of any given liver mRNA exists somewhere in the tube. The real trick is to find the one you want out of that mess - a process called screening (see "Screening").
genomic library is similar in concept to a cDNA library, but differs in three major ways - 1) the library carries pieces of genomic DNA (and so contains introns and flanking regions, as well as coding and untranslated); 2) you need bacteriophage or cosmids, rather than plasmids, because... 3) the inserts are usually 5-15 kb long (in a l library) or 20-40 kb (in a cosmid library). Therefore, a genomic library is most commonly a tube containing a mixture of  phages. Enough different phages must be present in the library so that any given piece of DNA from the source genome has a 99% probability of being present.

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.


A table of data summarizing mortality in a population.


A type of fibrous connective tissue that joins bones together at joints.
Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place.


A ligand is a substance which is capable of binding specifically and reversibly with a binder; The molecule which binds to a protein molecule such as a receptor size, on another molecule. As a ligand binds through the interaction of many weak, noncovalent bonds formed to the binding site of a protein, the tight binding of a ligand depends upon a precise fit to the surface-exposed amino acid residues on the protein. A ligand is termed an antigen when the binder is an antibody.

Ligand excess

Ligand excess is the presence of excess ligand, in relation to the binder concentration. Severe ligand excess can result in increased solubility of ligand-binder complexes, decreased apparent reactivity, and underestimation of the ligand quantity

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.


An enzyme, T4 DNA ligase, which can link pieces of DNA together.
The pieces must have compatible ends (both of them blunt, or else mutually compatible sticky ends), and the ligation reaction requires ATP.


The process of splicing two pieces of DNA together. In practice, a pool of DNA fragments are treated with ligase (see Ligase) in the presence of ATP, and all possible splicing products are produced, including circularized forms and end-to-end ligation of 2, 3 or more pieces. Usually, only some of these products are useful, and the investigator must have some way of selecting the desirable ones.


Type of phytoestrogen found in flax, rye and various vegetables. May lower LDL cholesterol, total cholesterol and triglycerides thereby protecting against heart disease and some cancers.


The visible part of the electromagnetic spectrum: red, orange, yellow, green, blue, indigo and violet. 
White light is a combination of all these colours.


a powerful flash of electricity between the negative electrical charges in clouds or between a cloud and the ground.

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

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.

Like terms

Two terms each of whose parts, with the exception of their coefficients, is the same.

Limbic system

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.


A straight set of points that extends off into infinity in both directions.

Linea alba (white line)

Tendinous band that runs along the midline of the rectus abdominis.


Any continuous line of descent; any series of organisms connected by reproduction by parent of offspring. A group of organisms, cells, or genes linked to one another through a continuous line of descent, i.e., through parent-offspring connections.

Linear Regression Coefficients

A least squares linear regression is a model based equation which describes a straight line. It is defined with two coefficients. Coefficient a is the y-intercept; the point on the y-axis that the line intersects. Coefficient b is the slope of the line.

Linear Binding Region

The linear binding region of an immunoassay standard curve is any region (usually midrange) which appears linear when the standard responses are plotted against their respective concentrations. The length of this region is only a portion of the usable concentration range of the test method and the length of this region (if present) varies among test methods. A linear binding region apparent in an immunoassay standard is not due to any fundamental relationships like the spectral properties of substances in the Beer-Lambert Law.

Line scale

Unstructured scale usually drawn as a line for marking a decision.
The subject marks on the line their perceived intensity of a stimulus, or rating of an object or sensation.

Line segment

Two points on a line, and all the points between those two points.

Lineweaver-Burk plot (Also “double reciprocal plot”)

In biochemistry, a graphical representation of the Lineweaver-Burk equation of enzyme kinetics.
The plot provides a useful graphical method for analysis of the Michaelis-Menten equation:

Taking the reciprocal gives

where V is the reaction velocity, Km is the Michaelis-Menten constant, Vmax is the maximum reaction velocity, and [S] is the substrate concentration.
The Lineweaver-Burk plot was widely used to determine important terms in enzyme kinetics, such as Km and Vmax before the wide availability of powerful computers and non-linear regression software, as the y-intercept of such a graph is equivalent to the inverse of Vmax; the x-intercept of the graph represents -1/Km. It also gives a quick, visual impression of the different forms of enzyme inhibition.

The double reciprocal plot distorts the error structure of the data, and it is therefore unreliable for the determination of enzyme kinetic parameters. Although it is still used for representation of kinetic data, non-linear regression or alternative linear forms of the Michaelis-Menten equation such as the Eadie-Hofstee plot are generally used for the calculation of parameters.

When used for determining the type of enzyme inhibition, the Lineweaver-Burk plot can distinguish competitive, noncompetitive and uncompetitive inhibitors. Competitive inhibitors have the same y-intercept as uninhibited enzyme (since Vmax is unaffected by competitive inhibitors the inverse of Vmax also doesn't change) but there are different slopes and x-intercepts between the two data sets. Noncompetitive inhibition produces plots with the same x-intercept as uninhibited enzyme (Km is unaffected) but different slopes and y-intercepts. Uncompetitive inhibition causes different intercepts on both the y and x axes but the same slope.


The tendency for certain alleles to be inherited together because they are located on the same chromosome; The proximity of two or more markers (e.g., genes, RFLP markers) on a chromosome.
The closer together the markers are, the lower the probability that they will be separated during DNA repair or replication processes (binary fission in prokaryotes, mitosis or meiosis in eukaryotes), and hence the greater the probability that they will be inherited together.

Linkage group

A pair of homologous chromosomes.

Linkage map

A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together (i.e., 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. Distance is measured in centimorgans (cM).

Linked genes

Genes that are located on the same chromosome.


A small piece of synthetic double-stranded DNA which contains something useful, such as a restriction site. A linker might be ligated onto the end of another piece of DNA to provide a desired restriction site.


One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.A lipid is a water insoluble (hydrophobic) substance and is the name of a large class of structurally and functionally diverse molecules. Important lipids include fatty acids (saturated and unsaturated), they are a component of phospholipids and fats
- phospholipids, main component of biological membranes composed of glycerol phosphate backbone, fatty acid substituents, and hydrophobic headgroups
- sterols, cholesterol derived membrane components, (sex) hormones, and bile salts (intestinal detergents)


A protein bonded to a lipid.This includes the low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) that transport fats and cholesterol in blood. These are protein based carriers of triglycerides (fats) and cholesterol in the blood circulation. The low-density lipoprotein particle LDL is also known as 'bad cholesterol' as it is the major carrier of blood plasma cholesterol and high levels of LDL particles are associated to increased risk of heart disease.


Small lipid particles that can be absorbed into skin.
They are sometimes used as a drug delivery system, as the drug or genetic material can be placed inside the particle, which is then released once it has been absorbed into the skin.

Liquid Chromatography Mass Spectrometry (LC-MS)

A technique that combines the physical separation capabilities of liquid chromatography (see Chromatography) (or HPLC) with the mass analysis capabilities of mass spectrometry.
LC-MS is a powerful technique used for many applications which has very high sensitivity and specificity. Generally its application is oriented towards the specific detection and potential identification of chemicals in the presence of other chemicals (in a complex mixture). (see Gas Chromatography - Mass Spectroscopy)

Liquid crystal display

Thin, flat, and low-power device made up of coloured pixels arrayed in front of a light source (or reflector).


When the telencephalic hemispheres (i.e. cerebral cortex) are relatively smooth (as opposed to "gyrencephalic" when the cortex is highly folded).
The difference is apparent when comparing a human brain with a squirrel brain. In humans (where gyrencephaly is the norm), lissencephaly is considered an abnormality of the brain and supposed to eb associated with some forms of mental retardation.


Listeria monocytogenes is a Gram-positive bacterium, found in at least 37 mammalian species, as well as 17 species of birds and possibly some fish and shellfish. The bacteria can be isolated from soil, and is resistant to heat, freezing and drying. Listeria has been associated with foods such as raw milk, soft-ripened cheeses, ice cream, raw vegetables, raw and cooked poultry, raw meat and raw and smoked fish. Unlike other pathogenic bacteria, such as salmonella, listeria can survive and grow at temperatures as low as 5°C (41°F).

Lithotripsy or Nephrolithotripsy

Crushing kidney stones with sound waves. The crushed material from crushing kidney stones has an easy direct pathway out of the body (down the ureter to the bladder and out the urethra). Nephrolithotripsy is an elective procedure that may spare a patient surgery.

Live gate

Gate through which any data from the flow cytometer/computer parallel interface must pass before acquisition. Any data outside the gate does not enter the computer.
Analysis: Gates used in analyzing the data. Data which fall outside the analysis gate bounds remain in memory but are not included in analysis.

Livestock water use

water used for livestock watering, feed lots, dairy operations, fish farming, and other on-farm needs.

Living modified organism

Any organism in which the genetic material [including both DNA and RNA] has been altered in a way that does not occur naturally by mating and/or natural recombination.Any organism or part thereof which is capable of regenerating itself on its own or in the body or cell of another organism and whose genetic material has been modified by modern biotechnology in a way which does not occur naturally by mating or recombination, [or any living organism or part thereof which had been a fossil but has been resuscitated through modern biotechnology]. These include subcellular particles such as plasmids, DNA fragments and vectors.
Any organism whose genome [has been altered by the insertion of] [contains] foreign DNA (or RNA). The DNA (or RNA) insert is gene construction created through chemical manipulations with certain DNA fragments isolated from different sources (organisms, taxa) or synthesized artificially.

Local optimum

A solution to a problem that is better than all other solutions that are slightly different, but worse than the global optimum.

Locked-in Syndrome

A condition resulting from interruption of motor pathways in the ventral pons, usually by infarction.This disconnection of the motor cells in the lower brain stem and spinal cord from controlling signals issued by the brain leaves the patient completely paralyzed and mute, but able to receive and understand sensory stimuli; communication may be possible by code using blinking, or movements of the jaw or eyes, which can be spared.


A small cavity or hollow space within an organism or in an organ. Also locules, loculi, locular, locule.

Locus (in DNA)

The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean regions of DNA that are expressed (see gene expression).

cLog P

Calculated log P (i.e. a theoretical value).

Log D

Log P when the test is performed at pH 7.4.

Log P

A measure of the lipophilicity of a compound; its partition coefficient between an organic solvent (usually octanol) and an aqueous buffer.
Conc. of non-ionised drug in octanol
Log P = Log  _____________________________________

Conc. of non-ionised drug in buffer

Values usually range from -6 to +6, high positive values indicating high lipid solubility and low or negative values indicating hydrophilic compounds. The test is performed at a pH at which the compound is non-ionised (compare Log D).


if y = bx, then logb y = x.


A five parameter logistic curve is a model based equation which describes a nonlinear curve such as that found with most immunoassay standard curves and in many biological ractions/relationships.
Logistic curves are defined with five coefficients. Coefficient a is the high asymptote. Coefficient b directs the shape of the curve. Coefficient c is the midpoint of the curve. Coefficient d is the low asymptote. Coefficient g is the asymmetry parameter. When the curve is symmetrical and the point of inflection is at the midpoint of the curve, coefficient g equals 1 and the equation becomes a four parameter logistic.

Logistic population growth

A model describing population growth that levels off as population size approaches carrying capacity.


The logit transform is a transformation which stretches the interval between successive responses of sample concentration logarithms disproportionally at the two asymptotic ends of the standard curve and makes the standard curve more linear. A logit transform also disproportionally increases the heteroscedasticity at the two asymptotes.

Longitudinal study

In survey research, a study in which a panel of individuals is interviewed repeatedly over a period of time once. Contrast with  cross sectional studies in which. data are obtained only .

Long latency or transcortical reflexes

The delayed and smaller electromyographic changes that are seen following the sudden stretch of a muscle.  The dorsal column-medial lemniscal system and corticospinal tract are thought to relay the afferent and efferent limbs of this reflex, which may have a role in load compensation.

Long-term self-renewal

The ability of stem cells to replicate themselves by dividing into the same non-specialized cell type over long periods (many months to years) depending on the specific type of stem cell.

Loop of Henle

The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.

Loosening of associations

Thinking characterized by speech in which ideas shift from one subject to another that is completely unrelated or only obliquely related, without the speaker's showing any awareness that the topics are unconnected.
Statements that lack a meaningful relationship may be juxtaposed, or the individual may shift idiosyncratically from one frame of reference to another. When loosening of associations is severe, speech may be incoherent. The term is generally not applied when abrupt shifts in topics are associated with a nearly continuous flow of accelerated speech (as in flight of ideas).
Loosening of associations may be seen in schizophrenia, manic episodes, and other psychotic disorders.


A vase-shaped or cup-shaped outer covering.
An organic or inorganic casing or shell incompletely surrounding an organism, usually loose fitting. Found in many protists, including some flagellates, ciliates, chrysophytes, and choanoflagellates, as well as in some animal cells.

Lotic water

flowing waters, as in streams and rivers.

Lower motoneuron

Lower motoneurons are those motorneurons that directly innervate the skeletal muscles. LMNs are found in some of the cranial nerve nuclei in the brainstem as well as in Layer X in the ventral (anterior) horn of the spinal cord.
(see Lower motoneuron syndrome Upper motoneurons)

Lower motoneuron syndrome

A group of symptoms resulting from disease of either the motoneuron or the muscle or both. Damage to the LMN results in muscle weakness, wasting with a loss of reflexes, more specifically in flaccid paralysis, lack of muscle tone and reflexes, muscle atrophy, and (initially) muscle fasciculations (small, uncoordinated muscle fiber contractions)
(see Lower motoneurons, Upper motoneuron syndrome)

Lowest Specific Binding

The lowest specific binding (LSB) standard is the standard expected to yield the lowest bound response. The LSB is used to calculate one of the method control monitor points in the absence of the appropriate baseline standard. In competitive binding assays, the highest concentration standard is the LSB. In sandwich binding assays, the lowest concentration standard is the LSB.

Low back pain

Pain in the lower back area that can relate to problems with the lumbar spine , the discs between the vertebrae , the ligaments around the spine and discs, the spinal cord and nerves , muscles of the low back, internal organs of the pelvis and abdomen , or the skin covering the lumbar area.

Low tone hypotonic

Having poor muscle tone, appearing floppy.

Lumbar (spinal) puncture or tap (LP)

A technique to sample the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord.
A needle is introduced between the lower bony vertebrae of the spinal column Lab tests on the fluid are used for diagnostic purposes such as presence of bacteria in meningitis, special proteins in multiple sclerosis, or blood cells.


The cavity of a tubular structure, such as endoplasmic reticulum or a blood vessel.


Surgery to remove abnormal tissue or cancer from the breast and a small amount of normal tissue around it. It is a type of breast-sparing surgery.
(Source: NIH National Cancer Institute;)


The invaginated respiratory surfaces of terrestrial vertebrates, land snails, and spiders that connect to the atmosphere by narrow tubes.

Lung cancer

Malignancy of the lung tissue.
Lung cancers arise in the epithelium lining the bronchi (the branching complex of air passages), by which air passes to the lungs, or in the fine air sacs at the periphery. The most common forms arise in bronchial glandular epithelium that has been altered by long exposure to cigarette smoke to form less specialized squamous cells, which eventually evolve into squamous-cell carcinomas. Structurally, unaltered glandular epithelia of bronchi may also undergo malignant transformation to give rise to adenocarcinomas, but these tumours do not appear to be related to cigarette smoking.
There are two basic kinds of lung cancer: small-cell carcinoma and non-small-cell carcinoma. The latter consists primarily of three types of tumour: squamous-cell carcinoma, adenocarcinoma, and large-cell carcinoma.
Small-cell carcinoma, also called oat-cell carcinoma, accounts for about 20 to 25 percent of all lung cancers. It is characterized by cells that are small and round, oval, or shaped like oat grains. Small-cell carcinoma is the most aggressive type of lung cancer; it tends to spread quickly, before symptoms are apparent.
Some 25 to 30 percent of primary lung cancers are squamous-cell carcinomas, also called epidermoid carcinomas. This tumour is characterized by flat, scalelike cells, and it often develops in the larger bronchi of the central part of the lung. Squamous-cell carcinoma tends to remain localized in the lung longer than other types of cancer and thus is generally more responsive to treatment.
Worldwide, adenocarcinoma accounts for some 25 to 30 percent of lung cancers; it is the most common type of lung cancer in the United States. Cells of adenocarcinoma are cube- or column-shaped, and they form structures that resemble glands and are sometimes hollow. Tumours often originate in the smaller, peripheral bronchi; symptoms at the time of diagnosis often reflect invasion of the lymph nodes, pleura, or the other lung or metastasis to other organs.
About 20 % of all lung cancers are large-cell carcinomas. There is some dispute as to whether it is a distinct type of cancer or merely a group of atypical squamous-cell carcinomas and adenocarcinomas. Large-cell carcinomas generally originate in the peripheral areas of the lung.

Lung compliance

The change in lung volume per unit pressure change


Type of carotenoid found in most green vegetables. Positively contributes to maintenance of eye vision.

Lutenising hormone (LH)

gonadotropin (hormone from the pituitary gland) that stimulates the ovaries or testes. A protein hormone secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.


Carotenoid related to the better known beta-carotene.
Lycopene gives tomatoes and some other fruits and vegetables their distinctive red colour. Nutritionally, it functions as an antioxidant. Lycopene is best absorbed by the body when consumed as tomatoes that have been heat-processed using a small amount of oil. This includes products such as tomato sauce and tomato paste.


A clear fluid, derived from interstitial fluid, that travels through the lymphatic system. It carries cells that help fight infections and other diseases. Also called lymphatic fluid.


Peptides and proteins secreted by (immune system response) stimulated T cells.
These hormone-like (peptide and protein) molecules direct the movements and activities of other cells in the immune system. Some examples of lymphokines are interleukin-1, interleukin-2, tumor necrosis factor (TNF), gamma interferon, colony stimulating factors, macrophage chemotactic factor, and lymphocyte growth factor. The suffix "-kine" comes from the Greek word kinesis, meaning movement.

Lymph node

A rounded mass of spongy lymphatic tissue that is surrounded by a capsule of connective tissue. Lymph nodes are located throughout the lymphatic system.  They remove filter lymph (lymphatic fluid) to remove dead cells, debris, and foreign particles from the circulation. They also store lymphocytes (white blood cells) and are sites at which foreign antigens are displayed to immunologically active cells.
Also called lymph gland.

Lymphatic circulation

The circulation of the lymphatic system, which is part of the immune system.
A secondary circulatory system that collects fluids from between the cells and returns it to the main circulatory system.

Lymphatic system

A network of glands and vessels that drain interstitial fluid from body tissues and return it to the circulatory system.
A system of vessels and lymph nodes, separate from the circulatory system.


A type of leukocyte (white blood cell) found in the blood, lymph nodes and certain organs. Lymphocytes are continuously made in the bone marrow. 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. (see also B lymphocytes and T lymphocytes).


Disintegration of a cell by rupture of its plasma membrane.

Lysogenic bacteria

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 / Lyzosome

A membrane-enclosed bag of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.Eukaryotic organelle which carries digestive enzymes. The lyzosome fuses with a vacuolar membrane containing ingested particles, which are then acted upon by the enzymes.


An enzyme in perspiration, tears, and saliva that attacks bacterial cell walls.

Lytic cycle

A type of viral replication cycle resulting in the release of new phages by death or lysis of the host cell.