Lebanese Baccalaureate - Life Sciences (LS) key terms

A thin double layer of lipids and proteins surrounding the cytoplasm that controls which substances enter and leave the cell and helps cells recognize one another.

The property of a membrane that lets some molecules pass through easily while blocking or slowing others, allowing the cell to keep a stable internal environment.

The movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration, until concentrations become balanced.

The shrinking and pulling away of the cell membrane from the cell wall when a plant cell loses water in a strongly concentrated solution.

Movement of substances across a membrane against their concentration gradient, using energy from ATP and often a carrier protein.

Adenosine triphosphate, the small energy-carrying molecule produced during cellular respiration that powers most energy-requiring reactions in the cell.

A protein that speeds up a specific chemical reaction by lowering its activation energy and that is not consumed in the reaction.

Loss of a protein's three-dimensional shape and therefore its function, caused by heat, extreme pH or other harsh conditions that break weak bonds.

The set of reactions in which a cell breaks down glucose, using oxygen in aerobic conditions, to release energy stored as ATP and produce carbon dioxide and water.

An anaerobic pathway that breaks down glucose without oxygen, producing a small amount of ATP plus products such as lactate or ethanol and carbon dioxide.

A type of nuclear division producing two daughter cells with the same chromosome number as the parent cell, used for growth, repair and asexual reproduction.

A two-stage nuclear division that halves the chromosome number and produces four genetically different gametes, with crossing-over and independent assortment creating variation.

A structure of DNA wrapped around proteins that carries the genes of a cell; human body cells contain 46, arranged in 23 pairs.

One of the alternative versions of a gene that can occupy the same position on a chromosome and may give different forms of a trait.

The combination of alleles an individual carries for a given gene, written as pairs of letters such as AA, Aa or aa.

The observable characteristics of an organism, resulting from its genotype interacting with the environment.

A characteristic that appears in a heterozygote even when only one copy of its allele is present, masking the effect of the recessive allele.

A characteristic that is expressed only when both alleles for that gene are recessive, so it is hidden in heterozygous individuals.

The exchange of segments between homologous chromosomes during prophase I of meiosis, which produces new combinations of alleles on a chromosome.

A permanent change in the DNA sequence of a gene or chromosome that may alter the protein produced and can be inherited if it occurs in gametes.

A photograph-like arrangement of all the chromosomes of a cell, ordered by size and shape, used to detect abnormalities such as an extra chromosome.

The ability of an organism to keep its internal conditions, such as temperature and blood glucose, within narrow limits despite changes in the environment.

A control mechanism in which a change in a body variable triggers a response that reverses the change and brings the value back toward its set point.

A chemical messenger secreted by an endocrine gland into the blood, which travels to target cells and alters their activity.

A cell that responds to a particular hormone because it carries specific receptor molecules that bind that hormone.

The control of water and dissolved salt balance in the body, carried out largely by the kidneys and adjusted by hormones such as ADH.

The microscopic functional unit of the kidney, where blood is filtered and useful substances are reabsorbed to form urine.

The pressure-driven filtering of blood in the kidney's glomerulus, which forces water and small molecules into the capsule while keeping cells and large proteins in the blood.

A rapid, automatic response to a stimulus that travels through a reflex arc without waiting for a decision from the brain.

A nerve cell specialized to receive, conduct and transmit electrical impulses, consisting of a cell body, dendrites and an axon.

The small gap between two neurons or between a neuron and a muscle, where a chemical neurotransmitter carries the signal across.

A chemical released from the end of an axon into a synapse that binds receptors on the next cell and either excites or inhibits it.

A Y-shaped protein made by certain white blood cells that binds specifically to an antigen and helps neutralize or destroy it.

Any molecule, often on the surface of a pathogen or foreign cell, that is recognized as non-self and triggers an immune response.

A type of white blood cell central to specific immunity; B lymphocytes produce antibodies and T lymphocytes act directly on infected cells.

The introduction of weakened or inactive antigens into the body to stimulate memory cells and give long-lasting protection against a disease.

The position an organism occupies in a food chain, such as producer, primary consumer or secondary consumer, based on how it obtains energy.

A network of interconnected food chains in an ecosystem that shows how energy and matter pass among many organisms.

An organism such as a fungus or bacterium that breaks down dead organic matter and returns mineral nutrients to the soil and ecosystem.

The circulation of an element such as carbon or nitrogen between living organisms and the non-living environment, including air, soil and water.

The process by which individuals with inherited traits better suited to their environment survive and reproduce more, so those traits become more common over generations.

An inherited feature or behavior that improves an organism's chance of surviving and reproducing in its particular environment.

A test in which all variables are kept constant except the one being changed, so that any observed effect can be attributed to that single factor.

The group in an experiment that is treated exactly like the experimental group but does not receive the treatment, providing a baseline for comparison.

The factor deliberately changed or chosen by the experimenter in an investigation, whose effect on the dependent variable is being measured.

The factor that is measured or observed in an experiment and is expected to change in response to the independent variable.