NEB Class 12 Biology key terms

The branch of biology that identifies, names, and classifies living organisms into hierarchical groups such as kingdom, phylum, class, order, family, genus, and species, based on shared characteristics and evolutionary relationships.

The internationally accepted system of giving each species a two-part Latin name: the capitalized genus name followed by a lowercase specific epithet, both italicized in print, as in Homo sapiens.

The basic unit of classification: a group of organisms that can interbreed under natural conditions to produce fertile offspring, and which are reproductively isolated from other such groups.

A taxonomic rank grouping one or more closely related species that share common structural features; it forms the first part of an organism's binomial scientific name.

An organism whose cell lacks a membrane-bound nucleus and other membrane-bound organelles; its genetic material lies free in the cytoplasm. Bacteria and cyanobacteria are examples.

An organism whose cells contain a true nucleus enclosed by a nuclear membrane, plus membrane-bound organelles such as mitochondria and chloroplasts. Protists, fungi, plants, and animals are eukaryotes.

Unicellular prokaryotic organisms, usually with a rigid cell wall, that reproduce mainly by binary fission and occupy almost every habitat; some are pathogens while many are beneficial decomposers.

Photosynthetic prokaryotes (blue-green algae) containing chlorophyll a and phycobilin pigments; they carry out oxygenic photosynthesis and can fix atmospheric nitrogen in specialized cells called heterocysts.

A kingdom of heterotrophic eukaryotes with chitinous cell walls that absorb nutrients from their surroundings; they include molds, yeasts, and mushrooms and reproduce by spores.

The mass of branching threadlike hyphae that forms the body of a fungus and absorbs nutrients from the substrate; mushrooms are reproductive structures produced by such a network.

A stable symbiotic association between a fungus and a photosynthetic partner, usually an alga or cyanobacterium; it is a classic example of mutualism and a pioneer organism on bare rock.

The principle stating that all living organisms are made of cells, that the cell is the basic structural and functional unit of life, and that all cells arise from pre-existing cells.

The selectively permeable phospholipid bilayer with embedded proteins that surrounds every cell, controlling the movement of substances in and out and enabling cell recognition and signaling.

Double-membraned organelles in eukaryotic cells where aerobic respiration occurs; the inner membrane is folded into cristae bearing the electron transport chain, and the matrix holds Krebs cycle enzymes.

A green double-membraned organelle of plant cells containing chlorophyll, thylakoid stacks (grana), and stroma, where light energy is captured and converted into chemical energy during photosynthesis.

The net movement of water molecules across a selectively permeable membrane from a region of higher water potential to one of lower water potential, driven by a concentration difference of solutes.

The movement of ions or molecules across a membrane against their concentration gradient, requiring energy from ATP hydrolysis and usually mediated by carrier or pump proteins.

Adenosine triphosphate, the universal energy currency of cells; hydrolysis of its terminal phosphate bond releases energy that drives endergonic reactions, transport, and mechanical work.

A biological catalyst, usually a globular protein, that speeds up a specific metabolic reaction by lowering its activation energy without being consumed, acting through an active site.

The specific region of an enzyme where substrate molecules bind and the catalytic reaction occurs; its shape is complementary to the substrate and determines enzyme specificity.

Loss of a protein's three-dimensional structure and function caused by heat, extremes of pH, or chemicals, which disrupt hydrogen and other weak bonds without breaking peptide bonds.

The process by which green plants, algae, and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen, with chlorophyll as the main pigment.

The light-dependent phase of photosynthesis occurring in the thylakoid membranes, in which water is split, oxygen is released, and ATP and NADPH are produced for the Calvin cycle.

The light-independent phase of photosynthesis in the chloroplast stroma, in which carbon dioxide is fixed by RuBP and reduced using ATP and NADPH to form carbohydrate.

The first stage of cellular respiration, occurring in the cytoplasm, in which one glucose molecule is split and oxidized to two molecules of pyruvate with a net gain of two ATP and two NADH.

A cyclic sequence of reactions in the mitochondrial matrix in which acetyl-CoA is oxidized, releasing carbon dioxide and producing NADH, FADH2, and a small amount of ATP.

Nuclear division producing two genetically identical daughter nuclei with the same chromosome number as the parent cell; it underlies growth, repair, and asexual reproduction.

A reduction division producing four haploid daughter cells from one diploid cell through two successive divisions; it generates gametes and creates genetic variation by crossing over and independent assortment.

The exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis, producing new combinations of alleles in the resulting gametes.

A threadlike structure of DNA and associated histone proteins that carries genes in a linear sequence; eukaryotic chromosomes are most condensed and visible during cell division.

A segment of DNA that carries the information for producing a functional product, usually a polypeptide or RNA, and is the basic unit of inheritance.

One of two or more alternative forms of a gene occupying the same locus on homologous chromosomes and giving rise to different expressions of a trait.

The genetic constitution of an organism with respect to one or more genes, describing which alleles it carries, whether homozygous or heterozygous.

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

The principle that the two alleles of a gene separate from each other during gamete formation, so each gamete carries only one allele for each trait.

The semiconservative process in which a DNA double helix unwinds and each strand serves as a template for synthesis of a complementary strand, producing two identical daughter molecules.

The synthesis of a messenger RNA molecule from a DNA template strand, catalyzed by RNA polymerase in the nucleus of eukaryotic cells.

The process in which ribosomes read the sequence of codons on mRNA and assemble the corresponding amino acids into a polypeptide, using transfer RNA to deliver them.

A sequence of three consecutive nucleotides in messenger RNA that specifies one amino acid or a start or stop signal during protein synthesis.

The evolutionary formation of new and distinct species from an ancestral population, usually following reproductive isolation brought about by geographic or genetic barriers.

The differential survival and reproduction of individuals that differ in heritable traits, so that advantageous variations become more common in later generations.

A sudden heritable change in the nucleotide sequence of DNA, which creates new alleles and is a primary source of genetic variation for evolution.

A self-regulating unit formed by a community of interacting organisms together with the non-living environment in which they live, through which energy flows and nutrients cycle.

A linear sequence showing the transfer of energy and matter from producers through herbivores to successive carnivores, with each step forming a trophic level.

A graphical representation of the numbers, biomass, or energy at each trophic level of a food chain, showing progressive decrease in energy toward higher levels.