OSAT Biological Sciences (110) key terms

The principle that all living organisms are made of one or more cells, that the cell is the basic unit of structure and function, and that new cells arise only from existing cells by division.

The active maintenance of a relatively stable internal environment, such as temperature, pH, or solute concentration, despite changes in external conditions. Achieved through feedback mechanisms operating at cell, tissue, and organ-system levels.

A control mechanism in which a change in a variable triggers a response that counteracts the change, returning the variable toward its set point. It is the dominant regulatory pattern in physiology, seen in thermoregulation and blood glucose control.

A control mechanism in which a change in a variable triggers a response that amplifies that change, driving the system further from its starting state. Examples include uterine contractions during childbirth and the clotting cascade.

A single-celled organism lacking a membrane-bound nucleus and other membrane-bound organelles. Bacteria and archaea are prokaryotes; their DNA typically forms a circular chromosome in a nucleoid region, and they reproduce by binary fission.

An organism whose cells contain a membrane-bound nucleus enclosing the genetic material and other membrane-bound organelles such as mitochondria and the endoplasmic reticulum. Includes protists, fungi, plants, and animals.

The explanation that mitochondria and chloroplasts descended from free-living prokaryotes engulfed by an ancestral host cell. Evidence includes their double membranes, circular DNA, and ribosomes resembling those of bacteria.

A membrane-bound organelle that carries out aerobic respiration, using the electron transport chain and chemiosmosis to generate most of the cell's ATP. Its inner membrane is folded into cristae, increasing surface area for ATP synthesis.

The photosynthetic organelle of plants and algae, containing thylakoid membranes stacked into grana and a fluid stroma. Light reactions occur in the thylakoids and the Calvin cycle occurs in the stroma.

The selectively permeable phospholipid bilayer surrounding every cell, studded with proteins and cholesterol. It regulates the movement of substances in and out and participates in recognition and signaling.

The net movement of particles from a region of higher concentration to one of lower concentration, driven by random molecular motion. It requires no energy input and continues until equilibrium is reached.

The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration. It governs cell volume in hypotonic, isotonic, and hypertonic surroundings.

Movement of a substance across a membrane against its concentration gradient, requiring energy, usually from ATP hydrolysis. The sodium-potassium pump is a classic example of primary active transport.

A protein, or occasionally an RNA molecule, that speeds a biochemical reaction by lowering its activation energy without being consumed. Enzymes are specific to substrates and sensitive to temperature and pH.

Loss of a protein's functional three-dimensional shape caused by heat, extremes of pH, or chemical agents that disrupt hydrogen bonds and other weak interactions. Denatured enzymes lose catalytic activity, often irreversibly.

Adenosine triphosphate, the universal energy currency of cells. Hydrolysis of its terminal phosphate bond releases free energy that powers endergonic reactions, transport, and mechanical work.

The catabolic process by which cells oxidize glucose and other fuels to produce ATP. Aerobic respiration proceeds through glycolysis, the Krebs cycle, and oxidative phosphorylation; anaerobic respiration ends in fermentation.

The first stage of cellular respiration, occurring in the cytoplasm, in which one glucose molecule is split into two pyruvate molecules with a net gain of two ATP and two NADH. It requires no oxygen.

Also called the citric acid cycle, a series of mitochondrial reactions that oxidize acetyl-CoA to carbon dioxide, generating NADH, FADH2, and a small amount of ATP. It supplies electrons to the electron transport chain.

The light-driven process by which autotrophs convert carbon dioxide and water into glucose and oxygen. Light reactions in the thylakoids produce ATP and NADPH, which the Calvin cycle uses to fix carbon.

The light-independent sequence of reactions in the chloroplast stroma in which CO2 is fixed by rubisco and reduced to carbohydrate using ATP and NADPH from the light reactions.

The division of a eukaryotic nucleus producing two genetically identical daughter nuclei with the same chromosome number as the parent. It supports growth, repair, and asexual reproduction and includes prophase, metaphase, anaphase, and telophase.

A two-part reduction division that produces four haploid gametes from one diploid cell. Crossing over in prophase I and independent assortment in metaphase I generate genetic variation among the products.

Exchange of corresponding chromosome segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis. It creates new allele combinations and contributes to genetic variation in offspring.

Mendel's principle that alleles of different genes are distributed to gametes independently of one another, because homologous chromosome pairs align randomly at metaphase I. It increases the number of possible gamete genotypes.

The principles of segregation, independent assortment, and dominance derived from Gregor Mendel's pea plant experiments. Segregation states that the two alleles of a gene separate during gamete formation.

One of two or more alternative forms of a gene at a given locus, arising by mutation. An organism's combination of alleles determines its genotype, which interacts with the environment to produce phenotype.

The specific combination of alleles an organism carries for a gene or set of genes, such as homozygous dominant, heterozygous, or homozygous recessive. Genotype is distinct from the observable phenotype.

The observable traits of an organism, produced by the interaction of its genotype with the environment. Phenotype includes morphology, physiology, and behavior and is the target of natural selection.

A grid diagram used to predict the genotypes and phenotype ratios of offspring from a cross, by combining the possible gametes of each parent. It applies the principles of segregation and independent assortment.

The semi-conservative process by which a DNA double helix is copied before cell division. Helicase unwinds the strands, DNA polymerase adds complementary nucleotides, and each daughter molecule retains one original strand.

Synthesis of an RNA molecule from a DNA template, catalyzed by RNA polymerase. In eukaryotes it occurs in the nucleus, and the resulting messenger RNA is processed before translation.

The process in which ribosomes read messenger RNA codons and assemble the corresponding sequence of amino acids into a polypeptide, using transfer RNA to deliver amino acids. It occurs in the cytoplasm or on the rough endoplasmic reticulum.

A heritable change in the nucleotide sequence of DNA, caused by replication errors, radiation, or chemicals. Mutations may be silent, missense, or nonsense, and they are the ultimate source of genetic variation.

The differential survival and reproduction of individuals whose heritable traits better suit their environment. Over generations it changes allele frequencies in a population, producing adaptation.

A heritable trait that increases an organism's fitness in its environment, or the evolutionary process producing such traits. Adaptations arise through natural selection acting on variation within populations.

The relative contribution an individual makes to the gene pool of the next generation, often measured as reproductive success. Higher fitness means more surviving offspring carrying the individual's alleles.

The condition under which allele and genotype frequencies in a population remain constant across generations, requiring no mutation, no migration, no selection, random mating, and a large population. Deviations indicate that evolution is occurring.

Random change in allele frequencies from one generation to the next, especially pronounced in small populations. It can fix or eliminate alleles regardless of their effect on fitness.

The evolutionary formation of new, reproductively isolated species from a common ancestor. Allopatric speciation involves geographic separation, while sympatric speciation occurs within a shared range.

The science of naming, describing, and classifying organisms into hierarchical groups such as domain, kingdom, phylum, class, order, family, genus, and species. Modern taxonomy aims to reflect evolutionary relationships.

The evolutionary history and branching relationships of a group of organisms, often depicted as a phylogenetic tree. Relatedness is inferred from shared derived characters and molecular sequence data.

Body parts in different species that share a common evolutionary origin, though they may differ in function, such as the forelimbs of mammals. They provide evidence for descent from a common ancestor.

Body parts with similar function but different evolutionary origins, such as insect and bird wings. They result from convergent evolution in similar environments rather than shared ancestry.

An autotroph that captures light or chemical energy and converts it into organic compounds, forming the base of every food web. Plants, algae, and some bacteria are producers.

A feeding position within a food chain or web, such as producer, primary consumer, or secondary consumer. Energy transfer between levels is inefficient, so biomass typically declines at higher levels.

A network of interconnected food chains that shows the feeding relationships among organisms in a community. It illustrates how energy and matter flow from producers to various consumers and decomposers.

A diagram showing the decrease in usable energy at each successive trophic level. Roughly ten percent of energy is transferred to the next level; the rest is lost mainly as heat through metabolism.

The maximum population size of a species that a given environment can sustain indefinitely, given available food, water, space, and other resources. Populations often follow logistic growth as they approach it.

Any environmental resource or condition that restricts the growth, abundance, or distribution of a population, such as food, water, light, or predators. Density-dependent factors intensify with crowding; density-independent factors do not.

An interaction in which organisms of the same or different species require the same limited resource, reducing fitness for both. It can drive resource partitioning or competitive exclusion.

A close, long-term interaction between individuals of two different species. Forms include mutualism, commensalism, and parasitism, classified by whether each partner benefits, is unaffected, or is harmed.

The predictable sequence of community change over time after a disturbance. Primary succession begins on bare rock with no soil; secondary succession begins where soil and some organisms already remain.

The movement of an element such as carbon, nitrogen, or phosphorus through living organisms and the physical environment. Unlike energy, matter is recycled repeatedly within ecosystems.