IB Biology Definitions

IB Biology Definitions

Core Definitions (SL & HL)

Cell Biology

Cell Theory: The unifying biological principle stating that all living organisms are composed of cells, cells are the smallest unit of life, and all cells arise from pre-existing cells.

Differentiation: The process during development where unspecialized cells become specialized in structure and function by expressing specific genes while suppressing others.

Stem Cells: Unspecialized cells characterized by the ability to continuously divide and differentiate into various specialized cell types.

Surface Area to Volume Ratio: A mathematical relationship that limits cell size, as a cell grows, its volume increases faster than its surface area, reducing the efficiency of material exchange.

Resolution: The ability of an optical instrument to distinguish between two points that are close together as separate, distinct entities.

Osmosis: The passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane.

Active Transport: The movement of substances across a cell membrane against their concentration gradient, requiring energy input in the form of ATP.

Simple Diffusion: The passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

Facilitated Diffusion: The passive movement of specific molecules across a cell membrane down a concentration gradient through specialized integral proteins.

Endocytosis: An active cellular process where the plasma membrane envelops external fluid or particles, budding inward to form an internal vesicle.

Exocytosis: An active process where intracellular vesicles fuse with the plasma membrane to release their contents outside the cell.

Mitosis: The division of a eukaryotic cell nucleus into two genetically identical daughter nuclei.

Cytokinesis: The physical division of the cytoplasm of a parental cell into two distinct daughter cells following the division of the nucleus.

Mutagen: An agent, such as a chemical or radiation, that increases the frequency of mutations by permanently altering genetic material.

Oncogene: A mutated gene that has the potential to cause cancer by contributing to the uncontrolled stimulation of the cell cycle.

Molecular Biology

Metabolism: The totality of all enzyme-catalyzed chemical reactions that occur within a living organism to maintain life.

Anabolism: The synthesis of complex molecules from simpler molecules, requiring an input of energy (condensation reactions).

Catabolism: The breakdown of complex molecules into simpler molecules, releasing energy in the process (hydrolysis reactions).

Enzyme: A globular protein that acts as a biological catalyst, speeding up chemical reactions by lowering the activation energy without being altered by the reaction.

Active Site: The specific region on the surface of an enzyme to which a substrate binds and where the catalytic reaction takes place.

Denaturation: A structural change in a protein that results in the permanent loss of its native three-dimensional shape and biological function, typically caused by extreme temperature or pH changes.

Cell Respiration: The controlled release of energy from organic compounds, such as glucose, in cells to form adenosine triphosphate (ATP).

Photosynthesis: The process by which metabolic pathways use light energy, water, and carbon dioxide to synthesize organic compounds like glucose.

Condensation Reaction: A chemical reaction in which two molecules combine to form a larger molecule with the simultaneous elimination of a water molecule.

Hydrolysis Reaction: A chemical reaction that breaks bonds in a molecule by utilizing a water molecule to split the compound into simpler components.

Cohesion: The binding together of two molecules of the same type, such as water molecules sticking to one another via hydrogen bonding.

Adhesion: The binding together of two different types of molecules, such as water molecules sticking to the cellulose walls of xylem vessels.

Proteome: The complete set of unique proteins expressed by a genome, cell, tissue, or organism at a specific time under given conditions.

Genetics

Gene: A heritable factor that consists of a length of DNA and influences a specific characteristic of an organism.

Allele: A specific, alternative form of a gene that differs from other alleles by only one or a few bases and occupies the same gene locus on a chromosome.

Genome: The entire genetic information of an organism, encompassing all of the DNA sequences contained within its chromosomes.

Gene Mutation: A permanent, random change in the base sequence of a section of DNA, which can result in the creation of a new allele.

Homologous Chromosomes: A pair of chromosomes that carry the same sequence of genes, with one chromosome inherited from each parent, though they may carry different alleles of those genes.

Genotype: The specific symbolic representation or combination of alleles possessed by an organism for a particular trait.

Phenotype: The observable physical or physiological characteristics of an organism, determined by its genotype and interaction with the environment.

Dominant Allele: An allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state, masking the presence of a recessive allele.

Recessive Allele: An allele that only influences the phenotype when it is present in the homozygous state and is masked in the presence of a dominant allele.

Codominant Alleles: Pairs of alleles that both affect the phenotype when present in a heterozygote, resulting in a joint or blended expression of both traits.

Homozygous: Having two identical alleles of a gene at a particular locus on homologous chromosomes.

Heterozygous: Having two different alleles of a gene at a particular locus on homologous chromosomes.

Carrier: An individual that has one copy of a recessive allele causing a genetic disease, meaning they do not exhibit symptoms but can pass the allele to offspring.

Sex Linkage: The pattern of inheritance shown by genes located on the sex chromosomes (usually the X chromosome) rather than on autosomes.

Clone: A group of genetically identical organisms or a group of cells derived from a single parent cell.

Ecology & Evolution

Species: A group of organisms that can potentially interbreed to produce fertile, viable offspring.

Population: A group of organisms of the same species who live in the same geographic area at the same time and are capable of interbreeding.

Community: A group of populations of different species living together and interacting with each other in a specific area.

Ecosystem: A biological community of interacting organisms and their physical, non-living (abiotic) environment.

Autotroph: An organism that synthesizes its own organic molecules from simple inorganic substances, typically using light or chemical energy.

Heterotroph: An organism that obtains organic molecules by consuming other living organisms or their organic products.

Detritivore: A type of heterotroph that obtains organic nutrients from detritus or dead organic matter via internal digestion.

Saprotroph: A heterotroph that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of external digestion.

Trophic Level: The feeding position that an organism or group of organisms occupies within a food chain or food web.

Evolution: The cumulative, gradual change in the heritable characteristics of a population over successive generations.

Clade: A monophyletic group of organisms that has evolved from a common ancestor, including the ancestor species and all of its descendants.

Natural Selection: The mechanism of evolution where organisms better adapted to their environment tend to survive and produce more viable offspring.

Adaptive Radiation: The rapid evolutionary diversification of a single ancestral line into many ecologically distinct niches.

Melanism: An increased development of dark-colored pigment in the skin or fur of an organism, often serving as an evolutionary adaptation against predation in polluted areas.

Analogous Structures: Superficial structural similarities between organisms that serve a similar function but evolved independently due to convergent evolution rather than common ancestry.

Homologous Structures: Anatomical features shared by different species that may perform different functions but originated from a common evolutionary ancestor.

Human Physiology

Homeostasis: The maintenance of a stable, constant internal environment within physiological limits, despite changes in the external environment.

Ventilation: The physical process of pumping fresh air into the alveoli and removing stale air from the lungs to maintain a concentration gradient for gas exchange.

Gas Exchange: The diffusion of oxygen and carbon dioxide across a respiratory membrane, such as between the alveoli and the surrounding blood capillaries.

Pathogen: An organism, virus, or microorganism that is capable of causing a disease in a host organism.

Antigen: A foreign substance, usually a protein or polysaccharide on a pathogen’s surface, that triggers a specific immune response by stimulating the production of antibodies.

Antibody: A globular protein produced by B-plasma cells that binds to a specific antigen to destroy or neutralize a pathogen.

Resting Potential: The electrical potential energy across the plasma membrane of a non-conducting neuron, typically around -70 millivolts, maintained by the sodium-potassium pump.

Action Potential: The rapid depolarization and repolarization of the plasma membrane of a neuron as an electrical impulse travels along the axon.

Synapse: The functional junction or gap across which a nerve impulse passes from an axon terminal of one neuron to a dendrite of another neuron or an effector cell.

Hormone: A chemical messenger secreted by endocrine glands directly into the bloodstream, which travels to a specific target organ or cell to elicit a physiological response.

Peristalsis: The waves of involuntary muscular contractions and relaxations along the walls of the digestive tract that force food forward through the lumen.

Absorption: The physiological process where digested food molecules and nutrients are absorbed from the lumen of the gastrointestinal tract into the blood or lymphatic vessels.

Assimilation: The process where absorbed nutrients are transported to cells and integrated into fluid or solid body tissues for metabolic use.

Artery: A thick-walled, muscular blood vessel that carries oxygenated blood away from the heart at high pressure to the rest of the body.

Vein: A thin-walled blood vessel equipped with internal valves that carries deoxygenated blood back to the heart under low pressure.

HL-Only Definitions

Nucleic Acids

Nucleosome: A structural unit of a eukaryotic chromosome consisting of a length of DNA coiled around a core of eight histone proteins.

Okazaki Fragments: Relatively short fragments of DNA synthesized discontinuously on the lagging strand during DNA replication, which are later joined together by DNA ligase.

Intron: A non-coding sequence of DNA within a gene that is transcribed into pre-mRNA but is spliced out before the mature mRNA is translated into a protein.

Exon: A coding sequence of DNA that remains in the mature mRNA molecule after splicing and is subsequently translated into a polypeptide chain.

Promoter: A specific non-coding sequence of DNA located upstream of a gene that acts as a binding site for RNA polymerase to initiate transcription.

Epigenetics: The study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than alterations in the underlying DNA sequence.

Metabolism

Competitive Inhibition: A form of enzyme inhibition where an inhibitor molecule structurally resembles the substrate and binds directly to the active site, preventing substrate binding.

Non-Competitive Inhibition: A form of inhibition where an inhibitor binds to an allosteric site away from the active site, altering the enzyme's shape so the active site can no longer bind the substrate.

End-Product Inhibition: A metabolic control mechanism where the final product of a biochemical pathway acts as a non-competitive inhibitor on an enzyme earlier in the pathway.

Chemiosmosis: The movement of ions across a semipermeable membrane down their electrochemical gradient, specifically the generation of ATP via the movement of hydrogen ions through ATP synthase.

Oxidative Phosphorylation: The metabolic pathway in mitochondria that uses energy released by the oxidation of nutrients to synthesize ATP.

Photolysis: The light-dependent chemical splitting of a water molecule into hydrogen ions, electrons, and oxygen during the light-dependent reactions of photosynthesis.

Plant Biology

Transpiration: The inevitable loss of water vapor from the leaves and stems of plants, primarily through the stomata, driving the upward movement of water through the xylem.

Translocation: The active transport of organic solutes, such as sucrose, through the phloem sieve tubes from a source to a sink.

Source: A plant organ, such as a mature leaf or storage root, that net exports organic solutes like sugars into the phloem.

Sink: A plant organ, such as a growing root, fruit, or developing seed, that consumes or stores organic solutes imported from the phloem.

Meristem: A region of plant tissue consisting of actively dividing, undifferentiated cells found at growing tips of roots and shoots responsible for plant growth.

Genetics & Evolution

Linkage Group: A group of genes whose loci are located on the same chromosome and therefore tend to be inherited together during meiosis unless separated by crossing over.

Polygenic Inheritance: A pattern of inheritance where a single characteristic or phenotype is controlled by the combined effects of two or more independent genes, resulting in continuous variation.

Gene Pool: The total collection of all the different alleles present within an interbreeding population at any given time.

Allopatric Speciation: The formation of a new species occurs because a geographical barrier isolates populations of the ancestral species, preventing gene flow.

Sympatric Speciation: The divergence of a population into separate species while remaining within the same geographic area, often driven by behavioral or temporal isolation.

Animal Physiology

Osmoregulation: The active regulation and maintenance of the osmotic pressure of an organism's fluids to control water and salt balance, primarily managed by the kidneys in humans.

Spermatogenesis: The complex sequence of continuous cellular divisions and transformations by which unspecialized germ cells develop into mature spermatozoa within the testes.

Oogenesis: The process in the ovaries that results in the production of mature female gametes (ova) from primordial germ cells, occurring in distinct cyclical stages.

Acrosome Reaction: The release of hydrolytic enzymes from the acrosome at the head of a sperm cell upon contact with the egg, allowing the sperm to penetrate the jelly coat (zona pellucida) for fertilization.

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