IB Biology SL key terms

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

A single-celled organism lacking a membrane-bound nucleus and membrane-bound organelles; its DNA lies free in the cytoplasm as a circular chromosome, and it typically has a cell wall and 70S ribosomes.

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

The division of a eukaryotic cell into membrane-bound regions with distinct chemical conditions, allowing incompatible reactions and processes to occur simultaneously in the same cell.

Two layers of phospholipid molecules arranged with hydrophobic tails facing inwards and hydrophilic heads facing outwards, forming the basic framework of all cell membranes.

The description of membranes as a fluid phospholipid bilayer with proteins, glycoproteins and cholesterol embedded in or attached to it, all able to move laterally within the layer.

A membrane protein embedded within the phospholipid bilayer, often spanning it, that may act as a channel, pump or receptor for substances or signals.

A protein with attached carbohydrate chains, located on the outer surface of the cell membrane, involved in cell recognition, adhesion and receptor function.

The net movement of particles from a region of higher concentration to a region of lower concentration, driven by random thermal motion and requiring no energy input.

Passive movement of substances across a membrane through specific channel or carrier proteins, down a concentration gradient and without expenditure of ATP.

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

The tendency of water to move from one place to another, measured in kilopascals; pure water at atmospheric pressure has a value of zero, and adding solute lowers it.

Movement of substances across a membrane against their concentration gradient by carrier proteins that use energy from ATP hydrolysis, for example the sodium-potassium pump.

A membrane protein that uses ATP to move three sodium ions out of the cell and two potassium ions in, maintaining electrochemical gradients across the plasma membrane.

A globular protein that acts as a biological catalyst, lowering the activation energy of a specific reaction by binding substrates at its active site.

The region of an enzyme, formed by a few amino acids, that binds the substrate through temporary bonds and where the catalytic reaction takes place.

The change in shape of an enzyme's active site as the substrate binds, which brings catalytic groups into position and tightens the enzyme-substrate complex.

Loss of a protein's three-dimensional shape caused by heat, extreme pH or other agents, which breaks bonds holding the structure and destroys the enzyme's catalytic ability.

Reduction of enzyme activity by a molecule resembling the substrate that binds to the active site and blocks substrate entry; its effect is reduced by raising substrate concentration.

A double-membraned organelle with folded inner cristae where aerobic respiration and ATP synthesis by oxidative phosphorylation occur; it contains its own DNA and 70S ribosomes.

A non-membrane-bound particle made of rRNA and protein that assembles amino acids into polypeptides during translation; it occurs free in cytoplasm or attached to rough endoplasmic reticulum.

A membrane-bound vesicle containing hydrolytic enzymes that digests worn-out organelles, ingested material and other substances at acidic pH inside the cell.

A structure of DNA wrapped around histone proteins that carries genes; in eukaryotes it is linear, while in prokaryotes the DNA is a single circular molecule.

A pair of chromosomes of similar size and shape carrying the same genes at corresponding loci, one inherited from each parent; they pair during meiosis.

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

Two successive nuclear divisions producing four haploid cells with half the chromosome number, in which crossing over and independent assortment generate genetic variation.

Exchange of segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis, producing new combinations of alleles on the resulting chromosomes.

Having one complete set of chromosomes, as in gametes and spores; the human haploid number is 23, written as n.

Having two complete sets of chromosomes, one from each parent, as in most body cells; the human diploid number is 46, written as 2n.

One of the different versions of a gene occupying the same locus on a chromosome, which may differ slightly in nucleotide sequence and produce different phenotypes.

A length of DNA that carries the coded information for making a particular polypeptide or functional RNA, and which is inherited as a unit.

The genetic makeup of an organism in terms of the alleles it carries for a particular gene or set of genes, such as homozygous dominant or heterozygous.

The observable characteristics of an organism resulting from the interaction of its genotype with the environment, including physical, biochemical and behavioural traits.

The synthesis of a messenger RNA molecule by RNA polymerase using one strand of DNA as a template, occurring in the nucleus of eukaryotic cells.

The assembly of a polypeptide chain at a ribosome, directed by the base sequence of mRNA read in codons, with transfer RNA delivering the matching amino acids.

The process by which individuals with inherited variations better suited to their environment survive and reproduce more successfully, so allele frequencies change over generations.

An inherited characteristic of an organism that increases its chance of survival and reproduction in a particular environment, arising through natural selection over many generations.

Change in the heritable characteristics of a population over generations, driven mainly by natural selection acting on variation and measured as shifts in allele frequencies.

A group of organisms that can interbreed to produce fertile offspring under natural conditions, and which is reproductively isolated from other such groups.

The two-part naming system for species in which the genus name is capitalised and the species name is not, both italicised, such as Homo sapiens.

The science of classifying organisms into a hierarchy of groups, from domain and kingdom down to genus and species, based on shared characteristics and evolutionary relationships.

The position an organism occupies in a food chain, determined by the number of energy-transfer steps between it and the producer at the first level.

An autotroph that converts light or chemical energy into organic compounds, forming the base of a food chain and supplying energy to all other trophic levels.

A heterotroph that obtains energy by feeding on other organisms, classified as primary, secondary or tertiary according to its position in the food chain.

A heterotroph such as a bacterium or fungus that breaks down dead organic matter and releases nutrients back into the ecosystem for reuse by producers.

A sequence of organisms in which each feeds on the one before it, showing the direction of energy flow from producer through successive consumers.

The continuous recycling of elements such as carbon and nitrogen between living organisms and the abiotic environment, driven by processes including decomposition, respiration and photosynthesis.

The process in which plants and other autotrophs use light energy to convert carbon dioxide and water into glucose and oxygen, storing energy in organic molecules.

The maximum population size of a species that an ecosystem can sustain indefinitely, determined by available resources and other limiting environmental factors.

A group of organisms of the same species living in the same area at the same time and able to interbreed with one another.

An environmental resource or condition, such as light, water, nutrients or space, that restricts the growth, distribution or population size of organisms.

Warming of the Earth's surface as greenhouse gases absorb and re-emit infrared radiation from the surface, rather than letting it escape into space.

A gas such as carbon dioxide, methane or water vapour that absorbs and re-emits long-wave infrared radiation, contributing to warming of the atmosphere.

Long-term shifts in temperature and weather patterns, currently driven largely by increased greenhouse gas emissions from human activities such as burning fossil fuels.

Meeting the needs of the present generation without compromising the ability of future generations to meet their own needs, balancing economic, social and environmental factors.