SACE Stage 2 Biology key terms

The maintenance of a stable internal environment within narrow limits, despite changes outside the body. It relies on receptors detecting change, a control centre processing it, and effectors producing a response.

A control mechanism in which a detected deviation from the set point triggers a response that reduces that deviation, returning the body to its normal range. Most physiological variables, including temperature and blood glucose, are regulated this way.

A control mechanism in which a change is amplified rather than reversed, pushing the system further from its starting state. It is uncommon and usually short-lived, such as oxytocin-driven contractions during childbirth or clot formation.

The target value around which a regulated variable is held, such as 37 degrees Celsius for core body temperature. Deviations above or below this value stimulate corrective responses.

A specialised cell, tissue or protein that detects a specific stimulus, such as temperature, stretch, light or a chemical messenger, and converts it into a signal the body can process.

A muscle, gland or organ that carries out the response commanded by a control pathway, for example a sweat gland releasing fluid or a muscle contracting to shiver and generate heat.

A chemical messenger secreted by an endocrine gland or cell into the bloodstream, which travels to target cells bearing matching receptors and alters their activity. Examples include insulin, glucagon, thyroxine and adrenaline.

Communication in which a cell releases a messenger into the blood or tissue fluid so it can act on distant target cells. It is typically slower and longer-lasting than nervous signalling.

A cell that responds to a particular signalling molecule because it carries the specific receptor for that molecule. Cells without the receptor are unaffected, which explains why one hormone can act on only some tissues.

A chemical released from the end of a neuron into a synapse, where it binds receptors on the next cell and either excites or inhibits it. Examples include acetylcholine, dopamine and serotonin.

The junction between a neuron and another neuron, muscle or gland cell, across which a signal passes by diffusion of a chemical messenger. The small gap ensures transmission is one-directional.

A brief, self-propagating reversal of membrane potential travelling along a neuron or muscle fibre. It results from sequential opening of sodium and potassium channels and is followed by a refractory period.

A shift of membrane potential towards a less negative value, usually caused by sodium ions entering the cell. If it reaches threshold, an action potential is triggered.

The membrane potential that must be reached for voltage-gated ion channels to open and trigger an action potential. Stimuli too weak to reach it cause only a local change that fades.

A short interval after an action potential during which the neuron cannot fire again, or needs a stronger stimulus to do so. It limits maximum firing frequency and keeps signals travelling in one direction.

A protein (or sometimes RNA) catalyst that speeds up a specific biochemical reaction by lowering its activation energy. Its activity depends on shape, temperature, pH and substrate concentration.

The region of an enzyme where substrate molecules bind and are converted to product. Its shape and chemical properties are complementary to the substrate, giving the enzyme specificity.

Loss of a protein's functional three-dimensional shape caused by heat, extreme pH or chemicals, which breaks bonds holding the structure together. The protein usually cannot perform its role afterwards.

The molecule or molecules on which an enzyme acts, binding at the active site to form an enzyme-substrate complex before being converted into products.

Adenosine triphosphate, the universal energy-carrying molecule of cells. Energy released when its terminal phosphate bond is broken drives processes such as active transport and muscle contraction.

The controlled release of energy from organic molecules such as glucose, producing ATP. Aerobic respiration uses oxygen and yields much more ATP than anaerobic pathways such as fermentation.

ATP production without oxygen, using partial breakdown of glucose in the cytoplasm. It yields small amounts of ATP and produces lactate in animals or ethanol and carbon dioxide in yeast.

The light-driven conversion of carbon dioxide and water into organic compounds, releasing oxygen. It occurs in chloroplasts and supplies the energy and carbon that most food chains depend on.

A plant cell organelle containing chlorophyll and stacked membranes where light energy is captured and converted into chemical energy. It also contains enzymes for carbon fixation.

Nuclear division producing two daughter cells that are genetically identical to the parent cell. It is used for growth, repair and asexual reproduction and maintains the chromosome number.

Two successive nuclear divisions producing four daughter cells, each with half the chromosome number of the parent. Crossing over and independent assortment during meiosis create genetic variation in gametes.

Exchange of corresponding segments between non-sister chromatids of a homologous pair during prophase I of meiosis. It produces new combinations of alleles on a chromosome.

The random orientation of each homologous pair at the equator during meiosis, so that chromosomes from the mother and father are distributed to gametes independently of one another.

One of two or more alternative versions of a gene occupying the same position on a chromosome. Different alleles may produce different phenotypes, and an individual may carry two copies.

The particular combination of alleles an individual carries for a gene or set of genes, for example homozygous dominant, heterozygous or homozygous recessive.

The observable characteristics of an organism, produced by the interaction between its genotype and its environment. Examples include eye colour, enzyme activity and body mass.

The process by which individuals with inherited traits that improve survival and reproduction leave more offspring, so those traits become more common in later generations. It requires heritable variation and differential reproductive success.

A permanent change in the DNA sequence of a gene or chromosome. Mutations are the original source of new alleles and may be harmful, neutral or occasionally beneficial.

The total collection of alleles present in a population at a given time. Its allele frequencies change over generations through selection, mutation, gene flow and genetic drift.

The variety of life at all levels, including genetic variation within species, the number of species in an ecosystem, and the range of ecosystems present. It supports ecosystem resilience.

A community of interacting organisms together with the non-living factors of their environment, such as light, water, temperature and soil. Energy and matter move through it via food webs and cycles.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer. Energy transfer between levels is inefficient, so fewer organisms occupy higher levels.

The net movement of particles from a region of higher concentration to one of lower concentration, driven by random motion. It requires no energy input from the cell.

The diffusion of water across a selectively permeable membrane from a region of higher water potential to one of lower water potential. It explains how cells gain or lose water in different solutions.

Movement of substances across a membrane against their concentration gradient, using carrier proteins and energy from ATP. It allows cells to accumulate ions or nutrients at higher concentrations than outside.

A difference in the concentration of a substance between two regions. Its steepness affects the rate of diffusion, and maintaining it often requires energy input.

The sequence by which a cell converts an external signalling molecule into an internal response, often through receptor activation and a chain of relay molecules. It amplifies and regulates the signal.

The ordered sequence of stages a cell passes through between divisions, including growth, DNA replication and mitosis. Checkpoints monitor conditions and can halt progress if problems are detected.

The extent to which an investigation actually measures or tests what it claims to. Valid conclusions require controlling relevant variables and using methods suited to the stated aim.

The consistency of measurements when an investigation is repeated. It is improved by taking multiple readings, using adequate sample sizes and reducing random error.

How close a measured or calculated value is to the true value. It is affected by systematic errors and by the precision of the instruments used.

A factor that is kept constant across treatment groups so it cannot influence the outcome. Controlling these factors lets a researcher attribute differences to the independent variable.

The factor deliberately changed or selected by the researcher in an investigation. It is the presumed cause whose effect on the dependent variable is being examined.

The factor measured or observed in an investigation to assess the effect of the independent variable. It must be defined operationally so that measurements are consistent.

A consistent bias in measurements, caused by faulty equipment, poor calibration or flawed technique, that shifts all readings in the same direction. Repeating does not fix it; the method must change.

Unpredictable variation in measurements caused by uncontrolled factors or limitations of reading instruments. It affects precision and can be reduced by averaging repeated measurements.

A comparison group that does not receive the treatment being tested, or receives a placebo, so the effect of the treatment can be isolated. It should otherwise match the experimental group.

A testable, evidence-based proposed explanation or prediction about the relationship between variables. It should be falsifiable and stated so that results can support or reject it.

A judgement about what the results mean, stating whether the hypothesis was supported and acknowledging limitations. It should be restricted to what the evidence and sample actually allow.

A principle guiding acceptable treatment of living things in research, including informed consent, minimising harm, confidentiality and humane handling of animals. It must be addressed before data collection begins.

Independent scrutiny of a scientific report by qualified experts before publication. It checks methods, interpretation and clarity, helping to filter flawed or unsupported work.

A well-tested explanation for a broad set of observations, supported by extensive evidence and accepted by most specialists. It can be revised or replaced when new evidence conflicts with it.

Procedures that prevent contamination of cultures and equipment by unwanted microorganisms, such as flaming loops, sterilising surfaces and working near a flame. It protects both the experiment and the worker.

A graph made from known concentrations of a substance, used to convert an unknown measurement such as absorbance into a concentration. It must be prepared under the same conditions as the samples.

An independent repeat of a measurement or treatment within an investigation. Replicates allow variation to be estimated and increase confidence that results are not due to chance.