LC Biology key terms

A testable statement predicting the outcome of an experiment, based on prior observation. It must be falsifiable, meaning data could show it to be wrong, and it is usually compared against a control.

A procedure in which all conditions are kept identical except the one factor being tested, so any change in the result can be attributed to that factor alone. A control group receives no treatment or a standard treatment.

Any factor in an experiment that can change. The independent variable is set by the investigator, the dependent variable is measured as the result, and controlled variables are held constant to keep the test fair.

A repeated run of the same experimental treatment. Replicates allow averages to be calculated and reduce the chance that a result is due to random variation or a single unusual sample.

A molecule produced by living organisms, grouped into four main classes: carbohydrates, lipids, proteins and nucleic acids. Each class has characteristic building blocks, solubility and roles in cells.

A reaction in which two molecules join with the loss of a small molecule, usually water. It builds polymers such as starch, proteins and DNA from their subunits; the reverse process, adding water to split them, is hydrolysis.

A protein that speeds up a specific biochemical reaction by lowering its activation energy. Each has an active site whose shape is complementary to its substrate, and activity is affected by temperature and pH.

Loss of a protein's three-dimensional shape, and therefore its function, caused by heat, extremes of pH or certain chemicals. In enzymes this destroys the active site so the substrate no longer fits.

The molecule on which an enzyme acts. It binds temporarily to the enzyme's active site to form an enzyme-substrate complex, is converted to product, and is then released unchanged.

A thin phospholipid bilayer studded with proteins that surrounds every cell. It is selectively permeable, controlling which substances enter and leave, and carries receptors and transport proteins.

The net movement of particles from a region of higher concentration to a region of lower concentration, caused by random motion. It requires no energy input and continues until concentrations are equal.

The movement of water across a selectively permeable membrane from a region of higher water potential to a region of lower water potential. It is passive and central to water uptake in plants and cells.

Nuclear division producing two daughter nuclei genetically identical to the parent, with the same chromosome number. It underlies growth, repair and asexual reproduction; in body cells of humans it maintains 46 chromosomes.

A reduction division 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.

A thread-like structure of DNA and protein found in the nucleus, carrying genes in a linear sequence. Human body cells contain 46 chromosomes arranged as 23 pairs; gametes contain 23 single chromosomes.

One of the alternative forms of a gene occupying the same position on homologous chromosomes. An organism may be homozygous, carrying two identical alleles, or heterozygous, carrying two different alleles for that gene.

The particular combination of alleles an organism carries for a gene or set of genes. It is written as pairs of alleles, such as AA, Aa or aa, and interacts with the environment to produce the phenotype.

The observable characteristics of an organism, such as eye colour or blood group. It results from the genotype expressed in a given environment and may follow dominant, recessive or co-dominant inheritance patterns.

Deliberate alteration of an organism's DNA, for example by inserting a gene from another species into a plasmid vector. The gene is cut out using restriction enzymes and joined using DNA ligase.

A small circular piece of DNA found in bacteria, separate from the main chromosome. Because it replicates independently and can carry inserted genes, it is widely used as a vector in genetic engineering.

The controlled release of energy from glucose in the presence of oxygen, producing carbon dioxide and water and yielding a large amount of ATP. In eukaryotes most stages occur in mitochondria.

Release of energy from glucose without oxygen. In human muscle it produces lactic acid, and in yeast it produces ethanol and carbon dioxide; both yield far less ATP per glucose than aerobic respiration.

Adenosine triphosphate, the universal energy currency of cells. Energy released from respiration is stored in its phosphate bonds and released when ATP is broken down to ADP and phosphate to drive cell processes.

The process in which green plants use light energy, absorbed mainly by chlorophyll, to convert carbon dioxide and water into glucose and oxygen. It stores energy in organic molecules and underpins nearly all food chains.

The green pigment in chloroplasts that absorbs mainly red and blue light for photosynthesis. It is contained in the thylakoid membranes, where light energy is captured and converted to chemical energy.

A community of interacting organisms together with the non-living factors of their environment, such as soil, water, light and temperature. Energy and nutrients flow between the living and non-living parts.

A sequence showing the transfer of energy from one organism to the next by feeding, beginning with a producer and ending with a top consumer. Arrows point in the direction of energy flow.

The recycling of nitrogen between the atmosphere, soil and living organisms. Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia, nitrifying bacteria form nitrates, and decomposers return nitrogen to the soil.

An organism, usually a bacterium or fungus, that breaks down dead organic matter and waste, releasing minerals such as nitrates and phosphates back into the soil for reuse by producers.

The maintenance of a stable internal environment, such as constant body temperature, blood glucose and water balance, despite changes outside the body. It relies on negative feedback loops involving nerves and hormones.

A control mechanism in which a change from the normal level triggers a response that reverses that change, restoring the set point. It is the basis of most homeostatic control, including temperature and blood glucose regulation.

A chemical messenger secreted by an endocrine gland into the blood, which travels to and acts on specific target cells bearing matching receptors. Hormonal responses are usually slower and longer-lasting than nerve responses.

The movement of oxygen into an organism and carbon dioxide out of it, across a moist, thin, well-vascularised surface. In humans this occurs in the alveoli of the lungs by diffusion.

The loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. It draws water up the xylem and is affected by light, temperature, humidity and wind.

Vascular tissue that transports water and dissolved minerals upward from the roots to the leaves. Its dead, hollow, lignified cells form continuous tubes and also provide mechanical support to the plant.

Small pores in the leaf epidermis, each surrounded by two guard cells, that allow carbon dioxide to enter and water vapour and oxygen to leave. They open in light and close when water is short.

A directional growth response of a plant to a one-sided stimulus. Phototropism is growth towards light and geotropism is growth in response to gravity; both are controlled by the plant hormone auxin.