NSSCAS Biology 8223 key terms

The number of times larger an image appears compared with the actual size of the object. Calculated by dividing the measured image size by the real object size, using the same units for both.

The smallest distance between two points at which they can still be seen as separate objects. Limited by the wavelength used, so electron microscopes resolve far finer detail than light microscopes.

A specialised structure inside a cell that carries out a particular function, such as a mitochondrion, chloroplast or ribosome. Some are membrane-bound compartments; others, like ribosomes, are not.

The detailed internal arrangement of a cell revealed by an electron microscope, including membranes, ribosomes and other structures too small to be seen with a light microscope.

A reaction in which two molecules join with the loss of a small molecule, usually water. It builds polymers such as polysaccharides, polypeptides and nucleic acids from their monomers.

A reaction in which a bond is broken by the addition of water. It reverses condensation, splitting polymers such as starch, proteins and nucleic acids back into their monomers.

A single small molecule that can be joined to many identical or similar units to form a polymer. Examples include glucose, amino acids and nucleotides.

A test for reducing sugars in which the sample is heated with blue copper(II) sulfate solution. A brick-red precipitate of copper(I) oxide forms if a reducing sugar is present.

A globular protein that acts as a biological catalyst, lowering the activation energy of a reaction. Its specificity comes from the shape and chemistry of its active site.

The small region of an enzyme where substrate molecules bind and the reaction takes place. Its shape and charge distribution are complementary to the substrate.

Loss of an enzyme's three-dimensional shape, and therefore its function, caused by heat, extremes of pH or other agents that break hydrogen and ionic bonds.

A substance with a shape similar to the substrate that binds to the active site and blocks substrate entry. Its effect is reduced by raising the substrate concentration.

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

Passive movement of substances across a membrane through channel or carrier proteins, down a concentration gradient. It needs no ATP but is limited by the number of transport proteins.

The net movement of water molecules through a partially permeable membrane from a region of higher water potential to one of lower water potential.

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

Movement of ions or molecules across a membrane against a concentration gradient using carrier proteins and energy from ATP hydrolysis, for example sodium-potassium pumps and root hair ion uptake.

Movement of two substances across a membrane together on one carrier protein, one moving down its gradient to drive the other up its gradient, as in glucose uptake in the ileum.

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

Two successive nuclear divisions that produce four haploid cells from one diploid cell, with genetic variation created by crossing over and independent assortment of chromosomes.

A pair of chromosomes of the same size and shape carrying the same genes at the same loci, one inherited from each parent. They pair and separate during meiosis.

The copying of DNA in which each new molecule contains one original strand and one newly built strand, so half the parental material is conserved in each daughter molecule.

The synthesis of a messenger RNA copy from one strand of a DNA gene, catalysed by RNA polymerase. It occurs in the nucleus of eukaryotic cells before translation.

The assembly of a polypeptide at a ribosome, in which transfer RNA molecules bring amino acids in the order specified by the base sequence of messenger RNA.

A sequence of three bases in messenger RNA that codes for one amino acid or signals a stop. The genetic code is degenerate because most amino acids have several codons.

One of the alternative forms of a gene occupying the same locus on a chromosome, differing slightly in base sequence and often producing a different phenotype.

The loss of water vapour from the aerial parts of a plant, mainly through stomata. It creates the pull that draws water up the xylem and cools the leaf.

Vascular tissue that transports water and mineral ions upward from the roots. Its dead, lignified vessel elements and tracheids form continuous tubes with pits for lateral movement.

The movement of dissolved sucrose and amino acids through the phloem from sources, such as leaves, to sinks, such as roots, fruits and growing regions.

The movement of air or water over a gas exchange surface to maintain steep concentration gradients of oxygen and carbon dioxide between the environment and the blood.

A tiny air sac in the lungs where gas exchange occurs. Its thin wall, large total surface area, moist lining and dense capillary network make exchange rapid.

The release of energy from glucose in the presence of oxygen, producing carbon dioxide and water. It yields far more ATP than anaerobic respiration and occurs partly in mitochondria.

Release of a small amount of energy from glucose without oxygen. In mammals it forms lactate; in yeast it forms ethanol and carbon dioxide. It allows glycolysis to continue.

Adenosine triphosphate, the universal energy currency of cells. Hydrolysis of its terminal phosphate bond releases energy for processes such as active transport and synthesis.

The first stage of photosynthesis, in which light energy excites electrons in chlorophyll, driving photolysis of water and producing ATP, reduced NADP and oxygen.

The Calvin cycle, in which carbon dioxide is fixed by combination with ribulose bisphosphate and reduced using ATP and reduced NADP to form triose phosphate and then glucose.

The environmental resource in shortest supply that holds a process such as photosynthesis below its maximum possible rate; increasing it raises the rate.

A control mechanism in which a deviation from the normal level triggers a response that reduces the deviation, restoring the value towards its set point.

The maintenance of a stable internal environment within narrow limits, despite changes in the external environment, using receptors, coordination centres and effectors.

A specialised cell or protein that detects a specific stimulus, such as temperature, light or a chemical, and generates a signal passed to a coordination centre.

The position an organism occupies in a food chain, defined by the number of transfers of energy from the producer, such as primary producer, primary consumer or secondary consumer.

The rate at which energy is converted into biomass by organisms in an ecosystem, usually expressed per unit area per unit time. Net productivity subtracts respiratory losses.

The maximum population size of a species that an ecosystem can support sustainably, determined by limiting resources such as food, water, space and shelter.