STPM Biology 964 key terms

A chemical joining in which two molecules bond together with the loss of one water molecule. It builds disaccharides from monosaccharides, triglycerides from glycerol and fatty acids, and polypeptides from amino acids.

A splitting reaction in which a water molecule is used to break a covalent bond between two subunits. Digestion of starch, proteins and lipids in the gut depends on it.

The covalent link formed between the anomeric carbon of one sugar and a hydroxyl group of another, releasing water. It joins monosaccharides into disaccharides and forms the branching in starch and glycogen.

The amide link formed between the carboxyl group of one amino acid and the amino group of the next, with loss of water. Its partial double-bond character makes the polypeptide backbone planar and rigid.

A lipid in which one fatty acid of a triglyceride is replaced by a phosphate-containing group. Its hydrophilic head and hydrophobic tails make it the main building block of cell membranes.

Describing a molecule that has both a hydrophilic region and a hydrophobic region. Phospholipids and membrane proteins behave this way, which drives them to arrange themselves at water-lipid boundaries.

The specific pocket of an enzyme where substrate binds and catalysis occurs. Its shape and charge distribution are complementary to the substrate, and it is destroyed by denaturation.

The change in shape of an enzyme's active site as the substrate binds, so the site moulds around the substrate and strains its bonds. It refines the older lock-and-key idea.

Loss of an enzyme's three-dimensional shape, and therefore its catalytic activity, caused by heat, extremes of pH, heavy metals or organic solvents. Hydrogen and ionic bonds holding the tertiary structure break.

A reversible inhibition in which a molecule resembling the substrate occupies the active site, so substrate cannot bind. Raising substrate concentration overcomes it, and the maximum rate is unchanged.

A reversible inhibition in which an inhibitor binds a site other than the active site and alters the enzyme's shape, so fewer enzyme molecules can catalyse the reaction. Adding more substrate does not relieve it.

A regulatory binding site on an enzyme separate from the active site. Binding of an effector there changes the enzyme's conformation and switches activity up or down, as in feedback inhibition of metabolic pathways.

The overall three-dimensional folding of a single polypeptide chain, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between distant side chains. It determines the protein's function.

The arrangement of two or more polypeptide chains, or a polypeptide with a non-protein prosthetic group, into one functional protein. Haemoglobin with its four globin chains is the standard example.

The description of the plasma membrane as a phospholipid bilayer with proteins floating in or spanning it, plus cholesterol and glycolipids. Both lipids and proteins can move laterally, giving the membrane fluidity.

Passive movement of substances across a membrane through specific carrier or channel proteins, down a concentration gradient and without energy from ATP. It is selective and shows saturation kinetics.

Movement of ions or molecules across a membrane against their concentration gradient using energy from ATP hydrolysis and a carrier protein. The sodium-potassium pump and root hair ion uptake are examples.

The tendency of water to move from one region to another, measured in kPa and equal to zero for pure water at atmospheric pressure. Water moves from higher to lower water potential, so solutes lower it.

The net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential. It is passive and central to turgor and water uptake.

The shrinking of a plant cell's protoplast away from the cell wall when it is placed in a solution of lower water potential and loses water by osmosis. It shows the wall is freely permeable but the membrane is not.

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

Two successive nuclear divisions that halve the chromosome number and generate genetically varied gametes. Crossing over and independent assortment in meiosis I create new combinations of alleles.

Exchange of corresponding segments between non-sister chromatids of a homologous pair during prophase I of meiosis. It breaks linkage and produces recombinant chromosomes, increasing genetic variation.

The random orientation of each homologous pair or of each pair of alleles at meiosis, so alleles of different genes are distributed to gametes independently. It applies only to genes on different chromosomes or far apart.

Inheritance of a gene carried on a sex chromosome, usually the X. Because males have one X, recessive alleles such as haemophilia or red-green colour blindness appear more often in males.

A situation in which both alleles of a heterozygote are fully expressed in the phenotype, as in human AB blood group. Neither allele masks the other, so a distinct third phenotype appears.

A cross between an individual showing a dominant phenotype and a homozygous recessive individual, used to reveal whether the unknown genotype is homozygous or heterozygous. Offspring ratios of 1:1 or all dominant indicate the answer.

An in vitro technique that repeatedly heats, anneals primers and extends DNA with a thermostable polymerase to amplify a chosen sequence exponentially. It is used in diagnosis, forensics and gene cloning.

A bacterial enzyme that cuts DNA at a specific recognition sequence, usually a palindrome, leaving sticky or blunt ends. It protects bacteria from foreign DNA and is the key tool of gene cloning.

Separation of charged molecules such as DNA fragments through a gel under an electric field. Smaller fragments move faster, and comparison with known standards gives sizes for mapping or fingerprinting.

Loss of water vapour from the aerial parts of a plant, mainly through stomata. It drives the upward pull of the transpiration stream and cools leaves, and is regulated by stomatal opening.

The tension created in the xylem water column when water evaporates from leaf mesophyll cells. Cohesion between water molecules and adhesion to xylem walls let this tension draw water up the stem.

Vascular tissue that conducts water and dissolved mineral ions from roots to shoots. Its dead, lignified vessels and tracheids provide support as well as transport, and flow is largely passive.

The transport of dissolved organic substances, especially sucrose and amino acids, through phloem sieve tubes from sources to sinks. It requires energy and is explained by pressure-flow or mass-flow models.

An arrangement in which two fluids flow in opposite directions across a exchange surface, maintaining a gradient along its whole length. Fish gills and the loop of Henle use it to maximise exchange.

The hairpin-shaped section of a kidney tubule that creates a concentration gradient in the medulla. Its descending limb is permeable to water, its ascending limb actively pumps out sodium chloride, enabling urine concentration.

Control of the water and solute content of body fluids within narrow limits despite changes in intake or loss. In mammals it depends on ADH acting on the collecting ducts and on the kidney's countercurrent multiplier.

A control mechanism in which a change in a factor triggers a response that reverses that change, restoring the set point. It maintains body temperature, blood glucose and hormone levels within narrow ranges.

The maintenance of a stable internal environment, such as pH, temperature and glucose concentration, despite external fluctuations. It relies on receptors, a control centre and effectors linked by negative feedback.

The ratio of an organism's or organ's exchange surface area to its volume. It falls as size increases, so large organisms need specialised exchange surfaces and transport systems.

The maximum population size of a species that a habitat can sustain indefinitely given the available resources. Populations growing logistically level off at this value as density-dependent factors intensify.

Population increase at a constant per capita rate, producing a J-shaped curve when resources are unlimited. It occurs during colonisation or after a crash, but cannot persist indefinitely.

The progressive replacement of species in a community over time until a stable climax is reached. Primary succession starts on bare rock; secondary succession follows disturbance where soil already exists.

The relatively stable end stage of succession, in which species composition stays roughly constant because the community is in equilibrium with the climate and soil of the area.

The variety of life at genetic, species and ecosystem levels. It is measured by species richness and evenness, and is threatened by habitat loss, invasive species, pollution and overexploitation.

The differential survival and reproduction of individuals whose inherited traits suit the environment better. Over generations it changes allele frequencies in a population and is the main mechanism of evolution.

A group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups. The definition is difficult to apply to asexual organisms and fossils.

Formation of new species when a population is divided by a physical barrier, so the separated groups diverge genetically and later become reproductively isolated. Geographic separation is the key first step.

A factor other than the independent variable that is kept constant in an experiment so it cannot affect the dependent variable. It makes any observed effect attributable to the variable being tested.

The factor deliberately changed or selected by the investigator in an experiment, for example temperature or light intensity. Its values define the treatment groups and are plotted on the x-axis.

A statement that there is no significant difference or relationship between the variables being studied. Statistical tests such as chi-squared or t-test estimate the probability of obtaining the data if it were true.