GCE A/L Biology flashcards

36 cards. Read the term and the definition here. The deck at the top of this page is optional.

Prokaryotic ribosome

A 70S ribosome made of 50S and 30S subunits, found in bacteria and archaea; it is the target of many antibiotics and differs from the eukaryotic 80S ribosome.

Water potential

The tendency of water to move from one region to another, measured in kPa; it equals solute potential plus pressure potential, and water moves from higher to lower water potential.

Competitive inhibition

An inhibitor with a shape similar to the substrate binds to the active site, blocking the substrate; it raises apparent Km but Vmax can still be reached at high substrate concentration.

Glycosidic bond

A covalent bond formed between two monosaccharides by a condensation reaction that releases water; it is broken by hydrolysis, as in the digestion of starch to maltose.

Crossing over

The exchange of corresponding segments between non-sister chromatids of homologous chromosomes at chiasmata during prophase I of meiosis, producing new allele combinations.

Independent assortment

The random orientation of homologous chromosome pairs at the metaphase I equator, so maternal and paternal chromosomes are distributed independently into gametes, increasing genetic variation.

Semi-conservative replication

DNA replication in which each new double helix contains one original parental strand and one newly synthesised strand, catalysed by DNA polymerase reading the template.

Cytokinesis in plants

Division of the cytoplasm in plant cells by formation of a cell plate from vesicles, which becomes the middle lamella and then new cell walls between daughter cells.

Test cross

Crossing an individual of unknown genotype with a homozygous recessive individual; the offspring phenotypes reveal whether the unknown parent was homozygous or heterozygous.

Codominance

A condition in which both alleles are fully expressed in the heterozygote phenotype, as in the AB blood group where IA and IB alleles both produce their antigens.

Transcription

The synthesis of messenger RNA from the DNA template strand by RNA polymerase in the nucleus, followed by splicing to remove introns before the mRNA leaves the nucleus.

Hardy-Weinberg principle

In a large, randomly mating population with no selection, mutation, or migration, allele and genotype frequencies remain constant; p + q = 1 and p² + 2pq + q² = 1.

Translocation in phloem

The movement of sucrose and amino acids through sieve tube elements from sources to sinks, driven by pressure differences and supported by companion cells.

Transpiration stream

The continuous movement of water up the xylem from roots to leaves, driven by evaporation from leaves and maintained by cohesion between water molecules and adhesion to xylem walls.

Limiting factor

The factor in shortest supply that restricts the rate of a process such as photosynthesis; increasing it raises the rate until another factor becomes limiting.

Guard cells

Specialised epidermal cells surrounding a stoma; they take up potassium ions and water, become turgid, and change shape to open the pore for gas exchange.

Bohr shift

The rightward shift of the haemoglobin dissociation curve caused by high carbon dioxide concentration, low pH, or high temperature, which reduces haemoglobin's affinity for oxygen and aids unloading.

Selective reabsorption

The active uptake of useful substances such as glucose and amino acids from the proximal convoluted tubule back into the blood, followed by osmotic reabsorption of water.

Negative feedback

A control mechanism in which a change from the set point is detected and effectors act to reverse that change, restoring the internal environment to its normal value.

Antibody structure

A Y-shaped immunoglobulin protein made of four polypeptide chains, with variable regions that bind specifically to one antigen; it is secreted by plasma cells during the humoral response.

Binary fission

Asexual reproduction in prokaryotes in which the circular chromosome replicates, the cell elongates, and the cell divides into two genetically identical daughter cells.

Memory cells

Long-lived lymphocytes produced after exposure to an antigen that remain in the blood and enable a faster, stronger secondary immune response on later infection or vaccination.

Log phase

The period of exponential population growth in a bacterial culture when nutrients are plentiful and waste products are low, so the population doubles at a constant rate.

Natural selection

The process by which individuals with inherited characteristics better suited to their environment survive and reproduce more, so advantageous alleles increase in frequency over generations.

Carrying capacity

The maximum population size that an environment can sustain indefinitely, determined by limiting factors such as food, space, and disease; it is the plateau of the logistic growth curve.

Mark-recapture estimate

A method of estimating population size in which a sample is marked, released, and resampled; population size equals total marked times second sample size divided by marked individuals recaptured.

Eutrophication

Nutrient enrichment of water, usually by nitrate and phosphate runoff, causing algal blooms whose decay depletes oxygen and kills aquatic animals.

Restriction enzyme

An enzyme that cuts DNA at a specific palindromic recognition sequence, producing sticky or blunt ends used to insert genes into plasmids during genetic engineering.

Polymerase chain reaction

A technique that amplifies DNA through repeated cycles of denaturation, primer annealing, and extension by Taq polymerase, producing many copies of a target sequence.

Gel electrophoresis

A technique that separates DNA fragments by size using an electric field; negatively charged DNA moves toward the positive electrode, and smaller fragments travel further through the gel.

Biological control

The use of a natural predator, parasite, or pathogen to reduce pest populations, avoiding chemical residues but requiring careful checks that the agent does not harm other species.

Yoghurt fermentation

The conversion of lactose in milk to lactic acid by Lactobacillus bacteria, lowering the pH and coagulating milk proteins to give yoghurt its thick texture and sour taste.

Benedict's test

A test for reducing sugars in which the blue solution is heated with the sample and turns brick red on formation of a precipitate of copper(I) oxide.

Active transport

The movement of ions or molecules across a membrane against their concentration gradient using carrier proteins and energy from ATP hydrolysis, as in mineral uptake by root hairs.

Chi-squared test

A statistical test comparing observed and expected frequencies; the calculated value is compared with a critical value at the appropriate degrees of freedom to judge significance.

Sex-linked inheritance

Inheritance of a gene located on a sex chromosome, usually the X; recessive X-linked conditions appear more often in males because they have only one X chromosome.