AQA A Level Biology Topic 6: Organisms Respond to Changes

A comprehensive set of vocabulary flashcards covering survival and response, receptors, nerve impulses, muscle contraction, and homeostasis based on AQA A Level Biology Topic 6.

Stimulus

A change in an organism's internal or external environment.

Tropism

Growth of a plant in response to a directional stimulus; can be positive (towards) or negative (away).

Indoleacetic acid (IAA)

A specific growth factor (auxin) that regulates cell elongation in shoots and inhibits it in roots.

Taxes (tactic response)

A directional response involving movement towards or away from a directional stimulus.

Kinesis (kinetic response)

A non-directional response where the speed of movement or rate of direction change changes in response to the intensity of a stimulus.

Chi-squared test

A statistical test used when data are categorical to see if there is a significant difference between observed and expected frequencies.

Pacinian corpuscle

A receptor specifically responding to mechanical pressure that illustrates the establishment of a generator potential.

Generator potential

The depolarisation of a receptor membrane (e.g., in a Pacinian corpuscle) caused by a stimulus; if it reaches threshold, it triggers an action potential.

Rods

Photoreceptors in the retina providing low visual acuity and monochromatic vision, but high sensitivity to light due to spatial summation.

Cones

Photoreceptors in the retina providing high visual acuity and colour vision (red-, green-, and blue-sensitive types) but lower sensitivity to light.

Myogenic

The property of cardiac muscle that allows it to contract and relax without receiving electrical impulses from nerves.

Sinoatrial node (SAN)

Known as the pacemaker; it releases regular waves of electrical activity across the atria to initiate heart contraction.

Atrioventricular node (AVN)

A node that delays the electrical impulse from the SAN to allow the atria to fully contract and empty before the ventricles contract.

Chemoreceptors

Receptors located in the aorta and carotid arteries that detect changes in blood $CO_2$ concentration or blood pH.

Resting potential

The state where the inside of an axon has a negative charge relative to the outside, typically maintained by the active transport of 3 $Na^+$ out and 2 $K^+$ in via a pump.

All-or-nothing principle

The principle that an action potential is only produced if depolarisation exceeds threshold, and all action potentials produced are of the same magnitude.

Saltatory conduction

The process in myelinated axons where the nerve impulse (depolarisation) occurs only at the nodes of Ranvier, increasing the speed of conductance.

Refractory period

The time following an action potential where $Na^+$ channels are closed/inactive, ensuring discrete, unidirectional impulses and limiting their frequency.

Cholinergic synapse

A synapse that uses the neurotransmitter acetylcholine (ACh).

Acetylcholinesterase

An enzyme that hydrolyses acetylcholine into products reabsorbed by the presynaptic neurone to stop overstimulation.

Spatial summation

When many pre-synaptic neurones collectively release sufficient neurotransmitter into one synaptic cleft to reach the threshold for an action potential.

Temporal summation

When one pre-synaptic neurone releases neurotransmitter many times over a short time to reach the threshold for an action potential.

Inhibitory synapses

Synapses that hyperpolarise the postsynaptic membrane by opening $Cl^-$ or $K^+$ channels, reducing the likelihood of an action potential.

Sarcomere

The functional unit of a myofibril, delineated by Z-lines, which contains actin and myosin filaments.

Actinomyosin crossbridge

A structure formed when a myosin head binds to an exposed binding site on an actin filament.

Phosphocreatine

A chemical that provides a source of inorganic phosphate ($P_i$) to rapidly phosphorylate ADP into ATP during short bursts of vigorous exercise.

Slow twitch muscle fibres

Muscle fibres specialized for slow, sustained contractions (aerobic), containing high concentrations of myoglobin, mitochondria, and capillaries.

Homeostasis

The maintenance of a stable internal environment within restricted limits by physiological control systems.

Negative feedback

A control mechanism where receptors detect a change from the optimum and effectors respond to counteract and return levels to normal.

Glycogenesis

The metabolic process of converting glucose into glycogen for storage in the liver.

Glycogenolysis

The metabolic process of hydrolysis of glycogen into glucose.

Gluconeogenesis

The metabolic process of converting non-carbohydrate sources, like amino acids or glycerol, into glucose.

Second messenger model

The process where hormones like adrenaline or glucagon bind to receptors, activating adenylate cyclase to produce cyclic AMP ($cAMP$).

Type I diabetes

A condition where $\beta$ cells in the islets of Langerhans produce insufficient insulin, usually due to an autoimmune response.

Type II diabetes

A condition where body cell receptors lose responsiveness to insulin, often associated with obesity.

Ultrafiltration

The process where high hydrostatic pressure in the glomerulus forces small substances into the Bowman's capsule to form glomerular filtrate.

Osmoregulation

The control of the water potential of the blood by negative feedback mechanisms involving the hypothalamus and ADH.

Antidiuretic hormone (ADH)

A hormone produced by the hypothalamus that increases the permeability of the collecting duct and DCT to water by adding aquaporins.