6. Organisms Respond to Changes in their Environments: Definitions (A Level only) (AQA A-Level Biology): Revision Notes

6. Organisms Respond to Changes in their Environments: Definitions (A Level only)

Acetylcholine: A neurotransmitter that facilitates communication between nerve cells.

Actin: A protein filament present in myofibrils that forms thin strands composed of two twisted chains.

Actinomyosin bridge: The connection established when a myosin head binds to a myosin binding site on an actin filament.

Action potential: A temporary alteration in electrical potential across an axon membrane that occurs when a nerve impulse is transmitted.

Adenylate cyclase: An enzyme that facilitates the transformation of ATP into cAMP.

Adrenaline: A hormone released by the adrenal glands during stress that raises blood glucose levels by stimulating enzymes in glycogenolysis.

Afferent arteriole: The blood vessel originating from the renal artery that delivers blood to the nephron and has a larger diameter than the efferent arteriole, creating a complex network of capillaries called the glomerulus.

All-or-nothing: A principle stating that stimuli exceeding a threshold will produce action potentials of identical magnitude, irrespective of stimulus intensity.

Anisotropic (A) bands: The dark regions in myofibrils containing overlapping actin and myosin filaments.

Antagonistic muscles: Muscle pairs that operate in opposing directions.

Antidiuretic hormone (ADH): A hormone produced by the hypothalamus and released by the posterior pituitary in response to decreased blood water potential, increasing water permeability in the distal convoluted tubule and collecting duct to enhance water reabsorption.

Ascending limb: The portion of the loop of Henle that ascends into the cortex, having a wider diameter than the descending limb with walls impermeable to water, where sodium ions are actively transported out.

Atrioventricular node (AVN): A cluster of cells positioned between the atria that delays the excitation wave and transmits it to the ventricles via the bundle of His.

Atrioventricular septum: A layer of non-conductive tissue separating the right atrium from the left ventricle in mammalian hearts.

Autonomic nervous system: A division of the motor nervous system that transmits impulses to muscles and glands, controlling involuntary functions through sympathetic and parasympathetic divisions. This system is crucial for maintaining homeostasis without conscious control.

Auxins: Plant hormones that regulate cell elongation.

Axon: An elongated fibre that transmits nerve impulses away from the cell body.

Bundle of His: A collection of Purkyne fibres extending from the AVN to the ventricular apex.

Cell body: The neuronal region containing organelles, particularly the nucleus and rough endoplasmic reticulum.

Central nervous system (CNS): The brain and spinal cord.

Chemoreceptor: A receptor in carotid artery walls that detects blood pH changes and sends impulses to the medulla oblongata, increasing impulse frequency when blood pH decreases.

Cholinergic synapse: An excitatory or inhibitory synapse between neurones or between neurones and effector organs that uses acetylcholine as its neurotransmitter.

Collecting duct: The nephron's final section that gathers urine from distal convoluted tubules and delivers it to the renal pelvis, with water permeability controlled by ADH.

Cone cells: Light receptor cells that convert light energy into generator potentials, concentrated in the fovea for detecting high-intensity light and producing colour vision, with each cone cell connecting to a single bipolar cell for high visual acuity.

Control mechanism: A self-regulating system comprising five components: optimum point, receptor, coordinator, effector, and feedback mechanism.

Coordinator: Processes receptor information and sends instructions to effectors.

Cyclic AMP (cAMP): A 'second messenger' in adrenaline's mechanism of action that activates protein kinase.

Dendrites: Short, branched extensions from the cell body that receive impulses from other neurones.

Dendrons: Cell body extensions that branch into smaller fibres called dendrites.

Depolarisation: A rapid, temporary change in neuronal membrane potential during nerve impulse transmission, making the axon interior less negative than the exterior.

Descending limb: The loop of Henle portion that descends into the medulla, narrower than the ascending limb with water-permeable walls, causing filtrate water loss as it descends.

Diabetes: A metabolic disorder characterised by poor blood glucose regulation, existing in Type I and Type II forms.

Distal convoluted tubule: The twisted nephron region between the loop of Henle and collecting duct that regulates blood pH through ion reabsorption and controls water and salt concentrations, with water permeability influenced by ADH.

Effector: An organ, tissue, or cell that generates a response to a stimulus.

Efferent arteriole: The blood vessel that removes blood from the glomerulus and subdivides into capillaries, having a smaller diameter than the afferent arteriole, creating hydrostatic pressure in the glomerulus.

Excitatory synapse: A synapse that generates new action potentials when neurotransmitters bind to receptor proteins on the postsynaptic neurone.

Fast-twitch muscle fibres: Muscle fibres that contract rapidly with greater power over shorter durations, suited for anaerobic respiration and intense activity.

Feedback mechanism: The process by which effector-induced system changes are detected by the receptor.

Fovea: The retinal point opposite the pupil that receives maximum light intensity, containing the highest cone cell concentration but no rod cells.

Generator potential: Depolarisation in a sensory receptor cell membrane following stimulus exposure.

Glomerular filtrate: The fluid formed by blood ultrafiltration into the renal capsule, containing water, glucose, mineral ions, and urea.

Glomerulus: A capillary network in the renal capsule specialised for blood filtration, later merging to form the efferent arteriole.

Glucagon: A hormone produced by pancreatic islet α cells that increases blood glucose by stimulating gluconeogenesis enzymes and glycogen-to-glucose conversion.

Gluconeogenesis: Glucose formation from non-carbohydrate sources such as amino acids and glycerol.

Glycogenesis: Glycogen formation from glucose in the liver.

Glycogenolysis: Glycogen breakdown into glucose in the liver.

Gravitropism: A plant's growth response to gravity.

Hormones: Cell signalling molecules produced by endocrine glands and released into blood, travelling to target cells and binding to specific receptors to initiate responses, typically having long-lasting effects.

Hyperpolarisation: A decrease in axon membrane potential, making it more negative than the resting potential.

Hypothalamus: The brain region near the pituitary gland that controls the autonomic nervous system and regulates body temperature and body fluid water potential.

H-zone: The lighter region at each A band's centre.

Indoleacetic acid (IAA): A plant growth factor and auxin type that controls cell elongation, stimulating shoot elongation while inhibiting root elongation.

Inhibitory synapse: A synapse that reduces action potential likelihood in the postsynaptic neurone by causing potassium ion (K⁺) exit and chloride ion (Cl⁻) entry, resulting in postsynaptic neurone hyperpolarisation.

Insulin: A hormone produced by pancreatic islet β cells that decreases blood glucose by activating glucose-to-glycogen conversion enzymes and increasing glucose transport channels in target cell membranes.

Intermediate neurone: A spinal cord neurone connecting sensory to motor neurones.

Iodopsin: The pigment present in cone cells.

Islets of Langerhans: Hormone-producing cell clusters in the pancreas, containing α-cells that secrete glucagon and β-cells that secrete insulin.

Isotropic (I) bands: The lighter myofibril regions containing non-overlapping actin and myosin filaments.

Kinesis: A non-directional stimulus response that alters an organism's movement speed and direction change rate.

Loop of Henle: A loop containing descending (medulla-dipping) and ascending (cortex-rising) limbs surrounded by blood capillaries, creating low water potential in the medulla for water reabsorption.

Medulla oblongata: The brain region controlling heart rate through two centres linked to the SAN, one connected to the sympathetic nervous system (increasing heart rate) and another to the parasympathetic nervous system (decreasing heart rate).

Motor neurone: A neurone carrying impulses from the CNS to effectors.

Myelin sheath: An electrically insulating layer of Schwann cell membranes that accelerates nerve impulse transmission.

Myofibrils: Small contractile muscle fibres that group together to form muscles, consisting of actin and myosin protein filaments.

Myogenic: Describes cardiac muscle tissue that initiates contraction independently without external nervous stimulation.

Myosin: A protein filament in myofibrils that forms thick strands with long tails and bulbous heads positioned laterally.

Myosin binding site: An actin site normally blocked by tropomyosin that becomes exposed during muscle contraction, allowing myosin head attachment.

Negative feedback: A feedback mechanism that counteracts the original stimulus and reverses condition changes, restoring the optimum point.

Negative tropism: Plant growth away from a stimulus.

Nephron: The mammalian kidney's functional unit.

Nerve impulse: A depolarisation wave travelling across axon membranes in a self-propagating manner.

Neuromuscular junction: An excitatory synapse between motor neurones and muscle fibres that uses acetylcholine.

Neurones: Nerve cells specialised for rapid nerve impulse transmission.

Neurotransmitters: Chemical messengers facilitating communication between neurones and target cells, stored in presynaptic neurone synaptic vesicles and released into synaptic clefts.

Nodes of Ranvier: Gaps between neighbouring Schwann cells in myelin sheaths where action potentials occur.

Optic nerve: A nerve transmitting electrical impulses from the retina to the brain.

Optimum point: The point of most effective system operation.

Osmoreceptors: Sensory receptor cells in the hypothalamus that detect decreased water potential.

Osmoregulation: Kidney regulation of blood water potential.

Pacinian corpuscle: A sensory receptor detecting mechanical pressure changes.

Parasympathetic nervous system: An autonomic nervous system branch active under normal, resting conditions that inhibits effectors, reducing activity.

Peripheral nervous system (PNS): Nerve pairs originating from the CNS that carry impulses to and from the CNS, divided into sensory and motor nervous systems.

Phosphocreatine: A muscle-stored compound serving as a phosphate reserve for ATP regeneration.

Phototropism: A plant's growth response to light.

Plant growth factors: Hormone-like substances (such as IAA) controlling plant growth in response to external stimuli.

Polarisation: The condition where an axon has a membrane potential of $-65$mV (resting potential).

Positive feedback: A feedback mechanism that amplifies the original stimulus and increases condition changes, moving the system further from the optimum point.

Positive tropism: Plant growth towards a stimulus.

Posterior pituitary gland: The gland responsible for ADH secretion into the bloodstream.

Postsynaptic neurone: The neurone following a synapse that contains specific receptor proteins on its membrane, complementary to the neurotransmitter.

Pressure receptors: Receptors in carotid artery and aorta walls that detect blood pressure changes and transmit impulses to the medulla oblongata, increasing impulse frequency when blood pressure rises.

Presynaptic neurone: The neurone preceding a synapse that releases neurotransmitters from synaptic vesicles into synaptic clefts.

Protein kinase: An enzyme catalysing glycogen-to-glucose conversion.

Proximal convoluted tubule: The twisted nephron portion between the renal capsule and loop of Henle, with epithelial cell walls adapted for glucose and water reabsorption into blood.

Purkyne tissue: Specialised cardiac muscle fibres conducting excitation waves from the AVN to ventricular apexes.

Receptor: A specialised structure detecting specific stimulus types.

Reflex: A rapid, automatic bodily response to sensory stimuli that serves as protection.

Reflex arc: The neuronal pathway in reflex actions: stimulus → receptor → sensory neurone → intermediate neurone → motor neurone → effector → response.

Refractory period: The time following an action potential during which additional action potentials cannot occur, ensuring unidirectional propagation and limiting action potential frequency to maintain discrete nervous impulses.

Renal (Bowman's) capsule: The cup-shaped structure at each nephron's beginning that surrounds the glomerulus, with an inner layer of podocytes through which blood filtration occurs.

Repolarisation: The restoration of resting potential ($-65$mV).

Response: An organismal change resulting from a stimulus.

Resting potential: An electrical potential difference of $-65$ mV across axon membranes, with the axon interior more negative than the exterior, describing the membrane as polarised.

Retina: The mammalian eye's inner layer containing light receptor cells that function as transducers, converting light energy into electrical energy.

Rhodopsin: The pigment in rod cells that breaks down to generate a generator potential.

Rod cells: Light receptor cells that convert light energy into generator potentials, located at retinal periphery for detecting low-intensity light and producing black-and-white images, with many rod cells connecting to single bipolar cells for low visual acuity.

Saltatory conduction: The process of nerve impulse propagation along myelinated neurones, where depolarisation occurs at nodes of Ranvier and action potentials jump between nodes, accelerating transmission.

Sarcomere: Each repeating unit of striations between adjacent Z-lines.

Sarcoplasm: The cytoplasm shared by muscle fibres, containing high concentrations of mitochondria and endoplasmic reticulum.

Schwann cells: Cells surrounding axons with various functions including electrical insulation, axon protection, nerve regeneration, and phagocytosis.

Second messenger model: The mechanism by which hormones (such as adrenaline or glucagon) affect cells by triggering second messenger production, such as cAMP.

Sensory neurone: A neurone transmitting impulses from receptors to the CNS.

Sinoatrial node (SAN): A cell group in the right atrium wall that generates electrical activity, often called the heart's pacemaker.

Skeletal muscle: Voluntary muscle responsible for movement, comprising most body muscle and attaching to the skeleton via tendons.

Sliding filament theory: The muscle contraction mechanism where myosin filaments pull actin filaments towards the sarcomere centre during contraction, with actin filaments sliding along myosin filaments, shortening the sarcomere and reducing muscle length.

Slow-twitch muscle fibres: Muscle fibres contracting slowly with less power over longer periods, suited for aerobic respiration and endurance activities.

Sodium-potassium pump: A carrier protein in axon plasma membranes that actively transports three sodium ions (Na⁺) out of the axon for every two potassium ions (K⁺) pumped in.

Spatial summation: A summation type involving neurotransmitter release from multiple presynaptic neurones, where neurotransmitter concentration exceeds threshold values and triggers postsynaptic neurone action potentials.

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

Stretch-mediated sodium channel: A sodium channel type whose permeability changes with distortion (such as pressure changes or stretching), found in sensory neurone plasma membranes at Pacinian corpuscle centres.

Summation: Neurotransmitter accumulation in synaptic clefts, enabling low-frequency action potentials to trigger new postsynaptic neurone action potentials, occurring in spatial and temporal forms.

Sympathetic nervous system: An autonomic nervous system branch active under stressful conditions that stimulates effectors, accelerating activity.

Synaptic cleft: A small gap between neurones across which nerve impulses are transmitted via neurotransmitters.

Synaptic vesicles: Secretory vesicles in presynaptic neurones that store neurotransmitters and release contents into synaptic clefts upon fusion with presynaptic membranes.

Target cells: Cells with specific receptor proteins on cell surface membranes that are complementary to particular hormone shapes.

Taxis: A directional stimulus response involving organism movement towards or away from stimuli.

Temporal summation: A summation type involving neurotransmitter release from single presynaptic neurones at high frequency, where neurotransmitter concentration exceeds threshold values and triggers postsynaptic neurone action potentials.

Threshold value: The minimum stimulus size required to generate an action potential.

Transducer cells: Cells converting one energy form into electrical signals.

Tropism: A plant's growth response to directional stimuli.

Tropomyosin: A muscle protein forming fibrous strands wrapped around actin filaments.

Type I diabetes: An insulin-dependent diabetes form where the body cannot produce insulin, having early, rapid onset and requiring insulin injection treatment.

Type II diabetes: An insulin-independent diabetes form where the body fails to respond to insulin due to glycoprotein receptor loss or unresponsiveness, sometimes involving insufficient insulin production, having late, slow onset and controlled through diet and exercise management.

Unidirectionality: Describes synaptic transmission characteristics where synapses transmit information in only one direction, from presynaptic to postsynaptic neurones.

Visual acuity: The sharpness of vision.