6. Organisms Respond to Changes in their Environments: Key Terms

• Acetylcholine**:** A neurotransmitter that facilitates communication between neurones.
• Actin**:** A protein filament in myofibrils forming thin, twisted chains.
• Actinomyosin bridge**:** A cross-bridge created when a myosin head binds to an actin filament's binding site.
• Action potential**:** A temporary electrical charge change across an axon membrane during a nerve impulse transmission.

• Adenylate cyclase**:** An enzyme that converts ATP into cAMP.
• Adrenaline**:** A hormone released by adrenal glands during stress, increasing blood glucose levels by activating glycogenolysis enzymes.
• Afferent arteriole**:** A renal blood vessel supplying blood to the nephron, branching into the glomerulus.

• All-or-nothing principle**:** The concept that action potentials are of the same size if a stimulus exceeds a threshold, regardless of strength.
• Anisotropic (A) bands**:** Dark bands in myofibrils formed by overlapping actin and myosin filaments.
• Antagonistic muscles**:** Muscle pairs that work in opposite directions.
• Antidiuretic hormone (ADH)****: A hormone produced by the hypothalamus, secreted by the posterior pituitary gland, which increases water reabsorption in the kidney's distal tubules and collecting ducts.
• Ascending limb**:** The section of the loop of Henle moving upwards into the cortex, impermeable to water, actively transporting sodium ions out.

• Atrioventricular node (AVN)****: Cells located between the atria that slow excitation waves and transmit them to the ventricles via the bundle of His.
• Atrioventricular septum**:** A non-conductive tissue layer between the right atrium and left ventricle.
• Autonomic nervous system**:** A motor nervous system branch controlling involuntary functions, divided into sympathetic and parasympathetic divisions.
• Auxins**:** Plant hormones regulating cell elongation.
• Axon**:** A long nerve fibre conducting impulses away from the cell body.
• Bundle of His**:** A collection of Purkyne fibres transmitting electrical signals from the AVN to the ventricles' apex.
• Cell body**:** The part of a neurone housing organelles, including the nucleus and rough ER.
• Central nervous system (CNS)****: Comprising the brain and spinal cord.
• Chemoreceptor**:** A receptor in carotid artery walls detecting pH changes and transmitting signals to the medulla oblongata.
• Cholinergic synapse**:** A synapse using acetylcholine as a neurotransmitter to facilitate or inhibit signals between neurones or other effectors.
• Collecting duct**:** The nephron's final section collecting urine and adjusting water permeability via ADH.
• Cone cells**:** Retinal light receptors providing colour vision and high visual acuity, found in the fovea.
• Control mechanism**:** A system maintaining optimal conditions, consisting of a receptor, coordinator, effector, and feedback loop.
• Coordinator**:** Processes receptor information and directs effectors.
• Cyclic AMP (cAMP)****: A secondary messenger activating protein kinase during adrenaline signalling.
• Dendrites**:** Branched extensions of the cell body receiving impulses from other neurones.
• Dendrons**:** Larger extensions of the cell body branching into dendrites.
• Depolarisation**:** A temporary reduction in the axon membrane's negative potential during a nerve impulse.
• Descending limb**:** The loop of Henle section descending into the medulla, permeable to water, allowing water loss from filtrate.
• Diabetes**:** A metabolic disorder affecting blood glucose regulation, classified as Type I or Type II.
• Distal convoluted tubule**:** Nephron segment regulating blood pH and adjusting water and ion reabsorption, influenced by ADH.
• Effector**:** A structure (organ, tissue, or cell) generating a response to a stimulus.
• Efferent arteriole**:** A narrower blood vessel carrying blood from the glomerulus, creating high glomerular pressure.
• Excitatory synapse**:** A synapse generating action potentials in postsynaptic neurones when neurotransmitters bind receptors.
• Fast-twitch muscle fibres**:** Muscle fibres contracting quickly and powerfully but for short durations, suited to anaerobic activity.
• Feedback mechanism**:** A system where the response to a stimulus is monitored by receptors to maintain balance.
• Fovea**:** The retina region with the highest light intensity and greatest concentration of cone cells.
• Generator potential**:** A change in sensory receptor membrane potential in response to a stimulus.
• Glomerular filtrate**:** Fluid filtered into the renal capsule, containing water, glucose, ions, and urea.
• Glomerulus**:** A capillary bundle in the renal capsule specialised for blood filtration.
• Glucagon**:** A hormone increasing blood glucose by promoting glycogen breakdown and gluconeogenesis.
• Gluconeogenesis**:** Glucose synthesis from non-carbohydrate sources like amino acids or glycerol.
• Glycogenesis**:** The process of forming glycogen from glucose in the liver.
• Glycogenolysis**:** The process of breaking down glycogen into glucose in the liver.
• Gravitropism**:** A directional plant growth response to gravity.
• Homeostasis**:** Maintaining a stable internal environment despite external changes.
• Hormones**:** Chemical messengers produced by endocrine glands, traveling via blood to target cells.
• Hyperpolarisation**:** A membrane potential decrease making the axon more negative than its resting potential.
• Hypothalamus**:** Brain region controlling temperature and water balance, influencing the autonomic nervous system.
• H-zone**:** The lighter region at the centre of an A band in myofibrils.
• Indoleacetic acid (IAA)****: A type of auxin that regulates cell elongation, promoting elongation in shoots and inhibiting it in roots.
• Inhibitory synapse**:** A synapse reducing the likelihood of an action potential by causing hyperpolarisation in the postsynaptic neurone.
• Insulin**:** A hormone produced by β-cells of the pancreas that decreases blood glucose levels by promoting glycogen synthesis and increasing glucose uptake by cells.
• Intermediate neurone**:** A neurone in the spinal cord connecting sensory and motor neurones.
• Iodopsin**:** The pigment found in cone cells.
• Islets of Langerhans**:** Pancreatic clusters of hormone-producing cells, including α-cells (glucagon secretion) and β-cells (insulin secretion).
• Isotropic (I) bands**:** Light bands in myofibrils containing non-overlapping actin filaments.
• Kinesis**:** A non-directional response to a stimulus where movement speed or direction changes.
• Loop of Henle**:** Nephron segment creating a low water potential in the medulla, enabling water reabsorption. It has descending (water-permeable) and ascending (impermeable to water) limbs.
• Medulla oblongata**:** Brain region regulating heart rate via two centres—one linked to the sympathetic system (increases rate) and the other to the parasympathetic system (decreases rate).
• Motor neurone**:** A neurone transmitting impulses from the CNS to effectors like muscles or glands.
• Myelin sheath**:** A layer of Schwann cell membranes insulating the axon and increasing nerve impulse speed.
• Myofibrils**:** Muscle fibre units containing actin and myosin filaments, responsible for contraction.
• Myogenic**:** Describes heart muscle tissue that initiates its own contractions without nervous input.
• Myosin**:** A thick filament protein in myofibrils with long tails and bulbous heads for forming cross-bridges during contraction.
• Myosin binding site**:** A site on actin that becomes accessible for myosin heads during muscle contraction.
• Negative feedback**:** A control mechanism reversing deviations from an optimum state to restore balance.
• Negative tropism**:** Growth of a plant away from a stimulus.
• Nephron**:** The kidney's functional unit responsible for filtering blood and forming urine.
• Nerve impulse**:** A self-propagating wave of depolarisation traveling along a neurone membrane.
• Neuromuscular junction**:** A specialised synapse between a motor neurone and a muscle fibre, using acetylcholine.
• Neurones**:** Nerve cells specialised for transmitting electrical impulses.
• Neurotransmitters**:** Chemical messengers facilitating communication between neurones or between neurones and effectors.
• Nodes of Ranvier**:** Gaps in the myelin sheath allowing action potentials to regenerate, enabling saltatory conduction.
• Optic nerve**:** Nerve transmitting visual signals from the retina to the brain.
• Optimum point**:** The ideal condition for a system to function effectively.
• Osmoreceptors**:** Hypothalamic receptors detecting changes in blood water potential.
• Osmoregulation**:** The kidney-regulated maintenance of blood water potential.
• Pacinian corpuscle**:** A sensory receptor detecting mechanical pressure changes.
• Parasympathetic nervous system**:** A branch of the autonomic nervous system that slows body activity under resting conditions.
• Peripheral nervous system (PNS)****: Nerves connecting the CNS to the body, divided into sensory and motor systems.
• Phosphocreatine**:** A compound stored in muscles as a reserve for regenerating ATP.
• Phototropism**:** Plant growth response directed by light.
• Plant growth factors**:** Hormone-like substances (e.g., IAA) influencing plant responses to stimuli.
• Polarisation**:** The resting potential state of a neurone membrane, typically -65 mV.
• Positive feedback**:** A mechanism amplifying deviations from the optimum condition.
• Positive tropism**:** Growth of a plant towards a stimulus.
• Posterior pituitary gland**:** Gland releasing ADH into the bloodstream.
• Postsynaptic neurone**:** A neurone receiving neurotransmitters via receptors on its membrane.
• Pressure receptors**:** Sensors in carotid arteries and the aorta detecting blood pressure changes.
• Presynaptic neurone**:** A neurone releasing neurotransmitters into the synaptic cleft.
• Protein kinase**:** An enzyme catalysing glycogen breakdown into glucose.
• Proximal convoluted tubule**:** Nephron segment reabsorbing glucose and water, lined with specialised epithelial cells.
• Purkyne tissue**:** Specialised fibres conducting electrical impulses from the AVN to the ventricles' apex.
• Receptor**:** A specialised cell or structure detecting specific stimuli.
• Reflex**:** A rapid, automatic response to a stimulus for protection.
• Reflex arc**:** The pathway of a reflex action involving sensory, intermediate, and motor neurones.
• Refractory period**:** The time after an action potential when further impulses cannot occur, ensuring discrete, unidirectional impulses.
• Renal capsule (Bowman's capsule)****: The nephron's initial structure surrounding the glomerulus and filtering blood.
• Repolarisation**:** Restoration of the resting potential after depolarisation.
• Response**:** A reaction to a stimulus.
• Resting potential**:** A neurone's stable electrical potential (-65 mV), with the inside more negative than the outside.
• Retina**:** The inner eye layer containing light receptor cells.
• Rhodopsin**:** The pigment in rod cells enabling light detection.
• Rod cells**:** Retinal light receptors sensitive to low light intensity, enabling black-and-white vision.
• Saltatory conduction**:** The rapid transmission of nerve impulses in myelinated neurones by jumping between nodes of Ranvier.
• Sarcomere**:** The functional unit of myofibrils between two Z-lines.
• Sarcoplasm**:** Muscle fibre cytoplasm containing mitochondria and endoplasmic reticulum.
• Schwann cells**:** Cells surrounding the axon, providing insulation and aiding nerve regeneration.
• Second messenger model**:** A signalling mechanism where hormones (e.g., adrenaline) trigger intracellular responses via molecules like cAMP.
• Sensory neurone**:** A neurone transmitting impulses from receptors to the CNS.
• Sinoatrial node (SAN)****: A group of pacemaker cells in the heart generating electrical activity.
• Skeletal muscle**:** Voluntary muscle attached to bones, enabling movement.
• Sliding filament theory**:** Muscle contraction mechanism involving actin sliding over myosin, shortening the sarcomere.
• Slow-twitch muscle fibres**:** Muscle fibres adapted for endurance and aerobic respiration.
• Sodium-potassium pump**:** A membrane protein actively transporting sodium out and potassium into axons, maintaining resting potential.
• Spatial summation**:** Summation caused by neurotransmitter release from multiple presynaptic neurones.
• Stimulus**:** A detectable environmental change triggering a response.
• Stretch-mediated sodium channel**:** Sodium channels responding to pressure or stretching, involved in sensory processes.
• Summation**:** The cumulative effect of neurotransmitter release triggering postsynaptic action potentials.
• Sympathetic nervous system**:** A branch of the autonomic nervous system stimulating activity during stress.
• Synaptic cleft**:** The gap between neurones across which neurotransmitters transmit impulses.
• Synaptic vesicles**:** Presynaptic vesicles storing neurotransmitters for release into the synaptic cleft.
• Target cells**:** Cells with receptors specific to a particular hormone.
• Taxis**:** Directional movement in response to a stimulus.
• Temporal summation**:** Summation caused by high-frequency neurotransmitter release from a single presynaptic neurone.
• Threshold value**:** The minimum stimulus level required to generate an action potential.
• Transducer cells**:** Cells converting stimuli into electrical signals.
• Tropism**:** Plant growth response to directional stimuli.
• Tropomyosin**:** A protein blocking myosin binding sites on actin, controlling muscle contraction.
• Type I diabetes**:** Insulin-dependent diabetes caused by a lack of insulin production, managed with insulin injections.
• Type II diabetes**:** Insulin-independent diabetes caused by insulin resistance or insufficient production, managed through diet and exercise.
• Unidirectionality**:** Synaptic transmission occurring only from the presynaptic to postsynaptic neurone.
• Visual acuity**:** The clarity of vision.
• Z-line**:** The line marking the boundary of each sarcomere in a myofibril.