AQA A-Level Biology Topic 6: Responding to Changes in Environment

A set of 50 vocabulary flashcards covering nervous coordination, muscle contraction, homeostasis, osmoregulation, and plant/nervous responses for AQA A-Level Biology Topic 6.

Stimulus

A change in the internal or external environment of an organism.

Receptor

Cells or proteins that detect a specific stimulus and convert it into a nerve impulse.

Resting potential

The potential difference across the membrane of a resting neuron, typically $-70\,\text{mV}$, maintained by the sodium-potassium pump.

Sodium-potassium pump

A membrane protein that actively transports $3\text{Na}^+$ out of and $2\text{K}^+$ into the neuron to maintain the electrochemical gradient.

Action potential generation

Occurs when a stimulus causes $\text{Na}^+$ voltage-gated channels to open, causing $\text{Na}^+$ ions to flood in and depolarise the membrane to threshold ($-55\,\text{mV}$).

Repolarisation

The process where voltage-gated $\text{Na}^+$ channels close and $\text{K}^+$ channels open, allowing $\text{K}^+$ ions to flood out and restore the negative charge.

Hyperpolarisation

A state when the potential difference across a neuron membrane becomes more negative than the resting potential due to excess $\text{K}^+$ efflux.

All-or-nothing principle

The principle stating that any stimulus below threshold fails to generate an action potential, while any stimulus above threshold generates an action potential of the same magnitude.

Saltatory conduction

The process by which myelination speeds up impulses as the impulse jumps between Nodes of Ranvier, skipping depolarisation in the myelin-insulated sections.

Synapse

The junction between two neurons where information is transmitted via neurotransmitters.

Pre-synaptic knob

The terminal structure of an axon where an action potential triggers voltage-gated $\text{Ca}^{2+}$ channels to open, causing $\text{Ca}^{2+}$ influx and synaptic vesicles to fuse with the membrane, releasing neurotransmitters.

Spatial summation

A mechanism where multiple pre-synaptic neurons release neurotransmitters at the same time onto a single post-synaptic neuron to reach threshold.

Temporal summation

A mechanism where a single pre-synaptic neuron releases neurotransmitters repeatedly in quick succession to reach threshold.

Inhibitory synapse

A synapse that causes hyperpolarisation of the post-synaptic membrane (e.g., via $\text{C}^-$ influx), making an action potential less likely.

Acetylcholinesterase

An enzyme that hydrolyses acetylcholine in the synaptic cleft to prevent continuous stimulation of the post-synaptic neuron.

Myofibril proteins

Actin (thin filaments) and Myosin (thick filaments).

Sarcomere

The functional unit of a muscle, measured from Z-line to Z-line.

Sliding filament theory

The theory explaining that myosin heads bind to actin, change shape (power stroke), and pull the actin filaments over the myosin, shortening the sarcomere.

Role of $\text{Ca}^{2+}$ in muscle contraction

Released from the sarcoplasmic reticulum; binds to troponin, causing it to change shape and move tropomyosin away from actin-myosin binding sites.

Role of ATP in muscle contraction

Provides energy for the power stroke and for the myosin head to detach from actin.

ATP regeneration pathways in muscle

Aerobic respiration, anaerobic respiration (lactate), and the phosphocreatine system.

Phosphocreatine

A rapidly available store of phosphate used to phosphorylate ADP to ATP anaerobically in intense exercise.

Slow-twitch muscle fibers

Muscle fibers adapted for endurance, featuring a high density of mitochondria, capillaries, and myoglobin, and resistant to fatigue.

Fast-twitch muscle fibers

Muscle fibers adapted for explosive power, featuring a high concentration of phosphocreatine and enzymes for glycolysis, which fatigue quickly.

Homeostasis

The maintenance of a relatively stable internal environment despite external fluctuations.

Negative feedback

A mechanism that reverses a change to return a system to its set point.

Islets of Langerhans

Clusters of endocrine cells in the pancreas containing alpha cells (which produce glucagon) and beta cells (which produce insulin).

Insulin

A hormone that lowers blood glucose by increasing permeability of cells to glucose and activating enzymes for glycogenesis (glucose $\rightarrow$ glycogen) and glucose respiration.

Glucagon

A hormone that raises blood glucose by activating enzymes for glycogenolysis (glycogen $\rightarrow$ glucose) and gluconeogenesis (amino acids/glycerol $\rightarrow$ glucose).

Adrenaline

A hormone that binds to receptors on liver cells to activate a second messenger ($\text{CAMP}$) to stimulate glycogenolysis.

Second messenger model

A mechanism where hormones (like glucagon or adrenaline) bind to receptors, triggering intracellular $\text{CAMP}$ to activate enzymes like protein kinase.

Type 1 Diabetes

An autoimmune destruction of beta cells in the pancreas, treated with insulin injections.

Type 2 Diabetes

A condition where body cells lose sensitivity to insulin, treated with diet/exercise or medication.

Ultrafiltration

Filtration of blood at high pressure from the glomerulus into the Bowman's capsule due to the afferent arteriole being wider than the efferent arteriole.

Selective reabsorption

The process where useful substances (glucose and amino acids) are reabsorbed into the blood at the proximal convoluted tubule (PCT).

PCT cell adaptations

Microvilli for increased surface area and a high density of mitochondria for active transport.

Loop of Henle role

Creates a low water potential in the medulla, allowing water to be reabsorbed from the collecting duct.

Counter-current multiplier

A mechanism in the loop of Henle where the descending limb is permeable to water while the ascending limb is impermeable to water but actively transports salts out.

ADH

Antidiuretic Hormone, released by the posterior pituitary gland when blood water potential is low.

Mechanism of ADH action

Binds to receptors on collecting duct cells, triggering the insertion of aquaporins into the membrane to increase water permeability.

CNS vs. PNS

CNS consists of the brain and spinal cord, whereas PNS consists of sensory and motor neurones.

Reflex arc

A rapid, automatic response pathway to a stimulus: stimulus $\rightarrow$ receptor $\rightarrow$ sensory neurone $\rightarrow$ relay neurone $\rightarrow$ motor neurone $\rightarrow$ effector.

Pacinian corpuscle

A mechanoreceptor that detects mechanical pressure; pressure deforms the lamellae, opening stretch-mediated $\text{Na}^+$ channels.

IAA

Indoleacetic acid, an auxin that controls plant growth and tropisms.

Phototropism

Plant growth in response to light, where IAA accumulates on the shaded side, causing cell elongation.

Gravitropism

Plant growth in response to gravity, where IAA accumulates on the lower side of roots, inhibiting growth so the root bends downwards.

Autonomic nervous system

The part of the nervous system that controls involuntary actions, split into sympathetic (fight/flight) and parasympathetic (rest/digest).

Myogenic

Refers to muscle tissue that generates its own contraction impulses (e.g., the SAN).

Heart rate conduction pathway

The sequence in which the SAN sends electrical waves, the AVN delays them, and the Bundle of His/Purkyne fibres conduct them to the apex.

Control of heart rate

Chemoreceptors (pH/$\text{CO}_2$) and pressure receptors send impulses to the medulla, which sends signals via the sympathetic or parasympathetic nervous system to the SAN.