Flashcards · 7402 · A Level Topic 6

Organisms respond to changes in their internal and external environments (A Level) flashcards

18 cards on A Level Topic 6, subtopics 3.6.1 to 3.6.4. Pick a subtopic or study them all.

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Taxis and kinesis: Taxis: directional movement in response to the direction of a stimulus. Kinesis: change in speed or rate of turning in response to stimulus intensity; random, not directional.
How does IAA cause positive phototropism in a shoot?: IAA moves to the shaded side, where it increases cell elongation, so the shaded side grows faster and the shoot bends towards the light. In roots, high IAA inhibits elongation.
How does a Pacinian corpuscle produce a generator potential?: Pressure deforms the lamellae and the neurone membrane, widening stretch-mediated sodium ion channels. Na⁺ diffuses in, depolarising the membrane. If threshold is reached, an action potential is sent.
Rods and cones: sensitivity and visual acuity: Rods: many share one bipolar neurone, so generator potentials summate; high sensitivity in dim light, low acuity. Cones: one cone to one bipolar neurone; low sensitivity, high acuity, colour vision from three types.
How is heart rate increased during exercise?: Chemoreceptors detect a rise in CO₂ (fall in blood pH); the cardiac centre in the medulla oblongata sends more impulses along sympathetic neurones to the SAN, so heart rate rises. Parasympathetic impulses slow it.

How is the resting potential maintained?: The sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in using ATP. The membrane is more permeable to K⁺, which diffuses out. Inside is about −70 mV relative to outside.
Ion movements in depolarisation and repolarisation: Depolarisation: voltage-gated Na⁺ channels open and Na⁺ diffuses in; past threshold (about −55 mV) positive feedback opens more, to about +40 mV. Repolarisation: Na⁺ channels close, voltage-gated K⁺ channels open, K⁺ diffuses out.
Three roles of the refractory period: Keeps action potentials discrete (separate), makes them travel in one direction only, and limits the maximum frequency of impulses.
Saltatory conduction: In a myelinated axon, depolarisation occurs only at the nodes of Ranvier, so the action potential jumps from node to node, which is faster.
Role of calcium ions at a cholinergic synapse: An action potential opens voltage-gated Ca²⁺ channels in the presynaptic knob. Ca²⁺ enters and causes vesicles to fuse with the membrane, releasing acetylcholine by exocytosis.

Roles of calcium ions and ATP in muscle contraction: Ca²⁺ from the sarcoplasmic reticulum moves tropomyosin, exposing binding sites on actin for myosin heads. ATP detaches the head from actin, is hydrolysed to reset the head, and powers pumping of Ca²⁺ back.
Structure of a sarcomere: The unit between two Z-lines. Thick myosin filaments form the A-band; thin actin filaments form the I-band; the H-zone holds only myosin. On contraction, the I-band and H-zone shorten, the A-band stays the same length.
Slow-twitch vs fast-twitch muscle fibres: Slow-twitch: contract slowly, for endurance; many mitochondria, much myoglobin and many capillaries; aerobic respiration. Fast-twitch: contract quickly and powerfully but tire quickly; more glycogen and phosphocreatine; anaerobic respiration.
Role of phosphocreatine: It is stored in muscle and gives its phosphate to ADP, regenerating ATP quickly during short bursts of intense activity.

Second messenger model (adrenaline and glucagon): Hormone binds a surface receptor → adenylate cyclase activated → ATP converted to cyclic AMP (second messenger) → cAMP activates protein kinase → enzymes break down glycogen to glucose.
Response to low blood water potential: Osmoreceptors in the hypothalamus shrink; more ADH is released from the posterior pituitary. ADH makes the DCT and collecting duct insert more aquaporins, so more water is reabsorbed by osmosis: small volume of concentrated urine.
Glycogenesis, glycogenolysis and gluconeogenesis: Glycogenesis: glucose → glycogen (stimulated by insulin). Glycogenolysis: glycogen → glucose (stimulated by glucagon and adrenaline). Gluconeogenesis: glucose made from non-carbohydrates such as glycerol and amino acids (stimulated by glucagon).
Role of the loop of Henle: The ascending limb actively transports Na⁺ and Cl⁻ out into the medulla, which lowers its water potential. Water leaves the descending limb and the collecting duct by osmosis, so it can be reabsorbed into the blood.