Control (A Level) flashcards

Topic 4: Control

Taxis and kinesis

Taxis: movement directed towards or away from the direction of a stimulus. Kinesis: random movement whose speed or rate of turning changes with stimulus intensity.

How does a Pacinian corpuscle produce a generator potential?

Pressure deforms the lamellae and stretches the neurone ending membrane; stretch-mediated sodium ion channels widen and Na⁺ diffuses in, depolarising the membrane. If the generator potential reaches threshold, an action potential is set up.

Rods and cones: sensitivity and acuity

Rods: many share one bipolar neurone, so generator potentials summate; high sensitivity in dim light, low acuity. Cones: usually one cone per bipolar neurone; need bright light, high acuity; three types give colour vision.

Effect of a higher IAA concentration on shoots and roots

Shoots: stimulates cell elongation. Roots: inhibits cell elongation. So a horizontal shoot bends up and a horizontal root bends down.

How does ABA close stomata in a water-stressed plant?

ABA stimulates transport of K⁺ and Cl⁻ out of the guard cells, raising their water potential. Water leaves the guard cells by osmosis, they shrink, and the stoma closes.

Ion movements during an action potential

Depolarisation: voltage-gated Na⁺ channels open and Na⁺ enters; past threshold (about −55 mV) positive feedback opens many more, to about +40 mV. Repolarisation: Na⁺ channels close, voltage-gated K⁺ channels open, K⁺ leaves.

What happens when an action potential reaches a cholinergic synapse?

Voltage-gated Ca²⁺ channels open and Ca²⁺ diffuses in; vesicles fuse and release acetylcholine by exocytosis; it diffuses across the cleft and binds receptors, opening Na⁺ channels. Acetylcholinesterase hydrolyses it.

Roles of ATP in muscle contraction

ATP binding detaches the myosin head from actin; hydrolysis of ATP by the head's ATPase resets the head; ATP also powers the pumps returning Ca²⁺ to the sarcoplasmic reticulum.

Route of the electrical wave in a heartbeat

SAN (pacemaker, right atrium) → wave across the atria, which contract → blocked by a fibrous layer → AVN delays it → bundle of His in the septum → Purkinje tissue → ventricles contract from the apex upwards.

Second messenger model for glucagon and adrenaline

The hormone binds a surface receptor; adenyl cyclase is activated and converts ATP into cAMP (the second messenger); cAMP activates protein kinase, which activates enzymes that break glycogen down to glucose.

Effect of histone acetylation on transcription

Histones are positive and DNA negative. Increased acetylation reduces their attraction, loosening the DNA so RNA polymerase and transcription factors can reach it: more transcription. Decreased acetylation packs DNA tightly: less transcription.

Restriction endonucleases and DNA ligase

A restriction endonuclease cuts DNA only at its own recognition sequence, often leaving sticky ends. DNA ligase joins sugar–phosphate backbones by forming phosphodiester bonds.