Homeostasis and coordination

Homeostasis

Maintaining a constant internal environment by negative feedback: receptor → coordinator → effector → correction.

Blood glucose

β cells of the islets of Langerhans secrete insulin when glucose rises (more uptake by cells, glycogenesis); α cells secrete glucagon when it falls (glycogenolysis, gluconeogenesis). Hormones act through receptors; glucagon uses the second messenger cyclic AMP.

The kidney

• Ultrafiltration in the glomerulus into Bowman's capsule.
• Selective reabsorption in the proximal convoluted tubule (all glucose, amino acids, much water and ions).
• The loop of Henle creates a high solute concentration in the medulla (countercurrent multiplier).
• ADH from the posterior pituitary increases the permeability of the collecting duct to water (aquaporins) when blood water potential is low.

The nervous system

• Resting potential (about −70 mV) maintained by the sodium–potassium pump.
• Action potential: depolarisation (Na⁺ channels open), repolarisation (K⁺ channels open), hyperpolarisation, refractory period. All-or-nothing.
• Myelinated neurones conduct faster by saltatory conduction between nodes of Ranvier.
• Synapses: an impulse causes Ca²⁺ entry, vesicles release acetylcholine, which binds to receptors on the post-synaptic membrane; acetylcholinesterase breaks it down.

Plant hormones

Auxins (cell elongation, tropisms), gibberellins (stem elongation, seed germination), abscisic acid (stomatal closure in drought).

Check yourself

1. Which hormone increases water reabsorption in the collecting duct?
2. What maintains the resting potential?
3. Why is conduction faster in myelinated neurones?