A-Level Biology: Stimuli, Receptors and Heart-Rate Control

Before a nerve impulse can travel along a neurone, something has to detect a change and something has to decide what to do about it. This material covers that front end of the 'organisms respond to changes in their environments' section of A level biology — the part that sits before neurones, synapses and muscle contraction.

It starts with simple organisms and plants. You distinguish a taxis, a directional movement towards or away from a stimulus, from a kinesis, where only the speed of movement or the rate of turning changes, using a woodlouse in a choice chamber as the worked example. For flowering plants you explain tropisms through indoleacetic acid (IAA): why it makes a shoot bend towards light by stimulating elongation on the shaded side, and why it makes a root bend downwards by inhibiting elongation on the lower side.

The receptor questions are about mechanism. The Pacinian corpuscle shows how pressure deforms the lamellae and opens stretch-mediated sodium channels to produce a generator potential. The retina contrasts rods and cones: rhodopsin and iodopsin, three types of cone, and how the wiring to bipolar neurones trades sensitivity (many rods converging on one neurone, allowing summation) against visual acuity (one cone per neurone). The simple reflex arc links receptor to effector through sensory, relay and motor neurones.

The final part is the heart. Cardiac muscle is myogenic: the wave of excitation starts at the sinoatrial node, is blocked by non-conducting tissue between atria and ventricles, is delayed at the atrioventricular node, and runs down the bundle of His and Purkyne tissue. Heart rate is then adjusted by the cardiac centre in the medulla oblongata, using information from chemoreceptors that respond to blood pH and baroreceptors that respond to pressure, and sending impulses along sympathetic nerves to speed the SAN or along the parasympathetic vagus nerve to slow it.

The material offers a multiple-choice quiz with explained answers, a flashcard deck of key terms, a printable written-work sheet of eight open questions answered by hand and checked, and an oral exam in which an examiner asks one question at a time and gives feedback at the end.

The content is based on the response-to-stimuli content of the DfE A level biology subject content, as taught in current exam-board specifications (for example AQA topic 6, survival and response, receptors and control of heart rate). It is practice material and does not replace your own board's specification.

• Distinguish taxes from kineses and explain their survival value
• Explain phototropism in shoots and gravitropism in roots in terms of IAA
• Describe a simple reflex arc and its protective role
• Explain how a Pacinian corpuscle produces a generator potential
• Compare rods and cones in pigment, sensitivity and visual acuity
• Explain the myogenic heartbeat and how the medulla adjusts heart rate through autonomic nerves

Practice material written by Zestly, based on the DfE A level biology subject content (organisms respond to changes in their environments: survival and response, receptors, control of heart rate).