Organisms Respond to Changes in their Environments
Organisms Respond to Changes in their Environments is the A-level Biology topic covering plant and animal responses, including tropisms and auxin, receptors such as the Pacinian corpuscle, the nervous system, resting and action potentials, synapses, and muscle contraction. Calculations of conduction speed, refractory period and rate of curvature are common in AQA exam questions.
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Method
- Distinguish a taxis (a directional response) from a kinesis (a non-directional change in the rate of movement/turning) before answering any behaviour question.
- For plant responses, explain phototropism and gravitropism in terms of unequal auxin distribution causing unequal cell elongation on either side of a shoot or root.
- For the Pacinian corpuscle, trace the pathway from stimulus (pressure) through deformation of stretch-mediated sodium channels to a generator potential, then to an action potential if threshold is reached.
- For action potential questions, work through the sequence in order: resting potential (maintained by the sodium-potassium pump), depolarisation (sodium channels open), repolarisation (potassium channels open), and the refractory period.
- For conduction speed calculations, use speed = distance / time, keeping units consistent (usually converting mm to m and ms to s).
- Explain differences in conduction speed (myelination, axon diameter, temperature) in terms of saltatory conduction or ion movement, rather than giving a one-word answer.
Worked example
A student placed two pairs of electrodes 60 mm apart along an isolated, myelinated sensory neurone and measured the time taken for a nerve impulse to travel between them as 0.60 milliseconds. Calculate the speed of conduction of the impulse, in m/s, to 2 significant figures.
- Convert distance to metres: 60 mm = 0.060 m.
- Convert time to seconds: 0.60 ms = 0.00060 s.
- Apply the formula: speed = distance / time = 0.060 / 0.00060.
- = 100 m/s.
- Final answer: 100 m/s (2 s.f.).
Practice questions
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Q1State what is meant by a taxis.Show answer
Answer: A directional response in which an organism moves towards or away from a directional stimulus.
Q2Name the ion channel that opens during the depolarisation phase of an action potential.Show answer
Answer: Voltage-gated sodium ion channels.
Q3State the resting potential typically maintained across an axon membrane.Show answer
Answer: About -70 mV (inside negative relative to outside).
Q4Using the term saltatory conduction, explain why a myelinated neurone conducts nerve impulses faster than an unmyelinated neurone of the same diameter.Show answer
Answer: The myelin sheath is an electrical insulator, so depolarisation can only occur at the (unmyelinated) nodes of Ranvier; the impulse jumps from node to node (saltatory conduction), which is much faster than continuous depolarisation along an unmyelinated membrane of the same diameter.
Q5A coleoptile curved through 42 degrees over a period of 70 minutes. Calculate the mean rate of curvature, in degrees per hour.Show answer
Answer: 36 degrees per hour (42 / 70 = 0.6 degrees per minute; 0.6 x 60 = 36)
Q6The absolute refractory period of an axon is 0.8 milliseconds. Calculate the maximum theoretical frequency, in Hz, at which action potentials could be generated along this axon.Show answer
Answer: 1250 Hz (1 / 0.0008 = 1250)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Give one way in which a taxis differs from a kinesis.
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A Pacinian corpuscle detects mechanical pressure applied to the skin. Using the idea of stretch-mediated sodium channels, describe and explain the sequence of events that links pressure on the skin to the production of a generator potential in the sensory neurone ending inside the corpuscle.
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The table shows the membrane potential of an axon recorded every 0.5 milliseconds during a single nerve impulse. Time (ms): 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0. Membrane potential (mV): -70, -70, -15, +35, +25, -55, -75. (a) State the resting potential of the axon and identify the time at which depolarisation begins. (b) Explain the ionic basis of the rising phase (depolarisation) and the falling phase (repolarisation) of the action potential shown.
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