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 exam questions on this topic.
Before you start
Make sure you're comfortable with these topics first:
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
Try each question, then tap to reveal the answer.
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.
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.
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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