DSE Biology | Nervous system and homeostasis lesson plan (for tutors)

DSE Biology | Nervous system and homeostasis lesson plan (for tutors) | From -70 mV to negative feedback

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students recite depolarisation and repolarisation but freeze when asked why it happens. This lesson builds the action potential ion by ion, then links it to temperature and glucose control.

Objectives — Students can: (1) explain the resting potential of -70 mV and the threshold of -55 mV, a gap of 15 mV; (2) describe depolarisation, repolarisation, hyperpolarisation and the refractory period, and compute that a 2 ms absolute refractory period caps firing at about 500 impulses per second; (3) explain the 80-fold speed gain of a myelinated axon (120 m/s) over an unmyelinated one (1.5 m/s); (4) trace a negative feedback loop for temperature and glucose.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Diffusion and active transport	How does sodium move against its gradient?	The sodium-potassium pump, using ATP
Membrane structure	How do ions cross the bilayer?	Through channel or carrier proteins
Hormones	How does a hormone change a cell?	It binds a receptor and alters cell activity

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Opener	Why can a neurone fire hundreds of times a second while muscle cannot stay contracted?	Guess first, then compare with the refractory period
10-24 min	Resting and action potential	From -70 mV to -55 mV, ion by ion	Draw the 15 mV gap
24-36 min	Synapse	Calcium in, vesicles fuse, transmitter diffuses, receptor binds, enzyme clears	Recite the order without gaps
36-46 min	Myelin and saltatory conduction	Myelinated 120 m/s against 1.5 m/s	Compute the 80-fold gain
46-56 min	Temperature and glucose feedback	Set point 37.0, sweating from 37.5, shivering from 36.5	Write the full loop
56-60 min	Exit check	Mark on the spot	Three correct

03 Teaching script (read it out)

Scene 1: The resting potential is a balance of pump and leak

The sodium-potassium pump uses one ATP to move 3 sodium out and 2 potassium in, leaving the outside positive and the inside near -70 mV. A stimulus opens voltage-gated sodium channels, sodium rushes in and the potential climbs to the -55 mV threshold, after which more channels open in a positive feedback loop. Sodium channels then inactivate while potassium channels open, potassium leaves and the potential drops, briefly overshooting below rest before the pump restores it. Answers must name the ion, its direction and its channel; “depolarisation” alone scores nothing.

Ask them to: sketch the trace from -70 mV to the overshoot in four panels, labelling the ion flow.

Scene 2: The synapse is a chemical sequence, so order is everything

Action potential reaches the axon terminal, calcium channels open and calcium enters, vesicles fuse with the presynaptic membrane, transmitter is released into the cleft, binds receptors on the postsynaptic membrane, ion channels open and a new potential starts, then the transmitter is broken down or taken back up. Transmission is one way because transmitter is stored only in the presynaptic knob and receptors sit only on the postsynaptic membrane. An inhibitory transmitter instead lets chloride in, making the membrane more negative and harder to excite.

Follow up: what happens if the enzyme that breaks down the transmitter is inhibited?

Scene 3: Negative feedback: deviation, sensor, coordinator, effector, response

The set point is 37.0 degrees. At 37.5 the hypothalamus triggers sweating and vasodilation to lose heat; at 36.5 it triggers vasoconstriction, shivering and a higher metabolic rate. Glucose works the same way: after a meal it rises to about 8.0 mmol per litre, insulin drives uptake and glycogen formation and it returns to about 6.5 within two hours (a fall of 1.5); fasting near 5.0 is corrected by glucagon. Full marks need all four links of the loop, and the effector must be named: glands and blood vessels, not the hypothalamus.

Ask them to: write the whole loop for a fever of 39.0 degrees, 2 above the set point.

04 Question types and worked steps

Type	Example	Steps	Answer
Description	Ion movements in the action potential	Channels and ions in time order	Sodium in means depolarisation
Explanation	Why myelin speeds conduction	Saltatory conduction, resistance, channel density	80-fold
Sequence	Five steps of synaptic transmission	Calcium, vesicles, transmitter, receptor, breakdown	Never reversible
Application	Temperature or glucose feedback	Sensor, coordinator, effector, response	All four links

Sanity check (teach this): The gap from -70 mV to -55 mV is 15 mV, the arithmetic behind every “the stimulus must be strong enough” question. During the 2 ms absolute refractory period no stimulus works, so the ceiling is about 500 impulses per second; write “about 500”, never “many”. Myelin forces the local current to jump between nodes of Ranvier, and 120 divided by 1.5 gives the 80-fold gain; speed also rises with axon diameter.

05 Independent practice (3 questions)

Question	Answer	What to watch
Gap between resting potential and threshold	15 mV	-55 minus -70
Maximum impulses per second with a 2 ms refractory period	500	1000 divided by the period
Ion that enters for an inhibitory transmitter	Chloride	It makes the membrane more negative

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying the pump repolarises instantly	“The pump pulls the potential back”	“Potassium efflux repolarises first; the pump only restores the gradients afterwards.”
Making the synapse two-way	“Transmitter travels both ways”	“Transmitter sits in the presynaptic knob, receptors on the postsynaptic side.”
Leaving out the effector	“The hypothalamus lowers body temperature”	“The hypothalamus coordinates; sweat glands and vessels are the effectors.”
Confusing refractory period with rest	“The refractory period is when the cell rests”	“It is when sodium channels are inactivated, so no stimulus works.”

07 Homework (paper, section and question numbers)

Biology chapter index — All chapters
Practice paper, Paper 1 Section B — Questions 7 to 9: action potential and feedback

08 Exit check (last 5 minutes)

Question	Answer	Decision
Resting potential and threshold	-70 mV and -55 mV	Both right before ecology
Why myelin speeds conduction	Saltatory conduction	One wrong: reteach the nodes of Ranvier
The four links of feedback	Sensor, coordinator, effector, response	Two wrong: restart act three

09 Tutor FAQ

Must every ion concentration be memorised?

Only the directions: sodium high outside, potassium high inside.

Why does the overshoot go below the resting potential?

Potassium channels close slowly, so potassium leaves in excess for a moment.

How should nerves and hormones be compared?

Nerves are fast and short lived, hormones slower and longer lasting; feedback loops often use both.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.