Cells constantly send and receive signals. This HL lesson covers C2.1 Chemical signalling (HL) and C2.2 Neural signalling.
You will study signal types, membrane and intracellular receptors, second-messenger cascades, and the action potential and synapse.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.
C2.1 Chemical signalling
Chemical signalling (HL)
Cells communicate with chemical signals: neurotransmitters, hormones, cytokines, and even bacterial quorum sensing.
Lipid-soluble signals (steroids) cross the membrane and bind intracellular receptors, often affecting transcription.
Water-soluble signals bind membrane receptors such as G-protein coupled receptors, triggering a second messenger (e.g. cAMP) cascade.
Agonists activate receptors; antagonists block them — the basis of many drugs.
Calculate
Your turn — calculate
1A nerve impulse travels 1.5 m along an axon in 0.02 s. Calculate the conduction speed.
m/s
Hint: speed = distance ÷ time = 1.5 ÷ 0.02.
C2.2 Neural signalling
The action potential
A neuron rests at about −70 mV. When a stimulus reaches threshold, voltage-gated sodium channels open and Na⁺ rushes in, causing depolarisation. Potassium then leaves to repolarise.
Impulses are all-or-none and travel by saltatory conduction along myelinated axons. Summation lets several sub-threshold inputs combine to fire an impulse.
Calculate
Your turn — calculate
2An axon is 0.9 m long and conducts at 45 m/s. Calculate the conduction time.
s
Hint: time = distance ÷ speed = 0.9 ÷ 45.
Sort it
Sort each item into its group
Sort each term into signal, receptor mechanism, or neural signalling.
Chemical signal
Receptor mechanism
Neural signalling
C2.2 Neural signalling
The synapse
At a synapse, an action potential triggers Ca²⁺ entry, releasing neurotransmitter that crosses the gap and binds receptors on the next cell.
Excitatory inputs depolarise; inhibitory inputs hyperpolarise. The postsynaptic neuron sums these to decide whether to fire — the basis of neural integration.
Quick check
Question
?Steroid hormones can bind intracellular receptors because they are...
Quick check
Question
?A G-protein coupled receptor typically triggers...
Quick check
Question
?The rising phase (depolarisation) of an action potential is caused by...
Match it
Match each item to its partner
Match each signalling term to its meaning.
Item
Partner
Quick check
Question
?Summation at a synapse allows...
Recap
The big ideas to take away
C2.1 (HL): signal types; membrane vs intracellular receptors; second messengers; agonists/antagonists
C2.2: resting and action potentials; Na in (depolarise), K out (repolarise)
C2.2: synapses release neurotransmitter; excitation, inhibition and summation