كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

C2.2Theme C — Interaction and interdependence

Neural signalling — IB Biology SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 5 of 9 2 quizzes

📚 IB Biology SL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

C2.2 Neural signalling — IB BIOLOGY SL notes: Describe neurone structure, the resting and action potentials, factors affecting conduction speed, and synaptic transmission.Central nervous system (CNS) = the brain and spinal cord, the coordinating centrePeripheral nervous system (P…

Peripheral nervous system (PNS)
Direction rule
Resting potential
Sodium-potassium pump
Different permeability
Negative proteins

Part 1 of 9Nervous system and neurones

Nervous system and neurones

Central nervous system (CNS)

Central nervous system (CNS) the brain and spinal cord, the coordinating centre

Peripheral nervous system (PNS)

Peripheral nervous system (PNS) all the nerves outside the CNS, reaching the sense organs and effectors

Nerve

Nerve a bundle of many neurone fibres

Impulse

Impulse not a current flowing in a wire but a brief reversal of the membrane potential that moves along the neurone

Neurone structure

A cell body holds the nucleus.
Dendrites receive impulses from other neurones.
A long axon carries impulses away from the cell body.
Axon terminals pass the signal on.

Direction rule

Direction rule an axon always carries impulses away from its cell body

Part 2 of 9Types of neurone

Feature	Sensory	Relay	Motor
Carries impulses	Receptor to CNS	Within the CNS	CNS to effector
Cell body position	Side branch in the middle of the fibre	Inside the CNS	At the end, inside the CNS
Fibres	Long dendron and long axon	Short, highly branched	Many dendrites and a long axon
Connects to	Relay neurone	Sensory and motor neurones	Muscle or gland

Part 3 of 9Resting potential

Resting potential

Resting potential the potential difference across the membrane of a neurone that is not transmitting, about -70 mV

The inside of the axon is negative compared with the outside.
A membrane at resting potential is polarised.

Sodium-potassium pump

Sodium-potassium pump uses ATP to move 3 Na⁺ out and 2 K⁺ in
This creates concentration gradients for both ions.

Different permeability

Different permeability the membrane is much less permeable to Na⁺ than to K⁺
K⁺ diffuses out faster than Na⁺ diffuses in, through channels by facilitated diffusion.
More positive ions are therefore outside than inside.

Negative proteins

Negative proteins negatively charged proteins inside the axon add to the negative charge

Part 4 of 9Action potential

Action potential = a momentary reversal of the membrane potential from about -70 mV to about +40 mV
- It is all-or-nothing, so its size does not depend on the stimulus.

A stimulus depolarises the membrane locally to the threshold of about -50 mV.

Voltage-gated Na⁺ channels open.

Na⁺ floods in and the inside becomes positive.

Na⁺ channels close and K⁺ channels open.

K⁺ leaves and the membrane repolarises.

The pump restores the resting distribution of ions.

Oscilloscope trace

Oscilloscope trace shows the rise, the fall and a brief overshoot below the resting level

Propagation

Propagation local currents depolarise the next section of the axon, so the impulse travels along it

Stimulus strength

Stimulus strength a stronger stimulus gives more frequent impulses and not bigger ones

Feature	Resting	Depolarisation	Repolarisation
Membrane potential	About -70 mV	Rises to about +40 mV	Falls back towards -70 mV
Channels open	Few leak channels	Voltage-gated Na⁺	Voltage-gated K⁺
Ion movement	Pump maintains gradients	Na⁺ diffuses in	K⁺ diffuses out
Inside charge	Negative	Positive	Negative again
Energy use	ATP for the pump	Passive diffusion	Passive diffusion

Part 5 of 9Speed of conduction

Myelination

Schwann cells wrap round the axon in layers of phospholipid membrane
The lipid has a high electrical resistance, so the sheath insulates the axon.
Only the gaps called nodes of Ranvier are uninsulated.
The impulse jumps from node to node by saltatory conduction.

Diameter

Diameter a wider axon offers less resistance, so it conducts faster

Comparison

Comparison a giant unmyelinated squid axon up to 1 mm wide is slower than a thin myelinated mammalian axon of 4-100 µm

Feature	Myelinated	Unmyelinated
Conduction	Saltatory	Continuous
Speed	Fast, up to about 100 m s⁻¹	Slow, about 1 m s⁻¹
Ion exchange	Only at the nodes	Along the whole length
Energy cost	Lower	Higher

Measuring speed
- stimulate at one point and record the arrival time at a second point
- Speed = distance between the electrodes divided by the time delay, in m s^-1.

Part 6 of 9Analysing correlations

1Correlation

Correlation an association between two variables, which never proves cause

Scatter graphs show positive, negative or no correlation.
Squid axon diameter and myelination are variables that can be correlated with speed.

2Pearson's correlation coefficient r

Pearson's correlation coefficient r for quantitative, normally distributed data, from -1 to +1
A value near +1 or -1 shows a strong linear correlation and 0 shows none.
The formula is given in the exam.

Analysing correlations

Plot a scatter graph.

State a null hypothesis that there is no correlation.

Calculate r from the sums of x, y, xy, x² and y².

Coefficient of determination R²
- r squared, expressed as a percentage
- Near 100% means the dependent variable can be predicted accurately from the independent variable.
- Near 0 means no prediction is possible.

Part 7 of 9Synapse structure

Synapse structure

Synapse

Synapse the junction between two neurones, separated by the synaptic cleft

Where neurones meet they do not touch.

Locations

Between neurones.
Between a receptor cell and a sensory neurone.
Between a motor neurone and a muscle fibre.

Part 8 of 9Synaptic transmission

Chemical transmission
- the impulse cannot jump the cleft, so it is converted to a chemical signal and back

An action potential depolarises the presynaptic membrane.

Ca²⁺ channels open and Ca²⁺ enters the presynaptic neurone.

Vesicles move to the membrane and release acetylcholine by exocytosis.

Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Na⁺ channels open and Na⁺ enters, depolarising the postsynaptic membrane.

If enough acetylcholine binds, the threshold is reached and an action potential travels on.

Acetylcholinesterase hydrolyses the acetylcholine to stop the stimulation.

One-way
- vesicles are only presynaptic and receptors only postsynaptic, so transmission goes one way

Part 9 of 9Acetylcholine and other transmitters

Acetylcholine (ACh)

Acetylcholine (ACh) made in the presynaptic neurone from choline and an acetyl group
Synapses that use it are cholinergic.
Acetylcholinesterase hydrolyses it to choline and acetate.
The products are absorbed back into the presynaptic neurone and rebuilt into ACh.

Other transmitters

Other transmitters over 40 are known, such as dopamine and noradrenaline

Neonicotinoids

Neonicotinoids insecticides that block acetylcholine receptors in insects

Key terms

Term	Meaning
Resting potential	The potential difference across the membrane of a neurone at rest, about -70 mV.
Action potential	A brief reversal of the membrane potential from about -70 mV to about +40 mV.
Depolarisation	The reversal of the membrane potential as Na⁺ enters the axon.
Saltatory conduction	The jumping of the impulse from one node of Ranvier to the next along a myelinated axon.
Synapse	The junction between two neurones, including the synaptic cleft.
Neurotransmitter	A chemical released by a presynaptic neurone that crosses the cleft and binds to receptors.
Acetylcholinesterase	The enzyme that hydrolyses acetylcholine in the synaptic cleft.
Sodium-potassium pump	A pump that uses ATP to move 3 Na⁺ out and 2 K⁺ in.

Avoid these mistakes

Saying an impulse is an electric current flowing along the nerve — it is a wave of membrane potential reversal

Saying a stronger stimulus gives a bigger impulse — it gives more frequent impulses

Saying the impulse jumps across the synapse — a neurotransmitter diffuses across the cleft

Saying myelin conducts the impulse — it insulates the axon and the impulse jumps between nodes

Last updated: 26 October 2026