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 HL Revision Notes & Topical Questions

10 min read 12 parts Lesson 6 of 11 2 quizzes

📚 IB Biology HL 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 HL notes: Explain neurone structure, resting and action potentials, impulse speed, synaptic transmission and integration of signals in the nervous system.Nervous system — the central nervous system (CNS) is the brain and spinal cord, and the pe…

Resting potential
Refractory period
All-or-nothing
Propagation
Oscilloscope
Correlation

Part 1 of 10Nervous system and neurones

Nervous system and neurones

Nervous system

Nervous system the central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is all the nerves.
It senses surroundings, coordinates responses and regulates body functions by electrical impulses along neurones.
A nerve is a bundle of neurones.

Neurone structure

A long axon carries impulses away from the cell body, often insulated by Schwann cells forming a myelin sheath.
The cell body holds the nucleus and organelles.
Dendrites extend from the cell body and axon terminals and connect to other neurones.

Neurone	Role	Structure
Sensory	Carries impulses from receptors to the CNS	Cell body branching off the middle, one long dendron and one long axon
Relay	Connects sensory and motor neurones inside the CNS	Short, highly branched axons and dendrites
Motor	Carries impulses from the CNS to effectors (muscles or glands)	Large cell body in the CNS, many branched dendrites for a large surface area

Part 2 of 10Resting potential

Impulse

Impulse it is a momentary reversal of the potential difference across the membrane, not an electrical current flowing like a wire.

Resting potential

Resting potential the membrane potential of a neurone not transmitting an impulse, about −70 mV, with the inside negative.

Causes

Sodium-potassium pumps use ATP to pump 3 Na⁺ out and 2 K⁺ in, creating concentration gradients.

Both ions diffuse back by facilitated diffusion through channels.

The membrane is much less permeable to Na⁺ than K⁺, so K⁺ leaves faster than Na⁺ enters.

More positive ions are outside, and negatively charged proteins inside add to the negative charge.

Polarised
- the membrane at resting potential is polarised, and depolarisation is the reversal of the potential difference.

Part 3 of 10Action potential

Action potential

Action potential the reversal of the potential from about −70 mV to about +40 mV when a neurone is stimulated.

Depolarisation

Action potential

A few sodium channels open and Na⁺ enters down its gradient, making the inside less negative.

At the threshold potential of about −50 mV, voltage-gated sodium channels open and many more Na⁺ enter.

The potential rises to about +40 mV.

Repolarisation

About 1 ms later, voltage-gated sodium channels close.

Voltage-gated potassium channels open and K⁺ diffuses out down its gradient, making the inside negative again.

A brief hyperpolarisation follows, when the potential is more negative than resting.

Potassium channels close and the pumps restore resting potential.

Voltage-gated channels are closed at rest, while some potassium channels are open at rest to produce the resting potential.

Refractory period

Refractory period the membrane cannot respond during hyperpolarisation, so action potentials are discrete and travel in one direction.

All-or-nothing

All-or-nothing an action potential is generated only if the threshold is reached.
A stronger stimulus raises the frequency of action potentials, not their size, so the brain detects stimulus size by frequency.

Part 4 of 10Propagation and local currents, oscilloscope traces

Propagation

Propagation depolarisation at one site causes the next section to reach threshold, repeating along the axon.

Local currents

Inside the axon, Na⁺ diffuses from the depolarised section to the neighbouring section, reducing its negative potential.
Outside the axon, Na⁺ moves from the resting section to the depolarised section.
These movements cause a wave of depolarisation and repolarisation.

Oscilloscope
- it shows time in ms on the x-axis and membrane potential in mV on the y-axis, measured with electrodes on each side of the membrane.
- Resting potential is a flat line at −70 mV.
- An action potential is a spike up to +30 to +40 mV, with the rising phase depolarisation and the falling phase repolarisation.
- A slow rise appears before the threshold because few sodium channels are open, then a brief hyperpolarisation follows.

Part 5 of 10Speed of nerve impulses

Factors

Myelination — myelinated neurones conduct far faster because the insulation allows saltatory conduction.
Axon diameter — a wider axon has less resistance and conducts faster.

Squid giant axons are unmyelinated and up to 1 mm wide,

but conduct more slowly than narrower myelinated mammalian axons.
- (human neurones are 4–100 µm)

Myelin

Schwann cells wrap the axon in layers of phospholipid membrane, giving high electrical resistance.
Nodes of Ranvier are the uninsulated gaps with clusters of ion channels and pumps.
Saltatory conduction = the impulse jumping from node to node, through local currents between nodes, up to 50 times faster than in an unmyelinated axon.

Part 6 of 10Correlation and Pearson's test

1Correlation

Correlation an association between variables, which does not prove causation.

2Scatter graph

Scatter graph plot the variables; correlation can be positive, negative or absent.

3Correlation coefficient (r)

Correlation coefficient (r) it ranges from −1 to +1, where ±1 is perfect, closer to ±1 is stronger and 0 means none.

Pearson's linear correlation test needs quantitative data with a normal distribution.

Make a scatter graph and check for a linear trend.

State a null hypothesis.

Calculate r

(nΣxy − ΣxΣy) ÷ √[(nΣx² − (Σx)²)(nΣy² − (Σy)²)].

If r is close to ±1, there is a strong linear correlation and the null hypothesis can be rejected.

Coefficient of determination (R²) = r squared, expressed as a percentage.
- Close to 1 means the dependent variable can be predicted from the independent variable, and close to 0 means no correlation.

Part 7 of 10Synapses and synaptic transmission

Synapse

Synapse the junction of two neurones (or a neurone and a receptor or muscle fibre) with a synaptic cleft between them,

where there is no physical contact.

Steps of transmission

An impulse depolarises the presynaptic membrane.

Calcium ions enter through calcium channels.

Vesicles move to and fuse with the presynaptic membrane and release neurotransmitter by exocytosis.

The neurotransmitter diffuses across the cleft and binds receptors on the postsynaptic membrane.

Sodium channels open and Na⁺ diffuses into the postsynaptic cell.

If the threshold is reached, an action potential starts in the postsynaptic neurone.

The neurotransmitter is broken down to prevent continued stimulation.

Unidirectional

Unidirectional neurotransmitter is released on one side and receptors are on the other, so transmission goes one way only.

Acetylcholine (ACh)

Acetylcholine (ACh) more than 40 neurotransmitters exist, including dopamine and noradrenaline.
ACh is made in the presynaptic neurone from choline and an acetyl group, and cholinergic synapses use it.
Acetylcholinesterase hydrolyses ACh to acetate and choline, which are reabsorbed to remake ACh.

Part 8 of 10Effects of exogenous chemicals on synapses

Neonicotinoids

Neonicotinoids synthetic nicotine-like pesticides.
They bind irreversibly to ACh receptors at insect cholinergic synapses because acetylcholinesterase cannot break them down.
Impulses cannot cross, causing paralysis and death.
They are not toxic to mammals because mammals have fewer cholinergic synapses and the compounds bind less strongly.
They are controversial because of harm to pollinators such as bees.

Cocaine

Cocaine it blocks reuptake of neurotransmitters, mainly by binding the dopamine transporter protein.
Dopamine builds up in the synapse, causing pleasure, and serotonin and norepinephrine also raise confidence and energy.
Regular users make more dopamine receptors, which gives increased sensitivity and depression when levels fall.

Part 9 of 10Inhibition and summation

Inhibitory synapse

Inhibitory synapse the neurotransmitter opens potassium channels so K⁺ leaves, the postsynaptic cell is hyperpolarised and the threshold is not reached.
Inhibition cancels the Na⁺ entry from excitatory synapses on the same cell body.
It prevents random impulses and allows specific pathways, such as a reflex pulling the arm away without moving the leg.
Inhibitory pathways develop over time, which refines fine movements.

Summation

Summation one impulse may release too little ACh, open too few channels and fail to reach the threshold, so impulses are added together.
Temporal summation = several impulses in quick succession at one synaptic knob.
Spatial summation = impulses arriving at once at different synaptic knobs on the same cell body.

Benefits of summation

It magnifies the effect of a stimulus.
It lets a combination of stimuli trigger a response.
It prevents the nervous system being overwhelmed.
Synapses slow transmission and pass impulses only when inputs are sufficient.

Part 10 of 10Pain and the brain

1Pain receptors (nociceptors)

Pain receptors (nociceptors) sensory receptors with free, unencapsulated nerve endings.
TRP channels open in response to high temperature, acid or chemicals such as capsaicin.
Positive ions enter, the threshold is reached and an action potential travels to the CNS.
Impulses reach the cerebral cortex, where pain is perceived and a protective response follows.
Sensitivity to damaging stimuli gives a survival and reproductive advantage.

2Cerebrum

Cerebrum about 80% of brain mass, controlling conscious activities such as vision, hearing, speech, thinking and memory.
It has two hemispheres, each with a cerebral cortex (grey matter) of neurone cell bodies.
Folding of the cortex increases its area for more neurones and connections.
Complex pathways give emergent properties such as consciousness, the qualitative perception of feelings and a complex awareness of the environment.

Key terms

Term	Meaning
Resting potential	the membrane potential of an unstimulated neurone, about −70 mV.
Threshold potential	the membrane potential, about −50 mV, at which an action potential is triggered.
Saltatory conduction	impulse jumping between nodes of Ranvier in myelinated axons.
Refractory period	the time after an action potential when the membrane cannot be stimulated.
Synaptic cleft	the gap between two neurones at a synapse.
Temporal summation	adding the effect of impulses arriving in quick succession.
Spatial summation	adding the effect of impulses from different synaptic knobs at once.
Neurotransmitter	a chemical that carries a signal across a synapse.
Hyperpolarisation	the membrane potential becoming more negative than resting potential.

Avoid these mistakes

A nerve impulse is a current flowing along the axon like in a wire — it is a wave of membrane potential reversal.

A stronger stimulus gives a bigger action potential — action potentials are all-or-nothing, and stimulus size is coded by frequency.

Sodium-potassium pumps are the only cause of resting potential — unequal ion permeability also contributes.

Impulses jump across the synaptic cleft — neurotransmitter diffuses across it.

Last updated: 26 October 2026