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]

B3.1Theme B — Form and function

Gas exchange — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 7 of 13 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

B3.1 Gas exchange — IB BIOLOGY SL notes: Explain how gas exchange surfaces, lungs and leaves are adapted for diffusion, and measure ventilation, transpiration and stomatal density.Gas exchange = the diffusion of oxygen and carbon dioxide between an organism and its environmentIn …

Respiration compared with gas exchange
Properties of a gas exchange surface
Small unicellular organisms
Airway order
Blood supply
Alveolar wall

Section 1 of 13Principles of gas exchange

1Gas exchange

Gas exchange the diffusion of oxygen and carbon dioxide between an organism and its environment

In animals oxygen is taken in and carbon dioxide released.
In plants carbon dioxide is absorbed and oxygen released in the day because of photosynthesis.

2Respiration compared with gas exchange

Respiration compared with gas exchange respiration is a chemical process in all living cells and gas exchange is diffusion across a respiratory surface

Rate of diffusion

It rises with a larger respiratory surface.
It rises with a steeper concentration gradient.
It falls as the diffusion distance increases.

4Properties of a gas exchange surface

Properties of a gas exchange surface permeable, thin, moist and with a large surface area
Gases must dissolve in the moisture before they diffuse.

Section 2 of 13Gas exchange in large organisms

Gas exchange in large organisms

Small unicellular organisms

Small unicellular organisms an amoeba has a large SA:V and a short diffusion distance, so diffusion alone supplies enough oxygen

Larger organisms

A smaller SA:V and a longer diffusion distance mean diffusion alone is not enough.
The outer surface is built for protection and is not suitable for exchange.
Active organisms need more oxygen per unit time.
They need specialised gas exchange organs and transport.

Maintaining the concentration gradient

A dense network of blood vessels gives a large surface area.
Continuous blood flow carries oxygen away from the surface and carbon dioxide towards it.
Ventilation with air in lungs or water in gills brings oxygen close to the surface and removes carbon dioxide.

Section 3 of 13Mammalian airways and alveoli

Airway order

Airway order air moves through the nose and mouth, the trachea, the two bronchi, the bronchioles and then the alveoli
The trachea has rings of cartilage that keep it open yet flexible.
The bronchi have cartilage and smooth muscle and are lined with ciliated epithelium and mucus that remove trapped particles.
Bronchioles have smooth muscle that dilates when more air is needed and constricts when an allergen is present.

Alveoli

Alveoli clusters at the ends of the bronchioles, spread evenly through the lung
Their large total area is about 70-100 m².
Type I pneumocytes are very flat and keep the diffusion distance short.
Type II pneumocytes secrete surfactant, which lowers surface tension and stops the alveoli collapsing and sticking together.
A moist lining lets gases dissolve before diffusing.

Blood supply

Blood supply each cluster is surrounded by a capillary bed
Deoxygenated blood arrives from a branch of the pulmonary artery.
Oxygenated blood leaves in a branch of the pulmonary vein.
This keeps the gradients of oxygen and carbon dioxide steep.

Alveolar wall

Alveolar wall a single layer of epithelial cells and not a cell wall

Section 4 of 13Ventilation

Ventilation

Ventilation replacing old air in the lungs with fresh air, which maintains the concentration gradients

Pressure rule

Pressure rule air always moves from higher to lower pressure, and a larger volume gives a lower pressure

Feature	Inhalation	Exhalation
External intercostals	Contract	Relax
Internal intercostals	Relaxed	Contract only in forced exhalation
Diaphragm	Contracts and flattens	Relaxes and domes upward
Ribcage	Moves up and out	Moves down and in
Thorax volume	Increases	Decreases
Lung pressure	Falls below atmospheric	Rises above atmospheric
Abdominal muscles	Relaxed	Contract in forced exhalation
Elastic fibres in alveoli	Stretched	Recoil to push air out

Antagonistic muscles

Antagonistic muscles muscles can only pull, so pairs work in opposition

Resting exhalation

Resting exhalation it is mostly passive because of the recoil of elastic fibres

Forced exhalation

Forced exhalation internal intercostals pull the ribs down and in, and abdominal muscles push organs up against the diaphragm

Section 5 of 13Measuring lung volumes

1Spirometer

Spirometer a chamber of water covered by a hinged lid that moves as the person breathes through a mouthpiece
A chamber of air gives lung volumes in different conditions.
A chamber of oxygen with soda lime to absorb carbon dioxide measures oxygen consumption.
Traces are drawn on a revolving drum or by a computer.

2Tidal volume

Tidal volume the volume of air breathed in and out in normal breathing, about 0.5 dm³ at rest

3Inspiratory reserve volume

Inspiratory reserve volume maximum inspiratory level minus tidal volume

4Expiratory reserve volume

Expiratory reserve volume tidal volume minus maximum expiratory level

5Vital capacity

VC = TV + IRV + ERV, typically 3-5 dm³ in an adult

6Ventilation rate

Ventilation rate breaths per minute

Exercise raises both the tidal volume and the ventilation rate.

Section 6 of 13Leaf structure and gas exchange

Leaf structure and gas exchange

Epidermis

Epidermis a single layer of tightly packed cells with a waxy cuticle
The lower epidermis holds most stomata.
Each stoma has two guard cells that open it when turgid and close it when flaccid.

Mesophyll

Mesophyll parenchyma cells with chloroplasts
Palisade mesophyll lies beneath the upper epidermis and is the main site of photosynthesis.
Spongy mesophyll has large air spaces for gas exchange.

Vascular tissue

Vascular tissue veins with xylem carrying water and minerals to the leaf and phloem carrying sugars away

Tissue	Location	Role in gas exchange
Waxy cuticle	Covers the upper epidermis	Cuts water loss so exchange occurs through stomata
Palisade mesophyll	Under the upper epidermis	Main photosynthesis site that uses CO₂
Spongy mesophyll	Middle to lower leaf	Air spaces allow gas diffusion and increase surface area
Lower epidermis	Underside	Holds most stomata where it is cooler, reducing water loss
Guard cells	Around each stoma	Open or close the pore
Veins	Within the mesophyll	Xylem supplies water for photosynthesis and transpiration

Section 7 of 13Transpiration

Transpiration

Transpiration loss of water vapour from the leaf by diffusion through open stomata

It is the unavoidable consequence of gas exchange.
Water vapour leaves by diffusion, not osmosis.
Closing the stomata reduces water loss but also reduces photosynthesis.

Benefits

Benefits evaporative cooling, uptake of mineral ions by the transpiration stream and turgor that supports leaves and stems
Turgid leaves do not wilt, so their surface area for photosynthesis stays large.

Factors increasing the rate

Air movement removes humid air near the stomata and steepens the gradient.
Higher temperature gives more kinetic energy, up to a point where stomata close.
Higher light intensity opens the stomata, until all are open and the rate levels off.
Lower humidity gives a steeper gradient, and in saturated air there is no net water loss.

Section 8 of 13Measuring transpiration with a potometer

Potometer

Potometer measures the rate of water uptake of a shoot, which stands for the rate of transpiration
Only a tiny fraction of the water taken up is used in photosynthesis.
A bubble potometer tracks the movement of an air bubble along a water-filled tube, or the time for it to move a set distance.
A mass potometer records the loss of mass of a water-filled tube attached to the shoot.

Varying one factor

Air movement by a fan on different settings.
Humidity by a plastic bag or a humidifier.
Light intensity by a lamp at different distances.
Temperature by a heater or air conditioner and a probe.

Control

Control keep every other variable constant, as a heater can also change light and humidity

Section 9 of 13Plan diagram of a leaf

Plan diagram of a leaf

Structures to identify

Structures to identify cuticle, upper and lower epidermis, guard cells, stomata, palisade mesophyll, spongy mesophyll, air spaces, chloroplasts and vascular bundles

Drawing rules

Draw tissue layers enclosed by lines and never individual cells.
Use a sharp pencil and clear continuous lines with no shading.
Make the drawing accurate, large and based on what is seen.

Labelling rules

Use a ruler for label lines.
No arrowheads and no crossing lines.
Write labels horizontally.

Section 10 of 13Stomatal density

Stomatal density = the number of stomata per unit area of leaf
- It shows the likely response to a dry spell and how the plant would behave if moved.

Paint clear nail varnish on the lower surface of a leaf.

Let it dry fully, for about 5 minutes.

Peel the film off with clear tape and stick it on a microscope slide.

Count the stomata in a field of view containing 15-100, counting partly visible ones at the edge.

Repeat for at least three fields and calculate the mean.

Measure the diameter of the field of view with a stage micrometer at the same magnification.

Calculate the area of the field as πr² and the density as the mean count divided by the area.

Reliability

Reliability repeats allow anomalies to be removed and a reliable mean to be calculated

Limitations

Limitations solvent-based varnish can damage cells and not all species give a strong imprint

Section 11 of 13More from the specification

1Alveolar adaptations in mammalian lungs

Extremely large surface area, thin single-cell squamous epithelium, and rich capillary networks maximize diffusion rates.

2Gas exchange vs cellular respiration

Gas exchange is the passive physical diffusion of O₂ and CO₂ across respiratory surfaces, distinct from biochemical ATP generation.

3Stomatal regulation in plants

Paired guard cells take in water molecules by osmosis to swell and open stomatal apertures, controlling transpiration and CO₂ entry.

4Determining stomatal density

Microscopic measurements using leaf epidermal impressions allow quantification of stoma count per unit surface area.

Section 12 of 13Key terms

Term	Meaning
Gas exchange	The diffusion of oxygen and carbon dioxide between an organism and its environment.
Alveolus	A tiny air sac in the lung where gas exchange takes place.
Surfactant	A substance from type II pneumocytes that lowers surface tension in the alveoli.
Tidal volume	The volume of air breathed in and out during normal breathing.
Vital capacity	The maximum volume of air exhaled after a deep breath, equal to TV + IRV + ERV.
Stoma	A pore in the leaf epidermis surrounded by two guard cells.
Transpiration	The loss of water vapour from a leaf through open stomata.
Potometer	An instrument that measures the rate of water uptake by a shoot.

Section 13 of 13Exam technique

Confusing respiration with gas exchange — respiration is a chemical process and gas exchange is diffusion across a surface

Saying transpiration is osmosis — water vapour diffuses out through the stomata

Saying expiration at rest needs muscle contraction — it is mostly passive recoil of elastic fibres

Saying the potometer measures transpiration exactly — it measures water uptake as an estimate

Last updated: 26 October 2026