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]

B2.3Theme B — Form and function

Cell specialization — IB Biology SL Revision Notes & Topical Questions

5 min read 13 parts Lesson 6 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

B2.3 Cell specialization — IB BIOLOGY SL notes: Explain how stem cells and gene expression produce specialised cells and how surface area to volume ratio limits cell size.Zygote — a diploid cell that divides by mitosis to form an embryoMorphogens — signalling chemicals that diffu…

Early embryonic cells
Stem cell niche
Totipotent
Pluripotent
Multipotent
Differentiation

Part 1 of 11Origin of cells in a new organism

Zygote

Zygote a diploid cell that divides by mitosis to form an embryo

Morphogens

Morphogens signalling chemicals that diffuse from a source
The concentration gradient tells cells their position.
Cells read the local concentration and so take different fates.

Early embryonic cells

Early embryonic cells unspecialised, and later they commit to one lineage

Part 2 of 11Stem cells

Stem cell

Stem cell a cell that can divide an unlimited number of times by mitosis

Self-renewal — it can divide repeatedly while staying unspecialised.
Differentiation — each new cell can remain a stem cell or develop into a specialised cell such as a blood cell or muscle cell.

Uses

Uses replacing damaged tissue, research into disease, and therapies such as bone marrow transplants

Stem cell niche

Stem cell niche a location in the body that keeps stem cells so tissues can regenerate and repair
Bone marrow supplies stem cells that replace red cells, white cells and platelets.
The hair follicle niche supports continual hair growth.
A niche must keep stem cells inactive until needed and stimulate them to divide and differentiate when required.

Part 3 of 11Potency

Feature	Totipotent	Pluripotent	Multipotent	Unipotent
Range of cell types	All, including placenta	All body cell types	A few related types	Only its own type
Example	Zygote and early embryo	Inner cell mass of the blastocyst	Bone marrow and hair follicle	Heart muscle cells
Can form a whole organism?	Yes	No	No	No

Totipotent

Totipotent the zygote and cells up to about the 16-cell stage can also form the placenta

Pluripotent

Pluripotent embryonic stem cells that cannot form extra-embryonic tissue

Multipotent

Multipotent adult stem cells already partly committed, such as marrow forming all blood cell types

Unipotent

Unipotent most animal body cells, and they can divide but give only their own type

Potency declines as development proceeds

Part 4 of 11Differentiation by gene expression

Genome

Genome every body cell nucleus contains the full set of genes

Differentiation

Differentiation specialisation by expressing a particular set of genes

Only a subset of genes is active and the others stay silent.
The proteins made decide the shape and function of the cell.
Haemoglobin genes are active only in developing red blood cells.

Permanence

Permanence differentiation is usually permanent in the adult

Division of labour

Division of labour specialised cells do particular jobs, so the organism works more efficiently

The zygote divides by mitosis.

Early cells are unspecialised stem cells.

Morphogen gradients signal position to each cell.

Specific genes are switched on and others off.

Specific proteins are made.

Cell structure and function become specialised.

Part 5 of 11Making a red blood cell

A stem cell divides in the bone marrow niche.

One daughter cell stays a stem cell.

The other commits to the red cell lineage.

Haemoglobin genes switch on.

The nucleus is lost as the cell matures.

A biconcave cell enters the blood.

Part 6 of 11Size and structure as adaptations

Size and structure as adaptations

Structure

Structure specialised cells differ in shape and in the organelles they hold
Cells that make a lot of protein have many ribosomes, rough ER and Golgi.

Red blood cell

Red blood cell small enough to squeeze through capillaries

Active white blood cell

Active white blood cell larger than a resting one, with room for rough ER and Golgi to make antibodies

Sperm

Sperm a long tail and a narrow streamlined head that reduces resistance

Egg

Egg the largest volume, to store food reserves for the embryo

Neuron

Neuron a large cell body makes the proteins that maintain a very long axon, which can exceed 1 m

Muscle fibre

Muscle fibre long and wide to generate force on contraction

Part 7 of 11Surface area to volume ratio

Metabolic rate

Metabolic rate depends on the volume of the cell, as more cytoplasm needs more exchange

Exchange rate

Exchange rate depends on the surface area of the membrane

Larger cells

Larger cells volume rises faster than surface area, so SA:V falls
Exchange becomes too slow, so wastes and heat build up.
A cube of side 1 µm has SA:V of 6 and a cube of side 2 µm has SA:V of 3.

Division

Division cells divide before growing too large

Single cells

Single cells high SA:V and short diffusion distances, so simple diffusion is enough

Multicellular organisms

Multicellular organisms large organisms evolved gas exchange, digestive and circulatory systems

Part 8 of 11Adaptations that increase SA:V

Red blood cell

Red blood cell biconcave disc without a nucleus, thin so oxygen diffuses quickly

Proximal tubule cell

Proximal tubule cell dense microvilli on the inner face and many mitochondria for active transport

Flattening

Flattening shortens the diffusion distance

Folds and projections

Folds and projections add membrane without much volume

Part 9 of 11Modelling SA:V with agar

Nature of science
- models simplify reality but still allow predictions to be tested

Make agar with dilute alkali and indicator and cut it into cubes of 0.5, 1 and 2 cm sides.

Calculate the surface area, volume and SA:V of each cube.

Place each cube in an equal volume of dilute acid that is stronger than the alkali.

Record the time for the full colour change, or the depth of change after a fixed time.

Convert times to rates and plot rate against SA:V.

Result
- the smallest cubes have the highest SA:V and change colour fastest

Part 10 of 11Applications and issues

Bone marrow transplant

Bone marrow transplant donor multipotent cells restore blood production after leukaemia treatment

Embryonic stem cells

Embryonic stem cells greater potency, but obtaining them destroys an embryo, which raises ethical debate

Tissue matching

Tissue matching cells from the patient avoid rejection by the immune system

Risk

Risk uncontrolled division could lead to tumours

Part 11 of 11More from the specification

1Stem cell therapy as a regenerative means

Stem cell therapy as a regenerative means differentiating pluripotent embryonic stem cells provides a revolutionary biomedical means to replace damaged pancreatic or cardiac tissues.

2Cell differentiation

The developmental process in multicellular eukaryotic cells where unspecialised stem cells express specific subsets of genes to acquire specialised structures.

3Division of labour

Specialisation enables distinct cell types to perform specialized metabolic functions efficiently rather than carrying out every physiological task.

4Cellular adaptations

Specialised cell types develop tailored structural adaptations.
- (e.g. biconcave shape and lack of nucleus in erythrocytes to carry oxygen)

5Metabolic surface requirements

As cell volume increases, specialized folded membranes increase surface area to satisfy high metabolic exchange requirements.

6Adaptation

The process by which populations become better suited to their environment.

Key terms

Term	Meaning
Zygote	The single diploid cell formed when two gametes fuse, the start of a new organism.
Morphogen	A signalling chemical whose concentration gradient tells embryonic cells which genes to express.
Stem cell	A cell that can divide repeatedly and give rise to specialised cell types.
Differentiation	Specialisation of a cell by expression of a particular set of genes.
Stem cell niche	A location in the body where stem cells are kept and can proliferate.
Totipotent	Able to become any cell type, including extra-embryonic tissue.
Pluripotent	Able to become any body cell type but not extra-embryonic tissue.
Surface area to volume ratio	Surface area divided by volume, which falls as a cell grows.

Avoid these mistakes

Saying specialised cells contain different genes — all cells have the same genome and differ in which genes are expressed

Saying a bigger cell is always better — a larger cell has a smaller SA:V and slower exchange

Mixing up pluripotent and totipotent — only totipotent cells can also form the placenta

Saying stem cells cannot divide after differentiation — a differentiated cell is no longer a stem cell

Last updated: 25 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.