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]

Theme D — Continuity and change

Theme D — Continuity and change — IB Biology SL Revision Notes & Topical Questions

5 min read 13 parts Lesson 1 of 14 Unit quiz

📚 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

Theme D — Continuity and change — IB BIOLOGY SL notes: Link DNA replication, protein synthesis, mutation, cell division, osmosis, reproduction, homeostasis and ecosystem stability in Theme D.TranscriptionRNA polymerase binds and unwinds the DNA at the gene.Free RNA nucleotides pa…

Template strand
Daughter strand
Anticodons
Somatic mutation
Germ-line mutation
Predicting an effect

Part 1 of 11From gene to polypeptide

Transcription

From gene to polypeptide

RNA polymerase binds and unwinds the DNA at the gene.

Free RNA nucleotides pair with the template strand and are joined into mRNA.

The mRNA leaves the nucleus through a nuclear pore.

Template strand

Template strand the DNA strand read in transcription, and the RNA made is complementary to it

Daughter strand

Daughter strand a newly built strand formed on a template in replication

Part 2 of 11Translation in order

The small ribosomal subunit binds to the mRNA.

The initiator tRNA pairs with the start codon AUG.

The large subunit joins.

A second tRNA pairs with the next codon.

A peptide bond links the two amino acids.

The ribosome moves on one codon and the process repeats.

At a stop codon the polypeptide is released.

Anticodons
- tRNA anticodons pair with mRNA codons

Part 3 of 11Mutations

Somatic mutation

Somatic mutation a mutation in a body cell

It is not passed to offspring but may cause cancer.

Germ-line mutation

Germ-line mutation occurs in cells that form gametes, so it is inherited and may cause genetic disease

Predicting an effect

Predicting an effect work out the likely effect of the base change on the polypeptide

Part 4 of 11Cell division

Mitosis

Mitosis keeps the chromosome number and genetic identity of the cells
New cells arise only by cell division.
Chromosomes condense so they can be moved without tangling.

Meiosis

Meiosis halves the chromosome number to make gametes
Crossing over and random orientation create variation.
Non-disjunction can cause abnormal chromosome numbers.

Part 5 of 11Osmosis

1Water movement

Water movement in osmosis water moves across a partially permeable membrane, and aquaporins are water channel proteins

2Red blood cell

Red blood cell shrinks in a hypertonic solution

3Percentage change in mass

Percentage change in mass used in osmosis practicals because the pieces start with different masses
If the graph crosses zero at 0.3 mol dm^-3, that is the isotonic concentration.

4Plasmolysis

Plasmolysis the membrane of a plant cell is pulled away from the wall

5Cells without walls

Cells without walls transplant kidney cells in pure water would swell and burst

6Contractile vacuole

Contractile vacuole an ATP inhibitor stops it, so the protist swells and may burst

7Hydration shell

Hydration shell the oxygen of water faces a sodium ion

8Repeats

Repeats more repeats reduce the standard error

Part 6 of 11Reproduction and inheritance

Asexual reproduction

Asexual reproduction genetically identical offspring from one parent without gametes

Germination

Germination the growth of a seed into a seedling once water, oxygen and a suitable temperature are available

Gametes

Gametes haploid, and fertilisation restores the diploid number
Each gamete carries only one allele of each gene.

Homozygous

Homozygous carrying two identical alleles, such as AA or aa

Part 7 of 11Homeostasis

After a long fast the glucose level is low.

Alpha cells of the pancreas detect this and secrete glucagon.

Liver cells break glycogen into glucose.

Glucose is released and the concentration returns to the set point.

Homeostasis

Glycogen

Glycogen a branched storage polysaccharide in liver and muscle cells

Type 2 diabetes

Type 2 diabetes judging lifestyle against genetic causes needs data on both

Paradigm shift

Paradigm shift a major change in scientific thinking when new evidence overturns accepted ideas

Part 8 of 11Ecosystem stability

Ecosystem stability

Requirements for a stable ecosystem

A supply of energy.
Recycling of nutrients by detritivores such as termites, worms and slugs, and decomposers such as fungi and bacteria.
Genetic diversity.
A suitable climate.

Tipping point

Tipping point deforestation reduces transpiration and rainfall, and the drier forest may become savanna

Keystone species

Keystone species sea otters eat urchins, and urchins graze kelp
Beavers create wetlands and elephants keep grassland open.

Mesocosm

Mesocosm a clear sealed container with water, substrate, producers, consumers and decomposers placed in the light

Oxygen, pH and population sizes are monitored over weeks.
A control keeps the test factor unchanged.
Terrestrial layers are gravel, charcoal, moss or filter paper, and compost with mosses and ferns.
Aquatic mesocosms use pond sediment, pond water, healthy plants and few water fleas or snails.
Only primary consumers are included, and the IB policy allows animals only if there is no alternative.

Part 9 of 11Sustainability

Sustainability

Maximum sustainable yield

Maximum sustainable yield the largest harvest that can be taken repeatedly without shrinking the population

Sustainable farming

Cover crops and small doses of fertiliser.
Biological control, such as ladybirds eating aphids.
GM crops and renewable energy.

Fertiliser

Fertiliser expensive and energy-intensive, so its price tracks energy costs

Part 10 of 11Pollution and restoration

Eutrophication

Eutrophication nutrients cause an algal bloom, light is blocked, decomposition uses oxygen, and a dead zone forms

Biomagnification

DDT moves from crops to waterways, plankton, fish and birds, thinning eggshells, and is now banned except for malaria control.
Methylmercury forms from mercury released by coal burning and gold mining, so tuna and swordfish are risky.

Plastics

Plastics turtles mistake bags for jellyfish, albatross chicks are fed plastic, and UV and waves break plastic into microplastics

Restoration

Reintroduce apex predators.
Connect habitats with corridors.
Limit human use.
Use controlled grazing or burning and allow succession, as at Hinewai Reserve in New Zealand.

Peat bog

Peat bog draining lets air in, so aerobic decomposers respire and release CO₂

Part 11 of 11More from the specification

Replication fidelity in inherited genes

Replication fidelity in inherited genes stable hydrogen bonds between nitrogenous base pairs ensure accurate copying of essential regulatory genes during meiosis.

Genetic code and triplet codons

A DNA molecule stores genetic information as a sequence of nucleotide bases;
- each triplet of bases codes for a specific amino acid in a polypeptide chain.

Transcription stage of protein synthesis

During transcription in the nucleus, RNA polymerase synthesizes a complementary messenger RNA (mRNA) molecule using the DNA template strand.

Translation stage at the ribosome

In the cytoplasm, the mRNA molecule binds to a ribosome where transfer RNA (tRNA) molecules with complementary anticodons deliver specific amino acids to assemble the polypeptide chain during translation.

Gene mutations and sickle cell anaemia

A single base substitution mutation in the beta-globin gene (GAG to GTG) changes the codon and replaces hydrophilic glutamic acid with hydrophobic valine,
causing sickle cell anaemia and haemoglobin aggregation.

Hydrogen bond

The bond between paired bases that holds the two strands together.

Coding strand

The DNA strand with the same base sequence as the mRNA (T for U).

Nuclear envelope

The double membrane around the nucleus, with pores.

Translation

Using the mRNA code to build a protein.

Sequence

The order of bases in DNA or amino acids in a protein.

Triplet

Three consecutive DNA bases that code for one amino acid.

Cytoplasm

The jelly in which the cell's chemical reactions happen.

Key terms

Term	Meaning
Template strand	The DNA strand that is read in transcription.
Somatic mutation	A mutation in a body cell that is not inherited.
Plasmolysis	The pulling away of the cell membrane from the cell wall in a plant cell.
Glycogen	A branched storage polysaccharide in liver and muscle cells.
Paradigm shift	A major change in scientific thinking after new evidence overturns accepted ideas.
Maximum sustainable yield	The largest harvest that can be taken repeatedly without shrinking the population.

Avoid these mistakes

Saying that mitosis halves the chromosome number — it keeps it, and meiosis halves it

Putting secondary consumers in a mesocosm — include only primary consumers

Saying that stable ecosystems do not change — they resist and recover from change

Saying a somatic mutation is inherited — only germ-line mutations are passed on

Last updated: 15 October 2026