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]

A2.1Theme A — Unity and diversity

Origins of cells — IB Biology SL Revision Notes & Topical Questions

5 min read 13 parts Lesson 7 of 11 1 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

A2.1 Origins of cells — IB BIOLOGY SL notes: Describe the conditions, steps and evidence for the origin of the first cells, LUCA and endosymbiosis.Cell = the smallest unit that can carry out all the functions of lifeEvery cell is enclosed by a plasma membrane.Every cell stores ge…

Greenhouse gases
UV radiation
Meteorites
Primordial soup
Protocell-first
Gene-first

Part 1 of 10Cells as the smallest units of life

Cell

Cell the smallest unit that can carry out all the functions of life

Every cell is enclosed by a plasma membrane.
Every cell stores genetic information in DNA, which is expressed during protein synthesis.

Functions of life

Functions of life metabolism, growth, response to stimuli, homeostasis, reproduction, excretion and nutrition

Viruses

Viruses not cells, because they lack metabolism, need no nutrition and rely on host cells to replicate

Part 2 of 10The challenge of the first cell

1Problem

Problem cells arise only from pre-existing cells, so the first cell had to arise without one

2Age

Age the Earth is about 4.5 billion years old and fossil evidence of cells goes back at least 3.5 billion years

Steps needed to make a cell

Simple organic compounds must be made from inorganic molecules.
The organic compounds must be assembled into polymers.
Some polymers must be able to self-replicate.
Membranes must surround the polymers to make a compartment with its own internal chemistry.

4Evidence

Evidence no direct evidence of the first cells survives, so models remain hypotheses

Part 3 of 10Conditions on the early Earth

Condition	Early Earth	Modern Earth	Relevance
Free oxygen	Almost none	About 21%	Organic molecules were not oxidised
Main gases	CO₂, N₂, H₂O, with CH₄ and NH₃ proposed	N₂ and O₂	Supplied C, H and N
Ozone layer	Absent	Present	High UV energy reached the surface
Energy	Lightning, UV, volcanic heat	Mainly sunlight	Drove synthesis reactions
Temperature	Hot and very variable	Mild	Faster reactions

Greenhouse gases

Greenhouse gases high CO₂ and methane raised surface temperatures

UV radiation

UV radiation with no ozone it reached the surface, damaged DNA and drove the formation of organic molecules

Water

Water oceans and tidal pools concentrated the reactants as they evaporated

Vents

Vents deep-sea alkaline vents offered heat, minerals and chemical gradients

Meteorites

Meteorites some carry amino acids and may have seeded the Earth with organic molecules

Primordial soup

Primordial soup the Oparin-Haldane idea that early oceans held a rich mix of simple organic compounds

Amino acids, simple sugars, nucleotides and fatty acids could have formed from heat or UV acting on the gases.

Part 4 of 10Theories for the origin of cells

1Protocell-first

Protocell-first cell-like compartments able to grow and divide arose first and later gained genetic material, probably RNA

2Gene-first

Gene-first a self-replicating nucleic acid, probably RNA, arose first and variants later developed membranes and metabolism

3Metabolism-first

Metabolism-first a self-sustaining system of chemical reactions arose first and later evolved into cells and genetic material
Many scientists favour it because life processes need energy from metabolic reactions.

4Testing

Testing the theories are hard to test
Early Earth conditions cannot be copied exactly.
The first cells were not fossilised.

Part 5 of 10Membranes and vesicles

Membranes and vesicles

Compartmentalisation

Compartmentalisation a membrane separates the genetic material and biochemistry from the surroundings

Fatty acids

Fatty acids likely first membrane molecules because they are amphipathic
In water a few lipid molecules form a monolayer at the surface.
More lipid forms bilayers with polar parts facing the water and non-polar tails facing each other.
Bilayers spontaneously form microspheres or vesicles with hydrophobic tails inwards.

Later evolution

Later evolution fatty acids could combine with glycerol to form triglycerides and then phospholipids, the main component of modern membranes

Part 6 of 10RNA as the first genetic material

1Needs of early life

Needs of early life a system that could replicate itself and a way to catalyse reactions

2RNA world hypothesis

RNA could both store genetic information and act as an enzyme
RNA folds into ribozymes that catalyse reactions.
DNA later took over storage because it is chemically more stable.
Proteins became the catalysts.

Evidence for RNA before DNA

RNA can assemble spontaneously from nucleotides and can replicate itself.
Ribose can be formed from methanal, a product of the Miller-Urey experiment.
Deoxyribose is made from ribose in an enzyme-catalysed reaction.
Ribozymes can join amino acids into proteins using an RNA template, and ribosomes still use RNA as their catalytic core.

4Fragility

RNA is chemically fragile

Part 7 of 10LUCA

LUCA

LUCA the last universal common ancestor, a single-celled organism that lived about 4 billion years ago

It sits at the base of the phylogenetic tree of life.

Evidence

All organisms share the same DNA bases, the same genetic code and the same amino acids in proteins.
Several genes are shared by eubacteria and archaea, so they were inherited from LUCA.
Organisms with similar DNA are more closely related than those with very different DNA.

Other early life

Other early life other organisms probably existed then but left no descendants
Descendants of LUCA outcompeted them for shared resources.

Dating

Dating fossils and rocks are dated by radiometric methods, and the molecular clock uses the rate at which mutations accumulate
Numbers from fossils and the molecular clock are estimates, not exact dates.

Part 8 of 10LUCA and hydrothermal vents

Habitat

LUCA may have lived near deep-sea hydrothermal vents, where chemosynthesis can supply energy

Fossil evidence

Fossil evidence haematite tubes in sedimentary rocks near vents in Quebec resemble structures made by modern iron-oxidising prokaryotes
Stromatolites and microfossils also show early prokaryotes.

Gene evidence

Gene evidence sequence data from modern vent species show a common ancestor

Likely features of LUCA

Anaerobic.
Converted carbon dioxide into glucose.
Used hydrogen as an energy source instead of sunlight.
Converted nitrogen into ammonia for amino acids.
Thermophilic, surviving very high temperatures.

Alternative hypotheses

Alternative hypotheses scientists continue to gather data that may support or refute this view

Part 9 of 10Endosymbiosis

Endosymbiosis

Endosymbiosis one cell living inside another, which explains the origin of mitochondria and chloroplasts

A large cell engulfed an aerobic bacterium that became the mitochondrion.
A later engulfed photosynthetic bacterium became the chloroplast.

Evidence

Evidence both organelles have a double membrane, their own circular DNA, 70S ribosomes and divide by binary fission

Common origin

Common origin it explains why all eukaryotes share a common origin

Part 10 of 10More from the specification

Theory
- A well-tested explanation supported by a large body of evidence.

Key terms

Term	Meaning
Primordial soup	The idea that early oceans held a rich mix of simple organic compounds.
Miller-Urey experiment	A 1950s laboratory simulation showing amino acids can form from simple gases with an energy input.
Vesicle	A small sphere bounded by a lipid bilayer that forms spontaneously from fatty acids in water.
RNA world	The hypothesis that early life used RNA both to store information and to catalyse reactions.
Ribozyme	An RNA molecule that acts as a catalyst.
LUCA	The last universal common ancestor from which all living organisms descend.
Hydrothermal vent	A crack in the sea floor releasing hot mineral-rich water, a proposed site for early life.
Endosymbiosis	The origin of mitochondria and chloroplasts from bacteria engulfed by a larger cell.

Avoid these mistakes

Saying that the Miller-Urey experiment created life — it only made simple organic molecules

Thinking viruses are cells — they lack metabolism and cannot reproduce without a host

Saying LUCA was the only organism alive at that time — other organisms probably existed but left no descendants

Thinking DNA came before RNA — RNA is thought to have been the first genetic material

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.