A2.1Theme A — Unity and diversity
Origins of cells — IB Biology HL Revision Notes & Topical Questions
10 min read 15 parts Lesson 7 of 14 1 quiz
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.
Lesson summary
A2.1 Origins of cells — IB BIOLOGY HL notes: Explain how conditions on early Earth could have produced carbon compounds and the first cells.Greenhouse atmosphere — it held more carbon dioxide and methane than today, which trap infrared radiation and raised surface temperatures.No…
- No free oxygen
- UV radiation
- Energy sources
- Concentration of reactants
- Extra supply
- Primordial soup hypothesis
Part 1 of 6Conditions on early Earth
1Greenhouse atmosphere
- Greenhouse atmosphere it held more carbon dioxide and methane than today, which trap infrared radiation and raised surface temperatures.
2No free oxygen
- No free oxygen without oxygen no ozone formed, and organic molecules were not quickly oxidised.
3UV radiation
- UV radiation it reached the surface, damaging DNA and raising mutation rates.
4Energy sources
- Energy sources heat, UV radiation, lightning and volcanic activity supplied energy to the gas mixture.
5Concentration of reactants
- Concentration of reactants oceans and tidal pools concentrated them as water evaporated.
6Extra supply
- Extra supply some meteorites carry amino acids and may have added organic material.
Part 2 of 6Origin of carbon compounds
1Primordial soup hypothesis
- Oparin and Haldane proposed that energy added to the early gases formed simple organic molecules.
- Molecules formed include amino acids, simple sugars, nucleotides and fatty acids.
- These were the building blocks of early cells.
2Polymers
- UV radiation could have catalysed the joining of simple molecules into proteins, complex sugars, mRNA and phospholipids.
Part 3 of 6Cells as units of life
Cells as units of life
Common features
- Common features every cell has a plasma membrane and stores genetic information in DNA that is expressed in protein synthesis.
Features of life
- Metabolic reactions such as respiration.
- Nutrition.
- Production of metabolic waste that is excreted.
- Reproduction and passing on genetic information, which allows evolution by natural selection.
- Response to stimuli.
- Growth.
- Viruses
- they are non-living because they have no cell structure, carry out no metabolism, need no nutrition and cannot replicate without a host cell.
Part 4 of 6Steps from non-living matter to the first cells
1Problem
- Problem cells form only from existing cells, so the first cells must have arisen from non-living components of the primordial atmosphere.
2Required properties of any cell
- Required properties of any cell a partially permeable membrane, passable genetic material and metabolism that releases energy for growth, maintenance and reproduction.
3
Stages
1
Simple organic compounds formed from inorganic molecules, as shown by Miller and Urey.
2
Simple organic compounds assembled into polymers.
3
Some polymers gained the ability to self-replicate.
4
Membranes enclosed the polymers to give compartments with a different internal chemistry.
Part 5 of 6Theories for the origin of cells
| Theory | Proposal | Later development |
|---|---|---|
| Protocell-first | Cell-like compartments able to do basic metabolism arose spontaneously and could grow and divide | Genetic material, probably RNA, was acquired later |
| Gene-first | A nucleic acid, probably RNA, arose able to replicate itself | Natural selection of variants led to a membrane and basic metabolism |
| Metabolism-first | A self-sustaining system of chemical reactions arose | It evolved into cells and genetic material; favoured by many scientists because cell processes need energy from metabolism |
Part 6 of 6Testing theories on the origin of cells
Testability
- Testability hypotheses and theories should be testable, so scientists experiment on the mechanisms behind each theory.
Limits
- Early Earth conditions cannot be reproduced exactly in a laboratory.
- The nature of the first cells is unknown because none fossilised.
- These limits make the hypotheses difficult to test.
Key terms
| Term | Meaning |
|---|---|
| Primordial soup | the hypothesis that energy acting on early gases and water formed organic molecules. |
| Protocell | a cell-like compartment able to grow and divide, possibly without genetic material. |
| Greenhouse effect | trapping of infrared radiation by gases such as carbon dioxide and methane. |
| Ozone | O3, formed from oxygen and UV radiation, which absorbs harmful UV. |
| Polymer | a large molecule made of many repeating monomers. |
| Metabolism | the chemical reactions in a cell that release energy and build molecules. |
Avoid these mistakes
1
The Miller–Urey experiment made living cells — it made only simple organic molecules such as amino acids.
2
Early Earth had an ozone layer — it had no free oxygen, so UV radiation reached the surface.
3
Viruses are cells — they lack cell structure and cannot carry out metabolism or replicate alone.
4
The origin of cells is settled — the theories are debated and difficult to test.
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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.