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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

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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
  1. Metabolic reactions such as respiration.
  2. Nutrition.
  3. Production of metabolic waste that is excreted.
  4. Reproduction and passing on genetic information, which allows evolution by natural selection.
  5. Response to stimuli.
  6. 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

TheoryProposalLater development
Protocell-firstCell-like compartments able to do basic metabolism arose spontaneously and could grow and divideGenetic material, probably RNA, was acquired later
Gene-firstA nucleic acid, probably RNA, arose able to replicate itselfNatural selection of variants led to a membrane and basic metabolism
Metabolism-firstA self-sustaining system of chemical reactions aroseIt 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

TermMeaning
Primordial soupthe hypothesis that energy acting on early gases and water formed organic molecules.
Protocella cell-like compartment able to grow and divide, possibly without genetic material.
Greenhouse effecttrapping of infrared radiation by gases such as carbon dioxide and methane.
OzoneO3, formed from oxygen and UV radiation, which absorbs harmful UV.
Polymera large molecule made of many repeating monomers.
Metabolismthe 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.