IB Biology HL — Origins of cells

Key Idea: Topic A2.1 (HL) is the origin of cells: how non-living chemistry on early Earth turned, step by step, into the very first living cells, and then into the complex (eukaryotic) cells we have today. It tells one connected story in five stages: 1. All life is cellular — the cell is the smallest unit of self-sustaining life — and conditions on early Earth (no oxygen, intense UV, volcanic gases, lots of energy) made the first origin possible. 2. Simple organic monomers (amino acids, nucleotides, sugars) formed abiotically — by prebiotic synthesis. 3. Membranes self-assembled into protocells, giving compartmentalisation. 4. An RNA world provided the first self-replicating molecule — RNA can both store information and catalyse. 5. All life traces back to LUCA, and later eukaryotes arose by endosymbiosis. This is HL-only material — SL students do not study it — and it is examined mostly on Paper 2 'outline/explain the origin of…' questions.

🧬 The cell as the unit of life & early Earth (1.3.1)

The cell is the smallest unit of self-sustaining life — every known living thing is made of one or more cells. So the origin of life is really the origin of the first cell, a unique, one-off series of events about 3.5–4 billion years ago. It could happen then because early Earth was very different from today: a reducing (oxygen-free) atmosphere, intense UV (no ozone shield), volcanic gases, and plenty of energy from lightning, heat and UV. The first cell needed abiogenesis — life from non-living chemistry — whereas today only biogenesis (life from life) occurs. The whole topic is the four-stage overview: monomers → polymers → self-replicating molecules → membrane-bound protocells.

Abiogenesis = Ancient one-off (no oxygen, the FIRST cell). Biogenesis = bio all around us now (cells only come from cells). The first origin needed the early-Earth conditions that no longer exist.

⚗️ Prebiotic synthesis of carbon compounds (1.3.2)

Step 1 of the story: where did the first organic (carbon) molecules come from, with no life to make them? They could form abiotically from simple inorganic precursors, and three sources are proposed. A Miller–Urey-type experiment sparks a reducing gas mixture and produces amino acids — showing monomers can form without life. Hydrothermal vents offer mineral catalysts plus energy in the deep sea. And some organics arrived from space on meteorites (e.g. the Murchison meteorite). The key rule: monomers must form before polymers — this is the very first step of the origin of cells.

Prebiotic synthesis only has to explain the building blocks (amino acids, nucleotides, sugars). Joining them into polymers (proteins, nucleic acids) comes next. Don't claim Miller–Urey made life or made proteins — it made monomers.

🫧 Protocells & compartmentalisation (1.3.3)

Put phospholipids or fatty acids in water and they spontaneously self-assemble into a bilayer vesicle — no enzymes required. This happens because of the hydrophobic effect: the water-fearing tails cluster away from water while the water-loving heads face it. A vesicle with an internal space is a protocell, and its great gift is compartmentalisation — an inside chemistry kept separate from the outside. That concentrates reactants, retains products, and protects any self-replicating molecules inside, so it would have been naturally selected. But protocells are not yet alive: they lack reliable heredity.

Tails hide from water → a bilayer builds itself → a bag with an inside. That 'inside' is compartmentalisation — the protocell's superpower. A protocell is a container, not yet a creature (no reliable heredity).

🧪 The RNA world (1.3.4)

Heredity needs a self-replicating molecule. The best candidate for the first one is RNA, because RNA can do two jobs at once: store genetic information (in its base sequence) and act as a catalyst (ribozymes). So RNA could catalyse its own replication, giving variation and selection — an 'RNA world' that came before DNA and proteins. Later there was a hand-over: DNA took over information storage (double-stranded → more stable, fewer mutations) and proteins took over catalysis (20 amino acids → far more versatile than 4 bases). Evidence RNA came first: the ribosome's catalytic core is a ribozyme (rRNA), RNA runs translation (mRNA, tRNA, rRNA), and cells still use RNA-based cofactors like ATP and NAD.

Key Idea: DNA stores information but can't catalyse. Protein catalyses but can't store the information to copy itself. RNA does both, so a single RNA molecule could copy itself — the only candidate that can start heredity on its own.

🌳 LUCA & the origin of eukaryotes (1.3.5)

All life today traces back to a single ancestral population — LUCA, the Last Universal Common Ancestor. The evidence is shared core biochemistry: the near-universal genetic code, DNA/RNA, ATP as energy currency, ribosomes, and common metabolic pathways are best explained by common ancestry from LUCA (dated by molecular clocks to roughly 4 billion years ago, probably near hydrothermal vents). Later, eukaryotic cells arose by endosymbiosis: a host cell engulfed a free-living aerobic bacterium that survived and became the mitochondrion; a separate engulfment of a photosynthetic cyanobacterium gave the chloroplast. The organelles still show four lines of evidence of their bacterial past — own circular DNA, 70S ribosomes, a double membrane, and division by binary fission.

LUCA = the shared starting point of all life (evidence = universal genetic code + shared biochemistry). Endosymbiosis = a later merger that made eukaryotic organelles (evidence = circular DNA, 70S ribosomes, double membrane, binary fission). LUCA is about a common ancestor; endosymbiosis is about a cell-inside-a-cell.

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