The cell as the unit of life & conditions on early Earth
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Question
Why is the cell called the unit of life?
Answer
It is the **smallest self-sustaining unit of life** — it can take in materials, release energy, grow and reproduce on its own. All known life is **cellular**.
Question
When and how did the first cells arise?
Answer
About **3.5–4 billion years ago**, by **abiogenesis** (life from non-living chemistry) on the early Earth — a **unique, one-off** series of events.
Question
Define biogenesis.
Answer
Living cells arise **only from pre-existing living cells** — the rule we observe **today**.
Question
Define abiogenesis.
Answer
Living matter arising from **non-living chemistry** — required for the **first** cell, under early-Earth conditions.
Question
How did early-Earth conditions differ from today?
Answer
**No free oxygen** (reducing atmosphere), **no ozone** (so **intense UV**), **volcanic gases** (CH₄, NH₃, H₂O, CO₂), **lightning**, **high temperatures**, and **liquid water**.
Question
Why did a lack of free oxygen help the first organic molecules?
Answer
With **no oxygen**, the molecules were **not destroyed by oxidation**, so they could **build up** instead of breaking down.
Question
List the four stages from non-living matter to the first cell, in order.
Answer
1) organic **monomers**, 2) **polymers**, 3) **self-replicating** molecules, 4) **membrane-bound protocells**.
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Topic 1.3 hub
Origins of cells
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