Endosymbiotic origin of organelles

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Mitochondria and chloroplasts began as **free-living prokaryotes** that were **engulfed** by a host cell and **survived inside it**, becoming organelles.

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'**Endo**' = inside, '**symbiosis**' = living together — one cell living permanently inside another, with both benefiting.

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An **aerobic (oxygen-using) bacterium** — it carries out aerobic respiration to release energy for the host.

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A **photosynthetic bacterium** — it makes food using light, in plant and algal cells.

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**Engulf** the bacterium -> it **survives** inside -> the relationship **benefits both** -> the bacterium is **kept** and becomes an organelle.

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**Endocytosis** — the host membrane folded around the bacterium, so it ended up inside a vesicle.

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Each organelle has its **own DNA**, its **own 70S ribosomes**, a **double membrane**, and divides by **binary fission**.

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**70S** — the smaller, bacterial type. The host cell's cytoplasm uses larger **80S** ribosomes.

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The **inner** membrane is the bacterium's own; the **outer** membrane came from the host's vesicle when the bacterium was engulfed.

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The cytoplasm uses **80S** ribosomes, but the chloroplast keeps the **70S** ribosomes of its free-living bacterial ancestor.

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By **binary fission** — splitting in two on their own, just like the free-living bacteria they descended from.

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Only some cells engulfed the **photosynthetic** partner (-> chloroplast); the **aerobic** partner (-> mitochondrion) was engulfed by the ancestor of nearly all eukaryotes.