# Photosynthesis and the Oxygen Revolution

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/photosynthesis-and-the-oxygen-revolution

## The Reaction That Changed Everything

Photosynthesis captures light and stores it as chemical bonds:

$$6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{light} \rightarrow \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}$$

It runs in two connected stages. The **light reactions**, in the thylakoid membranes, split water, release oxygen, and produce ATP and NADPH. The **Calvin cycle**, in the stroma, spends that ATP and NADPH to fix carbon dioxide into sugar. The second stage is often called light-independent, which is true only in the narrow sense that light is not directly required, it stops within minutes in the dark, once the ATP and NADPH run out.

The Calvin cycle's arithmetic is worth knowing because it explains the cost. Fixing 3 molecules of CO₂ yields one three-carbon sugar and consumes 9 ATP and 6 NADPH, so a full six-carbon glucose costs 18 ATP and 12 NADPH.

The **oxygen** released is a waste product, and it transformed the planet. The **Great Oxidation Event**, about 2.4 billion years ago, was arguably the largest pollution event in Earth's history: oxygen was toxic to most existing anaerobes and drove a mass extinction. But it also made **aerobic respiration** possible, which yields far more energy per glucose than fermentation, roughly 30-32 ATP against 2, and that energy budget is what large, complex, active organisms run on.

Oxygen also produced the **ozone layer**, which absorbs ultraviolet radiation. Until it existed, the land surface was effectively sterilised by UV, and life was confined to water. The colonisation of land had to wait for a by-product of photosynthesis to build a shield.

> **Common pitfall:** treating the Calvin cycle as something that happens at night. It needs ATP and NADPH from the light reactions, so it stops shortly after darkness falls. "Light-independent" means it does not use photons directly, not that it runs without light.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why does the Calvin cycle stop within minutes of darkness, despite being called light-independent?

A. It depends on ATP and NADPH that only the light reactions supply
B. Its key stromal enzymes are rapidly broken down in the dark
C. It directly requires a photon to activate carbon fixation steps
D. Stomata close immediately so carbon dioxide cannot enter the leaf

**Answer:** A. It depends on ATP and NADPH that only the light reactions supply

**Why:** The cycle uses no photons itself, which is what the name means, but it burns ATP and NADPH that only the light reactions make, and the cell holds only a small pool. Once that pool is spent the cycle halts.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/why-does-the-calvin-cycle-stop-within-minutes-of-darkness-despite

### 2. Aerobic respiration yields about 30 ATP per glucose, while fermentation yields 2. Set how many times more energy the aerobic route extracts.

**Answer:** 15 (within ±1.5)

**Why:** Aerobic respiration extracts about $30/2 = 15$ times more ATP per glucose. That margin is what pays for the size, activity and complexity of animals, anaerobes simply cannot afford them.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/aerobic-respiration-yields-about-30-atp-per-glucose-while

### 3. The oxygen released by photosynthesis comes from carbon dioxide.

**Answer:** False

**Why:** False, the oxygen comes from **water**, split in the light reactions. This was settled by isotope labelling: label the oxygen in water and it appears in the released O₂; label it in CO₂ and it appears in the sugar instead.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/the-oxygen-released-by-photosynthesis-comes-from-carbon-dioxide

### 4. Arrange these events in the order they occurred in Earth’s history.

**Answer:**

1. Anoxygenic photosynthesis evolves in early bacteria
2. Oxygenic photosynthesis begins releasing free oxygen
3. The Great Oxidation Event transforms the atmosphere
4. An ozone layer forms and shields the land surface
5. Life colonises the land

**Why:** Photosynthesis came first, oxygen accumulated, ozone formed from that oxygen, and only then could life leave the water. The colonisation of land was gated by a by-product of a metabolic pathway, a striking case of biology changing its own planet.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/arrange-these-events-in-the-order-they-occurred-in-earths-history

### 5. The Great Oxidation Event was caused by a metabolic waste product, yet it enabled the emergence of complex multicellular life. What explains this apparent paradox?

A. The toxin drove an immediate evolutionary switch towards fermentation
B. Oxygen enabled high-yield aerobic respiration and UV-shielding ozone
C. Photosynthesis consumed toxic carbon dioxide to cool down the planet
D. Anaerobic organisms adapted to utilise excess photons on dry land

**Answer:** B. Oxygen enabled high-yield aerobic respiration and UV-shielding ozone

**Why:** Treating oxygen solely as a toxin misses that its high redox potential allowed roughly 15 times more ATP production per glucose than fermentation, while atmospheric chemistry generated the protective ozone layer needed for terrestrial life.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/the-great-oxidation-event-was-caused-by-a-metabolic-waste-product

### 6. Sort each consequence by whether the Great Oxidation Event caused it directly.

**Answer:**

- Direct consequence: Mass extinction of obligate anaerobes, Formation of the ozone layer, Aerobic respiration becomes possible, Banded iron formations are deposited
- Not a consequence of oxygenation: The genetic code becomes a triplet code

**Why:** Free oxygen poisoned anaerobes, oxidised dissolved iron into the banded formations, formed ozone, and opened the aerobic route. The triplet code was established far earlier and has nothing to do with atmospheric chemistry.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/sort-each-consequence-by-whether-the-great-oxidation-event-caused-it

### 7. Match each molecule to its role in photosynthesis.

**Answer:**

- Chlorophyll → Absorbs the light energy
- Water → Is split, supplying electrons and releasing oxygen
- NADPH → Carries reducing power to the Calvin cycle
- Rubisco → Fixes carbon dioxide onto a five-carbon acceptor

**Why:** Chlorophyll captures the photon, water supplies electrons and yields oxygen, NADPH carries reducing power across to the stroma, and rubisco performs the actual fixation, a slow enzyme that plants compensate for by making enormous quantities of it.

Page: https://tryals.app/practice/biology-i/photosynthesis-and-the-oxygen-revolution/match-each-molecule-to-its-role-in-photosynthesis