For two thousand years, people were sure that rotting meat made maggots and stale broth made microbes — that life simply appeared out of non-living matter. You are about to test whether they were right with two sealed bottles and eight days.
Leave meat out and maggots appear. Leave broth out and it turns cloudy with microbes. The obvious conclusion, held since antiquity: living things arise on their own from non-living material.
Francesco Redi cracked the first piece in 1668. He left meat in jars — some open, some sealed, some covered with gauze. Maggots appeared only where flies could reach the meat. The maggots came from eggs, not from the meat itself.
But microbes were harder. They are everywhere and invisible. Even careful experimenters kept finding new growth in broth they thought was clean — and spontaneous generation held on for another two centuries.
Then Louis Pasteur designed the experiment your two bottles are modelling. The question he was really asking: can a cell make itself from nothing?
Pasteur's genius was control. If two bottles differ in only one thing, then any difference in the result must be caused by that one thing. Decide the role of each factor below — tap your answer.
Boiling killed everything living in both bottles of broth. They began identical and lifeless. One stays sealed. One has its lid removed and sits open to the room.
Eight days later, one of them is cloudy with microbes and the other is still clear. Before you read on, commit to an answer.
The open bottle turns cloudy. Where did its microbes come from?
Microbes from the air. The room is full of invisible bacteria and spores riding on dust. The open bottle simply gives them a landing pad and a meal; once a few arrive, they divide and the broth clouds. The sealed bottle stays clear because nothing can get in — not because it lacks air, but because it lacks arrivals.
The trap is option A. It feels intuitive — "give it air and life appears" — and it is exactly the spontaneous-generation idea Pasteur set out to kill.
IB exam tip: a question that asks you to explain the result needs the mechanism — microbes enter from the air and reproduce. A question that only asks you to state the result needs just the outcome: the open bottle becomes cloudy.
Press play. The red specks are microbes in the air. The bottle on the left is open; the bottle on the right is sealed.
Same broth, same air, same shelf. The only difference is the lid — and the lid decides whether anything ever arrives.
Drag the day, or press play, and watch the two bottles change. This is the qualitative data the practical asks you to record — clarity, surface film, sediment.
Notice the covered bottle never changes. With nothing able to land in it, there is nothing to grow.
The cloudiness in the open bottle is not new life appearing. It is existing microbes, arrived from the air, dividing again and again. Every cell in that cloud has a parent cell.
This is biogenesis: living cells arise only from pre-existing living cells. Its opposite, life from non-living matter, is abiogenesis — and that is what your sealed bottle rules out.
Pasteur made the proof airtight with a swan-neck flask. Its bent neck let air reach the broth freely, but trapped dust and microbes in the curve. Air alone produced nothing. Only when microbes could reach the broth did it cloud.
That result is one of the pillars of cell theory in IB Biology: all living things are made of cells, the cell is the smallest unit of life, and all cells arise from pre-existing cells.
IB Biology · Paper 2 short-answer style. Try each part before revealing the mark scheme.
A student boils two identical bottles of nutrient broth and seals both. Bottle 1 is kept sealed; Bottle 2 has its lid removed. After 8 days Bottle 2 is cloudy with a surface film, while Bottle 1 remains clear.
STATE = give a specific answer, no explanation required. Award 1 mark each, max 2.
Accept: "exposure to air/dust" for the IV. Do not accept: "the broth" or "8 days" as the dependent variable.
EXPLAIN = give reasons / mechanism — both what happened and why. Award 1 mark per point, max 4.
Accept: "microbes / micro-organisms / bacteria" interchangeably. Do not accept: "the air created life" or "oxygen made it cloudy".
EVALUATE = weigh strengths and weaknesses and reach a judgement. Award any 3 of the following.
Accept: a clear statement that more replicates increase reliability. Do not accept: "do it for longer than 8 days" alone as the improvement.
Fill in the blanks. Stuck? Tap Reveal answers.
The open bottle turns cloudy because microorganisms enter from the and reproduce, while the sealed bottle stays . This shows that life does not arise by spontaneous ; instead, every cell comes from a pre-existing , a principle called .
Two bottles, one difference, eight days — and a two-thousand-year-old idea falls.
Change one variable, keep everything else the same. Only then can the result be pinned on that one cause.
IB: State = name the IV. Explain = why controlling the rest makes it fair.
The cloud is microbes from the air, dividing. Living cells arise only from pre-existing living cells — never from non-living broth.
IB: State = cells come from cells. Explain = microbes enter and reproduce.
Two bottles is a start, not a proof. Repeats turn a single observation into evidence — the heart of how science decides.
NOS: controlled experiments let spontaneous generation be falsified.