All of the air had to be removed from the leaf discs before placing them in the beaker. Suggest one reason why.
How to approach this question
The experiment measures the time taken for the discs to rise. What must be their starting position for this to be possible? What is trapped in the air spaces of a leaf that makes it float?
Full Answer
Leaves naturally have air spaces within their spongy mesophyll layer, which allows for gas exchange. The air trapped in these spaces makes the leaf discs buoyant, so they would normally float on the surface of the solution. The aim of the experiment is to measure the time it takes for them to rise, which is only possible if they start at the bottom. By removing the air (often done by placing the discs in a syringe with the solution and pulling the plunger), the discs become denser than the solution and will sink. They will only rise again once they have produced enough oxygen from photosynthesis to make them buoyant.
Common mistakes
✗ Saying "to remove oxygen" - while air contains oxygen, the key is removing the trapped gas to alter the disc's density/buoyancy.
✗ Saying "to make the experiment fair" - this is too vague. You need to explain the practical reason.
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