Evidence for What Photosynthesis Requires

Photosynthesis produces glucose.
Plants use some glucose immediately.
They can convert some of it into starch for storage.

If starch appears in part of a leaf after an investigation, that can give Maya evidence that photosynthesis happened there.

But there is a problem.
A plant may already contain starch made before the investigation began.
Maya needs to remove this old starch before she can use newly produced starch as evidence.

Before the investigation begins, Maya places the plant in darkness for about 24–48 hours.

Without light, the plant cannot continue producing glucose by photosynthesis.
Its cells still need energy for respiration, so the plant uses its stored carbohydrate.

After this period, very little stored starch should remain in its leaves.

Removing stored starch before an investigation is called destarching.

Now, if Maya later finds starch in a leaf, she has much better evidence that it was made during the investigation.

Destarching therefore happens before Maya creates the conditions she wants to compare.

Maya begins with a destarched plant.

She chooses one leaf.
She covers part of the leaf on both sides with opaque material, such as aluminium foil.
No light can reach the covered region.

The rest of the same leaf remains uncovered.

Maya then places the plant in bright light for about four hours.

One part of the leaf receives light.
The covered part does not.

Everything else about the two regions is as similar as possible because they are parts of the same leaf.

Now Maya needs to find out where starch has been produced.

Maya removes the leaf from the plant.

First, she places it in boiling water for a short time.
This kills the leaf and stops its chemical reactions.

Next, she needs to remove the green chlorophyll.
The green colour would make the result of the iodine test difficult to see.

Maya places the leaf in ethanol.
Ethanol removes the chlorophyll.

Ethanol is flammable, so it must not be heated directly over a flame.
Instead, the ethanol is heated using a hot-water bath.

Once the chlorophyll has been removed, the leaf becomes pale.
Maya rinses it in water to soften it.

She spreads the leaf out and adds iodine solution.

Iodine tests for starch.
Where starch is present, iodine changes from orange-brown to blue-black.

Maya compares the two parts of the leaf.

The area that received light turns blue-black.
It contains starch.

The covered area does not turn blue-black.
It does not contain starch.

The important difference between the two regions was access to light.

The evidence therefore shows that light is required for photosynthesis.

Maya now investigates carbon dioxide.

Again, she begins with two similar destarched plants.

She places one plant inside a transparent sealed container with normal air. This is the control.

She places the second plant inside another transparent sealed container. Inside this container is soda lime, which absorbs carbon dioxide from the air. Soda lime should be handled as directed because it can irritate or damage skin and eyes.

Both containers receive the same light and are kept at the same temperature. The important difference is carbon dioxide availability.

After about four hours, Maya takes one leaf from each plant and uses the same starch test as before.

The leaf from the plant in normal air turns blue-black. It contains starch.

The leaf from the container with very little carbon dioxide does not turn blue-black. It has not produced detectable starch.

The evidence therefore shows that carbon dioxide is required for photosynthesis.

Maya investigates chlorophyll using a variegated plant.

A variegated leaf has differently coloured regions. Green areas contain chlorophyll. Pale or white areas contain little or no chlorophyll.

Maya destarches the plant, then places it in bright light for about four hours. Both the green and non-green regions receive the same light and have access to carbon dioxide.

Maya then takes a variegated leaf and uses the same starch test as before.

The regions that were originally green turn blue-black.

The pale regions do not.

Both regions received light and carbon dioxide. The important difference was the presence of chlorophyll.

The evidence therefore shows that chlorophyll is required for photosynthesis.

Maya has used two different kinds of evidence across these photosynthesis pages.

In the rate investigations, she measured oxygen produced in a fixed amount of time.

That evidence answers:

How quickly is photosynthesis happening?

On this page, Maya looked for starch after controlled comparisons.

That evidence answers:

Did photosynthesis happen under this condition?

The starch evidence shows that photosynthesis requires light, carbon dioxide and chlorophyll.

Different scientific questions need different evidence.

These exercises are for remembering the GCSE knowledge from Maya's investigations.

Use the lesson while you learn each task. Complete slide 1 before moving to slide 2 to check the completed answer. Correct anything you missed, then try the task again later from memory.

Slide 1 is the exercise.

Reconstruct the starch-test sequence and explain the purpose of each important step.

Your finished sequence should include:
destarch the plant before the investigation;
place the leaf in boiling water;
use ethanol in a hot-water bath to remove chlorophyll;
rinse the leaf in water;
add iodine solution;
recognise blue-black as a positive result for starch.

Also explain why ethanol is heated in a water bath rather than directly over a flame.

Move to slide 2 only after you have completed the task.

Slide 1 is the exercise.

Use the three comparisons to recall the result and conclusion.

Light:
compare the illuminated region with the region excluded from light.

Carbon dioxide:
compare the plant in normal air with the plant kept with very little carbon dioxide.

Chlorophyll:
compare the green region of a variegated leaf with the pale region.

For each comparison, record:
which part turns blue-black;
which part does not;
what the evidence shows photosynthesis requires.

Move to slide 2 only after you have completed all three comparisons.

Slide 1 is the exercise.

A scientist compares two similar destarched plants.

Plant A is kept in normal air.
Plant B is kept in the same light and temperature, but carbon dioxide is removed from the air around it.

After the same amount of time, both leaves are tested for starch.

Plant A turns blue-black.
Plant B does not.

Answer three questions:
1. Which plant is the control?
2. What is the important difference between the two conditions?
3. Why do the starch results support the conclusion that carbon dioxide is required for photosynthesis?

Move to slide 2 only after you have written your answers.