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Unlock This CoursePlants make their own food through photosynthesis, converting light energy into glucose. This glucose is often stored as starch in leaves. By testing for starch, we can prove that photosynthesis has taken place. This practical is one of the most important experiments in biology because it shows us direct evidence of how plants make food.
Key Definitions:
Starch is the main storage product of photosynthesis in leaves. When plants photosynthesise during the day, they convert glucose into starch for storage. By testing for starch, we can prove that photosynthesis has happened and see which parts of the leaf were photosynthesising most actively.
Testing leaves for starch involves several careful steps. Each step has a specific purpose and must be done in the right order to get accurate results.
The starch test follows a standard method that removes chlorophyll so we can see the iodine colour change clearly. Here's exactly how to do it safely and effectively.
1. Destarch the plant by keeping it in darkness for 24-48 hours
2. Place the plant in bright light for several hours
3. Select a healthy leaf for testing
1. Boil water in a beaker
2. Dip the leaf in boiling water for 30 seconds
3. Place leaf in hot ethanol to remove chlorophyll
4. Rinse leaf in cold water
1. Spread leaf flat on a white tile
2. Add iodine solution drop by drop
3. Observe colour changes
4. Record results
Always wear safety goggles when using hot ethanol. Ethanol is flammable, so keep it away from flames. Use tongs to handle hot leaves and never heat ethanol directly - use a water bath instead. If iodine gets on skin, wash immediately with cold water.
The colour changes you see tell you exactly where starch is present in the leaf. This gives us a map of where photosynthesis was happening most actively.
When iodine solution meets starch, it creates a distinctive blue-black colour. Areas without starch stay orange-brown (the normal colour of iodine). This creates a clear pattern showing starch distribution.
These areas contain starch, proving photosynthesis occurred here. You'll typically see this in green parts of leaves that had access to light. The darker the blue-black colour, the more starch is present.
These areas have no starch, meaning little or no photosynthesis happened here. This occurs in white/yellow parts of variegated leaves or areas that were covered and couldn't access light.
Scientists use the starch test in controlled experiments to prove that specific factors are needed for photosynthesis. By changing one factor at a time, we can see its effect on starch production.
Each experiment tests whether a particular factor is essential for photosynthesis by removing it and seeing if starch is still produced.
Cover part of a leaf with aluminium foil for several hours, then test for starch. The covered area should show no starch, proving light is needed.
Test variegated leaves with white and green areas. Only green areas should show starch, proving chlorophyll is needed.
Keep a plant in a sealed container with soda lime (absorbs CO₂) for several hours, then test for starch. No starch should be found.
Geranium plants with variegated leaves are perfect for starch testing. Their leaves have green areas (with chlorophyll) and white/cream areas (without chlorophyll). When tested with iodine, only the green areas turn blue-black, clearly showing that chlorophyll is essential for photosynthesis and starch production. This creates a natural experiment where the plant acts as its own control.
Sometimes the starch test doesn't work as expected. Understanding common problems helps you get better results and understand what might have gone wrong.
If your results aren't clear, there are several possible causes and solutions to try.
If no blue-black colour appears anywhere, the plant might not have been in light long enough, or the iodine solution might be too old. Try leaving the plant in bright light for longer and use fresh iodine solution.
If the leaf is still green, the chlorophyll wasn't removed properly. Heat the ethanol longer or use fresh ethanol. Make sure the leaf is completely white/yellow before adding iodine.
Understanding starch production in plants has important applications in agriculture, food production and environmental science.
The principles behind starch testing help us understand crop production, plant health and environmental impacts on food security.
Farmers use knowledge of photosynthesis to maximise crop yields. They ensure plants get enough light, water and carbon dioxide. Understanding starch production helps predict harvest times and crop quality.
Rising CO₂ levels affect plant photosynthesis and starch production. Scientists study these changes to understand how climate change might affect global food production and plant ecosystems.
A single large tree can produce enough oxygen for two people for one day through photosynthesis. The starch test we do in the lab is happening naturally in billions of leaves around the world every single day, producing the oxygen we breathe and the food that supports all life on Earth.