Iodine Detects Starch Grains (AQA A Level Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
Practical Skill: Using Iodine to Detect Starch Grains
Starch is the storage polysaccharide of plants
Sugars formed during photosynthesis are stored as starch inside starch grains
These starch grains are commonly found in:
the stroma of chloroplasts
storage organs such as potato tubers
the seeds of cereals and legumes
Starch grains are large enough to be seen with a light microscope but they first require staining in order to be seen easily
Iodine (I2) in potassium iodide (KI) solution can be used to stain starch grains
This can be done by adding a drop of potassium iodide onto the specimen (mounted on a slide) before placing a coverslip over the top
This makes the starch grains darker in colour, making them easier to see
Remember, iodine in potassium iodide solution has a light orange-brown colour but turns blue-black in the presence of starch
Examiner Tips and Tricks
Remember - only a very small amount of stain is required (usually just a drop or two will be enough)! Also, when adding a cover slip, remember to slowly lower the cover slip onto the specimen at an angle to avoid any air bubbles getting trapped (these could create ‘artefacts’ and obstruct your view of the specimen).
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