AQA A Level Biology复习笔记2.2.3 Iodine Detects Starch Grains

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


Exam Tip

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).