Practical Skill: Using Iodine to Detect Starch Grains

Question 1

Sugars formed during photosynthesis are stored as starch inside starch grains
These starch grains are commonly found in:

Answer 1

the stroma of chloroplasts
storage organs such as potato tubers
the seeds of cereals and legumes

Question 2

Starch grains are large enough to be seen with a light microscope but they first require

Answer 2

staining in order to be seen easily

Question 3

What is used as a stain and how can this be added

Answer 3

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

Question 4

This makes the starch grains darker in colour, making them easier to see

iodine does this

Question 5

Remember, iodine in potassium iodide solution has a light orange-brown colour but turns …………. in the presence of starch

Answer 5

blue-black