Topic 3 - Photosynthesis Experiments

Question 1

how do you test a leaf for starch?

Answer 1

1. dunk the leaf in boiling water - this stops any chemical reactions inside the leaf
2. put the leaf in a boiling tube with some ethanol and heat in a water bath until it boils - this gets rid of any chlorophyll
3. rinse the leaf in cold water and add iodine solution - if starch is present the leaf will turn blue-black

Question 2

what does the starch test show?

Answer 2

whether photosynthesis has taken place - if photosynthesis has happened starch is present

Question 3

how would you show chlorophyll is needed for photosynthesis?

Answer 3

1. use variegated leaves - only the green parts contain chlorophyll
2. test the leaf for starch
3. only the parts that were green turn blue-black - suggesting that only the parts containing chlorophyll can photosynthesus

Question 4

how would you show carbon dioxide is needed for photosynthesis?

Answer 4

1. set up a plant in a sealed bar jar with soda lime
2. the soda lime absorbs carbon dioxide
3. leave the plant
4. test it for starch - it will not turn blue-black, showing that photosynthesis hasn’t happened

Question 5

what variables have to be controlled in photosynthesis experiments?

Answer 5

temperature, carbon dioxide and light

Question 6

how would you show that light is needed for photosynthesis?

Answer 6

1. put a plant in a place without light for 48 hours - so it has used up its starch stores
2. cut a leaf from the plant and test for starch - the leaf won’t turn blue-black, showing light is needed for photosynthesis

Question 7

how is Canadian pondweed used to measure rate of photosynthesis?

Answer 7

the rate at which the pondweed producses oxygen corresponds to the rate at which its photosynthesising - the faster the rate of oxygen production, the faster the rate of photosynthesis

Question 8

how would you use Canadian pondweed to measure the effects of light intensity on photosynthesis?

Answer 8

1. set up a test tube with Canadian pondweed and water. attach a capillary tube with a gas syringe on the end
2. place a source of light a set distance from the pondweed
3. leave the pondweed to photosynthesis for a set amount of time
4. at the end of this amount of time, use the syringe to draw the oxygen through the capillary tube. measure the amount of oxygen produced
5. repeat this experiment with the light at different distances