photosynthesis practical

Question 1

how can you test leaves for starch?

Answer 1

by using iodine solution on a leaf that has had its chlorophyll removed by boiling in ethanol

Question 2

what are the 5 steps for the iodine starch experiment?

Answer 2

1) take a leaf
2) heat in boiling water for 1 minute
3) turn off the bunsen burner- transfer the leaf to a test tube containing ethanol and place it in the hot water
4) soften the leaf by transferring it to the beaker of hot water
5) place the leaf on a white tile and cover in iodine solution

Question 3

how do you destarch a leaf?

Answer 3

placing it in the dark for 24 hours

Question 4

why do the leaves need to be destarched?

Answer 4

so they are free from starch at the start of the experiment- this means that any starch in the leaf you test was produced from the glucose made in photosynthesis during your experiment

Question 5

what does the experiment show?

Answer 5

the conditions needed for photosynthesis

Question 6

how does this experiment show that light is required?

Answer 6

• part of the leaf is covered with foil- only the parts of the leaf exposed to light will produce starch and turn black

Question 7

how does this experiment show that chlorophyll is needed?

Answer 7

a variegated leaf- the white parts of the leaf lacks chlorophyll is used and starch is only produced in the areas that were green

Question 8

how does this experiment show that carbon dioxide is required?

Answer 8

• one leaf is enclosed in a conical flask (or plastic bag) containing soda lime to absorb the CO2
• this leaf is compared t another enclosed in a flask but without the soda lime
• only the leaf that had access to CO2 produces starch

Question 9

the pondweed experiment can vary what 2 factors and how?

Answer 9

• light intensity- distance of a bench lamp
• CO2 concentration- masses of sodium bicarbonate

Question 10

how to conduct the pondweed experiment?

Answer 10

a length of pondweed is put in a beaker of water and the number of bubbles released from the stalk in one minute is counted

Question 11

what improvements can be made to the pondweed experiment?

Answer 11

• putting the pondweed into a boiling tube of water- then placed in a beaker with a thermometer- acts as a water bath- reduces temperature change during the experiment
• collecting the gas in an inverted measuring cylinder and measure the volume of oxygen formed per unit time
• adding a fixed mass of sodium bicarbonate to the water- provides a higher concentration of carbon dioxide