PHOTOSYNTHESIS
What is photosynthesis
It is the process whereby green plants make their own food using water, carbon dioxide and energy
from the sun
Balanced chemical equation
for photosynthesis
Testing for starch in the leaf
Plants are living things that are able to make their own food (glucose) and store it in form of
starch
Iodine solution and starch form blue black color when they mix. The test for starch therefore is to add iodine solution
in a leaf to see if it goes to blue black, the leaf must be treated as follows
1.
Heat some water to boiling point in a beaker
2.
Use forceps to dip a leaf in
hot water for 30sec. this kills the cytoplasm
and denatures the enzymes and make the leaf more permeable to iodine solution.
3.
Remove the leaf from the leaf
from the beaker
4.
Push the leaf to the bottom of
the test tube and cover it with alcohol
(ethanol). Place the tube in hot water for alcohol to boil and dissolve out most of the chlorophyll.
This makes colour change with iodine easier to see
5.
Remove the leaf from the test tube and dip
it once more into the hot water to soften it
6.
Spread the decolorized leaf on
a white tile and drop iodine solution
onto it. The parts containing starch will turn into blue black and those parts with starch will stain brown colour of iodine
Sample question
and answer
You have been provided
with a variegated leaf and iodine
solution.
Carefully follow
the instruction given below
a)
Heat some water in a beaker to
boiling and then turn off the source of heat
b)
Use forceps to dip the leaf in hot water
for 1 minute
c)
Then push the leaf into bottom
of the test tube and cover it with ethanol.
d)
Place the tube in hot water
until the alcohol boils together with the leaf.
e)
Remove the leaf from the test
tube containing ethanol and deep it into hot water
f)
Spread the decolorized leaf on
a white tile and pour few drops of iodine solution
on it
Questions
i. What as the aim of the
experiment?
ii. Why was the leaf
dipped in hot water for 1 minute?
iii. Give the reason
why the leaf was boiled in ethanol.
iv. Why was the leaf
dipped in hot water again?
v. Give the interpretation of the result observed when few drops of iodine
solution were poured onto the decolorized
leaf
vi.
Mention the physiological process
investigated in the experiment.
vii. Draw a well label the diagram of a leaf as it appeared before the experiment and after the iodine solution
was added
Answers
i. The aim of the experiment whether chlorophyll is necessary for photosynthesis.
ii. The leaf was dipped for 1minute in hot water so as to stop the reaction
taking place in the leaf.
iii. The leaf was boiled again in methanol to dissolve the chlorophyll present
in it.
iv. The leaf was then dipped
in hot water so as to remove
alcohol and soften
it.
v. When few drops
of iodine solution were poured on the decolorized leaf, blue black colour occurred in some parts of the leaf.
This shows the presence of starch.
vi. The physiological process investigated in the experiment was photosynthesis
Diagram of a leaf before and after the experiment.
