Forrest Zhao
05/25/07
Mr. Happer
Investigating Photosynthesis
Introduction
Relevance:
Photosynthesis is an extremely important mechanism and my experiment is a step
in understanding the exact mechanism of it. It has been shown that chlorophyll like
substances are found in cancerous tumors. Scientists try to kill the cancerous cells by
using light, similar to the way photosynthetic chlorophyll absorbs sunlight and damages
the plant tissue. Also, increased knowledge in the mechanics of photosynthesis could lead
to a better understanding of how to build more efficient solar panels.
Goals:
I have two main goals in this lab. The first one is to see which color light allows
photosynthesis to function properly or even function at all. My other goal is to see if the
photosynthesis can function properly in a partially covered (with black paper) leaf.
General Expectation and Models:
I expect that the leaf covered with green cellophane would have the least amount of
starch as green light is mostly reflected off the leaf because chlorophyll cannot absorb it.
The leaf covered with red paper would contain almost as much starch as a normal leaf as
chlorophyll absorbs red light very well. The one covered with yellow cellophane would
be as poor as the green cellophane covered leaf as yellow light is also mostly reflected by
the chlorophyll. As for the leaf half covered with black paper, I would expect that
photosynthesis will still take place, though not as efficiently as it normally would.
Variables:
Controlled: type of plant
size of leaf (taken for sample)
size of pot
# times watered per week
amount of water give each time
type of alcohol and iodine solution used
Measured: The amount of starch present in each leaf
Materials
scissors
black construction paper
potted plant
tape
red, green and yellow cellophane
6 large test tubes
glass-marking pencil
forceps
beaker
5 Petri dishes
iodine solution
paper towels
Procedure
1. Cut the colored cellophane into leaf shapes. Do this for each type of color, one for
each leaf. Then cut out the black construction paper into two rectangles.
2. Use tape to secure each cellophane piece to the leaf. Half cover another leaf with
black paper and tape it down. Then place the plant in the sunlight and water it.
Wait until next class.
3. Cut off the cellophane and black paper covered leaves.
4. Take 5 test tubes, fill them halfway with alcohol, put the leaves in and label the
test tubes.
5. Fill the beaker ¾ full of water and heat it on a hotplate. Place the test tubes on the
hotplate.
6. Heat the test tubes until the alcohol turns a bright green. This means that most of
the chlorophyll has been extracted.
7. Take out the leaf with the forceps and gently rinse it in cold water.
8. Put the leaf on a Petri dish and cover it with iodine solution. Wait for 2 or 3
minutes.
9. Take the leaf out and rinse it in water again. Place on paper towel (evidence of
starch are the darkest spots on leaf).
10. Repeat for every other leaf and record the results.
Observations
Qualitative:
Normal Leaf:
Dark patches in the center of the leaf
The rest of the leaf is a lighter shade of brown.
Red Cellophane covered leaf:
The whole leaf is light brown
Leaf half covered with black paper:
Almost the whole leaf was a darker shade of brown
Yellow cellophane covered leaf:
Mostly dark brown
Patches of light brown can be seen as well
Green cellophane covered leaf:
Top half is lighter than the bottom
Some whiter spots on the leaf.
Discussion
The results were all very different from what I had expected. According to
Absorption of Light by Chlorophyll, chlorophyll absorbs the blue-violet and red part of
the light spectrum extremely well. The green part is reflected and the yellow part is
barely absorbed. According to the results of my experiment, the red cellophane covered
leaf had barely any starch but the yellow cellophane covered leaf had a large amount of
starch. The expectation that was proven correct was the fact that a leaf would continue
photosynthesis even if part of it was completely blocked from light.
I have learned many new aspects of both biology and how to conduct experiments
in general from this experiment. First of all, I learned that plants can’t use just any type of
light to utilize photosynthesis; the light has to contain wavelengths of sufficient length
(sufficient energy). I also learned that a half covered leaf still make starch, just not as
well as a normal leaf. By conducting this experiment, I have also learn that the controlled
variables have to be strictly controlled in order for the experiment to bring satisfactory
results. I start my experiment on a Friday and watered it, but left over the weekend
without watering it. The following Monday, the whole plant withered. That is why my
results were backwards. I also learned that to you have to repeat the experiment several
times to find enough data.
My experiment’s purpose was to generally find out which type of light
chlorophyll favors and if photosynthesis works with a half covered leaf. After finding the
answer, you could ask exactly why chlorophyll absorbs blue, violet and red light and
reflects green light. This question explores the physical property of light. You could also
ask why photosynthesis still works with a half covered leaf. How does it make enough
energy to survive?
A good follow up to my experiment would be to use two plants, cover one plant
with one color cellophane and leave the other plant untouched. This way, we can see the
effects of photosynthesis on the whole plant and not just one leaf, therefore, collecting
more data and having more to compare. It would also be interesting to measure the area
of the starch in each leaf by placing the leaf under a microscope. This could give us a
better understanding of just how much more starch a leaf in favorable conditions can
produce as compared to a non favorable one. And the most important thing is to repeat
the experiment many times to decrease the chance of getting “false” data due to human
errors and/or freak accidents.
Appendix I
Photosynthesis: (photo=light, synthesis=putting together)
A process in which plants turn solar energy into chemical energy.
We are depriving the plant of sunlight to prevent photosynthesis from occurring.
Plant cell: (cell: small enclosed cavity or space)
The smallest structural unit in a plant that is capable of independent functioning.
We are finding out how many plant cells die per square in the leaves of the plant.
Chloroplast: (chloro: green, plast: small structure/body)
Organelles that capture energy from sunlight and convert it into chemical energy in a
process called photosynthesis.
By depriving the plant of sunlight, we are disabling the chloroplast’s function.
Chlorophyll: (chloro:green, phyll: leaf)
The plant cell’s principle light absorbing molecule. Essential for photosynthesis.
Chlorophyll is an important molecule in the process of photosynthesis.
Forceps: (came from ‘formiceps’)
An instrument, as pincers or tongs, for seizing and holding objects, such as in a surgical
operation.
In our experiment, forceps are used for grabbing delicate materials such as the leaf.
Autotroph: (autos=self, trophe=nutrition)
Organisms which make their own food, such as plants.
All plants are autotrophs, as they obtain their food from photosynthesis.
Pigment:
A colored material that absorbs certain colors and transmits or reflects other colors.
Chlorophyll is a light absorbing pigment in the chloroplast and can be removed from the
leaf by heating it in alcohol.
Thylakoid: (Greek word: thulakos)
Saclike photosynthetic membranes that is the structural unit of the grana in chloroplasts.
Thylakoids are inside the chloroplasts of plant cells, they are normally in stacks call
granum.
Photosystems: (phos=light, systema=assembly)
Clusters in the chloroplast that take in chlorophyll and other pigments.
Photosystems, pigments, and chlorophyll are all light-absorbing molecules.
Stroma:
The dense colorless framework of a chloroplast.
The stroma is also inside the chloroplast, it is the space outside the Thylakoid membranes.
White Light:
Light that is composed of all the different wavelengths (entire color spectrum).
Sunlight is an example of white light and in this experiment, we are breaking down white
light into different colors to see which colors are necessary for photosynthesis.
Color:
Each color has its own unique wavelength. Colors of shorter wavelengths have higher
energy.
Because different colors have different energies, it is reasonable to assume that different
colors of light changes the output of photosynthesis.
Iodine: (Iode=rust colored)
A nonmetallic element belonging to the halogens; used especially in medicine and
photography and in dyes.
In our experiment, we are using iodine to detect starch in the plant which shows that
photosynthesis has happened.
Alcohol: (Al-kuhl=distillate)
Any of a series of hydroxyl compounds, the simplest of which are derived from saturated
hydrocarbons, have the general formula CnH2n+1OH, and include ethanol and methanol.
We are boiling alcohol and water with the leaf to remove the chlorophyll in order to
detect starch in the leaf.
ATP: (adenesine=a white, crystalline, water-soluble nucleoside, C10H13N5O4, of adenine
and ribose. Tri=three Phosphate=a salt of phosphoric acid)
A chemical that stores and releases energy in a cell.
The ATP gives the cell short but powerful bursts of energy.
Electron:
A negatively charged particle located outside the atomic nucleus.
The electrons are responsible for taking in the energy released during the light based
reactions.
Light dependent reactions:
Reactions in photosynthesis that require light.
The energy released from the light based reactions are later combined with carbon
dioxide to form starch.
NADP: (nicotinamide=a colorless, crystalline, water-soluble solid, C6H6N2O, the amide
of nicotinic acid dinucleotide= 2 building blocks of DNA/RNA)
One of the carrier molecules that transfers high-energy electrons from chlorophyll to
other molecules.
The NADP is instrumental in the process of photosynthesis as it carries part of the puzzle
needed for the Calvin cycle (creates starch).
ATP synthase: (synthase=any of various enzymes that catalyze the synthesis of a
substance without involving the breaking of a high-energy phosphate bond )
A large protein that uses energy from hydrogen ions to bind ADP and a phosphate group
to form ATP.
Appendix II
The Photosynthesis Equation
6CO2 + 6H2O + light = C6H12O6 + 6O2
My experiment is based on this essential procedure all plants go through. This diagram
gives us the big idea of photosynthesis: how plants use sunlight, carbon dioxide and
water to make high energy sugars and oxygen.
Light and Pigments
One of the main ideas that I am trying to found out is how different colored light effects
photosynthesis. This idea is shown in the diagram above. It shows that the chlorophyll in
the plant absorbs the blue-violet and red visible light very well and reflects green light. In
my experiment, I am trying to find out if photosynthesis can happen with only one color
light and if different colors of light have different effects on photosynthesis.
The Chloroplast
This diagram shows the inside of a chloroplast. This is where photosynthesis takes place.
The chloroplast is important in my experiment because both the light based reactions and
the Calvin Cycle occur here and the starch is made here.
Primary Colors
http://www.steve.gb.com/images/science/additive_colours.png
White light is composed of the whole color spectrum. The sunlight is composed
of white light and when that light is absorbed by the plant, the green light is reflected
back out and all the other colors are absorbed in the plant. We know this because the
plant leaves are green.
http://www.colourware.co.uk/cpfaq/pic1.gif
Sunlight is composed of UV, infrared and visible light. The energies of these
lights are the greatest at the left side of diagram and weakest at the right side. When
different color cellophane placed on the leaf, only that color light gets in so only that
amount of energy gets into the leaf (green light is an exception because it is always
reflected back out because of the green chlorophyll.
References
http://www.steve.gb.com/images/science/additive_colours.png
http://www.colourware.co.uk/cpfaq/pic1.gif
Miller KR, Levine JS. Biology. Upper Saddle River, New Jersey: Prentice Hall; 2006:
206
Miller KR, Levine JS. Biology. Upper Saddle River, New Jersey: Prentice Hall; 2006:
207
Miller KR, Levine JS. Biology. Upper Saddle River, New Jersey: Prentice Hall; 2006:
208