Photosynthesis Lab (Day 2)- April 23
Background Plants are capable of growing in many different environments. Dandelions
grow in cracks in a sidewalk, pine trees grow high on mountainsides, water
lilies grow from shallow lake bottoms. Though plants flourish in very different
environments, most plants have one thing in common -- they need light to
survive! Unlike certain animal species, no species of green plant can survive
for long in total darkness. To make sugar, plants capture light energy in the
process of photosynthesis. In this investigation, you will observe how
important light energy is to the process of photosynthesis.
Photosynthesis is the process by which plants use the energy in sunlight to
synthesize material from air and water for growth. Photosynthesis is the
building up of sugar molecules using carbon dioxide and water as the raw
materials. The energy for the process comes from light and a green pigment
called chlorophyll which allows the plant to transfer the energy from light to
sugar. The chemical equation for this process is shown below.
6CO2 + 6H2O C6H12O6 + 6O2
Light Energy
This equation simply means that carbon dioxide from the air and water
combine in the presence of sunlight to form sugar and oxygen is released as
a by-product of this reaction.
Photosynthesis Lab: What to Measure?
What to Measure?
Now, how would you measure the rate of photosynthesis?
Again, let's look at the equation for photosynthesis:
CO2 + H2O + light --> O2 + glucose
There are 2 things we can quickly measure in this experiment (the dependent variables): amount
of oxygen produced, or, amount of carbon dioxide used.
If the rate of photosynthesis increases, the rate of oxygen production goes up, and the rate of
CO2 consumption rises too! If the rate of photosynthesis goes down, then we can expect the
opposite effect: oxygen production drops and carbon dioxide is not used as quickly.
But how can we measure oxygen or carbon dioxide levels. It's quite easily done with Elodea!
Measuring Oxygen
In water, oxygen that is produced by the Cobamba plant is released as bubbles from its
leaves. The rate of oxygen produced can be measured by either counting the number of bubbles
released in a certain amount of time (bubbles/min), or by trapping the oxygen gas in an inverted
syringe or tube and measuring the volume of oxygen produced in a certain amount of time
(cm3/min).
It's important that you give the plant a few minutes to photosynthesize before starting your
measurements. This ensures that the plant is making oxygen at a constant rate. (You should also
check this visually, before starting to count the bubbles).
Instructions for the lab:
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Get a sprig of Cobamba plant. Mrs. Kaur will give you the stems.
Place the plant under the funnel as shown and filled with water. Add a pinch
of baking soda powder to 200ml of water. Then fill the test tube with water and
place on top of the funnel as shown in figure 1 make sure there is no air in the
test tube.
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Place a light source 15 cm from the plant. After one minute, count and record
the number of oxygen bubbles rising from the cut end of the stem. Count
bubbles seen for five minutes. Let the set up sit for 5 min before starting to
count the bubbles from 10cm, the same for 15cm.
Move the lamp so it is 30 cm from the plant. After one minute count and
record bubbles for five-minutes. Repeat the same procedure, but this time
move the light source 45 cm away and count the bubbles seen. Record the
results.
Light a thin piece of wood with a match, and blow out the flame. Remove the
test tube, invert it, and insert the glowing wooden splint into the test tube.
Note any evidence of it re-lighting. What does this indicate?
Distance of light source
15 cm
30 cm
45 cm
Number of bubbles
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Using graph paper display the data in the table by plotting a bar or line graph
showing the rate of oxygen production along the vertical (Y) axis verses the
distance of the light source along the horizontal (X) axis.
Questions to answer after this activity is finished:
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What is Photosynthesis?
During process of sugar production in this project carbon dioxide combines
with water to form glucose sugar and what gas is released?
Would this investigation have worked if a light source was not present?
Does distance (intensity) of light from the planet increase the rate oxygen
production?