Estimating Rates of Photosynthesis
INTRODUCTION
Photosynthesis is the process by which organisms that contain the pigment chlorophyll convert
light energy into chemical energy which can be stored in the molecular bonds of organic
molecules (e.g., sugars). Photosynthesis powers almost all trophic chains and food webs on the
Earth. The process involves the use of light energy to convert carbon dioxide and water into
sugar and oxygen. In eukaryotes, photosynthesis happens in organelles called chloroplasts. All
photosynthesis uses as the primary light trapping pigment chlorophyll a. Different
photosynthetic organisms like plants and algae use a variety of secondary pigments to increase
the wavelengths of light they can absorb. The net process of photosynthesis is described by the
following equation:
6 CO2
+
6 H2O
+
light energy
C6H12O6
+
6 O2
This equation simply shows that carbon dioxide and water combine in the presence of sunlight to
form sugars; oxygen is released as a by-product of this reaction.
Cellular Respiration is the general process by which organisms oxidize organic molecules (e.g.,
sugars) and derive energy (ATP) from the molecular bonds that are broken. It is the process of
converting the chemical energy of organic molecules into an energy form immediately usable by
organisms. Glucose may be oxidized completely if sufficient oxygen is available. All organisms,
including plants and animals, oxidize glucose for energy. In eukaryotic organisms, aerobic
respiration occurs in organelles called mitochondria. Respiration is the opposite of
photosynthesis, and is described by the equation:
C6H12O6
+
6 O2
6 CO2 +
6 H2 O +
36 ATP
Simply stated, this equation means that oxygen combines with sugars to break molecular bonds,
releasing the energy (in the form of ATP) contained in those bonds. In addition to the energy
released, the products of the reaction are carbon dioxide and water.
In eukaryotic cells, cellular respiration begins with the product of glycolysis (pyruvic acid) being
transported into the mitochondria. A series of metabolic pathways (the Krebs cycle and others) in
the mitochondria result in the further breaking of chemical bonds and the liberation of ATP. CO2
and H2O are end products of these reactions. The theoretical maximum yield of cellular
respiration is 36 ATP per molecule of glucose metabolized.
Photosynthesis
6 CO2 + 6 H2O + light energy
Light
Energy
C6H12O6 + 6 O2
NADPH
Light
Reactions
Calvin
Cycle
H+ and
ATP
Sugar
(C6H12O6)
containing
stored energy
CHLOROPLAST
H2O
O2
CO2
Electron
Transport
Chain
ATP Energy
NADH
Pyruvic
Acid
Krebs
Cycle
H+
Glycolysis
MITOCHONDRION
Aerobic Respiration
C6H12O6 + 6 O2
6 CO2 + 6 H2O + ATP
FIGURE 12-1
In order to measure the rate of photosynthesis or respiration you can measure the production rate
of any product or the consumption rate of any reactant. In this lab, we are going to focus on
oxygen. As oxygen is liberated as a waste product of photosynthesis, we can measure the amount
of oxygen produced and use that value to estimate the rate at which photosynthesis is taking
place. Similarly, since oxygen is required for aerobic respiration, measuring the amount of
oxygen consumed by an organism that is respiring aerobically will give us a value representing
the rate of respiration.
MATERIALS
test tubes with stands
2% potassium bicarbonate solution
Elodea shoots
plastic tubing
ruler
forceps
meter stick
LED lamps
PROCEDURE
The Color of Light and the Rate of Photosynthesis
Because white light consists of a spectrum of colors and because chlorophyll is green, different
colors of light (different wavelengths) affect the rate of photosynthesis. Plants should
photosynthesize at higher rates when exposed to light of wavelengths that chlorophyll absorbs
well and at lower rates when exposed to wavelengths that are not as well absorbed by
chlorophyll. We will test this idea with four different setups. In each setup, you will have a test
tube filled with 70 ml of 2% potassium bicarbonate (NaHCO3) solution which will provide an
adequate concentration of CO2 for photosynthesis. You will use only one Elodea shoot to control
for the differences that might exist between shoots.
The LED lamps that we are using emit very little heat. If we used incandescent bulbs, the heat
from the lamp would cause the gas to expand and make it look like more gas was being produced
than actually is produced by photosynthesis. There is a ruler attached to a pipette with a
submerged tip so that you can measure the movement of the water over time. The water will be
‘pushed’ through the pipette by the pressure of the oxygen gas produced by the Elodea. The
summary equation for photosynthesis shows a molecule of oxygen is produced for every
molecule of carbon dioxide converted into glucose. If oxygen gas simply replaced carbon
dioxide gas molecule for molecule, you would expect no change in the total amount of gas in the
system. But in our system, the carbon dioxide is coming out of the liquid solution. Its removal
from the system does not affect the partial pressure of the gas in the tube – only the oxygen being
liberated which is in the form of gas. The amount of oxygen liberated is a good way to measure
the rate of photosynthesis. For each setup you will measure the amount of oxygen liberated at 3
minute intervals for a total of 15 minutes. After turning on the light, you should wait 5
minutes before taking your first reading so that the system equilibrates.
You will have four different colored LED lamps. You will test the photosynthetic rate for each of
the four different colors of light at 15 cm distance between the light source and the Elodea. You
should pose specific hypotheses with your group as to the expected outcome in each case. Your
prediction should be based on your knowledge of the absorption spectra for chlorophyll a and
chlorophyll b (the two principle photosynthetic pigments in Elodea) from the spectrophotometry
lab.
Light Color
Prediction for Relative Photosynthetic Rate by color
WHITE
BLUE
RED
GREEN
Light Color (mm of O2)
TIME (min)
WHITE
BLUE
RED
GREEN
0
0
0
0
0
3
6
9
12
15
Graph the “Amount of Oxygen Liberated” (Rate of Photosynthesis) versus “Time” for all four
light colors on one graph.
DISCUSSION QUESTIONS
1. Why is measuring the liberation of oxygen a good way to measure photosynthetic rate? Why
not just measure the amount of sugar produced?
2.
List five factors that might influence the rate of oxygen production or consumption in
Elodea. Explain how you think each will affect the rate?