1
The Effect of Different Variables
on the Rate of Photosynthesis
Introduction
Visible light (remember ROYGBIV?) powers photosynthesis. Visible light has medium-sized wavelengths, which have enough energy to cause chemical change, but not enough energy to destroy biological molecules. Infrared radiation, microwaves, and radio waves have longer wavelengths, and do not have enough energy to power photosynthesis.
When visible light is absorbed by photosynthetic pigments like chlorophyll a and b, electrons within each Photosystem are boosted to a higher energy level. This energy is used to produce ATP and NADPH. These energy-storing molecules help incorporate the carbon in CO2 into organic molecules (such as glucose) in a process called carbon fixation.
The amounts of CO2 as well as intensity of light will impact the rates of photosynthesis in your plants. The CO2 is used in the dark reactions of the photosynthetic process to make glucose and the light helps to excite electrons in each photosystem of the light reactions.
Purpose
In this lab, we will investigate the effect of wavelength of light, CO2availability, or the intensity of light on the rate of photosynthesis. We will estimate the rate of photosynthesis by observing oxygen production in leaves exposed to these conditions.
Overview of experimental procedure
First, you will cut small disks out of the leaves and put them inside an empty syringe. Next, you will add dish detergent and a 2% sodium bicarbonate (NaHCO3) solution to the syringe, create a vacuum inside the syringe, and force the NaHCO3 solution into the air spaces of the leaf disks (by doing this, you will also be forcing oxygen gas out of the air spaces in the leaf disks). Once NaHCO3 has entered and oxygen gas has exited the leaf disks, they will be denser than the NaHCO3 solution, and will sink to the bottom of the solution. Next, you will transfer the leaf disks to a cup filled with NaHCO3 solution and covered by a light filter. After placing the cup under a lamp, you will observe the time it takes for the leaf disks to rise to the top of the NaHCO3solution.
Cut disks out of leaves Put leaf disks in an empty syringe
STEPS OF BASIC PROCEDURE IN PICTURES!
- Add dish detergent and 2% NaHCO3 solution to the syringe and create a vacuum inside the syringe. This will force NaHCO3 into the air spaces in the leaf disks and force O2 out of the air spaces in the leaf disks.
- The leaf disks will become more dense 3. Transfer leaf disks to a cup containing
than the solution and sink to the bottom. NaHCO3 solution and covered by a light filter (filter not shown). Place under a lamp.
- Observe how long it takes for all of the leaf disks to float.
The biology behind the lab
Leaves tend to float in aqueous (water-based) solutions because leaves contain air spaces, which make the leaves less dense than the solution. In this lab, you will be forcing NaHCO3 solution into and O2 out of the air spaces in leaf disks. This will cause the disks to become more dense than the solution they are in, and the disks will sink to the bottom of the solution.
In order for photosynthesis to occur, water, CO2, and light need to be present. In this experiment, the NaHCO3 solution serves as the source of CO2 and water, and the lamp serves as the light source.
The rate of photosynthesis can be determined by measuring the amount of CO2 used or the amount of O2 produced over time. In this experiment, we will indirectly measure the amount of O2 produced over time as the leaf disks photosynthesize, oxygen gas produced during photosynthesis will be released into the air spaces of the disks. As the disks fill up with oxygen, they will become less dense than the NaHCO3 solution and rise to the surface. The time it takes for the leaf disks to float will be inversely related to the rate of photosynthesis.
Pre-labassignment #1
1.Write the net photosynthesis equation.
2.In this experiment, what is the source of each of the reactants in the net photosynthesis equation?
3.How will you know that photosynthesis is occurring in the leaf disks?
- Predict the relationship between the time for the disks to float and the rate of photosynthesis.
5.How will you estimate the rate of photosynthesis?
EVERYONE WILL DO THE SAME CONTROL.
USE THE QUESTIONS BELOW TO DESIGN YOUR EXPIEREMENT
6.Describe how you would test either the effect of wavelength of light, CO2 availability, light intensity, or temperature on the rate of photosynthesis (using the experimental protocol described above). Discuss how many times you would repeat the procedure, and what you would change each time.
Based on your answer to question #6
Research question:
Hypothesis:
Materials Available to You
labeling tape1 beaker (500 mL)
scissorsRed, green, and blue markers and transparency sheets
transparent tapealuminum foil
clear plastic cups (266 mL)NaHCO3
electronic balanceplastic cups for measuring
spatula100 mL graduated cylinder
tap waterdish detergent
1 plastic pipet (1 mL)stirring rod
plastic straw or hole puncherLeaves
1 plastic syringe (10 mL)adjustable lamp with 150 watt bulb
timermetric ruler
Procedure for Control
1.Label the 500 mL beaker:0.2M NaHCO3 solution
Label 3 of the clear plastic cups:Heat absorber
Perform steps 2-13 for each cup:
2. Add 200 mL of pre-prepared 0.2M NaHCO3solution to your 500mL beaker.
- Add 150 mL of the solution to a 4th plastic cup and surround it with the three plastic cups filled halfway each with water. These are your “heat absorbers” to make sure the temperature of your cup does not get too hot.
PREPARING THE DISKS
4.Add 1 drop of dish detergent to the NaHCO3 solution and stir to dissolve (the dish detergent will help the NaHCO3 enter the leaf.) If suds develop, dilute the solution by making more 0.2M NaHCO3 solution and adding it to the beaker.
5.Remove a leaf from the plant and punch out 10 uniform leaf disks with a hole puncher or straw. Avoid the major leaf veins.
6.Remove the plunger of the syringe and place the leaf disks inside the barrel. Replace the plunger, being careful not to crush the leaf disks. Depress the plunger until only the leaf disks and a small volume of air remains in the barrel.
7. Draw 7 mL of NaHCO3 solution into the barrel, and tap the barrel to suspend the leaf disks in the solution.
8.Hold the syringe upright (open end up), and gently depress the plunger to remove any air left in the barrel.
9.Hold a finger firmly over the opening of the syringe, pull back on the plunger, and wait for about 10 seconds. the NaHCO3has entered the leaf disks, the disks will sink to the bottom of the barrel. You may have to create the vacuum several times before all the leaf disks sink. Shake the syringe if the leaf disks stick to the barrel or each other.
10.Pour the contents of the barrel into one of the plastic cups covered with a light filter. Add 150 mL of NaHCO3 solution. All of the leaf disks should settle at the bottom of the cup.
11.Place the cup under a lamp with a 150 watt bulb. Adjust the lamp so that the bulb is 10 cm away from the bottom of the cup.
12.Turn on the lamp, start the timer, and record the number of disks that are floating at minute 0
(the beginning of data collection).
13.When 1 minute has elapsed, record the number of disks that are floating and then swirl the cup to dislodge any disks that are stuck to the side of the cup or each other. Use a clean pipet to dislodge the disks if necessary. Do not record data past 3 minutes.
14.Repeat steps 2-13 for the remaining cups.
DATA
Table 1.1 Number of disks floating at 20 second time increments
TIME / Control / IV 1 / IV 2 / IV 30
0:20
0:40
1:00
1:20
1:40
2:00
2:20
2:40
3:00
Graph 1.1
1.What are the independent and dependent variables in your experiment?
2.Which leaf disks belong to the experimental group and which a leaf disks belong to the control group?
3.Identify as many controlled variables as possible.
4. What is the conclusion of your testing? How did your variable impact the rate of photosynthesis? Did it speed up or slow down the light or dark reactions? How would you know?
5.Are there any additional variables that need to be controlled or monitored? If so, list them below, and then modify your materials list and procedure so that you can control/monitor the variables.Be sure to tell me what additional materials you will need!