IB Biology Higher Level
Lab Report
Title:
Investigate the effects of sodium bicarbonate and light intensity on the rate of
photosynthesis of green spinach leaves through the observation of floating leaf discs
Student Name: Jane Jody Johnson
Date: October
Title: Investigate the effects of sodium bicarbonate and light intensity on the rate of photosynthesis of
green spinach leaves through the observation of floating leaf disks
Background information:
When light is absorbed by leaf pigments such as chlorophyll a or b, electrons within each photosystem
are boosted to a higher energy level. The leaves then produce ATP, to reduce NADP to NADPH, and
incorporate CO2 into organic molecules in a process called carbon fixation. When leaves are put in the
water and undergo the process of the light-dependent reaction, oxygen is produced and released into
the interior of the leaf which causes them to rise. However, cellular respiration is also occurring at the
same time, consuming the oxygen produced in photosynthesis. The leaves must have a carbon source
provided to them to allow photosynthesis to carry on. The amount of light the plant receives
coupled with the amount of available CO2 will influence the rate at which photosynthesis occurs. The
aim of this experiment is to measure the effect of light intensity and availability of carbon and their
effects of the rate of photosynthesis.
Hypothesis
If the leaf is exposed to higher intensity of light in the presence of a carbon source, then it takes
less time for the leaf disks to float to the surface of the solution due to an increased rate of O2
production through photosynthesis.
Control variables
Temperature
Volume of infiltration solution
Species of spinach
Variables
Independent Variables
Intensity of light
Presence of CO2
Size of leaf discs
1
Dependent Variables
Time it takes for disks to float
Controlling variables:
1. Control variables:
a) Temperature- the experiment is set up in the same laboratory, so the temperature is kept
approximately constant the whole time.
b) Volume of infiltration solution- the solution is sucked out by the syringe, and it reads to 3 cc.
c) The species of spinach is Spinacia oleracea.
d) Leaf discs we cut into uniform circles using a standard hole punch.
e) Existence of bicarbonate in the solution- 6.3g of bicarbonate is measured with a 6.3±0.02g
balance
2. Independent variable:
a) Intensity of light- the same light is used in every trial, with the distance from the sample
measured from a metric ruler to 20±0.05cm and 10±0.05cm
b)
3. Dependent variable:
a) The time it takes for the disks to float to the top is kept by a standard stopwatch, and
recorded every minute with an uncertainty of ±0.1s.
Materials
 6.3 g Sodium Bicarbonate (baking soda)
 Dawn brand original scent liquid soap
 1 Plastic Syringe (10 cc or larger)
 Spinach leaves(Spinacia oleracea)
 4 clear plastic cups
 1 Stopwatch (iPod)
 1 Regular 15W microscope light
 1 Hole punch
 2 beakers (30mL)
 1mL or 5mL plastic disposable pipette
 Metric ruler
 Stirring rod
 Iron stand
VI. Procedures
1. Label 4 cups with the following:
- 20 cm CO2 light
- 10 cm CO2 light
- 10 cm Water/ soap light
- CO2 dark
2. Label the 2 beakers: 1 and 2
3. Weigh 6.3g of baking soda and mix it in 300mL of water in beaker number 1, stir with the stirring
rod.
4. Mix a squirt of soap with 200mL of water in beaker number 2, also stir with the stirring rod.
2
5. Using the plastic pipette, add one drop of the diluted soap solution in beaker 1 into beaker
number 2. Avoid suds, and if your solution generates suds, dilute it with more bicarbonate.
6. Hole punch 10 uniform leaf disks for each of the 4 trials,
pay attention to avoid major veins in the leaf.
7. Remove the plunger of the syringe and place your leaf disk in the
syringe barrel.
8. Replace the plunger into the syringe and slowly push on the plunger
to remove as much air as you can from the barrel without crushing
the leaf disks.
9. Dip the opening of the syringe into the bicarbonate solution in
beaker number 1, and pull back the plunger in order to obtain 3cc of the bicarbonate solution in
the syringe. Tap the syringe to suspend the leaf disks in the solution.
10. Carefully push the plunger to leave as little air as possible inside the syringe. Hold a finger over the
syringe opening, draw back on the plunger to create a vacuum. Hold this for 10 seconds.
11. While holding the vacuum, swirl the leaf disks to suspend them in the solution, then let off the
vacuum.
12. If you need to, repeat the vacuum steps 2-3 times more, until all the disks in the syringe sink in the
solution.
13. If the disks still do not sink, add more soap to the solution and repeat steps 7-12
14. Pour the disks and the solution into the correct cup
15. Add the bicarbonate solution from beaker number 1 until the cup is 3/4 full
16. Place cup with leaf disks under a 15W microscope light above on an iron stand
17. For trial 1 “20 cm CO2 light”, as mentioned in step one, place the light 20cm directly above the
bottom of the iron stand; for trial 2 and 3, place the light 10cm above the bottom of the iron stand
18. As soon as the leaf disks in the cup are exposed to the light, begin timing
19. Record the number of disks that are floating at the end of every minute
20. Gently swirl the disks with a stirring rod to prevent them from sticking on the side of the cup and
repeat the step until all the disks are floating or until you have reached 30 minutes
21. Repeat the water/soap under the light (10cm) except replace the bicarbonate solution with the
diluted soap in the plunger
22. For the dark, follow all steps and keep the cup under the light of 10cm above for 14 minutes,
counting the disks every minute. At 14 minutes, shut off the light and place the cup in the dark
23. Every minute, count how many disks are still floating until all the disks have sunk or you have
reached 30 minutes
24. Gently swirl the disks with the pipette to be certain all disks have been properly displaced
3
Results
Raw data collection
The number of disks floating over time under different light intensity and different
solutions
Time (min)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Trial 1
20 cm CO₂ Light
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
2
2
2
2
2
2
2
2
2
2
2
2
1
Number of floating disk
Trial 2
Trial 3
10 cm CO₂ Light
Water/soap 10 cm
1
0
1
0
1
0
2
0
2
0
2
0
1
0
2
0
2
0
2
0
2
0
1
0
1
0
2
0
3
0
4
0
4
0
5
0
6
0
7
0
7
0
9
0
9
0
10
0
10
0
10
0
10
1
10
1
10
1
10
1
4
Trial 4
10cm / CO2 Dark
1
0
0
0
0
0
0
0
0
0
1
2
3
4
5
5
5
4
4
3
2
2
2
2
1
1
0
0
0
0