An investigation was carried out to determine how limiting factors can change the rate of photosynthesis.
Aim: To determine whether factors such as light intensity and concentration of sodium hydrogen carbonate (NaHCO3) will affect the rate of photosynthesis in the water plant, elodea.
Hypothesis: The higher the light intensity the faster the rate of photosynthesis as this provides more energy available for plants photosynthesis to take place. In addition, the higher the concentration of NaHCO3, the faster the rate of photosynthesis will become. This is due to the NaHCO3 providing more CO2 for photosynthesis as it is increasing the limiting factor.
Materials:
- Elodea
- 500ml beakers
- Lamp
- 1m ruler
- Glass funnel
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[0.5M] & [1M] of Sodium Hydrogen Carbonate (NaHCO3)
- Test tube
- Stop watch
Apparatus:
Method:
Light intensity:
- Elodea was cut up into a 10cm length piece and bottom of stem was cut off allowing water to flow through the xylem (use the same piece, allowing more accurate results)
- ¾ of a 500mL beaker was filled with tap water
- A glass funnel was placed in the beaker, along with the elodea (as shown in the apparatus above)
- A test tube was filled ¾ with water
- The test tube was then quickly flipped over and was placed on top of the funnel (a shown in the apparatus above)
- Used a marker to mark where the water level of the test tube started
- 10 seconds was allowed for the elodea to begin the process of photosynthesis
- The beaker containing the elodea was then moved to a certain distance
- After the lamp was switched on and was left turned on for 5 minutes
- After 5 minutes, the new water level was recorded
- This process was repeated at various distances
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This process was repeated again, however as a replacement for changing distance away from lamp, concentration of NaHCO3 were changed (excluding the lamp)
Results:
Light intensity
Concentration of Sodium Hydrogen Carbonate (NaHCO3)
Discussion:
The aquatic plant, Elodea, was used in this investigation because it gives off oxygen in bubbles as it carries on the process of photosynthesis.
As shown in the diagram, the rate of photosynthesis can be determined by the oxygen present in the test tube. The more oxygen present, the faster the rate of photosynthesis.
Light intensity:
“The rate of photosynthesis generally increases with an increase in light intensity until some other factor becomes limiting.”- (Green Book). This statement was confirmed by the results that were established. The results from the Longmans co-ordinated biology book (refer to appendix), also support this statement. The further the elodea was away from the lamp, the lower the light intensity became, and hence the rate of photosynthesis was slower. It was also observed that when the elodea was closer to the lamp, more bubbles were formed and were rising to the top of the test tube.
As light is a limiting factor of photosynthesis, the closer the elodea is to the lamp, the more light is available there is, causing the rate of photosynthesis to become greater when the elodea is closer to the lamp. The further the elodea is away from the lamp, the less light is absorbs as the light from the lamp does not travel in a straight line and spreads out, causing a strain to the limiting factor of light, hence a decrease in the rate of photosynthesis.
Light intensity will also affect how fast the light dependent stage of photosynthesis will form its products. The higher the light intensity, the more energy will become available for the electrons to be “excited”, therefore photophosphorylation will take place at a higher rate. An increase in the rate of photophosphorylation will mean more ATP will be transported to the light independent stage to be used. The light independent stage will also occur at a higher rate increasing the rate of photosynthesis in the elodea. “Due to the high rate of reaction of the photophosphorylation, causing more oxygen will be produced.” – (3)
A low light intensity can reduce the amount of oxygen released because the electrons will be less excited and photophosphorylation will take place at a lower rate, hence less oxygen will be produced. Less ATP and reduce NADP will be available for the light independent stage to use and this in turn will reduce the rate at which the dark stage produces its products. This can be seen in the results, as the light intensity increase, the reaction rate increases as more bubbles (O2) were produced in a more rapid rate.
Concentration of NaHCO3:
Carbon dioxide concentration will directly affect the rate of photosynthesis as it is used in the photosynthesis reaction. Varying the concentration of NaHCO3 will change the rate of photosynthesis as it determines the availability of CO2 present.
As determined by the results, the higher the concentration of NaHCO3, the faster the rate of photosynthesis. This is due to the availability of CO2 provided by the NaHCO3. There is a significant difference between the controlled concentration and [0.5M] of NaHCO3, however as the concentration of NaHCO3 increases, the smaller the difference becomes, hence the graph starts to plateau between [0.5M] and [1M]. This is also be seen by results of the secondary data – (refer to appendix). This is due to other limiting factors being used up, causing a stress to the balance of limiting factors, this is called photosynthetic saturation.
Limitations:
The most significant limitation was the elodea itself: It was planned that the same sized elodea would be used for all the trials at a given distance. This proved unreliable because firstly using different elodea from different parent plants for each different temperature because the changes in the different elodea could not be pointed out even though all of them were left to acclimatize under similar conditions for the same length of time. Factors like different amount of leaves per length or the health of the plant could have affected the number amount of oxygen produced.
Also the oxygen released from the shoot was being measured not the amount released from the site of photosynthesis i.e. the leaves. This was inaccurate as firstly the shoot may have been cut at an angle which allowed air bubbles to clog ip the end. This would have caused any produced oxygen not to be let out of the plant, therefore not all the oxygen being produced as a result of photolysis of water in the light stage were measure.
The elodea is a respiring plant and it uses the oxygen produced from photosynthesis fro respiration. This could have resulted in some of the oxygen not been released as it was being used up. Therefore an assumption was made that all the oxygen being produced is being released, but this wasn’t the case and theoretically it would have lead to inaccuracy in measurements.
Conclusion:
Plants use green pigments called chlorophylls to trap light energy. The chlorophylls give a plant its green color. Inside the cells that have chloroplasts, the light energy is used to make a simple sugar called glucose. The process by which plants use light energy to make glucose is called photosynthesis.
During this process of sugar production, carbon dioxide combines with water to form glucose and oxygen is released. Oxygen that is produced in photosynthesis is given off as a gas. If a lot of oxygen is being given off, photosynthesis is occurring rapidly. If little oxygen is being given off, photosynthesis is occurring slowly. The amount of trapped light energy and the amount of carbon dioxide available affects the rate of photosynthesis.
The hypothesis was confirmed with the results of this investigation. The results were recorded accurately, however some errors were made, causing results to be slightly inaccurate.
Factors affecting accuracy of results:
- Some of the oxygen give off is used for respiration by the plant.
- Some of the oxygen dissolved into the water.
- Some was used by small invertebrates that were found living within the pieces of elodea.
- The higher light intensities should be quite accurate but the smaller light intensities would be less accurate because the light spreads out. The elodea will also get background light from other light sources.
- The lights are also a source of heat which will affect the experiments with only a small distance between the light and the elodea. This heating could affect the results.
- Using the same piece of elodea for each experiment causes photosynthesis rate decreased over time. However using a different piece of elodea for each experiment would create the problem as each piece of elodea would not have the same surface area.
Improvements:
- It could be repeated more times to help get rid of any anomalies. A better overall result would be obtained by repeating the experiment more times because any errors in one experiment should be compensated for by the other experiments.
- All the experiments should be done sequentially.
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(1)
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- Mansfield state high school, year 12 Biology, Term II: Cell Biology (Green Book)
- Biozone year 12, (pgs 45-49)
Appendicies:
Variables:
Secondary data source:
Results from a Longmans Co-ordinated biology book
Glossary:
Photophosphorylation: The production of ATP using the energy of sunlight
Notes from today’s lesson
Diffusion/osmosis
- permeability of a cell membrane
- movement of particles across membrane
- diffusion: movement of high concentrated particles to low concentrated particles across membrane
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osmosis: movement of H2O particles
- The cell is in a hypertonic solution, water will move out of the cell. The cell shrinks PLAYMOLYSIS → CRENATION (in blood)
- The cell is in a hypotonic solution, water moves into the cell. The cell swells
CYTOLYSIS → HEMOLYSIS
- The cell is in an ISOTONIC solution. NO net movement of water molecule occurs DYNAMIC EQUILIBRIUM → No change
Passive activity – O2, CO2, H2O, Amino Acids – enters the membrane
Active – Protein, Carbohydrates, Ions
Exocytosis, vesicles, endocytosis, pinocytosis – cell “drinking”, phagocytosis
Active transport:
- Requires ATP
- Occurs when substances transported:
- against concentration gradient
- are large molecules
- Includes exo & endocytosis and membrane pumps
In metabolism, NAD+ is involved in redox reactions, carrying electrons from one reaction to another.
