5.6.10 PRACTICAL: Investigating Factors Affecting the Rate of Photosynthesis Flashcards
What plant can be used for this investigation?
aquatic plants, such as Elodea or Cabomba (types of pondweed)
How can the effect of limiting factors on the rate of photosynthesis be investigated?
- Light intensity – change the distance (d) of a light source from the plant (light intensity is proportional to 1/d2)
- Carbon dioxide concentration – add different quantities of sodium hydrogencarbonate (NaHCO3) to the water surrounding the plant, this dissolves to produce CO2
- Temperature (of the solution surrounding the plant) – place the boiling tube containing the submerged plant in water baths of different temperatures
How do you maintain control variables?
- Whilst changing one of these factors during the investigation (as described below), ensure the other two remain constant
- For example, when investigating the effect of light intensity on the rate of photosynthesis, a glass tank should be placed in between the lamp and the boiling tube containing the pondweed to absorb heat from the lamp – this prevents the solution surrounding the plant from changing temperature
What is the apparatus needed?
Distilled water
Test tube
Beaker
Lamp
Aquatic plant, algae or algal beads
Ruler
Sodium hydrogen carbonate solution
Thermometer
Test tube plug
Syringe
Describe the method.
Ensure the water is well aerated before use by bubbling air through it
- This will ensure oxygen gas given off by the plant during the investigation form bubbles and do not dissolve in the water
Ensure the plant has been well illuminated before use
- This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes of rate are due to the independent variable
Set up the apparatus in a darkened room
- Ensure the pondweed is submerged in sodium hydrogen carbonate solution (1%) – this ensures the pondweed has a controlled supply of carbon dioxide (a reactant in photosynthesis)
- Cut the stem of the pondweed cleanly just before placing into the boiling tube
- Measure the volume of gas collected in the gas-syringe in a set period of time (eg. 5 minutes)
- Change the independent variable (ie. change the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating) and repeat step 5
- Record the results in a table and plot a graph of volume of oxygen produced per minute against the distance from the lamp (if investigating light intensity), carbon dioxide concentration, or temperature
What are the results for light intensity?
- The closer the lamp, the higher the light intensity (intensity ∝ 1/d2)
- Therefore, the volume of oxygen produced should increase as the light intensity is increased
At a point, the volume of oxygen produced will stop changing even if the light is moved closer
- This is when the light stops being the limiting factor and the temperature or concentration of carbon dioxide is limiting the rate of photosynthesis
- The effect of these variables could then be measured by increasing the temperature of water (by using a water bath) or increasing the concentration of sodium hydrogen carbonate respectively
The results should be displayed on a graph of light intensity vs. rate of photosynthesis
- Rate of photosynthesis = volume of oxygen produced ÷ time elapsed
What are the limitations?
Algae is often used in experiments on photosynthesis and respiration rates but it can be very hard to maintain consistency in the number of algae and it can be hard to handle directly in the water
- Immobilised algae beads are beads of jelly with a known surface area and volume that contain algae, therefore it is easier to ensure a standard quantity
- Immobilised algae beads are easy and cheap to grow, they are also easy to keep alive for several weeks and can be reused in different experiments
- The method is the same for algae beads though it is important to ensure sufficient light coverage for all beads
