Factors affecting photosynthesis - experiments

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  • Factors affecting rate of photosynthesis
    • Temperature:
      • Keep all other factors constant. Use a light intensity that produces a high rate of photosynthesis. Alter temperature of water bath.
      • Measure volume of gas produced in a known period of time at each temperature. This is not completely accurate as warmer water will reduce the solubility of oxygen gas.
    • Carbon dioxide conc:
      • Keep all other factors constant. Vary number of drops of sodium hydrogencarbonate solution. Measure vol of gas produced in a known period of time at each concentration of carbon dioxide.
    • Investigating rate using changes in density of leaf discs:
      • 1. Use a straw to cut several leaf discs from cress cotyledons.
      • 2. Place 5 or 6 leaf discs in a 10cm3 syringe and half-fill the syringe with dilute sodium hydrogencarbonate solution.
      • 3. Hold the syringe upright, place your finger over the end of the syringe and gently pull on the plunger to pull the air out of the air spaces of spongy mesophyll in the leaf discs.
        • Air is replaced by sodium hydrogencarbonate solution. As the density of the discs increases, they sink to the bottom of the syringe.
      • 4. Once all of the discs have sunk, transfer the contents of the syringe to a small beaker. Illuminate from above using a bright light.
        • Time how long it takes for one leaf disc to float to the top of the solution. Reciprocal of the time taken (1/t) is a measure of the rate of photosynthesis.
    • 5. Repeat the procedure twice more at this light intensity and find the mean rate of photosynthesis.
    • 6. Repeat at different light intensities.
    • 7. Put results in a table.
    • Indicator solution can be used to measure uptake of CO2 to measure rate.
      • Some indicators are sensitive to small pH changes.
      • Hydrogencarbonate indicator is red when neutral, yellow at pH6 and purple/red at pH's slightly above 7.


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