Investigating effect of light on photosynthesis

  • Created by: Steff06
  • Created on: 05-05-16 17:43
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  • Investigating effect of light on photosynthesis
    • Measuring rate of photosynthesis:
      • Measure volume of oxygen produced per unit time.
      • Measure rate of uptake of carbon dioxide.
      • Rate of increase in dry made of plants.
    • Limitations of measuring rate by measuring volume of oxygen:
      • Some oxygen produced will be used up in respiration by the plant.
      • May be some dissolved nitrogen in the gas collected.
    • Photosynthometer/Audus microburette used.
    • Apparatus must be airtight and there are no air bubbles in the capillary tubing.
    • Gas collects in the flared end of the capillary tubing. The gas bubble can be moved into the part of the capillary tube against the scale and its length can be measured.
    • The length can be converted into the volume if we know the radius of the capillary bone.
    • Volume of gas collected = length of bubble x Pir2
    • If the same apparatus is used:
      • Diameter and radius is constant so we can compare the rate of photosynthesis using the length of the gas bubble each time.
      • The water bath keeps the temperature constant.
      • Sodium hydrogencarbonate solution added to the water in the tube and provides carbon dioxide.
      • Investigation must be carried out in a dark room so that the only light available to the plant is from the light source.
      • A low energy bulb is used as these do not release much heat.
    • 1. To fill the apparatus with tap water, remove the plunger from the syringe and allow a gentle stream of tap water into the barrel of the syringe until the whole barrel and plastic tube are full of water.
      • Replace the syringe plunger and gently push water out of the flared end of the capillary tubing until the plunger is nearly at the end of the syringe and there are no air bubbles in the water in the capillary tubing.
    • 2. Cut a piece of well-illuminated Elodea (Canadian pondweed) about 7cm long and make sure that bubbles of gas are emerging from the cut stem. Place this, cut end upwards into a test tube containing same water the pondweed was kept in.
      • Add 2 drops of hydrogencarbonate solution to the water of the test tube and stand the test tube in a beaker of water at 20 degrees. Use a thermometer to measure temp of the beaker at intervals during the investigation, adding cold water is needed.
    • 3. Place a light source as close to the beaker as possible and measure the distance (d) from the piece of pondweed to the light source and note this. Light intensity (I) follows the equation: I = 1/d2
      • Leave the apparatus with the capillary tube positioned so that it is not collecting gas given off by the plant for 5-10 minutes so that the plant adjusts to these conditions.
    • 4. Position the capillary tube over the cut end of the plant and after a known period of time (5-10 minutes) gently pull the syringe plunger so the bubble of gas collected is in the capillary tube near the scale. Read and note length of the bubble and gently push in the plunger so that the bubble is expelled.
    • 5. Reposition the capillary tube to collect more gas from the plant and repeat step 4 twice more.
    • 6. Move the light source further from the plant. Measure the distance and calculate the light intensity or use a light meter to measure light intensity. Allow a 5-10 minute acclimisation period and then repeat steps 4 and 5.
    • 7. Continue the investigation with different light intensities. Put the data in a table and plot a graph of rate of photosynthesis calculated by volume of oxygen per minute produced against light intensity (1/d2).


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