Investigating the rate of photosynthesis

Aim :

To investigate the effect of different light intensities on the rate of photosynthesis

Independent variable :

• Distance of light source to pondweed

• Light intensity (experienced by pondweed)

Dependant variable :

• no. Of oxygen bubbles produces

  OR

• The volume of oxygen produced

• Time

Control variable :

• temperature (keep in a water bath)

• Pondweed

• Carbon dioxide levels ( if done in a large container and in a timely manner )

• Solution it is submerged in

• Duration of the period you time for

Hypothesis :

The rate at which the pondweed produces oxygen must be directly proportional to the rate at which it photosynthesises

Method :

• Set out the practical as follows

• Locate a testube and fill it with water so that the pondweed may be fully submerged in it

• Submerge pondweed

• Clamp the test tube to a clamp stand

• Close the testube with a bung from which place a capillary tube that is connected at the end to a syringe

• Place a meter ruler with the zero end in line with the pondweed and then place the light source at the first interval for measurement

• Turn the light source on

• light source is placed at a specific distance form the pondweed

• Allow the pondweed to adjust for a fixed amount of time (before experiment)

• Leave it to photosynthesis for a fixed period of time

• During this period of time count the no. Of oxygen bubbles produced

• Record within your table

• Repeat previous steps for each redetermined increment of distance (e.g for 80,60,40,20 cm)

• Take repeats and average of the data so as to reduce random errors

OR

• Set out the practical as follows

• Locate a testube and fill it with water so that the pondweed may be fully submerged in it

• Submerge pondweed

• Clamp the test tube to a clamp stand

• Close the testube with a bung from which place a capillary tube that is connected at the end to a syringe

• Place a meter ruler with the zero end in line with the pondweed and then place the light source at the first interval for measurement

• Turn the light source on

• light source is placed at a specific distance form the pondweed

• Allow the pondweed to adjust for a fixed amount of time (before experiment)

• Leave it to photosynthesis for a fixed period of time

• the oxygen produced will be collected in the capillary tube

• At the end of the fixed time use the syringe to draw the gas bubble in the tube up alongside the ruler

• Measure and record this length as the volume of oxygen produced

• Take repeats and average of the data so as to reduce random errors

Hazard :

• ensure no cuts are exposed and wash hand afterwards as there is a slight by risk of infection

• Dispose of the pondweed carefully as it is not native to the UK

Calculating the rate of photosynthesis

Rate of photosynthesis = the volume of oxygen produced per unit time

(Total length of bubble) / (time taken to produce it)