Rate of Photosynthesis

By Will Stark on Feb 07, 2017

This experiment provides a low cost solution for investigating the rate of photosynthesis for students who are blind or visually impairedI. It could also be used with sighted students. 
 
A common experiment when studying plant biology is investigating factors affecting the rate of photosynthesis. Plants produce oxygen which is invisible and odorless, so the experiment is usually carried out with water plants; the bubbles of oxygen produced are observed. The experiment is usually carried out with a common pond weed, and sighted students are instructed to count tiny bubbles: the faster the rate of bubble production, the faster the rate of photosynthesis. 
 
This presents a problem for students with VI since the bubbles are often too small to see and therefore counting them and gathering valid results is not possible. A typical arrangement is shown in the photo on the top right, with a lamp and a beeker of water with pondweed in it.

Preparation:

A pipette with a narrow end is required. Cut the end, and seal using a flame. This creates a tube sealed at one end. Then cut the other end of the pipette to create a large opening which can be placed over an upturned funnel,
a plastic pipette        a pipette with the end melted closed        a pipette with the large end cut in the water with pond weed in a beaker

Materials

As well as the pipette, students will also need a large beaker (1000ml) a transparent glass funnel, pond weed, sodium hydrogencarbonate and a desk lamp.
  • Pipette
  • Large beaker (1000ml)
  • Transparent glass funnel
  • Pond weed
  • Sodium hydrogencarobate
  • Desk lamp

Procedure

This experiment can be adapted so that results can be gathered in 20 minutes or less. 
  1. Add sodium hydrogencarbonate to the water which introduces carbon dioxide to the water to speed up the rate, and fill the large beaker with this water. Use an aquatic plant such as elodea, but plants such as hottonia palustris also work well. Cabomba may be used in some countries, but it is restricted in the UK, due to it being an invasive species and a risk to local plant populations if it is released into the environment. 
  2. Place the pond weed under the funnel. Lower the funnel to rest on the lower surface of the large beaker, placing small pieces of putty to hold the funnel up to allow water to circulate.
  3. Fill the pipette with water and hydrogen carbonate in the large beaker and invert, so it rests on the funnel. It needs to be be full of water.
  4. Direct the light from the desklamp onto the pond weed. As the plant produces oxygen, the bubbles with move up the funnel into the pipette and displace the water. The amount of water displacement allows students to investigate factors affecting the rate of photosynthesis.
the beaker with water and pond weed, with the pipette showing the water level after 20 minutes   The two layers of glass provides protection from the heat of the lamp, and the water level moves down rapidly as the gas is collected, making it possible to gather results in well under an hour.   Collage of photosynthesis

NGSS Standards:

HS.Matter and Energy in Organisms and Ecosystems

HS-LS1-5.    Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.

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