MadSci Network: Botany

Re: How does the pH of water affect photosynthesis of elodea?

Date: Thu May 12 16:14:32 2005
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1115786456.Bt

In a classic classroom demonstration of oxygen production in photosynthesis
using Elodea, a little bit of sodium bicarbonate (baking soda) is added to the
solution to provide a ready supply of carbon for photosynthesis. The amount of
dissolved carbon dioxide in a small beaker will quickly run out when Elodea is
rapidly photosynthesizing so the sodium bicarbonate provides an abundant supply. 

The behavior of dissolved carbon dioxide is complex because dissolved carbon
dioxide reacts with water to form carbonic acid (H2CO3), which then can convert
to bicarbonate ion (HCO3-) or carbonate ion (CO3-2) as the pH changes. The graph
at the bottom of the first webpage shows how the relative amount of those three
change as the pH changes. Another complicating factor is that plants absorb
mineral nutrient cations and anions and can either raise or lower solution pH
depending on the dissolved ions present. Elodea photosynthesis or cellular
respiration also can cause a shift in solution pH.

The usual technique when working with Elodea photosynthesis is to capture the
gas evolved in an inverted, water-filled test tube. The volume of the gas can
then be measured. The amount of gas is estimated to be directly proportional to
the amount of photosynthesis. The gas will not be pure oxygen so your O2 sensor
could be used to determine the exact amount of O2 produced.

General recommendations for an experiment are as follows: 

1. Use replication and statistics, e.g. five beakers of Elodea per pH, at a
minimum determine means and standard deviations, graph results

2. Use an electric light source, such as fluorescent, that will be more constant
than sunlight. Incandescent lamps might heat up the solution too much and cause
dissolved gas to come out of solution.

3. Standardize your methods including the distance of the container with the
Elodea from the light source, the Elodea fresh weight per beaker, the water
volume per beaker, the type of water, the amount of sodium bicarbonate per
beaker and the water temperature. 

4. Calibrate your O2 sensor and pH meter with appropriate standards.

5. Gently blot the Elodea dry with paper towels before determining the fresh
weight. Express your rate of photosynthesis on a fresh weight basis. 

6. Probably the best way to adjust pH is to use a pH buffer. A pH buffer reduces
the changes in pH caused by the plant. You might try a potassium phosphate
buffer at a concentration no more than 5 millimolar. If the buffer concentration
is too high the Elodea will lose water by osmosis. If extremely high,
plasmolysis occurs. Your chemistry teacher might be able to help you prepare


The Carbonate System

Carbon Dioxide and Carbonic Acid

Re: what and how can i measure the amount of oxygen plants create?

How do I prepare a phosphate buffer solution with a specific pH?

Photosynthesis in Elodea

Re: How do plants alter the pH of a fish tank?

Re: When is a cell flaccid compared to turgid compared to plasmolysed?

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