Interdependence of Plants and Animals

Introduction

Plants and animals share many of the same chemicals throughout their lives. In most ecosystems, O₂, CO₂, water, food and nutrients are exchanged between plants and animals. In this lab, you will be designing your own experiments to determine the relationships between two organisms—a plant (Elodea) and an animal (a snail).

Several hypotheses have been discussed in the past about possible relationships between the Elodea and the snail. You will test to determine how oxygen and CO₂ are exchanged among Elodea plants, snails, and the water both exist in.

To perform the necessary tests, you will need to determine the presence of CO₂. An easy way to do this is to monitor the pH of the pond water. If CO₂ dissolves in water, it forms carbonic acid, H₂CO₃, and the pH decreases. If CO₂ is removed from pond water, the amount of carbonic acid goes down and the pH increases. One can use a computer to monitor the pH and determine whether CO₂ is released into the pond water or is taken from the water. Dissolved oxygen (DO) can be monitored with the aid of a Dissolved Oxygen Probe. Increases or decreases in the amount of dissolved oxygen can be rapidly assessed with this probe.

Objectives

In this experiment, you will

• Use a Dissolved Oxygen Probe to measure the dissolved oxygen in water.
• Use a pH Sensor to measure the pH of water.
• Use pH measurements to make inferences about the amount of CO₂ dissolved in water.
• Determine whether snails consume or produce oxygen and CO₂ in water.
• Determine whether plants consume or produce oxygen and CO₂ in the light.
• Determine whether plants consume or produce oxygen and CO₂ in the dark.