Generation of Action Potentials in Nitella Giant Algal Cells: A shocking Demonstration of Membrane Depolarization

Carol Bernon

Overview

In this simple demonstration, an action potential generated by a 9-volt battery can be shown to halt, temporarily, cytoplasmic streaming in Nitella cells. The underlying principles involved can be applied to the conduction of a wave of depolarization across other membranes, such as the axons of neurons.

Biological Concepts

•

• Impulses

•

• Membrane permeability

• Action potential (AP)

• Depolarization, repolarization

• Resting potential

• Cytoplasmic streaming (cyclosis)

Actin/myosin interaction

Class Time

If cells are screened for cytoplasmic streaming before the demonstration is set up, class time may take as little as 15 minutes.

Background Information

A variety of plants and algae use voltage changes to initiate an action potential. Nitella have giant cells that exhibit cytoplasmic streaming, a process which facilitates the movement of materials in the cytoplasm. The mechanism that drives this process is similar to the system involved in muscle contraction, and is controlled by the interaction of actin and myosin filaments. When an action potential is generated (depolarization), there is a rapid change in membrane permeability to calcium ions. The increase in cytoplasmic calcium initiates a series of reactions beginning with the activation of a protein kinase. The protein kinase catalyses the phosphorylation of myosin which in turn inhibits the myosin-actin interaction. Cyclosis then ceases temporarily until calcium is removed from the cytoplasm and repolarization (the resting potential) of the membrane is again established.

Studies with the related giant alga Chara have shown that other stimuli, including mechanical, chemical and temperature changes, may elicit the same interference with cytoplasmic streaming. It is believed that this response may be an adaptive advantage that limits the loss of cytoplasm when the cells are under stress.

Materials

• Microscope attached to video camera and monitor (If a video system is not available, a stereomicroscope may be used at separate lab stations)

• Nitella cells (available from major scientific supply companies)

• Silver wire (if available) for electrodes

• 9-volt battery with two small alligator clip leads; optional switch and rheostat

• tape

• petri dish or other container for cells and water

Teacher Preparation

Attach the alligator clips of two small leads to thin silver wire electrodes and tape them parallel to each other so that the electrodes are about one centimeter apart, or just wide enough to be placed along each side of a Nitella cell. The giant cells may be five centimeters long and one millimeter wide. The smaller nodal cells may also be used. Attach the other end of the leads to a 9-volt battery and complete your circuit with a switch and/or rheostat if available. Place Nitella cells in a petri dish on the stage of the microscope and locate cells in which cytoplasmic streaming occurs.

Students may make predictions on the effects of sending a small electric current through the cells. Then, while students are watching the monitor, place the electrodes in the water on each side of the cell and turn on the switch. Cytoplasmic streaming will immediately stop. Wait for recovery and repeat with the same cell or others.

Extensions/Variations

• Relate this process to the depolarization of a nerve cell.

• Study the relationship between recovery time and frequency of depolarizations.

• Relate the process of cyclosis to the function of actin and myosin in muscle contraction.

• Try this demonstration using other stimuli, such as chemicals, temperature change, or mechanical change (pinch the cell).

• Discuss this reaction in terms of its evolutionary significance.

• Examine the way other plants, such as Venus fly trap or sensitive mimosa, use voltage changes to initiate action potentials.

Resources

Drewes, Charles. Iowa State University. Personal communication.

Wayne, Randy. 1993. Excitability in Plant Cells. American Scientist. 81:140-151

About the Author

Carol Bernon is a biology teacher at Barnstable High School, 744 West Main Street, Hyannis MA. 02601. Use the e-mail address gbernon@capecod.net.

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