Lesson 3

How does C. elegans keep from drying up in high salt?

Driving Questions:

What happens to wild type and mutant worms after 24 hours on low and high salt?

How can glycerol protect worms in a high-salt environment?

By the end of this lesson, we hope that you will be able to:

Make accurate 24-hour observations of the worms in different environments, and compare their activity to your 15-minute observations from Lesson 2.
Construct an explanation for how glycerol can protect worms in a high-salt environment

Glycerol: keeping the worms alive

Why would salt be considered a weapon to a slug?

What happens when salt is poured on a slug? (not recommended)

What happens when a fresh water fish is put into a salt water aquarium? (also not recommended)

The fish or slug will “dry up” or shrivel because water is being pulled out of the organism by the salt. Water will travel from an area of low salt concentration (inside the animal) to an area of high salt concentration (outside the animal) through the skin of the fish or slug until the salt concentration is equalized on both sides. This will dry out the organism and cause it to shrivel.

This process of water traveling from an area of low solute concentration to an area high solute concentration across a semi-permeable membrane is called osmosis.

A difference between Wild Type and Mutant Worms

What does this graph show about the difference between the wild type and mutant worms?

Scientists have measured a difference between wild type and mutant worms and have found that the mutant worms contain much more glycerol than the wild type worms when grown on low salt—about 35 times as much.

The Role of Glycerol

Water

These are water molecules. Each one is made of 2 hydrogen atoms (in white) and 1 oxygen atom (in red), hence H2O.

Hydrogen Bonds

Water molecules create weak bonds between themselves. Attraction occurs between the hydrogen of one atom and the oxygen of another atom. This is what gives water some of its special properties, such as surface tension.

Glycerol

This is glycerol. It is made of carbon, oxygen and hydrogen (C3H8O3). See all of those hydrogen atoms in white and oxygen atoms in red? They are able to create hydrogen bonds with water.

Glycerol and Water

Glycerol "holds on" to water through hydrogen bonding.

Therefore, a worm with high levels of glycerol will retain water better than a worm with low levels of glycerol.

Where does glycerol come from?

Glucose is found in every cell and is used for energy. Under the right circumstances, one glucose molecule can be converted into two glycerol molecules. This is a multi-step process regulated by many proteins.

To keep it simple, we will call the protein which is the last enzyme in the pathway "glycerolase."

If a worm needs to manufacture some glycerol, then glycerolase needs to be produced. Where does glycerolase come from? It is coded for in a gene found in the DNA of the organism. When glycerolase is being produced, we say the glycerolase gene is being expressed.

Gene expression refers to the genes being used to produce the proteins they code for.

The effects of glycerol in C. elegans are modeled in this video. We use:

Dialysis tubing, which is a semipermeable membrane through which water can pass freely, but not larger molecules such as glycerol
Low glycerol solution (3%)
High glycerol solution (50%)
Salt beds
Scale

In this simulation, one dialysis tube is filled with a 3% glycerol solution and another is filled with a 50% glycerol solution. Both tubes are weighed and placed on a bed of salt overnight. The next day, we weigh the tubes again and calculate how much mass is lost from each tube overnight. Will higher glycerol concentrations keep water from leaving the tube? Watch to find out.

Worm Observations - 24 hours after chunking

In Lesson 2, we moved wild type and mutant worms from a low salt to a high salt environment. The wild type worms didn't respond well to the new environment. After 15 minutes they had stopped moving, eating, and most had curled up in a shriveled C shape. The mutant worms, however, kept moving and eating as if nothing had happened.

In today's lesson, we learned that the mutant worms produce high levels of glycerol constantly. Glycerol creates hydrogen bonds with water keeps the worm hydrated. This becomes important when moved to a high salt environment.

What do you think will happen to the worms after 24 hours in the new environment? Did we kill the wild type worms? Are the mutant worms still moving and eating? Let's find out.

Watch the following videos and fill out the second row of your observation data table.

Wild type on LOW salt

Wild type on HIGH salt

Mutant worms on LOW salt

Mutant worms on HIGH salt

Lesson 3 Comprehension Questions

A. How does glycerol help protect a worm in a high salt?

B. Think about the wild type worms on high salt that were very challenged at the 15 minute observation. How are they behaving after 24 hours?

C. What do you think is happening inside the wild type worms that account for this behavior?

D. What do you think is happening inside the mutant worms?

Next Step: Go to Lesson 4

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