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Osmoregulation in the Water Environment Lab
Summary
All living organisms must maintain their internal solute and water concentrations despite ever changing
external conditions. To better understand osmoregulation, the balance of water and solute concentrations in
animals, you will look at the two major groups of fishes, those with skeletons made of cartilage and those with
skeletons made of bone. Fishes inhabit both fresh and marine water, and solute and water concentrations vary
considerably for marine and freshwater fishes. Both groups face fundamentally different problems brought upon by
environmental conditions. Furthermore, sharks and their relatives, the cartilaginous fishes, differ from their bony,
marine fish relatives in methods of osmoregulation.
In this lab, you will investigate the following questions: How do freshwater and marine habitats influence
solute and water balance? Are there different mechanisms for maintaining solute and water balance in freshwater
and marine organisms? Why are most aquatic organisms only able to live in either fresh or saltwater, but not both?
Ocean Literacy Principals and Fundamental Concepts
5f. Most of Earth’s water (97%) is in the ocean. Seawater has unique properties: it is saline, its freezing point is
slightly lower than fresh water, its density is slightly higher, its electrical conductivity is much higher, and it is slightly
basic. The salt in seawater comes from eroding land, volcanic emissions, reactions at the seafloor, and atmospheric
deposition.
Learning Objectives
[b] Students will compare and contrast the osmoregulation challenges of freshwater and saltwater bony fishes.
[b] Students will compare and contrast the osmoregulation challenges of sharks and bony fishes.
[b] Students will understand the mechanisms of osmoregulation for sharks, marine fishes, and freshwater fishes.
Vocabulary
[b] Diffusion
[b] Osmosis
[b] Osmoconformer
[b] Osmoregulator
[b] Hypertonic
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[b] Isotonic
[b] Hypotonic
Materials
[b] Four Dialysis Tubes (30-cm strips)
[b] 0.2 M and 0.5 M Sucrose Solutions (in 500-mL beakers)
[b] Distilled Water (or tap water)
[b] Eight String pieces (2 per dialysis bag)
[b] Food Coloring Dyes
[b] One Small Funnel
[b] Eight 8-oz clear plastic cups
[b] Sharpie Marker
[b] Calculator
[b] Triple-beam balance (or electronic balance)
Safety Precautions
Students should use “Eye Safety” (art) during the lab.
Timeframe
[b] One 55-minute period
Background
The modes of regulation vary considerably for marine and freshwater animals. Some animals are
osmoconformers, meaning they are unable to maintain internal solute concentration. They can experience
significant osmotic pressures if environmental conditions change. Other organisms, the osmoregulators, are able to
control their internal solute and water concentrations, to a degree, and thus avoid some osmotic pressures. These
mechanisms of osmoregulation can vary widely between bony and cartilaginous fishes.
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In this lab, you will use different concentrations of sucrose solutions in dialysis tubes to simulate the internal
solute concentrations of freshwater and saltwater fishes (including both cartilaginous and bony fishes). A 0.2 M
sucrose solution will be used as an equivalent solute concentration found in bony fishes, and a 0.5 M sucrose
solution will be used as an equivalent solute concentration found in sharks and seawater.
After completing this lab, you should be able to answer the following questions. How are the osmotic
challenges facing both freshwater and marine organisms different? How do sharks and their relatives different in
their mechanisms of osmoregulation than their bony fish relatives? What are some mechanisms freshwater and
marine organisms use for osmoregulation?
Learning Procedure
[Step 1] Fill two 8-oz cups with 0.2 M sucrose solution. Label the cups Fish #1 and Fish #2. Fill two 8-oz cups with
0.5 M sucrose solution. Label these cups Shark#1 and Shark #2.
[Step 2] Obtain four 30-cm strips of dialysis tubing that is pre-soaked in water.
[Step 3] Tie one end of each of the four dialysis tubing strips.
[Step 4] Using the small funnel, pour approximately 25-ml of sucrose solution from each 8-oz cup into one of the
four dialysis tubes. Each dialysis tube should be about ¾ full.
[Step 5] Remove some of the air from each bag, using your fingers, leaving some space for expansion to occur and
then tie each dialysis bag. Carefully dry the outside of each dialysis bag.
[Step 6] Weigh each dialysis bag and record the initial mass (grams) in Table 4.1. Be careful not to mix-up your
dialysis bags.
[Step 7] Fill two 8-oz cups with distilled tap water. Label these two cups freshwater. Fill the final two 8-oz cups with
0.5 M sucrose solution. Label these two cups saltwater.
[Step 8] Immerse your fish #1 dialysis bag and shark #1 dialysis bag each in one of the cups labeled freshwater.
Immerse your fish #2 dialysis bag and shark #2 dialysis bag each in one of the cups labeled saltwater.
[Step 9] Record the time and then soak all four dialysis bags in the freshwater and saltwater cups for 30-minutes.
[Step 10] After 30-minutes, remove each dialysis bag from the water cups. Carefully dry the outside of each dialysis
bag.
[Step 11] Weigh each dialysis bag and record the final mass (grams) in Table 4.1.
[Step 12] Calculate the percentage weigh change of each dialysis bag and record it in Table 4.1.
% Change in Mass = [(Final Mass (g) – Initial Mass (g)) / Initial Mass (g)] x 100 %
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Analyzing Results
Table 4.1
Dialysis Tube
Contents
Initial Mass (g)
Final Mass (g)
Change in Mass (g)
% Change in Mass
Freshwater
Fish #1
Click here to enter text.
Click here to enter text.
Click here to enter text.
Click here to enter text.
Saltwater
Fish #2
Click here to enter text.
Click here to enter text.
Click here to enter text.
Click here to enter text.
Freshwater
Shark #1
Click here to enter text.
Click here to enter text.
Click here to enter text.
Click here to enter text.
Saltwater
Shark #2
Click here to enter text.
Click here to enter text.
Click here to enter text.
Click here to enter text.
Create a column graph showing the initial and final mass of your fishes and sharks in Graph 1. Mr. Rainbeau will go
over this in class.
Graph 4.1
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Chart Title
A
x
i
s
T
i
t
l
e
Fish #1
Fish #2
Shark #1
Shark #2
Axis Title
Evaluating Results
Freshwater fish and saltwater fish each face different osmoregulation issues in their habitats. Both fish have similar
internal salt solute concentrations, yet they each experience different problems regulating their salt and water
contents. Explain the salt and water regulation issues faced by marine and freshwater fish. Then, using the picture
below, summarize the ways each type of fish solves these problems.
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Sharks, with few exceptions, cannot tolerate freshwater environments for long periods of time. Using the picture
below as a reference, describe how sharks regulate their salt and water content. How does this differ from
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saltwater bony fishes? Why do sharks have an even harder time living in freshwater (or low salt concentrations)
than do other fishes?
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