A Report on the Effects of Osmolarity on Body Volume

March 16, 2007

Introduction
The experiment focused on the different effects of osmosis on body volume in osmoregulators and osmoconformers. Osmosis is the diffusion of water across a selectively permeable membrane.1 and this diffusion of water is usually from a region of high concentration to a region of low concentration. A solution with a high concentration of solutes is called a hypertonic solution, while that with a low concentration of solutes is called a hypotonic solution. A solution with equal concentrations of solute and solvent is called an isotonic solution. Therefore in osmosis water moves, across a selectively permeable membrane, from a hypotonic solution to a hypertonic solution till both solutions are isotonic.

This is important biologically because all the cells in an organism "live" in an aqueous environment. A difference in the concentration gradient between a cell and the solution it is suspended in can result in osmosis. Animal cells do not have strong cell walls and if placed in a hypotonic solution, the cells swell as water diffuses into the cell till the cell finally bursts. In a hypertonic solution, animal cells shrivel up as water diffuses out of the cell. The best environment for an animal cell is an isotonic environment.

In order for animals to ensure that their cells are maintained constantly in an isotonic environment, two methods of regulating osmosis have evolved. The two methods are Osmoregulation and Osmoconformism. Osmoregulators have specific internal methods to control and maintain the concentration gradient in their body fluid. These methods either prevent extreme water loss or extreme water absorption when there is a change in the concentration gradient. Osmoconformers do not have specific internal control methods, and as such their rate of osmosis changes according to the changes in the concentration of their external environment. Most osmoconformers are marine invertebrates (with the exception of hagfish, which is a vertebrate)4; while most osmoregulators are terrestrial or freshwater animals5. Marine animals are isotonic to their environment5, so they have no need to regulate water flow in and out of their cells. However, terrestrial and freshwater animal cells are not isotonic to their environments so in order to prevent water loss/gain due to osmosis they must become osmoregulators. Osmoregulators must either get rid of excess water (hypotonic solution) or take in excess water (hypertonic solution) in order to maintain their cell osmotic gradient. 5

The purpose of this experiment was to test and observe the effects of varying concentrations of saline solution on two different species. One species (the Nereis worm species) was an osmoconformer, and the other species (the Mercenaria clam species) was an osmoregulator.2 The Nereis worm species is an inhabitant of the intertidal zone of marine habitats6 (the coastal area of the ocean that is submerged at high tide and exposed at low tide7) and is thus a saltwater inhabitant and an osmoconformer. The Mercenaria clam species is an inhabitant of estuaries8 (a body of water with both freshwater and saltwater flowing into it9) and is thus a freshwater inhabitant and an osmoregulator.

The experiment was conducted on the basis that, if osmosis were occurring in both species, there would be a noticeable change in the body weight (i.e. volume) of the species over a period of time. The hypothesis of the experiment was that the Nereis worms were osmoconformers and as such their body weight would fluctuate with the tonicity of the saline solutions; while the Mercenaria clams were osmoregulators and as such there would be little or no change in their body weight.

Materials and Methods
Procedures were followed exactly as detailed in Foundations of Biology: Cell and Organ Physiology (Faculty of the Department of Neurobiology & Behavior, pp 15-7)2 with the following exceptions.

1. No worms were used in the experiment, instead sample data from previous experiments was used. 2. The volume of saline solution used was halved. Instead of 1liter, 500ml was used.

Results The amount of salt and water used to create the different solutions of salt and water is shown: |Concentration |Instant |Water | | |Ocean | | |Parts per |Percentage |In grams |In ml | |thousand | | | | |22.5 |75% (hypotonic)|13.36 |500 | |30 |100% (isotonic)|17.813 |500 | |37.5 |125% |22.266 |500 | | |(hypertonic) | | |

Table
1
In general, the data showed that the worms varied in weight according to the variations in the saline solution concentration, while the clams maintained a relatively constant weight throughout the experiment. Table 2 shows the results obtained in the group experiment of the clams. This initial table shows only the observed weight of the clams over the 60 minute period. |Time in |Merceneria Clams | |min. | | | |Weight in grams | | |75% |100% |125% | |0 |34.5 |40.47|41.25| |10 |34.6 |40.43|41.23| |20 |34.6 |40.43|41.21| |30 |34.54|40.41|41.18| |40 |34.52|40.40|41.18| |50 |34.52|40.40|41.16| |60 |34.50|40.37|41.15|

Table 2

Table 3 below also shows group data. It shows the change in the weight of the clams over the 60 minute interval. The values are negated because there was a loss in the weight of the clams. The values shown are the change in weight from the original weight at time 0, not the change in weight from the previous values. |Time in |Merceneria Clams | |min. | | | |Change in Weight in | | |grams | | |75% |100% |125% | |0 |0 |0 |0 | |10 |-0.1 |-0.02 |-0.02 | |20 |-0.1 |-0.02 |-0.04 | |30 |-0.04 |-0.06 |-0.07 | |40 |-0.02 |-0.07 |-0.07 | |50 |-0.02 |-0.07 |-0.09 | |60 |0. |-0.1 |-0.1 |

Table 3
In this table, there was slight variation in the weight of the clams during the experiment. However, the highest change in all 3 concentrations of saline solution was a 0.1 difference from the original weight.
Table 4, shows the entire class data for the change in weight of the clams over the 60 minute time interval.

In this table, there was a general decrease in the weight of the clams in the 75% saline solution, some weight gain in both the 100% and 125% saline solutions. The highest average decrease in the weight of the clams was recorded in the 75% saline solution and was 0.08 grams. In the 125% saline solution the lowest average change in weight was 0.00g. However, from the table it can be seen that there was some weight loss in the 125% saline solution. Table 5 shows the percent change in weight of the clams at different salinities.

Overall, the percent change in the weight of the clams was less than 1%. The highest average percent change was 0.36% and was observed in the 125% saline solution. The lowest average percent change was 0.00% in the 75% saline solution. The overall trend was a decrease in the weight of the clams.

For the worms, since the data used was from previous years, there were no initial group readings. Table 6, shows the entire class data for the change in weight of the worms over the 60 minute time interval. In this data, there was an increase in the weight of the worms in the 75% salinity and 100% salinity solutions. In the 125% salinity solution there was a decrease in the weight of the worms. The highest average decrease in the weight of the clams was recorded in the 125% saline solution and was 1.12 grams. The lowest average decrease in weight was 0.01 grams and this was recorded in the 100% saline solution. Table 7 below shows the percent change in weight of the worms at different salinities.

Overall, the percent change in the weight of the worms was higher than that of the clams. The highest average percent change was 11.87% and was observed in the 125% saline solution. The lowest average percent change was 0.07% in the 100% saline solution. The graphs below show a comparison of all the data obtained in the experiments. Graph 1 shows a comparison of all the average changes in weight of the organisms (clams and worms).

Graph 1 The greatest change in weight is noted in the 125% saline solution for the worms. It was a decrease in weight. The lowest noted change in weight is the 75% saline solution for the clams. It was a slight decrease in weight. The trend-line for the 75% saline solution for clams is almost a straight line. The highest slope is a negative slope; that of the 125% saline solution for worms. There was a large decrease in weight in the worms in that concentration.

Graph 2 below shows a comparison of the percent change in the weight of the organisms (both clams and worms).

Graph 2 The graph shows that the clams in all the concentrations of saline solution had almost the same percent change in weight. There was very little increase or decrease in the weight of the clams, and all three trend-lines are extremely close together and very straight. For the worms, the 100% saline solution slope came close to matching that of the clams, but was not as straight. The highest positive slope was that of the 75% saline solution, indicating and increase in weight. While the highest negative slope was that of the 125% saline solution, indicating a decrease in weight.

Discussion
The results showed that overall, the worms lost more weight than the clams. As can be seen from both graphs, the worms in the 125% saline solution lost the most weight of all the organisms. The worms also gained the most weight of all the organisms, in the 75% saline solution. Both graphs also show that the clams had the least amount of weight gain or loss.

Graph 2 provides a better picture of the situation. In Graph 2, the slopes of all three saline solution concentrations for the clams are almost on top of each other and form a straight line. This indicates two things: first, the clams had similar percent changes in weight; and the percent change in weight was almost 0%. This supports the theory that the clams are osmoregulators. There was almost no change in all the clams, regardless of the concentration of the saline solution. In graph 2 it can be seen that only the trend-line of the 100% saline solution of the worms came close to matching that of the clams, but even then it was not exactly as straight as the clams. This is to be expected though, because the 100% saline solution was prepared to be isotonic to the organism's bodies. So, the reason why the clams had a very low percent change in weight is because their bodies were isotonic to the solution, so there was little need for osmosis.

As mentioned before, it was predicted that the osmoconformers (the worms) would have the most noticeable change in weight and that was the result obtained from the experiment. The worms had the most noticeable weight loss, this occurred in the 125% saline solution. This was probably due to the fact that the 125% saline solution was hypertonic to their body composition. As osmoconformers the worms had to adjust the amount of water in their bodies to become isotonic to the solution, so water diffused out of the worms and into the saline solution causing the worms to decrease noticeably in weight i.e. osmosis occurred. The worms also had the most noticeable weight gain, this occurred in the 75% saline solution. This was probably due to the fact that the 75% saline solution was hypotonic to their body composition. As osmoconformers the worms had to adjust the amount of water in their bodies to become isotonic to the solution, so water diffused into the worms from the saline solution causing the worms to increase noticeably in weight i.e. osmosis occurred.

The clams in the experiment were not predicted to gain or lose any weight. The results from the experiment supported this prediction. Graph 1 shows that the average weight loss of the clams in all 3 concentrations of saline solution was less than 0.2g. This is attributed to the fact that the clams are osmoregulators. Since the clams were covered by a hard calcium carbonate shell, they could have simply closed their shells tightly to prevent hypertonic or hypotonic saline solution from entering the shells and disturbing homeostasis. Clams have strong adductor muscles that help them hold their shells tightly closed.1

Graph 2 is a better graph to note the results of the experiment because it shows the percent change in weight of the organisms. This gives an idea of the amount of weight gained or lost relative to the total weight of the organism. The graph supports the hypothesis that the clams are osmoregulators, and the worms are osmoconformers. The clam body weight remained almost the same throughout the experiment, while that of the worms fluctuated depending on the concentrations of the saline solution.

To provide more support for the theory, some changes could be made to the procedure in this experiment. For example, during the preparation of the saline solution, apart from mixing the salt and water, the solution could be heated to promote the dissolution of salt in the solution. This would mean more accurate saline solutions. Also it is possible that some of the data obtained might be flawed because of the fact that while removing and weighing the clams or worms, the organisms might not have been dried correctly before weighing. Although in this experiment that did not seem to cause any errors in the overall graphs, some of the data (like the weight loss in the 75% saline solution for clams) did not seem to match the predicted results. This experiment could be performed with greater accuracy to provide more proof for the hypothesis.

References
1 Campbell N.A. and Reece J.B. Biology: Sixth Edition. Benjamin Cummins: 2002. 658, 925-6. 2 Department of Neurobiology and Behavior. Stony Brook University. Foundations of Biology: Cell and Organ Physiology. Pacific Crest: 2007. 12- 8 3 Dilger, K. T. 2007. Personal Communication. From Lecture on Osmolarity given 2/8/07 for Bio 203, Cell and Organ Physiology, SUNY Stony Brook.
4 Osmoconformer - Wikipedia, the free encyclopedia. Wikimedia Foundation, Inc. Osmoconformer. . 3/12/07.
5 Biology Notes: Osmoconformer and osmoregulators . Pagewise, Inc. Biology Notes: Osmoconformers and osmoregulators. < http://ca.essortment.com/osmoconformerso_rjdp.htm>. 3/12/07.
6 Benthic Habitat Mapping - Clam Worm. National Oceanic and Atmospheric Administration-Coastal Services Center. Nereis. spp. . 3/12/07.
7 Intertidal Zone- Wikipedia, the free encyclopedia. Wikimedia Foundation, Inc. Intertidal Zone. < http://en.wikipedia.org/wiki/Intertidal_zone>. 3/12/07.
8 Benthic Habitat Mapping - Hard Clam. National Oceanic and Atmospheric Administration-Coastal Services Center. Mercenaria mercenaria. . 3/12/07.
9 Estuary- Wikipedia, the free encyclopedia. Wikimedia Foundation, Inc. Estuary. . 3/12/07.