Osmolarity And Body Volume

Introduction:

Osmosis is the passage of water from a region of high water concentration through a semi permeable membrane to a region of low water concentration. There are three basic states in which animal cells exist when referring to osmosis, tonicity, and osmotic balances within an organism; hypotonic, hypertonic, and isotonic. If too much water enters the cells they will swell and possibly burst and this is known as a hypotonic state. If too much water is lost from the cell, it could shrivel and die. This state is known as a hypertonic state. Water flows through the cell constantly in both directions (in and out). If there is no difference in the concentration of the solution inside the cell and outside the cell, there will be no overall net movement of water keeping the cell isotonic.

It is obviously in the cells best interest to remain isotonic to its environment, therefore the cells must be bathed in a solution having the same osmotic strength as that of the cytoplasm within the cells. Water and salts must be regulated throughout the body and this function is provided by the kidneys through a process known as osmoregulation. The regulation of water and salts in the extracellular and intracellular fluids is important to the survival of all living organisms. More specifically, it is the balance of the water and salts between these two fluids in any given cell that is key to the survival of living organisms. Different types of organisms accomplish osmotic regulation in different ways, but they are typically classified into two main groups, osmoconformers and osmoregulators, based on the way they regulate their body fluid osmotic concentration relative to the environmental osmotic concentration. Osmoconformers regulate their internal osmolarity in such a way that it remains equal to the osmolarity of its surrounding environment. They don't have to actively change their osmotic state. Osmoregulators have a different osmolarity than the environment and they must adjust it depending on the type of environment they are in; discharge excess water if they live in hypotonic (a solution having a lower solute concentration) environments or take in water if they live in hypertonic (a solution having a higher solute concentration) environments.

In this experiment we will be observing the osmoregulation of two organisms; the Nereis vexilossa (the sand worm) and Mercenaria mercenaria (the hard clam). The worms are osmoconformers and maintain a constant osmolarity using specialized excretory and osmoregulatory organs known as nephridia. They live in a fairly stable environment therefore it isn't as necessary for them to develop specialized mechanisms for osmoregulation. This form of osmoregulation works for them because of their stable environment, but the downside, is that they aren't capable of withstanding major changes in their environment. The clams, on the other hand, are osmoregulators. They live in the intertidal zone meaning that they are regularly exposed to drastically different environments (sea water at high tide and air at low tide) and thus have developed specialized mechanisms to maintain a constant osmolarity in the midst of their ever-changing environment. While this mechanism enables them to live in almost any type of environment, internal regulation is costly and uses a lot of energy; therefore they must take in much more energy just to maintain homeostasis.

We observed both the worm and the clam in different environmental conditions to see how their bodies responded to the changing solute concentrations in the different environments they're placed in and to gain a better general understanding of the regulation of water and salt in their bodies. We measured the amount of water loss or gain in these two organisms and determined how much fluid they exchanged with their environment to maintain a proper balance of water and salts in their body. We hypothesized that the clams, as the osmoregulators, would regulate internally and the worms, as the osmoconformers, would regulate their bodies according to the environment.

We expected this because osmoconformers have no specialized mechanisms for fluid exchange and thus allow their body fluid composition to vary with that of the environment and readily take in or expel water to keep their osmolarity the same as that of the environment and osmoregulators are used to an ever changing environment and must actively work to maintain homeostasis apart from whatever concentrations are present in the environment and are less likely to take in any fluids from the environment or expel any into it..

We predicted that the worms would experience the greatest change in weight due to them being osmoconformers and regulating their osmolarity according to that of the environment and that the weight of the clams would remain relatively the same because their internal processes should go on uninterrupted by the changing environment.

Methods:

The procedures that we followed for this lab can be found in the Foundation of Biology: Cell and Organ Physiology - A Laboratory Manual (Department of Neurobiology and Behavior, Stony Brook University).

Results:

Our results generally showed that the worms experienced a significant amount of weight change and that the clams experienced a very small fluctuation in weight. Refer to the graphs and tables on the following page for specific and detailed data.

Discussion:

Looking at the group results, it seems they support the hypothesis. The worms experienced a relatively noticeable change in weight. Through the process of osmosis, the Nereis worms passively take in or release water to regulate their body osmolarity when placed in different concentrations of seawater. Thus, we believed that as we placed them in different concentrations of seawater they would be constantly taking in and expelling fluid to match their internal osmolarity with that of the environment. In