A. What is excretion--The process whereby the organism gets rid of metabolic wastes that are produced by the normal metabolic processes. In the sense which we will use it, we also include osmotic and ionic regulation.

1. The end products of metabolism--The end products are mostly materials containing nitrogen. Proteins are a polypeptide, that is, a polymer of amino acids. There are 21 amino acids. An amino acid is a carbon skeleton that contains a carboxyl group (COOH) on one end of the carbon chain and a amine group (NH2) on the other end . The carbon chain between has a side group that determines the nature of the amino acid. When two amino acids are joined, a water molecule is released by the joining of a hydrogen atom from the amine and a hydroxyl group from the carboxyl group. A polypeptide chain can be broken by breaking off an end group or splitting the chain between two amino acid groups. If the latter is done, water is required. As proteins "wear out", the end product of their breakdown is usually ketoacetic acid and ammonia (NH3).

2. The nature of the nitrogenous waste products

a. As mentioned above, one of the most common wastes is ammonia. The substance is fairly soluble, but at the same time toxic. For most organisms, this is not a problem, in as much as they live in an aqueous medium and the ammonia simply diffuses from the organism into the medium surrounding the organism along a concentration gradient. If the organism moves or if the media moves over the organism, this gradient is fairly steep.

b. Another form of waste product is urea. Urea was the first organic material to be synthesize (an early German biochemist did this). It is less soluble than ammonia and at the same time less toxic. To get rid of the material though, an organism must use some of its water. At the same time urea can be used to maintain the osmotic pressure of body fluids.

c. A crystalline waste product formed is uric acid. This material is insoluble and the least toxic of the materials mentioned. No water is needed to get rid of it, but it usually is mixed with other materials, often with fecal materials in the cloaca of the animal (see grasshopper). It is a white crytalline material.

d. Purines--there are a number of materials that fit into the purine category. One of these is guanine. This material can be stored in epidermal/dermal cells in some organism is associated with body color. It reflects light.

3. The relationship of the above to environmental conditions.

a. Ammontelic-excretion of ammonia in environments where water balance is not a problem.

b. Ureotelic--excretion of urea. Associated often with terrestrial organisms where there is some need for fluid retention

c. Uricotelic--excretion of uric acid. Associated with terrestrial organisms where fluid retention is very important.

d. The term "telic" can be attached to any form of waste product, eg., purinotelic, guanotelic, etc.

4. Diffusion and osmosis

a. Diffusion is the movement of a substance along a concentration gradient, i.e., from a higher concentration to a lower concentration.

b. Osmosis is the movement of water through a selectively semi-permeable membrane from a region of lower solute concentration to a region of higher solute contration. The emphasis here is on the solute concentration, but it is easier to remember osmosis if you think that when you have a high solute concentration the number of water molecules is low and when you have a low solute concentration, the number of water molecules is high. Therefore you can think of osmosis of a special case of diffusion as water is moving from high water to low water. What makes it different is the selectively semi-permeable membrane. Many substances are too large to move through a cell membrane, or move more slowly, eg., divalent ions as compared to monovalent ions.

5. Active and passive transport.

a. Passive transport is simply a type of diffusion and as a result costs the organism no energy loss on its part.

b. Active transort involves the use of ATP and therefore costs the organism.

6. Osmotic regulation and ionic regulation

a. Even if an organism is in osmotic balance, it may not be in ionic balance. This is particularly so because proteins are retained within cells and within the body fluids and thus contribute greatly to the osmotic pressure exerted by those fluids, whereas they are normally not outside the organism. Thus if they are inside, then as a result, there must be an inbalance of ions. In as much as many ions are required by most organisms, particularly in the function of nerves, the organism must regulate its ion content, even though it might be in osmotic balance.

B. Regulation versus conformation

1. Most marine organisms are osmotic conformers. This means that they allow their body fluids to change in osmotic pressure as the osmotic fluid of their environment changes. In as much as the marine environment remains rather stable (rough 33-35 0/00 salinity ), the actual fluctuations remain rather minimal.

2. Organism that are subjected to greater salinity changes are osmotic regulators. They may allow their body fluids to vary, but only over a short segment of values exerienced in the environment. At lower values of salinity, they begin to expend metabolic energy to maintain their body fluids at a set level. Organisms that do this typically lived in esuarine environments, where they are subjected to large scale salinity changes.