Osmosis

What is osmosis?
This is the process by which water moves from one side of a semi-permeable (lets some substances through, like water, but not others, like the dissolved salts) to the other. Solutions of different osmotic pressures will become "balanced" if left alone for a time, as the water molecules move from the side of greater concentration of water (hence, less salty) to the other side of lesser concentration of water (or, more salty).
A organism is said to be hypotonic (lesser balance) when the internal body fluid is not as salty as the surrounding water medium.
On the other hand, it is said to be hypertonic when the internal fluid is saltier than the surrounding water medium.
When the internal body fluid is the same saltiness as the external medium, those 2 fluids are said to be isotonic.

The osmotic pressure of a solution (very difficult to measure) can be computed from the freezing point depression for that solution. This is possible as the salts that increase osmotic pressure depresses the freezing point of that liquid. This number, called fpd, is given as a positive number, meaning that this is the amount of temperature that the salts in a given solution has lowered the freezing point from 0 degrees Centigrade, which is the freezing point of pure water.
Hence, the fpd of sea water, with a salinity of 34.7 o/oo is 1.91, which tells us that sea water freezes at -1.91 degrees Centigrade.
Likewise, the fpd of human blood is 0.56, or human blood will freeze at -0.56 degrees Centigrade.
This tells us that the higher the salinity of a fluid, the higher its fpd, or the lower temperature at which that liquid freezes, when making comparisons between 2 liquids of different salinities.

The chemistry of sea water has implications to organisms that live in the oceans.
1. Evolution of plants and animals.
Since all of the elements that make up the biological organism are present in sea water, it is most probable that both plants and animals (and probably the most primitive forms of life, the precursors to plants) evolved in the oceans.
2. Most organisms are constantly "battling" to maintain the level of water and salts in their bodies within whatever narrow limits of tolerance.
3. This maintenance of a suitable salt/water balance expends considerable amounts of energy, whether in using special mechanisms or in the manufacture of a covering that prevents water and salts from entering or leaving the body.

Chemistry considerations for plants, in general.
1. Most of the living space in the oceans available to plants are shallow (upper 50-80 meters) since light is of primary concern to plants. Thus, the marine plants have "gone in the direction" of becoming "floating plants," called phytoplankton.
2. However, all plants do need a number of constituents in sea water.
a. Physical conditions, such as proper temperature, right light quantity and quality, suitable pH range, suitable substrate (for attached plants), and suitable salinity range.
b. Macronutrients, such as carbon, hydrogen, oxygen, magnesium, potassium, phosphorus, and nitrogen, that are found in good concentration and required by all plants.
c. Micronutrients, such as iron, calcium, zinc, manganese, and copper, essential in the construction and action of enzymes, only necessary in minute quantities.
3. Certain elements needed only by certain plants: silicon for diatoms for their shells, sodium for blue-green algae, and molybdenum for many others.
4. Accessory growth factors, such as vitamin B12.
5. The critical determinants for plant productivity in the oceans are phosphorus, nitrogen, and iron (sounds familiar? these are also required by plants for good production on land!).