Freshwater Protists Osmotic Problems

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

The protists are a very early form of life on our planet. Keep reading to learn more about an important adaption that freshwater protist contain that keeps them alive in water.

What are Protists?

Hi! I am a paramecium, a freshwater protist.
Microscope photograph of a paramecium.

A paramecium is a type of freshwater protist. Pretty cute for a single-celled organism. Not to mention, he is a really great friend, too. This is a Paramecium bursaria. See that green stuff inside of him? Those spots are actually algae that he lets hop along for the ride. The algae give him food, and he gives them protection.

All life on our planet is divided into one of six kingdoms. Protists are members of Kingdom Protista. They are the mostly single-celled organisms that do not fit into any of the other kingdom. The term ''protist'' is Greek for ''the first''. While these organisms do not represent the very first forms of life on our planet (that honor goes to the bacteria), the protists are the first eukaryotic forms of life. Eukaryotes are complex cells that contain a nucleus and other organelles. All multicellular forms of life are comprised of eukaryotic cells. Protists are believed to be the precursors of more advanced organisms found in the plant, animal, and fungus kingdoms. Our paramecium friend, for example, is a protozoan, which means ''first animal''.

The Process of Osmosis

Protists are found in pretty much any environment on Earth where there is liquid water. While life for any tiny creature on our planet is probably hard, freshwater protists face a unique hurdle. Their own environment is trying to kill them!

Osmosis is the process by which water flows across a semi-permeable membrane. This is a natural phenomenon which neither you nor the protists have any control over. Water passing through a membrane isn't a bad thing in and of itself. So, let's break down the process to see why it could be harmful to our single-celled friends.

Think about adding food coloring to a glass of water. Even if you don't stir the water, the food coloring will eventually spread out so that it is evenly dispersed throughout the water. This is what molecules in nature like to do, spread out and become evenly distributed. The same thing occurs when you spray air freshener in one corner of your room. The molecules spread and you can eventually smell the air freshener throughout the room.

The red molecules in this glass will eventually distribute themselves evenly throughout the water.
Photograph of red food coloring being added to water.

Here is where the ''semipermeable membrane'' part of the definition comes into play. A semipermeable membrane only lets some substances, such as water, pass through. Other substances, such as the internal contents of the cell, cannot. This usually works in the cell's favor since it would prefer that its insides stay right where they are.

However, as we just discussed, molecules prefer to be evenly spaced. So, if there are more molecules inside the cell than outside, the molecules would really like to move away from each other and spread out. But, the molecules inside the cell cannot spread out across the semipermeable membrane. In this case, the water takes over and moves into the cell in an attempt to dilute the concentration of the molecules within the cell. This continues until the concentrations of molecules inside and outside the cell are the same.

Molecules in nature prefer to be evenly spread out. In this case, the molecules cannot move because they are trapped by a semi-permeable membrane. So, instead, the water moves across the membrane until the concentrations of molecules are the same on both sides.
Diagram of osmosis.

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