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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.</p><div class=

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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.
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.
Microscope photo of a plant-like protist.

Luckily for him and other freshwater protists, Mother Nature has created a contractile vacuole. A contractile vacuole is an organelle that stores the excess water that makes its way into the cell through osmosis.

When the vacuole fills up, it expels its water back out to the environment. This prevents the paramecium from taking on too much water and allows him to live happily in his freshwater environment. Saltwater protists do not have this problem, by the way.

The concentration of molecules in their environment nearly matches the concentration of molecules within their cell membrane. Thus, the water flows back and forth evenly across the membrane without creating any potential danger for the cell.

Lesson Summary

Protists are the mostly single-celled organisms that do not fit into any of the other kingdoms of life. They are eukaryotic cells that are believed to be the precursors to other, more advanced forms of life. Eukaryotes are complex cells that contain a nucleus and other organelles. All multicellular forms of life are made up of eukaryotic cells.

Osmosis is the process by which water flows across a semipermeable membrane. A semipermeable membrane allows some substances to pass through while others cannot. Freshwater water protists live in an environment where there are fewer molecules outside of their cell than inside. In this situation, water, through the process of osmosis, flows into the cell in an attempt to even out the concentrations of molecules inside the cell and out. To avoid taking on too much water, freshwater protists contain a contractile vacuole that stores and expels any excess water.

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