Sometimes superb job of keeping in your

Sometimes Mother Nature needs to prevent things from going in or out, and sometimes Mother Nature needs to let everything (or just some things) through.

This lesson explores permeable, impermeable, and semi-permeable membranes and provides examples in nature for each.

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Mother Nature’s Three Membranes

Depending on your personality and your mood, you may feel like throwing a party and inviting the whole neighborhood. Other times you might just want to have your closest friends over for some dinner. And still other nights, you just want to be alone.Mother Nature is just like you in this matter. Sometimes everything needs to be let inside, sometimes just certain things, and still other times, nothing at all.

And just like you, Mother Nature can leave the door wide open, shut the door with a sign saying ‘please knock,’ or bolt the door to everyone and everything.In nature, there are three kinds of membranes that serve the same purpose as a door. A membrane, depending on where it is located, is a thin piece of tissue or flexible material that acts as a barrier or a divider between two regions.

Membranes can be fully permeable, which means they let all substances through; semi-permeable, they let only certain substances through; or impermeable, nothing gets through.

Permeable Membrane

A permeable membrane lets everything through, similar to how you let everyone into your home for the block party. An example of a permeable membrane in nature is the cell wall in plant cells.

Cell walls provide support and protection for plant cells. They are fully permeable to water, molecules, and proteins. This allows water and nutrients to be freely exchanged between the plant cells. If you look at this diagram of the permeable membrane, both the large circles and the small stars can freely move across.

Permeable membrane. The plasmodesmata are holes in the cell wall that allow the free movement of water and other molecules.
Diagram of a permeable membrane.

Semi-Permeable Membrane

Cell membranes are semi-permeable. Just like your private dinner party, only certain molecules can cross the cell membrane.

This allows the cell some control over what enters and leaves. Water, small molecules, and molecules without a charge can pass freely. Large molecules and strongly positively or negatively charged molecules must stay outside. In this diagram of a semi-permeable membrane, the large circles are too big to pass through the membrane, but the small stars have free movement.

Semi-permeable membrane. In the cell membrane, the protein channel is only letting the dark green hexagons pass through.
Impermeable membrane. Egg shells are examples of impermeable membranes.
What membrane. Animal cells only have the cell
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