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Semipermeable Membrane: Definition & Overview

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Semipermeable membranes are vital parts of biological systems - they're a big part of how the cells of plants and animals work. Learn what a semipermeable membrane is and how they are used in biological and human-made systems.

What is a Semipermeable Membrane?

A permeable membrane is a biological or synthetic material that has tiny holes in it, allowing small particles (including water molecules and ions) to move through it. A semipermeable membrane is a membrane that only allows certain types of particles to move through it under certain conditions.

You can think of it as being like a sheet of biological fabric with holes in it, where some molecules can fit through the holes and others can't. However, semipermeable membranes can be a bit more complicated than that, because the way particles move can depend on the conditions on either side. For example, particles can move fast, slowly, or not at all depending on how the concentration, pressure, and temperature compares on each side of the membrane.

Generally, particles move through a semipermeable membrane from areas of high concentration to areas of low concentration. This is a process called diffusion.

Examples of Semipermeable Membranes

Plants and animals are made up of cells. It is here that we find the most common example of a semipermeable membrane in action - a process called osmosis. Cells are surrounded by membranes. These membranes are made up of phospholipids (a type of lipid or fat) and proteins. Cell membranes are semipermeable, which means molecules can move through them. This is pretty important for cells to survive.

Osmosis is where solvent molecules (usually water) move from one side of a cell membrane to the other. This happens because the concentration of a solute is higher on one side. When two nearby concentrations are different, materials will tend to spread out to equalize the concentration. That's why, for example, if you put a drop of food coloring in some water it will gradually spread through the whole water.

One side of the membrane is often kept at a lower concentration on purpose to force desirable molecules to keep moving into the cell. The cell removes the molecules as soon as they arrive to keep osmosis happening. At least, that's the simple explanation.

It turns out that osmosis is a little more complicated than that. As we looked at the process more carefully, we found that there's really a change in pressure that helps force water through the membrane - kind of like when you force icing through a pastry bag nozzle - but it's still true that in most cases, molecules go from high concentration areas to low concentration areas.

Osmosis is how cells in the roots of a plant absorb water. It's also how plants get their turgid, upright shape. Water provides pressure, which makes plant stems firm and stand up straight.

Osmosis is also why there are freshwater fish and different saltwater fish. If you put a fish in an environment they're not suited for, osmosis will happen opposite to the way the fish expects and they will die.

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