Aquaporins: Definition & Function

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  • 0:04 The Cell Membrane
  • 0:54 Selective Permeability
  • 2:00 Diffusion
  • 3:56 Aquaporins
  • 4:50 Special Functions
  • 5:35 Lesson Summary
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Lesson Transcript
Instructor: Christie Spadafora

Christie has a B.S. and an M.S. in Biology. She teaches life and chemical science courses at college, high school, and middle school levels in MA.

This lesson focuses on special water channel proteins called aquaporins. Aquaporins allow water to travel across a cell membrane, thus controlling the water balance within a cell.

The Cell Membrane

Bacteria, algae, dogs, tulips, humans! If it's alive, each one of its cells has a cell membrane. Cell membranes are composed of two layers of phospholipids, with some proteins and carbohydrates scattered throughout, forming a layer of protection and separation for the cell's inner contents.

The cell membrane has numerous functions but, for the purpose of this lesson, we'll focus on one. The cell membrane has the job of deciding what is allowed to enter and exit the cell. Aquaporins are special channels made of protein and deposited into the cell membrane. Similar to a tunnel, they let molecules travel through them, entering or exiting the cell. But before we discuss these channels, let's review selective permeability and diffusion.

Selective Permeability

The doorbell rings and you head over to see who's at the door. If you recognize the person, chances are you will let them into your home. But if you see a stranger at the door, you might decide to speak to them through the screen door instead. Just as you're selective when choosing who is allowed in your home, the cell membrane is selective when deciding what is allowed to enter or leave the cell it is protecting. The cell membrane is permeable, meaning molecules can cross it, but it's also selective, meaning it only lets certain molecules cross it. This property is appropriately termed selective permeability.

Some molecules are no brainers, like oxygen. Obviously, the cell wants to let oxygen in, or it will die. But other molecules, like water, are a bit trickier. Too much water and the cell can explode like a water balloon. Not enough water and the cell could shrivel up and die. The key is to let just the right amount of water in to keep the cell healthy and thriving. How is this accomplished? To understand this, we first must understand diffusion.


Have you ever smelled delicious aromas coming from the kitchen, even though you were in another room in the house? Imagine an apple pie being baked in the kitchen while you're upstairs in your bedroom. The odor molecules of the apple pie are concentrated at their source (near the oven), but can freely travel from the oven to other parts of the house, including upstairs to your nose.

This phenomenon, molecules freely moving from a space where there are a lot of them (high concentration) to spaces where there are not a lot of them (low concentration), is called diffusion. Diffusion explains how cream spreads evenly throughout a cup of coffee and how food coloring spreads out within a cup of water.

Diffusion also allows many important molecules to enter and exit the cells within living organisms. For example, all human cells need oxygen. This tiny molecule can diffuse freely from the bloodstream into any cell, wiggling its way across the cell membrane. Conversely, all human cells produce carbon dioxide as a waste product. Carbon dioxide can freely diffuse across the cell membrane, leaving a cell and entering the blood stream. From here, it will travel to your lungs, where you'll breathe it out for good.

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