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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.

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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.

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.

Approximately two-thirds of the human body is made of water, so it’s clearly important. The diffusion of water across a selectively permeable membrane is given a special name: osmosis. Often times, water will osmose across the cell membrane to help the cell maintain a proper water balance, as well as a proper concentration of the several important molecules (like sugar, sodium, or calcium) within the cell. Maintaining a proper water balance is really important (think back to earlier in this lesson: we don’t want a cell to shrivel or to explode!) and it’s called osmoregulation. Often times, cells must react quickly to osmoregulate. This is where aquaporins come into play.

Aquaporins

You chug a giant soda, or run a marathon and lose a lot of water through sweat. Such drastic situations require your cells to react quickly to maintain correct levels of water and other important molecules. Of course, water can osmose across the cell membrane, but this isn’t always fast enough for cells to stay balanced. This is where aquaporins step in to save the day! 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.

Aquaporins are highly specific and let only water and nothing else pass through. ‘Aquaporin’ is therefore a great name for this protein, since ‘aqua’ means water and ‘porin’ means tunnel. Most cell membranes have many aquaporins, so the cell can react quickly to consistently maintain proper balance within the cell.

Special Functions

Osmoregulation is important to all cells. However, some cells need extra aquaporins.

The cell membranes of kidney cells, for example, are loaded with extra aquaporins, since it’s the job of kidney cells to filter blood and help maintain the right balance of water within blood. Aquaporins are important to other organisms too. Stop for a moment and think about organisms that are constantly submerged in water, like fish. Aquaporins play a critical role in their daily lives so their cells can osmoregulate with the seawater or freshwater environments in which they swim. Plants use aquaporins too. If plant cells don’t have enough water, you’ll end up with a wilted piece of lettuce or a saggy celery stalk.

Lesson Summary

All organisms are made of cells and each cell is protected by a selectively permeable cell membrane. One of the many jobs of these membranes is to regulate what’s allowed to enter the cell and what is allowed to leave the cell. Since water is critical to life, the cell membrane must regulate how much water is coming and going at all times. Although water is able to osmose, or diffuse across the membrane, aquaporins are often used to help speed up the process.

Aquaporins are protein tunnels built into the cell membrane that allow nothing but water to pass through them. Aquaporins are a critical part of osmoregulation for Earth’s many organisms.

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