Why Does Water Pass Quickly Through the Cell Membrane?

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  • 0:03 Cell Membrane
  • 1:36 Osmosis
  • 2:12 Control of Water Movement
  • 3:17 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Cells have a membrane that helps keep the inside and outside separate. Water moves in and out of the cell so the concentration is equal on both sides. To prevent too much movement, the human body controls water levels in the blood.

The Cell Membrane

The cell membrane helps protect the cell by separating the inside from the outside. It also helps the cell move things in and out. The cell membrane is made of what is called a phospholipid bilayer, which means it has two layers of phospholipids. Phospholipids are special molecules made with a phosphate head and lipid tail, a waxy or fatty organic compound.

The phosphate is hydrophilic (hydro = water, philic = loving), meaning it can dissolve in water. The lipid is hydrophobic (phobic = afraid), meaning it cannot dissolve in water. Have you ever heard the saying oil and water don't mix? Well, oils are a type of lipid, which is why you can't completely mix oil and water.

The phospholipid bilayer has the water-loving phosphates on the outside of the bilayer, and the lipids are sandwiched in between. This allows the phosphates to be exposed to water, either in the cell or the environment, while the lipids are protected from water.

This arrangement means that only small, uncharged molecules can pass through the membrane. The membrane is called semipermeable, meaning that some things can pass through without assistance, while other things cannot. Water is a charged molecule, so it cannot get through the lipid part of the bilayer. In order to allow water to move in and out, cells have special proteins that act as a doorway. These proteins are called aquaporins (aqua = water, porin = pore).


So, why does water move across a cell membrane? Water, like many molecules, wants to be at equilibrium; it wants to have an equal concentration on either side of the membrane. The movement of water across a membrane to reach equilibrium is called osmosis.

Osmosis, for the most part, happens instantaneously. Your cells are constantly working to maintain equilibrium. When the concentration of water on the outside of the cell is greater than the concentration on the inside, water will quickly move into the cell to even up the concentrations. The reverse will happen if there is a lower concentration of water outside the cell.

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