Fluid mosaic model: cell membranes article
What’s it made up of?
What makes the cell membrane fluid?
- Temperature: The temperature will affect how the phospholipids move and how close together they are found. When it’s cold they are found closer together and when it’s hot they move farther apart.
- Cholesterol: The cholesterol molecules are randomly distributed across the phospholipid bilayer, helping the bilayer stay fluid in different environmental conditions. The cholesterol holds the phospholipids together so that they don’t separate too far, letting unwanted substances in, or compact too tightly, restricting movement across the membrane. Without cholesterol, the phospholipids in your cells will start to get closer together when exposed to cold, making it more difficult for small molecules, like gases to squeeze in between the phospholipids like they normally do. Without cholesterol, the phospholipids start to separate from each other, leaving large gaps.
- Saturated and unsaturated fatty acids: Fatty acids are what make up the phospholipid tails. Saturated fatty acids are chains of carbon atoms that have single bonds between them. This makes them straight and easy to pack tightly. Unsaturated fats are chains of carbon atoms that have some double bonds between them. Double bonds create kinks in the chain, making them not as easy to pack tightly. There are two possible kinks that can occur:
- Cis-unsaturated fats, where both sides of the chain remain on the same side
- Trans-unsaturated fats, where the sides of the chain are opposite from each other
What can go through the cell membrane?
- Small, nonpolar molecules (ex: oxygen and carbon dioxide) can pass through the lipid bilayer and do so by squeezing through the phospholipid bilayers. They don't need proteins for transport and can diffuse across quickly.
- Small, polar molecules (ex: water): This is a little more difficult than the molecule type above. Recall that the interior of the phospholipid bilayer is made up of the hydrophobic tails. It won’t be easy for the water molecules to cross, but they can cross without the help of proteins. This is a somewhat slower process.
- Large, nonpolar molecules (ex: carbon rings): These rings can pass through but it is also slow process.
- Large, polar molecules (ex: simple sugar - glucose) and ions: The charge of an ion, and the size and charge of large polar molecules, makes it too difficult to pass through the nonpolar region of the phospholipid membrane without help.