Amphipathic Molecules: Definition & Example

Lesson Transcript
Instructor: Joseph Said
This lesson defines what amphipathic molecules are, reveals where they are found in nature, and explains their properties. The lesson will also discuss the biological roles of amphipathic molecules.

Definition of Amphipathic Molecules

Have you even dropped oil into water and watched the oil residues ball up and form little oil droplets? Or have you ever wondered why the big colorful blob of oil in a lava lamp never mixes with the water portion? Why does oil never seem to associate with water?

Oils and fats, which in science are called lipids, are known as amphipathic molecules. These molecules have two distinct ends to them: a water-loving (hydrophilic) side and a water-fearing (hydrophobic) side. While the hydrophilic sides of a lipid will associate with the water in a solution, the hydrophobic sides of the lipid all cluster together to 'hide' from the water. Lipids therefore cluster together and form spheres where the hydrophobic sides are in the center away from the water while the hydrophilic sides are on the outside, associating with the water.

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Amphipathic Molecule Structure

What makes one group of atoms hydrophilic and another hydrophobic? Why can't all molecules associate with water molecules? The ability of the atoms within a group to form hydrogen bonds with the water molecules around them makes them hydrophilic. Oxygen and nitrogen atoms readily form hydrogen bonds with water molecules, so any organic molecules that have oxygen or nitrogen atoms bound to their carbon skeletons will be hydrophilic.

For example, if we take the molecule for cholesterol, we see the OH group on the left is hydrophilic and will form hydrogen bonds with water, while the ring structures, which only consist of hydrogen and carbon atoms, are hydrophobic and will not associate with the water.


Biological Roles of Amphipathic Molecules

Are lipids necessary for biological processes? Absolutely! Without lipids our cells would not even have a cell membrane, which would be like having a house with no walls, floors, or a ceiling. Without lipids it is unlikely that life would exist as we know it.

There are many different kinds of lipids with different functions. Lets start by examining phospholipids, which compose the cell membranes of animals. They form lipid bilayers, with one set of hydrophilic heads facing the exterior of the cell membrane and the other set facing the interior (as you can see on the diagram on screen). The hydrophobic portions of the lipid bilayer - the lipid tails - face towards one another, which allows them to hide away from the water inside and outside of the cell.

lipid bilayer

Saturated vs. Unsaturated Lipids

Have you ever wondered what the difference is between saturated and unsaturated fats when you look at the nutrition facts on the back of your chocolate bar? The tails of lipids, which are hydrophobic, consist of long strands of hydrocarbons. A hydrocarbon is a long strand of carbons bound together in a chain with hydrogen atoms bound to each carbon. Carbon can form 4 covalent bonds, which means that a single carbon can bind up to 4 hydrogen atoms.

However, since we have a chain of them each carbon except the one at the end is bound to a carbon in front of it and a carbon behind it, like so: c-c-c-c-c-c. This takes up two bonding positions. The other two bonding positions are taken up with hydrogen atoms. If each carbon has two hydrogen atoms bound to it with the exception of the one on the end that can have three, we say that the hydrocarbon chain is saturated. If, however, one or more of the carbons forms a double bond with the carbon in front of it or behind it, we say it is unsaturated because fewer hydrogen atoms can bind to the chain.

If the cell membrane only consists of saturated lipids, the membrane can become rigid and stiff, which is not good for the cell. The double bonds in unsaturated lipids cause kinks to form that physically create space between the lipids, which makes the membrane more fluid.

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