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.
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.
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.
Cholesterol is another amphipathic molecule, and it takes a different form than phospholipids. Cholesterol consist of a hydrophobic ring structure with a small hydrophilic portion on one side of the molecule. The functions of cholesterol are diverse and range from being precursor molecules to hormones like testosterone and estrogen, to aiding in cell membrane fluidity, by physically taking up space in the cell membrane by creating physical gaps between phospholipids. While cholesterol is necessary for these cellular functions, too much of it can clog your veins and arteries, leading to heart attacks.
Oils vs. Fats
As we discussed earlier, the more saturated a lipid is the more rigid it becomes, and the more unsaturated it is the more fluid it becomes. Oils are for the most part unsaturated lipids because they are able to maintain a liquid state at room temperature. Fats like butter and lard are solid at room temperature because they are more tightly packed together (and thus solid). Generally, the more saturated a fat is, the less healthy it is for you.
Amphipathic molecules are oils and fats, which are known as lips in science. They have both hydrophilic (water loving) and hydrophobic (water-fearing) portions, which means that one side wants to associate with water and the other side does not. Amphipathic molecules found in the body include lipids, which compose the cell membranes of cells, and cholesterol, which is necessary for hormone production and creates fluidity in the cell membrane.
It is important to distinguish between saturated lipids - if each carbon has two hydrogen atoms bound to it with the exception of the one on the end that can have three, and unsaturated lipids, if one or more of the carbons forms a double bond with the carbon in front of it or behind it, because they both play important biological roles and are slightly different in terms of structure. At room temperature, saturated lipids tend to be solid while unsaturated ones tend to be liquid.
Saturated and unsaturated lipids both have their uses. For example, if a cell membrane was only made of saturated lipids, the cell membrane would be come too rigid. While lipids can take many different forms, they share the same basic properties that make them amphipathic. For example, cholesterol is very hydrophobic due to its ring structure, with only a small portion that is hydrophilic and is necessary as a precursor molecule to the hormones in our bodies. So you see, it doesn't just cause heart attacks.
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