In this lesson, we will learn what gives phospholipids a dual personality. How can this molecule be both hydrophobic and hydrophilic, and why is this important to a cell?
Hydrophobic vs. Hydrophilic
You're probably familiar with what happens to a bottle of Italian dressing when it sits on the counter for too long. The oil floats in a layer on top with a watery layer underneath. And if you don't shake it up, you're likely to end up with an oily mess all over your salad.
Oil is a hydrophobic substance. 'Hydro' comes from the Greek word for water, and 'phobic' is literally the fear of it.
Although oil is not exactly scared of water, it does run away from it and huddles together so that the least amount of oil is touching water at any time. This gives you the two separate layers of oil and water that you see in Italian dressing. Anything in that watery layer of the salad dressing is hydrophilic. Hydrophilic means a love of water. It comes from the Greek 'hydro,' for water, and 'philia,' a love for it.
The cell membrane separates the intracellular and extracellular membranes
Of course, hydrophobic and hydrophilic substances are not unique to your salad dressings.
One of the most important examples of a substance that's both hydrophobic and hydrophilic interacting in nature is actually within all of our cells. Our bodies, and all living things on Earth, are made up of one or more cells, the basic fundamental unit of life.
Our bodies are composed of trillions of cells. Some belong to our bodies, like skin cells and heart cells, and some don't, like the single-celled bacteria living in all of our guts. All cells, however, have a few things in common, and one of these things is the need to create a barrier between the outside world and the inside of the cell. Plant, animal, and bacterial cells alike do this by having a cell membrane that separates the intracellular environment from the extracellular environment outside the cell.
Although some components of the cell membrane are different, one of the most uniformly important and conserved components of a cell membrane is an important molecule that has a dual personality - it's both hydrophobic and hydrophilic. Behold a phospholipid.
A phospholipid is named for its two main parts, a phosphate group and a lipid. It's usually drawn with the phosphate group as a circle, and this is referred to as the hydrophilic head. Due to its negative charge, this phosphate group is also polar.
The phosphate group and the lipid are connected by a glycerol group.
Most phospholipids have two lipid tails made of hydrocarbon fatty acids bound to the glycerol by an ester linkage. These hydrophobic tails are nonpolar.
So what do you think would happen if we took a couple phospholipids and threw them into our bottle of Italian dressing?
Birds of a feather flock together, and hydrophobic molecules like to stick together, too. So do hydrophilic molecules. Therefore, the hydrophobic tails of a phospholipid would want to stay with the oil layer of the salad dressing, while the hydrophilic head of the phospholipid would be attracted to the water layer. It would look like the phospholipid was doing a hand stand in the salad dressing.
phosphate group and lipid that make up a phospholipid are joined together by a glycerol group
The Phospholipid BilayerCell membranesphospholipid bilayer
How is a phospholipid bilayer organized in the cell membrane? Let's think about what happened when we dropped a few phospholipids into the bottle of salad dressing.
What would happen if we organized a phospholipid bilayer inside this bottle? What would it look like?
In a bilayer, the hydrophilic heads of the phospholipids will want to be touching water at all times. So the first layer will form a ring with the heads facing water. This leaves the hydrophobic tails touching the inside of the circle. However, the intracellular environment of a cell is aqueous, or water-based. So the tails aren't going to like this. How can we account for it? Well, that's why a second layer of phospholipids is so important.
A second layer of phospholipids forms an inner ring with the hydrophobic tails touching each other and the hydrophilic heads oriented towards the outside and inside of the membrane. This is a phospholipid bilayer, which allows for an aqueous environment both inside and outside the cell but still creates a barrier between the cell and its surroundings.
Two layers of hydrophobic and hydrophilic phospholipids surround a cell
This barrier is both protective and selective against foreign objects, but it's not completely impermeable. In other lessons, we'll discuss methods of transporting molecules across the cell membrane.
The arrangement of the phospholipid bilayer is essential to cell organization and creation of the cell membrane, which separates the intracellular environment from the extracellular environment. Although different organisms can have slightly different phospholipids or other cell membrane components, they share a common structure. The hydrophilic phosphate heads like water, so they touch the inside and the outside of the cell where the environments are aqueous. The fatty acid tails form a hydrophobic region in the middle, which is free of water. This cell membrane helps in structuring the cell and also controls which substances can cross it.
After watching this lesson, you should be able to:
- Differentiate between hydrophobic and hydrophilic
- Describe the structure of a phospholipid and how it behaves
- Understand the structure and function of a phospholipid bilayer