How Lipids Form Cell Membranes


By Robert HazenGeorge Mason University

Lipids are a great type of biological molecules, which include fats, oils, and waxes, and are characterized by their insolubility in water. That’s a property which makes them ideally suited to form cell membranes. This diverse group also finds use in cells as energy-storage molecules. They’re sometimes used as hormones that regulate the activity of organs. They are also light-absorbing pigments, and other specialized functions that are performed by lipids.

Structure of phosphatidyl-ethanolamine, a lipid
Lipids are primarily hydrocarbons. (Image: Nothingserious/Public domain)

The Simplest Lipids

Lipids are made primarily from carbon and hydrogen, with a little bit of oxygen. In other words, they’re primarily hydrocarbons. The simplest of the lipids are called fatty acids; they have a hydrocarbon backbone, from 4 to 36 carbon atoms long. At one end, instead of being terminated by a carbon with 3 hydrogen atoms.

This forms an interesting problem, because you now have one end of the molecule where there’s a carbon and 3 hydrogens, and that’s repelled by water. But the other end of the molecule has an O sticking off it, and an OH group sticking off it, and those groups are attracted to water. You form a bipolar molecule, a molecule in which one end is attracted to water, and the other end is repelled by water.

Saturated Fats

In some fatty acids, the chain of carbon atoms has only a single C‑C bond, and the fatty acid is said to be saturated. Because these are straight molecules, the molecules of saturated fatty acids pack very well together. They have a relatively high melting point, a relatively high boiling point, so they provide thickeners in many fatty foods, such as candy bars. 

You’ve probably seen the various kinds of vegetable shortenings, and so forth, in which you have a thick material that basically thickens whatever you’re cooking. A material like this will typically have 30% or more saturated lipids, saturated fats. 

Other fatty acids may have one or more double C=C bonds. Remember, you can have a carbon-carbon bond in which two pairs of electrons are shared, and that causes a kink in an otherwise straight chain. When you put a double bond in, it suddenly has that kink. As a result, these molecules don’t pack together nearly as efficiently, and they have higher melting and boiling points. 

A typical example would be vegetable oil. A vegetable oil, which is liquid and can be used in cooking in that regard, typically only has 10% saturated fats, a much higher percentage of unsaturated fats, which gives it that liquid characteristic.

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How Do Fat Cells Work?

In our bodies, and those of many other organisms, fatty acids provide the building blocks for fat cells. These are energy-storing cells. They also provide insulation in many organisms. 

Fat cells are like tiny fuel-storage tanks filled with lipids, which is basically hydrocarbon energy. The energy content of these lipids, indeed, is very similar to that of fossil fuels. If you consume more calories, more energy, than you need, some of that extra energy is going to go into making fat cells, and that’s how you store energy. 

That’s what happens when you overeat, or if you have a metabolism that tends to produce fat cells spontaneously. On the other hand, if you consume fewer calories than you need, some of the extra fat in your body is going to be burned to produce CO2 and water and energy. 

Just like the first law of thermodynamics—the total amount of energy going into the system is equal to the amount of energy going out. That’s why, when you exercise and diet, you can lose weight. Some of that fat-cell energy is going into the energy that you use to run your body.

A Cornerstone of Cell Membranes

When we talk about lipids, we have to look at phospholipids, a vital class of lipids that are used to manufacture cell membranes. A typical phospholipid molecule looks a bit like a bobby pin. It’s got two long fatty-acid units coming off one end, and the other end is rounded. 

The cell membrane
Cell membrane can somehow be likened to a liquid crystal structure. (Image: Dhatfield/Public domain)

The exposed ends of the fatty acid are hydrophobic, that is, they have CH3 groups; but the rounded end incorporates a phosphate group, which likes to have water around. Here we have a long molecule that has one end that likes water, and the other end that doesn’t like water. When a large number of these phospholipid molecules are placed in water, they spontaneously rearrange themselves to reduce their energy. 

Remember, energy is reduced by placing the hydrophilic end close to water, and the hydrophobic end as far away from water as possible. So how would you do it? The solution is to form a double layer of lipids, a lipid bilayer, with the hydrophobic ends facing inward towards each other, and the hydrophilic ends, that like water, on the outside. This arrangement efficiently separates the inside of the cell from the outside. 

A cell membrane is something like a liquid crystal: you have all of these molecules, which themselves are not in an exact crystalline arrangement, but they all line themselves up. As soon as they’re put into water, they line themselves up in this lipid bilayer, and so they have, in a sense, a 2D arrangement in which the molecules are lined up in a very specific way.

A Brief Example

Let’s take an example, it gives you only a hint at the remarkable complexity and the adaptability of lipids.

Cell membranes have to have places where raw materials can come into the cell and waste molecules are disposed of; these places are called receptors. Deep-sea organisms have to adapt to extremely high pressure, at which normal cells would collapse.

How do you make a cell membrane that can hold receptors, these openings, that won’t be crushed under high pressure? What happens in high-pressure organisms is remarkable. It turns out that deep-sea phospholipids have more C=C double bonds. 

That is, they have more of those kinks that make it harder to focus the cells together. If you have, for example, two matches, they line up nicely next to each other. But if you would like to make them spread farther apart, you can slightly break each match, and then they don’t fit together quite so closely; and so at high pressure, you keep your openings farther apart. That’s how they do it: C=C double bonds, in the high-pressure form.

Common Questions about Lipids

Q: What are lipids?

Lipids are primarily made of carbon and hydrogen, and they include fats, oils, and waxes.

Q: What are fatty acids?

Fatty acids are the simplest of the lipids. A typical example of these lipids is vegetable oil. They also provide the building blocks for fat cells in the human body.

Q: What happens in a person’s body when they overeat?

Consuming more calories results in the body receiving more energy than it needs. Therefore, some of that extra energy will go into making fat cells, where that energy will be stored.

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