#8- The Macromolecules of Life (IV): Lipids

Fats are important, too. There I said it!

You have reached the final, yet equally essential macromolecule in every living organism! Good job! 🙌👏

In this post, I will discuss what lipids are, the varieties of lipids, a bit on the chemical structures, and their importance which most people tend to miss when going on a diet. Don’t get me wrong, I agree that it is good to stay fit and active, but eating a little bit of everything is good for our bodies, too. Most importantly, don’t go fat-shaming and being fatphobic, like c’mon, have a heart.

Now let’s go check out some lipids!

What are Lipids?

These fellas are a class of diverse macromolecules that do not dissolve in water but are readily soluble in other lipids and organic chemicals like, ethanol, diethyl ether, and chloromethane to name a few. They are composed of carbon (C), hydrogen (H), and oxygen (O). When compared to carbohydrates, however, their quantity of oxygen is low in number.

Fun Stuff!: Using just water to wash your hands, you will see that the oiliness doesn’t leave until you add some soap. When I was in school and carried hand sanitiser gel (2009 onwards), I found that the gel removed all the oiliness from my hand which was, and is, pretty cool! And you know what? The gel’s major ingredient was 70% ethanol, an organic compound.

The four different types of lipids present in nature: cholesterol, fatty acid, triglyceride, and phospholipid. Source in caption.
Figure 1- The skeletal formula of the four different types of lipids present in nature: cholesterol, fatty acid, triglyceride, and phospholipid. Source- By Lmaps at the English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4304232

The above figure shows the main types of lipids that we will look into in detail (not much for phospholipids since I described them in Post #1).

Before I go any further, you must be wondering how I was able to read the number of carbon atoms in the skeletal formula style in my previous posts. It’s simple really, you just count them at the corners, every bent angle, and the ends of the line. For example, the fatty acid (See Fig. 1) has 18 carbon atoms, including the one in the — COOH group. Now give it a go with the other molecules in this post (answers at the end!), my previous posts, and the ones coming soon 😄

Do give your answers at the end of this post, even if you got them wrong. You will get the hang of it 💪

Time to see the triglycerides and fatty acids!

Triglycerides and Fatty Acids

Formation of a triglyceride between the three hydrogens in glycerol’s  — OH groups and a part of the  — COOH groups in three separate fatty acid chains creating ester bonds after condensation. It is reversed by hydrolysis when water is added. Source in caption.
Figure 2- Formation of a triglyceride between the three hydrogens in glycerol’s — OH groups and a part of the — COOH groups in three separate fatty acid chains creating ester bonds after condensation. It is reversed by hydrolysis when water is added. Source- Advanced Biology, by Kent, 2nd Edn.

Alright, you just found out what a triglyceride really, or somewhat, looks like in Figure 1, but allow me to introduce them to you more properly. Triglycerides, or triglycerols, as the chemistry nerds like to call them, are formed by the combination of fatty acids connected to a single glycerol backbone via condensation to form an ester bond (See Fig. 2).

Glycerol is a molecule with three carbon atoms bonded to an alcohol group.

Fatty acids consist of a long hydrocarbon chain and a — COOH group which loses a proton in water, hence putting the acid in fatty acid.

Easy to Remember!: Anything with ‘-ol’ suffix is an alcohol (e.g. ethanol), and a compound with ‘-ic acid’ suffix is an organic acid, like, acetic acid, which is just your good ol’ vinegar.

Triglycerides are split into two based on their physical states, which you mostly find in the kitchen:

  • Oils: Liquid at room temperature, like olive oil
  • Fats: Solids at room temperature, like butter

While looking closely at the fatty acid chains in Figure 2, you notice that there are a few ‘n’ in subscript, which means that the chains can be identical in length, have varying lengths, contain different bonds, and many more. The combinations are limitless like the stuffing for your sandwich at Subway. This brings us to fatty acids in the following paragraph.

As mentioned earlier, fatty acids contain a hydrocarbon chain and a carboxyl group (See Fig. 1). They appear in two distinct forms, saturated and unsaturated fatty acids.

  • Saturated: Absence of double bonds between carbon atoms in the hydrocarbon chain; only C — C; allows close packing of triglycerides; tend to be solid fats
  • Unsaturated: Presence of double bond(s) between carbon atoms in the hydrocarbon chain; only C=C
The cis- and trans-forms seen in a hydrocarbon. In cis-form, the — CH₂ are on the top and H groups are at the bottom. In trans-form, the groups are not on the same side. Source in caption.
Figure 3- The cis- and trans-forms seen in a hydrocarbon. Source- Advanced Biology, by Kent, 2nd Edn.

Unsaturated fatty acids are further split into two based on the number of C=C bonds — monounsaturated, and polyunsaturated (2 or more C=C) fatty acids. The atoms around the C=C bonds can be arranged in either cis- or trans-forms that can affect the shape of the chain and also the physical state of that triglyceride (See Fig. 3).

When a triglyceride has many unsaturated cis-fatty acids, it will be an oil since the C=C bonds cause a bend, or kink, in the hydrocarbon chain that stops them from packing closely as they share weak bonds. Therefore, all the vegetable oils appear as liquids at room temperature, except for coconut oil, as far as I know (give me a simple reason why coconut oil is like this in the comments section 👀)

On the other hand, triglycerides containing plenty of unsaturated trans-fatty acids will remain solid at room temperature. This is due to the absence of C=C bonds that make the chains straighter, and perfect for being compact, almost like the saturated fatty acids.

You now understand what triglycerides and fatty acids are, and now you shall see why they are important:

  • High source of energy due to more hydrogen than in carbs
  • Can be stored as energy reserve under the skin of mammals as adipose tissue (See Fig. 4)
  • Good insulators for mammals in harsh, cold climates
  • Absorbs shock, for example, protecting your vulnerable kidneys from getting bashed in
  • Provides buoyancy for unicellular aquatic organisms in the form of oil droplet
A cute Baikal seal native to Siberia, Russia, looking its absolute best, has plenty of blubber under its skin to protect it from the freezing cold weather and water. Source in caption.
Figure 4- The cute Baikal seal native to Siberia, Russia, looking their absolute best, has plenty of blubber under its skin to protect it from the freezing cold weather and water. Source- https://otlibrary.com/baikal-seal/

Moving on to phospholipids!

Phospholipids

The term phospholipids sure does sound familiar if you have read Post #1. If not, do click on the link to know their basic structure and functions in a cell membrane.

As a quick refresher, phospholipids are a major part of cellular membranes and they have these essential components given below (See Fig. 1):

  • Glycerol backbone + Phosphate group + Choline group = Hydrophilic head
  • Two fatty acid chains = Hydrophobic tails

The ability to have dual polarities makes them amphipathic in nature.

The membranes of many animal cells contain the five important phospholipids — Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and sphingomyelin. They make up around 50% of the lipids in plasma membranes and their composition varies based on the type of cell you are looking at (See Table 1).

Other than the plasma membrane, phospholipids like sphingomyelin are also found in the myelin sheaths of neurons for insulating their long axons and amplifying the speed of neural impulses (See Fig. 5).

A simple diagram depicting the typical components of a neuron. In this post, pay attention to the long axon covered by the myelin sheath. Source in caption.
Figure 5- A simple diagram depicting the typical components of a neuron. In this post, pay attention to the long axon covered by the myelin sheath. Source- By Jennifer Walinga — https://opentextbc.ca/introductiontopsychology/chapter/3-1-the-neuron-is-the-building-block-of-the-nervous-system/, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=97847412

Signalling a Death Wish!: When a cell wants to die, a.k.a., undergo apoptosis, it translocates phosphatidylserine from the inner leaflet of the plasma membrane to the outside so it can get killed by phagocytosis.

Cholesterol

Oh yes! Cholesterol, the one that gets the bad rep when someone says, “Don’t eat the egg yolk because it has cholesterol!”. Honestly, “Eating too many things that are good for you can be bad” is the best way to tell someone to cut back on certain foodstuffs rather than trying to tell them to eliminate it completely from their diet. Well, eggs definitely have cholesterol, but they are super essential for strengthening animal plasma membranes and serves as a raw material for the production of vitamin D.

The myelin sheath of neurons not only has sphingomyelin and other kinds of lipids, but it also has plenty of cholesterol which are indeed important for keeping neurons healthy and functional.

To know a tiny bit more about cholesterol, see Post #1!

Now back to the facts.

Cholesterol and other sphingolipids (sphingomyelin and glycolipids) cluster on the surface of the plasma membrane to form semisolid regions called lipid rafts. These structures partition the membrane to create functional domains where proteins can interact to carry out cell signalling, cell movement, and endocytosis.

But Wait! There’s More

I mentioned the four types of lipids and kept it as simple as I could, but I wanted to mention two more which you most likely know.

  • Waxes: Solid at room temperature; waterproofing ability; instead of glycerol backbone like the triglycerides, it has a large alcohol molecule; examples: some leaves of plants, insect shells, beeswax, and the natural wax ingredient in some hair-styling products
  • Steroids: Have four rings made with carbon and hydrogen exactly like cholesterol (See Fig. 1) and can possess varying side chains; examples: oestrogen and testosterone, and many other animal hormones

Don’t forget! All these lipids come from your diet so make sure you have a good healthy meal 👍

Approximate lipid composition in membranes of liver cell (hepatocyte), red blood cell (RBC/erythrocyte), myelin of neurons, mitochondrion, endoplasmic reticulum (ER), and Escherichia coli. Source in caption.
Table 1- Approximate lipid composition in the membranes of the liver cell (hepatocyte), red blood cell (RBC/erythrocyte), myelin of neurons, mitochondrion, endoplasmic reticulum (ER), and Escherichia coli. Source- Molecular Biology of the Cell, by Alberts et al., 6th Edn.

Phew! This marks the end of the Macromolecules of Life series. Fab work for reading all my posts so far 🙌 You now know what these four molecules look like and how they make every living organism thrive amazingly, yourself included. Without these, all life will cease to exist!

Once again, fats are essential for our bodies to make everything in there work perfectly, from maintaining healthy neurons and plasma membranes to protecting your major organs. Always have a little bit of everything and don’t cut out carbs and fats just because they are “bad”. They are super important and are only bad if you have too much of it. One must eat in moderation and be active, like taking a walk around your house and doing your chores, walking to the market, or even doing a solid workout. Do whatever makes you comfortable to remain healthy and active. Stop believing in those silly fad diets promoted by celebrities and influencers. It will only do more harm than good for you! That’s all I ask. Believe in real science and not the pseudo (fake) stuff.

Thank you for reading, and make sure to clap, share, and follow/subscribe for a weekly dose of simplified microbiology! 😄

Glossary

Hydrocarbon- A combination of both hydrogen and carbon atoms only

Proton- A positively-charged hydrogen atom which lost its only electron; sometimes called hydrogen ion

Hydrophilic- Water-loving

Hydrophobic- Water-fearing

Neurons- Cells that work and belong to the nervous system by transmitting electrical impulses

Apoptosis- Process where a cell commits to die by expressing specific signals to get itself eliminated

Phagocytosis- Modification of cell membrane to ‘eat’ an outside material or even pathogens

Endocytosis- Process of bringing stuff into the cell; includes phagocytosis, pinocytosis (drinking), and receptor-mediated endocytosis

Sources

  • Chapter 2: The Chemicals of Life, Section 2.8: Lipids. From the textbook Advanced Biology, by Michael Kent, 2nd Edn.
  • Part III: Cell Structure and Function, Chapter 15: The Plasma Membrane, Section 15.1: Structure of the Plasma Membrane. From the textbook The Cell: A Molecular Approach, by Geoffrey M. Cooper, 8th Edn.
  • Me remembering stuff from my school and university lectures

Answers (In order of appearance)

  • Cholesterol: 27 C
  • Triglyceride: 54 C
  • Phospholipid: 42 C

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Treveni Mukherjee

A University of Leeds alumna with an Integrated Masters degree in Microbiology excited to simplify the subject for the curious and also for students ✌😄