Fats get a bad rap, but cellular components lipids wrap! Lipids include fats, oils, waxes, steroids, & more & they’re like “anti-water” You might have heard that your body’s mostly water, and that’s true. The stuff inside your cells & outside your cells is mostly water, so lots of the molecules in your body are “designed” to get along well with water. But you need “waterproof” barriers to separate the inside from the outside and in this niche lipids reside! 

A lot of the molecules in our bodies like to hang out with water & will happily put on a water-coat (dissolve in water). We call such water-loving molecules HYDROPHILIC. What makes something hydrophilic? Charge or partial charge. You see, water, H2O, might “look” neutral – you don’t see any + or – signs indicating it’s an ion (charged molecule) but its charge is unevenly distributed. 

Molecules are made up of atoms and atoms are made up of charged parts (positive protons and negative electrons) and neutral parts (neutrons). The reason some molecules are charged is that they have uneven numbers of protons and electrons. An excess of electrons gives you a negatively-charged molecule (ANION) – too few electrons and you get a positively-charged molecule (CATION).

Some molecules, like water, have an even number of protons and electrons, so they’re neutral overall, but the electrons like to hang out at certain parts more than others so those parts become partly negative and the other parts, where the electrons spend less time, become partly positive. 

Oxygen is more electron-hogging than hydrogen, so the O in H2O is partly negative and the H’s partly positive. And this creates a charge imbalance called a dipole – and we call molecules with dipoles POLAR. 

Because opposite charges attract, the O will be attracted to positive things – either fully-charged anions or molecules with dipoles (even other water molecules which gives you things like the surface tension that makes water “sticky”)

The nucleic acids we looked at a couple weeks ago (DNA & RNA) are very hydrophilic because they have negatively-charged phosphates in their backbone as well as polar sugars.

Proteins, which are made up of amino acid building blocks have some hydrophilic parts, but they also have some hydrophobic parts – some of the amino acids have hydrophilic side chains (their unique part), making them happy to hang out with water.

But other amino acids have hydrophobic side chains. Hydrophobic molecules are ones that avoid water. Don’t let the name scare you off – water doesn’t even “scare” off these molecules despite the “phobia” in the name. The molecules aren’t really “scared” of water – they’d just rather hang out with anything else. And the water doesn’t want to hang out with them either.

The reason for this is that hydrophobic molecules (or at least the hydrophobic parts of molecules (it’s not all or nothing) don’t have charges (full or partial) so no one stands to gain. There’s no charge attractions possible and water doesn’t want to give up the attractions it can find elsewhere to hang out with something that can’t make it happy. And the hydrophobic molecules don’t want to hang out around charge, so they team up to reduce their contact with water through so-called hydrophobic interactions. 

Electrons move about randomly so if a lot happen to be in one place, they can induce a shift in the electrons in the molecules next to them (which will want to get away from the negativity, leaving the area partly positive & thus attractive to the first) -> leads to a chain reaction of little shifts so you get induced dipole interactions. Lots of temporary, weak attractions that combine to give you a stronger attraction. These are sometimes called van der Waals (vdw) interactions. and you can learn more about them here: http://bit.ly/2C6oQIT

Lipids are largely non-polar because they’re made from chains of carbon and hydrogen, which share electrons pretty fairly. This makes them pretty “boring” as is – they lack so-called “functional groups” that give them “special powers” like enabling them to react and/or combine with other molecules. So, instead of just plain chains, the “starter kit” for a lipid is typically a fatty acid. It’s a hydrocarbon chain with a carboxylic acid (C=O)-OH group stuck onto the end. That carboxylic acid *is* reactive, so now you can make different things from these fatty acids. (you can think of the carboxylic acid kinda like putting the bump on a lego)

If you take pure hydrocarbons & water, they won’t mix – they’ll split into a lipid side & a water side. This is the basis of many organic chemistry extraction techniques because lipid-soluble things will side with the lipid & water-soluble things will side with the water. We take advantage of this sort of thing in RNA extraction/purification: http://bit.ly/2Xj4Zyc

That wouldn’t be very helpful in your body though. Instead, we need things that can get along with both & give molecules a chance to move between them. Molecules with both hydrophilic & hydrophobic parts are called amphiphilic (aka amphipathic) and they include things like soaps, detergent, and phospholipids. 

A common example is surfactants like soap & detergents – these SURFace ACTing agENTS accumulate at the water’s SURFace and ACT to to change the surface’s properties, such as lowering the water’s surface tension (how sticky the water is to other water molecules) – by interfering with these water-water bonds, it gives other molecules a chance to hang out with water -> helps things dissolve

At low concentrations, they’ll spread out throughout the  water, but when the concentration gets high enough, they’ll “team up” to hide from water, creating micelles – spheres with the tails in the middle, away from water, & heads on the water-facing side. A lot of the gunk you want to get off pots & pans is lipid-y – it can get dissolved in the surfactant tails and trapped in the micelle center, allowing you to wash it off. more here: http://bit.ly/2XKuhWs

In order to do this, there needs to be enough of the surfactant molecules to make a sphere and they have to be able to find one another. So the concentration of surfactant helps determine when micelles will form. The “tipping point” concentration is called the Critical Micelle Concentration (CMC)

Phospholipids are the type of amphiphilic molecules that make up cellular membranes. They’re similar to soaps & detergents in that they have hydrophilic heads & hydrophobic tails, but they have 2 tails, and phosphate-containing heads.

They’re made from combining fatty acids with glycerol or sphingosine & phosphoric acid, then modifying the resulting phosphatidic acid to give you different head groups (e.g. ethanolamine, choline, or serine). Because they have 2 tails to coordinate, as well as different heads, it’s harder for them to coordinate w/one another to form little spheres – instead they arrange themselves into bilayer “sandwiches” with the tails in the middle. 

They can still form spheres (in this case called liposomes not micelles) but the center of the sphere, facing the inner layer’s hydrophilic heads, is watery because the inner layers heads face it. So it wouldn’t make a good soap, but it does make a good cell coating!

It’s not just the heads of phospholipids that can vary, they can have different tails, which can influence things like their fluidity. The reason for these differences can involve how “saturated” the chains are. Saturated chains have the maximum number of hydrogens per carbon, whereas “unsaturated” chains can take more – instead of bonding to hydrogen, some of the carbons use the electron they’d usually use to bind to hydrogen to bind to their neighboring carbon even more strongly – a double bond. These double bonds introduce kinks in the chain, so they can’t pack together as tightly, so the lipids are more more flowy.

You might have heard the terms saturated & unsaturated in the context of diets. The “fats and oils” we usually think of are triglycerides, aka triacylglycerols. Like phospholipids they’re made of fatty acids joined to glycerol but instead of using 2 of glycerol’s OH’s to link to fatty acids & 1 to link to a phosphate group, they use all 3 to link to fatty acids -> so they have 3 tails & no “head” that lets them hang with water – they’re really hydrophobic. 

Fats are solids at room temp whereas oils, having more double bonds, are usually liquids. They differ in how long their hydrocarbon chains are & how saturated they are, & this affects their melting point. Longer chains offer more opportunities for hydrophobic interactions whereas double bond kinks disrupt those interactions. The more interactions, the more energy you need to break them up. Heat is a form of energy so the more energy you need to break them up. So longer and/or more saturated -> higher melting point (more likely to be solid at room temp)

You can make oils solid by converting some of the double bonds to single bonds through partial hydrogenation – this is how you get things like Crisco. A “side effect” of this process is that some of the double bonds don’t get fully saturated buy instead switch from cis to trans configuration – basically, they go from being locked in like this: \=/ to being locked in like this \=\.

The chains don’t all have to be the same – “simple triglycerides” have 3 of the same, and “mixed triglycerides” mix & match.

Fats are useful for energy storage & cushioning as well as storing fatty acid “legos” to be used to make all sorts of lipids. 

A few other kinds of lipids are:

  • waxes -> a fatty acid joined to an alcohol (something with an OH). Their hydrophobicness makes them important water repellents for some plants & animals. Some examples are spermaceti, beeswax, & carnuba wax
  • signaling molecules like prostaglandins – hormones with a variety of functions including helping regulate blood pressure & clottiness 
  • terpenes, including isoprene, the building block of natural rubber we looked at the other day
  • steroids – have a 4 fused ring backbone (3 6-sides rings & 1 5-sided). They have various roles including acting as hormones & influencing membrane fluidity (e.g. cholesterol)

Cholesterol is able to influence membrane fluidity because phospholipid membranes are far from “just” phospholipids – they’re more like “mosaics” of phospholipids interspersed with proteins & cholesterol, and things like sugar chains sticking off some places. Cholesterol basically messes up the nicely packed phospholipids, so increases membrane fluidity

Cytoplasmic proteins (the ones that live in water parts of your cells) hide their hydrophobic parts in the center but membrane proteins are opposite – they hide their hydrophilic parts in the center. If they go all the way through the membrane they can form channels & pores to allow passage. Other membrane-friendly proteins don’t go through but stick to one side or the other. These can play important roles in cell signaling because they can detect changes from outside the cell and relay them throughout the cell.

In addition to the membranes surrounding the main parts of our cells, we use membranes to cordon off “rooms” inside of our cells – such as the nucleus, where DNA is held, and mitochondria, where energy is produced. The presence of such membrane-bound compartments differentiates eukaryotes (plants, animals, etc.) from prokaryotes (bacteria & archaea) which don’t have such compartments. 

This has been your weekly Bri-fing from the Bench for the IUBMB – be sure to follow them for great biochemistry from around the world. 

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