Sunday, August 29, 2021


Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Evaluate comparative models of various cell types with a focus on organic molecules that make up cellular structures.

Article includes ideas, images, and content from Troy Smigielski (2021-08)

(Is that really the best word they could come up with for… anything?!)

Remember all that about carbohydrates being a source of energy in living things? And… if more carbohydrates are consumed than is needed, the excess energy gets stored as fat? Remember that?

If carbohydrates are full of energy and you overeat them and they get stored in fat, what do you think fat has a whole lot of?  Yeah… Energy!

Lipids function to store energy, provide insulation, and are important components of the cell membrane.

Lipids are hydrophobic which means they hate water. Not like in an emotional way. But… you know what it means!

Okay… what about those monomer building blocks? If lipids are a biomolecule and all biomolecules are made up of monomers, then… Lipids must have a monomer. 

The monomer of a lipid is a triglyceride.A triglyceride is one glycerol + 3 fatty acids
(That would make a lot more sense if we knew what glycerol and fatty acids were!)

A picture's worth 1000 words (so, they say)…

The two main elements in the fatty acid chains are hydrogen and carbon.

Structurally, a lipid is a long chain of hydrocarbons. These long hydrocarbon chains partially differentiate lipids from other biomolecules.

Lipids are composed of carbon, hydrogen and oxygen atoms, and in some cases contain phosphorus. Nitrogen, sulfur and other elements also occur occasionally.

The elements forming many common lipids are…

H hydrogen
O oxygen
C carbon

Also, in the special case of cell membranes, P phosphorus


Lipids occur in many places and fall into several main categories.

Examples of lipids are:
  • Fats
  • Oils
  • Cell Membranes
  • Waxes
  • Cholesterol
  • Steroids
  • Some hormones

So… big long hydrocarbon chains… Something about glycerol… Fats… oils… waxes…

And about that hydrophobic thing? What about that?

Fats and Oils are Lipids

So, water is a relatively dense compound of hydrogen and oxygen. (Insert long, complex discussion of electronegativity and such here… you know… basically stuff chemists care alot about.) Therefore, the water molecule is polar.

The lipid molecule is NOT polar.
So, "Lipids are non-polar molecules, which means their ends are not charged. Because they are non-polar and water is polar, lipids are not soluble in water. That means the lipid molecules and water molecules do not bond or share electrons in any way" (source, 2021-08-29).

So, it won't dissolve into water. AND, its structure causes it to take up a lot of space. Since it takes up a lot of space, it is relatively less dense than water. So…

What happens if you mix oil and water? Well, chefs know what happens when you mix oil and vinegar. And vinegar is like water (Insert long, complex discussion of acids here. Or take my word for it.) Oil and vinegar won't mix! Just look at your Italian Dressing bottle, if you don't believe me!

Okay, hang on! Let's go this way… Water is a polar molecule meaning that one "end" has a positive charge and the other a negative charge. So, opposite charges attract. Like when hair sticks to a comb or a yarn cap because of static. In essentially the same way, the positive end of one water molecule sticks to the negative end of another water molecule. In time, all of the water sticks to other water. NOT to the non-polar lips. Since water sticks to water, all of the oil ends up isolated.

So… back to basics! (Yes, please, back to basics!)

Lipids are hydrophobic which means they hate water and will not dissolve in it.

Okay… fun stuff… what else?

Fats In Food

There are two main types of fats in the food what we eat.

Saturated fats only have single bonds and are solid at room temperature. These are typically found in meats, dairy, butter, and cheese.

Sources of Saturated Fats

Unsaturated fats have at least one double bond and are liquid at room temperature. These are typically found in fish, plants, oils, and nuts.

Sources of Unsaturated Fats

Okay, that's great… and… why:

Back to that chemistry stuff, I'm afraid!

The saturated fats have a long, straight structure of single bonds between the H-C-H segments. This makes it tough for the body to break down.

The unsaturated fats have a long structure, too, but at least one of the segments has a double-bond and (because of more complex chemistry stuff), the chain bends. This makes it easier for the body to break down.

Generally, unsaturated fats are "better for you" than are saturated fats.

Lipids in Cell Membranes

The cell membrane functions to regulate what comes into the cell and what goes out of the cell. It is designed to let some things in and some things out. It relies, in part, on special lips to do this.

Phospholipids are a special kind of lipid that makes up the cell membrane.

They are arranged in a bilayer with their hydrophilic (water-lover) heads pointing outward hydrophobic (water-fearing) tails pointing inward.

Woo! There's some big words up there! Definition please!

hydrophilichaving a tendency to mix with, dissolve in, or be wetted by water.

hydrophobictending to repel or fail to mix with water.

So, the phospholipids have one end that is hydrophilic and then long, hydrophobic tails. This arrangement is very important in regulating what can and cannot enter your cells.

Small, hydrophobic molecules can fit between gaps in the phospholipid heads. However, large, charged, or hydrophilic molecules cannot pass through the membrane. 

But, the purpose of the cell membrane is to regulate what goes in and out. If most hydrophilic compounds are unable to pass through, how do they get into and out of your cells? For instance, water can pass in and out of cells as needed to keep everything in balance.

Well, phospholipids are only a part of the cell membrane. The cell membrane is more complex, and some of its parts are designed to allow things to pass through. 

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