Check your understanding of the structures of lipids. Visit the Dallas Learning Cloud, scroll down the page until you see the following, then click on the Launch Activity bar.

Lipid molecules are slightly more difficult to describe than other macronutrients. They aren’t considered to be polymers because the smaller molecules that are used to put them together don’t repeat in long chains like in carbohydrates or proteins. Instead, lipids are divided into different categories based on similar molecular shape.

 Why is shape important in Biology?

Shape 10. Show or describe how you could group these lipids based on similar chemical structure.

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The examples of unsaturated fatty acids you have seen so far are called cis fatty acids (definition:an unsaturated fatty acid with the carbon chain before and after the carbon-carbon double bond on the same sides of the molecule) because the carbon chain is arranged on the same side of the double bond. As a result, the molecules make a concave shape at the double bond. This kink in the shape is important in the increased fluidity between these fatty acid molecules. There is, however, a chemical method of arranging the double bond such that the molecule makes a zigzag shape. This type of fatty acid is called a trans fatty acid. (definition:an unsaturated fatty acid with the carbon chain before and after the carbon-carbon double bond on opposite sides of the molecule) This video summarizes the differences between these isomers.

 Modeling Lipids

Using the following interactive, build two fatty acids side-by side: palmytic acid, linoleic acid, linolenic acid and oleic acid. Some helpful hints on how to build double bonds for virtual molecules are available in the Resource Room. 

Take a screenshot of the two molecules.

Compare the structure of these fatty acids in a Venn diagram. Just like for other graphic organizers, the comparison should 

• be meaningful;
• be well-organized and easy-to-follow;
• show your understanding of the vocabulary.

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Functions of Lipids

An easy way to remember some of the functions of lipids in cells is with the mnemonic, SHIPS:

• S - Storage and Source of energy 
• H - Hormonal roles 
• I - Insulation 
• P - Protection 
• S - Structural components 
   

Fluid Mosaic Model of Membranes

The amphipathic property of many lipid molecules is interesting, especially when many of these molecules group together. Let’s explore this further now because it’s an important concept for how cells are organized.

The following interactive shows the behaviour of saturated fatty acids. As the fatty acids move about, they take an organized structure that’s called a micelle. (definition:a spherical arrangement of amphipathic molecules, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends facing outward.)

 Micelle Factors

You can change some of the different variables in this interactive: type of solvent, starting location of fatty acid chains, or starting orientation of fatty acid chains. 

Choose one of these variables to change. Record detailed observations. Run the simulation at least 3 times with the exact same setup to account for variations in the random motion of the molecules. Then interpret and analyse your results: describe and explain the effect of the independent variable you chose on at least one dependent variable you recorded.

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Micelles exist in the body for groups of fatty acids, like those stored in adipose tissue. Elsewhere, phospholipids containing two fatty acid chains come together but form a different shape. Phospholipids make a structure called the phospholipid bilayer. (definition:a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends facing outward.) 

Phospholipid bilayers are found in the membranes in the cell. Membranes define the compartments of the cell. These include the cell membrane and cell organelles: the nucleus, endoplasmic reticulum, vacuoles, lysosomes, golgi apparatus, mitochondria and chloroplasts. Membranes help to control what molecules are located on either side of them. These membranes are described best as a fluid mosaic. (definition:A lipid bilayer, interspersed with proteins and carbohydrate-linked molecules.)) The term fluid is appropriate because the different components, especially the phospholipids, can move past each other and freely. It allows membranes to bend and stretch in response to their environment. Imaging tools have helped Biologists put together a generalized model of a typical membrane. A model of a fluid mosaic membrane could look something like this:

The term mosaic is also appropriate because membranes are made of different components. Even within the bilayer of the same membrane, there are different components in inner and outer layers of phospholipids.

Watch the following video to identify five different components that are part of a membrane. Pay close attention to where they’re located within the membrane as well as what their role is in helping a membrane to function properly.