18 3.5 Lipids

Created by: CK-12/Adapted by Christine Miller


Image shows a cheeseburger and fries in a cardboard lunchbox.
Figure 3.5.1 Lipids can be unhealthy if consumed in large quantities.

It glistens with fat, from the cheese to the fries. Both cheese and fries are typically high-fat foods, so this meal is definitely not recommended if you are following a low-fat diet. We need some fats in our diet for good health, but too much of a good thing can be harmful to our health, no matter how delicious it tastes. What are fats? And why do we have such a love-hate relationship with them? Read on to find out.

Lipids and Fatty Acids

Fats are actually a type of lipid. Lipids are a major class of biochemical compounds that includes oils, as well as fats. Among other things, organisms use lipids to store energy.

Lipid molecules consist mainly of repeating units called fatty acids. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids. Both types consist mainly of simple chains of carbon atoms bonded to one another and to hydrogen atoms. The two types of fatty acids differ in how many hydrogen atoms they contain and the number of bonds between carbon atoms.

3.5 Cultural Connection: Fats in Tanning

Image shows a plains first nations rifle guncase made from the hide of a buffalo. It has beadwork and fringes.
Figure 3.5.2 The Plains First Nations used buffalo brains to tan their buffalo hides. These tanned hides where soft, flexible, and waterproof.

Ancient civilizations all over the world have used fats in the hide tanning process.  If raw hides (animal skins) aren’t tanned, they get very brittle and can breakdown.  Tanning results in a hide that is soft, flexible, and resists decay.

One method of tanning is called “brain tanning”.  It’s name is quite self-explanatory — a mixture of boiled animal brains is used to tan a hide.  A type of fat in the brain, called lecithin, is a natural tanning agent.  Once the hide has been rubbed with the brain mixture, it is smoked and then it is ready for use!

Brain tanning is preferred in many cultures because it creates hides which are waterproof and it doesn’t create environmentally harmful byproducts.

Saturated Fatty Acids

In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. All the carbon-to-carbon atoms share just single bonds between them. This causes the molecules to form straight chains, as shown in the figure below. The straight chains can be packed together very tightly, allowing them to store energy in a compact form. Saturated fatty acids have relatively high melting points, which explains why they are solids at room temperature. Animals use saturated fatty acids to store energy.  Some dietary examples of saturated fats include butter and lard.

Diagram shows examples of the shapes of different types of fatty acids. Saturated fatty acids form long straight chains. Monounsaturated fatty acids have a slight curve and saturated fatty acids can have multiple curves or bends.
Figure 3.5.3 Fatty acids can be saturated, monounsaturated, or unsaturated. This affects their state (solid or liquid) at room temperature.

Unsaturated Fatty Acids

In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible. Instead, they form double or even triple bonds with other carbon atoms. This causes the chains to bend (see Figure 3.5.3). The bent chains cannot be packed together very tightly. Unsaturated fatty acids have relatively low melting points, which explains why they are liquids at room temperature. Plants use unsaturated fatty acids to store energy.

Monounsaturated fatty acids contain one less hydrogen atom than the same-length saturated fatty acid chain. Monounsaturated fatty acids are liquids at room temperature, but start to solidify at refrigerator temperatures. Good food sources of monounsaturated fats include olive oils, peanut oils, and avocados.

Polyunsaturated fatty acids contain at least two fewer hydrogen atoms than the same-length saturated fatty acid chain. Polyunsaturated fatty acids are liquids at room temperature and remain in the liquid state in the refrigerator. Good food sources of polyunsaturated fats include safflower oils, soybean oils, and many nuts and seeds.

Types of Lipids

Lipids may consist of fatty acids alone, or they may contain other chemical components, as well. For example, some lipids contain alcohol or phosphate groups. Types of lipids include triglycerides, phospholipids, and steroids. Each type has different functions in living things.


Triglycerides are formed by combining a molecule of glycerol with three fatty acid molecules, as shown below. Glycerol (also called glycerine) is a simple compound known as a sugar alcohol. It is a colourless, odorless liquid that is sweet tasting and nontoxic. Triglycerides are the main constituent of body fat in humans and other animals. They are also found in fats derived from plants. There are many different types of triglycerides, with the main division being between those that contain saturated fatty acids and those that contain unsaturated fatty acids.

Image shows a model of a triglyceride. The glycerol molecule runs vertically along the left, and three saturated fatty acids run out horizontally from each of the three carbons in the glycerol molecule.
Figure 3.5.4 Triglycerides consist of a glycerol molecule (along the left side) with three attached fatty acids (coming off the right side). This diagram shows a saturated fatty acid, the storage form of fat in animals.


In the human bloodstream, triglycerides play an important role in metabolism as energy sources and transporters of dietary fat. They contain more than twice as much energy as carbohydrates, the other major source of energy in the diet. When you eat, your body converts any calories it doesn’t need to use right away into triglycerides, which are stored in your fat cells. When you need energy between meals, hormones trigger the release of some of these stored triglycerides back into the bloodstream.


Image shows a model of a phospholipid molecule. The phosphate group is at the top of the diagram, it is connected to a glycerol molecule below. The phosphate and glycerol molecule are grouped together and enclosed in a red circle. Two fatty acids are hanging below, attached to two neighbouring carbons on the glycerol molecule. The diagram notes that the glycerol/phosphate portion of the molecule is hydrophilic, and the fatty acids are hydrophobic.
Figure 3.5.5 A phospholipid is made up of a phosphate group connected to glycerol, which is connected to two fatty acids.

Phospholipids are a major component of the cell membranes of all living things. Each phospholipid molecule has a “tail” consisting of two long fatty acids, and a “head” consisting of a phosphate group and glycerol molecule (see Figure 3.5.5). The phosphate group is a small, negatively-charged molecule causing it to be hydrophilic, or attracted to water. The fatty acid tail of the phospholipid is hydrophobic, or repelled by water. These properties allow phospholipids to form a two-layered cell membrane, which is also called a bilayer.


As shown in Figure 3.5.6, a phospholipid bilayer forms when many phospholipid molecules line up tail to tail, forming an inner and outer surface of hydrophilic heads. The hydrophyilic heads point toward both the watery extracellular space and the watery inside space (lumen) of the cell.  The hydrophobic fatty acids are nestled in the inner space of the bilayer.

Diagram shows a phospholipid bilayer. It consists of two mats of phospholipids layered on top of one another. The top mat has the hydrophilic heads oriented up, and the bottom layer has the hydrophilic heads oriented down, causing the hydrophobic regions of the two layers to come into contact.
Figure 3.5.6 Cell membranes consist of a double layer of phospholipid molecules.


Image shows a ball and stick diagram of the steroid progesterone. Progesterone consists of four fused carbon rings.
Figure 3.5.7 Progesterone is an example of a steroid.


Steroids are lipids with a ring structure. Each steroid has a core of 17 carbon atoms, which are arranged in four rings of five or six carbons each (pictured in Figure 3.5.7). Steroids vary by the other components attached to this four-ring core. Hundreds of steroids are found in plants, animals, and fungi, but most steroids have one of just two principal biological functions. Some steroids (such as cholesterol) are important components of cell membranes, while many other steroids are hormones, which are messenger molecules. In humans, steroid hormones include cortisone — a fight-or-flight hormone — and the sex hormones estrogen, progesterone and testosterone.

Feature: My Human Body

During a routine checkup with your family doctor, your blood was collected for a lipid profile. The results are back, and your triglyceride level is 180 mg/dL. Your doctor says this is a little high. A blood triglyceride level of 150 mg/dL or lower is considered normal. Higher levels of triglycerides in the blood have been linked to an increased risk of atherosclerosisheart disease, and stroke.

Image shows a blue plate holding yogurt, soy beans, olives, pimentos, chickpeas, flatbread and various other diced vegetables.
Figure 3.5.8 Changing your diet can help keep blood lipid levels healthy.

If a blood test reveals that you have high triglycerides, the levels can be lowered through healthy lifestyle choices and/or prescription medications. Healthy lifestyle choices to control triglyceride levels include:

  • Weight loss: If you are overweight, losing even 5 or 10 pounds (about 2.2 to 4.5 kg) may help lower your triglyceride level.
  • Calorie reduction: Extra calories are converted to triglycerides and stored as fat, so reducing your calories should also reduce your triglyceride level.
  • Reduction in sugars and refined foods: Simple carbohydrates, such as sugars and foods made with white flour, can increase triglyceride levels.
  • Healthier fats: Trade saturated fats found in animal foods for healthier unsaturated fats found in plants and oily fish. For example, substitute olive oil for butter and salmon for red meat.
  • Cut back on alcohol: Alcohol is high in calories and sugar. It has a strong effect on triglyceride levels.
  • Regular exercise: Aim for at least 30 minutes of physical activity on most or all days of the week.

If healthy lifestyle changes aren’t enough to bring down high triglyceride levels, drugs prescribed by your doctor are likely to help.

3.5 Summary

  • Lipids are a major class of biochemical compounds that includes oils and fats. Organisms use lipids for storing energy and for making cell membranes and hormones, which are chemical messengers.
  • Lipid molecules consist mainly of repeating units called fatty acids. Depending on the proportion of hydrogen atoms they contain, fatty acids may be saturated or unsaturated. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.
  • Types of lipids include triglycerides, phospholipids, and steroids. Each type consists of fatty acids and certain other molecules. Each also has different functions.
  • Triglycerides contain glycerol (an alcohol), in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
  • In addition to fatty acids, phospholipids contain phosphate and glycerol. They are the main component of cell membranes in all living things.
  • Steroids are lipids with a four-ring structure. Some steroids (such as cholesterol) are important components of cell membranes. Many other steroids are hormones. An example of a human hormone is cortisone, which is the fight-or-flight hormone.

3.5 Review Questions

  1. What are lipids?
  2. Compare and contrast saturated and unsaturated fatty acids.
  3. Identify three major types of lipids. Describe differences in their structures.
  4. How do triglycerides play an important role in human metabolism?
  5. Explain how phospholipids form cell membranes.
  6. What is cholesterol? What is its major function?
  7. Give three examples of steroid hormones in humans.
  8. Which type of fatty acid do you think is predominant in the cheeseburger and fries shown above? Explain your answer.
  9. Which type of fat would be the most likely to stay liquid in colder temperatures: bacon fat, olive oil, or soybean oil? Explain your answer.
  10. Why do you think that the shape of the different types of fatty acid molecules affects how easily they solidify?  Can you think of an analogy for this?
  11. High cholesterol levels in the bloodstream can cause negative health effects. Explain why we wouldn’t want to get rid of all of the cholesterol in our bodies.

3.5 Explore More

Cortisone and Healing – An overview of the science, by Sportology and OrthoCarolina, 2015

What is fat? – George Zaidan, TED-Ed, 2013


Figure 3.5.1

cheeseburger by kayleigh harrington on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.5.2

Buffalo_Hide_Beaded_Guncase by Unknown onWikimedia Commons, is used under the Missouri History Museum‘s MHS Open Access Policy. Image is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.5.3

Fatty acids by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 3.5.4 

Fat_triglyceride_shorthand_formula by Wolfgang Schaefer on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.5.5

Phospholipid_Structure by OpenStax on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.5.6

Phospholipid_Bilayer by OpenStax on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.5.7

Progesterone, 5alpha-Dihydroprogesterone 3D ball by Jynto is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 3.5.8

Healthy plate by Edgar Castrejon on Unsplash is used under the Unsplash License (https://unsplash.com/license).


Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.2. Phospholipid structure [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.3. Phospolipid bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Mayo Clinic. (n.d.). Arteriosclerosis / atherosclerosis [online article]. https://www.mayoclinic.org/diseases-conditions/arteriosclerosis-atherosclerosis/symptoms-causes/syc-20350569

Mayo Clinic. (n.d.). Heart disease [online article]. https://www.mayoclinic.org/diseases-conditions/heart-disease/symptoms-causes/syc-20353118

Mayo Clinic. (n.d.). Stroke [online article]. https://www.mayoclinic.org/diseases-conditions/stroke/symptoms-causes/syc-20350113

Sportology/OrthoCarolina. (2015, February 26). Cortisone and healing – An overview of the science. YouTube. https://www.youtube.com/watch?v=zqSoyaDu4b0&feature=youtu.be

TED-Ed. (2013, May 22). What is fat? – George Zaidan. YouTube. https://www.youtube.com/watch?v=QhUrc4BnPgg&feature=youtu.be



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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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