MCAT Biomolecules – Making Sense of Lipids on the MCAT
- Apr 09, 2018
- MCAT Biology, MCAT Blog
- Reviewed By: Liz Flagge
Lipids on the MCAT
Of the four main categories of biomolecules (proteins, carbohydrates, nucleic acids, and lipids), lipids often fall between the cracks. This is understandable, because proteins are absolutely essential as the building blocks of the body (and even their components, amino acids, are the highest-yield biochemistry topic for the MCAT), carbohydrate metabolism is key for understanding how cells produce energy, and nucleic acids are how genetic material is stored. Next to biological standouts like these, it’s no surprise that lipids can be easy to overlook!
That said, lipids are tested on the MCAT, which is we cover them extensively in our online MCAT Course and with private MCAT tutors. You should absolutely expect to see at least a few questions testing them directly, and maybe even a few more where they’re useful background information. In this blog post, we’ll cover some of the general expectations that the MCAT has for your lipid knowledge and point out some sub-topics worth paying close attention to.
As is always the case for biomolecules, the two overarching themes to focus on are structure and function.
In terms of structure, there are four main categories to be familiar with: (1) fatty acids and fatty acid derivatives, (2) cholesterol and its derivatives, (3) eicosanoids, and (4) terpenes and terpenoids.
Fatty acids, cholesterol, and their derivatives are “frequent fliers” on the MCAT—that is, they are core content that you should be closely familiar with. Fatty acids, as exemplified below by palmitic acid, have a polar carboxylic acid head and a long, hydrophobic tail. They can be saturated (as is the case for palmitic acid), meaning that their hydrocarbon tail only contains single bonds, or they can be unsaturated, meaning that at least one double bond is present. As shown below, palmitic acid can be described as (16:0), which is a type of notation that indicates that it has 16 carbons and 0 C=C double bonds.
A molecule of glycerol (a three-carbon structure with three –OH groups) can form three ester bonds with fatty acids, resulting in structures known as triacylglycerols, or triglycerides, as shown below. Triacylglycerols can be modified to form structures like phospholipids (with two fatty acid chains and a phosphate group), which are the major component of the plasma membrane.
Cholesterol is a crucial component of the plasma membrane and is the basis from which several important hormones (known as steroids) are synthesized. Cholesterol and its derivatives, such as testosterone, have a characteristic four-ring structure that you can use to identify them automatically.
Eicosanoids are 20-carbon signaling molecules that have a characteristic 5-carbon ring flanked by long lipid chains. Prostaglandins, a category of eicosanoids, play a crucial role in modulating inflammation. You can immediately recognize them by their characteristic shape:
Finally, terpenes are composed of repeating isoprene (C5H8) units. Many biologically important compounds can be described as terpenes and terpenoids. The MCAT does not expect you to know them thoroughly, but you should be able to recognize the basic pattern.
In the above concise description of the different structures of lipids, we briefly touched on some of their functions. The following functions of lipids are especially important to familiarize yourself with for Test Day:
1. Triacylglycerols and fatty acids are a source of energy that can be stored and released through beta-oxidation, which is a highly productive form of energy metabolism. (In fact, this is why we use fats for long-term energy storage in the body).
2. Phospholipids (modified triglyceride derivatives) are the major underlying structural component of the bilayer plasma membrane of the cell.
3. Cholesterol in the cell membrane modulates its fluidity, increasing fluidity at low temperatures and decreasing it at high temperatures.
4. Cholesterol-derived hormones are known as steroid hormones. Steroid hormones can pass through the cell membrane, binding within the cell to nuclear receptors and affecting gene transcription. This allows them to exert slow-onset, long-lasting effects.
Most of the key functions of lipids turn out to be related to their structure. All lipids have a considerable nonpolar component. This is why hydrophobic molecules can also sometimes be described as lipophilic (= “loving lipids”). Additionally, lipids may have a polar area, making them “amphipathic” (a term used to describe molecules with both polar and nonpolar areas). The amphipathic properties of phospholipids is what allows them to form the bilayer plasma membrane effectively, and the nonpolar nature of steroid hormones is what allows them to pass through that plasma membrane. In contrast, large and polar peptide hormones must interact with the cell via membrane-bound receptors. This is a classic example for the MCAT of how low-level structural properties affect high-level physiological dynamics, and as you study, be sure to keep an eye out for examples like this!
This is far from a comprehensive overview of lipids (an entire chapter of biochemistry is needed for that), but hopefully it has given you a sense of where to start and how to organize your studying. If you’re just getting started with your prep, Next Step offers a free MCAT practice bundle that includes a half-length diagnostic, access to our first full-length practice test, and a demo of our online course. You can sign up for the free practice bundle here. If you’re looking for more comprehensive prep, we also offer one-on-one tutoring programs as well as an online MCAT course. Not sure where to start? Set up a free consultation with one of our experienced Academic Managers. They will go over your prep needs and help you decide what prep options are right for you.
Written by Blueprint MCAT (formerly Next Step Test Prep) MCAT experts.
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