Lipids and Membranes for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about lipids and membranes, plus practice questions and answers
(Note: This guide is part of our MCAT Biochemistry series .)
Table of Contents
Part 1: Introduction to lipids and membranes
Part 2: Lipid structures
a) Insolubility of lipids
b) Signaling lipids
c) Structural lipids
Part 3: Cell membranes and components
a) Phospholipid bilayers
b) The fluid mosaic model
c) Major components of the cell membrane
d) Transporters
Part 4: High-yield terms
Part 5: Passage-based questions and answers
Part 6: Standalone questions and answers
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Part 1: Introduction
In the early twentieth century, one of the most common therapies to control seizures in pediatric patients suffering from epilepsy was not a medication but a diet. Minimizing carbohydrates while consuming more fat seemed to alleviate seizures while still providing children the nutrients they needed to properly develop.
Fats, also known as lipids, are relatively simple molecules responsible for a variety of functions in our body, including energy storage and transmitting signals. The diversity of roles that lipids hold often makes them a challenging topic for students during their MCAT prep. Lipids are everywhere in your body—from hormones, cellular structures, and more. Indeed, they are a likely topic to show up on your exam.
In this guide, we will explain everything you need to know about lipids to be successful on the MCAT. At the end, we’ll include some practice passages and discrete questions so you can apply your knowledge in the exact context the AAMC will on test day.
Let’s get started!
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Part 2: Lipid structures
a) Insolubility of lipids
Carbohydrates are characterized by their 1:2:1 ratio of carbon, hydrogen, and oxygen. Amino acids and proteins possess distinctive amine and carboxyl functional groups. Lipids are characterized by their hydrophobic alkyl structures and are primarily composed of carbon and hydrogen atoms.
Below we have testosterone, a signaling lipid, and a triglyceride, an energy-storing lipid.
Figure: Testosterone and a triglyceride.
Although they may look very different, they are both hydrophobic lipids. The hydrocarbon ring structure of testosterone and the hydrocarbon tail of the triglyceride will cause the molecules to be nonpolar and insoluble in polar solvents (such as water). This is a unifying feature of all lipids and has special implications for how lipids are used in the body.
Due to their nonpolar properties, lipids have assumed many different functions within organisms. All cell membranes are composed of lipids (as we will discuss), and many animals use lipids as a form of energy storage. Waxes are protective secretions that serve as a waterproofing compound for many plants and animals. The structure of waxes is characterized by long, alkane chains.
b) Signaling lipids
Signaling lipids are specialized lipids involved in signal transduction pathways, the passing of information between and within cells. These signaling lipids are further divided into two categories: steroids and fat-soluble vitamins.
Steroids are characterized by their four-ring structure, which includes three cyclohexanes and one cyclopentane. As you’d expect, these hydrocarbon rings make steroids nonpolar. The most commonly tested steroid on the MCAT, cholesterol, is one you should be very familiar with. It plays an integral role in our cell membranes (more on this later) and is a precursor for many molecules, including steroid hormones. Steroid hormones are specially secreted by endocrine glands and act as hormones. You can find more information about steroid hormones in our guide on the endocrine system.
Figure: The cyclic backbone of steroids.
Testosterone, the hormone we mentioned earlier, is just one of the many steroid hormones used by your body. Take another look at testosterone’s structure and see if you can identify the rings that characterize steroids.
Terpenes and terpenoids are aromatic secretions following the formula (C₅H₈)₁₁. Terpenes are typically rich in double bonds between carbons, allowing these molecules to undergo cyclization reactions. Squalene is an especially important terpene molecule as it is the biological precursor of steroids in the human body, including cholesterol.
Prostaglandins are an additional class of hormones derived from lipids. In contrast to steroid hormones, which often use cholesterol as a precursor, prostaglandins are derived from arachidonic acid. Prostaglandins contain at least one five-carbon ring.
Figure: An example of a prostaglandin.
Vitamins are nutrients that must be acquired through the diet. They often function as cofactors for enzymes. They can be divided into two categories: water-soluble vitamins and fat-soluble vitamins.
When in excess, water-soluble vitamins will be excreted in the urine, whereas fat-soluble vitamins will be stored in fat tissue. For the MCAT, you should know that vitamins B and C are water-soluble, while vitamins A, D, E, and K are fat-soluble.
c) Structural lipids
Like cholesterol, structural lipids play a crucial role in the cell membrane. These lipids are amphipathic, which simply means they have hydrophobic and hydrophilic regions on the same molecule. Their hydrophobic region is a nonpolar tail (typically composed of alkyl hydrocarbons), while the hydrophilic region is a polar head (such as a phosphate or glycerol group).
Figure: An example of a structural lipid with a hydrophilic head and two hydrophobic tails.
You may recall similar-sounding words with quite different meanings. For instance, an amphoteric compound can react as an acid or a base, and an amphiprotic substance can donate or receive a proton. It’s important not to get these terms confused!
For the MCAT, the two main structural lipids you should be familiar with are phospholipids and sphingolipids.
Phospholipids, or phosphatides, are the primary component of the phospholipid bilayer of the cell membrane. They contain a hydrophilic, polar phosphate head group joined through an ester linkage to a hydrophobic nonpolar fatty acid tail. An important property to keep in mind of these lipids is their degree of saturation. In this context, saturation refers to the number of single bonds a carbon atom has with other molecules.
Saturated fatty acid tails only have single bonds, so every carbon atom is bonded to four other atoms. These fully saturated tails form van der Waals interactions with other saturated fatty acid tails around them. Due to the symmetric, orderly nature of these alkyl chains, they can more easily form a cohesive structure and tend to be solid at room temperature.
Figure: An example of a saturated and unsaturated fatty acid.
Unsaturated fatty acid tails, on the other hand, contain one or more double bonds. These double bonds introduce “kinks” in the structure of the alkyl chain. This makes them less likely to stack and solidify. Thus, at room temperature, unsaturated fatty acids tend to be found in liquid form.
Why is this distinction important? As we will later discuss, the saturation of these fatty acid tails impacts the fluidity of the cell membrane. In general, unsaturated fatty acids are more prevalent in fluid regions of the cell membrane compared to saturated fatty acids.
Glycerophospholipids are an important type of phospholipids. Instead of a single phosphate head group, these contain a glycerol backbone that forms ester linkages to two fatty acids and a polar head group. Sphingolipids have a similar structure to glycerophospholipids—but instead of glycerol, they contain a sphingosine backbone in addition to the typical polar head group and nonpolar fatty acid tail.
Triglycerides, also known as triacylglycerols, are an additional group of lipids, which are composed of a glycerol head group and three fatty acid tails. They are primarily used for energy storage in specialized cells called adipocytes, and feature prominently in metabolism.
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