Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (2023)

Learn key MCAT concepts related to lipids and membranes and practice questions and answers

Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (1)

(Note: this guide is part of ourMCAT BiochemistrySerie.)

Index

Part 1: Introduction to lipids and membranes

Part 2: Lipid structures

a) Insolubility of lipids

b) Signal of lipids

c) structural lipids

Part 3: Cell membranes and components

a) Fosfolipidio bilayers

b) The liquid mosaic model

c) Main components of the cell membrane

d) shipping company

Part 4: High Performance Conditions

Part 5: Passage-Based Questions and Answers

Part 6: Independent Questions and Answers

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Part 1: Introduction

In the early 20th century, one of the most common therapies used to control seizures in pediatric patients with epilepsy was diet rather than medication. Minimizing carbohydrates while consuming more fat appears to ease seizures while providing children with the nutrients they need for proper development.

Fats, also known asLIPIDS, are relatively simple molecules responsible for a variety of functions in our body, including energy storage and signaling. The variety of roles that lipids typically perform makes them a challenging topic for students as they prepare for the MCAT. Lipids are all over the body, from hormones to cell structures and much more. In fact, they are likely to show up on your exam.

In this guide, we 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 provides on test day.

Let's start!

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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 have distinct amine and carboxyl functional groups. Lipids are characterized by their hydrophobic alkyl structures and are composed mainly of carbon and hydrogen atoms.

Next we have testosterone, a signaling lipid, and triglyceride, an energy-storing lipid.

Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (2)

Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (3)

Although they may look very different, both are hydrophobic lipids. The hydrocarbon ring structure of testosterone and the hydrocarbon tail of triglycerides make the molecules nonpolar and insoluble in polar solvents (such as water). This is a common feature of all lipids and has particular implications for how lipids are used in the body.

Due to their nonpolar properties, lipids have taken on many different roles within organisms. All cell membranes are made of lipids (as we will see), and many animals use lipids as a form of energy storage.waxesthey are protective secretions that serve as an impregnating agent for many plants and animals. The structure of waxes is characterized by long chains of alkanes.

b) Signal of lipids

lipid signalthey are specialized lipids involved in signal transduction pathways, the transmission of information between and within cells. These signal lipids fall into two categories: steroids and fat-soluble vitamins.

steroidsthey are distinguished by their four-membered ring structure, which includes three cyclohexanes and one cyclopentane. As expected, these hydrocarbon rings make steroids nonpolar. The most 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 that later) and is a precursor to many molecules, including steroid hormones.steroidsthey are specifically secreted by the endocrine glands and act as hormones. For more information on steroid hormones, seeour guide to the endocrine system.

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Testosterone, the hormone mentioned above, is just one of many steroid hormones used by your body. Take another look at the structure of testosterone and see if you can identify the rings that characterize steroids.

Terpenomiterpenoideare aromatic secretions of formula (C₅H₈)₁₁. Terpenes are usually rich in double bonds between carbons, which allows these molecules to undergo cyclization reactions.squalalIt is a particularly important terpene molecule as it is the biological precursor to steroids in the human body, including cholesterol.

prostaglandinsare another class of lipid-derived hormones. Unlike steroid hormones, which often use cholesterol as a precursor, prostaglandins are derived from arachidonic acid. Prostaglandins contain at least one five-carbon ring.

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vitaminsThey are nutrients that must be ingested through food. They often act as cofactors for enzymes. They can be divided into two categories:water soluble vitaminsmiFettlösliche Vitamina.

In excess, water-soluble vitamins are excreted in the urine, while fat-soluble vitamins are stored in adipose 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 areamphipathic,which simply means that they have both hydrophobic and hydrophilic regions in the same molecule. Its 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).

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You can remember words that sound similar and have very different meanings. For example, an amphoteric compound can react as an acid or a base, and an amphiprotic substance can donate or accept a proton. It is important not to confuse these terms!

For MCAT, phospholipids and sphingolipids are the two main structural lipids you should be familiar with.

phospholipid,ophosphatide,They are the major components of the phospholipid bilayer of the cell membrane. They contain a polar hydrophilic phosphate head group linked by an ester bond to a hydrophobic, nonpolar fatty acid tail. An important property of these lipids is their degree of saturation. In that context,saturationrefers to the number of single bonds a carbon atom has with other molecules.

Crowded fatty acidTails only have single bonds, so each 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 ordered and symmetrical nature of these alkyl chains, they can more easily form a cohesive structure and tend to be solid at room temperature.

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unsaturated fatty acidTails, 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 clump up and solidify. Therefore, unsaturated fatty acids tend to be in liquid form at room temperature.

Why is this distinction important? As we will see later, the saturation of these fatty acid tails affects the fluidity of the cell membrane. In general, unsaturated fatty acids are more abundant in liquid regions of the cell membrane than saturated fatty acids.

glycerophospholipidThey are an important class of phospholipids. Instead of a single phosphate head group, they contain a glycerol backbone that forms ester bonds with two fatty acids and a polar head group.sphingolipidthey are similar in structure to glycerophospholipids, but contain a sphingosine backbone instead of glycerol, in addition to the typical polar headgroup and nonpolar fatty acid tail.

triglycerides,Also know astriacylglycerin,They are an additional group of lipids composed of a glycerol head group and three fatty acid tails. They serve mainly to store energy in so-called specialized cells.adipocytes,and play an important role in metabolism.

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Part 3: Cell membranes and components

a) Fosfolipidio bilayers

As we discussed, phospholipids are amphipathic, which means they have a polar and a nonpolar region in the same molecule. When placed in polar solutions, such as the body's aqueous environment, these structures take on some interesting shapes to minimize the free energy of the interactions.

Since water is polar, the hydrophobic regions of the lipid aggregate while the hydrophilic regions interact with the solvent. This allows structures such as micelles to form.micellesThey are spherical structures composed of a single layer of fatty acids arranged in their lowest energy state. The hydrophobic tails cluster within the bilayer, while the hydrophilic heads protect them from the polar environment.bicelasThey resemble micelles in that they are lipids arranged in their lowest energy state. However, they are more disc-shaped and contain a flat area. Both micelles and rods are used in research to model the cell membrane, but rods tend to be preferred as they more closely mimic natural cell structure.

A cell membrane sees phospholipids in another arrangement: a bilayer. As its name suggests, adouble layerIt is an assembly of two layers of phospholipids. In a bilayer, the polar heads of the outer phospholipid layer continue to interact with the aqueous environment, while the polar heads of the inner phospholipid layer are oriented in the opposite direction to interact with the cytosol. The alkyl tails of both phospholipid layers can create their own hydrophobic interactions and remain protected by the hydrophilic heads.

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Phospholipids may need to be moved across the cell membrane or between layers of a lipid bilayer. Random movement of phospholipids within a single layer can occur spontaneously and will occur over long periods of time. The same applies to the exchange of phospholipids between an outer and inner bilayer.

Special enzymes can be used to speed up the process of exchanging phospholipids between layers (or "booklets").I'm going crazyMoving a phospholipid from an outer leaf to an inner leaf.Failure PhaseMoving a phospholipid from an inner leaf to an outer leaf. As these are energetically demanding processes, ATP is required for the function of these two enzymes.

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b) The liquid mosaic model

While phospholipids are the main component of the cell membrane, the cell membrane contains additional key components, including signaling carbohydrates, transmembrane proteins, and more. Eastliquid mosaic modeldescribes the structure of these additional components within the "sea" of relatively liquid phospholipids that make up the bilayer. Carbohydrates, proteins, and less fluid lipids aggregate to form "rafts" that generally migrate freely around the membrane surface.

c) Main components of the cell membrane

cholesterol

In addition to being a precursor to steroids, cholesterol also plays a crucial role in maintaining cell membrane fluidity and stability. In particular, cholesterol can exhibit different behaviors depending on environmental conditions. At high temperatures, cholesterol limits the movement of phospholipids in the bilayer, which prevents the membrane from becoming excessively fluid and losing its shape. At low temperatures, the rigid structure of cholesterol serves to disrupt the van der Waals interactions between the fatty acid tails. This prevents the membrane from becoming too stiff and prone to tearing.

carb sign

Remember that carbohydrates tend to be hydrophilic structures. They can also bind to proteins on the extracellular surface of a cell to facilitate intercellular signaling.

Carbohydrates may play a crucial role in immune signaling and recognition. For example, blood group antigens (ABOs) are sphingolipids that have different carbohydrate sequences.

transmembrane protein

In the cell membrane, transmembrane proteins span the entire membrane thanks to a region of hydrophobic amino acids. They often serve as channels that can facilitate transport or as receptors involved in signal transduction pathways. G protein-coupled receptors (GPCRs) are just one example of transmembrane proteins that you may encounter frequently.

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Transmembrane channels can be classified into subtypes based on their mode of action.Ligand-gated ion channelsThey are channels that allow the passage of ions into or out of the cell, depending on the binding of a ligand or signaling molecule to a receptor. Just as signaling molecules bind to GPCRs to initiate a signal transduction pathway, signaling molecules can bind to ion channels to induce them to open and allow ion flow.

Voltage Gated Ion ChannelsThey are channels that allow ions to enter or leave the cell depending on the charge present in the cell membrane. An example is voltage-gated ion channels, which open in response to depolarization during an action potential, allowing sodium ions to flow into the cell. (For more information on action potentials, seeour guide to the nervous system.)

membrane associated proteins

Additional proteins can bind or associate with the polar heads of phospholipids. Although they do not span the inner hydrophobic region of the lipid bilayer, they may still play important roles in cell signaling and transport. The receptor portion of a GPCR is an example of a membrane-associated protein.

d) shipping company

As the name suggests, conveyors perform transportation processes. These transporters are usually specialized transmembrane proteins that transport molecules in and out of the cell.

These processes can be classified according to their energy consumption.passive transportoccurs when a molecule moves from an area of ​​high concentration to an area of ​​low concentration and no energy is required. Secondly,Active transportoccurs when a molecule moves from an area of ​​low concentration to an area of ​​high concentration. This process requires energy.

For the MCAT, you must be able to identify and describe subsequent transportation processes.

passive transport

Simple diffusion, facilitated diffusion, and osmosis are different types of passive transport.easy diffusionis the passive transport of a molecule without the aid of a carrier whilefacilitated diffusionuses a transporter to passively move a molecule across the cell membrane. The latter is useful when a molecule cannot cross the semi-permeable cell membrane.

Osmosisis a type of simple diffusion that refers to the movement of water out of a regionbass Resolvedconcentration toohoch ResolvedConcentration. This movement of water can lead to hypotonic, hypertonic, or isotonic conditions within the cell. In human cells, water molecules require the presence ofaguaporinato cross the cell membrane.aguaporinaThey are transmembrane channels that provide a polar environment for the passage of water molecules.

Active transport

As already mentioned, active transport processes are not spontaneous and require energy. However, the exact sources of this energy can vary.primary active transportThe processes obtain their energy through the hydrolysis of ATP. An example of this process is the sodium-potassium pump (also known as NaᐩKᐩ ATPase), which uses ATP to move sodium and potassium ions. Together with passive transporters known as "leakage channels", the NaᐩKᐩ ATPase maintains sodium and potassium gradients and cell membrane potential.

secondary active transportThe processes direct the unfavorable movement of a solute, such as an ion, against its concentration gradient, while simultaneously moving another solute (favorably) down its gradient.symportrefers to a form of active transport where these particles move in the same direction.counter-portalit is another form of active transport where they move in opposite directions.

Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (11)

Lipids and Membranes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (12)

About the Author

vikram shawis head of tutoring services at Shemmassian Academic Consulting. She achieved a perfect MCAT score (528) and brings years of professional tutoring experience to help our students maximize their test scores.

Credit: Sahil Chawla

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Part 4: High Performance Conditions

lipid signal: specialized lipids involved in signal transduction pathways or in the transmission of information between and within cells; These include steroids and fat-soluble vitamins.

Amphipathic:refers to the presence of hydrophobic and hydrophilic regions in the same molecule

Phospholipid:the main component of the cell's phospholipid bilayer; contains a hydrophilic polar phosphate head group attached through an ester bond to a nonpolar hydrophobic fatty acid tail

Triglycerides:lipids composed of a glycerol skeleton and three fatty acid tails; They serve mainly to store energy and play an important role in metabolism.

micelles: spherical structures composed of a single layer of fatty acids arranged in their lowest energy state

Liposomes:an empty spherical vesicle surrounded by a bilayer

fluid mosaic template: describes the structure of cell membrane components within the "sea" of relatively liquid phospholipids that make up the bilayer

passive transport: occurs when a molecule moves from an area of ​​high concentration to an area of ​​low concentration and no energy is required

Active transport: occurs when a molecule moves from an area of ​​low concentration to an area of ​​high concentration and the process requires energy; They can be classified into primary and secondary active transport.

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Part 5: Passage-Based Questions and Answers

ABCA7 is a member of the ATP-binding cassette A (ABCA) subfamily, an evolutionarily conserved group of cellular transmembrane transport proteins. In particular, ABCA7 is believed to play an important role in lipid homeostasis in immune cells.

CD1d is a non-classical major histocompatibility complex protein involved in lipid antigen presentation to invariant natural killer (iNK) T cells, a specialized subpopulation of T cells. iNK-T cells, but also activate downstream pro-inflammatory pathways in cells expressing CD1d. CD1d is present on a variety of immune cells, including thymocytes, hepatocytes, macrophages, and splenocytes. Previous studies have shown that CD1d is located in lipid raft microdomains on the surface of these cells and that disruption of lipid rafts prevents CD1d antigen presentation.

Building on previous knowledge about the role of ABCA7 in immune cell lipid homeostasis, the researchers investigated the relationship between ABCA7 and CD1d antigen presentation. To do this, the researchers use a strain of wild-type mice (B6) and a strain of ABCA7 knockout mice (ABCA7-/-). After isolating specific cells from these mice, the authors stained the lipid rafts using fluorescently labeled cholera toxin B (CTB) or fluorescently labeled anti-caveolin-1 antibodies. CTB is a protein that binds to lipid raft components, while caveolin is a lipid raft component. Using these techniques, researchers generate the following data:

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The researchers also want to investigate whether ABCA7 plays a role in the organization of the lipid raft on the cell surface and whether this affects CD1d antigen presentation. Their experiments produce the following data.

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PREPARER AND RESPONSIBLE: NOWYHED, H., CHANDRA, S., KIOSSES, W. ET AL. ABCA7 ATP-BINDING CASSETTE TRANSPORTER REGULATES NKT CELL DEVELOPMENT AND FUNCTION BY CONTROLLING CD1D EXPRESSION AND LIPID RAFT CONTENT. SCI REP 7, 40273 (2017). SEE THE FULL TEXT OF THE ARTICLE HERE: HTTPS://WWW.NATURE.COM/ARTICLES/SREP40273. ARTICLE IS NOT COPYRIGHTED BY SHEMMASIAN Academic Consulting. DISCLAIMER: SHEMMASIAN ACADEMIC CONSULTING DOES NOT OWN THE ENTRIES PRESENTED HERE. CREATIVE COMMON LICENSE: HTTP://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/. CHANGES HAVE BEEN MADE TO THE ORIGINAL ARTICLE TO CREATE AN MCAT STYLE PASSAGE.

Question 1: Which of the following biomolecules are found in the cell membrane?

I. Kohlenhidrato

II. LIPIDS

third protein

IV. Nucleic acids

A) I alone

B) II and IV

C) I, II, IV

D) I, II, III

Question 2:Based on the information in this passage, what important function of cell biology might lipid rafts directly affect?

A) Signal transduction

B) cell-cell adhesion

C) Translation and Transcription

D) Zellmigration

Question 3: Based on the data presented in Figure 1, how might loss of ABCA7 function affect CD1d antigen presentation on splenocytes?

A) Loss of ABCA7 has no effect on CD1d antigen presentation

B) Loss of ABCA7 enhances CD1d antigen presentation

C) Loss of ABCA7 decreases CD1d antigen presentation

D) Loss of ABCA7 alters the nature of antigens presented by CD1d

question 4:Based on the results presented in Figure 2, what role does ABCA7 normally play in CD1d antigen presentation in the context of lipid raft organization?

A) ABCA7 promotes the localization of CD1d in lipid rafts on the cell surface

B) ABCA7 inhibits the localization of CD1d in lipid rafts on the cell surface

C) ABCA7 has no effect on CD1d localization to cell surface lipid rafts

D) ABCA7 influences the lipid composition of lipid rafts and promotes CD1d localization to lipid rafts on the cell surface

Question 5: Based on the information presented in the passage, what role is ABCA7 likely to play in regulating CD1d antigen presentation on splenocytes?

I. ABCA7 helps transport lipid raft components to the cell surface and affects CD1d antigen presentation

II. ABCA7 controls the assembly of lipid rafts on the cell surface and influences CD1d antigen presentation

third ABCA7 influences CD1d expression on the cell surface and influences CD1d antigen presentation

a) only me

B) Only II

C) I and II only

D) I, II, III

Answer Key for Passage-Based Questions:

1. Option D is correct.Carbohydrates are found on the surface of the cell membrane and are involved in immunological recognition (I). Lipids, such as cholesterol and phospholipids, are the most abundant components of the membrane (II). Proteins such as transmembrane proteins are also very common in the membrane (III). Nucleic acids are practically absent (IV).

2. Alternative A is correct.The passage reads: "Disruption of lipid rafts prevents CD1d antigen presentation." Therefore, we can conclude that lipid rafts can directly affect the signal transduction involved in the antigenic response (option A is correct). In this case, lipid rafts affect signal transduction by preventing CD1d antigen presentation. Not enough information is presented in the passage to support other possible answers (possibilities B, C, and D are incorrect).

3. Alternative C is correct.The data in Figure 1 show that knockout of ABCA7 in splenocytes reduces fluorescence staining. CTB or cholera toxin B binds to components of lipid rafts. This suggests that knockout of ABCA7 in splenocytes reduces the amount of lipid rafts present on the cell surface and thus suppresses CD1d antigen presentation by splenocytes (selection C is correct).

4. Alternative A is correct.The data in Figure 2 show that knockout of ABCA7 reduces colocalization of caveolin-1 and CD1d. As caveolin-1 is a component of lipid rafts, these data suggest that ABCA7 promotes CD1d localization to cell surface lipid rafts (selections B and C are incorrect). There are insufficient data to support the claim that ABCA7 affects the lipid composition of lipid rafts (option D is incorrect).

5. Alternative C is correct.The data in Figure 1 show reduced CTB staining on the surface of ABCA7 knockout splenocytes compared to wild-type splenocytes. Since CTB binds to lipid raft components, these data suggest that there is decreased expression of lipid raft components on the surface of ABCA7-hit splenocytes. Therefore, it is reasonable to conclude that ABCA7 helps transport lipid raft components to the cell surface (I). The data in Figure 2 demonstrate reduced colocalization of CD1d and caveolin-1 on the surface of ABCA7 knockout splenocytes compared to wild-type splenocytes. Since CD1d is a component of lipid rafts, it is reasonable to conclude that ABCA7 controls the assembly of lipid rafts on the cell surface (II). There are no data on the expression of CD1d on the surface of splenocytes; therefore, statement III is false.

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Part 6: Independent Questions and Answers

Question 1: Which of the following cellular metabolic processes would result in the highest?the netenergy gain?

A) Glycolysis of two glucose molecules.

B) β-Oxidation of an unsaturated fatty acid of 20 carbon atoms

C) β-Oxidation of a 20-carbon saturated fatty acid

D) B y C

Question 2: Naᐩ/Kᐩ ATPase supports or performs what kind of transport process?

A) primary active transport

B) Secondary active transport by symporters

C) Secondary active transport through antiports

D) facilitated diffusion

Question 3: What is the fate of excess water-soluble vitamins in the body?

a) They are excreted in the urine.

B) They are stored in adipose tissue

C) They are stored in the lymph nodes

D) They are converted to glucose and used to maintain blood sugar levels.

Question 4: Which of the following statements isNoThis right?

A) Cholesterol is a precursor to steroids.

B) Cholesterol is a precursor of vitamin D

C) Cholesterol maintains the fluidity and stability of the cell membrane

D) Cholesterol aids in immunological and intracellular recognition.

Lipid Practice Questions Solution Key:

  1. Answer option C is correct.Saturated fats lack double bonds. Because they are smaller, oxidation of a saturated fatty acid provides more energy than an unsaturated fatty acid of the same length (choice B is correct). Glycolysis provides little energy (choice A is incorrect).

  2. Answer option A is correct.The Naᐩ/Kᐩ ATPase is the sodium-potassium pump, a common example of primary active transport (choice A is correct). Secondary active transport uses the energy of a particle moving down its concentration gradient to drive the transport of a particle moving up its concentration gradient (options B and C are incorrect). Facilitated diffusion is a type of passive transport (option D is incorrect).

  3. Answer option A is correct.Excess water-soluble vitamins are excreted in the urine (choice A is correct). Excess fat-soluble vitamins are stored in adipose tissue (option B is incorrect).

  4. Option D is correct.Carbohydrates, not cholesterol, can be found bound to proteins on the extracellular surface of a cell to aid in recognition (choice D is correct). The other statements about cholesterol are true (A, B, and C are false).

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