Biological Membranes on the MCAT

Biological membranes are enclosing tissue that acts as a barrier within or around the cell. These membranes are semipermeable and control the exit and entry of materials from cells.

This guide will introduce biological membranes, key terms and definitions, and things you should remember as you prepare for the MCAT

Let’s get started!

Biological Membranes on the MCAT: What Do You Need to Know?

Biological membranes are covered in the Biochemistry section of the MCAT

Introductory biochemistry accounts for 25% of the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem) and 25% of the Chemical and Physical Foundations of Biological Systems section (Chem/Phys).

It’s hard to predict the exact number of questions about biological membranes that will appear on the MCAT. However, you can expect it to appear in both the Bio/Biochem and Chem/Phys sections.

Important Sub-Topics – Biological Membranes

You might vaguely remember biological membranes and their constituents from your introductory biochemistry courses. Cell membranes separate the internal environment of the cell from the external environment. How the membrane is structured allows it to be selectively permeable to certain materials, protecting the cell from toxic substances.

Biological membranes can be complicated, so let’s break them down into important sub-topics you can concentrate on.

1. Cellular Membrane: Composition and Structure

Cellular membranes are characterized by having a phospholipid bilayer. This bilayer refers to two membrane layers of phospholipids arranged where the fatty acid tails are facing inward, creating a hydrophobic core, and the phosphate heads face the polar extracellular & intracellular environment. 

The amphipathic characteristics of the phospholipids allow the cell membrane to form spontaneously, reducing the energy and increasing the entropy of the system!

The cell membrane itself has numerous components. However, for the MCAT, you will be focusing on the main three – phospholipids, proteins, and cholesterol:

Term

Definition

Phospholipids 

The most prominent phospholipids in the cell membrane are glycerophospholipids, which contain 2 hydrophobic fatty acid tails and 1 polar phosphate head attached to a glycerol backbone. 

Proteins

Can be divided into 1) integral proteins and 2) peripheral proteins. Integral proteins have interactions with the hydrophobic core, while peripheral proteins only interact with the phosphate heads on the cell surface. 

Cholesterol

Because of the hydroxyl group on its structure, cholesterol aligns similarly to how phospholipids orient themselves in the membrane. It also has important implications when it comes to regulating membrane fluidity.

To achieve cell communication and proper structural integrity of the membrane, cellular membrane junctions are required. Three main types of cell junctions exist:

Type of Cell Junction

Description

Gap Junction

These are essentially channel proteins between the cells. Gap junctions are utilized for instant cell communication.

Tight Junction

These connections are very rigid and tight between cells to prevent the passage of unwanted particles and agents between cells!

Desmosomes

These connect neighboring cells and also serve as attachment points for the cell’s intermediate filaments. When intermediate filaments are connected, this aids in maintaining structural integrity in areas of high mechanical stress!

For more in-depth content review on compostion and structure, check out these detailed lesson notes created by top MCAT scorers. 

2. Cellular membrane: Dynamics

The cell membrane is dynamically moving. It can be defined by its semipermeability and fluidity.

This semipermeability is due to the phospholipid bilayer. These phospholipids have the capacity to move within the membrane through intramembrane and intermembrane movement.

Intramembrane movement refers to the lateral movement of the phospholipids, whereas intermembrane refers to the swapping of phospholipids between layers, which usually requires enzymatic aid.

  • Flippase: (out → in)
  • Floppases: (In → out)
  • Scramblases: (out, in → in, out)

Allows for strict regulation of what particles/agents are allowed entry to the cell. While usually associated with proteins, the cell membrane itself is also a mechanism in regulating what enters into the cell. Nonpolar molecules have a much easier time crossing the membrane because of the favorable interactions that occur within the hydrophobic membrane core.  Polar molecules have a much harder time crossing and usually require the use of transport proteins. 

Fatty acid saturation and cholesterol also play a major role in membrane fluidity. Depending on whether fatty acids lack or contain a double bond within the long hydrocarbon chain, has an impact on the membrane’s fluidity:

Unsaturated FA (“kink” present): This prevents fatty acid stacking resulting in INCREASED membrane fluidity

Saturated FA (no “kink” present): This promotes fatty acid stacking resulting in DECREASED membrane fluidity

Cholesterol also has a dual function in regulating membrane fluidity, depending on the temperature. In high temperatures, cholesterol’s rigid structure limits phospholipid movement. The steroid ring interacts with and partly immobilizes the regions of the phospholipid fatty acid chains that are closest to the polar head groups. Overall, this decreases cell membrane fluidity. In lower temperatures, the ring structure sterically hinders fatty acid stacking, which increases membrane fluidity.

For more in-depth content review on membrane dynamics, check out these detailed lesson notes created by top MCAT scorers. 

3. Cellular Membrane: Membrane Transport

Membrane transport is essential to receive the necessary nutrients, compounds, and substances that the cell needs to survive and thrive.

Membrane transport of proteins can be categorized in 2 main ways:

Description

Example

The exergonic, spontaneous  movement of substances DOWN their concentration gradient. It can be further subdivided into simple diffusion (no protein required) and facilitated diffusion (protein required).

Osmosis

A special type of passive transport which involves the movement of water molecules to areas of high solute

concentration (i.e. high osmotic

pressure/gradient)!

The endergonic, non-spontaneous movement of substances UP and AGAINST their concentration gradient. Further subclassed into primary (1˚) & secondary

(2˚) active transport, differing in energy source.

Primary (1˚) Transport 

Involves the DIRECT use of an energy source, often in the form of ATP. 

Secondary (2˚) Transport: Coupled Transport

Involves the INDIRECT use of energy that has been released from the flow of another substance down its concentration gradient.

There is also vesicular transport which involves the creation or fusion of vesicles with the cell membrane. The vesicles are enveloped with the phospholipid bilayer membrane, which allows for vesicle creation or fusion. There are three types of vesicular transport:

  • i. Exocytosis: Exiting of substances from cell
  • ii. Endocytosis: Entering of substance into cells
  • iii. Pinocytosis: Entering of fluids into cells

For more in-depth content review on membrane transport, check out these detailed lesson notes created by top MCAT scorers. 

Key Terms and Definitions – Biological Membranes

Here are some of the more important key terms and definitions to remember for this general guide to Biological Membranes!

Term

Definition

Fluid Mosaic Model

Describes the cell membrane as a combination of phospholipids, cholesterol, and proteins

Hydrophobic

Lacking affinity for water, insoluble in water

Hydrophilic

Strong affinity for water, soluble in water

Phospholipids

Major membrane lipids, composed of a hydrophilic head and two hydrophobic tails

Amphiphilic

A chemical compound possessing both hydrophilic and hydrophobic properties

Cholesterol

A sterol amphiphilic lipid, important in the composition of the cellular membrane

Additional FAQs – Biological Membranes on the MCAT

What is Considered a Biological Membrane?

A biological membrane is a structure consisting of a bilayer of lipid molecules. A biological membrane acts as a selectively permeable membrane that separates the interior of the cell from the exterior environment.

What are the Three Main Functions of Biological Membranes?

Cellular membranes have numerous functions. However, there are three main functions of a biological membrane. Firstly, to keep toxic substances outside of the cell. Secondly, these cellular membranes have specific receptors and channels to regulate the transport of substances, such as nutrients and ions, in and out of the cell. Lastly, cellular membranes act to receive chemical messages from other cells.

What are Phosphatides MCAT?

Phosphatides, also known as phospholipids, are the primary component of the phospholipid bilayer of the cell membrane.

What are the Three Types of Biological Membranes MCAT?

Three different types of lipids give rise to biological membranes. These are phospholipids, cholesterols, and glycolipids.

Additional Reading Links – Study Notes for Biological Membranes on the MCAT

For more in-depth content review about biological membranes on the MCAT, check out these detailed lesson notes created by top MCAT scorers!

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