MCAT Biology : Neurotransmitters

Study concepts, example questions & explanations for MCAT Biology

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Example Question #51 : Nervous System And Nervous Tissue

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following molecules can be found inside of a cell? 

I. Inositol trisphosphate

II. Protein kinase A

III. Epinephrine 

Possible Answers:

II only

II and III 

I and II 

I only

III only

Correct answer:

I and II 


Inositol trisphosphate (IP3) and protein kinase A are both second messengers in the G protein-coupled receptor pathway. Since they are second messengers, they amplify and transmit the signal inside of the cell. Epinephrine, however is a hydrophilic (impermeable) neurotransmitter and hormone that binds to the receptor located on the surface of the cell. 

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