MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
The content in this chapter should be relevant to about 6% of all questions about general chemistry on the MCAT.
This chapter covers material from the following AAMC content categories:
1D: Principles of bioenergetics and fuel molecule metabolism
3A: Structure and functions of the nervous and endocrine systems and ways in which these systems coordinate the organ systems
4C: Electrochemistry and electrical circuits and their elements
5E: Principles of chemical thermodynamics and kinetics
The mitochondria are powerhouses of energy. Their primary purpose is to manufacture a deliverable and usable form of energy. By now, you are well aware of the complex processes by which the potential energy in the chemical bonds of carbohydrates, amino acids, and lipids is converted into the potential energy of the phosphate bond in adenosine triphosphate (ATP). ATP is then delivered to different parts of the cell, where it is used to energize most of the processes essential to the maintenance of life.
The mitochondria generate tremendous amounts of ATP—in humans, the average daily turnover of ATP is more than 50 kilograms! Without a continuous supply and replenishment of ATP, we wouldn’t survive. ATP powers the contraction of our heart muscle and maintains the membrane potential essential for neurological function, among thousands of other essential roles. How do the mitochondria manufacture these packets of life-sustaining energy? Remember that mitochondria rely on their double-membrane structure to carry out the electron transport chain and oxidative phosphorylation. As such, mitochondria truly act as batteries of the cell. In fact, note the similarity between the proton-motive force of the mitochondria and the electromotive force of electrochemistry. Are these two terms the same thing or—at the very least—similar in nature?
Indeed, mitochondria and batteries do function in similar ways. Specifically, mitochondria function most similarly to concentration cells. In both concentration cells and mitochondria, a concentration gradient of ions between two separated compartments—connected to each other by some means of charge conduction—establishes an electrical potential difference (voltage). This voltage, called electromotive force in concentration cells and proton-motive force in the mitochondria, provides the drive to move charge from one compartment to the other, creating current. In the concentration cell, an oxidation—reduction reaction takes place, and electrons move in the direction that causes the concentration gradient to be dissipated. In the mitochondria, the charge buildup is in the form of a hydrogen ion (proton) gradient between the intermembrane space and the matrix. Embedded in the inner membrane is ATP synthase, which serves a dual role as a proton channel and a catalyst for the formation of the high-energy phosphate bond in ATP. As the hydrogen ions flow down their electrochemical gradient, energy is dissipated, and this energy is harnessed by ATP synthase to form ATP.
In this final chapter of MCAT General Chemistry Review, we will focus our attention on the study of various electrochemical cells. Utilizing our knowledge of oxidation—reduction reactions from Chapter 11, we will study how these principles can be applied to create different types of electrochemical cells, including galvanic (voltaic), electrolytic, and concentration cells. Regarding the thermodynamics of electrochemistry, we will focus on the significance of reduction potentials and examine the relationship between electromotive force, the equilibrium constant, and Gibbs free energy.