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Pentose Phosphate Pathway: MCAT Test Prep (Simple)

May 12, 2022
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What is the Pentose Phosphate Pathway?The Oxidative PhaseThe Non-Oxidative Phase

Reviewed by:

Rohan Jotwani

Former Chief Resident in Anesthesiology, Weill Cornell Medicine, & Admissions Officer, Columbia University

Reviewed: 5/11/22

Time to tackle the pentose phosphate pathway for the MCAT? Read on to learn what you should know about the pathway that contributes to DNA synthesis and NADPH production. 

Carbohydrate metabolism is a basic tenant of human biology—it’s how we convert food into the energy and biochemical materials we need to do anything and everything. Medical professionals must understand the complex implications of the foods we eat, especially in a world with growing rates of cardiovascular disease and diabetes. 

Accordingly, the MCAT will heavily test you on the biochemical reactions involved in metabolism, including the pentose phosphate pathway. Read on to learn what you should know about the pentose phosphate pathway for the MCAT.

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What is the Pentose Phosphate Pathway?

The pentose phosphate pathway for the MCAT is a process parallel to glycolysis. Occurring in the cytoplasm, glycolysis is the first step in cellular respiration, in which glucose is broken down to release energy and pyruvate acid.

The pentose phosphate pathway is one of many side reactions that occur from glycolysis but, unlike glycolysis, does not require the input of ATP for energy- which is convenient for the cell! 

It uses glucose shunted away from glycolysis to carry out two phases: the oxidative phase, which generates NADPH and ribulose 5-phosphate, and the non-oxidative phase, which generates ribose 5-phosphate.

Ribose 5-phosphate is used to synthesize the nucleotides in DNA and RNA. You might have heard DNA referred to as the “blueprint of life.” This is because DNA contains instructions for producing proteins, which are the molecules that make up everything from our organs to our hormones. 

RNA is the intermediate molecule that DNA uses to transcribe proteins and, like DNA, is a nucleic acid built with a sugar backbone. Given their biological importance, processes involved in the production of DNA and RNA, such as the pentose phosphate pathway on the MCAT, are highly regulated processes.

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The Oxidative Phase 

As per its name, this phase involves oxidation, which is the loss of electrons from a molecule or atom during a chemical reaction. This phase breaks down and oxidizes glucose 6-phosphate, which is shunted from the glycolysis process, into a 5-carbon molecule called ribulose 5-phosphate. This is a precursor to ribose 5-phosphate, which is used in the synthesis of RNA and DNA molecules. 

As a byproduct of this oxidation of glucose 6-phosphate, CO2 is produced, and two molecules of NADP+ are reduced into NADPH. NADPH acts as a reducing agent and donates electrons in reactions that produce fatty acids, amino acids, nucleotides, and more. Its reducing power also allows it to contribute to cellular stores of antioxidants.

Below is a diagram outlining the steps of the oxidative phase of the pentose phosphate pathway for the MCAT:

The steps of the oxidative phase of the pentose phosphate pathway for the MCAT. The oxidative phase goes from glucose-6-phosphate to 6-phosphogluconolactone in which NADP+ is reduced to NADPH. Then 6-phosphogluconolactone is oxidized to 6-phosphogluconate, in which an H+ is released from H2O. Finally, ribulose 5-phosphate is produced in which NADPH and CO2 are released.

The Non-Oxidative Phase 

The non-oxidative phase starts with ribulose 5-phosphate, which is the product of the oxidative phase and converts it into ribose 5-phosphate through a series of reactions. This pentose phosphate sugar molecule (see the connection to the pathway’s name?) is used in the synthesis of RNA and DNA.  

Unlike the oxidative phase, the non-oxidative phase is conveniently reversible. What this means is that ribose 5-phosphate can be interconverted with intermediate molecules from glycolysis, such as fructose-6-phosphate. 

This reversibility is excellent news for the cell because it allows it to produce more ribose 5-phosphate for DNA and RNA production when necessary. It also provides an alternative method for generating materials for glycolysis. Think of it as the pentose phosphate pathway’s payback to glycolysis for donating a glucose 6-phosphate back in the oxidation phase.

Below is a diagram outlining the steps of the non-oxidative phase of the pentose phosphate pathway for the MCAT:

The steps of the non-oxidative phase of the pentose phosphate pathway for the MCAT. In this phase, ribulose 5-phosphate is converted to ribose 5-phosphate and xylulose 5-phosphate. A series of steps demonstrate how ribose 5-phosphate can be interconverted with fructose-6-phosphate and glyceraldehyde 3-phosphate, which are intermediate molecules from glycolysis.

Conclusion

Our cells depend on the pentose phosphate pathway to produce the molecules necessary to synthesize DNA and RNA. It also provides other essential materials such as NADPH. Understanding the biochemical pathways involved in metabolism will be necessary for your medical career, and the MCAT will assess whether you have the foundational knowledge to thrive. 

When you’re first hitting the books, a diagram of the pentose phosphate pathway for the MCAT can seem daunting. The conversion of one molecule with a long and indistinct name into another molecule with an equally long and indistinct name can make it hard to understand why you need to know all this.

But thankfully, you don’t need to memorize each step of the pentose phosphate pathway in detail. Instead, focus on understanding the key outputs of each phase and why they’re beneficial for the cell. Once you understand the big picture, the components of the pentose phosphate pathway for the MCAT will begin to come together.

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