Metabolism: Cellular Respiration, Energy Flow, and Glycolysis

Vocabulary practice flashcards covering cellular metabolism, ATP thermodynamics, redox reactions, and the complete enzymatic pathway of glycolysis based on lecture slides.

Human Energy Storage

The approximately $500\,MJ$ of energy stored by a healthy adult (equivalent to about $3.7\text{ gallons}$ of gasoline), which provides sufficient energy to survive for roughly $40\text{ days}$ given access to water.

ATP (Adenosine Triphosphate)

The primary cellular energy currency composed of adenine, ribose, and three phosphate groups ($\alpha$, $\beta$, and $\gamma$), providing the chemical energy source for most cellular reactions.

Phosphoanhydride Bond

A highly reactive chemical linkage between phosphate groups in ATP that lacks resonance and possesses high electron density; the bond connecting the $\gamma$-phosphate is the most easily cleaved to power cellular processes.

Cellular Steady State of ATP

The non-equilibrium state in which a living cell maintains an energetically unfavorable concentration of ATP that exceeds chemical equilibrium by a factor of $10^9$.

Oxidation

The loss of electrons from an atom or molecule during a chemical reaction, which can involve high-energy or low-energy electrons depending on the energy required for electron removal.

Reduction

The gain of electrons by an atom or molecule during a chemical reaction.

Coupled Redox Reactions

The simultaneous pairing of oxidation and reduction processes, wherein the electron(s) lost by one chemical species are concurrently gained by another.

Carbon Oxidation Spectrum

The series of redox states of carbon ranging from most reduced in methane to progressively more oxidized states in methanol, formaldehyde, formic acid, and ultimately carbon dioxide.

Acetyl-CoA

The mitochondrial metabolic intermediate produced from pyruvate (derived from glycolysis) or fatty acids that enters the citric acid cycle.

Glycolysis

The cytosol-localized metabolic pathway divided into three phases (Activation of Glucose, Splitting the Sugar, and Generating Energy) that breaks down one glucose molecule into two pyruvate molecules.

Hexokinase

The enzyme that catalyzes Step 1 of glycolysis by transferring a phosphate group from ATP to glucose, creating glucose 6-phosphate to trap the negatively charged sugar inside the cell and regulate cellular glucose entry.

Phosphoglucose Isomerase

The enzyme that catalyzes Step 2 of glycolysis by rearranging glucose 6-phosphate (an aldose) into fructose 6-phosphate (a ketose) in an easily reversible, non-rate-limiting reaction.

Phosphofructokinase (PFK)

The enzyme catalyzing Step 3 of glycolysis by phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate using ATP; it is the most critical and essentially irreversible regulatory control point, allosterically inhibited by high ATP levels and activated by AMP or ADP.

Aldolase

The enzyme that catalyzes Step 4 of glycolysis by cleaving the six-carbon ring of fructose 1,6-bisphosphate into two distinct three-carbon sugar phosphates: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

Triose Phosphate Isomerase

The enzyme catalyzing Step 5 of glycolysis by isomerizing dihydroxyacetone phosphate into a second molecule of glyceraldehyde 3-phosphate, causing all subsequent glycolytic reactions to occur twice per glucose.

Glyceraldehyde 3-Phosphate Dehydrogenase

The enzyme catalyzing Step 6 of glycolysis by removing $2\,e^-$ to reduce $NAD^+$ to NADH while coupling the exergonic oxidation to the endergonic addition of inorganic phosphate ($P_i$) to form 1,3-bisphosphoglycerate.

Substrate-Level Phosphorylation

The direct synthesis of ATP by transferring a high-energy phosphate group from a phosphorylated substrate (such as 1,3-bisphosphoglycerate or phosphoenolpyruvate) to ADP, overcoming the positive $\Delta G$ of ATP formation.

Phosphoglycerate Kinase

The enzyme catalyzing Step 7 of glycolysis by transferring the high-energy acyl-phosphate from 1,3-bisphosphoglycerate to ADP, producing 3-phosphoglycerate and generating ATP to recoup the initial energy investment of Steps 1 and 3.

Phosphoglycerate Mutase

The enzyme catalyzing Step 8 of glycolysis by shifting the phosphate ester linkage from carbon 3 to carbon 2 of 3-phosphoglycerate, producing 2-phosphoglycerate in a reversible isomerization.

Enolase

The enzyme catalyzing Step 9 of glycolysis by removing a molecule of water from 2-phosphoglycerate via dehydration to create a high-energy enol phosphate bond in phosphoenolpyruvate (PEP).

Pyruvate Kinase

The enzyme that catalyzes Step 10 of glycolysis by transferring the phosphate group from phosphoenolpyruvate to ADP to produce ATP and pyruvate; it is subject to feed-forward activation by fructose 1,6-bisphosphate and is inactivated in the liver during low-glucose conditions.

Feed-Forward Activation

A regulatory mechanism in glycolysis where fructose 1,6-bisphosphate produced upstream in Step 3 stimulates the downstream activity of pyruvate kinase in Step 10.

Net Output of Glycolysis

The final yield generated from the breakdown of one single molecule of glucose, consisting of $2\text{ molecules of pyruvate}$, $2\text{ molecules of ATP}$, and $2\text{ molecules of NADH}$.