Cellular Respiration: Harvesting Chemical Energy

Vocabulary practice flashcards covering key terms, pathways, enzymes, and mechanisms in Chapter 9: Cellular Respiration and Fermentation.

Fermentation

A catabolic process that is a partial degradation of sugars occurring without oxygen (anaerobic).

Cellular Respiration

An aerobic catabolic pathway that consumes oxygen ($O_2$) and organic fuel such as glucose ($C_6H_{12}O_6$) to yield ATP, $CO_2$, and $H_2O$, releasing energy in the form of ATP and heat.

Redox Reactions

Chemical reactions that transfer electrons between reactants; also known as oxidation-reduction reactions.

Oxidation

The loss of electrons from a substance during a redox reaction.

Reduction

The addition of electrons to a substance during a redox reaction, which reduces the amount of positive charge.

Reducing Agent

The electron donor in a redox reaction, which becomes oxidized in the process.

Oxidizing Agent

The electron acceptor in a redox reaction, which becomes reduced in the process.

$$NAD^+$$

Nicotinamide adenine dinucleotide in its oxidized form; a coenzyme that functions as an electron acceptor and oxidizing agent during cellular respiration.

NADH

The reduced form of $NAD^+$ that carries stored energy extracted from food molecules and donates electrons to the electron transport chain.

Electron Transport Chain

A collection of multiprotein complexes embedded in the cristae of the mitochondrion that shuttles electrons down a free-energy gradient to $O_2$, breaking the fall of electrons into manageable, energy-yielding steps.

Glycolysis

A ten-step metabolic pathway occurring in the cytosol ("splitting of sugar") that breaks down one glucose molecule into two molecules of pyruvate, functioning with or without oxygen.

Substrate-Level Phosphorylation

The mode of ATP synthesis occurring in glycolysis and the citric acid cycle where an enzyme directly transfers a phosphate group from an organic substrate molecule to ADP.

Energy Investment Phase

The initial phase of glycolysis in which the cell expends $2\,\text{ATP}$ molecules to phosphorylate glucose intermediates.

Energy Payoff Phase

The second phase of glycolysis in which $4\,\text{ATP}$ and $2\,\text{NADH}$ (plus $2\,H^+$) are generated, producing a net gain of $2\,\text{ATP}$ and $2\,\text{pyruvate}$.

Pyruvate Oxidation

The step linking glycolysis to the citric acid cycle where pyruvate enters the mitochondrion via a transport protein and is converted to acetyl CoA, producing $CO_2$ and NADH.

Acetyl CoA

Acetyl coenzyme A; the two-carbon molecule derived from pyruvate that feeds its acetyl group into the citric acid cycle.

Citric Acid Cycle

An eight-step metabolic pathway located within the mitochondrial matrix (also called the Krebs cycle) that oxidizes organic fuel derived from pyruvate, generating $1\,\text{ATP}$, $3\,\text{NADH}$, and $1\,\text{FADH}_2$ per turn.

Oxaloacetate

The four-carbon molecule that combines with the acetyl group of acetyl CoA to form citrate, and is regenerated by the final step of the citric acid cycle.

Oxidative Phosphorylation

The production of ATP powered by redox reactions in the electron transport chain and chemiosmosis; accounts for almost $90\%$ of all ATP generated during cellular respiration.

Chemiosmosis

An energy-coupling mechanism that uses the energy stored in a hydrogen ion ($H^+$) gradient across a membrane to drive cellular work, specifically the synthesis of ATP.

ATP Synthase

A molecular machine in the inner mitochondrial membrane composed of a rotor, stator, rod, and catalytic knob that uses the exergonic flow of $H^+$ to synthesize ATP from ADP and inorganic phosphate ($P_i$).

Proton-Motive Force

The $H^+$ concentration and electrical gradient maintained across the inner mitochondrial membrane, emphasizing its capacity to perform cellular work.

Alcohol Fermentation

A form of anaerobic catabolism wherein pyruvate is converted to ethanol in two steps (the first releasing $CO_2$), regenerating $NAD^+$ for glycolysis.

Lactic Acid Fermentation

A form of anaerobic catabolism wherein pyruvate is directly reduced by NADH to form lactate as an end product with no release of $CO_2$, regenerating $NAD^+$ for glycolysis.

Facultative Anaerobes

Organisms, such as yeast and many bacteria, that can survive using either fermentation or cellular respiration depending on the presence of oxygen.

Feedback Inhibition

The primary regulatory mechanism of cellular respiration where end products or high energy levels (such as ATP and citrate) inhibit strategic enzymes to slow catabolism, while low energy levels (such as AMP) stimulate them.

Phosphofructokinase

An essential regulatory allosteric enzyme in glycolysis that acts as a pacemaker; it is stimulated by AMP and inhibited by ATP and citrate.