Cellular Respiration and Metabolism

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Metabolism

The totality of an organism's chemical reactions.

Metabolic Pathway

A series of chemical reactions that begins with a specific molecule and ends with a product, with each step catalyzed by a specific enzyme.

Catabolic Pathways

Metabolic pathways that release energy by breaking down complex molecules into simpler compounds, such as the breakdown of glucose in cellular respiration.

Anabolic Pathways

Metabolic pathways that consume energy to build complex molecules from simpler ones, such as the synthesis of glucose in photosynthesis.

Bioenergetics

The study of how organisms manage their energy resources.

Kinetic Energy

The form of energy associated with the relative motion of objects.

Heat (Thermal Energy)

Kinetic energy associated with the random movement of atoms or molecules.

Potential Energy

The energy that matter possesses because of its location or structure.

Chemical Energy

The potential energy available for release in a chemical reaction.

First Law of Thermodynamics

The principle stating that energy cannot be created or destroyed, only transferred or transformed; also known as the principle of conservation of energy.

Second Law of Thermodynamics

The principle stating that every energy transfer or transformation increases the entropy (disorder) of the universe, with some energy becoming unusable and often lost as heat.

Adenosine Triphosphate (ATP)

An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed; composed of ribose, adenine, and three phosphate groups.

ATP Cycle

The revolving door process through which energy passes during its transfer from catabolic to anabolic pathways, regenerating ATP by the addition of a phosphate group to ADP.

Catalyst

A chemical agent that speeds up the rate of a reaction without being consumed by the reaction.

Enzyme

A macromolecule serving as a catalyst, which is typically a catalytic protein.

Activation Energy ($E_A$)

The initial free energy required to start a chemical reaction, which enzymes lower without altering the change in free energy ($\Delta G$).

Cofactors

Nonprotein enzyme helpers that may be inorganic, such as a metal in ionic form, or organic.

Coenzyme

An organic cofactor required for an enzyme to function properly, which includes vitamins.

Fermentation

A partial degradation of sugars that occurs without $\text{O}_2$, utilizing substrate-level phosphorylation instead of an electron transport chain to produce ATP.

Aerobic Respiration

A catabolic pathway that consumes organic molecules and $\text{O}_2$, yielding ATP through cellular respiration.

Anaerobic Respiration

A catabolic process similar to aerobic respiration that produces ATP using an electron transport chain with a final electron acceptor other than $\text{O}_2$, such as sulfate.

Oxidation

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

Redox Reaction

A chemical reaction involving the transfer of electrons between two species, where one undergoes oxidation and the other undergoes reduction.

Reduction

The gain of electrons by a substance in a redox reaction, which reduces the amount of positive charge on that substance.

$$\text{NAD}^+$$

A coenzyme that acts as an electron acceptor and oxidizing agent during the stepwise breakdown of glucose and other fuels in cellular respiration.

NADH

The reduced form of $\text{NAD}^+$ that represents stored energy tapped to synthesize ATP via the electron transport chain.

Glycolysis

The splitting of glucose into two molecules of pyruvate occurring in the cytoplasm, consisting of an energy investment phase and an energy payoff phase, functioning whether or not $\text{O}_2$ is present.

Substrate-Level Phosphorylation

The enzyme-catalyzed formation of ATP by the direct transfer of a phosphate group to ADP from a phosphorylated intermediate metabolic substrate.

Oxidative Phosphorylation

The production of ATP using energy derived from the redox reactions of an electron transport chain and chemiosmosis, accounting for almost $90\%$ of ATP generated by cellular respiration.

Acetyl Coenzyme A (Acetyl CoA)

The entry compound for the citric acid cycle in cellular respiration, formed from a two-carbon fragment of pyruvate attached to a coenzyme.

Krebs Cycle (Citric Acid Cycle)

A chemical cycle that completes the metabolic breakdown of glucose to $\text{CO}_2$, generating $1\,\text{ATP}$, $3\,\text{NADH}$, and $1\,\text{FADH}_2$ per turn.

Electron Transport Chain

A sequence of electron carrier proteins located in the inner mitochondrial membrane that shuttles electrons down an energy-yielding cascade to $\text{O}_2$, producing $\text{H}_2\text{O}$.

Chemiosmosis

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

ATP Synthase

A complex of several membrane proteins that functions in chemiosmosis using the energy of a proton gradient to phosphorylate ADP to ATP.

Proton-Motive Force

The potential energy stored in the form of a proton ($\text{H}^+$) gradient generated across a membrane by the pumping of hydrogen ions during electron transport.

Alcohol Fermentation

A type of fermentation in which glycolysis is followed by the reduction of pyruvate to ethanol, releasing $\text{CO}_2$ and regenerating $\text{NAD}^+$.

Lactic Acid Fermentation

A type of fermentation in which glycolysis is followed by the direct reduction of pyruvate by NADH to form lactate with no release of $\text{CO}_2$, regenerating $\text{NAD}^+$.

Obligate Anaerobes

Organisms that carry out only fermentation or anaerobic respiration and cannot survive in the presence of $\text{O}_2$.

Facultative Anaerobes

Organisms, such as yeasts and many bacteria, that can survive using either fermentation or cellular respiration depending on oxygen availability.

Phosphofructokinase

An allosteric regulatory enzyme in glycolysis that acts as a metabolic pacemaker, stimulated by AMP and inhibited by ATP and citrate.

The Stages of Cellular Respiration

Glycolysis, the Krebs cycle, and oxidative phosphorylation, which together convert glucose into ATP through both aerobic and anaerobic pathways.