Cellular Energetics: Enzymes, Cellular Respiration, and Photosynthesis

Comprehensive vocabulary flashcards covering thermodynamic principles, enzyme mechanisms, cellular respiration stages, and photosynthetic light and dark reactions.

Metabolism

The sum of all chemical reactions in a cell or organism.

Metabolic pathway

A sequence of enzyme-controlled reactions that converts a starting molecule into products.

Catabolic pathway

A pathway that breaks complex molecules into simpler ones and often releases energy.

Anabolic pathway

A pathway that builds complex molecules from smaller ones and requires energy.

Kinetic energy

Energy associated with motion.

Potential energy

Stored energy associated with position or structure.

Chemical energy

Potential energy stored in chemical bonds and molecular arrangements.

Thermodynamics

The study of energy transformations.

First law of thermodynamics

Energy cannot be created or destroyed; it can be transferred or transformed.

Second law of thermodynamics

Every spontaneous energy transfer or transformation increases the total entropy of a system and its surroundings.

Entropy

A measure of energy dispersal and the number of possible microscopic arrangements.

Open system

A system that exchanges both energy and matter with its surroundings.

Closed system

A system that exchanges energy but not matter with its surroundings.

Free energy ($G$)

Energy available to do work at constant temperature and pressure.

$$\Delta G$$

Change in Gibbs free energy: $G(\text{products}) - G(\text{reactants})$.

Exergonic reaction

A reaction with negative $\Delta G$ that releases free energy and is thermodynamically favorable.

Endergonic reaction

A reaction with positive $\Delta G$ that requires an input of free energy.

Spontaneous reaction

A thermodynamically favorable reaction ($\Delta G < 0$); it may still proceed slowly.

Equilibrium

State in which forward and reverse reaction rates are equal; $\Delta G = 0$ for the reaction under those conditions.

Energy coupling

Using energy released by an exergonic process to drive an endergonic process.

ATP

Adenosine triphosphate; a nucleotide used for immediate energy transfer in cells.

ATP structure

Adenine, ribose sugar, and three phosphate groups.

ATP hydrolysis

$ATP + H_2O \rightarrow ADP + \text{inorganic phosphate}$; releases free energy.

ATP regeneration

$ADP + \text{inorganic phosphate}$ are combined to make ATP using energy from metabolism or light.

Phosphorylation

Addition of a phosphate group to a molecule, often changing its activity or energy.

Substrate-level phosphorylation

Direct transfer of a phosphate from an organic intermediate to ADP to make ATP.

Oxidative phosphorylation

ATP formation powered by electron transport and chemiosmosis during respiration.

Photophosphorylation

ATP formation powered by light-driven electron transport and chemiosmosis.

Catalyst

A substance that increases reaction rate without being consumed.

Enzyme

A biological catalyst, usually a protein, that lowers activation energy.

Activation energy

The energy barrier reactants must overcome to reach the transition state.

Transition state

A high-energy, unstable arrangement of atoms during a reaction.

Active site

The region of an enzyme where substrates bind and catalysis occurs.

Substrate

The reactant on which an enzyme acts.

Enzyme-substrate complex

Temporary association formed when substrate binds an enzyme.

Induced fit

Change in enzyme shape upon substrate binding that promotes catalysis.

Enzyme specificity

An enzyme recognizes particular substrate structures and catalyzes particular reactions.

Effect of enzymes on $\Delta G$

Enzymes do not change $\Delta G$ or the position of equilibrium; they lower activation energy.

Optimal temperature

Temperature at which an enzyme typically has its highest activity under specified conditions.

Optimal pH

pH at which an enzyme typically functions best.

Denaturation

Disruption of a protein’s three-dimensional structure, often reducing its function.

Competitive inhibition

An inhibitor competes with substrate for binding at the active site.

Noncompetitive inhibition

An inhibitor reduces enzyme activity by binding away from the active site; in pure noncompetitive inhibition maximum rate decreases.

Allosteric regulation

A regulator binds at a site other than the active site and changes enzyme activity.

Allosteric activator

A molecule that binds an enzyme and increases its activity.

Feedback inhibition

A pathway’s final product inhibits an enzyme in an earlier step.

Cofactor

A nonprotein helper required for some enzymes, such as a metal ion.

Coenzyme

An organic cofactor, often vitamin-derived, that helps enzyme reactions.

Saturation (enzyme)

Condition in which nearly all enzyme active sites are occupied and rate approaches its maximum.

Extreme pH effect on enzyme activity

Changes in protonation can disrupt bonding, active-site shape, or protein structure.

Kinetic rate of spontaneous reactions

A negative $\Delta G$ does not eliminate a high activation-energy barrier.

Cellular function of ATP coupling

ATP hydrolysis can be coupled to transport, movement, and biosynthesis.

Non-equilibrium in living cells

Cells continually exchange energy and matter and maintain gradients and chemical reactions.

Cellular respiration

A set of pathways that oxidizes fuel molecules and captures energy as ATP.

Aerobic respiration

Respiration that uses oxygen as the final electron acceptor.

Overall aerobic respiration equation

$$C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{energy}$$

Oxidation

Loss of electrons.

Reduction

Gain of electrons.

Redox reaction

A reaction involving transfer of electrons between substances.

$$NAD^+$$

Oxidized electron carrier that accepts electrons to become NADH.

NADH

Reduced electron carrier that donates electrons to respiratory electron transport.

FAD

Oxidized electron carrier that accepts electrons and hydrogen to become $FADH_2$.

$$FADH_2$$

Reduced electron carrier that delivers electrons to the electron transport chain.

Four stages of aerobic glucose respiration

Glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

Glycolysis location

Cytosol.

Glycolysis inputs

Glucose, $NAD^+$, ADP, phosphate; 2 ATP are invested in early steps.

Glycolysis outputs per glucose

2 pyruvate, 2 net ATP, and 2 NADH.

Glycolysis oxygen requirement

Glycolysis does not directly require $O_2$.

Energy investment phase

First part of glycolysis, in which 2 ATP are consumed.

Energy payoff phase

Later part of glycolysis, in which 4 ATP and 2 NADH are produced per glucose.

Net ATP in glycolysis

4 ATP produced minus 2 ATP invested = 2 net ATP.

Pyruvate oxidation location in eukaryotes

Mitochondrial matrix.

Pyruvate oxidation outputs per glucose

2 acetyl-CoA, 2 $CO_2$, and 2 NADH.

Acetyl-CoA

Two-carbon acetyl group attached to coenzyme A that enters the citric acid cycle.

Citric acid cycle alternative name

Krebs cycle or tricarboxylic acid (TCA) cycle.

Krebs cycle location in eukaryotes

Mitochondrial matrix.

Krebs cycle turns per glucose

Two turns because each glucose produces two acetyl-CoA molecules.

Krebs cycle outputs per turn

3 NADH, 1 $FADH_2$, 1 ATP or GTP equivalent, and 2 $CO_2$.

Krebs cycle outputs per glucose

6 NADH, 2 $FADH_2$, 2 ATP/GTP equivalents, and 4 $CO_2$.

Total NADH before ETC per glucose

10 NADH: 2 from glycolysis, 2 from pyruvate oxidation, and 6 from the Krebs cycle.

Total $FADH_2$ before ETC per glucose

2 $FADH_2$ from the Krebs cycle.

ETC location in eukaryotes

Inner mitochondrial membrane.

Electron transport chain

Series of carriers that pass electrons and use released energy to pump protons.

Final electron acceptor in aerobic respiration

Oxygen, which is reduced to water.

Chemiosmosis

Use of an electrochemical proton gradient to drive ATP synthesis.

Proton-motive force

Stored electrochemical energy due to an $H^+$ concentration and charge difference across a membrane.

Mitochondrial proton pumping direction

From mitochondrial matrix into the intermembrane space.

Proton flow through mitochondrial ATP synthase

From intermembrane space back into the matrix.

ATP synthase

Membrane enzyme that couples proton movement down a gradient to ATP synthesis.

Oxidative phosphorylation ATP yield

Often approximately 26–28 ATP per glucose in eukaryotic cells.

Typical total ATP yield per glucose

About 30–32 ATP in many eukaryotic cells; actual yield varies.

Effect of blocking oxygen on respiration

Electron transport stalls without its final electron acceptor, weakening proton pumping and oxidative phosphorylation.

Role of NADH and $FADH_2$

They carry electrons from fuel oxidation to the ETC.

Indirect ATP synthesis by ETC

Electron transfer creates a proton gradient that ATP synthase uses.

Fermentation

Pathway that reoxidizes NADH to $NAD^+$ so glycolysis can continue without a functioning respiratory ETC.

Lactic acid fermentation

Pyruvate is reduced to lactate, regenerating $NAD^+$.

Alcoholic fermentation

Pyruvate is converted to acetaldehyde and $CO_2$; acetaldehyde is reduced to ethanol, regenerating $NAD^+$.

ATP yield from fermentation plus glycolysis

2 net ATP per glucose, all from glycolysis.

ATP generation by fermentation reactions

No ATP is directly added beyond glycolysis; its key role is regenerating $NAD^+$.

Lactic fermentation products per glucose

2 lactate; 2 net ATP come from glycolysis.