Unit 2: Cellular Energy, Photosynthesis, and Cellular Respiration

Comprehensive vocabulary flashcards covering cellular energy, thermodynamics, enzyme kinetics and regulation, photosynthesis pathways, and cellular respiration mechanisms from Unit 2.

Energy

The ability to do work.

Work

The movement of matter.

Potential Energy

Stored energy that may potentially be used.

Kinetic Energy

The energy of motion that is currently being used.

First Law of Thermodynamics

The physical law of energy conservation stating that energy cannot be created or destroyed, only transformed.

Second Law of Thermodynamics

The physical law stating that all energy transformations are inefficient, resulting in a loss of usable energy in the form of heat (entropy).

Metabolism

The biochemical reactions of a cell, defined as the sum of all anabolic and catabolic reactions in a living organism.

Endergonic Reaction

An anabolic reaction requiring an energy input that converts simple reactants into complex, higher-energy products, creating stored potential energy.

Exergonic Reaction

A catabolic reaction that releases energy, breaking down complex molecules into simpler, lower-energy products and losing stored potential energy.

Oxidation

The loss of one or more electrons by an atom or molecule during a redox reaction.

Reduction

The gain of one or more electrons by an atom or molecule during a redox reaction.

Electron Transport Chain

A series of membrane proteins participating in sequential redox reactions, passing electrons from a donor to an acceptor while releasing energy in small steps.

Adenosine Triphosphate (ATP)

The primary cellular energy currency; a nucleotide composed of an adenine base, a ribose sugar, and three phosphate groups.

Coupled Reactions

Pairs of biochemical processes where an exergonic reaction provides the energy required to drive an endergonic reaction.

Phosphorylation

The transfer of a phosphate group from one molecule (commonly $\text{ATP}$) to another, altering its reactivity or protein conformation.

Sodium-Potassium ($\text{Na}^+ / \text{K}^+$) Pump

An active transport membrane mechanism that consumes $1\,\text{ATP}$ per cycle to export three $\text{Na}^+$ ions from the cytoplasm and import two $\text{K}^+$ ions into the cell.

Catalyst

A substance—such as a protein, metal, or RNA—that accelerates the rate of a chemical reaction without being consumed or destroyed in the process.

Enzyme

A reusable protein that acts as a biological catalyst to accelerate chemical reactions by lowering their activation energy.

Activation Energy

The initial threshold amount of energy required to begin a chemical reaction.

Active Site

The specific physical pocket or cleft on an enzyme where substrate molecules bind.

Allosteric Site

A regulatory binding site on an enzyme distinct from the active site where noncompetitive inhibitors or effectors attach.

Enzyme-Substrate Complex

The temporary molecular association formed when substrate molecules bind to the active site of an enzyme.

Conformational Change

A structural change in the shape of a protein (such as an enzyme upon substrate binding) that facilitates catalysis or biological function.

Cofactor

A nonprotein helper molecule—either an inorganic metal ion or an organic molecule—that binds permanently or reversibly to an enzyme to assist its catalytic function.

Coenzyme

An organic cofactor, such as a vitamin, required by certain enzymes to facilitate catalytic reactions.

Competitive Inhibition

An enzyme regulation mechanism where an inhibitor competes directly with the substrate for binding at the active site.

Noncompetitive Inhibition

An enzyme regulation mechanism where an inhibitor binds to an allosteric site rather than the active site, inducing a conformational change that prevents normal activity.

Feedback Inhibition

A regulatory mechanism in which the final product of a multi-step metabolic pathway binds allosterically to an early enzyme to halt its own synthesis.

Autotroph

An organism that utilizes inorganic substances to produce organic compounds (carbon-based molecules).

Heterotroph

An organism that acquires necessary carbon and nutrients by consuming preexisting organic molecules.

Stoma

A specialized microscopic pore on a plant leaf that mediates gas exchange, allowing $\text{CO}_2$ to enter while permitting $\text{O}_2$ and $\text{H}_2\text{O}$ to exit.

Stroma

The fluid-filled inner space of a chloroplast surrounding the thylakoid stacks, where the carbon reactions (Calvin cycle) take place.

Granum

A stack of disc-shaped, pigment-embedded thylakoid membranes within a chloroplast.

Chlorophyll a

The most abundant photosynthetic pigment in plants; reflects blue-green light and plays the primary role in transferring solar energy to chemical energy.

Carotenoids

Accessory photosynthetic pigments (including carotenes and xanthophylls) that absorb wavelengths missed by chlorophyll and reflect red, orange, and yellow light.

Photosystem

A functional unit embedded in the thylakoid membrane consisting of a reaction center chlorophyll a surrounded by approximately $300$ chlorophyll a molecules and $50$ accessory antenna pigments.

Light Reactions

The 'photo-' stage of photosynthesis occurring in thylakoid membranes that converts kinetic solar energy into potential chemical energy stored in $\text{ATP}$ and $\text{NADPH}$, producing $\text{O}_2$ from $\text{H}_2\text{O}$.

Calvin Cycle

The '-synthesis' carbon reactions occurring in the chloroplast stroma that use $\text{ATP}$, $\text{NADPH}$, and $\text{CO}_2$ to synthesize PGAL molecules over three steps: carbon fixation, PGAL synthesis, and RuBP regeneration.

Rubisco

The chloroplast enzyme that catalyzes the initial fixation of $\text{CO}_2$ onto RuBP in the Calvin cycle, but can also competitively bind $\text{O}_2$.

Photorespiration

A wasteful metabolic pathway that occurs when Rubisco fixes $\text{O}_2$ instead of $\text{CO}_2$, consuming $\text{ATP}$ and $\text{NADPH}$ while releasing fixed $\text{CO}_2$.

Aerobic Cellular Respiration

An oxygen-dependent catabolic pathway ($\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\,\text{ATP}$) that completely oxidizes glucose to yield high amounts of cellular energy.

Cristae

The extensive foldings of the inner mitochondrial membrane that house the complexes of the electron transport chain and ATP synthase.

Mitochondrial Matrix

The compartment enclosed by the inner mitochondrial membrane where the transition step and the Krebs cycle occur.

Glycolysis

The initial, universal cytosolic pathway of cell respiration that splits one glucose into two pyruvate molecules, yielding a net $2\,\text{ATP}$ and $2\,\text{NADH}$.

Transition Step (Pyruvate Oxidation)

The mitochondrial matrix stage where two 3-carbon pyruvates are oxidized into two 2-carbon acetyl CoA molecules, releasing two $\text{CO}_2$ and producing two $\text{NADH}$.

Krebs Cycle

The matrix-based metabolic cycle in which 2-carbon acetyl CoA combines with 4-carbon oxaloacetate to form citrate; produces $4\,\text{CO}_2$, $6\,\text{NADH}$, $2\,\text{FADH}_2$, and $2\,\text{ATP}$ per glucose.

ATP Synthase

A transport and enzymatic complex that uses the energy of an electrochemical proton ($\text{H}^+$) gradient across a membrane to drive chemiosmotic phosphorylation of $\text{ADP}$ to $\text{ATP}$.

Anaerobic Respiration

A form of respiration that utilizes glycolysis and a membrane-bound electron transport chain with an inorganic terminal electron acceptor other than $\text{O}_2$ (such as $\text{NO}_3^-$, $\text{SO}_4^{2-}$, or $\text{CO}_2$), generating a medium amount of additional ATP.

Fermentation

An anaerobic cytosolic metabolic pathway that reduces pyruvate to regenerate $\text{NAD}^+$ from $\text{NADH}$, producing zero additional ATP beyond the net yield of glycolysis.

Alcoholic Fermentation

A fermentation pathway wherein pyruvate is converted into ethanol and $\text{CO}_2$, oxidizing $\text{NADH}$ back to $\text{NAD}^+$.

Lactic Acid Fermentation

A fermentation pathway wherein pyruvate is reduced directly to lactic acid (lactate) to regenerate $\text{NAD}^+$ from $\text{NADH}$.

Chemical Work

The cellular task of synthesizing or breaking apart covalent bonds between molecules.

Transport Work

The cellular process of pumping ions and other solutes across biological membranes against concentration gradients.

Mechanical Work

Physical cellular movements such as muscle contraction, the beating of cilia, or the movement of chromosomes during cell division.

ATP Hydrolysis

An exergonic reaction ($ATP + H_2O \rightarrow ADP + P + \text{energy}$) in which a water molecule cleaves the terminal phosphate bond to release stored energy.

Substrate-Level Phosphorylation

A metabolic process forming $ATP$ through the direct enzymatic transfer of a phosphate group from a phosphorylated intermediate substrate directly to $ADP$.

Chemiosmosis

The generation of $ATP$ driven by the flow of protons ($H^+$) down an electrochemical gradient across a biological membrane through ATP synthase.

Energy Investment Phase

The preparatory stage of glycolysis that consumes $2\,\text{ATP}$ to split glucose into two three-carbon PGAL molecules.

Energy Harvest Phase

The payoff stage of glycolysis that produces $4\,\text{ATP}$ and $2\,\text{NADH}$, resulting in a net gain of $2\,\text{ATP}$ per glucose.

Terminal Electron Acceptor

The final electron-receiving molecule in an electron transport chain, such as $O_2$ in aerobic cellular respiration.

NADH Theoretical ATP Yield

The total quantity of cellular energy synthesized in the mitochondrial electron transport chain per oxidized coenzyme carrier, equal to $3\,\text{ATP}$.

FADH2 Theoretical ATP Yield

The total quantity of cellular energy synthesized in the mitochondrial electron transport chain per oxidized coenzyme carrier, equal to $2\,\text{ATP}$.

Photolysis

The light-catalyzed breakdown of $H_2O$ at Photosystem II that supplies replacement electrons to $P_{680}$ while releasing $O_2$ as a byproduct.

Chlorophyll b

An accessory photosynthetic pigment in plants that absorbs blue and red wavelengths while reflecting yellow-green light.

Photosystem II

The first multi-protein pigment complex in the thylakoid membrane containing reaction center $P_{680}$ that absorbs photons and oxidizes water.

Photosystem I

The second multi-protein pigment complex in the thylakoid membrane containing reaction center $P_{700}$ that re-energizes electrons to reduce $NADP^+$ into $NADPH$.

Positive Feedback Loop

A metabolic regulatory mechanism in which a downstream product or change reinforces and increases the rate of its initial production process.

Endosymbiosis Theory

The evolutionary model proposing that eukaryotic organelles such as mitochondria and chloroplasts originated as free-living prokaryotic cells engulfed by an ancestral host cell.

Alternative Respiration Substrates

Non-glucose macromolecules, such as proteins, lipids, and complex carbohydrates, that are enzymatically converted into intermediates to enter glycolysis or the Krebs cycle.

NADH Mitochondrial Transport Cost

The active cellular energy requirement of $1\,\text{ATP}$ per cytosolic $NADH$ needed to transport glycolysis-derived electrons into the mitochondrial matrix.

RuBP (Ribulose 1,5-bisphosphate)

The five-carbon starting and ending compound in the chloroplast stroma that binds incoming $CO_2$ during the initial fixation phase of the Calvin cycle.