What distinguishes exergonic reactions from endergonic reactions in biochemical pathways?

Exergonic reactions have a negative ΔG and release free energy spontaneously, whereas endergonic reactions have a positive ΔG and require an input of free energy to proceed.

Where do the light-dependent reactions of photosynthesis take place in plant cells?

They occur on the thylakoid membranes within chloroplasts, where chlorophylls and electron transport complexes are embedded.

Why does enzyme activity drop sharply at temperatures above the optimal thermal threshold?

Thermal kinetic energy disrupts weak hydrogen bonds and hydrophobic interactions stabilizing the tertiary structure, causing denaturation and loss of functional active site geometry.

How do biological enzymes accelerate metabolic chemical reactions?

Enzymes act as biological catalysts by lowering the activation energy (Ea) required to reach the transition state, without altering the equilibrium constant or the overall change in Gibbs free energy (ΔG).

Where does glycolysis occur within eukaryotic cells, and does it require oxygen?

Glycolysis takes place in the cytoplasm (cytosol) and is an anaerobic process that does not require molecular oxygen.

Where does the Calvin cycle take place and what is its primary catalytic enzyme?

The Calvin cycle occurs in the stroma of chloroplasts, and its initial carbon-fixation step is catalyzed by ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO).

What is the functional difference between an inorganic cofactor and an organic coenzyme?

A cofactor is any non-protein chemical component required for enzyme catalysis, including inorganic metal ions (such as Mg2+, Zn2+, or Fe2+/Fe3+); when the non-protein cofactor is an organic molecule (often derived from vitamins, such as NAD+, FAD, or coenzyme A), it is specifically called a coenzyme.

What are the end products of lactic acid fermentation versus alcoholic fermentation?

Lactic acid fermentation reduces pyruvate to lactate with no gas produced; alcoholic fermentation decarboxylates pyruvate to acetaldehyde and reduces it to ethanol, releasing CO2.

How do mitochondrial uncoupling proteins (e.g., UCP1 / thermogenin) generate non-shivering heat?

Uncoupling proteins allow protons to re-enter the mitochondrial matrix without passing through ATP synthase, dissipating the proton motive force directly as heat instead of trapping it as ATP.

What is the function of the water-splitting complex (photolysis) in Photosystem II?

The oxygen-evolving (water-splitting) complex in Photosystem II oxidizes two water molecules into molecular oxygen (O2), four protons (H+ released into the thylakoid lumen), and four electrons, replacing the photoexcited electrons lost by the P680 reaction-center chlorophyll pair and contributing to the lumen proton gradient.

How does the mitochondrial electron transport chain establish a proton motive force?

Electrons from NADH (entering at Complex I) and FADH2 (entering at Complex II) pass through mobile carriers (ubiquinone and cytochrome c) to Complex III and Complex IV; free energy released during electron transfer drives Complexes I, III, and IV to pump protons (H+) from the mitochondrial matrix into the intermembrane space (Complex II does not pump protons), establishing an electrochemical proton gradient.

What reaction occurs during the carbon fixation phase of the Calvin cycle?

RuBisCO fixes one CO2 molecule onto a 5-carbon ribulose-1,5-bisphosphate (RuBP) molecule, creating an unstable 6-carbon intermediate that splits into two 3-phosphoglycerate (3-PGA) molecules.

What occurs during pyruvate oxidation before entering the Krebs cycle?

Pyruvate is transported into the mitochondrial matrix, decarboxylated (releasing CO2), oxidized to transfer electrons to NAD+ forming NADH, and coupled to Coenzyme A to yield acetyl-CoA.

How does a competitive inhibitor alter the Michaelis-Menten kinetic parameters Vmax and Km?

A reversible competitive inhibitor binds to the active site, increasing the apparent Km (more substrate is required to reach half-maximal velocity, 1/2 Vmax) while leaving Vmax unchanged because sufficiently high substrate concentrations outcompete the inhibitor.

How does ATP synthase utilize chemiosmosis to produce ATP?

Protons flow down their electrochemical gradient from the intermembrane space back into the matrix through the Fo rotor subunit, driving the catalytic F1 subunit to phosphorylate ADP into ATP.

What does the induced-fit model describe regarding enzyme-substrate interactions?

Substrate binding induces a conformational change in the enzyme active site that aligns catalytic residues precisely around the substrate, stabilizing the transition state.

How many molecules of ATP and NADPH are consumed to produce one net G3P exported from the Calvin cycle?

Synthesizing one net exported glyceraldehyde-3-phosphate (G3P, a 3-carbon sugar) requires fixing 3 CO2 molecules onto 3 RuBP and consuming 9 ATP and 6 NADPH (generating 6 G3P in the reduction phase, of which 1 G3P exits the cycle and 5 G3P are rearranged to regenerate 3 RuBP).

How does the Second Law of Thermodynamics explain the constant energy requirement of living organisms?

Every energy transfer increases the universal entropy (disorder); organisms must continuously consume external free energy to maintain internal biological order and prevent cellular decay.

How do C4 plants anatomically and biochemically bypass photorespiration?

They spatially separate carbon fixation: PEP carboxylase fixes CO2 into oxaloacetate in mesophyll cells, which is transported as malate to bundle-sheath cells where CO2 is released around RuBisCO.

What causes photorespiration in C3 plants and why is it energetically wasteful?

When stomata close to conserve water, internal O2 levels rise and RuBisCO oxygenates RuBP instead of carboxylating it, consuming ATP and organic carbon without producing sugars.

How does the First Law of Thermodynamics apply to biological systems?

Energy cannot be created or destroyed, only transformed; photosynthetic organisms convert radiant solar energy into chemical bond energy in carbohydrates, which heterotrophs transform into ATP and heat.

How does alteration of pH away from an enzyme's optimum diminish catalytic activity?

Changes in H+ concentration alter the ionization states of amino acid R-groups in the active site, disrupting ionic bonds and inhibiting substrate binding.

How do CAM plants adapt their photosynthetic carbon fixation to arid environments?

They temporally separate fixation: stomata open at night to fix CO2 into malic acid stored in central vacuoles; stomata close by day while stored CO2 is released for the Calvin cycle using sunlight.

How does allosteric feedback inhibition regulate unneeded metabolic synthesis?

The end product of a multi-step biosynthetic pathway binds to an allosteric regulatory site on the initial commitment enzyme, inhibiting its activity and preventing resource overproduction.

How does ATP hydrolysis power non-spontaneous endergonic reactions in cells?

ATP hydrolysis is exergonic (ΔG°' ≈ -30.5 kJ/mol under standard biochemical conditions, and even more negative in living cells); cells couple this energy release to endergonic reactions primarily by transferring a terminal phosphate group to a reactant to form a reactive phosphorylated intermediate.

What are the total products generated by the Krebs cycle from one glucose molecule (two turns)?

Two turns of the Krebs cycle yield 4 CO2, 6 NADH, 2 FADH2, and 2 ATP (or GTP) via substrate-level phosphorylation.

What is the final electron acceptor in aerobic cellular respiration?

Molecular oxygen (O2) acts as the terminal electron acceptor at Complex IV, combining with electrons and free protons to form H2O.

What are the net inputs and net outputs of glycolysis per molecule of glucose?

Net inputs are 1 glucose, 2 NAD+, and 2 ADP + 2 Pi; net outputs are 2 pyruvate, 2 NADH + 2 H+, 2 H2O, and 2 net ATP (2 ATP invested in the preparatory phase and 4 ATP synthesized in the payoff phase via substrate-level phosphorylation).

Why is fermentation essential for cells when molecular oxygen is unavailable?

Fermentation oxidizes NADH back into NAD+ by transferring electrons to pyruvate or acetaldehyde, replenishing the NAD+ pool required for glycolysis to continue generating ATP.

Under what condition do chloroplasts engage in cyclic photophosphorylation?

When the chloroplast requires additional ATP relative to NADPH (such as to meet the Calvin cycle's 9 ATP : 6 NADPH consumption ratio), photoexcited electrons from PSI (P700) cycle through ferredoxin back to the cytochrome b6f complex and plastocyanin, driving proton pumping and ATP synthesis without producing NADPH or O2.

What is the primary role of Photosystem I during non-cyclic photophosphorylation?

PSI re-excites electrons via the P700 reaction center and passes them through ferredoxin to NADP+ reductase, reducing NADP+ to NADPH in the stroma.

How does a noncompetitive inhibitor affect the enzymatic parameters Vmax and Km?

In pure noncompetitive inhibition, the inhibitor binds to an allosteric site on either the free enzyme (E) or the enzyme-substrate complex (ES) with equal affinity, decreasing Vmax by reducing the concentration of catalytically active enzyme while leaving Km unchanged.

AP Biology: Cellular Energetics, Photosynthesis, Respiration & Enzymes

AP Biology Unit 3 review covering enzyme catalysis and inhibition, Gibbs free energy, glycolysis, the citric acid cycle, oxidative phosphorylation, light-dependent reactions, and the Calvin cycle.

Level: basic
Languages: en → en
Cards: 32
Markdown: AP Biology: Cellular Energetics, Photosynthesis, Respiration & Enzymes (.md)

Flashcards (32)

#	Front (Term / Question)	Back (Definition / Answer)
1	What distinguishes exergonic reactions from endergonic reactions in biochemical pathways?	Exergonic reactions have a negative ΔG and release free energy spontaneously, whereas endergonic reactions have a positive ΔG and require an input of free energy to proceed.
2	Where do the light-dependent reactions of photosynthesis take place in plant cells?	They occur on the thylakoid membranes within chloroplasts, where chlorophylls and electron transport complexes are embedded.
3	Why does enzyme activity drop sharply at temperatures above the optimal thermal threshold?	Thermal kinetic energy disrupts weak hydrogen bonds and hydrophobic interactions stabilizing the tertiary structure, causing denaturation and loss of functional active site geometry.
4	How do biological enzymes accelerate metabolic chemical reactions?	Enzymes act as biological catalysts by lowering the activation energy (Ea) required to reach the transition state, without altering the equilibrium constant or the overall change in Gibbs free energy (ΔG).
5	Where does glycolysis occur within eukaryotic cells, and does it require oxygen?	Glycolysis takes place in the cytoplasm (cytosol) and is an anaerobic process that does not require molecular oxygen.
6	Where does the Calvin cycle take place and what is its primary catalytic enzyme?	The Calvin cycle occurs in the stroma of chloroplasts, and its initial carbon-fixation step is catalyzed by ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO).
7	What is the functional difference between an inorganic cofactor and an organic coenzyme?	A cofactor is any non-protein chemical component required for enzyme catalysis, including inorganic metal ions (such as Mg2+, Zn2+, or Fe2+/Fe3+); when the non-protein cofactor is an organic molecule (often derived from vitamins, such as NAD+, FAD, or coenzyme A), it is specifically called a coenzyme.
8	What are the end products of lactic acid fermentation versus alcoholic fermentation?	Lactic acid fermentation reduces pyruvate to lactate with no gas produced; alcoholic fermentation decarboxylates pyruvate to acetaldehyde and reduces it to ethanol, releasing CO2.
9	How do mitochondrial uncoupling proteins (e.g., UCP1 / thermogenin) generate non-shivering heat?	Uncoupling proteins allow protons to re-enter the mitochondrial matrix without passing through ATP synthase, dissipating the proton motive force directly as heat instead of trapping it as ATP.
10	What is the function of the water-splitting complex (photolysis) in Photosystem II?	The oxygen-evolving (water-splitting) complex in Photosystem II oxidizes two water molecules into molecular oxygen (O2), four protons (H+ released into the thylakoid lumen), and four electrons, replacing the photoexcited electrons lost by the P680 reaction-center chlorophyll pair and contributing to the lumen proton gradient.
11	How does the mitochondrial electron transport chain establish a proton motive force?	Electrons from NADH (entering at Complex I) and FADH2 (entering at Complex II) pass through mobile carriers (ubiquinone and cytochrome c) to Complex III and Complex IV; free energy released during electron transfer drives Complexes I, III, and IV to pump protons (H+) from the mitochondrial matrix into the intermembrane space (Complex II does not pump protons), establishing an electrochemical proton gradient.
12	What reaction occurs during the carbon fixation phase of the Calvin cycle?	RuBisCO fixes one CO2 molecule onto a 5-carbon ribulose-1,5-bisphosphate (RuBP) molecule, creating an unstable 6-carbon intermediate that splits into two 3-phosphoglycerate (3-PGA) molecules.
13	What occurs during pyruvate oxidation before entering the Krebs cycle?	Pyruvate is transported into the mitochondrial matrix, decarboxylated (releasing CO2), oxidized to transfer electrons to NAD+ forming NADH, and coupled to Coenzyme A to yield acetyl-CoA.
14	How does a competitive inhibitor alter the Michaelis-Menten kinetic parameters Vmax and Km?	A reversible competitive inhibitor binds to the active site, increasing the apparent Km (more substrate is required to reach half-maximal velocity, 1/2 Vmax) while leaving Vmax unchanged because sufficiently high substrate concentrations outcompete the inhibitor.
15	How does ATP synthase utilize chemiosmosis to produce ATP?	Protons flow down their electrochemical gradient from the intermembrane space back into the matrix through the Fo rotor subunit, driving the catalytic F1 subunit to phosphorylate ADP into ATP.
16	What does the induced-fit model describe regarding enzyme-substrate interactions?	Substrate binding induces a conformational change in the enzyme active site that aligns catalytic residues precisely around the substrate, stabilizing the transition state.
17	How many molecules of ATP and NADPH are consumed to produce one net G3P exported from the Calvin cycle?	Synthesizing one net exported glyceraldehyde-3-phosphate (G3P, a 3-carbon sugar) requires fixing 3 CO2 molecules onto 3 RuBP and consuming 9 ATP and 6 NADPH (generating 6 G3P in the reduction phase, of which 1 G3P exits the cycle and 5 G3P are rearranged to regenerate 3 RuBP).
18	How does the Second Law of Thermodynamics explain the constant energy requirement of living organisms?	Every energy transfer increases the universal entropy (disorder); organisms must continuously consume external free energy to maintain internal biological order and prevent cellular decay.
19	How do C4 plants anatomically and biochemically bypass photorespiration?	They spatially separate carbon fixation: PEP carboxylase fixes CO2 into oxaloacetate in mesophyll cells, which is transported as malate to bundle-sheath cells where CO2 is released around RuBisCO.
20	What causes photorespiration in C3 plants and why is it energetically wasteful?	When stomata close to conserve water, internal O2 levels rise and RuBisCO oxygenates RuBP instead of carboxylating it, consuming ATP and organic carbon without producing sugars.
21	How does the First Law of Thermodynamics apply to biological systems?	Energy cannot be created or destroyed, only transformed; photosynthetic organisms convert radiant solar energy into chemical bond energy in carbohydrates, which heterotrophs transform into ATP and heat.
22	How does alteration of pH away from an enzyme's optimum diminish catalytic activity?	Changes in H+ concentration alter the ionization states of amino acid R-groups in the active site, disrupting ionic bonds and inhibiting substrate binding.
23	How do CAM plants adapt their photosynthetic carbon fixation to arid environments?	They temporally separate fixation: stomata open at night to fix CO2 into malic acid stored in central vacuoles; stomata close by day while stored CO2 is released for the Calvin cycle using sunlight.
24	How does allosteric feedback inhibition regulate unneeded metabolic synthesis?	The end product of a multi-step biosynthetic pathway binds to an allosteric regulatory site on the initial commitment enzyme, inhibiting its activity and preventing resource overproduction.
25	How does ATP hydrolysis power non-spontaneous endergonic reactions in cells?	ATP hydrolysis is exergonic (ΔG°' ≈ -30.5 kJ/mol under standard biochemical conditions, and even more negative in living cells); cells couple this energy release to endergonic reactions primarily by transferring a terminal phosphate group to a reactant to form a reactive phosphorylated intermediate.
26	What are the total products generated by the Krebs cycle from one glucose molecule (two turns)?	Two turns of the Krebs cycle yield 4 CO2, 6 NADH, 2 FADH2, and 2 ATP (or GTP) via substrate-level phosphorylation.
27	What is the final electron acceptor in aerobic cellular respiration?	Molecular oxygen (O2) acts as the terminal electron acceptor at Complex IV, combining with electrons and free protons to form H2O.
28	What are the net inputs and net outputs of glycolysis per molecule of glucose?	Net inputs are 1 glucose, 2 NAD+, and 2 ADP + 2 Pi; net outputs are 2 pyruvate, 2 NADH + 2 H+, 2 H2O, and 2 net ATP (2 ATP invested in the preparatory phase and 4 ATP synthesized in the payoff phase via substrate-level phosphorylation).
29	Why is fermentation essential for cells when molecular oxygen is unavailable?	Fermentation oxidizes NADH back into NAD+ by transferring electrons to pyruvate or acetaldehyde, replenishing the NAD+ pool required for glycolysis to continue generating ATP.
30	Under what condition do chloroplasts engage in cyclic photophosphorylation?	When the chloroplast requires additional ATP relative to NADPH (such as to meet the Calvin cycle's 9 ATP : 6 NADPH consumption ratio), photoexcited electrons from PSI (P700) cycle through ferredoxin back to the cytochrome b6f complex and plastocyanin, driving proton pumping and ATP synthesis without producing NADPH or O2.
31	What is the primary role of Photosystem I during non-cyclic photophosphorylation?	PSI re-excites electrons via the P700 reaction center and passes them through ferredoxin to NADP+ reductase, reducing NADP+ to NADPH in the stroma.
32	How does a noncompetitive inhibitor affect the enzymatic parameters Vmax and Km?	In pure noncompetitive inhibition, the inhibitor binds to an allosteric site on either the free enzyme (E) or the enzyme-substrate complex (ES) with equal affinity, decreasing Vmax by reducing the concentration of catalytically active enzyme while leaving Km unchanged.