Apply Cellular Energy Processes — Apply principles of cellular respiration and photosynthesis. - TEAS Science

SECTION 1

Historical Context & Motivation

The question of how living organisms obtain and transform energy has driven some of the most consequential discoveries in biology and chemistry. Before the molecular era, scientists recognized that plants required light and air to grow, and that animals consumed food to sustain life, yet the underlying biochemical mechanisms remained elusive. The elucidation of photosynthesis and cellular respiration as complementary metabolic processes required centuries of incremental experimentation, spanning the disciplines of physiology, organic chemistry, and eventually molecular biology. These discoveries collectively established that all living systems obey the laws of thermodynamics, channeling free energy through precisely regulated enzymatic pathways to power biosynthesis, transport, and mechanical work.

1771 • Priestley's Observations

Joseph Priestley demonstrated that plants "restore" air that has been depleted by a burning candle, providing the first experimental evidence that vegetation produces a gas essential for combustion and animal life—later identified as oxygen.

1842 • Mayer's Energy Conservation

Julius Robert von Mayer proposed that plants convert solar energy into chemical energy, articulating a biological application of the first law of thermodynamics and establishing the conceptual link between light and biomass.

1937 • Krebs Cycle Elucidated

Hans Krebs mapped the tricarboxylic acid (TCA) cycle, revealing the cyclical series of oxidation reactions in the mitochondrial matrix that systematically extracts electrons from acetyl-CoA for downstream ATP production.

1954 • Calvin Cycle Described

Melvin Calvin, Andrew Benson, and James Bassham used radioactive carbon-14 tracers to delineate the light-independent reactions of photosynthesis, demonstrating how CO₂ is fixed into three-carbon sugars in the chloroplast stroma.

1961 • Mitchell's Chemiosmotic Hypothesis

Peter Mitchell proposed that ATP synthesis is driven by an electrochemical proton gradient across a membrane, unifying the mechanisms of oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts under a single bioenergetic paradigm.

These historical milestones converge on a central question that remains foundational for biology and medicine: how do cells couple exergonic and endergonic reactions to efficiently transduce energy from the environment into the universal currency of ATP? Understanding these pathways at a mechanistic level is essential for interpreting metabolic disorders, pharmacological interventions targeting mitochondrial function, and ecological models of primary productivity—all of which appear in graduate-level assessments.

SECTION 2

Core Principles & Definitions

Both photosynthesis and cellular respiration are governed by fundamental thermodynamic and biochemical principles. Energy transformations in biological systems are not exceptions to physical law; rather, they represent exquisitely evolved mechanisms for harnessing free-energy changes to perform work. The following foundational concepts underpin every stage of these metabolic pathways, from the initial capture of photons in photosystem II to the final transfer of electrons to molecular oxygen in the mitochondrial electron transport chain.

Free Energy (ΔG) and Coupled Reactions

A reaction proceeds spontaneously when ΔG < 0 (exergonic). Cells couple exergonic reactions—such as glucose oxidation—to endergonic ones—such as ATP synthesis—so that the overall ΔG remains negative. This coupling is the thermodynamic engine of metabolism.

Redox Chemistry as Electron Shuttling

Both pathways rely on oxidation-reduction reactions. Electron carriers such as NAD⁺/NADH, FAD/FADH₂, and NADP⁺/NADPH serve as mobile electron shuttles, transferring high-energy electrons between metabolic intermediates and membrane-bound complexes.

Chemiosmosis and the Proton-Motive Force

ATP synthase harnesses the electrochemical gradient (Δp) generated by electron transport chains. The proton-motive force comprises both a chemical component (ΔpH) and an electrical component (Δψ), driving rotational catalysis of ATP from ADP + Pᵢ.

Compartmentalization

Metabolic efficiency depends on spatial organization. Glycolysis occurs in the cytoplasm; the Krebs cycle in the mitochondrial matrix; the electron transport chain on the inner mitochondrial membrane. In chloroplasts, light reactions occur on thylakoid membranes while the Calvin cycle operates in the stroma.

Reciprocal Relationship

Photosynthesis and cellular respiration are essentially inverse processes at the level of net chemistry. The products of one (O₂ and C₆H₁₂O₆) serve as substrates for the other, establishing a global biogeochemical cycle that sustains aerobic life.

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KEY TAKEAWAY

Think of photosynthesis and cellular respiration as two halves of a rechargeable battery system. Photosynthesis acts as the charger, using solar energy to reduce CO₂ into glucose—thereby storing energy in chemical bonds. Cellular respiration is the device drawing current: it oxidizes glucose, releasing that stored energy in controlled increments to phosphorylate ADP into ATP. The elegance of the system lies in the stepwise transfer of electrons through carrier molecules and membrane complexes, which prevents catastrophic energy loss and maximizes thermodynamic efficiency—analogous to a step-down transformer managing voltage in an electrical grid rather than discharging all energy at once.

SECTION 3

Visual Overview of Photosynthesis & Cellular Respiration

This diagram illustrates the reciprocal relationship between photosynthesis (left, green border) and cellular respiration (right, cyan border). Note how the products of photosynthesis—glucose and O₂—feed directly into respiration, while CO₂ and H₂O cycle back. The ATP yield breakdown (lower right) shows that the vast majority of ATP is generated via oxidative phosphorylation on the inner mitochondrial membrane.

The diagram above captures the essential reciprocity between the two pathways. In photosynthesis, light energy is transduced into chemical bond energy through a two-stage process: the light-dependent reactions on the thylakoid membranes generate ATP and NADPH while splitting water to release O₂, and the Calvin cycle in the stroma uses these energy carriers to fix atmospheric CO₂ into glyceraldehyde-3-phosphate (G3P), the precursor to glucose. Cellular respiration then reverses this energy flow: glycolysis in the cytoplasm cleaves glucose into two molecules of pyruvate, the Krebs cycle fully oxidizes the carbon skeleton to CO₂ in the mitochondrial matrix, and the electron transport chain (ETC) on the inner mitochondrial membrane captures the released electrons in a stepwise fashion, ultimately reducing O₂ to H₂O and generating the proton gradient that drives ATP synthase. The net yield of approximately 30–32 ATP molecules per glucose reflects the thermodynamic efficiency of this multi-compartment system.

SECTION 4

Mechanistic Framework: Energetics & Equations

Although biological energy transformations involve dozens of enzymatic steps, the net stoichiometry can be expressed concisely. Understanding these summary equations—and the free-energy changes they represent—is critical for answering TEAS-level questions about metabolic efficiency, oxygen consumption, and carbon cycling.

NET PHOTOSYNTHESIS

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

This is an endergonic reaction (ΔG° ≈ +2870 kJ/mol). The positive ΔG indicates that external energy input—in this case, photons absorbed by chlorophyll—is required to drive the reaction forward. Each molecule of glucose synthesized stores approximately 2870 kJ of chemical potential energy in its C–H and C–O bonds.

NET AEROBIC CELLULAR RESPIRATION

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ≈30–32 ATP

This is an exergonic reaction (ΔG° ≈ −2870 kJ/mol). The negative ΔG means the reaction releases free energy spontaneously. The cell captures roughly 40% of this energy as ATP (≈30.5 kJ/mol per ATP), with the remainder dissipated as heat—consistent with the second law of thermodynamics.

ANAEROBIC FERMENTATION (LACTIC ACID)

C₆H₁₂O₆ → 2C₃H₆O₃ + 2 ATP

In the absence of oxygen, pyruvate is reduced to lactate (in animals) or ethanol + CO₂ (in yeast). Fermentation regenerates NAD⁺ to sustain glycolysis but yields only 2 ATP per glucose—a dramatically lower yield than aerobic respiration, underscoring the energetic advantage of an electron transport chain coupled to O₂ as the terminal electron acceptor.

CHEMIOSMOTIC ATP SYNTHESIS

ADP + Pᵢ + H⁺(intermembrane space) → ATP + H₂O

ATP synthase (Complex V) catalyzes this phosphorylation using the proton-motive force (Δp). Approximately 3–4 H⁺ must flow through the enzyme per ATP molecule synthesized. The proton-motive force itself is generated by Complexes I, III, and IV pumping protons from the matrix into the intermembrane space as electrons cascade through the chain.

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Why ≈30–32 ATP Instead of 36–38?

Older textbook values (36–38 ATP) assumed an idealized P/O ratio and did not account for the energy cost of transporting ATP, ADP, and Pᵢ across the inner mitochondrial membrane via the adenine nucleotide translocase and the phosphate carrier. Modern estimates using revised P/O ratios (approximately 2.5 for NADH and 1.5 for FADH₂) yield the accepted range of 30–32 ATP per glucose. This distinction is commonly tested on graduate-admission science assessments.

SECTION 5

Stage-by-Stage Breakdown

A detailed understanding of each stage—its location, substrates, products, and regulatory checkpoints—is indispensable for applying these concepts to exam questions that require you to trace carbon atoms, predict the effects of metabolic inhibitors, or calculate ATP yields under varying conditions.

This stage-by-stage flowchart details cellular respiration from glycolysis through oxidative phosphorylation (top row), photosynthesis stages (bottom left), and anaerobic fermentation (bottom right). Electron carrier yields at each stage are highlighted in amber. Note that glycolysis is the only stage occurring in the cytoplasm; all subsequent aerobic steps are compartmentalized within the mitochondrion.

Stage	Location	Key Inputs	Key Outputs	ATP Yield
Glycolysis	Cytoplasm	Glucose, 2 NAD⁺, 2 ATP	2 Pyruvate, 2 NADH, 4 ATP	2 net ATP
Pyruvate Oxidation	Mitochondrial matrix	2 Pyruvate, 2 NAD⁺, 2 CoA	2 Acetyl-CoA, 2 NADH, 2 CO₂	0 (indirect)
Krebs Cycle (×2)	Mitochondrial matrix	2 Acetyl-CoA, 6 NAD⁺, 2 FAD	4 CO₂, 6 NADH, 2 FADH₂, 2 GTP	2 ATP (via GTP)
Oxidative Phosphorylation	Inner mitochondrial membrane	10 NADH, 2 FADH₂, O₂	H₂O, NAD⁺, FAD	≈26–28 ATP
Light Reactions	Thylakoid membranes	H₂O, NADP⁺, ADP + Pᵢ, light	O₂, NADPH, ATP	Variable (for Calvin cycle)
Calvin Cycle	Chloroplast stroma	CO₂, ATP, NADPH	G3P, ADP, NADP⁺	Consumes 9 ATP / 3CO₂

Summary of Metabolic Stages: Inputs, Outputs, and ATP Yields

SECTION 6

Worked Example: Tracking ATP & Electron Carriers

The following worked example demonstrates how to trace the complete oxidation of one molecule of glucose through aerobic respiration, tallying every ATP, NADH, and FADH₂ produced, and then converting reduced electron carriers to ATP equivalents using current P/O ratios. This type of accounting is frequently tested on graduate-admission science exams.

Total ATP Yield from One Molecule of Glucose

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Step 1 — Glycolysis (Cytoplasm)

Glucose is phosphorylated twice (costing 2 ATP), then cleaved into two molecules of glyceraldehyde-3-phosphate (G3P). Each G3P is oxidized and phosphorylated, yielding 2 NADH and 4 ATP total for both G3P molecules. The net ATP from glycolysis is 4 − 2 = 2 ATP.

Glycolysis: 2 ATP (net) + 2 NADH
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Step 2 — Pyruvate Oxidation (Mitochondrial Matrix)

Each pyruvate is decarboxylated by pyruvate dehydrogenase, releasing one CO₂ and transferring electrons to NAD⁺ to produce NADH. The acetyl group is attached to coenzyme A, forming acetyl-CoA. Since two pyruvates enter per glucose, this step generates 2 NADH and releases 2 CO₂.

Pyruvate oxidation: 0 ATP + 2 NADH + 2 CO₂
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Step 3 — Krebs Cycle (×2 turns, Mitochondrial Matrix)

Each turn of the Krebs cycle oxidizes one acetyl-CoA (2 carbons) completely to 2 CO₂, generating 3 NADH, 1 FADH₂, and 1 GTP (equivalent to 1 ATP) via substrate-level phosphorylation. Two turns per glucose double these values.

Krebs cycle (×2): 2 ATP (GTP) + 6 NADH + 2 FADH₂ + 4 CO₂
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Step 4 — Tally Electron Carriers

Summing across all preceding stages: glycolysis contributes 2 NADH, pyruvate oxidation contributes 2 NADH, and the Krebs cycle contributes 6 NADH plus 2 FADH₂. Total: 10 NADH and 2 FADH₂ per glucose.

Carriers entering ETC: 10 NADH + 2 FADH₂
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Step 5 — Oxidative Phosphorylation (Inner Mitochondrial Membrane)

Using the revised P/O ratios: each NADH yields approximately 2.5 ATP, and each FADH₂ yields approximately 1.5 ATP (FADH₂ donates electrons at Complex II, bypassing the proton-pumping step at Complex I). Therefore: (10 × 2.5) + (2 × 1.5) = 25 + 3 = 28 ATP from oxidative phosphorylation. Note: the 2 NADH from glycolysis may yield only 1.5 ATP each if transported via the glycerol-3-phosphate shuttle (rather than the malate-aspartate shuttle), reducing the total by 2 ATP in some cell types.

Oxidative phosphorylation: ≈26–28 ATP
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Step 6 — Grand Total

Adding substrate-level phosphorylation ATP (2 from glycolysis + 2 from Krebs) to oxidative phosphorylation ATP (26–28): the total yield is approximately 30–32 ATP per molecule of glucose fully oxidized under aerobic conditions.

Total: ≈30–32 ATP per glucose

SECTION 7

Photosynthesis vs. Cellular Respiration — Detailed Comparison

While photosynthesis and cellular respiration are often described as "reverse" reactions, this characterization, though thermodynamically accurate at the net-equation level, obscures important mechanistic distinctions. The two pathways employ different enzymes, different electron carriers, and operate in different organelles with distinct membrane architectures. The following table provides a side-by-side comparison of their key features, designed to clarify the parallels and divergences that commonly appear on standardized science assessments.

Feature	Photosynthesis	Cellular Respiration
Organelle	Chloroplast	Mitochondrion (and cytoplasm for glycolysis)
Overall ΔG°	+2870 kJ/mol (endergonic)	−2870 kJ/mol (exergonic)
Energy Source	Light (photons)	Chemical bond energy (glucose)
Carbon Pathway	CO₂ fixed → organic molecules (reduction)	Organic molecules oxidized → CO₂ (oxidation)
Electron Carrier	NADP⁺ / NADPH	NAD⁺ / NADH; FAD / FADH₂
O₂ Role	Released as byproduct (from H₂O)	Consumed as terminal electron acceptor
H₂O Role	Split (photolysis) to donate electrons	Formed as final product at Complex IV
ETC Location	Thylakoid membrane	Inner mitochondrial membrane
Proton Gradient Direction	H⁺ pumped into thylakoid lumen	H⁺ pumped into intermembrane space
Organisms	Photoautotrophs (plants, algae, cyanobacteria)	Nearly all eukaryotes and many prokaryotes

Side-by-Side Comparison of Photosynthesis and Cellular Respiration

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KEY TAKEAWAY

Despite their inverse stoichiometries, photosynthesis and respiration share a common bioenergetic strategy: both use electron transport chains to establish a proton gradient, and both drive ATP synthesis through chemiosmosis. This mechanistic parallelism reflects their deep evolutionary connection—the endosymbiotic origins of both mitochondria and chloroplasts. Think of the proton gradient as a dam holding back water: the ETC pumps water (protons) uphill, and ATP synthase is the turbine that converts the flow back downhill into useful work (ATP). The dam analogy works for both organelles; only the energy source filling the reservoir differs (light vs. chemical oxidation).

SECTION 8

Connections to Advanced Metabolic Concepts

The core pathways of photosynthesis and cellular respiration serve as metabolic hubs that interconnect with lipid metabolism, amino acid catabolism, gluconeogenesis, and the pentose phosphate pathway. Graduate-level assessments often test your ability to recognize these cross-connections and predict the metabolic consequences of mutations, toxins, or pharmacological agents that target specific enzymes or complexes.

Concept	Core Pathway	Advanced Extension
Substrate-level phosphorylation	Direct transfer of phosphate to ADP (glycolysis, Krebs)	Contrasted with oxidative phosphorylation; becomes the sole ATP source during ischemia or mitochondrial dysfunction
Electron transport inhibitors	Cyanide blocks Complex IV; rotenone blocks Complex I	Understanding inhibitor sites allows prediction of NADH/FADH₂ accumulation and ATP collapse; relevant to toxicology and pharmacology
Uncoupling proteins (UCP)	Proton leak dissipates gradient as heat	Thermogenin (UCP1) in brown adipose tissue; 2,4-DNP as a chemical uncoupler; implications for metabolic rate and obesity research
C₃ vs. C₄ vs. CAM photosynthesis	Calvin cycle directly fixes CO₂ via RuBisCO	C₄ plants spatially separate CO₂ fixation (mesophyll) from the Calvin cycle (bundle sheath) to minimize photorespiration; CAM plants temporally separate these processes
Metabolic integration	Pyruvate as a branch point: to acetyl-CoA, lactate, or ethanol	Fatty acids enter as acetyl-CoA (β-oxidation); amino acids can be deaminated and enter at multiple Krebs cycle intermediates (e.g., α-ketoglutarate, oxaloacetate)

From Core Bioenergetics to Advanced Metabolic Concepts

Understanding these extensions helps contextualize TEAS questions that present clinical or experimental scenarios. For instance, a question might describe a patient with lactic acidosis and ask which metabolic stage is impaired, or it might present data on photosynthetic rate under varying CO₂ concentrations and require you to explain the plateau in terms of RuBisCO saturation kinetics. By grounding your understanding in the fundamental pathways covered in this lesson, you can reason through such scenarios systematically rather than relying on rote memorization.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Photosynthesis is described as an endergonic process and cellular respiration as an exergonic process. Explain what this means in terms of free energy (ΔG), and describe how the products of each process serve as the reactants of the other.

Show answer

An endergonic process has a positive ΔG, meaning it requires an input of energy to proceed—in photosynthesis, this energy comes from absorbed photons. An exergonic process has a negative ΔG, meaning it releases free energy spontaneously—in cellular respiration, the oxidation of glucose releases approximately 2870 kJ/mol. The reciprocal relationship is elegant: photosynthesis produces glucose (C₆H₁₂O₆) and oxygen (O₂), which are precisely the substrates consumed by cellular respiration. Conversely, respiration produces CO₂ and H₂O, which are the inputs for photosynthesis. This cyclical exchange of matter, driven by solar energy input, sustains the biosphere's carbon and oxygen cycles.

Question 2: BASIC CALCULATION

Calculate the maximum number of ATP molecules produced from the complete aerobic oxidation of one molecule of glucose. Assume the malate-aspartate shuttle is used for cytoplasmic NADH, and apply P/O ratios of 2.5 for NADH and 1.5 for FADH₂.

Show answer

The breakdown is as follows. Substrate-level phosphorylation contributes 2 ATP from glycolysis and 2 ATP (as GTP) from the Krebs cycle, totaling 4 ATP. Electron carriers generated: glycolysis produces 2 NADH, pyruvate oxidation produces 2 NADH, and the Krebs cycle produces 6 NADH + 2 FADH₂. With the malate-aspartate shuttle, all 10 NADH enter the ETC at Complex I. ATP from oxidative phosphorylation: (10 NADH × 2.5 ATP/NADH) + (2 FADH₂ × 1.5 ATP/FADH₂) = 25 + 3 = 28 ATP. Grand total: 4 + 28 = 32 ATP per glucose. If the glycerol-3-phosphate shuttle were used instead (as in skeletal muscle and brain), the 2 cytoplasmic NADH would yield only 1.5 ATP each, reducing the total to 30 ATP.

Question 3: INTERMEDIATE

A researcher adds oligomycin (an ATP synthase inhibitor) to isolated mitochondria actively oxidizing NADH. Predict the immediate effects on: (a) the proton gradient across the inner membrane, (b) the rate of electron transport, and (c) oxygen consumption.

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(a) With ATP synthase blocked, protons can no longer flow back through the enzyme into the matrix. Consequently, the proton gradient (Δp) across the inner mitochondrial membrane will increase, as pumping continues briefly before back-pressure builds. (b) The elevated proton gradient opposes further proton pumping by Complexes I, III, and IV, creating thermodynamic resistance. Electron transport therefore slows dramatically because the complexes cannot pump protons against a gradient that has become too steep. (c) Oxygen consumption decreases substantially because electron flow to the terminal acceptor (O₂ at Complex IV) is coupled to proton pumping. Since electron transport is inhibited by the elevated Δp, fewer electrons reach O₂, and O₂ consumption drops. This experiment classically demonstrates that electron transport and ATP synthesis are tightly coupled through the proton gradient.

Question 4: APPLIED

During intense anaerobic exercise, a sprinter's muscle cells switch from aerobic respiration to lactic acid fermentation. (a) Why does this switch occur? (b) How does the ATP yield per glucose change? (c) Why must NAD⁺ be regenerated, and how does fermentation accomplish this?

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(a) During intense exercise, oxygen delivery to muscle cells cannot keep pace with demand. Without sufficient O₂ as the terminal electron acceptor, the electron transport chain stalls, NADH accumulates, and oxidative phosphorylation ceases. Cells switch to fermentation to sustain glycolysis. (b) The ATP yield drops from approximately 30–32 ATP per glucose (aerobic) to only 2 ATP per glucose (anaerobic), because fermentation captures energy solely from substrate-level phosphorylation in glycolysis. (c) NAD⁺ must be regenerated because glycolysis requires it as an electron acceptor at the glyceraldehyde-3-phosphate dehydrogenase step. Without NAD⁺ recycling, glycolysis itself would halt, eliminating even the 2-ATP yield. Lactic acid fermentation regenerates NAD⁺ by reducing pyruvate to lactate via lactate dehydrogenase, transferring the electrons from NADH back to the organic substrate rather than to the ETC. This allows glycolysis to continue producing ATP, albeit at a much lower efficiency.

Question 5: CRITICAL THINKING

Plants possess both chloroplasts and mitochondria. During the daytime, a leaf cell performs photosynthesis and cellular respiration simultaneously. Construct a conceptual argument explaining whether the net O₂ exchange of this cell is positive, negative, or zero, and describe how you would experimentally distinguish the gross rate of photosynthesis from the net rate of gas exchange.

Show answer

During daylight with adequate light intensity, the rate of photosynthetic O₂ production typically exceeds the rate of mitochondrial O₂ consumption, yielding a net positive O₂ exchange (net O₂ release). However, the gross rate of photosynthesis is always higher than the apparent (net) rate because mitochondrial respiration simultaneously consumes some of the O₂ produced. At the light compensation point, photosynthesis and respiration rates are exactly equal, and net gas exchange is zero. To experimentally distinguish gross from net photosynthesis, one can measure O₂ evolution in the light (net photosynthesis), then measure O₂ consumption in the dark (respiration rate, assuming it remains constant), and calculate gross photosynthesis as: Gross Photosynthesis = Net Photosynthesis + Respiration. Alternatively, isotopic labeling with ¹⁸O₂ can trace mitochondrial oxygen consumption independently of photosynthetic O₂ release, providing a direct measurement of both processes occurring concurrently. This distinction between net and gross photosynthesis is central to understanding plant carbon budgets and ecosystem productivity models.

SUMMARY

Lesson Summary

Summary

Cellular energy processes revolve around two reciprocal metabolic pathways. Photosynthesis captures light energy in chloroplasts, using the light-dependent reactions on thylakoid membranes to split H₂O, release O₂, and produce ATP and NADPH, which then power the Calvin cycle in the stroma to fix CO₂ into G3P and ultimately glucose. Cellular respiration then oxidizes glucose through glycolysis (cytoplasm), pyruvate oxidation and the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane), yielding approximately 30–32 ATP per glucose.

Both pathways share the bioenergetic strategy of chemiosmosis: electron transport chains generate a proton-motive force across a membrane, and ATP synthase harnesses this gradient to phosphorylate ADP. When O₂ is unavailable, cells resort to fermentation (lactic acid or alcoholic), which regenerates NAD⁺ for glycolysis but yields only 2 ATP per glucose. Mastering the inputs, outputs, locations, and regulatory logic of each stage equips you to tackle TEAS questions involving metabolic inhibitors, ATP accounting, carbon tracing, and the ecological significance of the photosynthesis–respiration reciprocal cycle.