Two Sides of the Same Coin
Photosynthesis and cellular respiration are the two most important metabolic processes on Earth. They are essentially reverse reactions of each other. Photosynthesis captures light energy and stores it in glucose: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Cellular respiration releases that stored energy by breaking down glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy).
Together, these processes form a cycle: the oxygen produced by photosynthesis is consumed by cellular respiration, and the carbon dioxide produced by respiration is consumed by photosynthesis. The glucose made by plants feeds both the plants themselves and the animals that eat them. Without photosynthesis, there would be no oxygen or food. Without cellular respiration, organisms could not extract energy from food to power life processes.
A key distinction: photosynthesis is performed only by autotrophs (organisms that make their own food) — primarily plants, algae, and some bacteria. Cellular respiration is performed by nearly all living organisms, including plants. Yes, plants perform both photosynthesis AND cellular respiration. During the day, photosynthesis typically exceeds respiration, so plants are net oxygen producers. At night, only respiration occurs.
Where Each Process Occurs
Photosynthesis takes place in chloroplasts, organelles found primarily in the mesophyll cells of leaves. The light-dependent reactions occur on the thylakoid membranes, where chlorophyll and other pigments absorb light. The light-independent reactions (Calvin cycle) occur in the stroma, the fluid-filled space surrounding the thylakoids.
Cellular respiration takes place mainly in mitochondria, though the first stage (glycolysis) occurs in the cytoplasm. The Krebs cycle occurs in the mitochondrial matrix (the fluid inside the inner membrane). The electron transport chain and oxidative phosphorylation occur on the inner mitochondrial membrane (cristae), which is highly folded to increase surface area for ATP production.
There is a beautiful parallel in the internal structure of these organelles. Both chloroplasts and mitochondria have double membranes and their own DNA. Both use electron transport chains embedded in internal membranes. Both generate a proton gradient to drive ATP synthase. These structural and functional similarities are explained by the endosymbiotic theory: both organelles likely evolved from ancient bacteria that were engulfed by primitive eukaryotic cells.
Energy Carriers: ATP, NADPH, and NADH
ATP (adenosine triphosphate) is the universal energy currency of cells. It stores energy in the bonds between its three phosphate groups. When a cell needs energy, it breaks the bond between the second and third phosphate groups, releasing energy and producing ADP (adenosine diphosphate) plus inorganic phosphate. Cellular respiration is the primary process that regenerates ATP from ADP.
In photosynthesis, the light reactions produce two energy carriers: ATP and NADPH (nicotinamide adenine dinucleotide phosphate, reduced form). These are used to power the Calvin cycle, which fixes carbon dioxide into glucose. NADPH acts as an electron carrier, donating high-energy electrons to reduce CO₂ into organic molecules.
In cellular respiration, the analogous electron carrier is NADH (nicotinamide adenine dinucleotide, reduced form) and its relative FADH₂. Glycolysis and the Krebs cycle produce NADH and FADH₂, which then donate their electrons to the electron transport chain. As electrons flow through the chain, their energy is used to pump protons and ultimately drive ATP synthesis. A single glucose molecule can yield approximately 30-32 ATP through aerobic respiration.
The Stages of Cellular Respiration
Glycolysis is the first stage and occurs in the cytoplasm. It splits one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each), producing a net gain of 2 ATP and 2 NADH. Glycolysis does not require oxygen and is the foundation of both aerobic and anaerobic respiration. It is believed to be the most ancient metabolic pathway, evolving before Earth's atmosphere contained significant oxygen.
The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix. Each pyruvate is first converted to acetyl-CoA (releasing one CO₂ and one NADH per pyruvate). Acetyl-CoA then enters the cycle, which fully oxidizes the remaining carbons, producing 2 CO₂, 3 NADH, 1 FADH₂, and 1 GTP (equivalent to ATP) per turn. Since glucose produces two pyruvates, the Krebs cycle turns twice per glucose, yielding a total of 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP.
The electron transport chain (ETC) and oxidative phosphorylation produce the vast majority of ATP — about 26-28 molecules per glucose. NADH and FADH₂ donate electrons to a series of protein complexes in the inner mitochondrial membrane. As electrons pass through Complexes I, III, and IV, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space. This creates a proton gradient that drives ATP synthase (Complex V), which produces ATP as protons flow back into the matrix. Oxygen serves as the final electron acceptor, combining with electrons and protons to form water.
Side-by-Side Comparison
Reactants: Photosynthesis uses CO₂ + H₂O + light. Cellular respiration uses glucose (C₆H₁₂O₆) + O₂. Products: Photosynthesis produces glucose + O₂. Cellular respiration produces CO₂ + H₂O + ATP. Energy: Photosynthesis converts light energy to chemical energy (endergonic, energy-storing). Cellular respiration converts chemical energy to ATP (exergonic, energy-releasing).
Location: Photosynthesis occurs in chloroplasts (thylakoids and stroma). Cellular respiration occurs in the cytoplasm (glycolysis) and mitochondria (Krebs cycle and ETC). Organisms: Photosynthesis is performed by plants, algae, and some bacteria. Cellular respiration is performed by virtually all living organisms.
Electron carriers: Photosynthesis produces NADPH (which donates electrons to the Calvin cycle). Cellular respiration produces NADH and FADH₂ (which donate electrons to the ETC). Both processes use ATP synthase driven by proton gradients (chemiosmosis), but in photosynthesis the gradient is across the thylakoid membrane, while in respiration it is across the inner mitochondrial membrane.
Anaerobic Respiration and Fermentation
When oxygen is absent, cells cannot use the electron transport chain. Instead, they rely on fermentation to regenerate NAD⁺ from NADH, allowing glycolysis to continue. Fermentation produces only 2 ATP per glucose (from glycolysis alone), compared to 30-32 from aerobic respiration — a massive efficiency difference.
There are two main types of fermentation. Lactic acid fermentation converts pyruvate directly to lactate (lactic acid). This occurs in your muscle cells during intense exercise when oxygen delivery cannot keep up with demand. It also occurs in certain bacteria, and is the basis for making yogurt, sauerkraut, and kimchi. Alcoholic fermentation converts pyruvate to ethanol and CO₂. This occurs in yeast and is the basis for brewing beer, making wine, and baking bread (the CO₂ makes bread rise).
Some organisms are obligate anaerobes — they cannot survive in oxygen. Others are facultative anaerobes — they can switch between aerobic respiration and fermentation depending on oxygen availability. Yeast is a classic example: it respires aerobically when oxygen is present (producing more ATP and growing faster) but switches to alcoholic fermentation when oxygen is depleted.
The Global Carbon and Oxygen Cycles
Photosynthesis and cellular respiration are the biological engines that drive the global carbon and oxygen cycles. Photosynthesis removes approximately 120 billion tons of carbon from the atmosphere each year, converting CO₂ into organic matter. Cellular respiration, decomposition, and combustion return carbon to the atmosphere. For billions of years, these processes maintained a rough balance that kept atmospheric CO₂ and O₂ at stable levels.
Human burning of fossil fuels has disrupted this balance by releasing carbon that was stored underground for millions of years, adding approximately 10 billion tons of extra carbon to the atmosphere annually. Deforestation compounds the problem by reducing the planet's photosynthetic capacity. Understanding the relationship between photosynthesis and respiration is essential for understanding climate change — it is not just biology, it is the chemistry of our planet's future.
For biology exams, remember that the outputs of one process are the inputs of the other. This cycle connects all living things on Earth in a single energy flow powered ultimately by the sun. If you need help comparing these processes or working through exam questions, ScanSolve can break down any biology concept step by step.