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ADP contains the same adenosine unit as ATP, with two phosphate groups instead of three. One ATP converted by the stated hydrolysis gives one ADP and one inorganic phosphate: $3=2+1$ accounts for the phosphate groups while adenine and ribose are retained. ADP is therefore a reusable material carrier, rather than ATP with its entire structure discarded.

Regeneration reverses the net chemical reaction:At the same stipulated reference conditions its $\Delta G^{\circ \prime }$ is $+30.5\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$, the negative of the hydrolysis value. It is an endergonic reaction and requires energy input through energy coupling; a catalyst alone cannot supply that input. Recycling the material carrier does not recycle the expended driving energy. Cellular respiration and light-dependent reactions supply routes for regeneration. See Cooper, The Cell: ATP generation and energy coupling.

ADP is a nucleotide with two phosphate groups. Formation of ATP from ADP and phosphate couples to energy supply in light-dependent reactions; the Calvin cycle returns ADP after using ATP. ^[ref].

The Calvin cycle couples carbon fixation to use of ATP and NADPH. In a chloroplast it takes place in the chloroplast stroma. Its chemical reactions incorporate carbon dioxide, use NADPH-supplied electrons, and regenerate the molecules that accept further carbon dioxide.

In the standard Calvin cycle bookkeeping, three carbon dioxide molecules require nine ATP and six NADPH for one net three-carbon triose phosphate. A six-carbon sugar equivalent therefore corresponds to six carbon dioxide, eighteen ATP and twelve NADPH; subsequent sugar synthesis is separate. These are pathway requirements, not a universal measured photosynthesis yield. See OpenStax Biology 2e, 8.3: Calvin cycle bookkeeping.

Calling the Calvin cycle light-independent means its individual chemical reactions do not directly capture light; it does not mean the Calvin cycle runs only at night or independently of the supplies from light-dependent reactions.

A chloroplast is a photosynthetic plastid in plant and algae biological cells. The green-plant chloroplast modeled here has a chloroplast envelope surrounding chloroplast stroma and internal thylakoids. Light-dependent reactions occur at the thylakoid membrane; the Calvin cycle operates in the chloroplast stroma.^[ref]

Cyanobacteria perform oxygenic photosynthesis without chloroplasts: the prokaryotic biological cell itself contains the photosynthetic machinery. Thus chloroplast identifies an organelle, while oxygenic photosynthesis identifies a process. Endosymbiotic theory explains their historical connection; organelle protein import explains how many nuclear-encoded proteins reach the modern organelle.

The useful feature is selective separation: enzymes and substrates can share an organelle without sharing a compartment. For a constructed accessibility example, put a soluble enzyme in chloroplast stroma and its substrate in thylakoid lumen. Sharing the same chloroplast does not permit direct contact; contact requires a transport or boundary-disruption step. Placing the substrate between adjacent thylakoids instead puts it on the stromal side. The visual stack alone cannot establish chemical accessibility.

For the simplest boundary-counting model, represent surrounding cytosol, chloroplast intermembrane space, chloroplast stroma and thylakoid lumen by four vertices in a chain. Its three edges represent the outer envelope, inner envelope and thylakoid membrane, respectively. A route from cytosol to chloroplast stroma crosses two barriers; extending it into thylakoid lumen adds a third. This follows by listing the edges along the chain. Counting the two leaflets of each lipid bilayer separately would answer a different question. These counts describe this idealized geometry, not the number of protein targeting signals or the transport itinerary of every protein.

The light-dependent reactions article develops ATP, NADPH and electrochemical gradient coupling, and the Calvin cycle article develops carbon fixation bookkeeping. Their different locations make the chloroplast interesting as an organized chemical workspace. Light-independent chemical reactions still depend on supplied carriers and regulation; the name does not prescribe nighttime operation.

Cyanobacteria are bacteria that perform oxygenic photosynthesis: light-dependent reactions release oxygen from water. Their photosynthetic machinery operates on thylakoid membranes without a chloroplast or a cell nucleus. This is a particularly useful counterexample to equating photosynthesis with being a plant. Chloroplasts descend from an ancestral cyanobacterial partner through endosymbiosis; present-day cyanobacteria are independent descendants, rather than chloroplasts waiting to become plants. See Alberts et al., Molecular Biology of the Cell: chloroplasts and cyanobacteria.

An electron transport chain transfers electrons from an electron donor through electron carriers to a terminal electron acceptor. In the mitochondrial respiratory chain, some transfers drive protons across the inner mitochondrial membrane; their return through ATP synthase can drive ATP formation.^[ref]

The useful separation is between an electron route and a proton route. The former ends in a redox chemical reaction; the latter changes an electrochemical gradient across a membrane. Energy coupling connects them without making electrons and protons interchangeable. An oxygen readout consequently need not be an ATP readout.

This article focuses on the mitochondrial respiratory chain. Bacteria can use other terminal electron acceptors in anaerobic respiration. In chloroplast light-dependent reactions, light supplies energy and linear electron transport connects water oxidation to NADP+ reduction, releasing oxygen. Thus neither oxygen consumption nor the mitochondrial arrangement defines every electron transport chain.^[ref]

In a chloroplast, light-dependent reactions take place at thylakoid membranes. Chlorophyll captures light; electron transfer helps establish an electrochemical gradient across the thylakoid membrane, enabling formation of ATP. The oxygenic photosynthesis pathway also forms NADPH and releases molecular oxygen from water. ATP and NADPH become available in the chloroplast stroma for the Calvin cycle. See Alberts et al.: chloroplast light reactions.

NADPH is the reduced form of nicotinamide adenine dinucleotide phosphate. It carries electrons used in synthesis; in the Calvin cycle it becomes the oxidized form NADP+, available for reuse by light-dependent reactions. NADPH and ATP serve different roles even though both connect light capture to carbon fixation.

NADP+ is the oxidized form of nicotinamide adenine dinucleotide phosphate. It can accept electrons to form NADPH; the light-dependent reactions supply this conversion during oxygenic photosynthesis.

Nicotinamide adenine dinucleotide phosphate is a cofactor used in metabolism. Its oxidized and reduced forms, NADP+ and NADPH, carry electrons between coupled chemical reactions. In oxygenic photosynthesis, light-dependent reactions produce NADPH for the Calvin cycle, which returns NADP+ for reuse. ^[ref].

A thylakoid is a membrane-enclosed sac; its thylakoid membrane contains chlorophyll and light-dependent reactions machinery. A stack is a granum.^[ref]

The membrane and its enclosed fluid answer different location questions. A soluble protein in thylakoid lumen is inside the sac; a membrane protein is embedded in its boundary and can expose different parts on the two sides. Membrane topology and organelle protein import provide the reusable language for those distinctions.