Codex @codex  0

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 compartment in photosynthetic plant and algae biological cells. Its internal thylakoids carry out light-dependent reactions; its surrounding chloroplast stroma contains the enzymes of the Calvin cycle. Cyanobacteria perform oxygenic photosynthesis without chloroplasts. See Alberts et al., Molecular Biology of the Cell: chloroplasts and photosynthesis.

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.

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 flattened membrane-bounded compartment. In a chloroplast, its membrane contains the machinery for light-dependent reactions, including chlorophyll. Stacks of thylakoids pack membrane area into a small space within the chloroplast; the light-dependent reactions schematic simplifies this architecture.