Short answer
Photosynthesis is the set of biochemical and biophysical processes by which photoautotrophs (plants, algae, cyanobacteria and some protists and bacteria) capture light energy and convert it into chemical energy, storing carbon as organic molecules and usually producing molecular oxygen (in oxygenic photosynthesis). It is the primary source of organic matter and most of the O2 in Earth's atmosphere and underpins virtually all food webs.

Comprehensive overview

1) Core definition and overall reaction
- Oxygenic photosynthesis (typical of plants, algae, and cyanobacteria):
  6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 (this is the conventional stoichiometric summary; the detailed balanced electron/proton accounting is more complex).
- Purpose: convert light energy into ATP and reducing power (NADPH) and use those to fix CO2 into carbohydrate (via the Calvin–Benson cycle).

2) Where it occurs (cellular anatomy)
- In eukaryotic photoautotrophs: chloroplasts.
  - Outer and inner envelope membranes.
  - Thylakoid membrane system (stacked grana, unstacked stroma lamellae) — site of the light reactions.
  - Stroma — site of the Calvin cycle and other dark reactions.
- In cyanobacteria: thylakoid-like membranes in the cytoplasm (no chloroplast envelope).
- Pigment-protein complexes embedded in thylakoid membranes capture light.

3) Two major sets of reactions
- Light-dependent reactions (light reactions)
  - Location: thylakoid membrane.
  - Inputs: light, H2O; Outputs: ATP, NADPH, O2 (in oxygenic organisms).
  - Key components:
    - Photosystem II (PSII) with reaction center P680 — absorbs light, drives water splitting (oxygen-evolving complex) producing O2, electrons and protons.
    - Plastoquinone (PQ) — mobile electron carrier.
    - Cytochrome b6f complex — pumps protons into lumen contributing to proton gradient.
    - Plastocyanin (PC) — transfers electrons to PSI.
    - Photosystem I (PSI) with reaction center P700 — absorbs light, raises electrons to reduce ferredoxin.
    - Ferredoxin (Fd) and ferredoxin–NADP+ reductase (FNR) reduce NADP+ to NADPH.
    - ATP synthase uses proton motive force across thylakoid membrane to synthesize ATP (photophosphorylation).
  - Two electron-flow modes:
    - Linear (non-cyclic) electron flow: water → PSII → PQ → Cyt b6f → PC → PSI → Fd → NADP+; yields NADPH and ATP, and releases O2.
    - Cyclic electron flow around PSI: electrons cycle from Fd back to PQ/Cyt b6f → increases proton pumping and ATP production without producing NADPH or O2 (helps balance ATP/NADPH demand).
  - Typical quantum cost: minimum ~8 photons per O2 (approximate, depends on system). ATP/NADPH stoichiometry varies with conditions; Calvin cycle typically requires more ATP than produced by strict noncyclic flow so cyclic flow or alternative pathways adjust.

- Light-independent reactions (Calvin–Benson cycle, often called “dark reactions” though they run in light)
  - Location: stroma.
  - Main purpose: fix CO2 into stable organic molecules using ATP and NADPH from light reactions.
  - Three phases:
    1. Carbon fixation: CO2 is fixed to ribulose-1,5-bisphosphate (RuBP, a 5-C sugar) by Rubisco (ribulose bisphosphate carboxylase/oxygenase), producing two molecules of 3-phosphoglycerate (3-PGA).
    2. Reduction: 3-PGA is phosphorylated (ATP) and reduced (NADPH) to form glyceraldehyde-3-phosphate (G3P or triose phosphate).
    3. Regeneration: RuBP is regenerated from some of the G3P using additional ATP so the cycle can continue.
  - Energetics (typical stoichiometry):
    - For fixing 3 CO2 (to produce one net G3P exported for biosynthesis): consumption ≈ 9 ATP and 6 NADPH.
    - For producing one hexose (C6) from CO2 (net): ~18 ATP and 12 NADPH (i.e., from 6 CO2).
  - Rubisco: most abundant enzyme on Earth; slow and imperfect—it also catalyzes oxygenation (see photorespiration).

4) Pigments and light absorption
- Chlorophylls: chlorophyll a (primary reaction center pigment) and chlorophyll b (accessory pigment) absorb mainly blue (~430–470 nm) and red (~640–680 nm) light. Chlorophyll a has peaks roughly near 430 nm (blue) and 662 nm (red).
- Carotenoids and phycobilins: accessory pigments that absorb blue/green and transfer energy to chlorophyll; also protect against photooxidative damage.
- Action spectrum vs absorption spectrum: action spectrum shows wavelengths that drive photosynthesis best; corresponds to pigment absorption but can be broader due to accessory pigments.

5) Variants and adaptations
- Anoxygenic photosynthesis (many bacteria): use bacteriochlorophylls and a single photosystem; do not split water and do not produce O2. Electron donors include H2S, H2, or organic compounds.
- C3 photosynthesis: standard Calvin cycle, common in temperate plants. Vulnerable to photorespiration when O2 competes with CO2 at Rubisco.
- C4 photosynthesis: spatial separation—CO2 initially fixed in mesophyll by PEP carboxylase (forms C4 acids like oxaloacetate/malate), then transported to bundle sheath cells where CO2 is released and refixed by Rubisco. Reduces photorespiration and concentrates CO2; efficient under high light, high temperature, and low CO2. Examples: maize, sugarcane.
- CAM (Crassulacean Acid Metabolism): temporal separation—stomata open at night to fix CO2 into organic acids (via PEP carboxylase), release CO2 during the day for the Calvin cycle, conserving water. Common in succulents and some epiphytes.
- Photorespiration: Rubisco oxygenation reaction yields 2-phosphoglycolate, a metabolically costly compound that must be recycled through the photorespiratory pathway (peroxisome, mitochondrion, chloroplast) releasing CO2 and consuming ATP; increased at high temperatures and low CO2.

6) Limiting factors and efficiency
- Short-term limiting factors: light intensity, spectral quality, CO2 concentration, temperature, water status (stomatal conductance).
- Long-term factors: nutrient availability (N, Mg, Fe for chlorophyll and electron carriers), leaf age, sink strength.
- Theoretical maximum efficiency of converting solar energy into biomass is limited (only certain wavelengths used, reflection, photorespiration, respiration losses). Real-world conversion efficiencies are typically a few percent for crop plants, though maximum theoretical for perfectly optimized systems is higher.
- Photoinhibition: excess light can damage photosystems (especially PSII), leading to protective responses (non-photochemical quenching, xanthophyll cycle) or repair mechanisms (D1 protein turnover).

7) Historical discoveries and experiments
- van Niel (1930s): generalized photosynthesis as light-driven reduction of an electron acceptor; showed oxygen does not come from CO2 but from H2O in oxygenic photosynthesis.
- Hill reaction (1937): isolated chloroplasts can reduce artificial electron acceptors in light, demonstrating light-driven electron flow and O2 evolution.
- Melvin Calvin and co-workers: elucidated the Calvin–Benson cycle using 14C tracing (1940s–1950s).
- Emerson enhancement effect, characterization of PSI and PSII, discovery of photophosphorylation and chemiosmotic coupling (Mitchell), identification of the oxygen-evolving complex, and elucidation of electron carriers (PQ, PC, cyt b6f).

8) Molecular players and electron carriers (key names)
- Reaction centers: P680 (PSII) and P700 (PSI).
- Oxygen-evolving complex (OEC) with Mn cluster splits water.
- PQ (plastoquinone), Cyt b6f, PC (plastocyanin), ferredoxin (Fd), NADP+ reductase (FNR).
- ATP synthase (CF1-CF0 complex).
- Rubisco (large and small subunits).
- Enzymes: PEP carboxylase (C4/CAM), Rubisco activase, sedoheptulose-bisphosphatase, fructose-1,6-bisphosphatase, transketolase, etc.

9) Measurement and research tools
- Gas exchange (IRGA) to measure CO2 uptake and transpiration.
- Chlorophyll fluorescence (e.g., PAM fluorometry) to probe photochemical efficiency (Fv/Fm), nonphotochemical quenching and PSII performance.
- Oxygen evolution measurements (electrodes).
- 14C labeling for carbon fluxes (historically used).
- Spectroscopy, absorbance and action spectra, HPLC for pigments, proteomics and molecular genetics.

10) Ecological and global significance
- Photosynthesis fixes atmospheric CO2 into organic carbon, forming the basis of food chains.
- Responsible for producing and maintaining atmospheric O2 (Great Oxidation Event ~2.4 billion years ago linked to evolution of oxygenic photosynthesis).
- Major player in the global carbon cycle and climate regulation; terrestrial and marine primary production sequesters CO2 (though much is returned by respiration).
- Marine phytoplankton (including cyanobacteria, diatoms, coccolithophores) contribute a large fraction of global primary production.

11) Applications and technological relevance
- Agriculture: improving photosynthetic efficiency is a route to increase crop yields (e.g., modifying photorespiratory pathways, optimizing Rubisco or CO2 concentrating mechanisms).
- Algal biofuels and biomass production.
- Artificial photosynthesis / solar fuels: mimicking natural light capture and water splitting to produce H2 or reduced carbon fuels.
- Carbon sequestration strategies and biosphere management to mitigate climate change.
- Synthetic biology: redesigning carbon-fixation pathways, engineering plants for improved water-use or nitrogen-use efficiency.

12) Evolutionary context
- Anoxygenic (single-photosystem) phototrophs likely predate oxygenic cyanobacteria.
- Oxygenic photosynthesis evolved in cyanobacteria (or their ancestors), and chloroplasts in eukaryotes arose by primary endosymbiosis of a cyanobacterium; secondary and tertiary endosymbioses spread photosynthesis to various algal groups.
- Evolutionary innovations: water-splitting OEC, Rubisco’s oxygenase problem, and various CO2-concentrating mechanisms (C4, CAM, pyrenoids in algae).

13) Current frontiers and research directions
- Engineering more efficient Rubisco or bypassing photorespiration to boost yields.
- Elucidating regulation of cyclic electron flow and balancing ATP/NADPH.
- Artificial water-splitting catalysts modeled on the Mn-cluster of OEC.
- Improving carbon-concentrating mechanisms in C3 crops (introducing C4 traits or algal pyrenoids).
- Understanding how climate change (rising CO2, temperature extremes) will alter photosynthetic productivity and ecosystem services.

14) Caveats and complexities
- The simplified overall equation and stoichiometries mask many intermediate steps, side reactions, and cellular costs (photorespiration, respiration, transport, biosynthetic allocations).
- Efficiency and responses are species- and environment-dependent.
- Rubisco’s dual carboxylase/oxygenase activities and its sulfur/nitrogen demands make it a central constraint on photosynthetic efficiency.

If you want more detail in any area
I can expand on: the detailed Z‑scheme electron flow and energetics, biochemical steps and enzymes of the Calvin cycle with intermediate structures, Rubisco structure and kinetics, differences between C3/C4/CAM at the biochemical and anatomical level, phycobilisomes and algal pigments, measurement techniques (chlorophyll fluorescence diagnostics), engineering approaches to improve photosynthesis, or the geologic/evolutionary history in greater depth. Which area would you like to explore first?
