Photosynthesis is the process of converting light energy into chemical energy (sugar). It can be understood in two stages: the light-dependent reactions that occur in the thylakoids of chloroplasts and the Calvin cycle (light-independent reactions) that occurs in the stroma.
Basic Knowledge
Chloroplast structure: Chloroplasts are enclosed by a double membrane consisting of an outer membrane and an inner membrane. Inside are thylakoids (grana, stacks of flattened membrane sacs) and the stroma (matrix).
Light-dependent reactions (thylakoid membrane): ① Photosystem II (P680) absorbs light → water is split (photolysis): 2H₂O→4H⁺+4e⁻+O₂. ② Electrons flow through the electron transport chain, forming an H⁺ concentration gradient → ATP synthesis (photophosphorylation). ③ Photosystem I (P700) absorbs light → NADP is reduced: NADP⁺+2H⁺+2e⁻→NADPH. Products: ATP, NADPH, and O₂ (a by-product).
Calvin cycle (Calvin cycle, stroma): ① CO₂ fixation: CO₂+RuBP(C₅) →[RuBisCo]→ 3-phosphoglycerate (3-PGA, C₃)×2. ② Reduction of 3-PGA: ATP and NADPH are consumed → glyceraldehyde 3-phosphate (G3P). ③ Regeneration of RuBP: ATP is consumed → RuBP is regenerated. Some G3P is used as sugar (such as glucose). For every 3 molecules of CO₂ fixed, 9 molecules of ATP and 6 molecules of NADPH are consumed, producing 1 molecule of G3P net.
C₄ plants and CAM plants: C₄ plants (corn and sugarcane) temporarily fix CO₂ as oxaloacetate (C₄), transport it from mesophyll cells to bundle-sheath cells, and then use it in the Calvin cycle. This is advantageous under hot, dry conditions. CAM plants (such as cacti) fix CO₂ at night and release it during the day. They minimize water loss through transpiration as much as possible.
Explain why water splitting (photolysis) is necessary in the light-dependent reactions of photosynthesis, from the perspectives of electron flow and O₂ production.
Answer: When Photosystem II (P680) absorbs light and releases excited electrons to the electron transport chain, P680 becomes electron-deficient. Electrons are supplied to P680 by the photolysis of water (2H₂O→4H⁺+4e⁻+O₂). O₂ is released as a by-product during this process. O₂ is not a product derived from CO₂; it is a by-product of water splitting.
- Light-dependent reactions = thylakoid membrane; ATP+NADPH+O₂ are produced
- Photolysis (water splitting) = source of O₂ and replacement of electrons in P680
- Calvin cycle = stroma; ATP and NADPH are consumed and sugar is produced
- CO₂-fixing enzyme = RuBisCo (the most abundant protein on Earth)
- C₄ plants = adapted to hot, dry conditions (corn and sugarcane)
- CAM plants = CO₂ fixed at night and the Calvin cycle occurs during the day (cacti)
- O₂ is a by-product derived from photolysis of water
Points to Note
① The O₂ produced by photosynthesis comes from water, not CO₂ (confirmed by isotope experiments). ② The Calvin cycle does not directly require light, but it stops when ATP and NADPH are depleted, so it cannot continue without light even though it is called a “light-independent reaction.” ③ RuBisCo (ribulose 1,5-bisphosphate carboxylase) is the rate-limiting enzyme of photosynthesis and the most abundant protein on Earth.
Practice
- Name the three products of the light-dependent reactions.
- Name the enzyme that fixes CO₂ in the Calvin cycle.
- Explain the advantage of C₄ plants compared with C₃ plants from the perspective of photosynthetic efficiency in hot, dry environments.