Questions
3-5 questions per paper
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Photosynthesis in higher plants is a fundamental physiological process that converts solar energy into chemical energy, sustaining life on Earth. It is a high-yield topic in biology exams that requires a clear understanding of the light-dependent and light-independent phases, along with the evolutionary adaptations in carbon fixation. Mastery of the chemical pathways and their regulating factors is essential for scoring well.
Light Reaction (Photochemical Phase)
The light reaction occurs in the thylakoid membranes where light energy is captured to produce ATP and NADPH. This stage is crucial as it creates the reducing power and energy currency required for the subsequent dark reactions.
- Light harvesting complexes (LHC) consist of hundreds of pigment molecules
- Reaction centers: P680 for PS II and P700 for PS I
- Photolysis of water provides electrons for PS II
- Non-cyclic photophosphorylation produces both ATP and NADPH
- Cyclic photophosphorylation occurs only in PS I and produces only ATP
Calvin Cycle (C3 Pathway)
The Calvin cycle is the light-independent phase occurring in the stroma, where CO2 is fixed to produce sugars. It involves three distinct stages: carboxylation, reduction, and regeneration, with RuBisCO acting as the primary enzyme.
- Primary CO2 acceptor is Ribulose 1,5-bisphosphate (RuBP)
- RuBisCO is the most abundant protein on Earth
- Three phases: Carboxylation, Reduction, and Regeneration
- One glucose molecule requires 6 turns of the cycle
- Requires 18 ATP and 12 NADPH per glucose molecule produced
C4 Pathway (Hatch-Slack Pathway)
The C4 pathway is an evolutionary adaptation in certain plants to minimize photorespiration in hot and dry climates. It utilizes Kranz anatomy, where the Calvin cycle is localized in bundle sheath cells to keep RuBisCO in a high CO2 environment.
- Primary CO2 acceptor is Phosphoenolpyruvate (PEP)
- First stable product is a 4-carbon compound: Oxaloacetic acid (OAA)
- Kranz anatomy features large bundle sheath cells
- PEP carboxylase (PEPCase) is the primary fixing enzyme
- More efficient than C3 pathway under high light and temperature
Formula Sheet
6CO2 + 12H2O + Light -> C6H12O6 + 6H2O + 6O2
RuBP + CO2 -> 2(3-PGA) [RuBisCO catalyzed]
PEP + CO2 -> OAA [PEP Carboxylase catalyzed]
Exam Tip
Always memorize the specific enzymes and primary CO2 acceptors for both C3 and C4 cycles as they are frequently tested in comparative table-based questions.
Common Mistakes
- Confusing the location of PS I and PS II (PS II is only in appressed regions of thylakoids).
- Miscalculating the ATP/NADPH requirements per glucose molecule for C3 vs C4 plants.
- Forgetting that photorespiration is a wasteful process that reduces the net yield of photosynthesis.
More Revision Notes
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