Photosynthetic Pathways (C3, C4, CAM)
⭐ Ultimate Botany & Plant Physiology Matrix
10 Parameters
Photosynthetic Pathways: C3 vs C4 vs CAM Plants
Comprehensive comparative matrix of CO2 acceptor enzymes, first stable products, Kranz anatomy, photorespiration rate, water use efficiency, and ATP costs.
| Feature / Diagnostic Trait | C3 Pathway (Calvin Cycle) | C4 Pathway (Hatch-Slack) | CAM Pathway (Crassulacean Acid) |
|---|---|---|---|
| Primary CO2 Acceptor | Ribulose-1,5-bisphosphate (RuBP; 5-carbon) | Phosphoenolpyruvate (PEP; 3-carbon) | Phosphoenolpyruvate (PEP; 3-carbon, at night) |
| Primary Carboxylating Enzyme | RuBisCO (acts as carboxylase & oxygenase) | PEP Carboxylase (PEPcase; no oxygenase activity) | PEP Carboxylase (at night); RuBisCO (during day) |
| First Stable Product | 3-Phosphoglyceric acid (3-PGA; 3-carbon) | Oxaloacetic acid (OAA; 4-carbon dicarboxylic) | Oxaloacetic acid (OAA; converted to malic acid) |
| Leaf Kranz Anatomy | Absent (mesophyll cells only; no bundle sheath chloroplasts) | PRESENT (concentric bundle sheath with large agranal chloroplasts) | Absent (large water-storing vacuoles in succulent mesophyll) |
| Spatial vs Temporal Separation | None (all reactions in mesophyll chloroplasts) | Spatial separation: PEPcase in mesophyll, RuBisCO in bundle sheath | Temporal separation: Night (PEPcase/malate), Day (RuBisCO/Calvin) |
| Photorespiration (C2 Cycle) | High (wastes up to 25–40% of fixed carbon under hot dry conditions) | Negligible to ABSENT (CO2 actively concentrated around RuBisCO) | Negligible to ABSENT |
| ATP Cost per Glucose Synthesized | 18 ATP + 12 NADPH (Cheapest in cool climates) | 30 ATP + 12 NADPH (Consumes extra 12 ATP for PEP regeneration) | 39 ATP + 12 NADPH (Highest ATP cost for vacuolar transport) |
| Water Use Efficiency (WUE: g H2O/g dry wt) | Low (~400 – 500 g H2O lost per g CO2 fixed) | Moderate (~250 – 300 g H2O lost) | EXTREMELY HIGH (~50 – 100 g H2O lost; 10x more efficient) |
| Optimum Temperature | 20°C – 25°C (Temperate) | 35°C – 45°C (Tropical & warm regions) | Over 40°C (Arid desert succulents) |
| Representative Plant Species | Wheat, Rice, Barley, Oats, Soybean, Potato | Maize (Corn), Sugarcane, Sorghum, Amaranthus | Pineapple, Agave, Kalanchoe, Vanilla, Cacti, Sedum |
Photosynthetic Pathway Tradeoffs
C3
Kranz Anatomy: No
Photorespiration: High (Wasteful)
Atp Per Glucose: 18 ATP
Wue: Lowest
C4
Kranz Anatomy: YES
Photorespiration: Negligible
Atp Per Glucose: 30 ATP
Wue: Moderate
CAM
Kranz Anatomy: No (Vacuoles)
Photorespiration: Negligible
Atp Per Glucose: 39 ATP
Wue: Highest (10x)
Photophosphorylation & Light Reactions
⭐ Thylakoid Electron Flux Matrix
6 Parameters
Light Reactions: Cyclic vs Non-Cyclic Photophosphorylation (Z-Scheme)
Comparative matrix of photosystems involved, photolysis of water, oxygen evolution, end products, and electron routing.
| Parameter | Non-Cyclic Photophosphorylation (Z-Scheme) | Cyclic Photophosphorylation |
|---|---|---|
| Photosystems Involved | Both PS II (P680) and PS I (P700) operating in series | PS I (P700) ONLY (PS II is inactive or absent) |
| Photolysis of Water & Oxygen Evolution | PRESENT (Oxygen-evolving complex of PS II hydrolyzes 2H2O → 4H+ + 4e- + O2) | ABSENT (No photolysis of water; zero oxygen released) |
| Electron Routing & Cycling | Unidirectional non-cyclic flow: H2O → PS II → Cytochrome b6f → PS I → NADP+ | Cyclic loop: PS I → Ferredoxin → Cytochrome b6f → Plastocyanin → back to PS I |
| High-Energy End Products | Produces BOTH ATP and NADPH | Produces ATP ONLY (No NADPH generated) |
| Thylakoid Membrane Localization | Granal thylakoid lamellae (appressed membranes house PS II) | Stroma thylakoid lamellae and non-appressed margins of grana (house PS I) |
| Triggering Conditions | Standard daytime photosynthetic steady state | When NADP+ is depleted, NADPH/NADP+ ratio is high, or light wavelength >680 nm (far-red) |
Electron Transport & Products
Key Point
Mode: Non-Cyclic
Photosystems: PS II + PS I
O2 Evolved: YES (Water split)
Products: ATP + NADPH
Key Point
Mode: Cyclic
Photosystems: PS I only
O2 Evolved: NO
Products: ATP only
Water Relations & Mineral Transport
⭐ Casparian Strip Checkpoint
4 Parameters
Water Transport in Roots: Apoplast vs Symplast vs Transmembrane Pathways
Comparative matrix of conduit components, living vs non-living spaces, resistance to water flow, and Casparian strip blockage.
| Feature | Apoplast Pathway | Symplast Pathway | Transmembrane Pathway |
|---|---|---|---|
| Conduit Components | Interconnected cell walls, intercellular spaces, and dead xylem lumens | Continuum of living cytoplasm interconnected through plasmodesmata | Crosses cell membrane twice per cell (vacuolar/membrane entry and exit) |
| Living Protoplasm Involvement | Non-living space (extracellular continuum; water never crosses membrane) | Living continuum (symplasmic transport through cytosolic bridges) | Crosses plasma membranes via aquaporins and lipid bilayers |
| Resistance to Water Flow | Lowest resistance (rapid mass flow of water along hydrostatic gradient) | Higher resistance (viscous cytoplasm and narrow plasmodesmata channels) | Highest resistance (permeability barriers of lipid bilayers) |
| Casparian Strip Barrier (Endodermis) | BLOCKED by hydrophobic suberin band; forced into symplast at endodermis | Bypasses Casparian strip unimpeded through intact plasmodesmata | Crosses endodermal membranes through selective transport proteins |
Pathway Mechanics
Apoplast
Living Cell: No (Walls)
Speed: Fastest
Casparian Effect: 100% Blocked
Symplast
Living Cell: Yes (Cytosol)
Speed: Moderate
Casparian Effect: Continuous flow
