Photosynthesis in Higher Plants - Master Notes

I. The Site & Pigments

Occurs in Chloroplasts. Mesophyll cells contain the maximum number of chloroplasts.

Pigments: Chlorophyll a (Bright/Blue-green), Chl b (Yellow-green), Xanthophylls (Yellow), Carotenoids (Yellow to Yellow-orange).
Reaction Center: A single Chlorophyll 'a' molecule forms the reaction center (P700 in PSI, P680 in PSII).

[i] Absorption vs Action Spectrum: Curvas coincide closely, showing that blue and red light are most effective for photosynthesis.

II. Light Reaction (Photochemical Phase)

Takes place in Grana Thylakoids. Results in ATP, NADPH, and O2.

Z-scheme: Non-cyclic electron transport involving both PSII and PSI.
Photolysis of Water: Happens at PSII. 2H2O -> 4H+ + 4e- + O2.
Cyclic Photophosphorylation: Happens in stroma lamellae; involves ONLY PSI. Produces only ATP.

[!] Chemiosmotic Hypothesis: ATP synthesis is linked to the development of a Proton Gradient across the thylakoid membrane. (Highest H+ concentration in the Lumen).

III. Dark Reaction (Calvin Cycle / C3 Pathway)

Takes place in the Stroma. Fixes CO2 into Glucose.

Carboxylation: CO2 + RuBP --(RuBisCO)--> 2 molecules of 3-PGA.
Reduction: Uses 2 ATP and 2 NADPH per CO2 fixed to form Glucose.
Regeneration: 1 ATP required to regenerate RuBP.

Total for 1 Glucose: 18 ATP and 12 NADPH.

IV. C4 Pathway (Hatch-Slack Pathway)

Adapted for dry tropical climates. Shows Kranz Anatomy (Large bundle sheath cells).

Initial CO2 Acceptor: PEP (3C) in mesophyll cells. Enzyme: PEPcase.
First Stable Product: OAA (4C).
Mechanism: PEPcase fixes CO2 -> OAA -> Malate -> Transported to Bundle Sheath -> Releases CO2 to Calvin Cycle.

[!] No Photorespiration: C4 plants have a mechanism that increases CO2 concentration at the RuBisCO site, suppressing its oxygenase activity.

V. Factors Affecting Photosynthesis

Blackman's Law of Limiting Factors: The rate is determined by the factor at its minimum level.
CO2 Concentration: Major limiting factor. C4 plants saturate at 360 ppm; C3 at 450 ppm.
Light: Saturation occurs at 10% of full sunlight. High intensity leads to photo-oxidation of chlorophyll.

[Q] Why is RuBisCO the most "confused" enzyme in the world?

[A] Explanation:

RuBisCO (Ribulose-1,5-bisphosphate Carboxylase-Oxygenase) has an affinity for both CO2 and O2. At high O2 concentrations or high temperatures, it acts as an Oxygenase, leading to Photorespiration, which is a wasteful process. Its "choice" depends on the relative concentration of the two gases.

[Q] Calculate the ATP/NADPH cost for 1 Glucose in C3 vs C4 plants.

[A] Calculation:

C3 Plants: 18 ATP + 12 NADPH.
C4 Plants: 30 ATP + 12 NADPH.
C4 plants require 12 extra ATP because they need to pump CO2 from mesophyll to bundle sheath (using energy to regenerate PEP from Pyruvate).

[Q] Why are C4 plants superior to C3 plants in tropical regions?

[A] Explanation:

C4 plants utilize Kranz Anatomy and the PEPcase enzyme (which has no affinity for O2). This ensures that CO2 is highly concentrated in the bundle sheath cells where RuBisCO is located, effectively shutting down photorespiration even at high temperatures and high light intensities.

[Q] Where exactly is the proton gradient established in the chloroplast?

[A] Explanation:

The proton gradient is established across the Thylakoid Membrane. Protons (H+) accumulate in the Lumen (inside the thylakoid) due to: 1. Water splitting, 2. PQ pumping H+ from stroma, and 3. NADP reductase consumes H+ in stroma. This creates a high H+ concentration in the lumen.

[Q] Explain the significance of Cyclic Photophosphorylation.

[A] Explanation:

It occurs when only light of wavelength > 680nm is available or when the plant needs extra ATP without producing NADPH. Since the Calvin cycle requires 3 ATP for every 2 NADPH, cyclic transport helps balance this energy ratio by providing the additional ATP.

[Q6] First stable product of C3 cycle.

3-PGA (3-Phosphoglyceric acid).

[Q7] First stable product of C4 cycle.

OAA (Oxaloacetic acid).

[Q8] Site of dark reaction.

Stroma of the chloroplast.

[Q9] CO2 compensation point.

Concentration at which photosynthesis rate equals respiration rate.

[Q10] Primary CO2 acceptor in C3.

RuBP (5-carbon ketose sugar).

[Q11] Primary CO2 acceptor in C4.

PEP (Phosphoenolpyruvate) found in mesophyll.

[Q12] Enzyme missing in C4 mesophyll.

RuBisCO (it is only present in bundle sheath).

[Q13] Who discovered the Oxygen evolution?

Joseph Priestley using a bell jar experiment.

[Q14] First action spectrum alga.

Cladophora (used by T.W. Engelmann).

[Q15] Magnesium is central to?

The porphyrin ring of the Chlorophyll molecule.

[Q16] Phycobilins found in?

Red algae (r-phycoerythrin).

[Q17] Effect of water stress on rate.

Decreases it by stomatal closure (reducing CO2 intake).

[Q18] Bundle sheath cell characteristics.

Thick walls, no intercellular spaces, high number of chloroplasts.

[Q19] Light saturation point.

About 10% of full sunlight intensity.

[Q20] Most abundant protein on Earth.

RuBisCO.

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Photosynthesis: 50 Mastery Facts

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Photosynthesis: Physicochemical process using light to make organics.

Priestley: Discovered Oxygen and the role of plants in restoring air.

Ingenhousz: Showed sunlight is essential for O2 evolution.

Sachs: Evidence for production of glucose/starch in green parts.

Engelmann: First action spectrum using a prism and Cladophora.

Van Niel: O2 evolved comes from water, not CO2.

Mesophyll cells: Primary site of photosynthesis in leaves.

Thylakoids: Flattened sacs where light reactions occur.

Stroma: Fluid matrix where dark reactions (Calvin cycle) occur.

Chlorophyll a: Bright or blue-green; primary pigment.

Chlorophyll b: Yellow-green accessory pigment.

Reaction Center: One Chl 'a' (P700 or P680).

Red & Blue light: Regions of maximum photosynthesis.

LHC: Light Harvesting Complex (Antennae).

PSII: Photosystem II (absorbs at 680 nm).

PSI: Photosystem I (absorbs at 700 nm).

Photolysis: Splitting of water by light energy at PSII.

Z-Scheme: Non-cyclic flow of electrons between PSII and PSI.

NADP Reductase: Found on the stroma side of the membrane.

Cyclic transport: Only PSI involved; occurs in stroma lamellae.

Proton Gradient: Established across the thylakoid membrane.

ATP Synthase: Enzyme that uses H+ flow to make ATP.

Lumen: Site of highest H+ concentration during photosynthesis.

ATP & NADPH: Products of light reaction used in dark reaction.

RuBisCO: Most abundant enzyme; located in stroma.

Calvin cycle: C3 pathway; occurs in all photosynthetic plants.

RuBP: 5-C primary acceptor of CO2 in C3 plants.

3-PGA: 3-carbon first stable product of C3 cycle.

Reduction stage: Consumes 2 ATP and 2 NADPH per CO2.

Regeneration stage: Consumes 1 ATP to reset RuBP.

Total ATP (C3): 18 ATP per glucose molecule.

Total NADPH (C3): 12 NADPH per glucose molecule.

Kranz Anatomy: Specialized anatomy of C4 plants.

PEP: 3-C primary acceptor of CO2 in C4 mesophyll.

PEPcase: Enzyme with very high affinity for CO2.

OAA: 4-carbon first stable product in C4 plants.

Bundle Sheath: Site of Calvin cycle in C4 plants.

Photorespiration: Wasteful C2 cycle occurring at high O2.

C4 plants: No photorespiration due to CO2 concentration pump.

Total ATP (C4): 30 ATP per glucose molecule.

Limiting Factor: The single factor slowest in rate.

Light intensity: Saturation at 10% of full sunlight.

C3 saturation: CO2 at 450 ppm.

C4 saturation: CO2 at 360 ppm.

Temperatue: Dark reactions are more temperature sensitive.

Greenhouse crops: Tomatoes and bell peppers benefit from CO2 enrichment.

H2O photolysis site: Associated with PSII on lumen side.

Oxidation: Water is oxidized; CO2 is reduced.

Photosynthetic unit: Also called Quantasome.

Net result: Solar energy converted into chemical energy.

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