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Photosynthesis in Higher Plants - Photochemical and biosynthetic phases

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It occurs in the chloroplasts, where the pigment chlorophyll traps light energy.

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The Photochemical Phase (Light Reaction) occurs in the thylakoid membranes (grana). It includes light absorption, water splitting (H2OH_2O photolysis), oxygen release, and the formation of high-energy chemical intermediates, ATPATP and NADPHNADPH.

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Photolysis of Water: The splitting of water is associated with PS IIPS\ II. It provides electrons to the electron transport chain, releasing protons and oxygen: 2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-.

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Cyclic and Non-cyclic Photophosphorylation: Non-cyclic (Z-scheme) involves both PS IPS\ I and PS IIPS\ II and produces both ATPATP and NADPHNADPH. Cyclic involves only PS IPS\ I and produces only ATPATP.

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Chemiosmotic Hypothesis: ATPATP synthesis is linked to the development of a proton (H+H^+) gradient across the thylakoid membrane. Protons accumulate in the lumen. The breakdown of this gradient through CF0−CF1CF_0-CF_1 particles (ATP synthase) releases energy to synthesize ATPATP.

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The Biosynthetic Phase (Dark Reaction/Calvin Cycle) occurs in the stroma. It is independent of direct light but dependent on the products of the light reaction (ATPATP and NADPHNADPH). It fixes CO2CO_2 into glucose.

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The Calvin Cycle (C3C_3 pathway): Consists of three stages: 1. Carboxylation (fixation of CO2CO_2 into 3−PGA3-PGA using the enzyme RuBisCO), 2. Reduction (formation of glucose), and 3. Regeneration of the CO2CO_2 acceptor (RuBPRuBP).

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The C4C_4 Pathway (Hatch-Slack Pathway): Occurs in plants like maize and sorghum which have 'Kranz anatomy'. The first stable product is Oxaloacetic Acid (OAAOAA, a 4−carbon4-carbon compound). It minimizes photorespiration and is more efficient at high temperatures.

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Photorespiration: A wasteful process occurring in C3C_3 plants when RuBisCO binds with O2O_2 instead of CO2CO_2 at high temperatures or high O2O_2 concentrations, leading to the loss of fixed CO2CO_2 as phosphoglycolatephosphoglycolate.

📐Formulae

6CO2+12H2O→Light/ChlorophyllC6H12O6+6H2O+6O26CO_2 + 12H_2O \xrightarrow{\text{Light/Chlorophyll}} C_6H_{12}O_6 + 6H_2O + 6O_2

2H2O→4H++O2+4e−2H_2O \rightarrow 4H^+ + O_2 + 4e^-

Requirement for 1 molecule of Glucose (C3 cycle): 6CO2+18ATP+12NADPH\text{Requirement for 1 molecule of Glucose (C3 cycle): } 6CO_2 + 18ATP + 12NADPH

Requirement for 1 molecule of Glucose (C4 cycle): 6CO2+30ATP+12NADPH\text{Requirement for 1 molecule of Glucose (C4 cycle): } 6CO_2 + 30ATP + 12NADPH

Net reaction of Light Phase: 12H2O+12NADP++18ADP+18Pi→6O2+12NADPH+12H++18ATP\text{Net reaction of Light Phase: } 12H_2O + 12NADP^+ + 18ADP + 18P_i \rightarrow 6O_2 + 12NADPH + 12H^+ + 18ATP

💡Examples

Problem 1:

Calculate the total number of ATPATP and NADPHNADPH molecules required to synthesize 5 molecules of glucose in a C3C_3 plant.

Solution:

For 1 molecule of glucose, a C3C_3 plant requires 18 ATP18\ ATP and 12 NADPH12\ NADPH. For 5 molecules: 5×18=90 ATP5 \times 18 = 90\ ATP and 5×12=60 NADPH5 \times 12 = 60\ NADPH.

Explanation:

The Calvin cycle turns 6 times to fix 6 CO26\ CO_2 for one glucose molecule. Each turn consumes 3 ATP3\ ATP and 2 NADPH2\ NADPH.

Problem 2:

Why is the C4C_4 pathway considered more energy-expensive than the C3C_3 pathway, yet more efficient in tropical conditions?

Solution:

A C4C_4 plant requires 30 ATP30\ ATP per glucose compared to 18 ATP18\ ATP in C3C_3. However, it avoids photorespiration.

Explanation:

In C4C_4 plants, the additional 12 ATP12\ ATP (total 30 ATP30\ ATP) are used to transport CO2CO_2 from mesophyll to bundle sheath cells to ensure RuBisCO always reacts with CO2CO_2, preventing the wasteful O2O_2 reaction (photorespiration) common in C3C_3 plants at high temperatures.