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Biology - Photosynthesis

Grade 10ICSE

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

🔑Concepts

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Photosynthesis is a physiological process by which plant cells containing chlorophyll produce food (glucose) from CO2CO_2 and H2OH_2O using light energy.

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The process occurs within the Chloroplast. The Thylakoids (Grana) are the site for the Light-Dependent Phase, while the Stroma is the site for the Light-Independent Phase (Dark Reaction).

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The Light-Dependent Phase (Photochemical Phase) involves the activation of chlorophyll by photons and the Photolysis of water: 2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2.

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Photophosphorylation is the process of converting ADPADP into ATPATP using light energy: ADP+Pi→ATPADP + P_i \rightarrow ATP.

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NADP (Nicotinamide Adenine Dinucleotide Phosphate) acts as a hydrogen acceptor to form NADPHNADPH during the light reaction.

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The Light-Independent Phase (Biosynthetic Phase or Calvin Cycle) involves the fixation of CO2CO_2 to produce glucose (C6H12O6C_6H_{12}O_6) using the ATPATP and NADPHNADPH generated in the light phase.

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Factors affecting the rate of photosynthesis include light intensity, CO2CO_2 concentration, temperature, and water content.

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The stomatal mechanism is often explained by the K+K^+ ion exchange theory, where the accumulation of K+K^+ ions in guard cells leads to endosmosis, making cells turgid and opening the stoma.

📐Formulae

6CO2+12H2O→ChlorophyllLight energyC6H12O6+6H2O+6O2↑6CO_2 + 12H_2O \xrightarrow[\text{Chlorophyll}]{\text{Light energy}} C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow

2H2O→Photons4H++4e−+O22H_2O \xrightarrow{\text{Photons}} 4H^+ + 4e^- + O_2

ADP+Pi→ATPADP + P_i \rightarrow ATP

NADP++e−+H+→NADPHNADP^+ + e^- + H^+ \rightarrow NADPH

💡Examples

Problem 1:

During an experiment to test for starch, why is the leaf boiled in methylated spirit/alcohol over a water bath?

Solution:

The leaf is boiled in alcohol to remove chlorophyll. This decolourises the leaf, making the subsequent color change with iodine (I2I_2) solution (blue-black for starch) easily visible.

Explanation:

Alcohol dissolves the magnesium-containing pigment chlorophyll. A water bath is used because alcohol is highly inflammable and should not be heated directly over a flame.

Problem 2:

Identify the gas evolved when an aquatic plant like Hydrilla is kept in a beaker under a funnel in sunlight, and give the chemical test for it.

Solution:

The gas evolved is Oxygen (O2O_2). Test: A glowing splinter introduced into the test tube containing the gas will burst into flames.

Explanation:

In the presence of sunlight, the plant undergoes photosynthesis. The O2O_2 is produced during the photolysis of H2OH_2O in the light-dependent phase.