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Photosynthesis in Higher Plants - Site of photosynthesis

Grade 11CBSEBiology

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

🔑Concepts

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Photosynthesis takes place in the green parts of the plant, primarily in the leaves within the mesophyll cells, which contain a high concentration of chloroplasts.

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Chloroplasts are double-membrane-bound organelles. The internal space is divided into the membranous system (grana and stroma lamellae) and the fluid matrix called the stroma.

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The membranous system (thylakoids/grana) is the site for light-harvesting and the 'Light Reaction', where light energy is converted into chemical energy in the form of ATPATP and NADPHNADPH.

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The stroma is the site for the 'Dark Reaction' (Light-independent reaction) or the Calvin Cycle, where CO2CO_2 is enzymatically reduced to form sugars (C6H12O6C_6H_{12}O_6).

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Chloroplasts align themselves within the mesophyll cells (usually along the walls) to optimize the absorption of incident light.

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Photosynthetic pigments such as Chlorophyll aa, Chlorophyll bb, Xanthophylls, and Carotenoids are located in the thylakoid membranes of the grana.

📐Formulae

6CO2+12H2O→ChlorophyllLightC6H12O6+6H2O+6O26CO_2 + 12H_2O \xrightarrow[Chlorophyll]{Light} C_6H_{12}O_6 + 6H_2O + 6O_2

Light Reaction: ADP+Pi+NADP++H2O→LightATP+NADPH+O2\text{Light Reaction: } ADP + Pi + NADP^+ + H_2O \xrightarrow{Light} ATP + NADPH + O_2

💡Examples

Problem 1:

During an experiment, a student observes that a plant is unable to synthesize sugar despite being provided with CO2CO_2 and light. Upon microscopic examination, the stroma of the chloroplast is found to be damaged. Explain why sugar synthesis is affected.

Solution:

The synthesis of sugar occurs in the stroma of the chloroplast through the Dark Reaction.

Explanation:

The stroma contains the enzymes (like RuBisCO) necessary for the Calvin Cycle. While the light-harvesting occurs in the thylakoids, the actual fixation of CO2CO_2 into C6H12O6C_6H_{12}O_6 requires the enzymatic environment of the stroma. If the stroma is damaged, the chemical energy (ATPATP and NADPHNADPH) produced in the grana cannot be utilized to form glucose.

Problem 2:

Which part of the chloroplast is responsible for the photolysis of H2OH_2O and where is the resulting O2O_2 released?

Solution:

The photolysis of H2OH_2O occurs on the inner side of the thylakoid membrane.

Explanation:

The Oxygen Evolving Complex (OEC) associated with Photosystem II (PSIIPS II) is located on the lumen side of the thylakoid membrane. Here, H2OH_2O is split into protons (H+H^+), electrons (e−e^-), and oxygen (O2O_2). The O2O_2 then diffuses out of the chloroplast.