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Biology - Photosynthesis and Plant Growth

Grade 8Cambridge (IGCSE)

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

🔑Concepts

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Photosynthesis is the fundamental process by which green plants manufacture carbohydrates from raw materials (CO2CO_2 and H2OH_2O) using energy from light.

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The process takes place in the chloroplasts, which contain the green pigment chlorophyll. Chlorophyll traps light energy and converts it into chemical energy.

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The balanced chemical equation for photosynthesis is 6CO2+6H2O→light+chlorophyllC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light + chlorophyll} C_6H_{12}O_6 + 6O_2.

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The glucose (C6H12O6C_6H_{12}O_6) produced is used for respiration, stored as starch, or converted into cellulose for cell walls and proteins for growth.

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Leaf adaptations for photosynthesis include a large surface area, the presence of stomata for gas exchange of CO2CO_2 and O2O_2, and a thin structure for short diffusion distances.

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The palisade mesophyll layer contains the highest concentration of chloroplasts to maximize light absorption.

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Limiting factors are environmental conditions that restrict the rate of photosynthesis. The three main factors are light intensity, CO2CO_2 concentration, and temperature (affecting enzyme activity).

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Plants require mineral ions for healthy growth: Nitrate ions (NO3−NO_3^-) are needed for making amino acids and proteins, while Magnesium ions (Mg2+Mg^{2+}) are essential for making chlorophyll.

📐Formulae

Carbon dioxide+Water→light + chlorophyllGlucose+Oxygen\text{Carbon dioxide} + \text{Water} \xrightarrow{\text{light + chlorophyll}} \text{Glucose} + \text{Oxygen}

6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

Light Intensity∝1d2\text{Light Intensity} \propto \frac{1}{d^2}

💡Examples

Problem 1:

Explain why a leaf turns blue-black when iodine solution is added after being boiled in ethanol.

Solution:

The blue-black color indicates the presence of starch.

Explanation:

Plants store excess glucose (C6H12O6C_6H_{12}O_6) as starch. Boilling in ethanol removes the chlorophyll so color changes are visible. The reaction between iodine and starch produces a dark blue-black complex, proving photosynthesis occurred.

Problem 2:

Predict the effect on a plant if the concentration of CO2CO_2 is increased in a greenhouse at a constant temperature of 25∘C25^{\circ}C and high light intensity.

Solution:

The rate of photosynthesis will increase until another factor becomes limiting.

Explanation:

CO2CO_2 is a reactant in the equation 6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2. Increasing its concentration allows for more frequent collisions between CO2CO_2 molecules and enzymes like Rubisco, increasing the reaction rate.

Problem 3:

A plant shows stunted growth and yellowing of the leaves (chlorosis). Which mineral ion is likely deficient?

Solution:

Magnesium ions (Mg2+Mg^{2+}) or Nitrate ions (NO3−NO_3^-).

Explanation:

A lack of Mg2+Mg^{2+} prevents the synthesis of chlorophyll, leading to yellow leaves. A lack of NO3−NO_3^- prevents the synthesis of proteins required for cell division and growth.