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Biology - Plant nutrition (Photosynthesis and leaf structure)

Grade 9IGCSE

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

🔑Concepts

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Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light. The chemical energy is stored in glucose (C6H12O6C_6H_{12}O_6).

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Chlorophyll is a green pigment found in chloroplasts that absorbs light energy for photosynthesis.

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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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Leaf structure is adapted for photosynthesis: The Palisade mesophyll contains a high density of chloroplasts to maximize light absorption.

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The Spongy mesophyll has large air spaces to allow for the diffusion of gases like CO2CO_2 and O2O_2 throughout the leaf.

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Stomata, controlled by guard cells, regulate gas exchange and water loss (H2OH_2O vapor) via transpiration.

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Limiting factors are components in shortest supply that restrict the rate of photosynthesis: light intensity, CO2CO_2 concentration, and temperature.

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

📐Formulae

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

Rate of Photosynthesis∝1distance2 (Inverse Square Law for light intensity)\text{Rate of Photosynthesis} \propto \frac{1}{\text{distance}^2} \text{ (Inverse Square Law for light intensity)}

💡Examples

Problem 1:

A student investigates the effect of light intensity on the rate of photosynthesis using an aquatic plant. If the distance of the light source is doubled, what happens to the light intensity?

Solution:

The light intensity decreases by a factor of 4.

Explanation:

According to the inverse square law, light intensity is inversely proportional to the square of the distance (I∝1d2I \propto \frac{1}{d^2}). If distance dd becomes 2d2d, the intensity becomes 1(2d)2=14d2\frac{1}{(2d)^2} = \frac{1}{4d^2}.

Problem 2:

Explain why a plant leaf turns yellow (chlorosis) when grown in soil deficient in Magnesium (Mg2+Mg^{2+}).

Solution:

Magnesium is a central component of the chlorophyll molecule.

Explanation:

Without Mg2+Mg^{2+} ions, the plant cannot synthesize chlorophyll (C55H72O5N4MgC_{55}H_{72}O_5N_4Mg). Lack of chlorophyll results in the loss of green pigment, leading to yellowing and a reduced rate of photosynthesis.

Problem 3:

Predict the effect of increasing temperature from 20∘C20^{\circ}C to 60∘C60^{\circ}C on the rate of photosynthesis.

Solution:

The rate will initially increase but then drop to zero as enzymes denature.

Explanation:

Photosynthesis is an enzyme-controlled reaction. Up to the optimum temperature (usually 30∘C−40∘C30^{\circ}C - 40^{\circ}C), kinetic energy increases reaction rates. Beyond this, the enzymes (like Rubisco) denature, changing the shape of their active sites and stopping the process.