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Plant Nutrition - Photosynthesis

Grade 11A LevelBiology

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 (CO2CO_2 and H2OH_2O) using energy from light.

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Chlorophyll is a green pigment found in chloroplasts that absorbs light energy and converts it into chemical energy for the synthesis of glucose (C6H12O6C_6H_{12}O_6).

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The process is divided into light-dependent reactions and light-independent reactions (Calvin Cycle).

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Limiting factors determine the rate of photosynthesis; these include light intensity, CO2CO_2 concentration, and temperature.

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Gaseous exchange occurs through the stomata, where CO2CO_2 diffuses into the leaf and O2O_2 diffuses out.

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Glucose produced during photosynthesis is used for respiration, converted into starch for storage, used to make cellulose for cell walls, or turned into sucrose for transport in the phloem.

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The internal structure of a leaf is adapted for photosynthesis: the palisade mesophyll contains many chloroplasts, and the spongy mesophyll has air spaces for gas diffusion.

📐Formulae

6CO2+6H2O→chlorophylllightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2

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

Glucose→Starch (Storage polymer)\text{Glucose} \rightarrow \text{Starch (Storage polymer)}

💡Examples

Problem 1:

Explain why the rate of photosynthesis levels off even if the light intensity continues to increase at a constant temperature of 20∘C20^\circ C and 0.04%0.04\% CO2CO_2 concentration.

Solution:

The rate levels off because light intensity is no longer the limiting factor. Another factor, such as CO2CO_2 concentration or temperature, has become the limiting factor.

Explanation:

According to the principle of limiting factors, the rate of a process is restricted by the factor at its least favorable value. Once light is abundant, the enzymes or the availability of CO2CO_2 molecules restrict further increases in the reaction rate.

Problem 2:

A student measures the oxygen production of an aquatic plant. At 10 cm10\text{ cm} from a light source, the plant produces 4040 bubbles per minute. If the distance is increased to 20 cm20\text{ cm}, what is the expected change in light intensity?

Solution:

The light intensity will decrease by a factor of 44 (it becomes 14\frac{1}{4} of the original intensity).

Explanation:

Light intensity follows the inverse square law: I∝1d2I \propto \frac{1}{d^2}. Since the distance doubled (10 cm10\text{ cm} to 20 cm20\text{ cm}), the intensity decreases by 22=42^2 = 4.

Problem 3:

Why must a leaf be boiled in ethanol before performing the iodine test for starch?

Solution:

The leaf is boiled in ethanol to remove the chlorophyll (C55H72O5N4MgC_{55}H_{72}O_5N_4Mg).

Explanation:

Chlorophyll is a green pigment that would mask the color change of the iodine solution (from brown to blue-black). Removing the pigment ensures that any color change resulting from the presence of starch is clearly visible.