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Life Processes in Plants - Leaves: the 'food factories' of plants

Grade 7CBSE

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

🔑Concepts

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Photosynthesis is the process by which green plants prepare their own food using carbon dioxide (CO2CO_{2}) and water (H2OH_{2}O) in the presence of sunlight and chlorophyll.

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Leaves are called 'food factories' because they contain chloroplasts, which house the green pigment called chlorophyll responsible for trapping solar energy.

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Stomata are tiny pores located mostly on the underside of leaves. They are surrounded by guard cells and facilitate the exchange of gases like CO2CO_{2} and O2O_{2}.

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The glucose produced during photosynthesis is either used for energy or converted into complex carbohydrates like starch for storage.

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Transpiration is the evaporation of water from the surface of leaves through stomata, which helps in cooling the plant and creates a suction pull to transport water and minerals from the roots.

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The presence of starch in leaves can be tested using the Iodine test; a leaf containing starch turns blue-black when iodine solution is added.

📐Formulae

6CO2+6H2O→ChlorophyllSunlightC6H12O6+6O26CO_{2} + 6H_{2}O \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} C_{6}H_{12}O_{6} + 6O_{2}

Carbon Dioxide+Water→ChlorophyllSunlightGlucose+Oxygen\text{Carbon Dioxide} + \text{Water} \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \text{Glucose} + \text{Oxygen}

💡Examples

Problem 1:

Why does a leaf turn blue-black when treated with iodine solution, and what does it indicate?

Solution:

The leaf turns blue-black because of the reaction between iodine and starch. This indicates that photosynthesis has occurred, and the plant has stored glucose as starch ([C6H10O5]n[C_{6}H_{10}O_{5}]_{n}).

Explanation:

Iodine is an indicator that reacts with the helical structure of starch molecules to form a dark blue-black complex. If a leaf has been performing photosynthesis, it will have starch deposits.

Problem 2:

Calculate the total number of atoms in a single molecule of glucose (C6H12O6C_{6}H_{12}O_{6}), the product of photosynthesis.

Solution:

6 (Carbon)+12 (Hydrogen)+6 (Oxygen)=24 atoms6 \text{ (Carbon)} + 12 \text{ (Hydrogen)} + 6 \text{ (Oxygen)} = 24 \text{ atoms}

Explanation:

By summing the subscripts of each element in the chemical formula C6H12O6C_{6}H_{12}O_{6}, we find the total number of atoms in one molecule of glucose: 6+12+6=246 + 12 + 6 = 24.

Problem 3:

A student observed that a plant kept in a dark room for 72 hours does not show the presence of starch. Explain why.

Solution:

In the absence of sunlight, the process of photosynthesis cannot occur as chlorophyll cannot trap energy to convert CO2CO_{2} and H2OH_{2}O into food. The plant consumes its stored starch for survival.

Explanation:

Sunlight is a mandatory requirement for the chemical reaction 6CO2+6H2O→C6H12O6+6O26CO_{2} + 6H_{2}O \to C_{6}H_{12}O_{6} + 6O_{2}. Without light, the plant is 'de-starched' as it uses up existing reserves.