Nutrition in Plants - Autotrophic nutrition and the process of photosynthesis

Hello, young scientists! Welcome to the fascinating world of Class 7 Science. Have you ever wondered where living organisms get the energy to run, play, think, and grow?

All living organisms need food. But while humans and animals rely on other living things for their meals, plants have a superpower: they can make their own food!

In this tutorial, we will unlock the secrets of how plants cook their food without a stove or oven, using a magical process called Photosynthesis. Grab your notebooks, and let's get started!

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1. What is Nutrition?

Before we look at plants, let's understand what nutrition means.

When you eat food like rice, chapati, fruits, or vegetables, your body absorbs components like carbohydrates, proteins, fats, vitamins, and minerals. These components are called nutrients.

Definition: Nutrition is the process of taking in food by an organism and its utilization by the body to build structure, grow, repair damaged parts, and provide energy for life processes.

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2. Modes of Nutrition: How Do Living Things Get Food?

Based on how organisms obtain their food, nutrition is broadly divided into two modes:

Mode of Nutrition	Meaning of the Word	Description	Examples
1. Autotrophic Nutrition	Auto = self <br> Trophos = nourishment	Organisms make their own food using simple raw materials from the surroundings.	Green plants, some bacteria
2. Heterotrophic Nutrition	Heteros = other <br> Trophos = nourishment	Organisms depend on plants or other animals for their food.	Humans, animals, fungi

Since green plants synthesize their own food, they are called Autotrophs.

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3. Photosynthesis: The Food Making Process in Plants

Have you ever wondered where the "food factory" of a plant is located?

The leaves are the primary food factories of plants! Therefore, all raw materials must reach the leaf for food synthesis to take place.

What does the word "Photosynthesis" mean?

Photo = Light

Synthesis = To combine or put together

So, Photosynthesis literally means *combining ingredients using light*.

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The 4 Essential Ingredients for Photosynthesis

Just like your parents need specific ingredients to bake a cake, a plant needs four essential ingredients to make its food:

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Sunlight (Energy)

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CO₂ ──► [ LEAF ] ◄── Water & Minerals

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Chlorophyll (Green Pigment)

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1. Water and Minerals (Sourced from the Soil)

How it gets in: Water and minerals present in the soil are absorbed by the roots.

Transportation: They are transported up to the leaves through continuous, pipe-like vessels running through the roots, stem, branches, and leaves.

2. Carbon Dioxide ($\text{CO}_2$) (Sourced from the Air)

How it gets in: Air contains carbon dioxide gas. Leaves take in $\text{CO}_2$ from the atmosphere through tiny openings on their lower surface called stomata (singular: *stoma*).

Guard Cells: Each stoma is surrounded by microscopic cells called guard cells that control the opening and closing of the pore.

3. Chlorophyll (The Green Magic)

Leaves possess a green pigment called chlorophyll.

Function: Chlorophyll acts like a solar panel! It traps the light energy coming from the sun so the leaf can use it.

4. Sunlight (The Ultimate Source of Energy)

Solar energy is captured by the leaves and stored in the plant in the form of food.

Fun Fact: The Sun is the ultimate source of energy for all living organisms! Without solar energy trapped by plants, there would be no food on Earth.

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🍳 Real-World Analogy: The "MasterChef Plant Kitchen"

To make it super easy to remember, let's compare photosynthesis to cooking in a restaurant kitchen:

The Chef: Chlorophyll (It manages the cooking process).

The Kitchen: The Leaf (The place where food is prepared).

The Fire/Gas Stove: Sunlight (The energy source that powers the cooking).

The Vegetables & Spices: Water & Carbon Dioxide (The raw materials).

The Delicious Meal: Glucose (Carbohydrate/Food).

Fresh Air Released: Oxygen (The delightful byproduct!).

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4. The Equation of Photosynthesis

During photosynthesis, in the presence of sunlight and chlorophyll, carbon dioxide and water combine to form carbohydrates (glucose) and oxygen.

Here is the word equation you must remember for your exams:

$$\text{Carbon Dioxide} + \text{Water} \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \text{Carbohydrate (Glucose)} + \text{Oxygen}$$

What Happens to the Produced Food?

Glucose: The simple sugar produced is used by the plant for immediate energy.

Starch Storage: The extra glucose is converted into a complex carbohydrate called starch and stored in different parts of the plant (leaves, roots, stems, seeds).

*Tip:* The presence of starch in a leaf confirms that photosynthesis has taken place!

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5. Why is Photosynthesis Essential for Life on Earth?

Imagine a world without photosynthesis. What would happen?

No Food: Plants are the primary producers. Without them, herbivores would starve, and carnivores would have no herbivores to eat.

No Oxygen: Oxygen, which is essential for the survival of all living organisms during respiration, is produced during photosynthesis.

Imbalance of Gases: Photosynthesis maintains the balance of carbon dioxide and oxygen in the Earth's atmosphere.

In short: In the absence of photosynthesis, life would be impossible on Earth!

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📝 Concept Summary Box

Autotrophic Nutrition: Mode of nutrition where organisms synthesize their own food (e.g., green plants).

Photosynthesis: Process of making food using $\text{CO}_2$, $\text{H}_2\text{O}$, Chlorophyll, and Sunlight.

Stomata: Tiny pores on leaf surfaces surrounded by guard cells for gas exchange.

Chlorophyll: Green pigment in leaves that traps solar energy.

Primary Output: Glucose (stored as Starch) and Oxygen gas.

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🛑 Check Your Understanding: Practice Questions

Try answering these questions yourself before reading the detailed solutions below!

Question 1 (Short Answer)

What are stomata? State their two main functions in plants.

Solution:

Definition: Stomata are tiny pores present mainly on the lower surface of leaves, surrounded by pair of guard cells.

Functions:

Gas Exchange: They allow Carbon Dioxide ($\text{CO}_2$) to enter the leaf for photosynthesis and allow Oxygen ($\text{O}_2$) to be released into the atmosphere.

Transpiration: They help in the release of excess water from the plant in the form of water vapour.

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Question 2 (Conceptual Application)

Why is the Sun considered the ultimate source of energy for all living organisms, even for non-vegans/carnivores? Explain in 3 steps.

Solution:

Direct Dependency: Green plants (autotrophs) use solar energy directly to prepare food through photosynthesis.

Indirect Dependency: Herbivores (like deer, cows) feed directly on green plants to obtain energy. Carnivores (like lions, tigers) feed on herbivores.

Conclusion: If the Sun did not exist, green plants could not make food. Without plants, herbivores would perish, leading to the death of carnivores. Thus, all living organisms—directly or indirectly—depend on solar energy.

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Question 3 (Experimental Question)

Rohan kept a potted green plant in a dark room for 72 hours, while Priya kept an identical plant in bright sunlight. They both performed an Iodine Test on the leaves of their plants. What will be their observations, and why?

Solution:

Rohan's Plant (Kept in Dark Room):

Observation: The leaf will not turn blue-black when iodine solution is added (it remains brown/light brown).

Reason: In the dark, sunlight was absent, so no photosynthesis occurred. The plant used up all its stored starch for survival, leaving no starch in the leaf.

Priya's Plant (Kept in Sunlight):

Observation: The leaf will turn blue-black upon adding iodine solution.

Reason: In the presence of sunlight, photosynthesis occurred successfully, leading to the formation and storage of starch. Starch turns blue-black when it reacts with iodine solution.

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*Keep exploring, keep asking questions, and happy learning!*