Nutrition in Plants (CBSE Class 7 Science)

Introduction to Nutrition in Plants

Nutrition in Plants- All living organisms, including plants, animals, and humans, require food to survive and perform their life processes. Food provides energy and essential nutrients needed for growth, repair, and maintenance of the body. While animals obtain food by consuming plants or other animals, plants have a unique way of making their own food. This process is called nutrition, and in plants, it primarily occurs through a process known as photosynthesis. This chapter explores how plants fulfill their nutritional needs, the types of nutrition in plants, and other related concepts.

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What is Nutrition?

Nutrition is the process by which organisms obtain and utilize food to meet their energy and growth requirements. In plants, nutrition involves the intake of raw materials like carbon dioxide, water, and minerals, which are used to produce food. Plants are autotrophs, meaning they can synthesize their own food, unlike animals, which are heterotrophs and depend on other organisms for food.

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Modes of Nutrition in Plants

Plants primarily follow an autotrophic mode of nutrition, but some plants exhibit heterotrophic nutrition. Let’s explore these in detail:

1. Autotrophic Nutrition
   • Plants make their own food using simple substances like carbon dioxide (CO₂), water (H₂O), and sunlight.
   • This process is called photosynthesis.
   • It occurs in the green parts of the plant, mainly the leaves, which contain a pigment called chlorophyll.
2. Heterotrophic Nutrition
   • Some plants cannot produce their own food due to a lack of chlorophyll or other reasons.
   • These plants depend on other organisms for nutrition. Examples include parasitic plants, insectivorous plants, and saprophytic plants (discussed later).

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Photosynthesis: The Food-Making Process

Photosynthesis is the process by which green plants prepare their own food. The word “photosynthesis” comes from two Greek words: “photo” (light) and “synthesis” (putting together). During photosynthesis, plants use sunlight to combine carbon dioxide and water to produce glucose (a type of sugar) and oxygen.

Equation of Photosynthesis:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂

Key Components of Photosynthesis:

1. Chlorophyll:
   • A green pigment present in the chloroplasts of plant cells.
   • It captures sunlight and gives plants their green color.
2. Sunlight:
   • Acts as the energy source for photosynthesis.
3. Carbon Dioxide (CO₂):
   • Taken in from the air through tiny openings on leaves called stomata.
4. Water (H₂O):
   • Absorbed by the roots from the soil and transported to the leaves through the xylem (a type of plant tissue).
5. Oxygen (O₂):
   • Released as a byproduct through the stomata into the atmosphere.

Steps of Photosynthesis:

1. Chlorophyll absorbs sunlight.
2. The energy from sunlight splits water molecules into hydrogen and oxygen.
3. Hydrogen combines with carbon dioxide to form glucose.
4. Oxygen is released as a waste product.

Importance of Photosynthesis:

• Provides food (glucose) for plants to grow and survive.
• Releases oxygen, which is essential for animals and humans to breathe.
• Maintains the balance of oxygen and carbon dioxide in the atmosphere.

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Structure of Leaves and Role in Photosynthesis

Leaves are the primary sites of photosynthesis and are often called the “kitchen” of the plant. Their structure is well-suited for this process:

• Stomata: Tiny pores on the surface of leaves (mostly on the underside) that allow the exchange of gases (CO₂ in, O₂ out).
• Chloroplasts: Small structures inside leaf cells containing chlorophyll.
• Veins: Contain xylem (for water transport) and phloem (for food transport).

The flat and broad shape of leaves maximizes the surface area for absorbing sunlight and gases.

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How Nutrients are Replenished in the Soil

Plants absorb water and minerals (like nitrogen, phosphorus, and potassium) from the soil through their roots. Over time, these nutrients get depleted as plants use them for growth. Farmers replenish these nutrients by:

1. Adding Manure: Organic matter like decomposed plant and animal waste that enriches the soil with nutrients.
2. Using Fertilizers: Chemical substances rich in specific nutrients (e.g., nitrogen, phosphorus) to boost soil fertility.
3. Crop Rotation: Growing different crops alternately to maintain soil nutrient balance (e.g., legumes add nitrogen to the soil).

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Other Modes of Nutrition in Plants

While most plants are autotrophic, some have adapted unique ways to obtain nutrients due to their environment or lack of chlorophyll. These are examples of heterotrophic nutrition in plants:

1. Parasitic Plants:
   • These plants live on other plants (hosts) and derive nutrients from them.
   • Example: Cuscuta (dodder) – a yellowish, thread-like plant with no chlorophyll that grows on other plants and sucks their nutrients using special structures called haustoria.
2. Insectivorous Plants:
   • These plants grow in nitrogen-deficient soils and trap insects to supplement their nitrogen needs.
   • Example: Pitcher Plant – Its leaves are modified into a pitcher-like structure with a lid. Insects are attracted to the pitcher, slip inside, and get digested by enzymes.
   • Example: Venus Flytrap – Has leaves that snap shut when an insect touches sensitive hairs, trapping and digesting the prey.
3. Saprophytic Plants:
   • These plants feed on dead and decaying organic matter.
   • Example: Indian Pipe (or ghost plant) – Lacks chlorophyll and absorbs nutrients from decaying matter with the help of fungi.
4. Symbiotic Plants:
   • Some plants live in a mutually beneficial relationship with other organisms.
   • Example: Lichens – A partnership between fungi and algae. The algae produce food via photosynthesis, while the fungi provide water and minerals.
   • Example: Leguminous Plants (e.g., peas, beans) – Have nitrogen-fixing bacteria (Rhizobium) in their root nodules. The bacteria convert atmospheric nitrogen into compounds the plant can use, and in return, the plant provides nutrients to the bacteria.

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Experiment to Show Photosynthesis Needs Chlorophyll

To prove that chlorophyll is essential for photosynthesis, we can perform the following experiment:

1. Take a plant with variegated leaves (e.g., croton, which has green and non-green patches).
2. Keep it in sunlight for a few hours.
3. Pluck a leaf and test it for starch (starch is produced during photosynthesis).
   • Boil the leaf in water, then in alcohol to remove chlorophyll.
   • Add iodine solution: Green areas (with chlorophyll) turn blue-black (indicating starch), while non-green areas (no chlorophyll) remain unchanged.
4. Conclusion: Only the parts with chlorophyll produce starch, proving chlorophyll is necessary for photosynthesis.

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Key Terms to Remember

• Autotrophs: Organisms that make their own food (e.g., green plants).
• Heterotrophs: Organisms that depend on others for food (e.g., animals, some plants).
• Chlorophyll: Green pigment that captures sunlight.
• Stomata: Tiny openings on leaves for gas exchange.
• Glucose: Simple sugar produced during photosynthesis.
• Xylem: Tissue that transports water and minerals from roots to leaves.
• Phloem: Tissue that transports food from leaves to other parts of the plant.

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Summary

Plants are remarkable organisms that sustain life on Earth by producing their own food through photosynthesis. This process involves using sunlight, water, and carbon dioxide to create glucose, with oxygen as a byproduct. While most plants are autotrophic, some have adapted heterotrophic modes like parasitism, insectivory, saprophytism, or symbiosis to survive in challenging environments. The leaves, with their chlorophyll and stomata, play a crucial role in photosynthesis. Additionally, plants rely on soil nutrients, which need to be replenished for healthy growth. Understanding plant nutrition helps us appreciate their role in the ecosystem and their importance to all living beings.

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