Introduction to the Topic

Every living organism, from the smallest bacteria to the largest blue whale, requires energy to survive, grow, and maintain its body. This energy comes from the food we eat. In Class VI, we learned that food contains essential components like carbohydrates, proteins, fats, vitamins, and minerals. These components are called nutrients. But have you ever wondered how plants get their food? While humans and animals depend on others for their meals, plants are the master chefs of the natural world.

In this chapter, Nutrition in Plants, we explore the fascinating world of biological self-sufficiency. We will delve into how plants use sunlight, water, and air to build their own bodies and, in turn, provide the foundation for all life on Earth. Understanding plant nutrition is not just about biology; it is about understanding the very energy cycle that keeps our planet breathing.

Key Concepts Explained

1. Mode of Nutrition in Plants

Nutrition is the mode of taking food by an organism and its utilization by the body. There are two primary modes of nutrition in the biological world:

  • Autotrophic Nutrition: The word 'auto' means self and 'trophos' means nourishment. Therefore, organisms that make food themselves from simple substances are called autotrophs. Green plants are the primary examples of autotrophs.
  • Heterotrophic Nutrition: Animals and most other organisms take in food prepared by plants. They are called heterotrophs (heteros = other). Since we cannot make food from carbon dioxide, water, and minerals like plants do, we fall into this category.

2. Photosynthesis: Food Making Process in Plants

Plants are like miniature factories. The leaves are the 'food factories' of plants. Therefore, all the raw materials must reach the leaf. The process by which green plants synthesize their own food (glucose) from carbon dioxide and water in the presence of sunlight and chlorophyll is called Photosynthesis.

The Raw Materials

  • Water and Minerals: These are absorbed by the roots from the soil and transported to the leaves by tiny vessel-like structures that run like pipes throughout the root, stem, and branches.
  • Carbon Dioxide: This gas is taken from the air through tiny pores present on the surface of leaves. These pores are called stomata. Each stoma is surrounded by 'guard cells' that regulate its opening and closing.
  • Chlorophyll: This is a green pigment found in leaves. It helps leaves capture the energy of the sunlight. This energy is used to synthesize food.

The Chemical Equation

During photosynthesis, chlorophyll-containing cells of leaves, in the presence of sunlight, use carbon dioxide and water to synthesize carbohydrates. The process can be represented in the following equation:

Carbon dioxide + Water —(Sunlight/Chlorophyll)—> Carbohydrate + Oxygen

The carbohydrate produced is eventually stored as starch. The presence of starch in leaves indicates the occurrence of photosynthesis. Oxygen, a byproduct of this process, is released into the atmosphere, which is vital for the survival of all living organisms.

3. The Sun: The Ultimate Source of Energy

Chlorophyll acts as a solar panel, capturing solar energy. This energy is then converted into chemical energy stored in food. Without the sun, there would be no photosynthesis, no food, and consequently, no life. Thus, the sun is the ultimate source of energy for all living organisms.

4. Other Modes of Nutrition in Plants

Not all plants are green, and not all can perform photosynthesis. Some plants depend on the food produced by other plants. These are known as heterotrophic plants.

  • Parasitic Plants: Have you seen a yellow wiry branched structure twining around the stem and branches of a tree? This is a plant called Cuscuta (Amarbel). It does not have chlorophyll. It takes readymade food from the plant on which it is climbing. The plant on which it climbs is called the host. Since it deprives the host of valuable nutrients, Cuscuta is called a parasite.
  • Insectivorous Plants: There are a few plants which can trap insects and digest them. The Pitcher plant is a classic example. The leaf is modified into a pitcher-like structure with a lid. When an insect lands in the pitcher, the lid closes and the insect gets entangled in the hair inside. The insect is then digested by digestive juices secreted in the pitcher. Why do they eat insects? These plants grow in nitrogen-deficient soil and fulfill their nitrogen requirements by consuming insects.
  • Saprotrophs: You might have seen fluffy umbrella-like patches growing in moist soils or on rotting wood during the rainy season. These organisms are called Fungi. They have a different mode of nutrition. They secrete digestive juices onto dead and decaying matter and convert it into a solution. Then they absorb the nutrients from it. This mode of nutrition is called saprotrophic nutrition.
  • Symbiosis: Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis. A great example is Lichens. In lichens, a chlorophyll-containing partner (an alga) and a fungus live together. The fungus provides shelter, water, and minerals to the alga and, in return, the alga prepares and provides food to the fungus.

5. How Nutrients are Replenished in the Soil

Plants absorb mineral nutrients from the soil, which leads to a decline in the amount of nutrients in the soil over time. Farmers often add manures and fertilizers to the soil to enrich it with nitrogen, potassium, and phosphorous. In nature, this replenishment happens through a biological process involving bacteria called Rhizobium. These bacteria live in the roots of leguminous plants (like peas, grams, and beans) and convert atmospheric nitrogen into a usable form for the plants. In return, the plants provide food and shelter to the bacteria—another beautiful example of a symbiotic relationship.

Summary & Key Takeaways

  • Nutrition: The process of intake and utilization of food by organisms.
  • Autotrophs: Green plants that make their own food via photosynthesis.
  • Photosynthesis: Requires sunlight, chlorophyll, carbon dioxide, and water to produce glucose and oxygen.
  • Stomata: Tiny pores on leaves for gas exchange, guarded by guard cells.
  • Heterotrophs: Organisms (humans, animals, parasites) that depend on plants for food.
  • Parasites: Plants like Cuscuta that live on and derive nutrition from a host plant.
  • Saprotrophs: Organisms like fungi that feed on dead and decaying organic matter.
  • Symbiosis: A mutually beneficial relationship between two different organisms, like Rhizobium and legumes.
  • Soil Health: Nutrients like nitrogen are replenished through fertilizers or natural nitrogen fixation by bacteria.