Mode Of Nutrition In Plants

As we learned in Class VI, **food** is essential for the survival and functioning of all living organisms. Food provides vital components like carbohydrates, proteins, fats, vitamins, and minerals, which are collectively called **nutrients**. These nutrients are necessary for our bodies to build structure, grow, repair damage, and obtain the energy needed for life processes.

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All living beings require food, but they obtain it in different ways:

• **Plants** have a unique ability among most organisms to **prepare their own food** using simple raw materials available in their environment.
• **Animals** and **humans** cannot make their own food. They depend on plants or other animals that consume plants for their nutrition. This means animals and humans are ultimately **dependent** on plants, either directly (by eating plants) or indirectly (by eating animals that eat plants).

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The way an organism takes in food and how its body uses it is called **Nutrition**. Based on how organisms obtain food, their mode of nutrition can be classified:

• Autotrophic Nutrition: This mode of nutrition involves organisms synthesising (making) their food themselves from simple, inorganic substances (like water, carbon dioxide, and minerals). The word "autotrophic" comes from 'auto' meaning self, and 'trophos' meaning nourishment. Organisms that perform autotrophic nutrition are called **Autotrophs**. **Green plants** are the primary examples of autotrophs.
• Heterotrophic Nutrition: This mode of nutrition involves organisms obtaining food by consuming food prepared by other organisms. 'Heteros' means other. Organisms that follow heterotrophic nutrition are called **Heterotrophs**. **Animals** and **most non-green plants** (and other non-plant organisms) are heterotrophs, as they rely on the food produced by autotrophs.

Since plants are capable of making their own food, they are considered the **producers** in most ecosystems, forming the base of the food chain.

Photosynthesis — Food Making Process In Plants

**Photosynthesis** is the specific process by which green plants create their own food. This vital process primarily takes place in the **leaves** of the plant, which can be thought of as the plant's "food factories".

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For photosynthesis to occur, raw materials must reach the leaves:

• Water and Minerals: These are absorbed from the **soil** by the **roots** of the plant. From the roots, they are transported upwards to the leaves through special **vessels**. These vessels are like continuous pipelines running throughout the root, stem, branches, and into the leaves, forming a pathway for nutrient transport.
• Carbon Dioxide: This gas is taken from the **air**. It enters the leaves through tiny pores located on the surface of the leaves. These pores are called **stomata** (singular: stoma). Each stoma is surrounded by a pair of cells called **guard cells** which regulate its opening and closing.

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The leaves contain a green pigment called **chlorophyll**. Chlorophyll is crucial because it captures the **energy from sunlight**. This captured solar energy is then used by the leaves to combine carbon dioxide and water to produce food, primarily in the form of **carbohydrates** (like glucose and starch).

Since this food synthesis process requires sunlight ('Photo' means light, 'synthesis' means to combine), the process is named **Photosynthesis**.

The process of photosynthesis can be represented by the following equation:

$\textsf{Carbon dioxide} + \textsf{Water} \xrightarrow{\textsf{Sunlight}} \textsf{Carbohydrate} + \textsf{Oxygen}$

Or more precisely:

$6\textsf{CO}_2 + 6\textsf{H}_2\textsf{O} \xrightarrow{\textsf{Sunlight}} \textsf{C}_6\textsf{H}_{12}\textsf{O}_6 + 6\textsf{O}_2$

($\textsf{Chlorophyll}$ is needed for the energy absorption, but is not consumed in the reaction itself).

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Photosynthesis is a fundamental process for life on Earth because:

• It produces food (carbohydrates) which directly or indirectly sustains almost all living organisms. The **Sun** is the ultimate source of energy that powers this process.
• It releases **Oxygen** as a byproduct, which is essential for the respiration (and survival) of most living organisms on Earth.

Without photosynthesis, there would be no food production by plants and no replenishment of oxygen in the atmosphere, making life as we know it impossible.

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While leaves are the main sites, photosynthesis can also occur in other green parts of the plant, such as **green stems and branches**, because they also contain chlorophyll. Desert plants, for instance, have modified leaves (spines/scales) to reduce water loss; their green stems take over the function of photosynthesis.

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Activity 1.1

This activity demonstrates that **sunlight is necessary for photosynthesis** (and thus for starch production). Two identical potted plants are taken. One is kept in the dark for 72 hours (to remove any pre-existing starch), and the other is kept in sunlight.

An **iodine test** is performed on a leaf from each plant (as learned in Class VI - testing for starch presence results in a blue-black colour). The leaf from the plant kept in the dark will show no starch (no blue-black colour), while the leaf from the plant in sunlight will test positive for starch (blue-black colour).

If the plant from the dark is then placed in sunlight for a few days, its leaves will again test positive for starch, proving that exposure to sunlight enables the plant to perform photosynthesis and produce starch.

The activity also points out that leaves that are not primarily green (like red, violet, or brown leaves) still contain chlorophyll. The green colour is simply masked by the presence of large amounts of other pigments. Photosynthesis still occurs in these leaves.

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Besides synthesising carbohydrates, plants also produce other components of food, like **proteins and fats**. Carbohydrates provide the basic carbon, hydrogen, and oxygen atoms needed for these compounds.

However, proteins are **nitrogenous substances**, meaning they contain nitrogen. Plants cannot directly use the abundant gaseous nitrogen ($N_2$) from the air.

They obtain nitrogen from the soil in a soluble form:

• Certain **bacteria** in the soil (like *Rhizobium*) have the ability to convert atmospheric nitrogen into a usable form that plants can absorb from the soil along with water.
• Farmers also add **fertilisers and manures** rich in nitrogen and other nutrients to the soil to ensure plants have sufficient supply.

With the necessary nutrients absorbed from the soil, plants can then synthesise proteins and vitamins.

Other Modes Of Nutrition In Plants

Not all plants are autotrophs. Some plants **lack chlorophyll** and therefore cannot perform photosynthesis to make their own food. These plants rely on food produced by other organisms and exhibit **heterotrophic modes of nutrition**.

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One such mode is **Parasitic Nutrition**. Plants with parasitic nutrition live on or inside another living organism (called the **host**) and derive readymade food from it, often harming the host in the process.

An example is **Cuscuta** (Amarbel). It is a plant with a yellow, wiry, branched stem that twines around other plants. Since it lacks chlorophyll, it absorbs nutrients directly from the host plant through special structures. By obtaining valuable nutrients from the host, Cuscuta is considered a **parasite**.

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Another interesting mode is seen in **Insectivorous Plants**. These are plants that can trap and digest insects to obtain nutrients, primarily nitrogen.

A well-known example is the **Pitcher plant**. Its leaves are modified into a pitcher-like or jug-like structure. The tip of the leaf forms a lid that can open and close the pitcher's mouth. Inside the pitcher, there are downward-pointing hairs. When an insect enters the pitcher, the lid closes, and the insect gets trapped by the hairs. The plant then secretes digestive juices into the pitcher to break down the insect and absorb the nutrients.

While pitcher plants are green and can perform photosynthesis, they often grow in soils that are deficient in certain nutrients, particularly nitrogen. Feeding on insects helps them supplement their nutrient intake, especially nitrogen. Therefore, insectivorous plants are sometimes considered **partial heterotrophs** because they photosynthesize but also obtain nutrients by consuming other organisms.

Saprotrophs

Some organisms obtain nutrients from **dead and decaying organic matter**. These organisms are called **Saprotrophs**, and their mode of nutrition is **Saprotrophic Nutrition**.

Examples include mushrooms and fungi (like the fluffy patches that grow on moist surfaces or rotting wood, especially during the rainy season).

Saprotrophs secrete digestive juices onto the dead and decaying matter, breaking it down into simpler substances, which they then absorb as nutrients.

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Activity 1.2

Mould (a type of fungi) is grown on a piece of moist bread. After a few days in a moist, warm place, fluffy patches appear on the bread. Observing these patches under a microscope reveals cotton-like threads, which are the fungal hyphae. These fungi are absorbing nutrients from the dead bread, demonstrating saprotrophic nutrition.

Fungi spores are present in the air. They land on surfaces and germinate when conditions are favourable (moisture, warmth, presence of organic matter), causing items like pickles, leather, and clothes to get spoiled during humid weather.

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Another type of relationship where organisms live together and share resources is called **Symbiosis**.

• Some **fungi** live in the roots of plants. The plant provides nutrients to the fungus, and the fungus helps the plant absorb water and certain minerals more effectively.
• **Lichens** are composite organisms formed by a symbiotic association between an **alga** (which contains chlorophyll and performs photosynthesis to provide food) and a **fungus** (which provides shelter, water, and minerals to the alga).

How Nutrients Are Replenished In The Soil

Plants constantly absorb mineral nutrients from the soil for growth and other functions. This process depletes the soil's nutrient content over time.

To maintain soil fertility and ensure healthy plant growth, nutrients need to be added back to the soil. This is done by spreading **manures** and **fertilisers** in fields, lawns, or pots. Manures and fertilisers contain essential nutrients like nitrogen, potassium, and phosphorus.

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Crop plants, especially, absorb a large amount of **nitrogen** from the soil, making the soil deficient in this crucial nutrient. As mentioned earlier, plants cannot directly use atmospheric nitrogen.

However, there are bacteria, like **Rhizobium**, that live in the root nodules of **leguminous plants** (e.g., grams, peas, moong, beans). These bacteria convert gaseous atmospheric nitrogen into a soluble, usable form that the host plant can absorb from the soil. In return, the plant provides the bacteria with food and shelter. This is a classic example of a **symbiotic relationship**.

This symbiotic association between Rhizobium and leguminous plants is very important for farmers. By growing leguminous crops, they can naturally enrich the soil with nitrogen, reducing the need to use expensive nitrogenous fertilisers.

In summary, most plants are autotrophs performing photosynthesis. However, some are parasitic (like Cuscuta) or saprotrophic (like fungi), obtaining nutrients from other organisms. Insectivorous plants (like the Pitcher plant) are considered partial heterotrophs. Animals are all heterotrophs, depending on plants and other animals for food. Nutrient levels in soil are maintained through decomposition, natural nitrogen fixation by bacteria, and the addition of manures and fertilisers.

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