Nutrition in Plants - Class 7 Notes & Olympiad Questions

Nutrition

a) Nutrition is an essential process that occurs in all living organisms, including plants and animals. It refers to how organisms obtain and use food to support their growth, energy needs, and overall well-being.
b) Just as we need to eat and drink to survive and stay healthy, other organisms also have their own ways of acquiring and utilising food.
c) Plants, for example, have a unique ability to make their own food through a process called photosynthesis.
d) Animals, on the other hand, have different ways of obtaining their nutrition. Some animals are herbivores, meaning they eat plants as their primary source of food. Others are carnivores and eat other animals, while some animals are omnivores and eat both plants and animals.

Modes of Nutrition

There are different modes of nutrition that organisms use to obtain their food and each one has its own unique adaptations and strategies.

1. Autotrophic Nutrition

a) Autotrophic nutrition is when an organism can produce its own food using simple substances from the environment.
b) These organisms are called autotrophs or producers.
c) The most common example of autotrophic nutrition is photosynthesis, which occurs in plants, algae, and some bacteria.
d) Through photosynthesis, these organisms convert sunlight, carbon dioxide, and water into glucose (sugar) and oxygen.

2. Heterotrophic Nutrition

a) Heterotrophic nutrition is when an organism relies on consuming preformed organic matter from other organisms to obtain its food.
b) These organisms are called heterotrophs or consumers. There are different types of heterotrophic nutrition:

Herbivores: Herbivores are animals that mainly eat plants for their nutrition. They have specialised teeth and digestive systems to process plant material.

Carnivores: Carnivores are animals that primarily eat other animals. They have sharp teeth and strong jaws to catch, kill, and consume their prey.

Omnivores: Omnivores are animals that consume both plants and animals. They have a more diverse diet and can eat a variety of foods.

Scavengers: Scavengers are organisms that feed on dead or decaying matter. They help recycle nutrients and play a crucial role in the ecosystem.

Decomposers: Decomposers, such as fungi and bacteria, break down dead organic material into simpler substances. They play a vital role in recycling nutrients back into the environment.

3. Saprophytic Nutrition

a) Saprophytic nutrition is a type of heterotrophic nutrition where organisms, known as saprophytes, obtain their nutrition by decomposing and feeding on dead or decaying organic matter.
b) They secrete enzymes that break down the organic material externally and then absorb the nutrients released.

4. Parasitic Nutrition

a) Parasitic nutrition occurs when an organism, called a parasite, feeds on another living organism, known as the host, to obtain its nutrition.
b) The parasite benefits from the host's resources while causing harm or damage to the host.

Autotrophic Nutrition

a) Autotrophic nutrition in plants is a fascinating process through which they make their own food using simple inorganic substances like carbon dioxide and water. This process is called photosynthesis and takes place in the presence of sunlight and a green pigment called chlorophyll.
b) Photosynthesis mainly occurs in the leaves of plants, which are often referred to as the "food factories" of the plant. However, it can also occur in other green parts of the plant, such as the stem.
c) Leaves have tiny openings called stomata on their upper surface, which allow carbon dioxide from the air to enter the leaf and reach the chlorophyll. Carbon dioxide is essential for photosynthesis. At the same time, water and minerals necessary for photosynthesis are absorbed by the roots from the soil and transported to the leaves through specialised vessels called xylem.
d) Inside the leaf, chlorophyll, which is contained within the chloroplasts, captures energy from sunlight. This energy is then used to combine carbon dioxide and water to produce glucose, a type of sugar that serves as the plant's food. The process of photosynthesis requires certain conditions, including carbon dioxide, sunlight, chlorophyll, and water.
e) The overall reaction of photosynthesis can be represented as follows:

f) During photosynthesis, carbon dioxide is absorbed, and oxygen is released into the air as a byproduct. The glucose produced during photosynthesis is often converted into starch for storage within the plant. The presence of starch in leaves is an indicator that photosynthesis has taken place.
g) Photosynthesis is a vital process for plants as it allows them to produce their own food, generate energy, and release oxygen into the environment, which is crucial for supporting life on Earth.

Synthesis of Proteins and Fats by the Plants

In addition to carbohydrates, plants have the ability to synthesise proteins and fats. Proteins are crucial for various functions in plants, including growth, repair, and enzymatic activities. Fats, also known as lipids, serve as an energy reserve and are involved in cell structure and signalling.

These processes require nitrogen, an essential component of these macromolecules. However, plants cannot directly absorb nitrogen from the air, despite its abundance. They rely on two main methods to obtain nitrogen:

1. Nitrogen Fixing Bacteria

a) Certain bacteria, such as Rhizobium, have a special ability to convert atmospheric nitrogen into water-soluble compounds like ammonia. These bacteria live in the roots of certain plants, such as legumes (e.g., peas, beans).
b) The plants form a symbiotic relationship with these bacteria, providing them with a habitat while receiving nitrogen compounds in return.
c) The plants can absorb these compounds along with water from the soil and use them to synthesize proteins and fats.

2. Fertilisers

a) Farmers often supplement the soil with fertilisers that contain a high concentration of nitrogen compounds.
b) These fertilisers can be synthetic, such as ammonium nitrate, or organic, such as manure.
c) By adding fertilisers to the soil, farmers ensure that plants have an ample supply of nitrogen for protein and fat synthesis.

Heterotrophic Nutrition

While most plants are autotrophs, meaning they can make their own food through photosynthesis, some plants have evolved to obtain their nutrients by consuming other organisms or organic matter. These plants are called heterotrophs and their mode of nutrition is called heterotrophic nutrition.

Heterotrophic nutrition in plants can occur in various ways:

1. Parasitic Plants

a) Parasitic plants are plants that live on or inside other organisms and obtain their nutrients from them. The organism from which a parasite gets its nutrients is called the host.
b) There are two main types of parasitic plants: total parasites and partial parasites.

(i) Total Parasites: Total parasites are plants that depend entirely on their host for nutrition. They no longer need green leaves and the ability to perform photosynthesis. An example of a total parasite is Cuscuta, also known as Dodder. Dodder is a vine-like plant that wraps itself around its host plant and extracts water, nutrients, and carbohydrates directly from the host.

(ii) Partial Parasites: Partial parasites are plants that have green leaves and can carry out photosynthesis to produce their own carbohydrates. However, they still rely on the host plant for water and minerals. An example of a partial parasite is mistletoe. Mistletoe grows on trees, such as mango trees, and has green leaves that allow it to make its own food through photosynthesis. However, mistletoe depends on the host tree for water and essential nutrients.

2. Insectivorous Plants

a) Insectivorous plants are a unique group of green plants that obtain their nutrition from both the soil and the atmosphere through photosynthesis, but they also rely on small insects for a part of their nutrient requirements because the soil they grow in is often nutrient-poor, especially in terms of nitrogen.
b) Insectivorous plants have developed unique adaptations to capture and digest insects. Here are a few examples of insectivorous plants:

(i) Venus Flytrap: The Venus flytrap has specialised leaves with sensitive trigger hairs. When an insect touches these trigger hairs, the leaves snap shut, trapping the insect inside. The plant then secretes digestive enzymes to break down the insect and absorb the nutrients.

(ii) Pitcher Plant: Pitcher plants have modified leaves that form pitcher-shaped structures. These pitchers contain a mixture of digestive enzymes and liquid that lures insects into the pitcher. Once inside, the insects drown in the liquid and are broken down by enzymes, providing the plant with nutrients.

(iii) Sundew: Sundews have leaves covered in glandular hairs that produce a sticky substance. When an insect lands on the leaf, it becomes trapped in the sticky substance. The sundew then releases enzymes to digest the insect and absorb the nutrients.

3. Saprophytic Plants

a) Plants that use saprophytic nutrition are known as saprophytes. These plants obtain their food by feeding on dead and decaying plant or animal matter.
b) Instead of relying on living organisms for their nutrition, saprophytes specialise in decomposing organic material and extracting nutrients from it.
c) Saprophytic plants are important in ecosystems because they play a vital role in the process of decomposition. By breaking down dead plant or animal matter, they help recycle nutrients back into the environment.
d) Some examples of saprophytic plants are Mushrooms and Bracket Fungi.

4. Symbiotic Nutrition

Symbiotic nutrition refers to a special type of relationship in which two different species show a close association and mutually benefit from each other's presence. For example:

(i) Certain plants and nitrogen-fixing bacteria: One example of symbiotic nutrition in plants is the association between certain plants and nitrogen-fixing bacteria. Nitrogen-fixing bacteria have the ability to convert atmospheric nitrogen into a form that plants can use for their growth and development. However, most plants cannot directly access atmospheric nitrogen.

In this symbiotic relationship, the plants provide a suitable environment for the nitrogen-fixing bacteria to live within their root nodules. The bacteria, in turn, take atmospheric nitrogen and convert it into a form of nitrogen that the plants can utilise.

(ii) Certain plants and mycorrhizal fungi: Another example of symbiotic nutrition in plants is seen in the association between plants and mycorrhizal fungi.

In this association, the plants provide the fungi with carbohydrates produced through photosynthesis. In return, the mycorrhizal fungi enhance the plant's ability to absorb water and nutrients from the soil.

Experimental Investigations on Photosynthesis: Understanding the Factors and Processes Involved

1. To test a leaf for starch

This experiment helps determine if a leaf has undergone photosynthesis and produced starch, which is a storage form of glucose.

The steps involved are:

a) Boil the leaf in water to stop any ongoing chemical reactions.
b) Place the boiled leaf in a container of boiling water and add some drops of iodine solution.
c) Observe the leaf for any colour change in response to the iodine solution.

Observation: If the leaf turns blue-black, it indicates the presence of starch, while a brown colour indicates the absence of starch.

Conclusion: The experiment helps determine if the leaf has undergone photosynthesis and produced starch, which is a storage form of glucose. The boiling process stops any ongoing chemical reactions, and the addition of iodine solution allows the detection of starch.

2. To show that light is necessary for photosynthesis

This experiment demonstrates that light is a crucial factor for photosynthesis to occur.

The setup involves:

a) Take two potted plants of the same species and place them in separate dark chambers.
b) Provide one plant with light while keeping the other plant in the dark.
c) After a period of time, test both plants for the presence of starch using the method described in the above experiment.

Observation: The plant exposed to light will show the presence of starch, while the plant kept in the dark will not.

Conclusion: The experiment confirms that light is a necessary factor for photosynthesis to occur. The plant exposed to light showed the presence of starch, indicating successful photosynthesis and the production of glucose. On the other hand, the plant kept in the dark did not show the presence of starch, suggesting that photosynthesis did not take place due to the absence of light.

3. Carbon dioxide is necessary for photosynthesis

This experiment investigates the necessity of carbon dioxide in photosynthesis.

The steps that can be followed are:

a) Take two potted plants of the same species and place them in separate chambers.
b) Provide one plant with a normal air supply, while the other plant receives air depleted of carbon dioxide (for example, by passing it through soda lime).
c) After a certain period of time, test both plants for the presence of starch using the method described in experiment 1.

Observation: The plant supplied with normal air will likely show the presence of starch, indicating that carbon dioxide is necessary for photosynthesis.

Conclusion: The experiment confirms that carbon dioxide is necessary for photosynthesis to occur. The plant supplied with normal air, which contains an adequate amount of carbon dioxide, showed the presence of starch, indicating successful photosynthesis and the production of glucose. However, the plant receiving air depleted of carbon dioxide did not show the presence of starch.