Class 7 Science Nutrition in Plants Chapter Notes

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Class 7 Science Chapter 1 Nutrition in Plants chapter notes covering autotrophic and heterotrophic nutrition, the structure of chloroplast, photosynthesis and its conditions, light and dark phases, stomata, synthesis of nutrients including nitrogen, replenishment of soil nutrients, and the parasitic, saprophytic, insectivorous and symbiotic modes of nutrition in plants.

Nutrition in Plants

All living organisms need energy to perform various life activities. They obtain this energy from the food they eat. This process of taking food and its utilisation by the body is called nutrition. The various components of food are called nutrients. These are carbohydrates, proteins, fats, vitamins, minerals, roughage and water.

Energy is stored in food as chemical energy. First, the food is broken into small soluble molecules inside the body of living beings. These small molecules then react with oxygen and energy is released. The energy released during this process is stored as ATP (adenosine triphosphate). This ATP is used by the body cells.

[Figure: Flow chart titled Nutrition in Plants. It branches into Autotrophic, Heterotrophic and Interdependence. Autotrophic leads to Photosynthesis, which needs conditions: CO₂ (through stomata), H₂O (through roots) and sunlight (through chlorophyll). Heterotrophic leads to Parasitic, Saprophytic and Insectivorous. Interdependence leads to Symbiosis, with examples lichens and legumes.]

MODES OF NUTRITION

Living organisms exhibit two main modes of nutrition.

Autotrophic nutrition
(Auto = self + trophos = nourishment)
The mode of nutrition in which organisms make their own food from simple inorganic substances is called autotrophic. Green plants make food for themselves. They are called autotrophs. They are also called producers.

Heterotrophic nutrition
(heteros = other + trophos = nourishment)
The mode of nutrition in which organisms obtain food from plants or other animals is called heterotrophic nutrition. All animals and non-green plants are called heterotrophs. They obtain their food from plants or other animals.

Difference between autotrophic and heterotrophic nutrition

Detailed structure of chloroplast

Chloroplasts are the food producers of the cell. They are found in plant cells. Animal cells do not have chloroplasts. Chlorophyll and other photosynthetic pigments are present inside the chloroplast. The chloroplast is surrounded by a double membrane which encloses a matrix. Embedded in the matrix are flattened sac-like structures called thylakoids. Thylakoids are made up of membranes. Photosynthetic pigments are present associated with thylakoids. One thylakoid stack is called a granum. The thylakoids have chlorophyll molecules on their surface. The matrix has enzymes for the dark reaction of photosynthesis. The light reaction takes place in the grana thylakoids.

[Figure: Structure of chloroplast with labels: outer membrane, inner membrane, stromal lamellae, thylakoid, stroma and starch/sugar.]

AUTOTROPHIC MODE OF NUTRITION

Green plants are autotrophs as they make their own food.

Photosynthesis

The process by which green plants make their food is called photosynthesis (photo = light + synthesis = to make). During photosynthesis, green plants prepare complex organic food substances from simple inorganic compounds. Green plants make all the nutrients that are needed by using carbon dioxide, water and minerals, with the help of chlorophyll (green pigment) and energy from sunlight.

Carbohydrates are synthesised from carbon dioxide from the air and water from the soil.

Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen
(from air) (from soil) (food stored in plant) (released into the atmosphere)

Amino acids and proteins are made by combining carbohydrates with nitrogen obtained in the form of nitrates from the soil.

During photosynthesis, green plants convert solar energy into chemical energy. The food is synthesised as glucose. It is distributed to different parts of the plant for immediate use or storage.

[Figure: Fig. 1: Schematic representation of photosynthesis. A plant with roots and root hairs absorbing water and minerals from soil, green leaves containing chlorophyll in chloroplasts, a stem transporting water and minerals to leaves and branches, sunlight with CO₂ from air entering the leaves, food in the form of glucose being formed, and oxygen released.]

Conditions necessary for photosynthesis

Chlorophyll: The chlorophyll is present in the chloroplasts. These cell organelles occur in the green parts of plants, mainly the leaves.
Sunlight: Sunlight provides energy for photosynthesis. It comes from the sun.
Water and minerals: These are obtained from the soil by the roots and are transported to the leaves.

Site of photosynthesis

Green leaves and stems of young plants contain green coloured structures, the chloroplasts. The green colour of chloroplasts is due to the presence of chlorophyll, a green coloured pigment. Chlorophyll traps energy from sunlight. Water is absorbed from the soil by the thin root hairs present in the roots. The water and minerals so absorbed are transported to other parts of the plant through the stem.

Leaves have tiny pores called stomata on the under surface. Carbon dioxide enters the leaves through stomata.

Photosynthetic pigments

Different types of photosynthetic pigments are present inside the chloroplast, viz., chlorophyll a, chlorophyll b, carotenes and xanthophyll. In some algae, chlorophyll c and chlorophyll d are also present. Chlorophyll a is present in all photosynthetic organisms, so it is called the universal photosynthetic pigment. It is also called the primary photosynthetic pigment because it carries out the primary reaction of photosynthesis, that is, converting light energy into chemical energy. Other pigments are called accessory pigments because they absorb light and transfer it to chlorophyll.

Photosynthetic bacteria have bacteriochlorophyll. Chlorophyll is absent in them. One important source of carotene is carrot. Carrot has beta-carotene, which is required by our body for producing vitamin A. An important source of xanthophyll is seaweeds.

The light phase and dark phase of photosynthesis

During the light dependent phase of photosynthesis, light energy from the sun is harvested by special pigments on the thylakoid membranes of the chloroplasts. Chlorophyll (specifically chlorophyll a) is the most important of these pigments. The pigments are not arranged haphazardly on the surface of the chloroplast, but they occur in a specific aggregated manner known as photosystems. The photosystems play a central role in the light dependent phase of photosynthesis. In the light dependent reaction of photosynthesis, an energy-rich electron from a special chlorophyll molecule in a photosystem is passed along a chain of molecules known as the electron transfer system, in a process called photophosphorylation. The energy lost by the electron, as it moves along the chain, is used to make ATP.

What are commonly called the dark reactions are the second phase of photosynthesis. These reactions are light-independent, but they happen both during the day and at night. This phase of photosynthesis is also called the Calvin cycle. The purpose of this phase is to store the cellular energy generated in the light phase as glucose.

Functioning (opening and closing) of stomata

Stomata open into small air cavities inside the leaves. Each stoma is guarded by two kidney shaped guard cells. The main function of stomata is to let the CO₂ from the atmosphere enter the leaf for photosynthesis. During the day time, when photosynthesis takes place, stomata remain open.

At night, when there is no light and no photosynthesis takes place, the stomata get closed to avoid loss of water by transpiration.

The opening and closing of stomata is carried out by guard cells. In sunlight, water from surrounding cells diffuses into the guard cells. The guard cells become turgid by the accumulation of water, their outer thin walls bulge out and the inner thick walls are drawn apart. The stomatal opening between the two guard cells widens, making the stoma open. At the time of closing, water diffuses out of the guard cells. The guard cells become flaccid, their inner thick walls become straight and the stoma gets closed.

Synthesis of nutrients other than carbohydrates

Plants synthesise only glucose (a simple carbohydrate) through photosynthesis. These simple carbohydrates are then used for the synthesis of other nutrients like proteins and fats. Complex carbohydrates and fats are synthesised from simple carbohydrates.

Plants need nitrogen for the synthesis of proteins. Though nitrogen is present in abundance in gaseous form in the air, plants cannot use this free nitrogen. They can absorb only the nitrate compounds, along with water. The soil contains some nitrifying bacteria. These bacteria prepare nitrate compounds from nitrogen and enrich the soil with nitrates. Farmers add nitrogen rich fertilisers (like urea and ammonium sulphate) to the soil to increase its fertility.

Note:
1. Nitrogen rich fertilisers provide nitrogen to the soil. Plants obtain nitrogen from the soil and use it for protein synthesis. Proteins are needed for plant growth.
2. Coloured leaves of coleus, which are red, brown or with patches of brown and pink, also have chlorophyll. But their green pigment is masked by the abundance of other pigments.

How nutrients are replenished in the soil

Soil supplies nutrients to the plants for growth. When crops are growing, they absorb nutrients from the soil. This results in the loss of nutrients in the soil, and the soil loses its fertility. The soil has to maintain its fertility to grow crops well. So the nutrients need to be restored or replenished in the soil.

Nutrients are replenished in the soil by the addition of manures and fertilisers. Manures are normally added to the soil before sowing the seeds. Fertilisers are added when crops are growing. Fertilisers contain nitrogen, potassium and phosphorus.

Leguminous plants such as gram, peas, beans, etc. have root nodules in which Rhizobium bacteria are present. These bacteria can take atmospheric nitrogen and convert it into a usable soluble form. Plants can use this nitrogen for making protein.

HETEROTROPHIC MODES OF NUTRITION IN PLANTS

Non green plants like fungi, some bacteria and some flowering plants are without chlorophyll. They cannot synthesise their food by photosynthesis. Like animals, they depend on green plants for their food. Such non green plants are called heterotrophs, and their mode of nutrition is called heterotrophic nutrition.

Based on the method of obtaining food, heterotrophic plants may be:
Parasitic plants
Saprophytic plants
Insectivorous plants
Symbiotic plants

Parasitic plants

Non-green plants that obtain their food from some other green plants are called parasitic plants. The plant that provides food is called the host plant. The parasitic plants absorb food from the root or the stem of the host plant. They develop special roots called haustoria, which penetrate the host tissues and reach the vascular bundles. Some parasitic plants are total parasites, while others may be partial parasites.

[Figure: Fig. 3: Dodder, a yellowish thread-like climber wrapped around a green host plant.]

Total parasitic plants completely depend on the host plant for their food. For example, dodder (cuscuta, amarbel) and Apodanthes are common total parasitic plants. [Fig. 3]

[Figure: Fig. 4: Mistletoe plant growing on a tree branch, with green leaves and clusters of small white flowers.]

Partial parasitic plants have green leaves and can synthesise their food. They depend on the host for water and minerals. Mistletoe is a partial parasite. [Fig. 4]

The plant that bears the largest flower in the world is a parasitic plant. Its flower may be 106 cm in diameter and about 10 kg in weight. The plant has no leaves. It was discovered by Sir Stamford Raffles on a tour to Sumatra and hence is named after him as Rafflesia. [Fig. 5]

[Figure: Fig. 5: A Rafflesia flower, a large reddish flower with a central cup and spotted surface.]

Note: The parasitic plants obtain food from the host plant and thus interrupt the food supply of the host. This weakens the host plant. Cuscuta covers woody plants and causes great damage to the timber industry.

Saprophytic plants

Saprophytic plants, or saprophytes, obtain their food from dead and decaying plants and animals. They release digestive enzymes on the decaying matter or animal excreta to break down their complex organic compounds into simple ones. These simple organic compounds are absorbed by the saprophytes. Fungi such as bread mould and mushrooms, and some bacteria, have saprophytic nutrition. [Fig. 6]

[Figure: Fig. 6: Saprophytic plants. (a) Bread mould, with a network of threads and dark round sporangia on stalks. (b) Mushroom, with labelled cap, gills, stem and mycelium.]

Note: The white cottony mass of fine threads that grows on stale bread, fruits, pickles, leather goods and cowdung, etc., is bread mould. Umbrella shaped non green, white or pinkish plants are called mushrooms. Some mushrooms are poisonous, but some are edible and form a delicacy.

Insectivorous plants

Insectivorous plants are green plants. They make their food by photosynthesis, but they trap and digest insects to meet their nitrogen need. These plants grow in nitrogen deficient soil and have devices to trap insects. Examples of insectivorous plants are pitcher plant, venus flytrap, sundew and bladderwort.

Pitcher plant (Nepenthes) has a long pitcher-like structure hanging from the tip of the leaf. It has a lid which can open and close the mouth of the pitcher. The inner surface is hairy. When an insect visits the pitcher for nectar, the lid closes. The insect is trapped and entangled in the hairs. The digestive juice is secreted in the pitcher and the insect is digested.

[Figure: Fig. 7: Pitcher plant, with several reddish pitcher-shaped leaves hanging from the stem.]

Symbiotic plants

When two organisms live together and share shelter and nutrients, their association is called symbiosis or symbiotic relationship, and the organisms are called symbionts. They mutually help each other.

Certain fungi live in the roots of some plants. The plant provides nutrients to the fungi, and the fungi help the plant to take up water and nutrients from the soil.

Lichens are symbiotic organisms in which an alga and a fungus live together and derive nutrients from each other. The alga is green and autotrophic. It makes food for both by photosynthesis. The fungus provides shelter, water and minerals to the alga. [Fig. 8]

Roots of leguminous plants (gram, moong, urd, peas and beans, etc.) have nodules. Bacteria belonging to the genus Rhizobium live in these nodules and convert atmospheric nitrogen into soluble nitrates. These nitrogenous compounds are used by leguminous plants. Rhizobium cannot make its food and hence depends on the plant for it. Thus leguminous plants and Rhizobium have a symbiotic relationship. [Fig. 9]

[Figure: Fig. 8: A lichen showing symbiotic relationship between fungus and a tree, growing on a tree trunk. Fig. 9: Root nodules on the roots of a leguminous plant.]

Note: Pulses are obtained from leguminous plants. Growing pulses in the field increases soil fertility, and farmers do not need to add nitrogen rich fertilisers to the soil.

Carbondioxide+ Water ¾¾Sun¾ligh¾t ®  Glucose+ Oxygen