Class 7 Science Nutrition in Plants Chapter Notes

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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 utilization by the body is called nutrition. The various components of food are called nutrients. These are carbohydrates, proteins, fats, vitamins, minerals, roughage & 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.

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 hetrotrophic nutrition.
All animals and non-green plants are called heterotroph. They obtain their food from plants or other animals

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

Autotrophic Mode of Nutrition
Green plants are autotrophs as they make their own food.

Photosynthesis
The process by which green plnats 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 nutrients that are needed by using carbon dioxide, water and minerals with the help of chlorophyll (green pigment) and energy from sunlight.
* Carbohydrates are synthesized from carbondioxide from air and water from soil.

Carbondioxide+ Water ¾¾Sun¾ligh¾t ®  Glucose+ Oxygen

(from air ) ( from soil )( chlorophyl ) ( food stored in plant) (released into the atmosphere)

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

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

Conditions Necessary for Photosynthesis

l Chlorophyll: The chlorophyll is present in the chloroplasts. These cell organelles occur in green parts of plants, mainly the leaves.

l Sunlight: Sunlight provides energy for photosynthesis. It comes from the sun.

l Water & Minerals: These 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. The 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. Carbondioxide 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 universal photosynthetic pigment. It is also called 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 pigment because they absorb light and transfer it to chlorophyll. A Photosynthetic bacteria has bacteriochlorophyll. Chlorophyll is absent in them. One important source of carotene is carrot. Carrot has betacarotene which is required by our body for producing vitamin A.

An important source of Xanthophyll is seaweeds Chlorophyll and other photosynthetic pigments are present inside chloroplasts which is surrounded by a double membrane enclosing matrix. Embeded in the matrix are present flattened sac like structures called thylakoids. They are made up of membranes. Photosynthetic pigments are present associated with thylakoids. At places, the thylakoids are stacked to form grana. The matrix has enzymes for the dark reaction of photosynthesis. The light reaction takes place in the grana thylakoids.

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 konwn as the electron transfer system in a process caled photophosphorylation. The energy lost by the electron, as it moves along the chain, is use 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 stomata is guarded by two kidney shaped guard cells. The main function of stomata is to let the CO2 from 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 became turgid by the accumulation of water, their outer thin walls bulge out and the inner thick walls are drawn apart. The stomatal opening betwene the two guard cells widens making the stoma open. At the time of closing of stomatal opening, water diffuses out of the guard cells. The guard cells become of flaccid, their inner thick walls became straight and stomata get closed.

Synthesis of Nutrients other than Carbohydrates

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

No te: 1. Nitrogen rich fertilisers provide nitrogen to the soil. Plants obtain nitrogen from soil and use it for protein synthesis. Proteins are needed for plant growth.

2. 2. Coloured leaves of coleus which are red, brown or with patches of brown and pink also have chlorophyll. But their green pigment is marked 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 loss of nutrients in the soil and soil losses its fertility. The soil has to maintain its fertility to grow well. So, the nutrients need to be restored or replenished in the soil.

Nutrients are replenished in the soil by

l Addition of manures and fertilizers in the soil. Manures are normally added to the soil before sowing the seeds. Fertilizers are added when crops are growing. Fertilizers contain nitrogen, potassium and phosphorous.

l Legumious 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 synthesis 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 stems of 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.

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]

Partial parasitic plants have green leaves and can synthesis 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]

No te: The parasitic plants obtain food from host plant and thus interrupt food supply of the host. This weakens the host plant. Cuscuta covers woody plants and causes great damage to 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]

No te: The white cottony mas of fine threads that grow on stale bread, fruits, pickles, leathers goods and cowdung etc. is bread mould. Umbrella shaped nongreen, 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 trap and digest insects to meet their nitrogen need. These plants grow in nitrogen deficient soil and have devices to trap insects. The examples of insectivorous plants are pitcher plant, venus flytrap, sundew and bladderwort.

Pitcher plant (Nepenthes) has long pitcher like structure hanging from the tip of leaf.

It has a lid which can open and close the mouth of pitcher.The inner surface is hairy when an insect visits the pitcher for nectar, the lid closes. The insect is trapped and entangled into the hairs. The digestive juice is secreted in the pitcher and insects is digested.

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 same plants. The plant provides nutrients to the fungi and 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 autotroph. 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 genus Rhizobium live in these nodules and convert atmosphere nitrogen into soluble nitrates. These nitrogenous compound 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]

No te: Pulses are obtained from leguminous plants. Growing pulses in the field increases soil fertility and farmers do not need to add nitrogen rich fertilizers to the soil.

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