Class 7 Science Nutrition in Plants Notes

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. 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

• 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.

• 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]

Note: 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.