Class 7 Science Chapter 1 Important Questions

Food is essential for all living organisms. Carbohydrates, proteins, fats, vitamins and minerals are components of food. These components of food are necessary for our body and are called nutrients.

All living organisms require food. Plants can make their food themselves but animals including humans cannot. They get it from plants or animals that eat plants. Thus, humans and animals are directly or indirectly dependent on plants.

Plants are the only organisms that can prepare food for themselves by using sunlight, water, carbon dioxide and minerals. The raw materials are present in their surroundings.

Nutrition is the mode of taking food by an organism and its utilisation by the body.
The nutrients enable living organisms to build their bodies, to grow, to repair damaged parts of their bodies and provide the energy to carry out life processes.

The mode of nutrition in which organisms make food themselves, called autotrophs. Such as Plants.
Animals and most other organisms take in ready-made food prepared by the plants. They are called heterotrophs (heteros = other).

Plants takes carbon dioxide from air through the tiny pores present on the surface of the leaves. These pores are surrounded by ‘guard cells’. Such pores are called stomata.

Plants are the only organisms that can prepare food for themselves by using sunlight, water, carbon dioxide and minerals. Water and minerals present in the soil are absorbed by the roots and transported to the leaves. Carbon dioxide from air is taken in through the tiny pores present on the surface of the leaves. And the solar energy is captured by the leaves.

Leaves are known as food factories of plants because leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. This energy is used to synthesise (prepare) food from carbon dioxide and water. Since the synthesis of food occurs in the presence of sunlight.
Besides leaves, photosynthesis also takes place in other green parts of the plant — in green stems and green branches. The desert plants have scale- or spine-like leaves to reduce loss of water by transpiration. These plants have green stems which carry out photosynthesis.

In the absence of photosynthesis there would not be any food. The survival of almost all living organisms directly or indirectly depends upon the food made by the plants. Besides, oxygen which is essential for the survival of all living organisms is produced during photosynthesis. In the absence of photosynthesis, life would be impossible on the earth.

Nitrogen is present in abundance in gaseous form in the air. However, plants cannot absorb nitrogen in this form. Soil has certain bacteria that convert gaseous nitrogen into a usable form and release it into the soil. These soluble forms are absorbed by the plants along with water.

There are some plants which do not have chlorophyll but they can survive. They cannot synthesise their food. Like humans and animals such plants depend on the food produced by other plants. They use the heterotrophic mode of nutrition. A yellow tubular structures twining around the stem and branches of many trees. This is a plant called Cuscuta (Amarbel). It does not have chlorophyll. It takes readymade food from the plant on which it is climbing. Since, it deprives the valuable nutrients, as parasite.

The mode of nutrition in which organisms take in nutrients in solution form from dead and decaying matter is called saprotrophic nutrition.

Fungi have a different mode of nutrition. They secrete digestive juices on the dead and decaying matter and convert it into a solution. Then they absorb the nutrients from it. This mode of nutrition in which organisms take in nutrients in solution form from dead and decaying matter is called saprotrophic nutrition. Plants which use saprotrophic mode of nutrition are called saprotrophs.

The fungal spores are generally present in the air. When they land on wet and warm things they germinate and grow. Fungi grow on pickles, leather, clothes and other articles that are left in hot and humid weather for long time. During the rainy season they spoil many things.

Some of the organisms live together and share shelter and nutrients. This is called symbiotic relationship. For example, certain fungi live in the roots of trees. The tree provides nutrients to the fungus and, in return, receives help from it to take up water and nutrients from the soil. This association is very important for the tree.

Some of the organisms live together and share shelter and nutrients. This is called symbiotic relationship.
In organisms called lichens, a chlorophyll-containing partner, which is an alga, and a fungus live together. The fungus provides shelter, water and minerals to the alga and, in return, the alga provides food which it prepares by photosynthesis.

Usually crops require a lot of nitrogen to make proteins. After the harvest, the soil becomes deficient in nitrogen. Nitrogen gas is available in plenty in the air, plants cannot use it in the manner they can use carbon dioxide. They need nitrogen in a soluble form. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a soluble form. But Rhizobium cannot make its own food. So, it lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen. Most of the pulses (dals) are obtained from leguminous plants. In return, the plants provide food and shelter to the bacteria.
Thus, they have a symbiotic relationship. This association is of great significance for the farmers. They do not need to add nitrogen fertiliser to the soil in which leguminous plants are grown.

The plants such as gram, peas, pulses are called ‘Leguminous plant’. This plants have root nodules in them which have a symbiotic association with bacteria such as rhizobium. This bacteria convert gaseous nitrogen of air into water soluble nitrogen compounds like nitrates. Some of these nitrogen compounds are used by the leguminous plants for their growth. Hence, farmer do not need to apply nitrogenous fertilisers in the fields during cultivation.

Parasite:
i). The organisms derive nutrition from the body of other living organisms host are parasites.
ii). Example: Cuscuta (Amar bel)

Saprotroph:
i). The plants which derive nutrition from dead and decaying organisms are called saprotrophs.
ii). Example: Mushrooms.

Carbohydrates in wheat dough encourage the growth of yeast and other saprophytic fungi. They breakdown carbohydrates into simpler compounds like CO2 and alcohol that leads to a foul smell.