Test: Class 7 General Science NCERT Based - 1 - UPSC MCQ

There is a symbiotic relationship between algae and fungus. For example, 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 prepares and provides food to the fungus. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form.

But Rhizobium cannot make its own food. So, it often lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen. In return, the plants provide food and shelter to the bacteria. They, thus, have a symbiotic relationship. (i.e., between plants and bacteria).

Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis. For example, certain fungi live inside the roots of plants.

The plants provide nutrients to the fungus and, in return, the fungus provides water and certain nutrients.

For example: lichens. The mode of nutrition in which organisms take in nutrients from dead and decaying matter is called saprotrophic nutrition. Such organisms with saprotrophic mode of nutrition are called saprotrophs.

For example fungi, that grow on pickles, leather, clothes and other articles that had been left out in hot and humid weather for a long time, is a saprotroph. Note that during the rainy season such fungal growth may spoil a lot of household things.

There are some plants which do not have chlorophyll. They cannot synthesise food. Like humans and animals such plants depend on the food produced by other plants. They use the heterotrophic mode of nutrition.

For example: Cuscuta (Amarbel). Plants cannot absorb nitrogen in gaseous form. Soil has certain bacteria that convert gaseous nitrogen into a usable form and release it into the soil. This is absorbed by the plants along with water.

Both carbohydrate and oxygen are formed in the process of photosynthesis.Besides leaves, photosynthesis also takes place in other green parts of the plant - in green stems and green branches.

Desert plants have scales or spine-like leaves to reduce loss of water by transpiration. These plants have green stems which carry out photosynthesis.

Leaves are the food factories of plants. Water and minerals present in the soil are absorbed by the roots and transported to the leaves.

Therefore, all the raw materials must reach the leaf. A plant needs sunlight, carbon dioxide, minerals and water to make food by photosynthesis. A green substance in plants called chlorophyll traps the energy from the sun needed to make food. Chlorophyll is mostly found in leaves, inside plastids.

Which are inside the leaf cells. The leaf can be thought of as a food factory. Leaves of plants vary in shape and size, but they are always the plant organ best suited to capture solar energy. Once the food is made in the leaf, it is transported to the other parts of the plant such as stems and roots.

The rearing of silkworms for obtaining silk is called sericulture. The female silk moth lays hundreds of eggs, from which hatch larvae which are called caterpillars or silkworms.

They grow in size and when the caterpillar is ready to enter the next stage of its life history called pupa, it first weaves a net to hold itself.

Then it swings its head from side to side in the form of the figure of eight (8). During these movements of the head, the caterpillar secretes fibre made of a protein which hardens on exposure to air and becomes silk fibre.