class 7 science ch 1 notes
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Modes of Nutrition
On the basis of a different mode of nutrition, organisms are categorized into two major types, i.e.
(i) Autotrophs (auto-self, troops-nourishment) Autotrophic nutrition is the mode of nutrition in which organisms make their own food from the simple substance (e.g. CO2 and H2O) by the process of photosynthesis. Therefore, plants are called autotrophs.
(ii) Heterotrophs (heteros-other) Humans and animals do not contain chlorophyll and are dependent on plants for their food in readymade form. Those organisms which cannot prepare their own food and take food from green plants or animals are called heterotrophs and the mode of nutrition is called heterotrophic nutrition.
Photosynthesis: Food Making Process in Plants
The process by which autotrophic green plants make their own food from simple inorganic substances (carbon dioxide and water) in the presence of sunlight and green pigment or chlorophyll is known as photosynthesis.
Site of Photosynthesis
The process of photosynthesis takes place in green leaves, therefore leaves are referred to as the food factories of plants. The photosynthetic process can occur in other green parts of the plant-like stem but is not enough for meeting all the needs of the plant.
Cells
All living organisms are made from small building units of catted cells. Cells are the structural and functional units of the body of all living organisms. They can only be seen under a microscope. The cell has a thin outer boundary called the cell membrane, a distinct, centrally located spherical structure called the nucleus, and jelly-the substance surrounding the nucleus called cytoplasm.
Products of Photosynthesis
The food produced by the process of photosynthesis is mainly carbohydrates. It produces glucose as a food material which later gets converted into starch. The presence of starch in leaves indicates the occurrence of photosynthesis.
Importance of Photosynthesis
If the plants do not perform photosynthesis, there would be no food on earth. Photosynthesis is also necessary for the production of oxygen gas in the atmosphere which is necessary for the respiration of organisms. Therefore, it can be said that no life is possible in the absence of photosynthesis.
Photosynthesis in Leaves of Various Colours
In green plants, chlorophyll absorbs light energy from the sun to perform photosynthesis. Besides some green colour plants like Croton, maple, Colocasia, etc., have leaves that are red, brown, and violet in colour (variegated). These colours are present in large amounts and mask the green colour of chlorophyll in leaves. Thus, these leaves also perform photosynthesis and synthesise starch in them.
Synthesis of Plant Food other than Carbohydrates
Starch or glucose is the simplest form of carbohydrate synthesised by the plants which are composed of carbon, hydrogen and oxygen. Sometimes these simplest forms of carbohydrates are utilised to synthesise other food nutrients like fats (oils), proteins, etc. Starch or glucose is rich in seeds like wheat, rice and various parts of plants like potato tuber. Sometimes the starch or glucose is stored in the form of oil in their seeds (oilseed), e.g. sunflower seed.