When we walk or ride a bicycle, we are using up energy. Even when we are not doing any apparent activity, energy is needed to maintain a state of order in our body. We also need materials from outside in order to grow, develop, synthesise protein and other substances needed in the body. This source of energy and materials is the food we eat.

How do living things get their food?

• The general requirement for energy and materials is common in all organisms, but it is fulfilled in different ways. Some organisms use simple food material obtained from inorganic sources in the form of carbon dioxide and water. These organisms, the autotrophs, include green plants and some bacteria.
• Other organisms utilise complex substances. These complex substances have to be broken down into simpler ones before they can be used for the upkeep and growth of the body. To achieve this, organisms use bio-catalysts called enzymes. Thus, the heterotrophs survival depends directly or indirectly on autotrophs. Heterotrophic organisms include animals and fungi.

1 Autotrophic Nutrition

• Carbon and energy requirements of the autotrophic organism are fulfilled by photosynthesis. It is the process by which autotrophs take in substances from the outside and convert them into stored forms of energy.
• This material is taken in the form of carbon dioxide and water which is converted into carbohydrates in the presence of sunlight and chlorophyll.
• Carbohydrates are utilised for providing energy to the plant. We will study how this takes place in the next section.
• The carbohydrates which are not used immediately are stored in the form of starch, which serves as the internal energy reserve to be used as and when required by the plant.
• A somewhat similar situation is seen in us where some of the energy derived from the food we eat is stored in our body in the form of glycogen.
•                Chlorophyll

6CO2 +6H2O→C6H12O6 +6O2

                        Sunlight (Glucose)

• The following events occur during photosynthesis process

(i) Absorption of light energy by chlorophyll.

(ii) Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen.

(iii) Reduction of carbon dioxide to carbohydrates.

• These steps need not take place one after the other immediately. For example, desert plants take up carbon dioxide at night and prepare an intermediate which is acted upon by the energy absorbed by the chlorophyll during the day.
• If you carefully observe a cross-section of a leaf under the microscope , you will notice that some cells contain green dots. These green dots are cell organelles called chloroplasts which contain chlorophyll.
• Now, let us study how the plant obtains carbon dioxide. Stomata, which are tiny pores present on the surface of the leaves. Massive amounts of gaseous exchange takes place in the leaves through these pores for the purpose of photosynthesis.
• But it is important to note here that exchange of gases occurs across the surface of stems, roots and leaves as well.
• Since large amounts of water can also be lost through these stomata, the plant closes these pores when it does not need carbon dioxide for photosynthesis. The opening and closing of the pore is a function of the guard cells.
• The guard cells swell when water flows into them, causing the stomatal pore to open. Similarly the pore closes if the guard cells shrink.
• Water used in photosynthesis is taken up from the soil by the roots in terrestrial plants.
• Other materials like nitrogen, phosphorus, iron and magnesium are taken up from the soil.
• Nitrogen is an essential element used in the synthesis of proteins and other compounds. This is taken up in the form of inorganic nitrates or nitrites. Or it is taken up as organic compounds which have been prepared by bacteria from atmospheric nitrogen.

2 Heterotrophic Nutrition

• Each organism is adapted to its environment. The form of nutrition differs depending on the type and availability of food material as well as how it is obtained by the organism.
• For example, whether the food source is stationary (such as grass) or mobile (such as a deer), would allow for differences in how the food is accessed and what is the nutritive apparatus used by a cow and a lion.
• There is a range of strategies by which the food is taken in and used by the organism. Some organisms break-down the food material outside the body and then absorb it. Examples are fungi like bread moulds, yeast and mushrooms.
• Others take in whole material and break it down inside their bodies. What can be taken in and broken down depends on the body design and functioning.
• Some other organisms derive nutrition from plants or animals without killing them. This parasitic nutritive strategy is used by a wide variety of organisms like cuscuta (amar-bel), orchids, ticks, lice, leeches and tape-worms.

How do Organisms obtain their Nutrition?

• Since the food and the way it is obtained differ, the digestive system is different in various organisms.
• In single-celled organisms, the food may be taken in by the entire surface.
• But as the complexity of the organism increases, different parts become specialised to perform different functions. For example, Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole .
• Inside the food- vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out.
• In Paramoecium, which is also a unicellular organism, the cell has a definite shape and food is taken in at a specific spot. Food is moved to this spot by the movement of cilia which cover the entire surface of the cell.