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| Summary |
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 Nutrition can be defined as the process by which an organism obtains food that is used to provide energy and materials for its life sustaining activities. |
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There are basically two modes of nutrition: |
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| - Autotrophic |
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| - Heterotrophic |
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Autotrophic Nutrition |
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| Organisms which synthesize their own food are called the autotrophs and the process is called autotrophic nutrition. They include all green plants and some bacteria such as the nitrifying bacteria. |
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Autotrophs are classified as: |
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| Photoautotrophic |
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| Organisms that synthesise, food with the help of the light energy of the sun, carbon dioxide and water by a process called photosynthesis are called photoautotrophic organisms. |
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| For example: |
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| Cyanobacteria (blue-green bacteria, prokaryotes), algae and all green plants. |
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| Organisms which synthesise food with the help of chemical energy are called chemotrophic organisms. Examples of chemosynthetic bacteria are nitrifying bacteria such as Nitrobacter and Nitrosomonas, hydrogen bacteria and iron bacteria. |
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Heterotrophic nutrition is the process of obtaining the food from other organisms. |
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Heterotrophic nutrition is broadly classified as holozoic, saprotrophic and symbiotic. |
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Holozoic nutrition involves the taking in of solid particles of food. It involves ingestion of food, its digestion, absorption and assimilation. |
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Saprotrophic nutrition is the process by which the organisms feed on dead and decaying matter. Examples of saprophytes are Rhizopus (bread mould), Mucor (pin mould), Yeast, Agaricus (mushroom), many bacteria etc. |
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| Examples of saprozoans are Mastigamoeba and Chilomonas. |
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Symbiosis is the association of two dissimilar organisms. The association may be mutually beneficial (mutualism), benefiting one organism and harming another (parasitism) and benefiting one but having no effect on the other (commensalism). |
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Parasitism is defined as an association between individuals of two different species which is beneficial to one and generally harmful to another. |
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The different types of nutrition put the different organisms under three categories: |
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| - Producers |
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| - Consumers |
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| - Decomposers |
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Photosynthesis is the most important life sustaining process of nature. |
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| It can be essentially summarised as follows: |
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In higher plants, the leaves along with the chloroplasts are considered the photosynthetic apparatus of the plant. Certain young regions may, however, contain chlorophyll. However, in the lower plants like algae, the entire plant body contains the pigments. The exchange of gases and uptake of water takes place through the general surface of the plant body. |
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Photosynthesis has two main reactions: |
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| - Light Reaction |
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| - Dark Reaction |
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Light reaction takes place only in the presence of sunlight. In this reaction, the light energy from the sun is absorbed by the photosynthetic pigments and is used to break down the water molecules. This is called the photolysis ('photo' is light and 'lysis' refers to break down) of water. |
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| This releases energy, at the same time setting free hydrogen and oxygen. The energy is temporarily stored in two types of molecules NADP (Nicotinamide Adenine Dinucleotide Phosphate) and ATP (Adenosine triphosphate). |
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Dark reaction |
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| In this stage, water and carbon dioxide are used to form carbohydrates. This is done with the help of a carbon acceptor - ribulose biophosphate (RuBP). Calvin cycle produces sugar molecules and regenerates the carbon acceptor which then enters another cycle. The hydrogen atoms set free during the light reaction are used to fix carbon dioxide. This stage is also called carbon assimilation. |
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Factors affecting photosynthesis are light intensity, carbon dioxide concentration, temperature, chlorophyll concentration, water and pollution. |
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All animals are heterotrophic. Holozoic nutrition is most common among all types / methods of heterotrophism. |
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| Holozoic nutrition involves the following steps: |
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Ingestion - Taking in of food or eating |
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Digestion - breaking down food into simpler forms. Digestion is both mechanical and chemical. The digestion may take place outside cells (extracellular) or within the cells (intracellular). |
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Absorption - Taking in of food in the soluble forms into the tissues. The absorption may be passive (without the use of energy) or active(uses energy). |
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Assimilation - Utilisation of the nutrients for the various functions. |
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Egestion - Removal of undigested or waste matter is called egestion. |
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Nutrition in amoeba is holozoic. The food is ingested at the point where it comes in touch with the cell surface with the help of pseudopodia. |
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Digestion in amoeba is intracellular and with the help of lysosomal enzymes. |
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| Since the food on digestion is converted into liquid diffusible form after digestion, it is readily absorbed by the cytoplasm. Then, it is circulated to all the parts of the cell and assimilated by building new materials. The undigested food is formed into faecal pellets and egested by exocytosis. |
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Nutrition in grasshopper is holozoic. It is a herbivore and feeds by biting off pieces of vegetation. The mouthparts of the grasshopper are of the chewing or the mandibular type. |
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| The different mouthparts are labrum or the upper lip, lingua or the hypopharynx, mandibles, maxillae and labium. |
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| The mandibles and the maxillae grind the food by moving it laterally. The labrum and labium help to hold the food between the mandibles and the maxillae. |
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Digestion takes place in the alimentary canal. |
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| The alimentary canal is divided into foregut, midgut and hindgut. Foregut is lined with chitin and consists of pharynx, oesophagus, crop and gizzard. The midgut consists entirely of stomach or ventriculus and is lined only by a peritrophic membrane. Hindgut is a coiled structure consisting of anterior ileum, middle colon and posterior rectum. The rectum opens to the exterior through the anus. The hindgut is lined with cuticle |
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| Digestion starts at the mouth with the mandibles and the maxillae chewing the food. It is also acted upon by the salivary carbohydrases which partially digests the food. The food then enters the oesophagus and then into the crop for temporary storage. The food is then passed into the gizzard which acts as the grinding chamber. |
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| The ground food then enters the stomach. The digestive enzymes secreted by the gastric caecae act upon the food in the stomach. These enzymes include amylase, maltase, invertase, tryptase and lipase. |
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| The digested food is absorbed through the stomach walls into the surrounding space which is called the hoemocoel. From here, it is transported to the different body parts. |
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| In the hindgut, absorption of water takes place and the undigested food is formed into almost dry pellets. These are excreted through the anus as faeces. |
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Human Digestive System |
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| Man is omnivorous - feeding on different kinds of food. |
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| The digestive system is an organ system. An organ system is a collection of organs which together carry out a life process. The digestive system consists of mouth cavity, pharynx, oesophagus, stomach, small intestine and large intestine. |
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Mouth cavity consists of tongue and two rows of teeth. Saliva secreted by three large salivary glands and other smaller glands gets mixed with the food by the action of the tongue and the teeth. The food is acted upon mechanically by the teeth, grinding it and chemically by the salivary enzymes. The food is formed into a bolus and swallowed into the oesophagus. It moves down the oesophagus by peristalsis. |
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From oesophagus, the food enters into the stomach where it is acted upon by the enzymes (protease and renin) of the gastric juice. It is now called the chyme. |
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From the stomach, the chyme enters into the duodenum (of small intestine). Duodenum receives the bile juice and pancreatic juice. These secretions help to further digest the carbohydrates, fats and proteins. In ileum, the food is completely broken down into the simplest of forms - proteins into amino acids and carbohydrates into monosaccharides. This digested mass is now called the chyle and it is in liquid form. |
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| The intestinal walls of ileum absorb the nutrients and also water. |
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The undigested food then passes into the large intestine where it is temporarily stored. More water is absorbed from the waste matter to form solid faeces. |
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The faeces are egested out of the anus by the movements of the colon. |
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The food that is digested is absorbed into the blood and lymphatic systems which transport the food to all the cells of the body where they are assimilated and used as required. |
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