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Nutrition In Plants And Animals



Nutrition is the act of providing nutrients to the body cells so that they can carry out activities required to keep them alive. Cells obtain nutrients from the food taken by the organism.

There are two main modes of nutrition as follows:

  • Autotropic nutrition
  • Heterotropic nutrition
  1. Autotrophic nutrition (auto = self, troph = nutrition)The process by which an organism makes its own food from simple raw materials is called autotrophism.
  • Photosynthetic nutrition is a process where green plants synthesise food using raw materials like water, carbon dioxide and mineral salts in the presence of sunlight. Since autotrophic plants are able to produce food, they are also known as producers.
  • Chemosynthetic nutrition is a process by which some non-green bacteria such as sulphur bacteria use chemical energy to manufacture their food. This energy is derived from chemical reactions brought out by bacteria. Chemosynthetic bacteria do not require light as the source of energy.
  1. Heterotrophic nutrition (hetero = different, troph = nutrition)
    Organisms that are not capable of preparing their own food but obtain their food from other sources are called heterotrophs. This mode of nutrition is called heterotrophic nutrition. Most of the animals and non-green plants are heterotrophic. As heterotrophs depend on other organisms for their food, they are called consumers. Consumers that consume herbs and other plants are called herbivores and those that consume animals are called carnivores. Omnivores eat both animal and plant material as food. A crow, for example, eats all kinds of food.
    Depending upon the mode of living and the mode of intake of food, organisms may be saprophytic, parasitic, insectivores, epiphytic and symbiotic.
  • Saprophytic (sapro = rotten, phyto = plants)
    Organisms that obtain food from dead and decaying organic matter are called saprophytes. Examples include fungi (moulds, mushrooms, yeasts) and many bacteria. Saprophytic organisms secrete enzymes that are released on the substrate, which digest and breakdown complex food materials into simpler ones. The soluble end-product is then absorbed back by the saprophyte.
  • Parasitic (para = feeding, sites = gains)
    Organisms that live on or inside other living organisms, called hosts, and obtain their food from them are called parasites.
    The mode of feeding depends upon the habit, habitat and modifications of the parasite.
    An ectoparasite lives external to the host. It may have certain devices, with the help of which it obtains food from the host, e.g. mosquito.
  • An endoparasite lives inside the body of a host and obtains nutrition from it, e.g. tapeworm.
    Semiparasite depends for its nutrition on its host partially. When partially dependent, it retains its leaves with which it prepares its own carbohydrates, e.g. Viscum and Loranthus. These semiparasites possess thick green leaves and remain firmly perched on branches of host by means of haustorial roots. The haustoria gets connected with the xylem stream of the host plant.
    Some plants are total parasites. They lack chlorophyll and depend totally on host plant for food and water. They absorb prepared food and water through haustoria, e.g. Cuscuta.
  • Holozoic (holo = whole, zoon = animal)
    Ingestion of complex organic substances without being degraded or decomposed is holozoic nutrition, e.g. amoeba.
    Insectivorous: Certain green plants growing in marshy places, capable of preparing their own food, capture and digest insects to obtain nitrogenous substances, e.g. Nepenthes (pitcher plant), Drosera (sun dew), Utricularia (bladderwort).
  • Epiphytes: Plants which grow on trees for support are called epiphytes. They have green leaves and can manufacture their own food by absorbing moisture from atmosphere through special aerial roots, e.g. orchids.
  • Symbiotic: It is a mode of nutrition in which two types of organisms live together and are mutually benefited, e.g. lichens.
  • Predators are those carnivores which chase and kill their prey to eat it, e.g. tiger.
  • Scavengers are those carnivores which consume dead and rotting meat, e.g. vultures and jackals.

Nutrition in Plants


Photosynthesis Green plants, which are autotrophic, synthesise food through the process of photosynthesis, using simple raw materials such as water, carbon dioxide and mineral salts in the presence of sunlight.

The overall equation of photosynthesis is as follows:

6CO2 + 12H2O  C6H12O6 + 6H2O + 6O2

Site and Process of Photosynthesis
The leaves are the most suitable organs for the process of photosynthesis. Anatomically, leaf has the outermost epidermal layers on both the surfaces that are covered by waxy cuticle.

In the internal layers, palisade and spongy parenchyma are found which contain green plastids, known as chloroplasts(see Figure 6.1). Palisade and spongy parenchyma together form mesophyll tissue. Palisade cells are elongated with more chlorophyll while spongy parenchyma contains almost spherical cells with fewer chloroplasts.


Palisade and Spongy Parenchyma


Requirements for Photosynthesis Photosynthesis requires chlorophyll pigments, carbon dioxide, water and sunlight.

 It is a green pigment in plants which is found in cell organelles called chloroplasts. It absorbs light of all wavelengths except green, which they reflect to be detected by our eyes. There are five types of chlorophylls, namely chlorophyll a, b, c, d and e. In plants, chlorophyll is mainly found in leaves. In lower plants such as algae, the whole plant is green and takes part in photosynthesis. Chlorophyll helps to trap light energy and convert it to chemical energy.

Carbon dioxide:
 Plants get carbon dioxide from their surroundings. Terrestrial plants use atmospheric CO2 in photosynthesis, while aquatic plants get it from water in the form of carbonate ions.

 It is an important raw material for photosynthesis. Plants absorb water from the soil through root hairs. Water is then translocated up to the leaves through the stem. Xylem tissues present in roots and stems are responsible for translocation of water. Aquatic plants absorb water and minerals from their general surface. Minerals absorbed along with water contribute not only to the process of photosynthesis but also in the development of the plant.

 When a photon, a particle of light, is absorbed by a molecule of chlorophyll, the energy level of the electrons in the molecule is increased and it reaches the excited state. This energy is used in splitting water molecules. The splitting of water in the presence of light is called photolysis.

Occurrence of Photosynthesis 
The structural unit of a chloroplast is the thylakoid. It is a flattened sac of membrane which contains molecules responsible for photosynthesis. Stacks of thylakoids are called grana. The space between the grana is called stroma. Chloroplast is a three-membrane-bound organelle, which contains grana along with their stroma (Figure 6.2).

Ultrastucture of Chloroplast


Stages of Photosynthesis There are two main stages in photosynthesis. The first stage is dependent on light (light reactions) and the other stage is independent of light (dark reactions).

  • Light reaction occurs in the grana of the chloroplast. During the light reaction, light strikes chlorophyll, and photons are absorbed so as to excite the electrons of the chlorophyll to a higher energy level. These activated electrons are harnessed to form ATP from ADP (the process of formation of ATP during photosynthesis is known asphotophosphorylation) and reduce NADP (nicotine adenine dinucleotide phosphate) to NADPH. Water is split in this process (called photolysis) into hydrogen and oxygen ions. Finally, oxygen is released. (Oxygen is released as a photochemical by-product of photosynthesis. Both plants and animals take in oxygen and give out CO2during respiration. Thus, CO2 and O2 balance in the atmosphere is maintained by the process of photosynthesis.)
  • Dark reactions are series of cyclic reactions, otherwise called Calvin cycle, that occur in the stroma of chloroplasts. In dark reaction, the NADPH molecules and ATP which were produced during the light reaction are utilised for the synthesis of carbohydrates from carbon dioxide. Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate. The resulting 6-carbon compound is unstable and hence breaks into two molecules of a 3-carbon compound called phosphoglyceric acid. This compound is finally converted into a 6-carbon compound called glucose. This process is called carbon fixation.

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