What is Nutrition?

It is a biological process of providing or obtaining food that is necessary for growth and health. It includes many physiological processes - absorption, ingestion, assimilation, catabolism, excretion, etc. In biology, the science that deals with nutrition's physiological and chemical processes are called "nutritional science." 

Different Modes of Nutrition

Generally, there are two types of mode of nutrition presents among the living creatures - Heterotrophic mode & Autotrophic mode

Autotrophic nutrition

In autotrophic nutrition, organisms use simple inorganic substances like water, carbon dioxide, etc., in the presence of air & sunlight, and chlorophyll to make their food in stomata by photosynthesis. Such organisms are called autotrophs (Example: Plants, Algae, Cyanobacteria & other unicellular organisms).

 Photosynthesis is the main mode of autotrophic nutrition. It is used to convert sunlight energy into food, such as glucose, stored in the form of starch in plant parts. The process of photosynthesis generally has three steps.

i. Absorption:  In photosynthesis, the sunlight is absorbed by chlorophyll present in the leaves.

ii. Conversion: The absorbed sunlight energy changes into chemical energy, and water absorbed by the roots will break down into hydrogen and oxygen molecules.

iii. Reduction: Carbon molecules of carbon dioxide get reduced; that is; hydrogen molecules bind with carbon molecules to form carbohydrates (sugar molecules).

Heterotrophic nutrition

In this mode of nutrition, two organisms depend on each other or autotrophs for their nutrition or food. They are not able to make their foods like Autotrophic organisms. All eukaryotes (except green plants) are heterotrophs; they obtain food from other organisms. Example- Fungus.

Generally, Heterotrophic nutrition has two steps.

  1. Digestion: Digestion is the conversion of solid food into soluble compounds which are easy to absorb.
  2. Respiration: The soluble products break down and form energy.

There are mainly four types of heterotrophic nutrition.

i. Holozoic nutrition: It is a mode of heterotrophic nutrition followed by the ingestion & internal gaseous liquids or solid food particles synthesize. Example: Protozoa, like amoeba and most of the free-living animals like humans. There are many stages of holozoic nutrition, which take place in separate organelles within an organism.

  1. Ingestion: The process of taking food into the body through the mouth. In protozoa, this process is most commonly done by phagocytosis.
  2. Digestion:  Digestion is the physiological and enzymatic breakdown by different genes of large complex and organic food particles into small, simpler particles.
  3. Absorption: Transportation may be active or passive of the bio-chemical product outside of the food-containing organelles and into the cytoplasm.
  4. Assimilation: Use of the absorbed particles for further metabolic processes.
  5. Egestion: The undigested material is eliminated from the body by a process called defecating.

ii. Saprotrophic nutrition or lyotropic nutrition: When the heterotroph takes their nutrition from dead or decaying organic matter, like dead plants or animals, the nutrition is called Saprotrophic nutrition. The organisms are generally decomposers and are also called saprophytes, saprotrophs, etc. Example: Fungus, bread mound, a few protists, and bacteria. For growth, saprophytic organisms need some optimal sources, such as;

  1. Presence of water: Fungus needs excess water for absorption.
  2. Presence of oxygen: Very few organisms can endure anaerobic conditions.
  3. Neutral-acidic pH: Acidic conditions are required.
  4. Low-medium temperature: Generally, it requires between 1°C--35°C.

The saprophytic/Saprotrophic organisms take nutrients generally provide proteins, vitamins, carbons, and ions. Example: Fungus can absorb carbohydrates like sucrose.

iii. Parasitism: When one organism takes all the benefits from the expenses of other organisms, the first type of organisms are called parasites to others & the process is called parasitism. For example, Cuscuta, Cassytha, some fungus, like - Fusarium, Pythium. Generally, there are three types of parasitic mode of nutrition:

  1. Ectoparasites: They live on the outer body surface of the host but not the cause of any disease. Example: -ticks, fleas, leeches, and lice.
  2. Endoparasites: They may be either intercellular (inhabiting spaces in the host's body) or intracellular (inhabiting cells in the host's body) and cause their disease. For example, bacteria (singular: bacterium) or viruses.
  3. Brood parasitism: In this mode, the one may not directly affect the host, but for their nature or behavioral changes, the host body is infected by their own. Example- We all know the common example of crow and cuckoo. The crows do not build their own nest; they deposit their eggs to cuckoo's nest. The mother cuckoo does not recognize her species and reduces one or more eggs to reduce the suspicion. The crow bird does not do anything, but the cuckoo hurt their species for the crow's behavior.

iv. Symbiosis: It is a long-term and close biological interaction between two different biological organisms called symbiotic association, for example, some fungus-like lichens & mycorrhiza. It can be communalistic, parasitic, or mutualistic. Symbiosis can be obligatory; that is, one or more species depends on each other for existence, or facultative (optional), that is, the species can generally live independently. Physical attachment can be classified into two types. When symbionts form in a single body, called conjunctive symbiosis & all other types are called disjunctive symbiosis.

 a. Ectosymbiosis

In the Symbiotic association, the symbionts live in the internal organs such as the digestive tube, ducts, glands, or outer surface of the host. Examples of this include ectoparasites such as lice.

A. Competition:

Deals with the fitness of the species, where one species is more fit than another. Limitation of at least one resource like food, water, and territory, or reproduction, partners used by both usually reasons for this type of interaction.

B. Mutualism or interspecies reciprocal altruism

When the relationship between two different individual species is beneficial for both and stays longer in terms is called mutualism. It may be obligate for both species or obligate for one but must be facultative for both.

b. Endosymbiosis

It is a relationship between two species where one lives inside another and behaves like a single species. It is believed that by this process, organelles such as mitochondria and chloroplasts arise within eukaryotic cells. Example-Rhizobium bacteria stay in the roots of leguminous plants for nitrogen fixation.

A. Commensalism:

It is an association between two species in which one is benefited, and the other one is unaffected or the same. Examples, baitfish and Manta Rays.

 B. Mimicry:

When a species adopts the identical characteristics of another species to alter its relationship strategies with the mimicked species for their advantages, they are called mimicry.

Example: Coral snakes and kingsnakes have similar looks. Kingsnakes are harmless, but coral snakes are venomous. So, king snakes are easy to catch for predators, but coral snakes can protect themselves. Thus, king snakes mimic coral snakes to protect themselves from the predator.

C. Amensalism

When the two different species associate with each other, where one species is fully destroyed or inhibited, the other is the same or unaffected.

An example of Amensalism is penicillin, killing bacteria.

D. Symbiogenesis

In Symbiogenesis, there is a formation of a new organism through the merging of two or more free-living organisms. Some biologists believe that Symbiogenesis is an important mechanism of evolutionary change.

Insectivorous Plants

Plants that trap insects by the modification of their leaves, in biology they are called insectivorous plants. They engulf the insect and digest it by their digestive enzymes for their nutrition.

Which are the important nutrients for plants?

They need two types of nutrients- macronutrients and micronutrients.

Macronutrients are mainstream molecules. Example- nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

The micronutrients are secondary nutrient sources. Example- Boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

How do plants absorb nutrients?

Plants absorb nutrients by their roots or root hairs and transport nutrients and water up through the xylem of the stem to the leaves for photosynthesis.

Context and Application

This topic significant in the professional exams as well as graduate examination, especially for,

  • Bachelor of Science in Food & Nutritional science
  • Bachelor of Science in Life Science
  • Bachelor of Science in Dietetics & Applied Nutrition
  • Master of Science in Food & Nutritional science
  • Master of Science in Dietetics & Applied Nutrition

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