Metabolic Wastes and Non-Metabolic Wastes

Types of Waste Metabolic and Non-Metabolic

Waste products can be classified as metabolic or non-metabolic. The main difference between the excretion types of waste metabolic and non metabolic is one substance is produced by the chemical processes of a living cell, and the other merely passes through the digestive tract of an organism without actually entering into its life processes.


Excretion is a natural process in which metabolic waste is eliminated from an organism. There are several kinds of solid wastes among them are types of waste metabolic and non metabolic of excretion. Vertebrates carried out the excretion by the kidney, lungs, and skin. 


Waste products can be classified as metabolic or non-metabolic. The main difference is one substance is produced by the chemical processes of a living cell, and the other merely passes through the digestive tract of an organism without actually entering into its life processes.

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Metabolic Wastes

Metabolic wastes are the substances leftover from metabolic processes (like cellular respiration) which are not absorbed by the organism and hence they should be excreted. Metabolic wastes can be categorized into gases, liquids, solids, and heat. Heat should be eliminated from our body. It is a by-product of metabolic activity to avoid elevation of body temperature — nitrogen compounds, water, CO2, phosphates, sulphates are examples of metabolic waste.

  • Gaseous Wastes

Gaseous waste includes oxygen, carbon dioxide and nitrogen gas. Oxygen is produced during photosynthetic reactions in green plants. Carbon dioxide is produced by all animals and by green plants in darkness. And, nitrogen gas is produced by denitrifying sulfur bacteria. 

  • Liquid Wastes

Water is the sole liquid waste that is produced as a metabolic by-product by all animals. 

  • Solid Wastes

There are many kinds of materials that may be classified as solid wastes. They are nitrogenous wastes, nitrite and nitrate compounds produced by nitrifying bacteria, by-products of protein and amino-acid metabolism by animals, sulfur and sulfates resulting from the metabolic activities of sulfur bacteria. There are many other substances like resins, fats and complex organic chemicals that are discharged from many plants — as in the latex from milkweeds and rubber trees. There are organic pigments like haemoglobin from the breakdown of biological pigments that become components of solid waste. Inorganic salts, like molecules and ions such as bicarbonates, carbonates and phosphates resulting from life-sustaining chemical reactions, will also be part of solid waste products.

Non- Metabolic Wastes

The materials which are undigested or unused by an organism are called non- metabolic wastes, which are produced by undergoing chemical makeup. Also, non-metabolic wastes are the substances that are ingested, absorbed, or otherwise taken into a living system in excess of the storage and needs capabilities of the organism. These substances are digestible (metabolizable) and also indigestible materials, and they may be excreted almost immediately, even though they are often used as food.


The excretion types of waste metabolic and non metabolic include active and passive mechanisms. Generally, gaseous wastes of the metabolic were eliminated through passive means without compromising direct expenditure of energy of the living system. There are two methods of disposal that may be classified into specific and nonspecific systems.


Specific Elimination Mechanisms

There are three pathways of excretion: the alimentary canal, (2) the respiratory system, and (3) the kidneys.

  • Alimentary Canal

The solid waste, which is indigestible in nature, is excreted through the alimentary canal. This act of elimination by this method is called an egestion

  • Respiratory System

The gaseous waste like carbon dioxide and ammonia is excreted through the respiratory pathway, diffuses from the cells of origin and goes to the external environment. For example, multicellular aquatic animals lose carbon dioxide through diffusion. However, it is difficult for diffusion in higher animals as their skin is too thick or hard.

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  • The Kidneys

Kidneys serve as a highly sophisticated channel for waste disposal. It has evolved very much, and they help regulate the levels of salt, water, and organic materials in the bodies of higher animals. Nitrogenous waste products (ammonia, urea, uric acid, urea, creatinine, creatine and amino acids), excess quantities of salts and water that consumed by the body, and various other organic materials produced by life-sustaining chemical reactions are excreted through the kidney. For instance, animals that absorb large quantities of water into their bodies (such as freshwater fishes) excrete large quantities of water in their urine. On the contrary, the desert animals do not have so much water and hence produce thick, semisolid urine.

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Nonspecific Mechanisms of Waste Disposal

Many other disposal mechanisms prevail throughout the animal and plant kingdoms to excrete the excess plant and animal material. From the plant's materials, leaves, the shedding and dropping of bark and twigs might represent disposal mechanisms. 


Specialized amoeba-like cells present in the blood and tissues of animals. They engulf specific wastes occurring from the intake of foreign particles into the bodies of animals or the disintegration of dead cells. Waste matter is then stored inside these small cells and are removed from contact with the organism or its metabolism. 

FAQs (Frequently Asked Questions)

1. Why do Higher Animals not Undergo Diffusion? 

Ans: Higher animals do not undergo diffusion as their skin is too hard or thick and nonvascular to function effectively in gas disposal. They have evolved form of gills and lungs having large surface areas for the diffusion of gaseous waste materials from the circulatory system to the outside environment. They developed many respiratory surfaces that will lead to an increase in the surface area for exchange. A respiratory surface is covered with moist and thin epithelial cells that allow carbon dioxide and oxygen to exchange. The respiratory surfaces must be moist as those gases can only cross cell membranes when they are in an aqueous state or dissolved in water. 

2. What is the Main Role of the Kidney in Excretion? 

Ans: The kidneys function in whole-body homeostasis. The primary function of the kidneys is to get rid of nitrogenous wastes. The kidneys are a pair of bean-shaped, reddish-brown organs that are located just above the waist. The waste products and excess fluid are excreted through the urine.