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What Is Excretion In Biology

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Difference Between Secretion And Excretion

Excretion-Introduction-Leaving Cert Biology
  • Excretion releases waste materials that cannot be reused, whereas secretion releases undesirable materials that can be reused.
  • The process of excretion is passive, whereas secretion is active.
  • Carbon dioxide, perspiration, tears, faeces, and urine are excreted by the body, whereas hormones, enzymes, and saliva are secreted by the body.
  • The lungs, skin, tear ducts, and rectum is engaged in excretion, whereas the Endocrine glands, digestive glands, salivary glands, liver, pancreas, and gallbladder are involved in secretion.
  • Tears, perspiration, and urine are materials released during the excretion process, whereas saliva and hormones are materials released during the secretion process.


It is the process of removing materials from a living organism.

It’s the transportation of material from one region to another.

In nature, excretion is a passive process.

In nature, secretion is active.

Tears, urine, carbon dioxide, and sweat are all excreted by humans.

Enzymes, saliva, and hormones are all examples of human secretions.

Body waste makes up the majority of excretion.

Secretion is a vital substance that our bodies can metabolize.

Plants excrete through their roots into their surroundings, as well as through the leaves and bark of their leaves and bark.

Latex, resin, gums, and other secretions are found in the plant body.

Nitrogenous Waste In Birds And Reptiles: Uric Acid

Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related compound guanine instead of urea. Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder it is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea.

Nitrogenous waste is excreted in different forms by different species. These include ammonia, urea, and uric acid.

The video below describes the origins of nitrogenous wastes, the reason they are problematic, and provides an overview of some of the different mechanisms of removal of nitrogenous wastes in different lineages of organisms:

The information below was adapted from OpenStax Biology 41.3

Microorganisms and invertebrate animals use more primitive and simple mechanisms to get rid of their metabolic wastes than the mammalian system of kidney and urinary function. Three excretory systems evolved in organisms before complex kidneys: vacuoles, flame cells, and Malpighian tubules.

Difference Between Excretion In Plants And Animals

Excretion in Plants
Excretion takes place but not with specific organs.Excretion takes place with a specific organ like skin, lungs, kidneys.
Water, carbon dioxide, latex, etc., are the excretory materials excreted by plants.Ammonia, urea, uric acid, and other nitrogenous wastes are excretory materials excreted by animals.
The excretory products of plants are used in rubber industries, in producing medicines, etc.The excretory products from animals such as cow dung, chicken dung, etc., are used as manure for plants.

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The Consequences Of A System That Does Not Excrete

If the waste disposal system of an organism were completely blocked, its consequences on the body would be as catastrophic as stopping the bodys food, oxygen, and water supply. Moreover, the metabolic process also releases toxins that are so harmful that they must be released from the body as soon as they are created by the cells. Hence the excretory functions of a body need to work continuously so that other chemical processes taking place inside an organism can work too. An overview of the eliminatory mechanism of the human body The waste materials released by the body can be categorized into two types:

  • 1. Metabolic

Metabolic waste is generated because of the chemical processes of a cell.

Nitrogenous Waste In Terrestrial Animals: The Urea Cycle

Excretory System

The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is 2 NH3 + CO2 + 3 ATP + H2O â H2N-CO-NH2 + 2 ADP + 4 Pi + AMP.

The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea. The amino acid L-ornithine gets converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.

The urea cycle converts ammonia to urea.

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How Is Excretion In Plants Useful To Human Being

Excretion in the plants leads in the production of different kinds of excreta which may be waste for the plant but are sometimes equally important for human use. … -Oxygen which is an excretory product of the process of photosynthesis which is an absolute requirement for human beings in order to survive on earth.

Difference Between Secretion And Excretion: Definition Sample Questions

Senior Content Specialist| Updated On -Apr 5, 2022

The difference between Secretion and Excretion is that excretion is the process of eliminating or ejecting anything that is no longer useful, whereas secretion is any substance released by an organism or secretion can be the act of concealing something. The active process of secretion is different from the passive process of excretion.

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Excretion Of Nitrogenous Wastes In Animals


1. Ammonotelism: The elimination of ammonia as the chief nitrogenous waste material is termed ammonotelism. Ammonia is formed by the oxidative deamination of amino acids. Ammonia is extremely poisonous and readily dissoluble in water. Hence it should be removed from the body quickly and in the form of a very dilute solution. In many invertebrates, ammonia is excreted through the surface of the body. In fishes, most of the ammonia is lost as NH+ across the epithelium of the gills, and kidneys excrete only minor amounts of nitrogenous wastes.

Examples of Ammonotelic Animals:Sycon, Hydra, Fasciola, Taenia, Nereis, Hirundinaria, Ascaris, Pila, Labeo, prawn, crocodiles, etc.

Fig: Hydra

2. Ureotelism: The elimination of urea as the chief nitrogenous waste material is termed ureotelism. Urea is less toxic than ammonia. Although ammonia excretion works fine in aquatic situations, it is unsuitable for terrestrial situations. As terrestrial animals cannot afford to spend more water for excretion, they excrete less toxic nitrogenous wastes like urea and uric acid. Ammonia produced by metabolism is converted into urea by the ornithine cycle in the liver of these animals and released into the blood, which is filtered and excreted out by the kidneys.

Examples of Ureotelic Animals: Since water intake is limited, many terrestrial amphibians, earthworms, mammals, and marine fishes mainly excrete urea and are called ureotelic animals.

Fig: Earthworm

Fig: Lizard

What Is Excretion In Biology

Excretion | Biology

Asked by: Dr. Dejuan Romaguera

Excretion, the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. … Excretion is a general term referring to the separation and throwing off of waste materials or toxic substances from the cells and tissues of a plant or animal.

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Excretion In Unicellular Organisms

In unicellular organisms, excretion is quite simple. Most unicellular organisms feed through the process of phagocytosis, during which the cell membrane is folded inward to create a food vesicle. This membrane-bound package is taken into the cytoplasm of the cell, where digestive enzymes are introduced. Once the food is digested, all that remains in the vesicle are the waste products left over. Most cells then use the process of exocytosis to remove this waste. Exocytosis is simply the reverse of phagocytosis, in that the food vesicle fuses with the cell membrane and the contents are dumped on the outside of the cell.

Many freshwater protists also have a contractile vacuole. This internal organelle collects the water flowing into the cell and forces it out by squeezing the vacuole. Freshwater organisms, typically unlike marine organisms, are hypertonic to their environment. While they must maintain a high level of dissolved substances compared to the water around them, this also cause the water to flow into their cells. Excreting this water ensures they maintain homeostasis.

Excretion Of Nitrogenous Waste

The theory of evolution proposes that life started in an aquatic environment. It is not surprising to see that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. Arid conditions probably led to the evolution of the uric acid pathway as a means of conserving water.

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Excretion In Human Beings

  • The body parts that the excretory system includes are

  • A pair of kidneys

  • A urinary bladder

  • A urethra

  • Kidneys are located in the abdomen , one on either side of the backbone.

    • Urine that produced in the kidneys passes through the ureters into the urinary bladder where it gets stored until released through the urethra.

    • On the other hand, plants have completely different process for excretion than those of animals.

    • Oxygen itself can be considered as a waste product generated during photosynthesis.

    • Many plant waste products are stored in leaves that fall off.

    • Some other waste products, in plants, are stored as resins and gums, especially in old xylem.

    Excretion In Animals Humans And Plants

    IGCSE Biology

    Excretion in Animals, Humans and Plants!

    Chemical reactions occur in the cells of living organisms all the time to carry out the life processes.

    The sum of these reactions is called metabolism. Metabolism produces useful products as well as toxic by-products.

    These toxic substances have to be removed as they are harmful if allowed to accumulate. The removal of metabolic waste products from the body of an organism is known as excretion.

    The major excretory products are carbon dioxide, excess water, and nitrogenous compounds like ammonia, urea, uric acid, etc. Carbon dioxide and water are produced in the process of tissue respiration. Nitrogenous compounds are formed from the breakdown of proteins and amino acids. Water and salts in excess of the bodys needs are also excreted.

    We acquire most of the water with our food and drink and some by metabolism, e.g., the water produced during cellular respiration. Other excretory products include chemicals from medicines, toxic substances, and circulating hormones that have already served their purpose. We will learn how metabolic wastes get eliminated.

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    Frequently Asked Questions On Excretion In Animals

    Frequently asked questions related to excretion in animals is listed as follows:

    Q.1. Why is excretion important for animals and plants?Ans: Excretion is very important for both animals and plants because of the removal of waste from the body by absorbing necessary nutrients from the food consumed to keep the body clean and healthy. Plants need to excrete oxygen and carbon dioxide, whereas animals need to excrete urea, uric acid, ammonia, etc., which are highly toxic if retains in the body.

    Q.2. What is excretion with examples?Ans: The process of expelling the amount of waste matter from our body is called excretion. Urine is an example of excretion.

    Q.3. What are the waste products of animals?Ans: Urea, uric acid, amino acid, ammonia, etc., are the waste products of animals.

    Q.4. What are the organs of excretion?Ans: Excretion takes place through various organs in different animals. Skin, lungs, kidneys, etc., are the organs of excretion.

    Q.5. Is Dung an animal?Ans: Dung is not an animal. It is a waste matter excreted by cows, chickens, etc.

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    Definition And Mechanism Of Excretion

    Excretion is defined as the process of discharging waste matter from an organism. Every organism has metabolic waste generated in its body. The process of elimination of such waste from the organism is called excretion. In humans and animals, this is generally carried out by the skin, kidneys, and lungs. This is in complete contrast to the function of secretion. After leaving the cell, the substance could still have specific functions.

    In mammals, urine is excreted through the urethra and this is part of the excretory system. In single-celled organisms, waste products get discharged through the cell surface.

    In the process of various activities of life like respiration, there are many chemical reactions that take place in the body. This process is called metabolism. This process produces waste products like carbon dioxide, water, salts, etc: When these wastes exceed a certain level, they become harmful to the organism and need to be expelled. The excretory organs remove these wastes.

    In the case of green plants, carbon dioxide and water are the products of respiration. The carbon dioxide thus released gets used during photosynthesis. Oxygen is released during photosynthesis, and thus exits through the stomata or the root cell’s walls. Plants use the process of transpiration and guttation to excrete excess water waste.

    Sweat glands in the skin produce sweat. The body is kept cool during hot conditions by the water in the sweat. The sweat contains salts and urea.

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    How Does Excretion Occur In Humans

    In human beings, excretion is performed by the excretory system consisting of a pair of kidneys, a pair of ureters, a urinary bladder and urethra. The urine is formed in kidneys and passes on to the urinary bladder via the ureters. The urinary bladder stores the urine until it is excreted through the urethra.

    Regulation Of Phosphate Homeostasisan Integrated View

    Introduction : Excretion | Biology | Science | Class 10

    Intestinal feed-forward and hormonal feedback systems are likely to be responsible for the control of phosphorus homeostasis . The short-term responses that occur within minutes to hours of feeding of a high-Pi meal are important in regulating phosphorus homeostasis via feed-forward mechanisms, whereas the longer-term changes occur as a result of alterations in circulating concentrations of PTH, 1,252D3, and the phosphatonins such as fibroblast growth factor 23.86,420423 Intestinal signals have been shown in rodents to rapidly alter renal Pi excretion in response to changes in duodenal Pi concentrations.421

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    Malpighian Tubules Of Insects

    Malpighian tubules are found lining the gut of some species of arthropods, such as the bee illustrated in Figure 22.11. They are usually found in pairs and the number of tubules varies with the species of insect. Malpighian tubules are convoluted, which increases their surface area, and they are lined with microvilli for reabsorption and maintenance of osmotic balance. Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered as in the case of nephridia urine is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph . Metabolic wastes like uric acid freely diffuse into the tubules. There are exchange pumps lining the tubules, which actively transport H+ ions into the cell and K+ or Na+ ions out water passively follows to form urine. The secretion of ions alters the osmotic pressure which draws water, electrolytes, and nitrogenous waste into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments, and uric acid is excreted as a thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water this is especially important for life in dry environments.

    Contractile Vacuoles In Microorganisms

    The most fundamental feature of life is the presence of a cell. In other words, a cell is the simplest functional unit of a life. Bacteria are unicellular, prokaryotic organisms that have some of the least complex life processes in place however, prokaryotes such as bacteria do not contain membrane-bound vacuoles. The cells of microorganisms like bacteria, protozoa, and fungi are bound by cell membranes and use them to interact with the environment. Some cells, including some leucocytes in humans, are able to engulf food by endocytosisthe formation of vesicles by involution of the cell membrane within the cells. The same vesicles are able to interact and exchange metabolites with the intracellular environment. In some unicellular eukaryotic organisms such as the amoeba, shown in Figure 22.9, cellular wastes and excess water are excreted by exocytosis, when the contractile vacuoles merge with the cell membrane and expel wastes into the environment. Contractile vacuoles should not be confused with vacuoles, which store food or water.

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    Flame Cells Of Planaria And Nephridia Of Worms

    As multi-cellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Planaria are flatworms that live in fresh water. Their excretory system consists of two tubules connected to a highly branched duct system. The cells in the tubules are called flame cells because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope, as illustrated in Figure 22.10a. The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface cilia also draw water from the interstitial fluid, allowing for filtration. Any valuable metabolites are recovered by reabsorption. Flame cells are found in flatworms, including parasitic tapeworms and free-living planaria. They also maintain the organisms osmotic balance.

    Earthworms have slightly more evolved excretory structures called nephridia, illustrated in Figure 22.10b. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore. They are more evolved than the flame cells in that they have a system for tubular reabsorption by a capillary network before excretion.

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