Introduction

Relevance of the Theme

Excretion is an essential and distinct function of all living beings, responsible for eliminating waste from the body. In animals, this process varies to meet the metabolic needs of each species. The study of the different forms of excretion is crucial in Biology, as it allows the understanding of the adaptation and evolution of animals in relation to their respective habitats.

Understanding the Types of Excretion, specifically, allows for the conception of the various adaptive strategies adopted by animals for the maintenance of homeostasis. Animals, from the simplest to the most complex, present a magnificent diversity of excretion mechanisms, addressed in this lecture note.

Contextualization

Excretion is a crucial topic in the High School Biology curriculum. It comes into play not only in the context of the discipline but also has synchrony with other parts of the curriculum such as Physiology, Ecology, and Genetics. Its study aligns mainly with the theme 'Functioning of Living Beings,' one of the fundamental pillars of Biology.

Understanding the types of excretion fits perfectly into the sequence of contents of the discipline, after the introduction to Biochemistry and the fundamental molecules of life (carbohydrates, lipids, proteins, and nucleic acids) and before the exploration of the specific physiology of different groups of animals.

Mastering excretion as a vital process contributes to a deeper understanding of the functioning of living organisms, extending to the studies of health issues, such as drug metabolism and dialysis therapies, and even environmental conservation, by analyzing the impacts of excreted waste on ecology.

Theoretical Development

Components

• Excretion: Biological process that removes unwanted waste products from the body's homeostasis. It is a vital function common to all living beings. Excretion is an essential step in the flow of energy in ecosystems and is influenced by genetic, dietary, and environmental factors.

• Types of Excretion:

  • Ammonia: It is the predominant degradation product when amino acids are used as an energy source, mainly in aquatic animals, such as fish and amphibians. It is highly toxic and soluble, requiring a large amount of water for excretion.

  • Urea: It is the main nitrogenous compound resulting from the metabolism of amino acids in mammals, adult amphibians, and cartilaginous fish (such as sharks and rays). It has moderate toxicity and is excreted in low water concentrations.

  • Uric Acid: It is the main excretory product of birds, reptiles, insects, and crustaceans. Its excretion requires little water and produces a water-insoluble compound called uric acid, allowing for storage in solid form, saving water in the body.

• Ammonia, Urea, and Uric Acid:

  • Ammonia: With the chemical formula NH₃, it is highly toxic and water-soluble. The great advantage of using ammonia as an excretory product is the energy savings in its formation process in the body. However, to excrete ammonia, a significant amount of water is required, so it is more common in aquatic environments.

  • Urea: With the chemical formula (NH₂)₂CO, it is less toxic and less soluble than ammonia, allowing for its excretion in smaller amounts of water. Urea is formed in the liver and transported to the kidneys, where it is excreted in mammals. Urea excretion is metabolically more expensive than ammonia, but does not require the amount of water used to excrete ammonia.

  • Uric Acid: With the chemical formula C₅H₄N₄O₃, it is the least toxic and insoluble in water. This allows animals to excrete uric acid in the form of a paste without the need for large amounts of water. The disadvantage of uric acid is that its formation is energetically expensive, requiring more metabolic steps to be formed from ammonia.

Key Terms

• Amino Acids: They are the basic units that make up proteins. During metabolism, the amino part of amino acids is removed, turning into ammonia, urea, or uric acid, depending on the organism.

• Toxicity: Toxicity refers to the ability of a substance to produce harmful effects on a living being. Ammonia, for example, is highly toxic and, if accumulated in the body, can lead to death.

• Solubility: The ability of a substance to dissolve in another (the solvent). Ammonia has high solubility in water, making it easily excreted by animals living in aquatic environments.

• Homeostasis: Dynamic balance of an organism's internal environment, maintaining stable conditions for life. Excretion is essential to maintain homeostasis, removing substances that can interfere with the body's proper functioning.

Examples and Cases

• Ammonia Excretion: Fish and amphibians excrete ammonia directly into the water. This process requires dilution of ammonia in large volumes of water to reduce its toxicity.

• Urea Excretion: Mammals and adult amphibians excrete urea, which is less toxic and less soluble in water. Urea excretion is more efficient in terms of water consumption.

• Uric Acid Excretion: Birds, reptiles, and insects excrete uric acid, which is insoluble in water. These animals can conserve water, as uric acid excretion does not require large water volumes.

Detailed Summary

Relevant Points

• Excretion: Vital process that removes unwanted waste products from the homeostasis of organisms. Excretion is a continuous and essential process for maintaining life.

• Ammonia (NH₃): Predominant degradation product when amino acids are used as an energy source. It has high toxicity and solubility, requiring large amounts of water for excretion. Ammonia excretion requires less energy expenditure but more water consumption.

• Urea ((NH₂)₂CO): Main nitrogenous compound resulting from the metabolism of amino acids in mammals, adult amphibians, and cartilaginous fish. It is less toxic and less soluble in water than ammonia, allowing for its excretion in smaller amounts of water. It has an intermediate metabolic cost.

• Uric Acid (C₅H₄N₄O₃): Main excretory product in birds, reptiles, insects, and crustaceans. It is the least toxic and insoluble in water, allowing for its storage and excretion without the need for large amounts of water. However, its formation is energetically more expensive.

• Amino Acids: The building blocks of proteins that, during metabolism, release the residues that become ammonia, urea, or uric acid.

Conclusions

• Excretion is a vital function in all animals, ensuring the elimination of toxic waste from metabolism.

• The choice between ammonia, urea, and uric acid as the main excretory products reflects adaptations to different habitats and strategies for water and energy conservation.

• Proper management of excretions is crucial for the survival of organisms, and the ability to efficiently excrete toxic substances is a fundamental evolutionary characteristic.

Exercises

1. What is excretion? Why is this process essential in all animals?

2. Compare the three types of excretion studied (ammonia, urea, and uric acid) in terms of toxicity, solubility, and water demand for excretion. In which types of animals is each one more common?

3. Explain the relationship between amino acids and excretory products, highlighting how the choice of each product is determined by amino acids.