Common Characteristics of Life: Cellular Respiration and Excretion

• Cellular respiration refers to the process in which organic fuel molecules (carbohydrates, lipids, proteins) are broken down within the cell, releasing chemical bond energy in the form of ATP.
• Cellular respiration does not produce the energy that keeps us alive; it releases the energy stored in food during photosynthesis.
• The purpose of cellular respiration is to synthesize (or generate) ATP.

The processes of ATP production (phosphorylation) and consumption (dephosphorylation) are common characteristics among living organisms.

• Cellular respiration and fermentation are catabolic (breakdown) pathways that provide energy.

A. Cellular respiration is classified into two types based on the use of oxygen.

1. Aerobic Respiration: This is the process in which oxygen is used within the cell to release the chemical bond energy in nutrients. Humans, animals, plants, bacteria, and fungi perform aerobic respiration.

The general equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) + Heat

• The Electron Transport Chain (ETC) functions, with oxygen being the final electron acceptor.

2. Anaerobic Respiration: This is the process in which glucose is broken down in the cytoplasm of the cell without the use of oxygen, releasing energy.

• Some bacteria obtain energy by using the ETC without the presence of oxygen.
• The final electron acceptor molecule is inorganic and different from oxygen.
• Less energy is obtained compared to aerobic respiration.

B. Fermentation: It is important to note that fermentation is not a type of anaerobic respiration. Fermentation is the process of releasing energy from nutrients without the use of oxygen. In other words, it is a partial breakdown of sugar that occurs without the help of oxygen.

• The Electron Transport Chain (ETC) is not used in fermentation, and the final electron acceptor molecule is organic.
• Based on the type of end product formed, fermentation is classified into two types: ethanol fermentation and lactic acid fermentation.

A. Ethanol Fermentation: This is the fermentation in which ethanol is formed from the pyruvates, the end product of glycolysis.

• It occurs in yeast, especially in brewer’s yeast, and in wine bacteria. The CO₂ released during this fermentation causes dough to rise and beer and champagne to foam.

The general equation is:

Glucose + 2 ATP → 2 Ethanol + 2CO₂ + TOTAL 4 ATP + Heat

B. Lactic Acid Fermentation: This is the fermentation in which lactic acid is formed from pyruvate, the end product of glycolysis.

• It occurs in yogurt bacteria, in skeletal muscle cells under conditions where sufficient O₂ is not available, and in mature red blood cells of mammals.
• The general equation is:

Glucose + 2 ATP → 2 Lactic Acid + TOTAL 4 ATP + Heat

Comparison of ATP Production Pathways for ATP Production in Terms of Certain Characteristics

Excretion refers to the process of removing waste products produced in cells and tissues as a result of metabolism from the cell or the body.

The goal is to create a stable internal environment (homeostasis).

Metabolic waste products resulting from metabolic processes:

• CO2, H2O, ammonia (NH3), urea, and uric acid.
• Nitrogenous waste products in organisms are ammonia (NH3), urea, and uric acid. Ammonia is produced during the breakdown of proteins and nucleic acids within cells or during the conversion of amino acids into carbohydrates. Highly toxic ammonia is converted into less toxic urea and uric acid for excretion in many organisms.
• The manner in which an organism eliminates its nitrogenous waste depends on the water availability of the environment it has adapted to. In most aquatic animals (e.g., most bony fishes), nitrogenous waste is excreted as ammonia, as it dissolves in water and is expelled with excess water. In unicellular organisms such as Paramecium, invertebrates like hydra and planaria, and vertebrates such as fish and frog larvae, nitrogenous waste is ammonia.
• Urea, being less toxic than ammonia, is excreted with less water. Cartilaginous fish (e.g., sharks), adult amphibians, and mammals excrete nitrogenous waste as urea.
• Uric acid is largely insoluble in water and is excreted as a semi-solid paste. Therefore, organisms in arid environments such as insects, reptiles, and birds excrete uric acid.

Nitrogenous waste products:

1. Solubility in water:
   • Ammonia > Urea > Uric acid
2. Water loss during excretion:
   • Ammonia > Urea > Uric acid
3. Toxicity levels:
   • Ammonia > Urea > Uric acid
4. ATP expenditure in production:
   • Uric acid > Urea > Ammonia

Organisms carry out excretion through different methods.

• Unicellular organisms excrete waste products through their cell membrane (surface).
• In freshwater organisms such as Paramecium, Euglena, and Amoeba, excess water is actively excreted via contractile vacuoles using ATP.
• In organisms living in saltwater, contractile vacuoles are absent. This is because their cells do not take in excess water; rather, they lose water.
• Terrestrial plants excrete waste by shedding their leaves, through guttation (via hydathodes), or by transpiration (via stomata or lenticels).
• In animals, excretion occurs through the digestive, respiratory, and excretory systems. Additionally, organs such as the skin and lungs assist in excretion in mammals. Undigested food wastes (feces) are removed by defecation, CO2 is expelled through the respiratory system, and water and dissolved waste products are removed through the excretory system.