BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

Excretion and osmoregulation are essential homeostatic processes characteristic of living organisms. Each of these processes plays a vital role in maintaining the constancy of the internal environment of an Organism in the face of a changing external environment.

Excretion

Excretion is the removal from the organism of End products of METABOLISM ("wastes"), the accumulation of which would hinder the maintenance of a steady state in the internal environment. It should be emphasized that we are referring specifically to metabolic products, i.e., substances formed within the Cells of the organism itself. This distinguishes excretion from defecation, i.e., the removal from the digestive tract of indigestible remains of food taken in from the outside. However, feces also contain catabolites (excretory products) such as Bile pigments formed by The breakdown of Hemoglobin in the Liver. Excretion should also be distinguished from secretion. The latter generally refers to the release of substances used by the organism for its own needs, such as Hormones, although a small fraction of these substances may also be excreted, as we will discuss later.

Osmoregulation

Osmoregulation is necessary to maintain a constant osmotic pressure within the body, i.e., in the Cytoplasm, tissue (interstitial) fluid, Blood Plasma, and lymph. This is essential for proper Cell function. As a rule, this involves maintaining the Water-electrolyte balance and, above all, the normal concentration in Body Fluids of ions such as Na⁺, K⁺, and Cl^-.

20.1. Significance of Excretion and Osmoregulation

The processes of excretion and osmoregulation perform A number of Functions, which can be described as follows.

1. Removal of metabolic by-products, which is necessary to maintain the equilibrium of biochemical reactions. Many reactions are reversible, and According to the law of mass action, the direction of a reaction is determined solely by the relative concentrations of reactants and products. For example, in the enzymatic reaction

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the continuous production of the vital metabolite C is ensured by the removal of the by-product D, which shifts the equilibrium toward the forward reaction.

2. Removal of wastes that, if allowed to accumulate, would negatively affect the METABOLIC ACTIVITY OF the organism. Many of these substances are toxic because they inhibit enzyme activity.

3. Regulation of the ionic composition of body fluids. In the aqueous environment of the body, salts behave as electrolytes and undergo dissociation. For example, NaCl ingested with food exists in body fluids in the form of sodium (Na⁺) and chloride (Cl^-) ions. If the concentrations of these and other ions are not maintained within narrow limits, many PHYSIOLOGICAL AND BIOCHEMICAL processes can be disrupted. For example, a decrease in Na⁺ concentration leads to impaired nervous coordination. In addition to Na⁺ and Cl^-, important roles in the body are played by the following ions: K⁺, Mg²⁺, Ca²⁺, Fe²⁺, H⁺, Cl^-, I^-, PO₄^3-, and HCO₃^-, whose concentrations must be strictly regulated as they participate in numerous processes, including enzyme action, Protein Synthesis, hormone and respiratory pigment production, membrane permeability, electrical activity, and Muscle contractions. Their influence on water content, osmotic pressure, and the pH of body fluids will be discussed below.

4. Regulation of Water content in body fluids. The water content of body fluids and its regulation represent one of the major challenges organisms faced when colonizing various ecological niches on the planet. To meet these challenges, a number of crucial Structural and functional adaptations have evolved. The mechanisms ensuring water acquisition, conservation, and elimination are highly diverse, but all of them, as we will see later, are of paramount importance for maintaining the osmotic pressure and volume of body fluid at a stable level. Before examining these mechanisms, it must be emphasized that the Osmotic Pressure of body fluids depends on the quantitative ratio between dissolved substances and the solvent, i.e., water. The regulation of the relative concentrations of solutes and water is called osmoregulation.

5. Regulation of hydrogen ion concentration in body fluids. The Nature of pH and Methods for its measurement are described in Appendix 1.1.5, while the Mechanisms of Excretion of ions (such as H⁺ and HCO₃^-) that have The most significant effect on this value are discussed in this chapter. For example, the pH of human urine can range from 4.5 to 8, thereby helping to maintain the pH of body fluids at a relatively constant level.

20.1.1. Products Subject to Excretion

The main products excreted by animals and plants, along with their sources, are as follows.

1. Nitrogenous compounds, particularly urea, ammonia, and uric acid. They are formed during the Breakdown of Proteins, Nucleic Acids, or excess Amino Acids. These excretory products are discussed in more detail in Section 20.2.

2. Oxygen, produced during Photosynthesis in plants, Algae, and certain Bacteria. Part of it may be utilized by the organism for Respiration.

3. Carbon dioxide, produced during cellular respiration. It can be used by autotrophic organisms as a carbon source.

4. Bile pigments, formed during the breakdown of heme in the liver.

20.1.2. Excretory Structures

The following structures are used by animals for excretion:

1) the outer cell membrane (Plasmalemma) in unicellular organisms;

2) Malpighian tubules (vessels) and tracheae by Arthropods;

3) Kidneys, liver, gills, and Skin by fish and amphibians;

4) kidneys, liver, Lungs, and skin by terrestrial vertebrates.

The cells of relatively simple organisms are usually in direct contact with the external environment, allowing their excretes to be immediately removed by diffusion. As animal Organization becomes more complex, specialized excretory Organs evolve to discharge metabolic wastes from the body into the surrounding environment. Among these, the skin, lungs, liver, and kidneys are the most vital in vertebrates. First, we will briefly examine The Role of the first three organs.

Skin

Water, urea, and salts are actively secreted from dermal capillaries into the ducts of Sweat Glands. Once on the skin surface, sweat evaporates, which leads to heat loss and AIDS in thermoregulation.

Lungs

Carbon dioxide (CO₂) and water vapor diffuse across the moist surface of the pulmonary alveoli. In mammals, the lungs are the sole organ responsible for the elimination of CO₂. Part of the water evaporated in the lungs represents metabolic water—that is, a byproduct of cellular respiration which might technically be considered an excretory product, though its precise origin is of little consequence given the large total volume of water contained within the body.

Liver

Given the numerous homeostatic Functions of the liver described in Section 19.6.2, it is hardly surprising that excretion is among them. The excreted products are bile pigments, which are formed during the breakdown of hemoglobin from Aging erythrocytes. These pigments enter the duodenum as a component of bile and are eliminated from the body with the feces, imparting their characteristic color. However, the most crucial excretory role played by the liver is The formation of urea from excess amino acids (Section 20.4).

20.1.3. Excretion in Plants

In plants, excretion does not present the same multitude of problems as it does in animals. This is due to fundamental differences in the physiology and lifestyle of plants and animals. Plants are primary producers, synthesizing all the Organic compounds they need in precise quantities. For example, a plant produces only as much protein as is required at any given moment. Proteins are never synthesized in excess, and consequently very little nitrogenous waste is generated from Protein Catabolism. Moreover, if proteins are broken down into amino acids, the latter can be reused to synthesize new proteins. Three end products generated through specific metabolic pathways—O₂, CO₂, and water—are utilized by plants as raw Materials for other reactions; this applies particularly to CO₂ and water, with water also serving as a solvent. The only gaseous product released by plants in large quantities is molecular oxygen. In the light, a plant produces far more O₂ than it requires for respiration, and this excess oxygen diffuses out into the surrounding environment.

Many organic metabolic end products in plants are sequestered within dead permanent Tissues (such as heartwood), as well as in leaves or bark, which are periodically shed. Perennial plants consist largely of dead tissues, allowing excretory products to accumulate within them without exerting any harmful effects on the metabolic activity of living tissues. Mineral salts absorbed by the plant as ions can be stored in a similar manner. Certain organic acids harmful to the plant are frequently bound to excess cations and precipitate out as safe, insoluble crystals that can be safely stored within plant cells. For instance, Calcium Ions and sulfate ions are absorbed simultaneously; however, while sulfate ions are immediately utilized for Amino acid synthesis, calcium remains in excess. Ca²⁺ ions readily react with oxalic and pectic acids to form harmless, insoluble products—calcium oxalate and calcium pectate. Substances slated for removal are eliminated not only via foliage, but also through petals, fruits, and seeds, although excretion is not The primary function of these structures. In aquatic plants, the bulk of metabolic end products diffuses directly into the surrounding water.