ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaienko 2005

Chapter 8. RESISTANCE AND ADAPTATION OF PLANTS AND ANIMALS

8.3. Resistance and adaptation of animals to environmental conditions

8.3.1. Animal respiration in water and air

Animals utilize the chemical energy of food compounds to sustain their vital activity. Most animals obtain energy through aerobic (oxygen-dependent) oxidation of these compounds. The End products of the Complete oxidation of major food components—namely CARBOHYDRATES, fats, and Proteins—are H₂O and _СО2г. The Oxidation of proteins also yields a certain amount of other end products.

Some invertebrates, like certain Bacteria, are capable of utilizing the energy of Organic compounds under anaerobic (oxygen-free) conditions. However, the ultimate energy yield under these conditions amounts to no more than 10% of the energy that would be produced by the complete oxidation of these compounds. It should be noted that the aerobic phase of carbohydrate breakdown is preceded by an anaerobic transformation stage, regardless of oxygen availability.

The process of oxygen uptake and CO₂ release by organisms is called respiration. Animals extract oxygen from their surrounding environment and release CO₂ back into it. The Water produced during oxidation becomes part of the Organism's total water pool. Aquatic animals utilize dissolved oxygen in water, while some of them, notably lungfishes, also use atmospheric oxygen. Terrestrial animals breathe the oxygen present in atmospheric air. Some animals can obtain sufficient amounts of oxygen through their body surface, but most possess specialized respiratory Organs. The primary—and in some cases the only—process that ensures oxygen transfer from the external environment to the Cells is diffusion, which is the movement of oxygen along a concentration gradient from regions of high concentration to low concentration. The elimination of CO₂ from cells likewise occurs along a concentration gradient, from higher to lower concentrations. In animals with a Circulatory system, oxygen diffusion is facilitated by Blood Circulation.

Oxygen content in air and water. The composition of atmospheric air under normal conditions is constant. Convection currents ensure its intensive mixing. A slight enrichment of the upper layers of the atmosphere at altitudes of about 100 km with hydrogen and helium is insignificant for the biosphere. The relative composition of dry atmospheric air is given in Table 8.2.

The composition of atmospheric air is maintained by the balance between oxygen consumption in organismal oxidative processes and its release during Photosynthesis (photolysis of H₂O). Concerns that the burning of fossil fuels (coal, oil, natural gas) will lead to a decrease in the oxygen content of the atmospheric

air are evidently unfounded. It has been calculated that if all known fossil fuel reserves were burned, the atmospheric oxygen content would decrease by no more than 20.8%.

Class="center">Table 8.2

RELATIVE COMPOSITION OF DRY ATMOSPHERIC AIR

The increase in atmospheric CO₂ resulting from fossil fuel combustion should not significantly impact the biota either. At the same time, even a slight rise in atmospheric CO₂ levels can affect the absorption of solar energy by the atmosphere, triggering the so-called greenhouse effect, which is capable of altering global climatic conditions. As is known, the atmosphere is more permeable to incoming shortwave radiation than to the longwave radiation emitted by the Earth. The Earth's longwave radiation is absorbed primarily by atmospheric CO₂ and water vapor. Calculations show that a twofold increase in atmospheric CO₂ would raise its Temperature by approximately 1.3 °C, assuming The amount of water vapor remains constant. Concurrently, a rise in atmospheric temperature promotes the retention of water vapor, leading to cloud formation that reflects solar radiation, which in turn reduces the amount of energy reaching the Earth's surface. However, an increase in atmospheric water vapor would also reduce the amount of outgoing longwave terrestrial radiation escaping into space, acting similarly to elevated CO₂ levels. Therefore, the potential greenhouse effect resulting from increased CO₂ concentrations is complex and requires detailed investigation.

It should be noted that the constancy of atmospheric composition is observed in the air above the Earth's surface. Soil often contains little oxygen due to the presence of oxidizable organic compounds or oxygen-consuming inorganic substances (e.g., iron sulfide). Such processes depend on the concentration of these substances, temperature, moisture, and other factors. Oxygen content may decrease, and CO₂ increase, in crevices, caves, burrows, etc.

Atmospheric pressure is of vital importance in maintaining the absolute amount of gas in atmospheric air. For instance, at sea level—where atmospheric pressure is 101.3 kPa (1 atm, 760 mmHg)—the partial pressure of oxygen in dry air (the fraction of the total gas pressure exerted by a specific gas) is 21.2 kPa (20.95%). At an altitude of 6,000 m, the atmospheric oxygen pressure drops to 10.6 kPa (still 20.95%), but this is insufficient for human survival.

The amount of gas dissolved in water depends on The Nature of the gas, its atmospheric pressure, temperature, the presence of other soluble substances, and so forth. Gas will pass into water until an equilibrium is established where an equal number of gas molecules enter and leave the water.

The solubility of gases in water varies. To characterize this property, the solubility coefficient is introduced, defined as the volume of gas in milliliters dissolved per liter of water under conditions where the pressure of the gas without water vapor is 101.3 kPa and the temperature is 0 °C. Typically, for biological studies, gas solubility is reported at a temperature of 15 °C. The solubility of major atmospheric gases in water is presented in Table 8.3.

Table 8.3

AMOUNTS OF OXYGEN, NITROGEN, AND CARBON DIOXIDE IN WATER AT 15 °C AND A PRESSURE OF 101.3 KPA

(according to Schmidt-Nielsen, 1982)

Note. Water is in equilibrium with pure anhydrous gas dissolving in it

The solubility of carbon dioxide in water is approximately 30 times greater than that of oxygen and 60 times greater than that of nitrogen.

Each gas in a mixture dissolves in proportion to its own partial pressure, independently of the presence of other gases. If the partial pressure of a gas changes, the amount of dissolved gas in the water changes accordingly: as pressure decreases, the dissolved gas escapes from the water until equilibrium is re-established, and conversely, as pressure increases, the amount of dissolved gas rises.

The solubility of gases decreases with increasing temperature and higher concentrations of other dissolved substances. Table 8.4 presents data on oxygen solubility at various temperatures in freshwater and seawater.

Table 8.4

AMOUNT OF OXYGEN DISSOLVED IN FRESHWATER AND SEAWATER

(after Schmidt-Nielsen, 1982)

Note. The water is in equilibrium with atmospheric air.

Since a gas dissolved in water is in equilibrium with its gas phase at a given partial pressure, the gas in the water has the same partial pressure.

Aquatic respiration. Aquatic animals have various adaptations for breathing. Most small animals obtain oxygen from water by diffusion across their body surface and lack specialized respiratory organs. Well-aerated water is in equilibrium with the atmosphere, and the O₂ tension in it is 21.27 kPa. At the same time, in small spherical organisms with a diameter of 1 mm, oxygen reaches the center of the sphere by diffusion provided that its partial pressure at the surface is at least 15.2 kPa. Calculations were made using Harvey's equation:

where F_О2 is the oxygen concentration near The surface of the sphere compared to that of the atmosphere; V _О2 is the oxygen consumption in milliliters per gram per minute (0.001 ml/min · g); K is the diffusion constant, which indicates the volume in milliliters of O₂ diffusing per minute through an area of 1 cm² at an O₂ gradient of 101.3 kPa/cm (for connective and many other animal Tissues, it equals 1.086 · 10^-7 cm² · Pa/min); and r is the radius of the sphere in centimeters.

Thus, small organisms (such as Protozoans and some Flatworms) with a diameter of 1 mm or less are capable of consuming oxygen via diffusion. Organisms with a very low metabolic rate (low oxygen demand), such as jellyfish, can also obtain oxygen in this way. Although jellyfish can grow quite large, they typically consist of less than 1% organic compounds, with the rest being water and salts. Furthermore, in jellyfish, metabolically active cells are located near the surface, where the oxygen diffusion pathway is relatively short.

In organisms with non-spherical shapes, the relative body surface area is larger, and the diffusion pathway is shorter. These flattened or thread-like organisms develop pseudopodia or more complex surface structures. Examples of such organisms include corals and Sponges. Thanks to this, they are able to acquire a sufficient amount of oxygen purely through diffusion.

If the respiratory surface is turned outward and forms evaginations, it is generally called a gill. Alternatively, gills may be enclosed within a cavity, as observed, for example, in fish.

Gills have thin walls that facilitate gas exchange. However, this results in a loss of mechanical rigidity. Consequently, in the air, gills cannot support their own weight and collapse. This is precisely why fish pulled out of water suffocate, even though there is more oxygen in the air than in water.

To supply oxygen to aquatic organisms, maintaining a water current over the gill surface is crucial. This requires either the movement of the gills themselves or the Displacement of the water. Gill movement is observed in small organisms, such as mayfly larvae (Ephemeroptera). However, moving gills requires increasing their mechanical strength, which leads to an increase in thickness, thereby reducing their gas-exchange capacity. Nevertheless, the large aquatic amphibian Necturus moves its gills, though these movements are slow.

Water movement over the gill surface is driven by ciliary action (e.g., in bivalves), flagellar pumping through openings (sponges), or pump-like mechanisms (crabs, fish). In some aquatic animals, water flow around the gill surface is facilitated by the locomotion of the entire body. A typical example is the continuous swimming of tunas. Squids and octopuses draw water into their mantle cavity and expel it through a siphon.

In addition to respiration, gills perform other Functions, notably osmoregulation (in fish, crabs, etc.) and the trapping of suspended food particles, directing them toward the Mouth (cilia of bivalves).

Gills are housed within the branchial cavity, which protects these organs and allows water to flow over the respiratory surface in the most efficient manner. A particularly advantageous arrangement is countercurrent blood flow, where the water current over the gill surface flows in the opposite direction to the blood flow within the gills. This allows the blood to be oxygenated up to the level present in the incoming water. If water and blood flowed in the same direction, the oxygen content in the blood could, at best, only reach the level of the exiting water.

Aquatic organisms have evolved several adaptations to increase water flow through the gills. For instance, bony fish use a combined action of the Oral Cavity and opercula, aided by appropriate Valves, to control the water flow. In some fast-swimming fish (such as tunas), water flows continuously over the gills (ram ventilation).

To clear solid particles that become trapped in the gills during ventilation, aquatic organisms can reverse the direction of water flow. To achieve this, they expand the buccal cavity while keeping their mouth closed. This causes a drop in pressure inside the oral cavity, triggering a cough-like reflex that removes particles from the respiratory tracts, similar to mammals.

Aerial respiration. In the atmosphere, as already noted, a high and constant concentration of oxygen is ensured. This enables the intensification of metabolic and energy-conversion processes.

Air-breathing animals possess three MAIN TYPES OF respiratory organs: gills, tracheae, and Lungs.

Gills. These organs are poorly adapted for breathing in air. Therefore, they are used by relatively few animals, specifically those that have transitioned to a terrestrial lifestyle comparatively recently. A typical example is the robber crab Birgus latro, which has rigid gills. The land crab Cardiosoma can survive on land and in water for extended periods. Another example of terrestrial gill-breathers is woodlice (Isopoda), which inhabit damp environments and have their gills enclosed in a cavity.

Some fish are capable of breathing air. For instance, the European eel (Anguilla) can crawl significant distances across land to migrate from one water body to another, especially at night and through damp grass. On land, eels keep their gill cavities filled with air, renewing it approximately once per minute. In water, these eels perform about 20 respiratory movements per minute. They exhibit a shift in the relative contribution of the gills and Skin to respiration when transitioning between water and air. In water, about 90% of total oxygen uptake occurs through the gills and 10% through the skin. In air, approximately 30% of oxygen is absorbed through the gills and 70% through the skin.

An example of a fish that obtains oxygen from the air primarily through its skin is the mudskipper (Periophthalmus). The electric eel (Electrophorus electricus) breathes air through its mouth. The spotted gar (Lepisosteus osseus) has gills and also uses its modified swim bladder for respiration, periodically filling it with air.

A special group of air-breathing fish is represented by lungfish. For instance, Protopterus and Lepidosiren inhabit water bodies

that are prone to drying out. During droughts, they remain dormant inside mucus cocoons in the mud, resuming normal life once the wet season arrives. In Protopterus, the majority of oxygen is taken up through the lungs and a smaller portion through the gills, whereas in Lepidosiren, almost all oxygen is supplied by the lungs with only a negligible fraction via the gills. Conversely, in Neoceratodus, the gills serve as the primary respiratory organ. This fish lives in rivers and streams, entering a dormant state during dry periods. When water is absent, it still fills its lungs with air, which significantly AIDS oxygen uptake into the body, though the utilization of _O2 decreases sharply.

Tracheae. This type of Respiratory system is characteristic of insects. It consists of a network of tiny tubules that deliver oxygen directly to the tissues. Consequently, blood circulation for The transport of O₂ is unnecessary. Gas Exchange in the tracheae occurs primarily via diffusion, while in many large and active insects, unidirectional airflow through the tracheal system takes place. This mechanism is far more efficient than the alternating intake and expulsion of air.

In most insects, the tracheae connect to the external environment through openings known as spiracles. These can be closed, allowing the insect to regulate gas exchange between the atmosphere and the air within the tracheal system. The tracheae branch extensively, reaching all PARTS OF THE body. The finest branches, termed tracheoles, have a diameter of up to 1 µm. In some insects (such as mayfly larvae, Ephemeroptera), the tracheal system is entirely closed, lacking any openings on the body surface. In such cases, gas exchange occurs through diffusion across the insect cuticle. Another variation of the tracheal system involves its placement within the rectal cavity. The insect ventilates the tracheae by drawing water into the rectum and expelling it. This type of system is found in dragonfly larvae.

Ventilation of the tracheae is facilitated by thin-walled expansions called air sacs, which connect to the larger tracheae and expel air when compressed.

In many insects, primary respiratory movements are performed by the abdomen. During flight, ventilation is intensified by Movements of the Thorax. These respiratory movements are synchronized with the opening and closing of the spiracles and are likewise regulated by a shortage of O₂ or an excess of CO₂.

Certain aquatic insects have retained an aerial mode of respiration—possessing tracheae that communicate with the external environment via spiracles brought into contact with atmospheric air at the posterior end of the body. Small insects (such as mosquito larvae) feature a closed tracheal system where gas exchange occurs via diffusion.

Many aquatic insects carry an air reserve On the surface of their body, with the spiracles opening directly into this air layer. The air is held in place by non-wettable body surfaces equipped with hydrophobic hairs. Adult diving beetles (Dytiscus) possess large air sacs beneath their wing covers (elytra). Backswimmers of the genus Notonecta carry their air store on the ventral surface.

Another method of air retention in aquatic insects relies on microscopic hydrophobic hairs covering a specialized body surface known as a plastron. A typical example of such an insect is the water bug Aphelocheirus, whose plastron surface bears up to 2 million hairs.

In the hemipteran insects Microvelia buenoi and Anisops, the abdomen contains large cells filled with Hemoglobin that binds oxygen. This stored oxygen is utilized while the insects are submerged underwater.

Lungs. These respiratory organs are located within depressions, invaginations, or internal Body Cavities. They are generally categorized into diffusion lungs and ventilation lungs. Diffusion lungs are characterized by the fact that air exchange with the atmosphere occurs solely through diffusion. This type of respiration is typical of relatively small animals—pulmonate snails, scorpions, certain crustaceans, and the like. Ventilation lungs are found exclusively in vertebrates.

In amphibians, the lungs are essentially a simple sac partitioned by several septa (folds) that increase the surface area. The lungs of mammals are subdivided into a vast number of tiny chambers known as alveoli.

In the lungs, oxygen passes from the alveolar air into the blood, while CO₂ moves from the blood into the lungs via diffusion across the walls of blood capillaries and alveoli.

The movement of air through the respiratory tract is driven by rhythmic breathing movements. These involve the Diaphragm, external intercostal Muscles, as well as the intercartilaginous Muscles of the Trachea and Bronchi. During a deep inspiration, the diaphragm and intercostal muscles contract simultaneously, alongside certain thoracic and shoulder-girdle muscles (such as the trapezius, rhomboid, pectoralis Major and minor, etc.) that elevate the Ribs higher than during quiet breathing. Upon completion of inspiration, the Respiratory Muscles relax and the chest cavity returns to its baseline position, resulting in expiration. Quiet expiration occurs passively without muscular participation. During a forced expiration, the internal intercostal muscles and Abdominal muscles contract.

The respiratory system of birds differs significantly from that of mammals. Avian lungs are connected to voluminous, thin-walled air sacs. Unlike in mammals, the branching of the bronchial system in birds does not terminate in alveoli; instead, it forms a continuous-flow loop, allowing air to pass uninterrupted through the lungs in close proximity to the gas-exchange surface.

To supply oxygen to avian embryos, respiration of the eggs takes place. At the blunt end of the egg, there is an air Cell located between the inner and outer shell membranes. This air space gradually enlarges as the egg loses water through evaporation. The hard calcium carbonate eggshell is porous; for instance, a medium-sized chicken egg features approximately 10,000 pores.

Just before hatching, the chick punctures the inner shell membrane of the air cell and begins breathing the air contained within it. Afterward, the chick pips the eggshell and breathes atmospheric air.

Oxygen transport by blood. In many invertebrates, oxygen is transported in the hemolymph or blood in a dissolved state. However, the amount of oxygen that can be carried in this manner is very small—in mammals, it amounts to approximately 0.2 ml per 100 ml of blood. Highly organized animals (all vertebrates without exception) possess specialized substances in their blood capable of reversibly binding oxygen in quantities nearly 20 times greater than what could be carried by simple dissolution in Blood Plasma.

The substances serving as oxygen carriers in the blood are iron- or copper-containing proteins (Table 8.5).

In some animals, these metalloprotein respiratory pigments are dissolved freely in the blood plasma, whereas in others (particularly vertebrates), they are enclosed within red Blood Cells (erythrocytes). In most mammals, mature erythrocytes are anucleate and measure 5–10 µm in diameter. In amphibians, reptiles, fish, and birds, erythrocytes are nucleated and are 100 to 200 times larger than those of mammals. The only vertebrates that lack both erythrocytes and hemoglobin are eel larvae (leptocephali) and certain Antarctic fish of the family Chaenichthyidae.

The Role of the skin in respiration. Cutaneous gas exchange plays a vital role in amphibians. Certain small salamanders of the family Plethodontidae completely lack lungs or gills, relying almost entirely on cutaneous respiration, supplemented by gas exchange across the mucous membranes of the oral cavity.

In frogs during the winter period, when oxygen consumption is low, the skin absorbs more oxygen than the lungs do. Conversely, during periods of active METABOLISM, pulmonary oxygen uptake vastly exceeds cutaneous uptake.

Unlike amphibian skin, the skin of reptiles is nearly impermeable to oxygen. Therefore, they rely primarily on pulmonary respiration. Nevertheless, certain sea snakes, such as Pelamis platurus, absorb up to 33% of their required oxygen through the skin.

In mammals, cutaneous gas exchange is exceptionally minor. Oxygen uptake through the skin is barely detectable by modern Analytical Methods, and only up to 1% of the CO₂ eliminated by the lungs is released cutaneously. In big brown bats (Eptesicus fuscus), the wing skin eliminates 0.4% of the total metabolically produced CO₂ at 18 °C. However, at 27.5 °C, this figure rises to 11.5%. At the same time, oxygen uptake through the wing membranes in these animals remains extremely low.

Table 8.5

RESPIRATORY Metalloproteins (after Schmidt-Nielsen, 1982, modified)