Respiration in Invertebrates

INTRODUCTION

Respiration is essential for all organisms and can be simplified as “breathing”. However, the exact definition of respiration varies depending on the specific type being discussed.

The exchange of gases, such as carbon dioxide and oxygen, that is vital for the body’s functioning is referred to as “Respiration”. Oxygen is necessary for sustaining life. Respiration can be divided into two types: pulmonary (visible breathing) and cellular (invisible). Pulmonary respiration, which involves activities like inhaling and exhaling or breathing, leads to the release of carbon dioxide as waste gas. When we inhale, the deficiency in oxygen caused by this release is replenished by the oxygen present in the inhaled air.

“Cellular respiration” is a process that occurs within cells, involving the release of carbon dioxide through cell walls and the influx of oxygen for energy production. Animals are categorized into two groups: Vertebrates, which possess a backbone, and Invertebrates, which do not have a backbone. Invertebrates consist of various phyla including Porifera, Cnidaria, Nematoda, Mollusca, Annelida, Arthropoda, and Echinodermata.

There are two groups among invertebrates based on their level of complexity and specialization.

1. Lower Invertebrates: This comprises of the phyla Protozoa, Porifera, Cnidaria, Platyhelminthes and Nematoda. These generally lack specialized organs or organelles to carry out various living functions such as respiration, excretion, reproduction, etc, hence their small size.
2. Higher Invertebrates: These are more complex in body structure and their organs are specialized. They include phyla such as Mollusca, Annelida, Arthropoda and Echinodermata.

CELLULAR RESPIRATION IN INVERTEBRATES

Cellular respiration is a metabolic process that occurs in cells and converts biochemical energy from nutrients into adenosine triphosphate (ATP), while also generating waste products. The reactions involved are catabolic, breaking down large molecules into smaller ones and releasing energy by breaking high-energy bonds. This process provides valuable energy for cellular activity and is classified as an exothermic redox reaction.

While cellular respiration is similar to a combustion reaction, it is divided into smaller redox reactions within the body. Unlike a combustion reaction, cellular respiration occurs in multiple steps within living cells. Animal and plant cells commonly use sugar, amino acids, and fatty acids as nutrients for respiration. In this process, molecular oxygen (O2) acts as an oxidizing agent or electron acceptor.

ATP has energy that can be used for different processes, including biosynthesis, locomotion, and molecule transportation across cell membranes.

AEROBIC RESPIRATION IN INVERTEBRATE CELLS

Aerobic respiration is a process that uses oxygen to generate ATP. It can utilize carbohydrates, fats, and proteins as substrates, but its main role is in breaking down pyruvate during glycolysis. The entry of pyruvate into the mitochondrion is essential for its complete oxidation in the Krebs cycle.

The process results in the production of ATP (adenosine triphosphate), NADH, and FADH2 through substrate-level phosphorylation. The simplified equation is C6H12O6 (s) + 6O2 (g) > 6CO2 (g) + 6 H2O (l) + heat, with a G value of -2880 kJ per mole of C6H12O6. The negative G indicates spontaneous occurrence. Oxygen serves as the final electron acceptor in the electron transport chain, converting the reducing potential of NADH and FADH2 into additional ATP. Oxidative phosphorylation accounts for most of the ATP production in aerobic cellular respiration.

Consuming pyruvate releases energy which is used to create a chemiosmotic potential through proton transportation across a membrane. This potential powers ATP synthase, enabling the synthesis of ATP from ADP and phosphate. Biology textbooks state that around 38 ATP molecules can be produced from one glucose molecule during cellular respiration (2 ATP from glycolysis, 2 ATP from the Krebs cycle, and approximately 34 ATP from the electron transport system).

Despite losses caused by leaky membranes and transportation costs of pyruvate and ADP into the mitochondria’s matrix, the maximum ATP yield per glucose is not fully realized. Current estimates range from 29 to 30 ATP. Cellular Respiration involves four distinct processes occurring in different cellular locations: glycolysis, oxidative decarboxylation of pyruvate, Kreb’s Cycle, and oxidative phosphorylation.

GLYCOLYSIS

Glycolysis is a cellular process that occurs in the cytosol of all living organisms. It can happen with or without oxygen, leading to the creation of pyruvate or lactate. This process transforms one glucose molecule into two pyruvate molecules and generates energy as two net ATP molecules. In total, four ATP molecules are made, but two are used during the preparatory phase. The Aldolase enzyme is vital in dividing glucose into two pyruvate molecules after its initial phosphorylation, which enhances its reactivity.

During glycolysis, ADP is phosphorylated by substrate-level phosphorylation, resulting in the production of four ATP. Simultaneously, pyruvate undergoes oxidation to generate two NADH. The overall reaction can be represented as: Glucose + 2 NAD+ + 2 Pi + 2 ADP > 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O + heat. Initially, one ATP molecule is used to add a phosphate group to glucose, forming glucose 6-phosphate. Glycogen phosphorylase also assists in converting glycogen into glucose 6-phosphate. Lastly, during energy metabolism, glucose 6-phosphate converts into fructose 6-phosphate.

Phosphofructokinase uses an additional ATP molecule to add a phosphate group to fructose 6-phosphate, resulting in the formation of fructose 1,6-disphosphate. Afterward, fructose 1,6-diphosphate divides into two three-carbon chain molecules which ultimately convert into pyruvate.

OXIDATIVE DECARBOXYLATION OF PYRUVATE

The pyruvate dehydrogenase complex (PDC) is responsible for the oxidation of pyruvate to acetyl-CoA and CO2. It is found in both eukaryotic cell mitochondria and prokaryotic cytosol. During this conversion, one molecule of NADH and one molecule of CO2 are generated.

The link reaction, also known as the transition step, acts as a bridge between glycolysis and the Krebs cycle, connecting these two processes. Aerobic metabolism is much more efficient than anaerobic metabolism and can produce up to 15 times more ATP from a glucose molecule. However, certain anaerobic organisms like Methanogen can still carry out anaerobic respiration by using alternative inorganic molecules as the final electron acceptors in the electron transport chain instead of oxygen. These organisms have the same initial glycolysis pathway, but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation.

The post glycolytic reactions in eukaryotic cells occur in the mitochondria, while in prokaryotic cells, they happen in the cytoplasm. This process is also referred to as the citric acid cycle or the tricarboxylic acid cycle. When there is oxygen present, pyruvate molecules resulting from glycolysis are converted into acetyl-CoA. Acetyl-CoA can then undergo either aerobic respiration or anaerobic respiration. If oxygen is available, aerobic respiration takes place in the mitochondria and leads to the Krebs cycle. However, if there is no oxygen, fermentation of pyruvate occurs.

The presence of oxygen leads to the production of acetyl-CoA, which then enters the citric acid cycle (Krebs cycle) in the mitochondrial matrix. During this process, acetyl-CoA is oxidized to CO2 and simultaneously reduces NAD to NADH. The electron transport chain uses NADH for ATP production through oxidative phosphorylation. To fully oxidize one glucose molecule, the Krebs cycle metabolizes two molecules of acetyl-CoA. This metabolic pathway produces two waste products: H2O and CO2. The citric acid cycle consists of eight steps and involves various enzymes and co-enzymes (refer to fig 1 & 2).

Throughout the entire cycle, acetyl-CoA(2 carbons) and Oxaloacetate(4 carbons) are transformed into Citrate(6 carbons), which then changes into Isocitrate(6 carbons). Isocitrate(6 carbons) undergoes further modifications and becomes -Ketoglutarate(5 carbons), Succinyl-CoA, Succinate, Fumarate, Malate, and eventually Oxaloacetate. In one cycle, the overall energy obtained consists of 3 NADH, 1 FADH2, and 1 GTP; the GTP has the potential to generate ATP. Hence, from one glucose molecule (in the form of 2 pyruvate molecules), a total of 6 NADH, 2 FADH2, and 2 ATP are acquired.

OXIDATIVE PHOSPHORYLATION

Oxidative phosphorylation occurs in the mitochondrial cristae (see fig 3) of eukaryotes. This process includes the electron transport chain, which oxidizes NADH from the Krebs cycle to create a chemiosmotic potential or proton gradient across the inner membrane. The ATP synthase enzyme uses this gradient to phosphorylate ADP and produce ATP. Eventually, the electrons are transferred to external oxygen and water is generated along with two protons.

Respiratory structures have evolved to meet the need for oxygen. Small organisms like protozoans exchange oxygen and carbon dioxide through their entire surfaces, while larger multicellular organisms have developed other strategies due to longer diffusion distances. Aquatic worms, for instance, elongate and flatten their bodies to enhance gas exchange with the external medium.

Sessile sponges depend on the ebb and flow of surrounding water for survival. In comparison, jellyfish, which can be quite large, have a low oxygen requirement due to their low organic matter content (less than 1 percent) and the proximity of their metabolizing cells to the surface, reducing diffusing distances. For organisms too big to obtain enough oxygen through diffusion from the environment, specialized respiratory structures are present, such as gills, lungs, specific regions of the intestine or pharynx (in certain fish), or tracheae (air tubes that penetrate the body wall in insects).

Respiratory structures commonly have a slender shape and a surface that is relatively large compared to their volume, allowing for semi-permeability. These structures usually contain a circulation of body fluids, such as blood flowing through the lungs. Two types of pumping mechanisms are often found: one to refresh the surrounding medium with oxygen, and another to ensure the circulation of body fluids within the respiratory structure.

Respiration in lower invertebrates

Many lower invertebrates have no specialized respiratory organs, so they respire through either a passive transport system or an active transport system. An example of simple diffusion is seen in Amoeba, where molecules move from areas of higher concentration to areas of lower concentration. On the other hand, active transport involves the movement of substances across the cell membrane against their concentration gradient (from low to high concentration). In cells, this typically means accumulating high concentrations of necessary molecules like ions, glucose, and amino acids. When this process requires chemical energy such as adenosine triphosphate (ATP), it is known as primary active transport.

Secondary active transport is a process that utilizes an electrochemical gradient for transportation. Unlike passive transport, which does not require cellular energy, active transport depends on cellular energy. Cells require energy to carry out active transport, making it a prime example of an energy-dependent process. Instances of active transport include the absorption of glucose in the human intestines and the intake of mineral ions by root hair cells in plants.

RESPIRATION MECHANISM IN AMOEBA

In Amoeba, gas exchange or direct respiration takes place through the cell membrane. The intake of oxygen and the expulsion of Carbon (IV) Oxide happen through the entire body surface, specifically the cell membrane, by simple diffusion.

The principle of diffusion, as previously mentioned, states that an organism absorbs oxygen from its surroundings and releases carbon dioxide with a higher concentration from within the cell. In Amoeba, Figure 4 illustrates how this internal structure enables such exchange. However, respiration in higher invertebrates is more intricate compared to lower invertebrates. These higher invertebrates frequently possess specialized respiratory organs like trachea, gills, and skin.

The primary role of these respiratory organs is to increase the surface-to-volume ratio, which helps with exchanging gases between the organism and its surroundings.

Respiratory System in Annelids (Earthworm)

Gaseous exchange in earthworms occurs in the capillaries found throughout their skin. Diffusion allows oxygen and carbon dioxide to move through the skin. To facilitate diffusion, the earthworm’s skin must stay moist. Goblet cells release body fluid and mucous to keep the skin moist. As a result, earthworms require damp or moist soil. This is why they typically come to the surface at night when temperatures are cooler and the air’s evaporating potential is lower. Since gases cannot evenly distribute through diffusion in the entire body, earthworms possess a well-developed blood vascular system for transporting respiratory gases internally. Refer to Figure 5: Internal Structure of Earthworm.

The respiratory system in grasshoppers is called the tracheal system. It involves the diffusion of oxygen directly from the atmosphere into air-filled tubes. In grasshoppers, the tracheal system consists of 10 pairs of spiracles located laterally on the body surface. Of these, 2 pairs are thoracic and 8 pairs are abdominal. The spiracles are protected by hairs to prevent foreign particles from entering and by valves that regulate their opening and closing. The spiracles lead to small spaces known as atria, which then extend into air tubes called tracheae. The tracheae are narrow tubes composed of a single layer of epithelial cells. These cells secrete spiral cuticular thickenings that provide support for the tubes.

The Tracheal tubes divide into smaller tracheoles that penetrate all the tissues and sometimes, even the cells of the insect. The tracheoles in the tissues are filled with fluid and do not have the cuticular thickenings. The primary tracheal tubes join together to create three main tracheal trunks-dorsal, ventral, and lateral. In certain areas, the trachea expands to form air sacs, which lack cuticle and function as air reservoirs. The first four pairs of spiracles participate in inspiration or the intake of oxygen-rich air. This air travels through the trachea and the air sacs to reach the tracheoles.

The tracheoles are filled with fluid and lack cuticles, which allows for a thin respiratory surface and easy diffusion of oxygen into cells. The accumulation of respiration by-products in cells causes fluid to enter the tracheoles, creating low pressure and drawing in more oxygen to the tissues. Chemoreceptors detect carbon dioxide and cause nearby muscles to contract, pushing air out.

The final six pairs of spiracles are responsible for expelling air during the breathing process. Consequently, in grasshoppers, there is a continuous circulation of air where oxygen is inhaled through the first four spiracles, while carbon dioxide is exhaled through the remaining six pairs of spiracles. Thus, in insects, the respiratory system operates separately from the circulatory system.

The respiratory system of gastropods displays a wide range of forms and was previously used to classify the group into subclasses. Most marine gastropods obtain oxygen through a single gill, which is supplied with oxygen by a flow of water through the mantle cavity. This flow takes a U-shaped path, serving the dual purpose of transporting waste away from the anus and preventing fouling, as the anus is positioned above the animal’s head. In contrast, pulmonate gastropods, which inhabit both land and freshwater environments, have a basic lung instead of a gill.

Gastropodal Respiration with Filamentous Gills

In gastropods, many ancient lineages have bipectinate gills with a shape resembling a bird’s feather. These gills consist of narrow filaments projecting from a central stalk. Abalone and keyhole limpets, for example, have two gills, which is thought to be the arrangement found in the earliest fossil gastropods. The water current required for these gills exits through a slit or notch on the upper surface of the shell, below which the anus is located. In most other gastropods, the right gill has been lost.

Groups like the turban shells have retained the primitive bipectinate form of their gill. In these creatures, water enters the mantle cavity from the left side of the head, flows over the gill, and exits on the right side. The anus is also located on the right side, which facilitates effective waste disposal. Bipectinate gills require membranes for support, and these can become clogged with debris and sediment. Consequently, gastropods with such gills can only thrive in relatively clean-water environments, such as those with water flowing over solid rock.

The majority of living gastropods have a unipectinate arrangement of gills, which allows them to thrive in muddy or sandy environments. This type of gill is firmly attached to the mantle wall along its entire length, with a single row of filaments extending into the water flow. Unipectinate gills are found in various snails, including those that live in marine, freshwater, and even terrestrial habitats. Examples of snails with unipectinate gills include periwinkles, conches, and whelks. Although the water current is oblique, similar to that of turban shells, many species have developed a siphon that is formed from the rolled-up edge of their mantle.

The siphon draws water into the mantle cavity and can be lengthy enough to reach through the substrate in burrowing species. Within the Ampullariidae amphibious group, the mantle cavity is divided into two sections. One side contains a unipectinate gill while the other side contains a lung. This allows these snails to respire using either air or water.

The lung of most pulmonates has a single opening, known as the pneumostome, which can either remain open or open and close during breathing. The highly vascularized roof of the lung allows for gas exchange to take place. While the majority of pulmonates are fully terrestrial and have the lungs described above, the Athoracophoridae have blind tubules in place of the mantle cavity, and the Veronicellidae respire through their skin (see fig 8) and no longer have a lung.

CONCLUSION

Respiration is essential for all levels of living organisms, from unicellular to multicellular. The structures involved in respiration are adapted to the organism’s habitat, highlighting their importance in survival.

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