Respiration in Invertebrates


Respiration is one of the characteristics of ALL LIVING THINGS. In the simplest terms, “respiration” simply means “breathing”. But more formally, the term depends on what type of respiration been referred to.

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Essentially, “respiration” refers to gaseous exchange of unwanted carbon dioxide and much-needed oxygen for the body’s use. In direct terms, oxygen is essential for life. There are two types of ‘respiration’: pulmonary (visible breathing) and cellular (which is not visible). Regular, visible (pulmonary) respiration is simply another term for breathing – or the inhalation-exhalation process. In this process, the unwanted gas expelled is mainly carbon dioxide. This is then replaced during the inhalation breathing phase by the oxygen content within the air inhaled.

The other kind of respiration is called “cellular respiration”. It occurs in the very local level of the cell itself, where a gaseous exchange takes place as unwanted carbon-dioxide is expelled through the cell walls and oxygen is diffused into the cell for energy purposes within the cell itself. Animals in general are divided into Vertebrates and Invertebrates. Vertebrates are animals with a vertebral column or backbone and Invertebrates are animals without a backbone. Invertebrates include organisms from the following phyla: Porifera, Cnidaria, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata, etc.

Invertebrates can be further divided into two groups according to their level of complexity and specialization. These are:

  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 is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process as they break high-energy bonds. Respiration is one of the key ways a cell gains useful energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction.

The overall reaction is broken into many smaller ones when it occurs in the body, most of which are redox reactions themselves. Although technically, cellular respiration is a combustion reaction, it clearly does not resemble one when it occurs in a living cell. This difference is because it occurs in many separate steps. While the overall reaction is a combustion, no single reaction that comprises it is a combustion reaction. Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and a common oxidizing agent (electron acceptor) is molecular oxygen (O2).

The energy stored in ATP can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes.


Aerobic respiration requires oxygen in order to generate ATP. Although carbohydrates, fats, and proteins can all be processed and consumed as reactants, it is the preferred method of pyruvate breakdown in glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle.

The product of this process is energy in the form of ATP (adenosine triphosphate), by substrate-level phosphorylation, NADH and FADH2 Simplified reaction: C6H12O6 (s) + 6O2 (g) > 6CO2 (g) + 6 H2O (l) + heat ? G = -2880 kJ per mole of C6H12O6 The negative ? G indicates that the reaction can occur spontaneously. The reducing potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen as the “terminal electron acceptor”. Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation.

This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system).

However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondria’s matrix and current estimates range around 29 to 30 ATP per glucose. Cellular Respiration is split into 4 distinct aspects that occur at different parts of the cell. These are glycolysis, Oxidative decarboxylation of pyruvate, Kreb’s Cycle and Oxidative phosphorylation.


Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms. This pathway can function with or without the presence of oxygen. Aerobic conditions produce pyruvate and anaerobic conditions produce lactate. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced; however, two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme Aldolase.

During the pay-off phase of glycolysis, four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP, and two NADH are produced when the pyruvate are oxidized. The overall reaction can be expressed this way: Glucose + 2 NAD+ + 2 Pi + 2 ADP > 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O + heat Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. Glycogen can change into glucose 6-phosphate as well with the help of glycogen phosphorylase. During Energy metabolism, glucose 6-phosphate turns into fructose 6-phosphate.

An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-disphosphate by the help of phosphofructokinase. Fructose 1,6-diphosphate then splits into two phosphorylated molecules with three carbon chains that later degrades into pyruvate.


Pyruvate is oxidized to acetyl-CoA and CO2 by the pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. In the conversion of pyruvate to acetyl-CoA, one molecule of NADH and one molecule of CO2 is formed.

This step is also known as the link reaction or transition step, as it links glycolysis and the Krebs cycle. Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism (which yields 2 molecules ATP per 1 molecule glucose). However some anaerobic organisms, such as Methanogen are able to continue with anaerobic respiration, yielding more ATP by using other inorganic molecules (not oxygen) as final electron acceptors in the electron transport chain. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation.

The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells. KREB’S CYCLE This is also called the citric acid cycle or the tricarboxylic acid cycle. When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, two processes can occur, aerobic or anaerobic respiration. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur.

In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and gets oxidized to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products, H2O and CO2, are created during this cycle. The citric acid cycle is an 8-step process involving different enzymes and co-enzymes (see fig 1 & 2).

Throughout the entire cycle, acetyl-CoA(2 carbons) + Oxaloacetate(4 carbons). Citrate(6 carbons) is rearranged to a more reactive form called Isocitrate(6 carbons). Isocitrate(6 carbons) modifies to become -Ketoglutarate(5 carbons), Succinyl-CoA, Succinate, Fumarate, Malate, and finally, Oxaloacetate. The net energy gain from one cycle is 3 NADH, 1 FADH2, and 1 GTP; the GTP may subsequently be used to produce ATP. Thus, the total energy yield from one whole glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH2, and 2 ATP.


In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae (see fig 3). It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. The electrons are finally transferred to exogenous oxygen and, with the addition of two protons, water is formed.

Respiratory structures are tailored to the need for oxygen. Minute life-forms, such as protozoans, exchange oxygen and carbon dioxide across their entire surfaces. Multicellular organisms, in which diffusion distances are longer, generally resort to other strategies. Aquatic worms, for example, lengthen and flatten their bodies to refresh the external medium at their surfaces.

Sessile sponges rely on the ebb and flow of ambient water. By contrast, the jellyfish, which can be quite large, has a low oxygen need because its content of organic matter is less than 1 percent and its metabolizing cells are located just beneath the surface, so that diffusing distances are small. Organisms too large to satisfy their oxygen needs from the environment by diffusion are equipped with special respiratory structures in the form of gills, lungs, specialized areas of the intestine or pharynx (in certain fishes), or tracheae (air tubes penetrating the body wall, as in insects).

Respiratory structures typically have an attenuated shape and a semipermeable surface that is large in relation to the volume of the structure. Within them there is usually a circulation of body fluids (blood through the lungs, for example). Two sorts of pumping mechanisms are frequently encountered: one to renew the external oxygen-containing medium, the other to ensure circulation of the body fluids through the respiratory structure.


Owing to their lack of specialized organs of respiration, many lower invertebrates respire by passive transport system or active transport system. Simple diffusion is the movement of molecules of a substance from a region of higher concentration to a region of lower concentration. Example is the simple diffusion in Amoeba. Active transport on the other hand is the movement of a substance across a cell membrane against its concentration gradient (from low to high concentration). In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport.

Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.


Gas exchange (direct respiration) occurs through the cell membrane in Amoeba. Oxygen intake and expulsion of Carbon (IV) Oxide is by simple diffusion through the entire body surface (cell membrane).

Based on the principle of diffusion earlier stated, surrounding oxygen diffuse into the organism and Carbon (IV) Oxide with a higher concentration within the cell diffuses out simultaneously. CO2 CO2 O2 O2 Figure 4: Internal structure of Amoeba.  Respiration in Higher Invertebrate takes a more complex form unlike lower invertebrates. Many higher invertebrates possess specialized respiratory organs such as trachea, gills and skin.

The essence of these respiratory organs is to increase the surface-to-volume ratio, thereby increasing the gaseous exchange between the organism and the environment.


In earthworms, gaseous exchange occurs at the capillaries located throughout the skin. Oxygen and carbon dioxide pass through the earthworm’s skin by diffusion. For diffusion to occur, the earthworm’s skin must be kept moist. Body fluid and mucous is released from goblet cells to keep its skin moist. Earthworms therefore, need to be in damp or moist soil. This is one reason hy they usually surface at night when it is possibly cooler and the evaporating potential of the air is low. Because gases cannot be distributed evenly by diffusion throughout its whole body, the earthworm has a well-developed blood vascular system for the transport of respiratory gases within the body. Figure 5: Internal Structure of Earthworm.

The respiratory system in grasshopper is called the tracheal system. It involves the diffusion of oxygen directly from the atmosphere into the air filled tubes. In grasshopper, 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 guarded by the hairs to keep the foreign particles out and by valves that function to open or close the spiracles as required. The spiracles open into small spaces called the atria that continue as air tubes called the tracheae. The tracheae are fine tubes that have a wall of single layered epithelial cells. The cells secrete spiral cuticular thickenings around the tube that gives support to the tubes.

The Tracheal tubes branch further into finer tracheoles that enter all the tissues and sometimes, even the cells of the insect. The ends of the tracheoles that are in the tissue are filled with fluid and lack the cuticular thickenings. The main tracheal tubes join together to form three main tracheal trunks-dorsal, ventral and lateral. At some places, the trachea enlarges to form air sacs which are devoid of cuticle and serve to store air. The first four pairs of spiracles are involved in inspiration or drawing in of air that is oxygen rich. This air passes through the trachea and the air sacs to reach the tracheoles.

The ends of the tracheoles are filled with fluid and also devoid of cuticles therefore, the respiratory surface is very thin and thus making the diffusion of oxygen into the cells easy. As respiration occurs in the cell, the products of respiration accumulate in the cell and this force the fluid in the tracheoles to enter the tissue. The exit of fluid creates low pressure in the tubes and draws in more oxygen to the tissues where it is needed. The carbon dioxide produced is detected by the chemoreceptors which make the muscles near the spiracles contact. This pushes the air out.

The last six pair of spiracles is involved in expiration of air. Thus, in grasshopper, there is ventilation or circulation of air as the oxygen rich air is inhaled through the first four spiracles and the carbon dioxide-rich air is exhaled through the remaining six pairs of spiracles. In Insects, therefore, the respiratory system is independent of the circulatory system.

The respiratory system of gastropods varies greatly in form. These variations were once used as a basis for dividing the group into subclasses. The majority of marine gastropods breathe through a single gill, supplied with oxygen by a current of water through the mantle cavity. This current is U-shaped, so that it also flushes waste products away from the anus, which is located above the animal’s head, and would otherwise cause a problem with fouling. In the pulmonate gastropods, which are found on both land and in freshwater, the gill has been replaced by a simple lung.


In gastropods in many ancient lineages, the gills are bipectinate, having an overall shape that is similar to a bird’s feather, with narrow filaments projecting either side of a central stalk. Gastropods such as abalone and keyhole limpets have two gills, which is believed to be the arrangement in the earliest fossil gastropods. The water current to supply these gills is evacuated through a slit or notch in the upper surface of the shell, below which the anus opens. In most other gastropods, the right gill has been lost.

In groups such as the turban shells the gill still retains its primitive bipectinate form, and in these animals, the water current is oblique, entering the mantle cavity on the left side of the head, flowing over the gill, and then being flushed out on the right side. The anus is also on the right side of the body, so that waste matter is efficiently carried away. Bipectinate gills have to be supported by membranes, and these can become fouled with debris and sediment, restricting such gastropods to relatively clean-water environments, such as water flowing over solid rock.

In living gastropods, a unipectinate arrangement is more common, allowing species to invade muddy or sandy environments. This type of gill is firmly anchored to the mantle wall along its length, with a single row of filaments projecting down into the water stream. Unipectinate gills are found in a wide range of snails, including marine, freshwater, and even terrestrial forms. Examples include periwinkles, conches, and whelks. The water current is oblique, as it is in the turban shells, but many have developed a siphon formed from the rolled-up margin of the mantle.

The siphon sucks in water to the mantle cavity, and may be long enough to extend through the substrate in burrowing species. In one amphibious group, the Ampullariidae, the mantle cavity is divided into two, with a unipectinate gill on one side, and a lung on the other, so that these snails can respire using air or water.

The lung has a single opening on the right side, called the pneumostome, which either remains permanently open, or opens and closes as the animal breathes. The roof of the lung is highly vascularized, and it is through this surface that gas exchange occurs. The majority of pulmonates, however, are fully terrestrial. Most have the typical lung arrangement described above, but in the Athoracophoridae, the mantle cavity is replaced by a series of blind tubules, while the Veronicellidae respire through their skin (see fig 8), and have lost the lung altogether.


It is very obvious that life cannot exist without respiration taking place and respiration occurs at all levels of living things; from unicellular organism down to multicellular organisms. Structures used in respiration are also very well suited to the habitat in which the organism is found as observed in this report. Respiration indeed plays a very crucial role in the survival of any living thing.


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