Which one of the following animals breathes through the skins?

1. Biological Respiration: Aerobic vs. Anaerobic (basic)

To understand the diversity of life, we must first understand how organisms power themselves. Respiration is the fundamental biochemical process by which living cells break down organic compounds, primarily glucose, to release energy. This energy is stored in a molecule called ATP (Adenosine Triphosphate), which acts as the universal "energy currency" for all cellular activities Science, class X, Life Processes, p.88. It is a common mistake to use the terms 'breathing' and 'respiration' interchangeably; however, breathing is simply the physical exchange of gases, while respiration is the chemical process of energy release that happens inside the cells Science-Class VII, Life Processes in Animals, p.132.

Nature has developed two main strategies for respiration depending on the availability of oxygen. Aerobic respiration uses oxygen to completely break down glucose into carbon dioxide (CO₂) and water (H₂O). Because the breakdown is complete, it releases a significant amount of energy, making it the preferred method for complex animals. On the other hand, anaerobic respiration occurs in the absence of oxygen. It is less efficient, yielding much less energy and producing different byproducts like lactic acid (in animal muscles) or ethanol and CO₂ (in organisms like yeast) Science, class X, Life Processes, p.99.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.88, 99; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.132

2. Diversity in Living Organisms: Kingdom Animalia (basic)

Welcome back! In our journey through Kingdom Animalia, we often look at physical features like backbones or fins, but how an animal "breathes" tells us a lot about its evolutionary complexity and habitat. While all animals need oxygen to break down food for energy, the methods they use vary significantly across the animal kingdom. Vertebrates—animals with backbones—are generally the most advanced and have developed specialized systems to dominate their environments Shankar IAS Academy, Indian Biodiversity, p.153.

The most fascinating group in this regard is Amphibia, represented by animals like the frog. Unlike most other creatures, frogs are master multitaskers; they use three different modes of respiration. On land, they use pulmonary respiration (lungs) and buccal respiration (the lining of the mouth). However, when they are underwater or hibernating, they rely on cutaneous respiration—breathing through their skin. For this to work, their skin must remain moist and highly vascularized (rich in blood vessels), allowing oxygen to diffuse directly from the water or air into the bloodstream NCERT Class VII Science, Life Processes in Animals, p.133.

To appreciate this diversity, let’s compare how different groups handle gas exchange:

Sources: Shankar IAS Academy, Indian Biodiversity, p.153; NCERT Class VII Science, Life Processes in Animals, p.133; NCERT Class X Science, Life Processes, p.89

3. Comparative Respiratory Organs across Species (intermediate)

At the heart of survival for any animal is the ability to extract oxygen from the environment to release energy from food. However, because nature is diverse, animals have evolved specialized machinery adapted to their specific habitats—whether they are submerged in water, burrowing in soil, or flying through the air.

Aquatic Respiration: For organisms living in water, the primary challenge is that the concentration of dissolved oxygen is significantly lower than the oxygen available in the atmosphere. To compensate for this, aquatic animals like fish have a much higher breathing rate than terrestrial animals. They utilize gills, which are feathery, highly vascularized structures that allow oxygen to diffuse from the water into the bloodstream as it flows over them Science, Class X, Chapter 5, p.89.

Terrestrial and Multi-modal Respiration: Land-dwelling animals have developed different strategies to prevent their respiratory surfaces from drying out. Most higher vertebrates (mammals, birds, reptiles) use lungs, though the complexity varies—birds, for instance, possess additional air sacs to meet their high metabolic demands during flight Science-Class VII, Life Processes in Animals, p.133. Smaller creatures like insects utilize a unique tracheal system, a network of tiny tubes that deliver air directly to tissues through openings called spiracles Environment, Shankar IAS Academy, Indian Biodiversity, p.155.

The most versatile respirators are amphibians like frogs. They bridge the gap between water and land by using three distinct methods:

• Cutaneous Respiration: Breathing through moist, thin, and highly vascularized skin (vital when underwater or hibernating).
• Buccal Respiration: Gas exchange occurring through the lining of the mouth.
• Pulmonary Respiration: Using lungs when active on land.

Sources: Science, Class X, Life Processes, p.89; Science-Class VII, Life Processes in Animals, p.133; Environment, Shankar IAS Academy, Indian Biodiversity, p.155

4. Thermoregulation: Ectotherms vs. Endotherms (intermediate)

In the study of animal physiology, thermoregulation is the process by which organisms maintain their internal body temperature within a tolerable range. This is critical because most biochemical processes, such as enzyme activity, are highly sensitive to temperature. Broadly, animals are classified into two groups based on how they manage this thermal balance: Ectotherms and Endotherms.

Ectotherms (commonly referred to as 'cold-blooded') rely primarily on external environmental sources to regulate their body temperature. They do not use their internal metabolism to generate heat. Instead, they use behavioral strategies, such as basking in the sun to gain heat or seeking shade to cool down Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.158. Because their body temperature fluctuates with the surroundings, their metabolic rate is generally lower. This category includes most fish, amphibians, and reptiles. However, this reliance makes them vulnerable; even a shift of 1°C to 2°C in environmental temperature can lead to significant cellular stress or force migration to more suitable habitats Environment, Shankar IAS Academy, Environmental Pollution, p.78.

Endotherms (or 'warm-blooded' animals), such as birds and mammals, maintain a relatively constant internal temperature regardless of the external environment. They achieve thermal homeostasis primarily through internal metabolic processes Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.419. While this allows them to remain active in diverse climates—from frozen tundras to scorching deserts—it comes at a high 'fuel' cost, requiring much more food than an ectotherm of the same size. To manage this heat, endotherms have evolved fascinating physical adaptations. For example, elephants use their large ears as cooling devices; by flapping them, they can lower their blood temperature by up to 5°C through an intricate web of blood vessels Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.154.

Sources: Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.158; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.419; Environment, Shankar IAS Academy, Environmental Pollution, p.78; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.154

5. Survival Adaptations: Hibernation and Aestivation (intermediate)

In the natural world, survival often depends on knowing when to stop. When environmental conditions become too extreme—whether it is the biting cold of winter or the scorching heat of summer—many animals enter a state of metabolic dormancy. This is a physiological 'pause button' that allows them to conserve energy and survive without food or water for extended periods.

Hibernation, often called 'winter sleep,' is a strategy used by endothermic (warm-blooded) and some ectothermic (cold-blooded) animals to survive low temperatures and food scarcity. During hibernation, an animal's body temperature drops, and its heart and breathing rates slow down significantly. For instance, animals in the Tundra biome, like rodents and certain bears, rely on thick insulating fur and stored body fat to survive these periods Environment and Ecology, MAJOR BIOMES, p.20. Even social mammals like bats utilize deep sleep and specific roosting behaviors to manage energy Environment, Indian Biodiversity Diverse Landscape, p.158.

Aestivation, conversely, is 'summer sleep.' It is a response to high temperatures and arid conditions where there is a risk of desiccation (drying out). Animals like snails, lungfish, and some amphibians bury themselves in cool mud or retreats to escape the intense heat and vertical rays of the summer sun Certificate Physical and Human Geography, The Earth's Crust, p.8. For amphibians like frogs, these dormant periods are manageable because of their unique respiratory flexibility; while they use lungs on land, they can rely entirely on cutaneous respiration (breathing through their moist, vascularized skin) when buried in mud or submerged in water during these periods Science-Class VII, Life Processes in Animals, p.133.

Sources: Environment and Ecology, Majid Hussain, MAJOR BIOMES, p.20; Environment, Shankar IAS Acedemy, Indian Biodiversity Diverse Landscape, p.158; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.8; Science-Class VII, NCERT, Life Processes in Animals, p.133

6. The Physiology of Amphibian Respiration (exam-level)

Amphibians, such as frogs, represent a fascinating evolutionary bridge between aquatic and terrestrial life. Unlike humans who rely almost exclusively on lungs, amphibians have developed a multi-modal respiratory strategy. This flexibility allows them to survive in water, on land, and even deep underground during hibernation. While the chemical goal of respiration remains the same across species — breaking down glucose in the presence of oxygen to release carbon dioxide, water, and energy (ATP) Science-Class VII, Life Processes in Animals, p.132 — the physical mechanisms amphibians use to capture that oxygen are unique.

An adult frog utilizes three distinct methods of gas exchange depending on its environment and activity level:

• Cutaneous Respiration (Skin): This is perhaps the most critical adaptation. An amphibian's skin is thin, moist, and highly vascularized (richly supplied with blood vessels). Oxygen from the air or water dissolves into the moisture on the skin and diffuses directly into the bloodstream. This method is the primary source of oxygen during hibernation or when the animal is submerged in water for long periods Science-Class VII, Life Processes in Animals, p.133.
• Buccopharyngeal Respiration (Mouth): If you watch a resting frog, you will see its throat pulsing. This is the frog using the lining of its mouth (buccal cavity) to exchange gases. The moist membranes here act similarly to the skin, absorbing oxygen from the air gulped into the mouth.
• Pulmonary Respiration (Lungs): On land, or when the frog is active and requires a sudden burst of energy, it uses its lungs. However, unlike humans who use a diaphragm to suck air in, frogs force air into their lungs by swallowing it, a process known as positive pressure breathing.

It is important to distinguish these adult mechanisms from the larval stage. Tadpoles, being fully aquatic, rely on gills to extract dissolved oxygen, much like fish Science-Class VII, Life Processes in Animals, p.133. As they undergo metamorphosis, these gills are resorbed and replaced by the lung-skin-mouth trio. This transition highlights how breathing mechanisms are specifically adapted to suit habitats Science-Class VII, Life Processes in Animals, p.134.

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.132; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.134

7. Solving the Original PYQ (exam-level)