NCERT Explained: Class VII Science, Chapter 10 - Respiration in Organisms: Why and How We Breathe

Introduction to the Topic

Have you ever wondered why we breathe faster after running a race or climbing a flight of stairs? Or why we can't hold our breath for more than a minute? Breathing is an essential part of life, but it is actually only one part of a much larger process called respiration. While we often use the terms 'breathing' and 'respiration' interchangeably, they represent different stages of how our bodies generate energy.

In the NCERT Class VII Science curriculum, Chapter 10, 'Respiration in Organisms,' we explore the fundamental biological process that keeps every living cell alive. This chapter takes us on a journey from the microscopic level of cellular chemistry to the complex mechanical systems of human lungs and the unique adaptations of insects, fish, and plants. Understanding respiration is crucial because energy is required for every activity we perform—whether it is walking, talking, sleeping, or even thinking. This post will break down the complexities of respiration into simple, manageable concepts to help students master this vital topic.

Key Concepts Explained

1. Why Do We Respire?

To understand respiration, we first need to understand that all living organisms are made of small microscopic units called cells. A cell is the smallest functional unit of an organism. To perform various functions like nutrition, transport, excretion, and reproduction, the cell needs energy. Where does this energy come from? It comes from the food we eat.

The food has stored energy, which is released during respiration. Therefore, all living organisms respire to get energy from food. When we breathe, we take in air rich in oxygen and breathe out air rich in carbon dioxide. This oxygen is transported to all parts of the body and ultimately to every cell. Inside the cells, oxygen helps in the breakdown of food. The process of breakdown of food in the cell with the release of energy is called cellular respiration.

2. Aerobic vs. Anaerobic Respiration

The breakdown of glucose (food) can happen in two ways, depending on whether oxygen is used or not:

• Aerobic Respiration: When the breakdown of glucose occurs with the use of oxygen, it is called aerobic respiration. Most organisms, including humans, perform this type of respiration. The chemical reaction is: Glucose + Oxygen → Carbon dioxide + Water + Energy.
• Anaerobic Respiration: Sometimes, food can also be broken down without using oxygen. This is called anaerobic respiration. Organisms like yeast can survive in the absence of air. They are called anaerobes. In these organisms, glucose breaks down into alcohol and carbon dioxide instead of water: Glucose (without oxygen) → Alcohol + Carbon dioxide + Energy.

A Special Case: Muscle Cramps
Our muscle cells can also respire anaerobically, but only for a short time when there is a temporary deficiency of oxygen. During heavy exercise, fast running, or heavy weight lifting, the demand for energy is high, but the supply of oxygen to produce that energy is limited. To fulfill the demand, muscle cells perform anaerobic respiration: Glucose (in muscle) → Lactic acid + Energy. The accumulation of lactic acid in the muscles causes muscle cramps. We get relief from cramps after a hot water bath or a massage because it improves blood circulation, increasing the supply of oxygen and breaking down lactic acid into carbon dioxide and water.

3. Breathing: The Physical Act

Breathing means taking in air rich in oxygen and giving out air rich in carbon dioxide with the help of respiratory organs. This involves two main steps:

• Inhalation: The taking in of air rich in oxygen into the body.
• Exhalation: The giving out of air rich in carbon dioxide.

The breathing rate is the number of times a person breathes in a minute. On average, an adult human being at rest breathes in and out 15–18 times per minute. During heavy exercise, the breathing rate can increase up to 25 times per minute because the body needs more energy, and thus more oxygen.

4. How Do We Breathe? (The Human Respiratory System)

Our respiratory system is a marvel of biological engineering. When we inhale, air passes through our nostrils into the nasal cavity. From the nasal cavity, the air reaches our lungs through the windpipe (trachea). The lungs are located in the chest cavity, which is surrounded by ribs on the sides.

A large, muscular sheet called the diaphragm forms the floor of the chest cavity. Breathing involves the movement of the diaphragm and the rib cage:

• During Inhalation: Ribs move up and outwards, and the diaphragm moves down. This movement increases space in our chest cavity and air rushes into the lungs. The lungs get filled with air.
• During Exhalation: Ribs move down and inwards, while the diaphragm moves up to its former position. This reduces the size of the chest cavity and air is pushed out of the lungs.

This is why you can feel your chest moving when you breathe deeply. The air we breathe out is not just carbon dioxide; it is a mixture of gases. In fact, exhaled air contains about 16.4% oxygen and 4.4% carbon dioxide, whereas inhaled air contains 21% oxygen and 0.04% carbon dioxide.

5. Breathing in Other Animals

Humans aren't the only ones who need to breathe! Other animals have developed fascinating ways to get oxygen:

• Cockroaches & Insects: They do not have lungs. Instead, they have small openings on the sides of their bodies called spiracles. These lead to a network of air tubes called tracheae for gas exchange. Oxygen-rich air rushes through spiracles into the tracheal tubes, diffuses into the body tissue, and reaches every cell.
• Earthworms: Earthworms breathe through their skins. The skin of an earthworm feels moist and slimy on touching. Gases can easily pass through it. Frogs also have a pair of lungs like human beings, but they can also breathe through their moist and slippery skin.
• Fish: Animals that live underwater use gills. Gills are projections of the skin that are well-supplied with blood vessels. They help the fish use oxygen dissolved in water.

6. Do Plants Respire?

Yes, plants also respire for their survival! While they produce oxygen during photosynthesis, they also need to break down glucose to get energy. They take in oxygen from the air and give out carbon dioxide. In plants, each part can independently take in oxygen from the air and give out carbon dioxide.

Leaves have tiny pores called stomata for the exchange of oxygen and carbon dioxide. Even the roots need oxygen to generate energy. Roots take up air from the air spaces present between the soil particles. This is why overwatering a plant can kill it—the water fills up the air spaces in the soil, and the roots cannot breathe!

Summary & Key Takeaways

• Respiration is essential: It is the process of releasing energy from food at the cellular level.
• Aerobic vs. Anaerobic: Aerobic uses oxygen (produces CO2 and water); Anaerobic happens without oxygen (produces alcohol or lactic acid).
• Breathing Mechanism: Inhalation (ribs up, diaphragm down) and Exhalation (ribs down, diaphragm up) facilitate the movement of air in and out of the lungs.
• Diverse Adaptations: Insects use tracheae, earthworms use skin, fish use gills, and plants use stomata and root hairs to breathe.
• Cellular Respiration: This is common to all living organisms and occurs in every single cell of the body.