NCERT Explained: Class VII Science, Chapter 10 - Respiration in Organisms

Introduction to the Topic

Welcome to NCERT Explained: Class VII Science, Chapter 10 - Respiration in Organisms. Have you ever wondered why we breathe heavily after running a race, or how plants and tiny microorganisms survive without lungs? All living organisms are made of microscopic units called cells. To perform everyday tasks—whether it is walking, reading, sleeping, or even thinking—every cell requires energy. Where does this energy come from? It comes from the food we eat, but that food must be broken down inside our bodies to release usable energy. This vital process is known as respiration.

Respiration is not just breathing; breathing is merely a physical part of a much larger biochemical journey. In this comprehensive guide, we will explore why all living beings respire, the difference between aerobic and anaerobic respiration, how humans and other animals breathe, and how respiration occurs in plants. Let us dive deep into the fascinating world of biological energy production!

Key Concepts Explained

1. What is Respiration and Why Do We Need It?

Every living organism needs energy to survive. The food we consume contains stored chemical energy. During digestion, food is broken down into simpler substances like glucose. Once glucose reaches the cells, it undergoes a breakdown process to release energy. This process of breakdown of food in the cell with the release of energy is called cellular respiration. Cellular respiration takes place in the cells of all organisms.

We can represent the basic chemical process of respiration using the following equation:

\[\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy (ATP)}\]

Without respiration, our cells would starve for energy, leading to the immediate shutdown of life-sustaining metabolic functions. This is why holding your breath for too long is impossible—your cells constantly demand oxygen to keep you alive!

2. Types of Respiration: Aerobic vs. Anaerobic

Depending on the availability of oxygen, respiration can be broadly classified into two main types:

• Aerobic Respiration: When breakdown of glucose takes place with the use of oxygen, it is called aerobic respiration. This is the most common type of respiration in humans, animals, birds, and plants. It releases a large amount of energy, and the end products are carbon dioxide, water, and energy.
• Anaerobic Respiration: Some organisms can survive in the absence of air (oxygen). Such organisms break down glucose without oxygen, a process known as anaerobic respiration. Though less energy is produced here, it is enough for survival in low-oxygen environments. Yeast is a classic example of an anaerobic organism (often called facultative anaerobes) that breaks down glucose into alcohol, carbon dioxide, and energy. Yeast is widely used in baking and brewing industries.

What happens when we exercise vigorously? During heavy exercise like running, sprinting, or heavy weightlifting, our body's demand for energy is very high, but the supply of oxygen from our lungs is limited. To meet this temporary shortfall in oxygen, our muscle cells resort to anaerobic respiration for a short duration. During this muscular anaerobic respiration, glucose is partially broken down into lactic acid:

\[\text{Glucose (in muscle)} \rightarrow \text{Lactic Acid} + \text{Energy}\]

The accumulation of lactic acid in our muscle cells is what causes painful muscle cramps after heavy physical exertion. A hot water bath or a gentle massage improves blood circulation, increasing the supply of oxygen to the muscle cells, which completely breaks down the accumulated lactic acid into carbon dioxide and water, providing relief.

3. The Mechanism of Breathing

Students often confuse breathing with respiration. Let us clear this up: Breathing is a physical process where an organism takes in oxygen-rich air from the environment and gives out carbon dioxide-rich air. Respiration, on the other hand, includes breathing plus the internal cellular breakdown of food to release energy.

Let us look at how humans breathe:

• Inhalation: When we breathe in air, it enters our nasal cavity and travels through the windpipe (trachea) into our lungs. During inhalation, our ribs move up and outwards, and the diaphragm (a large, muscular sheet forming the floor of the chest cavity) moves downwards. This increases the space in our chest cavity, lowering the pressure inside, and air rushes into the lungs.
• Exhalation: When we breathe out, our ribs move down and inwards, while the diaphragm moves back up to its original dome shape. This decreases the chest cavity space, pushing the air out of the lungs through the nose.

On average, an adult human being at rest breathes in and out 15 to 18 times in a minute. During heavy exercise, the breathing rate can increase up to 25 times per minute because the body needs more oxygen to burn food faster for energy.

4. Breathing in Other Animals

Not all animals have lungs like humans. Nature has designed wonderful respiratory organs suited to different habitats:

• Earthworms: Earthworms have a soft, moist, and slim skin. Gases can easily pass through their skin via diffusion. Hence, earthworms breathe through their skin, which is why their skin feels moist and slimy.
• Insects: Insects like cockroaches, grasshoppers, and butterflies have a network of tiny air tubes called tracheae spread throughout their bodies for gas exchange. Air enters through small openings on the sides of the body called spiracles.
• Frogs: Frogs have a dual life. On land, they breathe using their lungs, just like humans. However, when they are underwater, they breathe through their moist and slippery skin.
• Fish: Fish live underwater and cannot breathe air directly. They possess special respiratory organs called gills. Gills are projections of skin rich in blood vessels. As water flows over the gills, dissolved oxygen is absorbed by the blood, and carbon dioxide from the blood is released into the water.

5. Respiration in Plants

Just like animals, plants also need energy to grow, and therefore, they respire. Plants take in oxygen from the air and give out carbon dioxide, exactly like animals do. This happens day and night.

How do plant parts exchange gases? Plants have tiny pores on their leaves called stomata. Oxygen enters the leaf through stomata and is used in cellular respiration. Roots also need oxygen; they take in oxygen trapped in the spaces between soil particles. If a potted plant is overwatered, the water fills up the soil spaces, cutting off oxygen to the roots, which can cause the plant to suffocate and die!

It is important to remember that during daylight hours, plants perform photosynthesis (making food using carbon dioxide, water, and sunlight) alongside respiration. During photosynthesis, they produce much more oxygen than they consume in respiration, which is why we feel that plants give out oxygen during the day.

Summary & Key Takeaways

Let us summarize the core learning points from Chapter 10 of Class VII Science:

• Respiration is essential for all living organisms to release energy from food for carrying out daily life processes.
• Cellular respiration occurs inside cells and can be aerobic (using oxygen) or anaerobic (without oxygen).
• Breathing is a part of respiration involving the intake of oxygen and release of carbon dioxide.
• In humans, inhalation involves the outward movement of ribs and downward movement of the diaphragm, while exhalation involves the reverse movement.
• Different animals use different organs for breathing: humans use lungs, earthworms and frogs use skin, insects use spiracles and tracheae, and fish use gills.
• Plants respire through stomata present on leaves and through root hair spaces in the soil.
• Temporary anaerobic respiration in human muscles during heavy exercise produces lactic acid, causing painful muscle cramps.