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Respiration
ICSE Class 7 Biology • Chapter 6 (Detailed Master Notes)
Chapter Overview
Every living cell needs energy to stay alive and function. This energy is obtained by breaking down food (glucose) inside the cells. This fundamental life-sustaining process is called Respiration. It occurs continuously, day and night, in all living cells across all plants and animals.
6.1 What is Respiration?
Respiration is a continuous biochemical process where living cells oxidize (break down) complex organic food, primarily glucose, to release usable cellular energy. It produces carbon dioxide and water as byproducts.
The General Chemical Equation (Aerobic)
$C_6H_{12}O_6 (\text{Glucose}) + 6O_2 \xrightarrow{\text{Enzymes}} 6CO_2 \uparrow + 6H_2O + \text{Energy (ATP)}$
(Food + Oxygen $\rightarrow$ Carbon Dioxide + Water + Energy)
The energy released during respiration is immediately captured and stored in tiny chemical molecules called ATP (Adenosine Triphosphate). ATP is often called the "energy currency" of the cell. Whenever a cell needs energy to perform work, it uses stored ATP.
6.2 Types of Respiration
Depending on whether oxygen is used or not, respiration is classified into two main types:
- Aerobic Respiration:
This is the most common form of cellular respiration. It occurs in the presence of oxygen. The glucose is completely broken down into carbon dioxide and water. Because the breakdown is complete, a large amount of energy is released.
Equation: Glucose + Oxygen $\rightarrow$ Carbon Dioxide + Water + High Energy. - Anaerobic Respiration:
This occurs in the absence of oxygen. The food is only partially broken down. Consequently, a much smaller amount of energy is released.
In some microorganisms like Yeast, glucose breaks down into ethyl alcohol and carbon dioxide. This process is called Fermentation and is used in baking and brewing.
Equation (in Yeast): Glucose $\rightarrow$ Ethyl Alcohol + Carbon Dioxide + Low Energy.
In human muscle cells during heavy exercise (when oxygen demand exceeds supply), glucose partially breaks down into toxic lactic acid, causing muscle fatigue and cramps.
6.3 Respiration in Plants
Plants also constantly respire, taking in oxygen and giving out carbon dioxide. However, unlike animals, plants do not have a specialized centralized respiratory system. Gas exchange happens independently in different plant parts:
- Leaves: The primary site for gas exchange. Oxygen enters and carbon dioxide exits through microscopic pores called stomata.
- Stems: In young green stems, stomata perform this function. In older, thick woody stems, gas exchange occurs directly through loose, porous openings in the bark called lenticels.
- Roots: Roots absorb oxygen that is trapped in the tiny air spaces between soil particles. The oxygen simply diffuses into the root hairs. If soil is permanently waterlogged, these air spaces fill with water, restricting oxygen access and effectively suffocating the roots.
6.4 Respiration vs Photosynthesis
These two fundamental biological processes are completely opposite yet perfectly complementary, ensuring the balance of oxygen and carbon dioxide in nature.
| Feature | Respiration | Photosynthesis |
|---|---|---|
| Purpose | To break down food and release energy for the organism. | To manufacture and store food (chemical energy). |
| Timing | Occurs continuously (day and night). | Occurs only during the day in the presence of sunlight. |
| Gas Exchange | Absorbs Oxygen ($O_2$), releases Carbon Dioxide ($CO_2$). | Absorbs Carbon Dioxide ($CO_2$), releases Oxygen ($O_2$). |
| Location | In all living cells of all organisms. | Only in cells that contain chlorophyll (chloroplasts). |
6.5 Respiration in Humans (Briefly)
In humans, the respiratory system is responsible for bringing oxygen into the body and removing carbon dioxide. The main organs include the nose, trachea (windpipe), bronchi, and the lungs. Breathing is the physical process of inhaling air, while cellular respiration is the chemical process that happens deep inside every single body cell using that inhaled oxygen.
AI Image Prompt: A simple educational diagram showing the human respiratory system. Outline of a human torso with the trachea leading down into two lungs. Show red and blue arrows indicating oxygen entering the lungs and carbon dioxide exiting. Include a small zoomed-in circle showing a single cell combining oxygen and sugar to produce ATP (energy).
Q1. True or False: Respiration is the exact same thing as breathing.
Answer: False. Breathing is simply the physical process of inhaling and exhaling air. Respiration is the complex biochemical breakdown of food to release energy that occurs inside the cells.
Q2. Why do athletes sometimes get muscle cramps after heavy exercise?
Answer: During heavy exercise, the body demands more energy than typical aerobic respiration can supply due to a lack of immediate oxygen. Muscle cells switch to anaerobic respiration, which partially breaks down glucose into lactic acid. The accumulation of this lactic acid in the muscles causes severe cramps.