Respiration in Organisms - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter explains how living organisms obtain energy from food through respiration, the different types of respiration, and the human respiratory system.
1. Why this chapter matters
Respiration is the process that keeps every living cell alive. Understanding how we breathe and how our cells produce energy is fundamental to biology. In CBSE exams, this chapter contributes 6-8 marks.
2. What is respiration?
Respiration is the process by which living organisms break down glucose to release energy.
Respiration is NOT the same as breathing. Respiration is a cellular process, while breathing is the physical act of taking in air.
3. Aerobic respiration
Aerobic respiration requires oxygen. It is the more common form of respiration and produces more energy.
Equation
Glucose + Oxygen = Carbon dioxide + Water + Energy
C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy
Where does it occur?
Aerobic respiration occurs in the mitochondria of cells. Mitochondria are called the 'powerhouses of the cell' because they produce energy.
Energy released
Aerobic respiration releases 38 ATP (energy units) per molecule of glucose.
4. Anaerobic respiration
Anaerobic respiration occurs without oxygen. It produces less energy.
In yeast
Yeast performs anaerobic respiration to produce alcohol and carbon dioxide.
Glucose = Alcohol + Carbon dioxide + Energy
This process is called fermentation and is used in:
- Making bread (CO2 makes the dough rise).
- Making alcohol (beer, wine).
- Making yoghurt and cheese.
In human muscles
During heavy exercise, when oxygen supply is insufficient, muscles perform anaerobic respiration.
Glucose = Lactic acid + Energy
- Lactic acid accumulation causes muscle cramps and fatigue.
- This is why you feel sore after intense exercise.
5. Comparison: aerobic vs anaerobic respiration
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen needed | Yes | No |
| Products | CO2 + H2O + Energy | Alcohol + CO2 or Lactic acid + Energy |
| Energy released | High (38 ATP) | Low (2 ATP) |
| Location in cells | Mitochondria | Cytoplasm |
| When it occurs | Normal conditions | Oxygen shortage |
| Examples | Humans, animals, plants | Yeast, muscle during exercise |
6. Breathing
Breathing is the physical process of taking in oxygen-rich air and releasing carbon dioxide-rich air.
Inhalation (breathing in)
- The rib cage moves upward and outward.
- The diaphragm contracts and moves downward.
- The chest cavity expands, creating lower pressure.
- Air rushes into the lungs.
Exhalation (breathing out)
- The rib cage moves downward and inward.
- The diaphragm relaxes and moves upward.
- The chest cavity contracts.
- Air is pushed out of the lungs.
7. The human respiratory system
Organs
- Nostrils (nose): Air enters through the nostrils, which have hair to filter dust.
- Nasal cavity: Warms and moistens the incoming air.
- Pharynx: Common passage for air and food.
- Larynx (voice box): Produces sound (vocal cords).
- Trachea (windpipe): Tube that carries air to the lungs.
- Bronchi: Trachea divides into two bronchi (one for each lung).
- Bronchioles: Bronchi further divide into smaller tubes.
- Alveoli: Tiny air sacs where gas exchange occurs.
The alveoli
- Each lung has millions of alveoli.
- Alveoli have thin walls and are surrounded by blood capillaries.
- Oxygen passes from alveoli into the blood.
- Carbon dioxide passes from blood into alveoli to be exhaled.
8. Breathing rate
| Condition | Breathing rate (breaths per minute) |
|---|---|
| At rest (adult) | 15-18 |
| During exercise | 25-40 |
| Newborn baby | 30-40 |
Breathing rate increases during exercise because the body needs more oxygen for increased respiration.
9. Respiration in other organisms
| Organism | Respiratory organ/mechanism |
|---|---|
| Earthworm | Moist skin (cutaneous respiration) |
| Fish | Gills |
| Insects (grasshopper) | Tracheal tubes (spiracles) |
| Frogs | Skin (underwater) and lungs (on land) |
| Plants | Stomata in leaves, lenticels in stems |
10. Worked examples
Example 1: Why do we breathe faster after running?
Running requires more energy. Muscles respire more actively, needing more oxygen and producing more CO2. The breathing rate increases to supply more oxygen and remove excess CO2.
Example 2: Why do we get muscle cramps after heavy exercise?
During heavy exercise, oxygen supply to muscles is insufficient. Muscles perform anaerobic respiration, producing lactic acid. The accumulation of lactic acid causes cramps.
Example 3: Differentiate between breathing and respiration.
Breathing is the physical process of inhaling and exhaling air. Respiration is the cellular process of breaking down glucose to release energy.
11. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Using breathing and respiration interchangeably | Breathing is physical; respiration is cellular |
| Thinking plants only photosynthesise, not respire | Plants respire ALL the time (24 hours) |
| Saying anaerobic respiration produces only energy | It produces alcohol/acid + CO2 + energy |
| Confusing inhalation and exhalation movements | Inhale: ribs up, diaphragm down. Exhale: ribs down, diaphragm up |
| Believing only lungs are used for breathing | Many organisms use skin, gills, or tracheae |
12. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Aerobic vs anaerobic respiration | 2-3 marks | 1 question |
| Human respiratory system diagram | 3 marks | 1 question |
| Breathing mechanism (inhalation/exhalation) | 2 marks | 1 question |
| Respiration in different organisms | 2 marks | 1 question |
| Muscle cramps and lactic acid | 2 marks | Occasional |
13. Self-test
- Write the equation for aerobic respiration.
- What is anaerobic respiration? Give one example.
- Draw a labelled diagram of the human respiratory system.
- Why do we get muscle cramps during heavy exercise?
- How is respiration in an earthworm different from respiration in a fish?
- What is the role of alveoli in the respiratory system?
14. Answer key
- Glucose + Oxygen = Carbon dioxide + Water + Energy.
- Respiration without oxygen. Example: yeast fermentation producing alcohol and CO2.
- Diagram should include: nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.
- During heavy exercise, muscles perform anaerobic respiration producing lactic acid, causing cramps.
- Earthworm uses moist skin (cutaneous respiration). Fish uses gills to extract oxygen from water.
- Alveoli are tiny air sacs where gas exchange (O2 into blood, CO2 out of blood) takes place.
15. Quick revision
- Respiration: cellular glucose breakdown for energy.
- Aerobic: with oxygen, high energy, in mitochondria.
- Anaerobic: without oxygen, low energy, in cytoplasm.
- Breathing: inhalation (ribs up, diaphragm down) and exhalation (ribs down, diaphragm up).
- Human respiratory system: nostrils to alveoli.
- Alveoli are the site of gas exchange.
- Different organisms: earthworm (skin), fish (gills), insects (tracheae).
- Muscle cramps caused by lactic acid from anaerobic respiration.
