Chapter Overview & Weightage
Respiration in Organisms is Chapter 10 of Class 7 Science. It introduces the concept of cellular respiration — how all living organisms, from yeast to humans, extract energy from food. This chapter typically carries 6–8 marks in Class 7 annual exams.
| Question Type | Marks | Topics |
|---|---|---|
| MCQ | 1 | Definitions, gases involved, types of respiration |
| Short Answer | 2–3 | Aerobic vs anaerobic, breathing organs in organisms |
| Long Answer | 4–5 | Human breathing process, comparison of organisms |
The distinction between breathing and respiration, and between aerobic and anaerobic respiration, are the two most tested concepts. Clear definitions of each will fetch you full marks on definition questions.
Key Concepts You Must Know
Breathing vs Respiration: These are NOT the same thing.
- Breathing = physical process of inhaling and exhaling air (mechanical, involves lungs)
- Respiration = chemical process inside cells that releases energy from glucose (cellular)
You breathe to get oxygen to cells. Cells use that oxygen for cellular respiration.
Aerobic Respiration: Uses oxygen to break down glucose. Releases large amounts of energy, produces carbon dioxide and water.
Anaerobic Respiration: Without oxygen. Releases small amounts of energy.
- In yeast/microbes: Glucose → Ethanol + Carbon dioxide + Energy (fermentation)
- In muscle cells: Glucose → Lactic acid + Energy (during intense exercise)
Breathing Rate: Number of breaths per minute. Normal adult rate: 15–18 breaths/min. Increases during exercise (more oxygen demand).
Breathing Organs in Different Organisms:
- Fish: Gills (extract dissolved oxygen from water)
- Insects: Spiracles and tracheae (network of tubes carrying air directly to cells — no lungs)
- Earthworms: Moist skin (gaseous exchange through skin surface)
- Frogs: Both lungs and moist skin
- Humans: Lungs
Diaphragm and Rib muscles: Drive breathing in mammals.
- Inhaling: Diaphragm contracts (moves down), rib muscles contract (ribs move up and out) → chest volume increases → air rushes in
- Exhaling: Diaphragm relaxes (moves up), ribs move inward → chest volume decreases → air pushed out
Important Formulas
This is the equation to memorise for exams. Reactants: glucose + oxygen. Products: carbon dioxide + water + energy.
(Glucose → Ethanol + Carbon dioxide + Energy)
Solved Previous Year Questions
PYQ 1 — Breathing vs Respiration (2 marks)
Q: Differentiate between breathing and respiration.
Solution:
| Breathing | Respiration |
|---|---|
| Physical process | Chemical process |
| Involves lungs (in humans) | Occurs inside every cell |
| Takes in oxygen, releases CO₂ | Breaks down glucose using O₂ |
| Does not release energy | Releases energy (ATP) |
PYQ 2 — Anaerobic Respiration (3 marks)
Q: What happens when you run very fast? Why do muscles start aching? Explain using the concept of anaerobic respiration.
Solution: When you run very fast, muscles need more energy than aerobic respiration can supply (because oxygen delivery is too slow). Muscles switch to anaerobic respiration: glucose breaks down without oxygen to produce lactic acid and a small amount of energy.
Lactic acid accumulates in muscle cells. This causes the burning/aching sensation. Once you rest, lactic acid is gradually broken down (partly converted back to glucose in the liver) — the ache disappears.
PYQ 3 — Breathing in Fish (2 marks)
Q: How do fish breathe? How is their method different from humans?
Solution: Fish breathe using gills. Water enters the mouth, passes over the gill filaments, and oxygen dissolved in water diffuses into the blood. Carbon dioxide diffuses out into the water.
Humans breathe using lungs — we take in air (a gas mixture) and exchange gases in air sacs (alveoli). Fish use water; humans use air as the oxygen source.
PYQ 4 — Why does bread dough rise? (2 marks)
Q: Explain why kneaded flour (dough) rises when left overnight.
Solution: Yeast is added to dough. At room temperature, yeast cells perform anaerobic respiration (fermentation) — they break down glucose in the flour and produce carbon dioxide gas. The CO₂ bubbles get trapped in the dough, making it rise and become light and porous. This is why bread has a spongy texture.
Difficulty Distribution
| Difficulty | % of Questions | Types |
|---|---|---|
| Easy | 50% | Definitions, equation of respiration, breathing organs |
| Medium | 35% | Comparing aerobic/anaerobic, real-life applications |
| Hard | 15% | Multi-part questions on mechanisms of breathing |
Expert Strategy
For all definition questions, write one identifying feature as the first sentence. “Respiration is the process occurring inside cells…” or “Breathing is the physical act of…” — this first sentence signals you know the key distinction.
For application questions (bread dough, running muscles, fermentation in industry), always name the type of respiration first, then explain what happens.
Learn all the breathing organs in a table — fish (gills), insects (spiracles/tracheae), earthworm (moist skin), frogs (skin + lungs), humans (lungs). Organisms often appear in MCQs pairing the wrong organism with the wrong organ.
Remember that the diaphragm is a muscle — it contracts and relaxes. When it contracts, it moves down (not up) and the chest expands. Students often get this direction wrong. Contraction = moves down = inhaling.
Common Traps
Trap 1 — Saying plants don’t respire: Plants respire 24 hours a day (taking in O₂, releasing CO₂). They also photosynthesise during the day (taking in CO₂, releasing O₂). At night, only respiration occurs — so plants consume O₂ at night. The myth that “plants give out O₂ at night” is wrong.
Trap 2 — Breathing rate vs heart rate: Breathing rate (how fast you breathe) and heart rate (how fast your heart beats) both increase during exercise, but they are separate processes. Don’t confuse them in an answer.
Trap 3 — Fermentation produces O₂: Fermentation (anaerobic respiration in yeast) produces ethanol and CO₂. No oxygen is produced. Only photosynthesis produces oxygen.