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Class 7 Science Chapter 6 Respiration in Organisms — Formulas & Key Points

Chapter 6 Respiration in Organisms is a high-weightage topic in CBSE Class 7 Science, typically carrying 3–5 marks in the annual exam and appearing in multiple-choice, match-the-following, and short-answer formats. Unlike many other biology chapters, this one centres on a handful of chemical equations and clear definitions rather than lengthy descriptions. Mastering the aerobic and anaerobic respiration formulas, understanding the distinction between breathing and cellular respiration, and knowing organism-specific mechanisms will secure full marks. Use this sheet for targeted revision.

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Key takeaways

  • ✓Aerobic respiration equation: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP) is the cornerstone formula tested in CBSE Class 7 Science papers.
  • ✓Anaerobic respiration in muscles produces lactic acid; in yeast it yields ethanol and carbon dioxide—know both pathways separately.
  • ✓Breathing is the physical intake of oxygen and removal of carbon dioxide; respiration is the biochemical release of energy at cellular level.
  • ✓Plants respire 24×7 through stomata and lenticels; photosynthesis happens only in sunlight—never confuse the two processes.
  • ✓Fish use gills to extract dissolved oxygen from water; insects use spiracles and tracheae—no lungs involved.
  • ✓Correct units matter: always write energy released as ATP or kilojoules (kJ), never leave it vague in exams.
  • ✓CBSETUTOR.ai offers instant doubt-solving by uploading a photo of any question—available at ₹999/month for all classes 6-12 with a 3-day free trial.

Core Chemical Equations & Formulas

NCERT Class 7 Science presents two fundamental biochemical equations that you must reproduce exactly in exams. Aerobic respiration occurs in the mitochondria when oxygen is available; anaerobic respiration takes over when oxygen supply is limited or absent. Both release energy stored in glucose, but the products and energy yield differ significantly. Write the word equations first; only in higher classes will you encounter full molecular formulas with C₆H₁₂O₆ notation. CBSE Class 7 papers expect word equations with correct arrows and plus signs. Many students lose half a mark by writing 'produces' instead of the arrow symbol or forgetting to mention energy/ATP explicitly.
  • Aerobic Respiration: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
  • Anaerobic Respiration (muscles): Glucose → Lactic acid + Energy (less ATP)
  • Anaerobic Respiration (yeast): Glucose → Ethanol + Carbon dioxide + Energy
  • Energy is measured in ATP molecules or kilojoules; aerobic yields ~38 ATP per glucose, anaerobic only ~2 ATP

Formula & Equation Reference Table

Below is a ready-reckoner table listing every equation and key statement from NCERT Chapter 6. Copy this table into your notebook or print it for last-minute revision. The 'When to Use' column tells you which question type demands that formula. CBSE examiners often ask 'State the end products of anaerobic respiration in yeast' or 'Why do we feel cramps during vigorous exercise?'—both require you to recall the anaerobic pathways. Mark the differences in products: lactic acid versus ethanol and carbon dioxide. This distinction alone is worth 2 marks in the annual paper.

Key Terms & Definitions

CBSE Class 7 Science examiners love 'Define the following' questions worth 1 mark each. Write definitions in one clear sentence using NCERT language. Breathing and respiration are often confused; breathing is the mechanical process (inhalation and exhalation), while respiration is the chemical breakdown of food to release energy. Cellular respiration happens inside every living cell, even in plants at night. Stomata are tiny pores on leaves; lenticels are similar openings on woody stems. Diaphragm and ribs work together to expand and contract the chest cavity. Gills in fish have thin walls and rich blood supply to absorb dissolved oxygen. Tracheae in insects are air tubes that deliver oxygen directly to tissues without needing blood transport. Learn these terms verbatim to score full marks in definition-based questions.
  • Respiration: The biochemical process of breaking down glucose in cells to release energy in the form of ATP.
  • Breathing: The physical process of inhaling oxygen-rich air and exhaling carbon dioxide-rich air.
  • Cellular respiration: Respiration occurring inside the mitochondria of each cell.
  • Aerobic respiration: Respiration in the presence of oxygen, yielding maximum energy.
  • Anaerobic respiration: Respiration without oxygen, yielding less energy and different by-products.
  • Stomata: Tiny pores on leaf surfaces through which plants exchange gases.
  • Lenticels: Small openings in the bark of woody stems for gas exchange.
  • Diaphragm: A dome-shaped muscle below the lungs that helps in breathing movements.
  • Gills: Respiratory organs in fish that extract dissolved oxygen from water.
  • Spiracles: Small openings on the sides of insects' bodies connected to tracheae.
  • Tracheae: Network of air tubes in insects that carry oxygen directly to body tissues.

Important Constants, Values & Units

Unlike physics or chemistry, Class 7 Science Chapter 6 has few numerical constants, but understanding typical ranges helps in descriptive answers. Human breathing rate at rest is roughly twelve to twenty breaths per minute; during exercise it can rise to thirty or more. One glucose molecule yields approximately thirty-eight ATP in aerobic respiration and only two ATP in anaerobic pathways—this twenty-fold difference explains why athletes train for aerobic endurance. Always write 'Energy' or 'ATP' as the product; never leave it blank. Units for energy in higher classes are kilojoules (kJ) or calories, but CBSE Class 7 papers accept the word 'energy'. Carbon dioxide concentration in exhaled air is about four percent, compared to 0.04 percent in inhaled air—a hundred-fold increase that proves respiration is happening. Write percentages with the percent symbol to avoid ambiguity.
  • Normal human breathing rate: 12–20 breaths/min at rest; 25–35 during exercise
  • ATP yield: ~38 ATP per glucose (aerobic); ~2 ATP (anaerobic)
  • CO₂ in exhaled air: ~4%; in inhaled air: ~0.04%
  • Oxygen in inhaled air: ~21%; in exhaled air: ~16%
  • Energy unit: ATP or kilojoules (kJ) in senior classes; 'energy' is acceptable in Class 7

Memory Tricks & Mnemonics

Remembering the products of aerobic versus anaerobic respiration becomes easy with simple mnemonics. For aerobic, think 'WE CARE': Water, Energy, Carbon dioxide, Aerobic, Requires oxygen, Energy high. For anaerobic in muscles, remember 'LAME': Lactic Acid, Muscles, Energy low. For yeast anaerobic, use 'YEEC': Yeast, Ethanol, Energy, Carbon dioxide. To recall that plants respire all the time, think 'Plants Party 24/7'—photosynthesis is the day job, respiration is round-the-clock. Fish Gills Get Oxygen: Fish use Gills to Get dissolved Oxygen. Insects Spiral Through Air: Insects use Spiracles, air goes Through tracheae. These silly sentences stick in memory far better than rote lists, especially under exam pressure. Teach them to a younger sibling; explaining cements your own recall.
  • Aerobic products → WE CARE: Water, Energy, CO₂, Aerobic, Requires O₂, Energy high
  • Anaerobic (muscles) → LAME: Lactic Acid, Muscles, Energy low
  • Anaerobic (yeast) → YEEC: Yeast, Ethanol, Energy, CO₂
  • Plants respire 24/7 → 'Plants Party all day and night'
  • Fish → Gills Get Oxygen; Insects → Spiracles Push air Through tracheae

Common Mistakes in Notation, Signs & Units

CBSE Class 7 Science answer keys penalise small errors that show conceptual confusion. Never write 'oxygen is produced' in respiration—oxygen is consumed, not produced; that is photosynthesis. Students often reverse the arrow or write equals signs instead of the reaction arrow; always use a proper arrow (→) between reactants and products. Do not abbreviate 'Carbon dioxide' as 'CO₂' unless you are confident with subscript notation; NCERT uses word equations at this level. Writing 'glucose breaks down into energy' misses the intermediate products water and carbon dioxide. Another frequent mistake: stating that plants do not respire, only photosynthesise. Plants respire continuously; photosynthesis occurs only in green parts in sunlight. In diagrams, label all parts—trachea, bronchi, alveoli, diaphragm—with correct spelling; 'diafram' or 'alvioli' will cost you marks. Finally, never say 'breathing releases energy'; breathing is mechanical, respiration releases energy.
  • Never write 'oxygen is produced' in respiration equations—it is consumed.
  • Use → (arrow), not = (equals), between reactants and products.
  • Write full words (Carbon dioxide, Water) unless your school explicitly teaches molecular formulas.
  • Do not omit 'Energy' or 'ATP' from the product side.
  • Avoid saying 'plants do not respire'—they respire 24/7 and photosynthesise in sunlight.
  • Spell diaphragm, alveoli, spiracles correctly in diagrams and definitions.
  • Do not confuse breathing (mechanical) with respiration (biochemical).

Organism-Specific Respiration Mechanisms

NCERT Chapter 6 dedicates sections to how different organisms exchange gases. Humans and mammals use lungs with a diaphragm and rib cage for breathing; the actual gas exchange happens in alveoli, tiny air sacs with thin walls and dense capillaries. Fish extract dissolved oxygen through gills, which have a large surface area and rich blood supply; water flows over gill filaments, oxygen diffuses in, carbon dioxide diffuses out. Insects rely on spiracles (small holes on their body surface) connected to a network of tracheae that deliver air directly to tissues, bypassing blood. Earthworms respire through moist skin because they lack lungs and gills. Plants exchange gases through stomata on leaves (mostly on the underside) and lenticels on stems; roots absorb oxygen dissolved in soil water through root hairs. Know one unique feature per organism type; this is a favourite match-the-following or one-word-answer area in CBSE papers.
  • Humans/Mammals: Lungs, alveoli, diaphragm, rib muscles; gas exchange in alveoli
  • Fish: Gills with thin filaments; extract dissolved O₂ from water
  • Insects: Spiracles and tracheae; air delivered directly to cells, no blood transport
  • Earthworms: Moist skin; oxygen diffuses through skin surface
  • Plants: Stomata (leaves), lenticels (stems), root hairs (roots); diffusion-based exchange

Solved Mini-Example 1: Aerobic vs Anaerobic in Muscles

A Class 7 student runs a 400-metre race. For the first 200 metres her muscles receive enough oxygen and respire aerobically, producing carbon dioxide, water, and plenty of ATP. In the final 200 metres, oxygen supply cannot keep up with demand, so her muscles switch to anaerobic respiration, breaking down glucose into lactic acid and releasing much less energy. After the race, she breathes heavily to take in extra oxygen, which helps convert accumulated lactic acid back into glucose in the liver—a process called repaying the oxygen debt. The lactic acid buildup causes the burning sensation and cramps she feels in her calves. This example shows why athletes focus on aerobic capacity and why untrained individuals feel sore after sudden exertion. Write both equations in your answer to score full marks: aerobic first, then anaerobic.
  • First 200 m (oxygen available): Glucose + Oxygen → CO₂ + Water + Energy (high ATP)
  • Final 200 m (oxygen limited): Glucose → Lactic acid + Energy (low ATP)
  • After race: Heavy breathing supplies O₂ to convert lactic acid back to glucose (oxygen debt repayment)
  • Cramps and burning caused by lactic acid accumulation in muscle tissue

Solved Mini-Example 2: Yeast Fermentation

A baker mixes yeast with flour and sugar to make bread dough, then leaves it in a warm place. Yeast cells respire anaerobically because the dough environment has limited oxygen. The anaerobic equation is Glucose → Ethanol + Carbon dioxide + Energy. The carbon dioxide gas forms bubbles that make the dough rise and become fluffy. The small amount of ethanol evaporates during baking, leaving soft bread with tiny air pockets. If the same yeast is used in brewing, the ethanol is retained to make beer or wine. This single equation explains both baking and brewing industries. In your CBSE exam, if a question asks 'Why does bread dough rise?', write the anaerobic respiration equation and state that carbon dioxide gas causes the rising. Do not forget to mention yeast by name and that this is anaerobic, not aerobic, respiration.
  • Yeast respires anaerobically in dough: Glucose → Ethanol + Carbon dioxide + Energy
  • Carbon dioxide bubbles make dough rise and create soft texture
  • Ethanol evaporates during baking; retained in brewing for alcohol production
  • Warm temperature speeds up yeast respiration, so bakers prove dough at ~30–35 °C

Solved Mini-Example 3: Comparing Breathing Rates

An NCERT practical activity asks students to count their breathing rate at rest and after exercise. Suppose a student counts fifteen breaths per minute while sitting and thirty breaths per minute after running in place for two minutes. The breathing rate doubles because muscles consume glucose faster, producing more carbon dioxide. High carbon dioxide in blood signals the brain to increase breathing rate, bringing in more oxygen and expelling excess carbon dioxide. This feedback loop is automatic and controlled by the respiratory centre in the brain. Plants do not show variable breathing rates because they lack a nervous system and their metabolic rate changes slowly. Understanding this cause-and-effect (exercise → more CO₂ → faster breathing) is key to answering 'Why' questions worth three marks in CBSE papers. Always link increased activity to increased respiration to increased breathing.
  • At rest: 15 breaths/min; after exercise: 30 breaths/min (example values)
  • Exercise increases cellular respiration → more CO₂ produced → brain triggers faster breathing
  • Faster breathing supplies more O₂ and removes excess CO₂
  • This is an automatic reflex controlled by the respiratory centre in the medulla of the brain

One-Glance Last-Minute Revision Box

Use this box the night before your exam or during the fifteen-minute reading time. It condenses the entire chapter into bullet points that fit on one page. Read each line aloud twice, visualise the process, then close your eyes and repeat from memory. Focus especially on the two anaerobic pathways—muscle versus yeast—and the organism-specific mechanisms. Write the aerobic equation on scrap paper before the exam starts so you have a reference anchor. Remember that respiration happens in all living cells all the time; photosynthesis is not the opposite of respiration but a separate process. If a diagram question appears, label parts neatly and spell technical terms correctly. CBSETUTOR.ai users often upload photos of tricky diagram-labeling questions at midnight before the exam and receive step-by-step solutions within seconds—a ₹999/month subscription covers every class 6–12 subject with a 3-day free trial, so you can try it risk-free before your next test cycle.
  • Aerobic: Glucose + Oxygen → CO₂ + Water + Energy (~38 ATP); occurs in mitochondria
  • Anaerobic (muscles): Glucose → Lactic acid + Energy (~2 ATP); causes cramps
  • Anaerobic (yeast): Glucose → Ethanol + CO₂ + Energy; used in baking and brewing
  • Breathing ≠ Respiration: breathing is mechanical intake/expulsion; respiration is biochemical energy release
  • Humans: lungs, alveoli, diaphragm | Fish: gills | Insects: spiracles, tracheae | Plants: stomata, lenticels
  • Plants respire 24×7; photosynthesis only in sunlight in green parts
  • Always write 'Energy' or 'ATP' on the product side; use → not =
  • Common mistakes: saying O₂ is produced, confusing breathing with respiration, misspelling diaphragm/alveoli

Frequently asked questions

What is the chemical equation for aerobic respiration in CBSE Class 7 Science?+
Glucose + Oxygen → Carbon dioxide + Water + Energy. This reaction occurs in the mitochondria of cells and releases approximately 38 ATP molecules per glucose molecule, providing maximum energy for the organism.
How is anaerobic respiration in muscles different from that in yeast?+
In muscles, glucose breaks down into lactic acid and energy (Glucose → Lactic acid + Energy), causing cramps. In yeast, glucose forms ethanol and carbon dioxide (Glucose → Ethanol + CO₂ + Energy), used in baking and brewing.
Do plants respire at night, or only during the day?+
Plants respire 24 hours a day, both day and night, releasing energy from glucose. Photosynthesis occurs only during daytime in sunlight. At night, plants take in oxygen and release carbon dioxide, just like animals.
Why do we feel muscle cramps during vigorous exercise?+
During intense activity, oxygen supply to muscles is insufficient for aerobic respiration. Muscles switch to anaerobic respiration, producing lactic acid. Accumulation of lactic acid causes the burning sensation and cramps we feel.
What is the difference between breathing and respiration?+
Breathing is the physical process of inhaling oxygen and exhaling carbon dioxide using lungs and diaphragm. Respiration is the biochemical process inside cells that breaks down glucose to release energy in the form of ATP.
How do fish breathe underwater without lungs?+
Fish use gills to extract dissolved oxygen from water. Water flows over thin gill filaments rich in blood vessels. Oxygen diffuses into the blood, and carbon dioxide diffuses out into the water, enabling fish to respire underwater.
Why does bread dough rise when yeast is added?+
Yeast respires anaerobically in dough, breaking down glucose into ethanol and carbon dioxide (Glucose → Ethanol + CO₂ + Energy). The carbon dioxide gas forms bubbles, making the dough rise and giving bread its soft, fluffy texture.
Which respiratory organs do insects use?+
Insects breathe through spiracles, small openings on their body surface connected to a network of air tubes called tracheae. Oxygen travels directly through tracheae to body tissues without involving blood, unlike mammals.
How do plants exchange gases if they have no lungs?+
Plants exchange gases through stomata (tiny pores on leaves), lenticels (openings on woody stems), and root hairs (on roots). Oxygen and carbon dioxide diffuse in and out based on concentration gradients during respiration and photosynthesis.
What is oxygen debt, and why do we breathe heavily after running?+
During intense exercise, muscles respire anaerobically, producing lactic acid. After stopping, we breathe heavily to take in extra oxygen that converts lactic acid back into glucose in the liver—this extra oxygen requirement is called oxygen debt.
Can I use molecular formulas like C₆H₁₂O₆ in Class 7 exams?+
NCERT Class 7 Science uses word equations (Glucose, Oxygen, Carbon dioxide, Water). Unless your school or sample papers explicitly show molecular formulas, stick to words to avoid notation errors and score full marks safely.
How can CBSETUTOR.ai help me revise Chapter 6 Respiration in Organisms?+
CBSETUTOR.ai offers 24×7 AI tutoring where you upload a photo of any diagram, equation, or question and receive instant step-by-step solutions. At ₹999/month for all subjects across classes 6–12, with a 3-day free trial, it is perfect for last-minute doubt clearing before exams.

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