What Actually Happens When You Breathe?
You’ve been breathing every second of your life — roughly 20,000 times a day — but have you ever stopped to think about why? Not the mechanical in-out of air, but what your cells are actually doing with it?
Every cell in your body needs energy to work. Muscle cells need energy to contract. Brain cells need energy to think. Even when you’re asleep, your heart cells need energy to keep beating. That energy comes from food — specifically from breaking down glucose. Respiration is the process by which living organisms break down food to release energy.
Here’s the key thing most Class 7 students miss: respiration is NOT the same as breathing. Breathing is just moving air in and out of your lungs. Respiration is the chemical process happening inside every single cell. We call the cell-level process cellular respiration, and it’s one of the most fundamental processes in all of biology.
The reason we breathe is because of cellular respiration — our cells need oxygen to break down glucose efficiently, and they produce carbon dioxide as a waste product that we need to get rid of. Breathing is the delivery system; respiration is the factory.
Key Terms & Definitions
Respiration — The process of breaking down food (glucose) inside cells to release energy. Happens in all living organisms, all the time.
Breathing (Ventilation) — The physical process of inhaling air into the lungs and exhaling it out. Only organisms with lungs (or similar organs) breathe.
Aerobic Respiration — Respiration that uses oxygen. Produces more energy. This is what happens in most cells most of the time.
Anaerobic Respiration — Respiration without oxygen. Produces less energy. Happens during intense exercise or in organisms that live without oxygen.
Glucose — The main fuel molecule for cellular respiration. Comes from digesting carbohydrates in food.
Diaphragm — The dome-shaped muscle below your lungs that controls breathing. When it contracts, your lungs expand and air rushes in.
Gill — The respiratory organ of fish and some aquatic organisms. Extracts dissolved oxygen directly from water.
Spiracle — The tiny breathing holes on the body of insects. They don’t have lungs — air enters directly through these pores.
Core Concepts
Aerobic Respiration — The Full Story
Aerobic respiration is like a very efficient burning process. When wood burns, it combines with oxygen and releases energy (heat and light). When glucose “burns” inside a cell, it combines with oxygen and releases energy — but this happens in controlled steps so the energy can be captured and used (not just lost as heat).
The simplified equation to remember:
This happens in the mitochondria of cells (you’ll study this in detail in Class 9).
Why does aerobic respiration need oxygen? Oxygen acts as the final “receiver” in the energy-releasing chain. Without it, the process breaks down mid-way and we get far less energy.
Anaerobic Respiration — When Oxygen Runs Out
During a 100m sprint, your muscles need energy faster than your blood can deliver oxygen. So they switch to anaerobic respiration. This produces energy quickly but incompletely.
In muscles, anaerobic respiration produces lactic acid as a byproduct. The buildup of lactic acid causes that burning sensation you feel in your legs after intense exercise.
In yeast, anaerobic respiration produces ethanol (alcohol) and carbon dioxide instead. This is the process behind bread rising (the CO₂ makes dough puff up) and fermentation in beverages.
How Different Organisms Breathe
Different organisms have evolved different structures to get oxygen into their bodies:
| Organism | Breathing Organ | Medium |
|---|---|---|
| Humans, mammals | Lungs | Air |
| Fish | Gills | Water |
| Insects | Spiracles (tracheae) | Air |
| Earthworms | Moist skin | Air (dissolved) |
| Plants | Stomata (leaves), lenticels (stems) | Air |
Earthworms are fascinating — they have no special respiratory organ at all. Oxygen simply diffuses through their moist skin directly into their blood. This is why earthworms come to the surface after rain (waterlogged soil has less oxygen) and why they die if their skin dries out.
The Human Breathing Mechanism
Let’s trace a single breath:
- Diaphragm contracts → dome flattens and moves down → chest cavity volume increases
- Lungs expand → air pressure inside falls below outside air pressure
- Air rushes in (inhalation) through nose → trachea → bronchi → bronchioles → alveoli
- In alveoli — oxygen crosses into blood, carbon dioxide crosses out of blood
- Diaphragm relaxes → dome returns up → chest cavity shrinks → air is pushed out (exhalation)
The alveoli are tiny air sacs (about 700 million in both lungs!) with walls just one cell thick. Their huge surface area and thin walls make gas exchange extremely efficient. If you spread out all the alveoli from one pair of human lungs, they’d cover a tennis court.
Solved Examples
Example 1 — Easy (CBSE Level)
Q: Why do we feel tired after running a long race?
Solution:
During intense running, muscles need more energy than aerobic respiration can supply. They switch to anaerobic respiration, which produces lactic acid as a byproduct.
Lactic acid accumulates in muscle tissue, causing the burning, tired feeling. After we stop running and breathe deeply (paying back the “oxygen debt”), the lactic acid is gradually broken down and tiredness reduces.
Answer: Lactic acid buildup due to anaerobic respiration causes tiredness.
Example 2 — Medium (CBSE Level)
Q: A student says “plants don’t respire because they make their own food through photosynthesis.” Is this correct? Explain.
Solution:
This is incorrect. Photosynthesis and respiration are two completely different processes.
- Photosynthesis happens only in green cells, only during the day (needs light), and stores energy by making glucose.
- Respiration happens in all cells of the plant, 24 hours a day (day and night), and releases energy from glucose.
During the day, a plant does both simultaneously. At night, only respiration continues. Plants need energy for growth, repair, and reproduction — just like animals.
Answer: Incorrect. All living cells — including plant cells — respire continuously to release energy.
Example 3 — Medium (CBSE + Olympiad Level)
Q: Why do fish die when taken out of water, even though there is more oxygen in air than in water?
Solution:
Fish breathe using gills. Gills are designed to extract dissolved oxygen from water — the water flows over thin gill filaments where gas exchange happens.
When a fish is taken out of water:
- The gill filaments collapse and stick together (they need water to stay spread out)
- The surface area for gas exchange drops drastically
- The fish cannot extract oxygen from air because its respiratory system is not designed for air-breathing
So even though air has ~21% oxygen (much more than the tiny dissolved amount in water), the fish’s gills cannot use it.
Answer: Fish gills are adapted for extracting dissolved oxygen from water, not gaseous oxygen from air. Out of water, gill filaments collapse and can’t function.
Example 4 — Hard (Olympiad / Higher Order Thinking)
Q: During bread-making, a baker adds yeast to the dough and keeps it in a warm place. The dough doubles in size. Explain the science behind this.
Solution:
Yeast is a unicellular fungus that performs anaerobic respiration (fermentation) in the absence of sufficient oxygen inside the dense dough:
The carbon dioxide gas produced gets trapped within the elastic gluten network of the dough. These CO₂ bubbles make the dough rise and become light and porous.
When the bread is baked, the yeast dies (high temperature), the alcohol evaporates, and the CO₂ escapes — leaving behind the porous, airy texture of bread.
Warmth speeds up the process because yeast (like all organisms) respires faster at higher temperatures — up to an optimum, after which enzymes denature.
Answer: Yeast undergoes anaerobic fermentation, releasing CO₂ that gets trapped in dough, causing it to rise.
Exam-Specific Tips
CBSE Class 7 Pattern
Respiration is a high-weightage topic in Class 7 Science. In CBSE board pattern, expect:
- 1-mark: definitions (aerobic vs anaerobic, diaphragm, spiracle)
- 2-mark: “why” questions (why fish die in air, why earthworms surface after rain)
- 3-mark: comparisons (photosynthesis vs respiration, aerobic vs anaerobic)
- 5-mark: diagram-based (label the human respiratory system)
The most PYQ-heavy subtopic is the comparison between aerobic and anaerobic respiration — know it cold.
For the diagram question (very common in CBSE): Practice labeling — trachea, bronchi, bronchioles, alveoli, diaphragm. Students often confuse bronchi (two tubes) with bronchioles (smaller branches). Remember: bronchi → bronchioles → alveoli (largest to smallest).
For Olympiad students: Be ready to explain why aerobic respiration yields more energy than anaerobic. The answer connects to how completely glucose is broken down — CO₂ and water are more stable (lower energy) than lactic acid or ethanol, meaning more energy was extracted.
Common Mistakes to Avoid
Mistake 1: Confusing respiration with breathing
“Respiration = breathing” is the #1 error in Class 7. Breathing is ventilation — moving air. Respiration is the chemical reaction in cells. A student who writes “respiration is inhaling oxygen and exhaling carbon dioxide” will lose marks because that’s describing breathing, not respiration.
Mistake 2: Thinking plants only do photosynthesis
Plants respire 24/7. They only photosynthesize during the day. At night, plants only respire — they take in O₂ and give out CO₂, just like us. That’s why you shouldn’t keep too many plants in a closed bedroom at night (limited air circulation, competing for oxygen).
Mistake 3: Writing the respiration equation backwards
Students sometimes write: “glucose + carbon dioxide → oxygen + water + energy.” That’s photosynthesis (nearly)! For respiration: glucose + oxygen → carbon dioxide + water + energy. The reactants and products are opposite for the two processes.
Mistake 4: Saying anaerobic respiration produces “no energy”
Anaerobic respiration produces less energy than aerobic, but it absolutely does produce some energy. If it produced none, organisms would get no benefit from it. The correct statement: aerobic respiration is more efficient.
Mistake 5: Forgetting that insects breathe through spiracles, not lungs
A very common MCQ trap. Insects have a network of tubes called tracheae that carry air directly to cells — they don’t need lungs or a circulatory system to transport oxygen. Air enters through spiracles on the body surface.
Practice Questions
Q1. What is the full form of ATP, and why is it called the “energy currency” of the cell?
ATP stands for Adenosine Triphosphate. It’s called the energy currency because just as money can be used to buy anything, ATP can be “spent” to power any cellular activity — muscle movement, nerve signals, protein synthesis, etc. Respiration produces ATP; cell activities consume it.
Q2. Give two differences between aerobic and anaerobic respiration.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Energy released | More (complete breakdown) | Less (incomplete) |
| End products | CO₂ + H₂O | Lactic acid (animals) or Ethanol + CO₂ (yeast) |
Q3. Why do we breathe faster during exercise?
During exercise, muscles need more energy. More cellular respiration occurs, consuming more oxygen and producing more carbon dioxide. The brain detects increased CO₂ levels in the blood and sends signals to breathe faster and deeper, bringing in more oxygen and expelling more CO₂.
Q4. An earthworm was kept on a dry surface for 30 minutes and then returned to moist soil, but it died. Why?
Earthworms breathe through their moist skin. Oxygen dissolves in the moisture on their skin and diffuses into their blood. On a dry surface, the skin dries out — oxygen cannot dissolve and diffuse. After 30 minutes, the earthworm would have suffocated. Even after returning to moist soil, if the dryness caused irreversible damage to skin cells, the earthworm cannot recover.
Q5. The equation for photosynthesis is:
How does this compare to the equation for aerobic respiration?
The two equations are exact reverses of each other:
- Photosynthesis: CO₂ + H₂O → Glucose + O₂ (uses energy, stores it in glucose)
- Respiration: Glucose + O₂ → CO₂ + H₂O (releases stored energy)
Photosynthesis builds glucose by using light energy. Respiration breaks down glucose to release that stored energy. Together, they form the basis of the carbon cycle on Earth.
Q6. During a biology experiment, a student puts germinating seeds in a sealed jar. After 24 hours, a burning matchstick placed inside the jar goes out immediately. What does this show?
The germinating seeds have been respiring aerobically — consuming oxygen and releasing carbon dioxide. After 24 hours, most of the oxygen in the sealed jar has been used up and replaced with CO₂. Since combustion (burning) requires oxygen, the matchstick goes out immediately. This is a classic demonstration that respiration consumes oxygen.
Q7. Why does dough rise when yeast is added, but the bread doesn’t smell of alcohol after baking?
Yeast performs anaerobic fermentation, producing ethanol and CO₂. The CO₂ gets trapped in the dough (causing it to rise), while the ethanol remains in the dough. During baking at high temperatures (180–220°C), two things happen: the yeast dies, and the ethanol (boiling point 78°C) evaporates completely. That’s why baked bread has no alcohol smell — it all vaporises during baking.
Q8. Can a fish survive in oxygen-rich water where all oxygen molecules have been replaced with oxygen-18 (a heavier isotope, but chemically identical)? Would its respiration work normally?
Yes, the fish would survive and respire normally. Oxygen-18 () is chemically identical to regular oxygen — it has the same number of protons and electrons, just two extra neutrons. Chemical reactions (including respiration) depend on electron configuration, not atomic mass. The fish’s cells would use exactly as they use regular . (This is actually how scientists trace respiration in research — using oxygen-18 as a label to track where the oxygen atoms end up.)
FAQs
Why do we breathe faster when we run?
Your muscles consume more oxygen during exercise. This produces more CO₂ as a byproduct. Your brain detects the rising CO₂ in blood (via chemoreceptors) and sends signals to increase breathing rate. The goal is to bring in more O₂ and flush out excess CO₂ faster.
Do plants breathe at night?
Yes. Plants respire 24 hours a day — they constantly need energy to live. At night, with no photosynthesis occurring, plants only perform respiration: they take in oxygen and release carbon dioxide, just like animals.
Why do fish die in a dirty aquarium even if water is there?
Fish need dissolved oxygen in water. In a dirty, overcrowded aquarium, bacteria break down organic waste and consume most of the dissolved oxygen. The fish suffocate even with water present. This is why aquarium pumps (aerators) are essential — they keep adding oxygen to the water.
What is the difference between respiration and combustion?
Both break down fuel (glucose or wood) using oxygen and release energy. But combustion is uncontrolled — all energy releases at once as heat and light. Cellular respiration is controlled — energy is released in small, manageable steps so cells can capture it as ATP. If glucose burned all at once in a cell, the heat would destroy the cell.
Why do we yawn when sleepy?
The traditional explanation — “we yawn to get more oxygen” — is largely a myth. The current scientific understanding is that yawning helps cool the brain and is triggered by sleepiness and boredom. However, your breathing rate and depth do decrease when drowsy, which can slightly raise blood CO₂. Yawning may partly help reset this.
Can humans do anaerobic respiration?
Yes — specifically in muscle cells during intense, short-duration exercise (sprinting, heavy lifting). The lactic acid produced is the reason for the “burn” and post-exercise soreness. Once you rest and breathe normally, your liver converts the lactic acid back to glucose over time.
What happens to the energy released in respiration?
Most is captured as ATP (adenosine triphosphate), which cells use to power everything — muscle movement, nerve signals, cell division, protein synthesis. Some energy is also released as heat, which is why your body stays warm and why you feel hot after exercise.