In this chapter, you will learn
- —Understand why living organisms need to respire
- —Differentiate between breathing and respiration
- —Learn about aerobic and anaerobic respiration with equations
- —Understand the human respiratory system and mechanism of breathing
- —Learn how gaseous exchange occurs in the lungs
- —Understand respiration in plants through stomata and lenticels
- —Know how different animals breathe - fish, earthworm, insects
Why Do We Respire?
All living organisms need energy to carry out life processes like growth, movement, repair, and reproduction.
This energy comes from the food we eat. But the energy locked in food molecules cannot be used directly. It must be released through a process called respiration.
Respiration is the process by which food (glucose) is broken down in cells to release energy.
Glucose + Oxygen → Carbon dioxide + Water + Energy
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Respiration happens in every living cell, 24 hours a day, 7 days a week. It never stops as long as the organism is alive.
Exam Tip
Respiration is NOT the same as breathing. Respiration is a chemical process that occurs in cells. Breathing is a physical process of inhaling and exhaling air.
Common Mistake
Students often think respiration only happens in animals. Plants also respire continuously - day and night!
Breathing vs Respiration
These two terms are often confused but are very different:
| Breathing | Respiration |
|---|---|
| Physical process | Biochemical process |
| Involves inhaling and exhaling air | Involves breakdown of glucose in cells |
| Occurs in lungs/respiratory organs | Occurs in every cell of the body |
| No energy is released | Energy is released |
| No enzymes needed | Enzymes are essential |
Exam Tip
The difference between breathing and respiration is one of the MOST asked questions in CBSE exams. Memorize at least 4 differences.
Aerobic Respiration
Aerobic respiration is the type of respiration that occurs in the presence of oxygen. It is the most common type of respiration in most organisms.
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Key features:
- Requires oxygen
- Complete breakdown of glucose
- Produces large amount of energy (38 ATP molecules)
- End products: CO₂ and H₂O
- Occurs in the mitochondria of cells
Mitochondria are called the "powerhouse of the cell" because this is where aerobic respiration occurs and energy is produced.
Exam Tip
Remember: Mitochondria = Powerhouse of the cell. This is where aerobic respiration takes place.
Anaerobic Respiration
Anaerobic respiration occurs without oxygen. It produces much less energy than aerobic respiration.
In yeast (fermentation):
Glucose → Alcohol + Carbon dioxide + Energy (less)
In muscles (during heavy exercise):
Glucose → Lactic acid + Energy (less)
Key features:
- Does NOT require oxygen
- Incomplete breakdown of glucose
- Produces much less energy (only 2 ATP)
- Products: alcohol + CO₂ (in yeast) OR lactic acid (in muscles)
Why do we get muscle cramps? During intense exercise, oxygen supply to muscles is insufficient. Muscles switch to anaerobic respiration, producing lactic acid. Accumulation of lactic acid causes cramps and pain.
Exam Tip
CBSE commonly asks: Why do we get cramps during exercise? Answer: Due to accumulation of lactic acid produced by anaerobic respiration in muscles.
Common Mistake
Anaerobic respiration in yeast produces ALCOHOL, but in human muscles it produces LACTIC ACID (not alcohol).
Human Respiratory System
The human respiratory system consists of the following organs:
1. Nostrils (Nose): Air enters through nostrils. Nose hair and mucus filter dust and germs. Air is warmed and moistened.
2. Pharynx and Larynx: Air passes through the throat. The larynx (voice box) contains vocal cords.
3. Trachea (Windpipe): A tube with C-shaped cartilage rings that keep it open. Lined with cilia and mucus that trap dust particles.
4. Bronchi: Trachea divides into two bronchi - one for each lung.
5. Bronchioles: Bronchi further divide into smaller tubes called bronchioles.
6. Alveoli: Tiny air sacs at the end of bronchioles. This is where gaseous exchange occurs. Alveoli have thin walls and are surrounded by blood capillaries.
7. Lungs: Spongy organs in the chest cavity. Protected by the ribcage. Separated from the abdomen by the diaphragm.
Exam Tip
Know the correct order: Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Mechanism of Breathing
Breathing involves two phases:
Inhalation (Breathing in):
- Diaphragm contracts and moves downward (flattens)
- Ribs move upward and outward
- Chest cavity expands
- Air pressure inside lungs decreases
- Air rushes into the lungs
Exhalation (Breathing out):
- Diaphragm relaxes and moves upward (dome-shaped)
- Ribs move downward and inward
- Chest cavity contracts
- Air pressure inside lungs increases
- Air is pushed out of the lungs
We breathe about 15-18 times per minute at rest. This rate increases during exercise.
Exam Tip
Remember the diaphragm movement: Inhale = diaphragm goes DOWN (contracts), Exhale = diaphragm goes UP (relaxes).
Gaseous Exchange in the Lungs
Gaseous exchange occurs in the alveoli of the lungs.
Why are alveoli ideal for gas exchange?
- Extremely thin walls (one cell thick) - gases pass easily
- Large surface area (millions of alveoli in each lung)
- Rich supply of blood capillaries
- Moist inner surface for dissolving gases
Process:
- Oxygen from inhaled air passes through alveolar walls into blood capillaries
- Oxygen binds to haemoglobin in red blood cells
- Carbon dioxide from blood passes into the alveoli
- CO₂ is then exhaled out
Inhaled air: ~21% oxygen, ~0.04% CO₂
Exhaled air: ~16% oxygen, ~4% CO₂
Exam Tip
Exhaled air has MORE CO₂ and LESS oxygen than inhaled air. It also has more water vapour (moisture).
Respiration in Plants
Plants also respire, just like animals. They take in oxygen and release carbon dioxide during respiration.
How do plants exchange gases?
- Stomata: Tiny pores on the surface of leaves. Each stoma is surrounded by two guard cells that control opening and closing.
- Lenticels: Small pores on the bark of stems and woody branches.
- Root hair: Roots absorb oxygen dissolved in soil water through root hair.
Important facts:
- Plants respire all the time (day and night)
- During the day, photosynthesis rate is higher than respiration, so plants appear to give out oxygen overall
- During the night, only respiration occurs, so plants release CO₂
- Each plant cell has mitochondria where respiration takes place
Exam Tip
Common exam question: Do plants respire at night? YES! Plants respire 24 hours. At night, they release CO₂ because there is no photosynthesis.
Common Mistake
Plants do NOT take in CO₂ for respiration. They take in OXYGEN for respiration (just like animals). CO₂ is used in photosynthesis, which is a separate process.
Respiration in Different Animals
Different animals have different respiratory organs based on their habitat:
| Animal | Respiratory Organ | Medium |
|---|---|---|
| Humans, mammals | Lungs | Air |
| Fish | Gills | Water (dissolved O₂) |
| Earthworm | Moist skin | Air |
| Insects (cockroach) | Spiracles and tracheae | Air |
| Frog | Lungs and moist skin | Air |
| Amoeba | Cell membrane (diffusion) | Water |
Exam Tip
This table is very important for exams. Know which animal uses which respiratory organ. Fish use GILLS, earthworms use MOIST SKIN, insects use SPIRACLES.
Fermentation
Fermentation is a type of anaerobic respiration carried out by microorganisms like yeast.
Glucose → Alcohol + Carbon dioxide + Energy (less)
Uses of fermentation:
- Making bread: Yeast ferments sugar in dough, producing CO₂ which makes bread rise (fluffy)
- Making wine and beer: Yeast converts sugar into alcohol
- Making idli and dosa batter: Fermentation by bacteria causes the batter to rise
- Making curd from milk: Lactobacillus bacteria ferment milk sugar (lactose)
Discovery: Louis Pasteur discovered fermentation in 1857 and showed it was caused by living organisms (yeast).
Exam Tip
Remember: Yeast is used in bread-making. CO₂ produced during fermentation makes the dough rise and bread becomes soft and fluffy.