💨 1. What is Respiration?
Respiration is the process by which all living organisms break down
glucose (food) to release energy. Important: Respiration ≠ Breathing!
Breathing is just the physical act of inhaling/exhaling air. Respiration is the chemical process inside
cells.
Word Equation for Respiration:
Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)
Chemical Equation (Aerobic):
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
This is the reverse of photosynthesis — plants make glucose (storing energy); respiration breaks it
down (releasing energy)
⚗️ 2. Types of Respiration
⬇
AEROBIC 🟢
(With oxygen)
Requires O₂
Occurs in cytoplasm + mitochondria
Products: CO₂ + H₂O + LOTS of energy
Examples: humans, animals, most plants, most bacteria
More efficient = complete glucose breakdown
ANAEROBIC 🔴
(Without oxygen)
Does NOT require O₂
Occurs only in cytoplasm
Products: Ethanol + CO₂ (yeast) OR Lactic acid (muscle cells)
Examples: yeast, bacteria in jars; our muscles during intense exercise
Less efficient = incomplete glucose breakdown
During heavy exercise, muscles run out of oxygen → switch to anaerobic respiration →
produce lactic acid as byproduct. This lactic acid accumulation causes the pain/burning
sensation in muscles! After rest, oxygen reaches muscles again and lactic acid is broken down.
🌬️ 3. Breathing in Humans
Breathing is the mechanical process of taking in air (inhalation) and pushing out air (exhalation). Here's
how it works:
INHALATION 😮💨 (Breathing IN)
⬇
Diaphragm contracts (moves down) + Rib muscles contract (ribs move up & out)
→ Chest cavity volume increases
→ Air pressure inside lungs drops below atmospheric pressure
→ Air rushes in from outside (high pressure → low pressure)
⬇
GAS EXCHANGE in Alveoli (tiny air sacs in lungs)
⬇
O₂ from air → diffuses into blood through alveolar walls (very thin, very large surface area)
CO₂ from blood → diffuses into alveoli to be breathed out
⬇
EXHALATION 😮 (Breathing OUT)
⬇
Diaphragm relaxes (moves up) + Rib muscles relax (ribs move in & down)
→ Chest cavity volume decreases
→ Air is pushed out (CO₂ + water vapour expelled)
📸 AI Image Prompt
A two-panel anatomical diagram showing the human respiratory system with breathing
mechanics: Panel 1 "Inhalation" — a body outline with the lungs shown inside the ribcage. The diaphragm
(dome-shaped muscle at the bottom) is shown flattened and moving downward (labeled "Diaphragm contracts
— moves down"), ribs have upward curved arrows showing "Ribs move up and out". The lungs are shown
expanded (larger). A blue arrow shows "Air rushes IN" down the trachea. Inside the lung, a zoomed circle
shows an alveolus: thin-walled sac surrounded by red capillaries, with "O₂ enters blood" (blue arrow
inward) and "CO₂ leaves blood" (red arrow outward). Panel 2 "Exhalation" — the diaphragm is shown
relaxed (dome-shaped upward), ribs show "Ribs move in and down", lungs are contracted (smaller), and
"Air rushes OUT" with upward arrow through trachea. Both panels show the pathway: Nasal cavity → Trachea
→ Bronchi → Lungs → Alveoli. Clean medical diagram style, blue and red color coding for O₂ and CO₂.
Fig. 6.1 — Human breathing mechanism and gas exchange in alveoli
🦟 4. How Different Organisms Breathe
| Organism |
Breathing Organ |
Special Note |
| Humans & most mammals |
Lungs (with diaphragm) |
Alveoli provide huge surface area for gas exchange |
| Fish |
Gills |
Extract dissolved O₂ from water; water flows over gills |
| Insects |
Tiny pores called Spiracles → connected to Tracheae (tubes) |
Tracheae carry O₂ directly to all body cells; no blood involvement in gas transport |
| Earthworms |
Moist skin (body surface) |
O₂ dissolves in skin mucus → diffuses in. Skin must stay moist! |
| Plants |
Stomata (leaves) and Lenticels (stems) |
During day: photosynthesis releases O₂. At night: only respiration occurs (takes in O₂, releases
CO₂) |
| Frogs (Amphibians) |
Lungs + Moist skin |
Can breathe through both; mainly through skin in water |
📝 5. Quick Revision
- Respiration ≠ Breathing. Respiration = cellular chemical process (glucose →
energy). Breathing = physical inhalation/exhalation
- Aerobic: With O₂ → CO₂ + H₂O + lots of energy. E.g. humans, animals
- Anaerobic: Without O₂ → Ethanol + CO₂ (yeast) OR Lactic acid (muscles during heavy
exercise)
- Muscle cramps from exercise = lactic acid buildup (anaerobic respiration)
- Yeast + anaerobic → ethanol + CO₂ → used in bread (CO₂ makes bread fluffy) and wine/beer (ethanol)
- Inhalation: Diaphragm down, ribs up/out, lungs expand, air rushes in
- Exhalation: Diaphragm up, ribs in/down, lungs contract, air pushed out
- Fish → gills. Insects → spiracles + tracheae. Earthworms → moist skin. Plants → stomata + lenticels
- Alveoli = tiny air sacs in lungs; large surface area → efficient O₂-CO₂ exchange
with blood
