Quick answer: Aquatic organisms breathe much faster than terrestrial ones because the amount of oxygen dissolved in water is very low compared with the amount of oxygen in air. A litre of air holds about 210 mL of oxygen, but a litre of water holds only about 5–8 mL, roughly 30 times less. So a fish must pass a lot of water over its gills quickly to get enough oxygen.
The key reason: less oxygen in water
| Medium | Oxygen available | Notes |
|---|---|---|
| Air | About 21% by volume (about 210 mL per litre) | Light, easy to move in and out |
| Water | About 5–8 mL per litre (varies with temperature) | Dense, takes more effort to pump |
NCERT point: Since the amount of dissolved oxygen in water is fairly low compared with the amount of oxygen in air, the rate of breathing in aquatic organisms is much faster than that in terrestrial organisms.
Cold water holds more oxygen than warm water, so fish in warm water need to breathe even faster.
Why this makes breathing faster
- Every litre of water a fish takes in carries only a small amount of oxygen.
- To meet its needs, the fish must move a large volume of water over its gills.
- So it opens and closes its mouth and gill covers at a much faster rate than a land animal breathes air.
How a fish breathes
Step 1: The fish opens its mouth and takes in water.
Step 2: It closes its mouth, and the water is pushed over the gills.
Step 3: Oxygen dissolved in the water diffuses into the blood in the gills, and carbon dioxide diffuses out.
Step 4: The water leaves through the gill openings, under the gill cover (operculum).
mouth → gills → out under the
opens (O₂ in, gill cover
CO₂ out)
How gills are suited for gas exchange
- Large surface area: gills have many thin filaments with tiny plates (lamellae).
- Thin walls and many blood capillaries: gases diffuse quickly.
- Counter-current flow (extra): blood in the gills flows in the opposite direction to the water. This keeps oxygen diffusing into the blood along the whole length of the gill, so fish can take up a large share (often over 70–80%) of the oxygen from the water.
Aquatic vs terrestrial respiration
| Feature | Aquatic (e.g. fish) | Terrestrial (e.g. humans) |
|---|---|---|
| Medium | Water with dissolved oxygen | Air |
| Oxygen available | Low | High |
| Rate of breathing | Faster | Slower (about 12–16 breaths a minute at rest in adults) |
| Respiratory organ | Gills (under a gill cover in bony fish) | Lungs inside the chest |
| Keeping surfaces moist | Water keeps gills moist | Lungs are inside the body to stop them drying out |
Exam tips
- The key phrase for the NCERT question is: "the amount of dissolved oxygen in water is fairly low compared with air".
- Add the result: so aquatic organisms must breathe faster to take in enough oxygen.
- Mention that fish take in dissolved oxygen through gills.
- For a 3-mark answer, add one point about how gills are adapted (large surface area, thin walls, rich blood supply).
Common mistakes
- Saying fish split water molecules to get oxygen. They use oxygen gas that is dissolved in the water.
- Saying aquatic animals breathe faster because they are more active. The reason is the low oxygen content of water.
- Writing that gills are inside lungs, or that fish have lungs.
- Forgetting that temperature affects how much oxygen water can hold.
Frequently Asked Questions
Why is the rate of breathing in aquatic organisms faster than in terrestrial organisms?
The amount of oxygen dissolved in water is much lower than the amount of oxygen in air, so aquatic organisms must breathe faster to get enough oxygen.
How much oxygen does water contain compared with air?
Air holds about 210 mL of oxygen per litre, while water holds only about 5–8 mL per litre, roughly 30 times less.
How do fish take in oxygen?
Fish take in water through the mouth and pass it over the gills. Oxygen dissolved in the water diffuses into the blood in the gills.
Why are gills good at taking in oxygen?
Gills have a large surface area, thin walls and many blood capillaries, and the blood flows opposite to the water, which helps oxygen diffuse in efficiently.
Why are human lungs inside the body?
Gas exchange needs moist surfaces. Keeping the lungs inside the chest stops them from drying out in air.