Respiration in Organisms

• What respiration is — and why it is NOT the same as breathing.
• Aerobic respiration: equation, products, where it happens.
• Anaerobic respiration: equation, when it happens, products.
• How different organisms breathe (lungs, gills, skin, spiracles).
• Why we breathe faster during exercise.

What is respiration?

• Respiration: a biochemical process occurring in all living cells where food (glucose) is broken down to release energy (ATP).
• Happens in the mitochondria of cells.
• NOT the same as breathing — breathing is just the mechanical movement of air in and out of lungs.

Respiration ≠ Breathing

• Respiration = cellular energy release (biochemical, in all cells).
• Breathing = taking in air (mechanical, in lungs/gills).

Aerobic respiration

• Aerobic respiration: breakdown of glucose in the presence of oxygen.
• Produces: CO₂ + H₂O + energy (ATP).
• Occurs in: mitochondria.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) Glucose + Oxygen → Carbon dioxide + Water + Energy

• Releases much more energy than anaerobic (≈ 38 ATP per glucose).
• Used by most organisms including humans, animals, plants, most fungi.

Anaerobic respiration

• Anaerobic respiration: breakdown of glucose without oxygen.
• Releases less energy than aerobic (≈ 2 ATP per glucose).

In yeast (alcoholic fermentation):

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy Glucose → Ethanol (alcohol) + Carbon dioxide + Energy

• Used to make bread (CO₂ makes dough rise), beer, wine, yogurt (partially).
• Yeast dies in high-alcohol concentrations.

In human muscle cells (lactic acid fermentation):

C₆H₁₂O₆ → 2C₃H₆O₃ + Energy Glucose → Lactic acid + Energy

• Happens during intense exercise when O₂ supply to muscles is insufficient.
• Lactic acid build-up causes muscle cramps and fatigue.
• Removed by the body once O₂ supply is restored (you breathe heavily to repay the "oxygen debt").

In some microbes:

• Certain bacteria in waterlogged soil, marshes, intestines — produce methane or other products.
• Biogas digesters use anaerobic bacteria to produce methane (CH₄) from organic waste.

Comparison: Aerobic vs Anaerobic

How different organisms breathe

Human breathing mechanism

• Respiratory system: nostrils → nasal cavity → trachea → bronchi → bronchioles → alveoli (lungs).
• Alveoli: tiny air sacs; thin walls + rich blood supply = efficient gas exchange.
  • O₂ diffuses from alveolar air → blood.
  • CO₂ diffuses from blood → alveolar air.
• Diaphragm (dome-shaped muscle) + intercostal muscles control breathing movements.

Breathing movements:

• Normal breathing rate: 15–18 breaths/min at rest; up to 25+ during exercise.

Why do we breathe faster during exercise?

• Muscles need more ATP → require more O₂ and produce more CO₂.
• Rising CO₂ levels in blood detected by brain's medulla oblongata.
• Medulla sends signals → breathing rate and depth increases.
• Heart rate also increases → blood delivers O₂ faster.

Oxygen debt

• During intense exercise: muscles switch to anaerobic (insufficient O₂) → lactic acid builds up.
• After exercise: breathe heavily to take in extra O₂ to break down accumulated lactic acid.
• This extra O₂ consumed = oxygen debt.

Respiration in plants

• Plants respire 24 hours a day (not just at night).
• During day: photosynthesis produces more O₂ than respiration consumes → net gas exchange appears opposite.
• At night: only respiration (no photosynthesis) → absorb O₂, release CO₂.
• Gas exchange occurs through stomata in leaves and lenticels in stems.

• What is the difference between respiration and breathing?
• Write the equation for aerobic respiration. Where does it occur in the cell?
• What is lactic acid fermentation? When does it happen in humans?
• How do earthworms breathe? Why must they stay in moist soil?
• Why do we breathe faster after running? What is oxygen debt?

Open the Practice tab for graded questions on Respiration in Organisms.