Respiration

Respiration is how living cells release energy from food. Every cell in every living thing — plant, animal, person — needs energy to move, grow, build new cells and repair damage. That energy comes from glucose (a sugar) reacting with oxygen inside the cells. So respiration is happening RIGHT NOW in every cell of your body.

Two important things to keep separate:

• Breathing is the physical movement of air in and out of the lungs.
• Respiration is the chemical reaction that releases energy from food INSIDE cells.

You can breathe without thinking about it; respiration happens automatically in every cell. They are not the same thing — students lose marks by confusing them.

The word equation for aerobic respiration:

Glucose + Oxygen → Carbon dioxide + Water + Energy

The chemical equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy.

Notice that aerobic respiration is the exact reverse of photosynthesis — photosynthesis takes in CO₂ and water and makes glucose and O₂; respiration takes glucose and O₂ and releases CO₂ and water.

Aerobic vs anaerobic respiration:

• Aerobic respiration — happens when there is enough oxygen. Produces lots of energy. Word equation above. Happens in most of our cells most of the time.
• Anaerobic respiration — happens when there is NOT enough oxygen (for example, when you sprint very fast and your muscles can't get oxygen quickly enough). In humans, it produces lactic acid in the muscles — which is why your muscles ache after hard exercise. It releases less energy.

In yeast and some bacteria, anaerobic respiration produces ethanol (alcohol) and CO₂. This is how bread rises and how chang'aa, beer and wine are made — the yeast respires anaerobically using sugar, releasing CO₂ (which makes bread rise) and ethanol (which makes the drink alcoholic).

The human breathing system:

Air enters through the nose or mouth, passes through the trachea (windpipe), splits into two bronchi (one for each lung), branches into smaller bronchioles, and ends at tiny air sacs called alveoli. Each alveolus is surrounded by tiny blood capillaries. Oxygen passes from the alveoli into the blood; carbon dioxide passes from the blood into the alveoli to be breathed out.

Inhalation and exhalation:

• Inhalation (breathing in): the diaphragm (a sheet of muscle below the lungs) contracts and flattens; the rib muscles pull the ribs up and out; the chest expands; air rushes in.
• Exhalation (breathing out): the diaphragm relaxes and domes up; the ribs drop down and in; the chest gets smaller; air rushes out.

Where does the energy go?

The energy released by respiration is used for:

• Movement — running, walking, lifting.
• Growth — building new cells.
• Repair — healing wounds.
• Heat — keeping body temperature at 37°C.
• Chemical reactions — making proteins, sending nerve signals, all the other work of living.

Common student mistakes to avoid:

• Saying respiration only happens in the lungs. It happens in every living cell.
• Calling breathing "respiration". Breathing is the AIR movement; respiration is the CHEMICAL reaction inside cells.
• Thinking plants don't respire. Plants respire all the time — day AND night. In daylight they ALSO photosynthesise, but they never stop respiring.
• Confusing the trachea (windpipe — air) with the oesophagus (food pipe — food).
• Forgetting water is a product. The equation produces CO₂ AND water (that's why your breath fogs a cold mirror).

CBC Grade 5 introduces breathing and the lungs; Grade 6 covers the difference between breathing and respiration, and the word equation; Grade 7–9 Integrated Science extends to aerobic vs anaerobic respiration, the breathing system in detail, and connections to fitness, smoking and lung disease — material that appears in KPSEA and KJSEA.