Cellular Respiration

Word equation:

glucose + oxygen → carbon dioxide + water (+ energy)

Balanced equation:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O (+ ~38 ATP)

Key features:

• Happens in EVERY living cell (mostly in the MITOCHONDRIA).
• Needs OXYGEN — delivered by breathing and the circulatory system.
• Releases LOTS of energy per glucose molecule (about 18× more than anaerobic).
• Waste products: CO₂ (breathed out) and water.

Don't confuse:

• BREATHING (ventilation) = moving air in and out of the lungs. Mechanical process.
• RESPIRATION = the chemical reaction in cells that releases energy. These are different!

When oxygen is scarce (e.g. during a sprint, or in yeast in a dough), cells switch to ANAEROBIC respiration.

In animals (human muscle):

glucose → lactic acid (+ a small amount of energy)

• No O₂ needed.
• Releases far less energy per glucose (~2 ATP vs ~38).
• Lactic acid builds up in muscles → causes muscle ACHE / FATIGUE.
• After exercise, you take in extra O₂ to break down the lactic acid (oxygen DEBT).

In yeast (used in biotechnology):

glucose → ethanol + carbon dioxide (+ small amount of energy)

• C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂.
• The CO₂ makes bread rise; the ethanol is the basis of beer/wine.

Why both pathways exist? Aerobic is far more efficient, but slow. When you need a burst of energy quickly (sprint), anaerobic kicks in — gets some ATP fast, accepts a build-up of waste, repays the oxygen debt later.

Respiration is happening in your cells RIGHT NOW. The ATP it produces powers:

• Muscle contraction (walking, breathing, heartbeat).
• Active transport across cell membranes.
• Building new molecules (proteins, DNA).
• Keeping warm (mammals release heat as a by-product).
• Nerve impulses.

In exercise:

• At rest or slow walking, you respire AEROBICALLY — your breathing rate is enough to supply O₂.
• In a sprint, your muscles need ATP faster than O₂ can be delivered → ANAEROBIC kicks in → lactic acid → muscle burn.
• Trained athletes have larger lungs and more mitochondria → can stay aerobic at higher effort → fewer aches.

MYP investigation: measure your own respiration rate by counting breaths per minute (or measuring CO₂ exhaled) before and after exercise. The rise shows how the body matches respiration to demand.