Where your body's energy comes from
Aerobic cellular respiration · Redox · Exothermic
Your cells burn sugar with the same overall equation as a campfire, just in dozens of controlled steps.
Reactants approach
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ΔH = -2,812 kJ · exothermic, releases heat
ΔH per mol dissolved glucose (about -2803 kJ for solid glucose).
Meet the molecules
- Reactantdissolved in water
C6H12O6
glucose
- Reactantgas
O2
oxygen
- Productgas
CO2
carbon dioxide
- Productliquid
H2O
water
What you notice
Every breath in brings oxygen to your cells, and every breath out carries away carbon dioxide made from the food you ate. In each cell, mitochondria break glucose down step by step and capture part of the released energy in ATP, the molecule that powers muscle contraction, nerve signals and protein building. The overall equation is the same as for burning sugar in a flame, but the energy comes out in many small steps instead of as a burst of heat and light. Well over half still ends up as heat, which keeps your body near 37 °C.
Try it
Count your breaths per minute at rest, then again right after a minute of jumping jacks; the faster breathing supplies extra O2 and removes extra CO2.
Step by step
- 1
Glycolysis
In the cytoplasm, glucose is split into two pyruvate molecules, giving a net 2 ATP and loading electrons onto NAD+ to make NADH.
- 2
Krebs cycle
In the mitochondria, pyruvate's carbon atoms are removed as CO2, which diffuses into the blood and leaves through your lungs. More NADH and FADH2 are made.
- 3
Electron transport
NADH and FADH2 pass electrons along a chain of membrane proteins. The energy pumps H+ across the membrane, and ATP synthase uses the flow back to make about 30 ATP per glucose.
- 4
Oxygen is the last stop
At the end of the chain, O2 accepts the electrons and H+ ions to become water. Without oxygen the chain stops, which is why you have to keep breathing.