What is cell respiration?
Cell respiration is the process in which cells break down food molecules, mainly glucose, to release energy. This is a catabolic process controlled by enzymes. The energy released is stored in a molecule called ATP (adenosine triphosphate), which the cell can use for movement, active transport, growth and building new molecules.
Key idea
Respiration does not "make" energy. It transfers the chemical energy in glucose into ATP, the immediate, usable energy currency of every living cell.
Two types of respiration
Respiration can happen with or without oxygen:
- Aerobic respiration uses oxygen, fully breaks glucose into carbon dioxide and water, and releases a large amount of energy (about 38 ATP per glucose).
- Anaerobic respiration (fermentation) happens without oxygen, only partly breaks down glucose, and releases much less energy (about 2 ATP per glucose).
Where it happens
The first stage, glycolysis, occurs in the cytoplasm and does not need oxygen. The later, energy-rich stages of aerobic respiration take place inside the mitochondria, which is why mitochondria are called the "powerhouse of the cell".
Why cells need a constant energy supply
Living cells never stop working, so they need a steady supply of ATP. ATP is used for active transport across membranes, for muscle contraction, for the synthesis of large molecules such as proteins, and for cell division. When ATP is used, it releases its energy and is recycled, then respiration recharges it again. Because cells store very little ATP, respiration must go on all the time. This is why an organism that stops respiring, even briefly, quickly runs out of usable energy and cannot survive.
Example
When you run, your muscle cells respire faster to release more ATP for contraction. Your breathing and heart rate rise to deliver extra oxygen and remove the carbon dioxide produced.
Remember
Respiration (a cellular chemical process that releases energy) is NOT the same as breathing (the movement of air in and out of the lungs). Aerobic respiration releases far more energy than anaerobic.