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Cellular Respiration and Energy Metabolism, Explained

Here's how your cells actually turn food into usable energy, step by step.

  1. What is cellular respiration?

    Cellular respiration is a series of chemical reactions in which a cell breaks down organic molecules like glucose using oxygen, producing ATP — the energy source needed for the activities of life.

  2. Glycolysis

    Glycolysis is the first stage of cellular respiration, taking place in the cytoplasm, where one molecule of glucose is broken down into two molecules of pyruvate, producing a small amount of ATP and NADH. It can proceed even without oxygen.

  3. The Krebs cycle and electron transport chain

    Once pyruvate enters the mitochondria, it goes through the Krebs cycle (TCA cycle), producing carbon dioxide and electron carriers; those electron carriers then feed into the electron transport chain on the mitochondria's inner membrane, which uses oxygen as the final electron acceptor to generate a large amount of ATP.

  4. What mitochondria do

    Mitochondria are equipped with a double-membrane structure and an inner membrane (the cristae) with a greatly expanded surface area, which lets them produce ATP efficiently — which is why they're often called the 'powerhouse of the cell.'

  5. Aerobic vs. anaerobic respiration

    When there's enough oxygen, aerobic respiration can yield up to about 36-38 ATP molecules from a single glucose molecule, but when oxygen is scarce, anaerobic processes — like lactic acid fermentation in muscle or alcoholic fermentation in yeast — yield far less ATP.

  6. The connection to body temperature

    Some of the energy generated during cellular respiration is released as heat, and that heat plays an important role in helping warm-blooded animals, including humans, keep their body temperature steady.

How does our body get its energy?

The glucose you get from eating food isn't, by itself, energy your body can use directly — it has to go through the chemical reactions of cellular respiration inside cells before it's converted into ATP, the form of energy your body can actually put to use for physical activity. Understanding each stage of cellular respiration reveals the principle behind how your body generates energy.

See the enzymes that drive cellular respiration, too

Every stage of cellular respiration, from glycolysis to the electron transport chain, cannot proceed properly without the catalytic action of a great many enzymes. If you're curious about the principle behind how enzymes speed up reactions, that's what enzyme basics covers.

Frequently Asked Questions

Is the muscle soreness you feel during exercise related to cellular respiration?

Yes. When intense exercise creates an oxygen shortage, muscle cells generate energy through the anaerobic process of lactic acid fermentation, and the lactic acid that builds up during this process is thought to be related to muscle fatigue.

Are cellular respiration and breathing (respiration in the everyday sense) the same thing?

No. Breathing — inhaling and exhaling air through your nose and lungs — is the process of bringing oxygen into your body and expelling carbon dioxide. Cellular respiration refers to the chemical reactions that actually use that oxygen to generate energy inside cells.