metabolism
Meaning of metabolism: Two opposing sets of reactions run at once, one tearing molecules apart for energy and the other spending that energy building new ones.
Definition of metabolism
Metabolism is the whole set of chemical reactions that keep an organism alive, converting food into usable energy, assembling the materials cells need, and clearing the waste left behind. It has two halves that run continuously side by side: catabolism, which breaks larger molecules down and releases energy, and anabolism, which uses that energy to build molecules up.
What are catabolism and anabolism?
| Catabolism | Anabolism | |
|---|---|---|
| Direction | Breaks large molecules into small ones | Builds small molecules into large ones |
| Energy | Releases it, most of it captured as ATP | Consumes it |
| Examples | Digesting starch to glucose, burning fat for fuel | Building muscle protein, storing glucose as glycogen |
The two never stop and rarely balance exactly. After a meal, building predominates; between meals and during exertion, breaking down takes over. Energy balance across weeks and months is simply the running total of the two. Nearly all the energy freed by catabolism is first parked in one carrier molecule, ATP, which every energy consuming process in the cell then spends.
What sets the rate?
The largest share of daily energy use is not exercise but the cost of staying alive: pumping salts across membranes, replacing worn proteins, driving the heart and lungs, and holding body temperature steady. That baseline tracks lean tissue mass more closely than body weight, which is why two people who weigh the same can differ. Thyroid hormone sets the overall pace, and too much or too little shifts it noticeably in either direction. Fever, injury, infection and healing all raise demand. Aerobic metabolism, which uses oxygen, extracts far more energy from a given amount of fuel than the oxygen free route muscle falls back on during a hard sprint.
Does the word mean the same thing for drugs?
Not quite. Drug metabolism means the chemical alteration a medicine undergoes inside the body, mostly in the liver, which generally converts it into something more water soluble and easier for the kidneys to remove. Sometimes that change destroys the drug's activity; sometimes it creates it, as with medicines deliberately given in an inactive form. Inherited differences in these liver enzymes explain why an identical dose can behave differently in two people. It is still cellular metabolism in the broad sense, but the question being asked is disposal rather than energy supply.
Also known as
Used in a sentence
Thyroid hormones have an important effect on metabolism.
Good to know
Metabolic rate varies with age, body composition, hormone levels, physical activity and illness, so two people of similar size can use energy at noticeably different rates.
Word origin
Where “metabolism” comes from
From Greek metabole, meaning change.